File: rfc1716.txt

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Network Working Group                                P. Almquist, Author
Request for Comments: 1716                                    Consultant
Category: Informational                            F. Kastenholz, Editor
                                                      FTP Software, Inc.
                                                           November 1994


                  Towards Requirements for IP Routers

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.





































Almquist & Kastenholz                                           [Page i]

RFC 1716          Towards Requirements for IP Routers      November 1994


Table of Contents


0.  PREFACE .......................................................    1
1.  INTRODUCTION ..................................................    2
1.1  Reading this Document ........................................    4
1.1.1  Organization ...............................................    4
1.1.2  Requirements ...............................................    5
1.1.3  Compliance .................................................    6
1.2  Relationships to Other Standards .............................    7
1.3  General Considerations .......................................    8
1.3.1  Continuing Internet Evolution ..............................    8
1.3.2  Robustness Principle .......................................    9
1.3.3  Error Logging ..............................................    9
1.3.4  Configuration ..............................................   10
1.4  Algorithms ...................................................   11
2.  INTERNET ARCHITECTURE .........................................   13
2.1  Introduction .................................................   13
2.2  Elements of the Architecture .................................   14
2.2.1  Protocol Layering ..........................................   14
2.2.2  Networks ...................................................   16
2.2.3  Routers ....................................................   17
2.2.4  Autonomous Systems .........................................   18
2.2.5  Addresses and Subnets ......................................   18
2.2.6  IP Multicasting ............................................   20
2.2.7  Unnumbered Lines and Networks and Subnets ..................   20
2.2.8  Notable Oddities ...........................................   22
2.2.8.1  Embedded Routers .........................................   22
2.2.8.2  Transparent Routers ......................................   23
2.3  Router Characteristics .......................................   24
2.4  Architectural Assumptions ....................................   27
3.  LINK LAYER ....................................................   29
3.1  INTRODUCTION .................................................   29
3.2  LINK/INTERNET LAYER INTERFACE ................................   29
3.3  SPECIFIC ISSUES ..............................................   30
3.3.1  Trailer Encapsulation ......................................   30
3.3.2  Address Resolution Protocol - ARP ..........................   31
3.3.3  Ethernet and 802.3 Coexistence .............................   31
3.3.4  Maximum Transmission Unit - MTU ............................   31
3.3.5  Point-to-Point Protocol - PPP ..............................   32
3.3.5.1  Introduction .............................................   32
3.3.5.2  Link Control Protocol (LCP) Options ......................   33
3.3.5.3  IP Control Protocol (ICP) Options ........................   34
3.3.6  Interface Testing ..........................................   35
4.  INTERNET LAYER - PROTOCOLS ....................................   36
4.1  INTRODUCTION .................................................   36
4.2  INTERNET PROTOCOL - IP .......................................   36


Almquist & Kastenholz                                          [Page ii]

RFC 1716          Towards Requirements for IP Routers      November 1994


4.2.1  INTRODUCTION ...............................................   36
4.2.2  PROTOCOL WALK-THROUGH ......................................   37
4.2.2.1  Options: RFC-791 Section 3.2 .............................   37
4.2.2.2  Addresses in Options: RFC-791 Section 3.1 ................   40
4.2.2.3  Unused IP Header Bits: RFC-791 Section 3.1 ...............   40
4.2.2.4  Type of Service: RFC-791 Section 3.1 .....................   41
4.2.2.5  Header Checksum: RFC-791 Section 3.1 .....................   41
4.2.2.6  Unrecognized Header Options: RFC-791 Section 3.1 .........   41
4.2.2.7  Fragmentation: RFC-791 Section 3.2 .......................   42
4.2.2.8  Reassembly: RFC-791 Section 3.2 ..........................   43
4.2.2.9  Time to Live: RFC-791 Section 3.2 ........................   43
4.2.2.10  Multi-subnet Broadcasts: RFC-922 ........................   43
4.2.2.11  Addressing: RFC-791 Section 3.2 .........................   43
4.2.3  SPECIFIC ISSUES ............................................   47
4.2.3.1  IP Broadcast Addresses ...................................   47
4.2.3.2  IP Multicasting ..........................................   48
4.2.3.3  Path MTU Discovery .......................................   48
4.2.3.4  Subnetting ...............................................   49
4.3  INTERNET CONTROL MESSAGE PROTOCOL - ICMP .....................   50
4.3.1  INTRODUCTION ...............................................   50
4.3.2  GENERAL ISSUES .............................................   50
4.3.2.1  Unknown Message Types ....................................   50
4.3.2.2  ICMP Message TTL .........................................   51
4.3.2.3  Original Message Header ..................................   51
4.3.2.4  ICMP Message Source Address ..............................   51
4.3.2.5  TOS and Precedence .......................................   51
4.3.2.6  Source Route .............................................   52
4.3.2.7  When Not to Send ICMP Errors .............................   53
4.3.2.8  Rate Limiting ............................................   54
4.3.3  SPECIFIC ISSUES ............................................   55
4.3.3.1  Destination Unreachable ..................................   55
4.3.3.2  Redirect .................................................   55
4.3.3.3  Source Quench ............................................   56
4.3.3.4  Time Exceeded ............................................   56
4.3.3.5  Parameter Problem ........................................   57
4.3.3.6  Echo Request/Reply .......................................   57
4.3.3.7  Information Request/Reply ................................   58
4.3.3.8  Timestamp and Timestamp Reply ............................   58
4.3.3.9  Address Mask Request/Reply ...............................   59
4.3.3.10  Router Advertisement and Solicitations ..................   61
4.4  INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ....................   61
5.  INTERNET LAYER - FORWARDING ...................................   62
5.1  INTRODUCTION .................................................   62
5.2  FORWARDING WALK-THROUGH ......................................   62
5.2.1  Forwarding Algorithm .......................................   62
5.2.1.1  General ..................................................   63
5.2.1.2  Unicast ..................................................   64


Almquist & Kastenholz                                         [Page iii]

RFC 1716          Towards Requirements for IP Routers      November 1994


5.2.1.3  Multicast ................................................   65
5.2.2  IP Header Validation .......................................   66
5.2.3  Local Delivery Decision ....................................   68
5.2.4  Determining the Next Hop Address ...........................   70
5.2.4.1  Immediate Destination Address ............................   71
5.2.4.2  Local/Remote Decision ....................................   71
5.2.4.3  Next Hop Address .........................................   72
5.2.4.4  Administrative Preference ................................   77
5.2.4.6  Load Splitting ...........................................   78
5.2.5  Unused IP Header Bits: RFC-791 Section 3.1 .................   79
5.2.6  Fragmentation and Reassembly: RFC-791 Section 3.2 ..........   79
5.2.7  Internet Control Message Protocol - ICMP ...................   80
5.2.7.1  Destination Unreachable ..................................   80
5.2.7.2  Redirect .................................................   82
5.2.7.3  Time Exceeded ............................................   84
5.2.8  INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ..................   84
5.3  SPECIFIC ISSUES ..............................................   84
5.3.1  Time to Live (TTL) .........................................   84
5.3.2  Type of Service (TOS) ......................................   85
5.3.3  IP Precedence ..............................................   87
5.3.3.1  Precedence-Ordered Queue Service .........................   88
5.3.3.2  Lower Layer Precedence Mappings ..........................   88
5.3.3.3  Precedence Handling For All Routers ......................   89
5.3.4  Forwarding of Link Layer Broadcasts ........................   92
5.3.5  Forwarding of Internet Layer Broadcasts ....................   92
5.3.5.1  Limited Broadcasts .......................................   94
5.3.5.2  Net-directed Broadcasts ..................................   94
5.3.5.3  All-subnets-directed Broadcasts ..........................   95
5.3.5.4  Subnet-directed Broadcasts ...............................   97
5.3.6  Congestion Control .........................................   97
5.3.7  Martian Address Filtering ..................................   99
5.3.8  Source Address Validation ..................................   99
5.3.9  Packet Filtering and Access Lists ..........................  100
5.3.10  Multicast Routing .........................................  101
5.3.11  Controls on Forwarding ....................................  101
5.3.12  State Changes .............................................  101
5.3.12.1  When a Router Ceases Forwarding .........................  102
5.3.12.2  When a Router Starts Forwarding .........................  102
5.3.12.3  When an Interface Fails or is Disabled ..................  103
5.3.12.4  When an Interface is Enabled ............................  103
5.3.13  IP Options ................................................  103
5.3.13.1  Unrecognized Options ....................................  103
5.3.13.2  Security Option .........................................  104
5.3.13.3  Stream Identifier Option ................................  104
5.3.13.4  Source Route Options ....................................  104
5.3.13.5  Record Route Option .....................................  104
5.3.13.6  Timestamp Option ........................................  105


Almquist & Kastenholz                                          [Page iv]

RFC 1716          Towards Requirements for IP Routers      November 1994


6.  TRANSPORT LAYER ...............................................  106
6.1  USER DATAGRAM PROTOCOL - UDP .................................  106
6.2  TRANSMISSION CONTROL PROTOCOL - TCP ..........................  106
7.  APPLICATION LAYER - ROUTING PROTOCOLS .........................  109
7.1  INTRODUCTION .................................................  109
7.1.1  Routing Security Considerations ............................  109
7.1.2  Precedence .................................................  110
7.2  INTERIOR GATEWAY PROTOCOLS ...................................  110
7.2.1  INTRODUCTION ...............................................  110
7.2.2  OPEN SHORTEST PATH FIRST - OSPF ............................  111
7.2.2.1  Introduction .............................................  111
7.2.2.2  Specific Issues ..........................................  111
7.2.2.3  New Version of OSPF ......................................  112
7.2.3  INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM -  DUAL  IS-IS
     ..............................................................  112
7.2.4  ROUTING INFORMATION PROTOCOL - RIP .........................  113
7.2.4.1  Introduction .............................................  113
7.2.4.2  Protocol Walk-Through ....................................  113
7.2.4.3  Specific Issues ..........................................  118
7.2.5  GATEWAY TO GATEWAY PROTOCOL - GGP ..........................  119
7.3  EXTERIOR GATEWAY PROTOCOLS ...................................  119
7.3.1  INTRODUCTION ...............................................  119
7.3.2  BORDER GATEWAY PROTOCOL - BGP ..............................  120
7.3.2.1  Introduction .............................................  120
7.3.2.2  Protocol Walk-through ....................................  120
7.3.3  EXTERIOR GATEWAY PROTOCOL - EGP ............................  121
7.3.3.1  Introduction .............................................  121
7.3.3.2  Protocol Walk-through ....................................  122
7.3.4  INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL ..............  124
7.4  STATIC ROUTING ...............................................  125
7.5  FILTERING OF ROUTING INFORMATION .............................  127
7.5.1  Route Validation ...........................................  127
7.5.2  Basic Route Filtering ......................................  127
7.5.3  Advanced Route Filtering ...................................  128
7.6  INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE ..................  129
8.  APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS ..............  131
8.1  The Simple Network Management Protocol - SNMP ................  131
8.1.1  SNMP Protocol Elements .....................................  131
8.2  Community Table ..............................................  132
8.3  Standard MIBS ................................................  133
8.4  Vendor Specific MIBS .........................................  134
8.5  Saving Changes ...............................................  135
9.  APPLICATION LAYER - MISCELLANEOUS PROTOCOLS ...................  137
9.1  BOOTP ........................................................  137
9.1.1  Introduction ...............................................  137
9.1.2  BOOTP Relay Agents .........................................  137
10.  OPERATIONS AND MAINTENANCE ...................................  139


Almquist & Kastenholz                                           [Page v]

RFC 1716          Towards Requirements for IP Routers      November 1994


10.1  Introduction ................................................  139
10.2  Router Initialization .......................................  140
10.2.1  Minimum Router Configuration ..............................  140
10.2.2  Address and Address Mask Initialization ...................  141
10.2.3  Network Booting using BOOTP and TFTP ......................  142
10.3  Operation and Maintenance ...................................  143
10.3.1  Introduction ..............................................  143
10.3.2  Out Of Band Access ........................................  144
10.3.2  Router O&M Functions ......................................  144
10.3.2.1  Maintenance - Hardware Diagnosis ........................  144
10.3.2.2  Control - Dumping and Rebooting .........................  145
10.3.2.3  Control - Configuring the Router ........................  145
10.3.2.4  Netbooting of System Software ...........................  146
10.3.2.5  Detecting and responding to misconfiguration ............  146
10.3.2.6  Minimizing Disruption ...................................  147
10.3.2.7  Control - Troubleshooting Problems ......................  148
10.4  Security Considerations .....................................  149
10.4.1  Auditing and Audit Trails .................................  149
10.4.2  Configuration Control .....................................  150
11.  REFERENCES ...................................................  152
APPENDIX  A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ................  162
APPENDIX  B. GLOSSARY .............................................  164
APPENDIX  C. FUTURE DIRECTIONS ....................................  169
APPENDIX D.  Multicast Routing Protocols ..........................  172
D.1  Introduction .................................................  172
D.2  Distance Vector Multicast Routing Protocol - DVMRP ...........  172
D.3  Multicast Extensions to OSPF - MOSPF .........................  173
APPENDIX E  Additional Next-Hop Selection Algorithms ..............  174
E.1. Some Historical Perspective ..................................  174
E.2. Additional Pruning Rules .....................................  176
E.3  Some Route Lookup Algorithms .................................  177
E.3.1 The Revised Classic Algorithm ...............................  178
E.3.2 The Variant Router Requirements Algorithm ...................  179
E.3.3 The OSPF Algorithm ..........................................  179
E.3.4 The Integrated IS-IS Algorithm ..............................  180
Security Considerations ...........................................  182
Acknowledgments ...................................................  183
Editor's Address ..................................................  186










Almquist & Kastenholz                                          [Page vi]

RFC 1716          Towards Requirements for IP Routers      November 1994


0.  PREFACE

This document is a snapshot of the work of the Router Requirements
working group as of November 1991.  At that time, the working group had
essentially finished its task.  There were some final technical matters
to be nailed down, and a great deal of editing needed to be done in
order to get the document ready for publication.  Unfortunately, these
tasks were never completed.

At the request of the Internet Area Director, the current editor took
the last draft of the document and, after consulting the mailing list
archives, meeting minutes, notes, and other members of the working
group, edited the document to its current form.  This effort included
the following tasks: 1) Deleting all the parenthetical material (such as
editor's comments). Useful information was turned into DISCUSSION
sections, the rest was deleted.  2) Completing the tasks listed in the
last draft's To be Done sections. As a part of this task, a new "to be
done" list was developed and included as an appendix to the current
document.  3) Rolling Philip Almquist's "Ruminations on the Next Hop"
and "Ruminations on Route Leaking" into this document.  These represent
significant work and should be kept.  4) Fulfilling the last intents of
the working group as determined from the archival material.  The intent
of this effort was to get the document into a form suitable for
publication as an Historical RFC so that the significant work which went
into the creation of this document would be preserved.

The content and form of this document are due, in large part, to the
working group's chair, and document's original editor and author: Philip
Almquist.  Without his efforts, this document would not exist.



















Almquist & Kastenholz                                           [Page 1]

RFC 1716          Towards Requirements for IP Routers      November 1994


1.  INTRODUCTION

The goal of this work is to replace RFC-1009, Requirements for Internet
Gateways ([INTRO:1]) with a new document.

This memo is an intermediate step toward that goal. It defines and
discusses requirements for devices which perform the network layer
forwarding function of the Internet protocol suite.  The Internet
community usually refers to such devices as IP routers or simply
routers; The OSI community refers to such devices as intermediate
systems.  Many older Internet documents refer to these devices as
gateways, a name which more recently has largely passed out of favor to
avoid confusion with application gateways.

An IP router can be distinguished from other sorts of packet switching
devices in that a router examines the IP protocol header as part of the
switching process.  It generally has to modify the IP header and to
strip off and replace the Link Layer framing.

The authors of this memo recognize, as should its readers, that many
routers support multiple protocol suites, and that support for multiple
protocol suites will be required in increasingly large parts of the
Internet in the future.  This memo, however, does not attempt to specify
Internet requirements for protocol suites other than TCP/IP.

This document enumerates standard protocols that a router connected to
the Internet must use, and it incorporates by reference the RFCs and
other documents describing the current specifications for these
protocols.  It corrects errors in the referenced documents and adds
additional discussion and guidance for an implementor.

For each protocol, this final version of this memo also contains an
explicit set of requirements, recommendations, and options.  The reader
must understand that the list of requirements in this memo is incomplete
by itself; the complete set of requirements for an Internet protocol
router is primarily defined in the standard protocol specification
documents, with the corrections, amendments, and supplements contained
in this memo.

This memo should be read in conjunction with the Requirements for
Internet Hosts RFCs ([INTRO:2] and [INTRO:3]).  Internet hosts and
routers must both be capable of originating IP datagrams and receiving
IP datagrams destined for them.  The major distinction between Internet
hosts and routers is that routers are required to implement forwarding
algorithms and Internet hosts do not require forwarding capabilities.
Any Internet host acting as a router must adhere to the requirements
contained in the final version of this memo.


Almquist & Kastenholz                                           [Page 2]

RFC 1716          Towards Requirements for IP Routers      November 1994


The goal of open system interconnection dictates that routers must
function correctly as Internet hosts when necessary.  To achieve this,
this memo provides guidelines for such instances.  For simplification
and ease of document updates, this memo tries to avoid overlapping
discussions of host requirements with [INTRO:2] and [INTRO:3] and
incorporates the relevant requirements of those documents by reference.
In some cases the requirements stated in [INTRO:2] and [INTRO:3] are
superseded by the final version of this document.

A good-faith implementation of the protocols produced after careful
reading of the RFCs, with some interaction with the Internet technical
community, and that follows good communications software engineering
practices, should differ from the requirements of this memo in only
minor ways.  Thus, in many cases, the requirements in this document are
already stated or implied in the standard protocol documents, so that
their inclusion here is, in a sense, redundant.  However, they were
included because some past implementation has made the wrong choice,
causing problems of interoperability, performance, and/or robustness.

This memo includes discussion and explanation of many of the
requirements and recommendations.  A simple list of requirements would
be dangerous, because:

o  Some required features are more important than others, and some
   features are optional.

o  Some features are critical in some applications of routers but
   irrelevant in others.

o  There may be valid reasons why particular vendor products that are
   designed for restricted contexts might choose to use different
   specifications.

However, the specifications of this memo must be followed to meet the
general goal of arbitrary router interoperation across the diversity and
complexity of the Internet.  Although most current implementations fail
to meet these requirements in various ways, some minor and some major,
this specification is the ideal towards which we need to move.

These requirements are based on the current level of Internet
architecture.  This memo will be updated as required to provide
additional clarifications or to include additional information in those
areas in which specifications are still evolving.





Almquist & Kastenholz                                           [Page 3]

RFC 1716          Towards Requirements for IP Routers      November 1994


1.1  Reading this Document


1.1.1  Organization

      This memo emulates the layered organization used by [INTRO:2] and
      [INTRO:3].  Thus, Chapter 2 describes the layers found in the
      Internet architecture.  Chapter 3 covers the Link Layer.  Chapters
      4 and 5 are concerned with the Internet Layer protocols and
      forwarding algorithms.  Chapter 6 covers the Transport Layer.
      Upper layer protocols are divided between Chapter 7, which
      discusses the protocols which routers use to exchange routing
      information with each other, Chapter 8, which discusses network
      management, and Chapter 9, which discusses other upper layer
      protocols.  The final chapter covers operations and maintenance
      features.  This organization was chosen for simplicity, clarity,
      and consistency with the Host Requirements RFCs.  Appendices to
      this memo include a bibliography, a glossary, and some conjectures
      about future directions of router standards.

      In describing the requirements, we assume that an implementation
      strictly mirrors the layering of the protocols.  However, strict
      layering is an imperfect model, both for the protocol suite and
      for recommended implementation approaches.  Protocols in different
      layers interact in complex and sometimes subtle ways, and
      particular functions often involve multiple layers.  There are
      many design choices in an implementation, many of which involve
      creative breaking of strict layering.  Every implementor is urged
      to read [INTRO:4] and [INTRO:5].

      In general, each major section of this memo is organized into the
      following subsections:

      (1)  Introduction

      (2)  Protocol Walk-Through - considers the protocol specification
           documents section-by-section, correcting errors, stating
           requirements that may be ambiguous or ill-defined, and
           providing further clarification or explanation.

      (3)  Specific Issues - discusses protocol design and
           implementation issues that were not included in the walk-
           through.

      Under many of the individual topics in this memo, there is
      parenthetical material labeled DISCUSSION or IMPLEMENTATION. This
      material is intended to give a justification, clarification or


Almquist & Kastenholz                                           [Page 4]

RFC 1716          Towards Requirements for IP Routers      November 1994


      explanation to the preceding requirements text.  The
      implementation material contains suggested approaches that an
      implementor may want to consider.  The DISCUSSION and
      IMPLEMENTATION sections are not part of the standard.

1.1.2  Requirements

      In this memo, the words that are used to define the significance
      of each particular requirement are capitalized.  These words are:

      o  MUST
         This word means that the item is an absolute requirement of the
         specification.

      o  MUST IMPLEMENT
         This phrase means that this specification requires that the
         item be implemented, but does not require that it be enabled by
         default.

      o  MUST NOT
         This phrase means that the item is an absolute prohibition of
         the specification.

      o  SHOULD
         This word means that there may exist valid reasons in
         particular circumstances to ignore this item, but the full
         implications should be understood and the case carefully
         weighed before choosing a different course.

      o  SHOULD IMPLEMENT
         This phrase is similar in meaning to SHOULD, but is used when
         we recommend that a particular feature be provided but does not
         necessarily recommend that it be enabled by default.

      o  SHOULD NOT
         This phrase means that there may exist valid reasons in
         particular circumstances when the described behavior is
         acceptable or even useful, but the full implications should be
         understood and the case carefully weighed before implementing
         any behavior described with this label.

      o  MAY
         This word means that this item is truly optional.  One vendor
         may choose to include the item because a particular marketplace
         requires it or because it enhances the product, for example;
         another vendor may omit the same item.


Almquist & Kastenholz                                           [Page 5]

RFC 1716          Towards Requirements for IP Routers      November 1994


1.1.3  Compliance

      Some requirements are applicable to all routers.  Other
      requirements are applicable only to those which implement
      particular features or protocols.  In the following paragraphs,
      Relevant refers to the union of the requirements applicable to all
      routers and the set of requirements applicable to a particular
      router because of the set of features and protocols it has
      implemented.

      Note that not all Relevant requirements are stated directly in
      this memo.  Various parts of this memo incorporate by reference
      sections of the Host Requirements specification, [INTRO:2] and
      [INTRO:3].  For purposes of determining compliance with this memo,
      it does not matter whether a Relevant requirement is stated
      directly in this memo or merely incorporated by reference from one
      of those documents.

      An implementation is said to be conditionally compliant if it
      satisfies all of the Relevant MUST, MUST IMPLEMENT, and MUST NOT
      requirements.  An implementation is said to be unconditionally
      compliant if it is conditionally compliant and also satisfies all
      of the Relevant SHOULD, SHOULD IMPLEMENT, and SHOULD NOT
      requirements.  An implementation is not compliant if it is not
      conditionally compliant (i.e., it fails to satisfy one or more of
      the Relevant MUST, MUST IMPLEMENT, or MUST NOT requirements).

      For any of the SHOULD and SHOULD NOT requirements, a router may
      provide a configuration option that will cause the router to act
      other than as specified by the requirement.  Having such a
      configuration option does not void a router's claim to
      unconditional compliance as long as the option has a default
      setting, and that leaving the option at its default setting causes
      the router to operate in a manner which conforms to the
      requirement.

      Likewise, routers may provide, except where explicitly prohibited
      by this memo, options which cause them to violate MUST or MUST NOT
      requirements.  A router which provides such options is compliant
      (either fully or conditionally) if and only if each such option
      has a default setting which causes the router to conform to the
      requirements of this memo.  Please note that the authors of this
      memo, although aware of market realities, strongly recommend
      against provision of such options.  Requirements are labeled MUST
      or MUST NOT because experts in the field have judged them to be
      particularly important to interoperability or proper functioning
      in the Internet.  Vendors should weigh carefully the customer


Almquist & Kastenholz                                           [Page 6]

RFC 1716          Towards Requirements for IP Routers      November 1994


      support costs of providing options which violate those rules.

      Of course, this memo is not a complete specification of an IP
      router, but rather is closer to what in the OSI world is called a
      profile.  For example, this memo requires that a number of
      protocols be implemented.  Although most of the contents of their
      protocol specifications are not repeated in this memo,
      implementors are nonetheless required to implement the protocols
      according to those specifications.

1.2  Relationships to Other Standards

   There are several reference documents of interest in checking the
   current status of protocol specifications and standardization:

     o  INTERNET OFFICIAL PROTOCOL STANDARDS
        This document describes the Internet standards process and lists
        the standards status of the protocols.  As of this writing, the
        current version of this document is STD 1, RFC 1610, [ARCH:7].
        This document is periodically re-issued.  You should always
        consult an RFC repository and use the latest version of this
        document.

     o  Assigned Numbers
        This document lists the assigned values of the parameters used
        in the various protocols.  For example, IP protocol codes, TCP
        port numbers, Telnet Option Codes, ARP hardware types, and
        Terminal Type names.  As of this writing, the current version of
        this document is STD 2, RFC 1700, [INTRO:7].  This document is
        periodically re-issued.  You should always consult an RFC
        repository and use the latest version of this document.

     o  Host Requirements
        This pair of documents reviews the specifications that apply to
        hosts and supplies guidance and clarification for any
        ambiguities.  Note that these requirements also apply to
        routers, except where otherwise specified in this memo.  As of
        this writing (December, 1993) the current versions of these
        documents are RFC 1122 and RFC 1123, (STD 3) [INTRO:2], and
        [INTRO:3] respectively.

     o  Router Requirements (formerly Gateway Requirements)
        This memo.

     Note that these documents are revised and updated at different
     times; in case of differences between these documents, the most
     recent must prevail.


Almquist & Kastenholz                                           [Page 7]

RFC 1716          Towards Requirements for IP Routers      November 1994


     These and other Internet protocol documents may be obtained from
     the:

     The InterNIC
     DS.INTERNIC.NET
     InterNIC Directory and Database Service

     +1 (800) 444-4345 or +1 (619) 445-4600

     info@internic.net


1.3  General Considerations

   There are several important lessons that vendors of Internet software
   have learned and which a new vendor should consider seriously.

1.3.1  Continuing Internet Evolution

      The enormous growth of the Internet has revealed problems of
      management and scaling in a large datagram-based packet
      communication system.  These problems are being addressed, and as
      a result there will be continuing evolution of the specifications
      described in this memo.  New routing protocols, algorithms, and
      architectures are constantly being developed.  New and additional
      internet-layer protocols are also constantly being devised.
      Because routers play such a crucial role in the Internet, and
      because the number of routers deployed in the Internet is much
      smaller than the number of hosts, vendors should expect that
      router standards will continue to evolve much more quickly than
      host standards.  These changes will be carefully planned and
      controlled since there is extensive participation in this planning
      by the vendors and by the organizations responsible for operation
      of the networks.

      Development, evolution, and revision are characteristic of
      computer network protocols today, and this situation will persist
      for some years.  A vendor who develops computer communications
      software for the Internet protocol suite (or any other protocol
      suite!) and then fails to maintain and update that software for
      changing specifications is going to leave a trail of unhappy
      customers.  The Internet is a large communication network, and the
      users are in constant contact through it.  Experience has shown
      that knowledge of deficiencies in vendor software propagates
      quickly through the Internet technical community.



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1.3.2  Robustness Principle

      At every layer of the protocols, there is a general rule (from
      [TRANS:2] by Jon Postel) whose application can lead to enormous
      benefits in robustness and interoperability:

                       Be conservative in what you do,
                  be liberal in what you accept from others.

      Software should be written to deal with every conceivable error,
      no matter how unlikely; sooner or later a packet will come in with
      that particular combination of errors and attributes, and unless
      the software is prepared, chaos can ensue.  In general, it is best
      to assume that the network is filled with malevolent entities that
      will send packets designed to have the worst possible effect.
      This assumption will lead to suitably protective design.  The most
      serious problems in the Internet have been caused by unforeseen
      mechanisms triggered by low probability events; mere human malice
      would never have taken so devious a course!

      Adaptability to change must be designed into all levels of router
      software.  As a simple example, consider a protocol specification
      that contains an enumeration of values for a particular header
      field - e.g., a type field, a port number, or an error code; this
      enumeration must be assumed to be incomplete.  If the protocol
      specification defines four possible error codes, the software must
      not break when a fifth code shows up.  An undefined code might be
      logged, but it must not cause a failure.

      The second part of the principle is almost as important: software
      on hosts or other routers may contain deficiencies that make it
      unwise to exploit legal but obscure protocol features.  It is
      unwise to stray far from the obvious and simple, lest untoward
      effects result elsewhere.  A corollary of this is watch out for
      misbehaving hosts; router software should be prepared to survive
      in the presence of misbehaving hosts.  An important function of
      routers in the Internet is to limit the amount of disruption such
      hosts can inflict on the shared communication facility.

1.3.3  Error Logging

      The Internet includes a great variety of systems, each
      implementing many protocols and protocol layers, and some of these
      contain bugs and misfeatures in their Internet protocol software.
      As a result of complexity, diversity, and distribution of
      function, the diagnosis of problems is often very difficult.


Almquist & Kastenholz                                           [Page 9]

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      Problem diagnosis will be aided if routers include a carefully
      designed facility for logging erroneous or strange events.  It is
      important to include as much diagnostic information as possible
      when an error is logged.  In particular, it is often useful to
      record the header(s) of a packet that caused an error.  However,
      care must be taken to ensure that error logging does not consume
      prohibitive amounts of resources or otherwise interfere with the
      operation of the router.

      There is a tendency for abnormal but harmless protocol events to
      overflow error logging files; this can be avoided by using a
      circular log, or by enabling logging only while diagnosing a known
      failure.  It may be useful to filter and count duplicate
      successive messages.  One strategy that seems to work well is to
      both:
      o  Always count abnormalities and make such counts accessible
         through the management protocol (see Chapter 8); and
      o  Allow the logging of a great variety of events to be
         selectively enabled.  For example, it might useful to be able
         to log everything or to log everything for host X.

      This topic is further discussed in [MGT:5].

1.3.4  Configuration

      In an ideal world, routers would be easy to configure, and perhaps
      even entirely self-configuring.  However, practical experience in
      the real world suggests that this is an impossible goal, and that
      in fact many attempts by vendors to make configuration easy
      actually cause customers more grief than they prevent.  As an
      extreme example, a router designed to come up and start routing
      packets without requiring any configuration information at all
      would almost certainly choose some incorrect parameter, possibly
      causing serious problems on any networks unfortunate enough to be
      connected to it.

      Often this memo requires that a parameter be a configurable
      option.  There are several reasons for this.  In a few cases there
      currently is some uncertainty or disagreement about the best value
      and it may be necessary to update the recommended value in the
      future.  In other cases, the value really depends on external
      factors - e.g., the distribution of its communication load, or the
      speeds and topology of nearby networks - and self-tuning
      algorithms are unavailable and may be insufficient.  In some
      cases, configurability is needed because of administrative
      requirements.


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      Finally, some configuration options are required to communicate
      with obsolete or incorrect implementations of the protocols,
      distributed without sources, that persist in many parts of the
      Internet.  To make correct systems coexist with these faulty
      systems, administrators must occasionally misconfigure the correct
      systems.  This problem will correct itself gradually as the faulty
      systems are retired, but cannot be ignored by vendors.

      When we say that a parameter must be configurable, we do not
      intend to require that its value be explicitly read from a
      configuration file at every boot time.  For many parameters, there
      is one value that is appropriate for all but the most unusual
      situations.  In such cases, it is quite reasonable that the
      parameter default to that value if not explicitly set.

      This memo requires a particular value for such defaults in some
      cases.  The choice of default is a sensitive issue when the
      configuration item controls accommodation of existing, faulty,
      systems.  If the Internet is to converge successfully to complete
      interoperability, the default values built into implementations
      must implement the official protocol, not misconfigurations to
      accommodate faulty implementations.  Although marketing
      considerations have led some vendors to choose misconfiguration
      defaults, we urge vendors to choose defaults that will conform to
      the standard.

      Finally, we note that a vendor needs to provide adequate
      documentation on all configuration parameters, their limits and
      effects.

1.4  Algorithms

   In several places in this memo, specific algorithms that a router
   ought to follow are specified.  These algorithms are not, per se,
   required of the router.  A router need not implement each algorithm
   as it is written in this document.  Rather, an implementation must
   present a behavior to the external world that is the same as a
   strict, literal, implementation of the specified algorithm.

   Algorithms are described in a manner that differs from the way a good
   implementor would implement them.  For expository purposes, a style
   that emphasizes conciseness, clarity, and independence from
   implementation details has been chosen.  A good implementor will
   choose algorithms and implementation methods which produce the same
   results as these algorithms, but may be more efficient or less
   general.


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   We note that the art of efficient router implementation is outside of
   the scope of this memo.














































Almquist & Kastenholz                                          [Page 12]

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2.  INTERNET ARCHITECTURE

This chapter does not contain any requirements.  However, it does
contain useful background information on the general architecture of the
Internet and of routers.

General background and discussion on the Internet architecture and
supporting protocol suite can be found in the DDN Protocol Handbook
[ARCH:1]; for background see for example [ARCH:2], [ARCH:3], and
[ARCH:4].  The Internet architecture and protocols are also covered in
an ever-growing number of textbooks, such as [ARCH:5] and [ARCH:6].

2.1  Introduction

   The Internet system consists of a number of interconnected packet
   networks supporting communication among host computers using the
   Internet protocols.  These protocols include the Internet Protocol
   (IP), the Internet Control Message Protocol (ICMP), the Internet
   Group Management Protocol (IGMP), and a variety transport and
   application protocols that depend upon them.  As was described in
   Section [1.2], the Internet Engineering Steering Group periodically
   releases an Official Protocols memo listing all of the Internet
   protocols.

   All Internet protocols use IP as the basic data transport mechanism.
   IP is a datagram, or connectionless, internetwork service and
   includes provision for addressing, type-of-service specification,
   fragmentation and reassembly, and security.  ICMP and IGMP are
   considered integral parts of IP, although they are architecturally
   layered upon IP.  ICMP provides error reporting, flow control,
   first-hop router redirection, and other maintenance and control
   functions.  IGMP provides the mechanisms by which hosts and routers
   can join and leave IP multicast groups.

   Reliable data delivery is provided in the Internet protocol suite by
   Transport Layer protocols such as the Transmission Control Protocol
   (TCP), which provides end-end retransmission, resequencing and
   connection control.  Transport Layer connectionless service is
   provided by the User Datagram Protocol (UDP).









Almquist & Kastenholz                                          [Page 13]

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2.2  Elements of the Architecture


2.2.1  Protocol Layering

      To communicate using the Internet system, a host must implement
      the layered set of protocols comprising the Internet protocol
      suite.  A host typically must implement at least one protocol from
      each layer.

      The protocol layers used in the Internet architecture are as
      follows [ARCH:7]:

      o  Application Layer
         The Application Layer is the top layer of the Internet protocol
         suite.  The Internet suite does not further subdivide the
         Application Layer, although some application layer protocols do
         contain some internal sub-layering.  The application layer of
         the Internet suite essentially combines the functions of the
         top two layers - Presentation and Application - of the OSI
         Reference Model [ARCH:8].  The Application Layer in the
         Internet protocol suite also includes some of the function
         relegated to the Session Layer in the OSI Reference Model.

         We distinguish two categories of application layer protocols:
         user protocols that provide service directly to users, and
         support protocols that provide common system functions.  The
         most common Internet user protocols are:
         - Telnet (remote login)
         - FTP (file transfer)
         - SMTP (electronic mail delivery)

         There are a number of other standardized user protocols and
         many private user protocols.

         Support protocols, used for host name mapping, booting, and
         management, include SNMP, BOOTP, TFTP, the Domain Name System
         (DNS) protocol, and a variety of routing protocols.

         Application Layer protocols relevant to routers are discussed
         in chapters 7, 8, and 9 of this memo.

      o  Transport Layer
         The Transport Layer provides end-to-end communication services.
         This layer is roughly equivalent to the Transport Layer in the
         OSI Reference Model, except that it also incorporates some of
         OSI's Session Layer establishment and destruction functions.


Almquist & Kastenholz                                          [Page 14]

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         There are two primary Transport Layer protocols at present:
         - Transmission Control Protocol (TCP)
         - User Datagram Protocol (UDP)

         TCP is a reliable connection-oriented transport service that
         provides end-to-end reliability, resequencing, and flow
         control.  UDP is a connectionless (datagram) transport service.
         Other transport protocols have been developed by the research
         community, and the set of official Internet transport protocols
         may be expanded in the future.

         Transport Layer protocols relevant to routers are discussed in
         Chapter 6.

      o  Internet Layer
         All Internet transport protocols use the Internet Protocol (IP)
         to carry data from source host to destination host.  IP is a
         connectionless or datagram internetwork service, providing no
         end-to-end delivery guarantees. IP datagrams may arrive at the
         destination host damaged, duplicated, out of order, or not at
         all.  The layers above IP are responsible for reliable delivery
         service when it is required.  The IP protocol includes
         provision for addressing, type-of-service specification,
         fragmentation and reassembly, and security.

         The datagram or connectionless nature of IP is a fundamental
         and characteristic feature of the Internet architecture.

         The Internet Control Message Protocol (ICMP) is a control
         protocol that is considered to be an integral part of IP,
         although it is architecturally layered upon IP, i.e., it uses
         IP to carry its data end-to-end.  ICMP provides error
         reporting, congestion reporting, and first-hop router
         redirection.

         The Internet Group Management Protocol (IGMP) is an Internet
         layer protocol used for establishing dynamic host groups for IP
         multicasting.

         The Internet layer protocols IP, ICMP, and IGMP are discussed
         in chapter 4.

      o  Link Layer
         To communicate on its directly-connected network, a host must
         implement the communication protocol used to interface to that
         network.  We call this a Link Layer layer protocol.


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         Some older Internet documents refer to this layer as the
         Network Layer, but it is not the same as the Network Layer in
         the OSI Reference Model.

         This layer contains everything below the Internet Layer.

         Protocols in this Layer are generally outside the scope of
         Internet standardization; the Internet (intentionally) uses
         existing standards whenever possible.  Thus, Internet Link
         Layer standards usually address only address resolution and
         rules for transmitting IP packets over specific Link Layer
         protocols.  Internet Link Layer standards are discussed in
         chapter 3.

2.2.2  Networks

      The constituent networks of the Internet system are required to
      provide only packet (connectionless) transport.  According to the
      IP service specification, datagrams can be delivered out of order,
      be lost or duplicated, and/or contain errors.

      For reasonable performance of the protocols that use IP (e.g.,
      TCP), the loss rate of the network should be very low.  In
      networks providing connection-oriented service, the extra
      reliability provided by virtual circuits enhances the end-end
      robustness of the system, but is not necessary for Internet
      operation.

      Constituent networks may generally be divided into two classes:

        o  Local-Area Networks (LANs)
           LANs may have a variety of designs.  In general, a LAN will
           cover a small geographical area (e.g., a single building or
           plant site) and provide high bandwidth with low delays.  LANs
           may be passive (similar to Ethernet) or they may be active
           (such as ATM).

        o  Wide-Area Networks (WANs)
           Geographically-dispersed hosts and LANs are interconnected by
           wide-area networks, also called long-haul networks.  These
           networks may have a complex internal structure of lines and
           packet-switches, or they may be as simple as point-to-point
           lines.





Almquist & Kastenholz                                          [Page 16]

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2.2.3  Routers

      In the Internet model, constituent networks are connected together
      by IP datagram forwarders which are called routers or IP routers.
      In this document, every use of the term router is equivalent to IP
      router.  Many older Internet documents refer to routers as
      gateways.

      Historically, routers have been realized with packet-switching
      software executing on a general-purpose CPU.  However, as custom
      hardware development becomes cheaper and as higher throughput is
      required, but special-purpose hardware is becoming increasingly
      common.  This specification applies to routers regardless of how
      they are implemented.

      A router is connected to two or more networks, appearing to each
      of these networks as a connected host.  Thus, it has (at least)
      one physical interface and (at least) one IP address on each of
      the connected networks (this ignores the concept of un-numbered
      links, which is discussed in section [2.2.7]).  Forwarding an IP
      datagram generally requires the router to choose the address of
      the next-hop router or (for the final hop) the destination host.
      This choice, called routing, depends upon a routing database
      within the router.  The routing database is also sometimes known
      as a routing table or forwarding table.

      The routing database should be maintained dynamically to reflect
      the current topology of the Internet system.  A router normally
      accomplishes this by participating in distributed routing and
      reachability algorithms with other routers.

      Routers provide datagram transport only, and they seek to minimize
      the state information necessary to sustain this service in the
      interest of routing flexibility and robustness.

      Packet switching devices may also operate at the Link Layer; such
      devices are usually called bridges. Network segments which are
      connected by bridges share the same IP network number, i.e., they
      logically form a single IP network.  These other devices are
      outside of the scope of this document.

      Another variation on the simple model of networks connected with
      routers sometimes occurs: a set of routers may be interconnected
      with only serial lines, to form a network in which the packet
      switching is performed at the Internetwork (IP) Layer rather than
      the Link Layer.


Almquist & Kastenholz                                          [Page 17]

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2.2.4  Autonomous Systems

      For technical, managerial, and sometimes political reasons, the
      routers of the Internet system are grouped into collections called
      autonomous systems.  The routers included in a single autonomous
      system (AS) are expected to:

      o  Be under the control of a single operations and maintenance
         (O&M) organization;

      o  Employ common routing protocols among themselves, to
         dynamically maintain their routing databases.

      A number of different dynamic routing protocols have been
      developed (see Section [7.2]); the routing protocol within a
      single AS is generically called an interior gateway protocol or
      IGP.

      An IP datagram may have to traverse the routers of two or more ASs
      to reach its destination, and the ASs must provide each other with
      topology information to allow such forwarding.  An exterior
      gateway protocol (generally BGP or EGP) is used for this purpose.

2.2.5  Addresses and Subnets

      An IP datagram carries 32-bit source and destination addresses,
      each of which is partitioned into two parts - a constituent
      network number and a host number on that network.  Symbolically:

         IP-address  ::=  { <Network-number>, <Host-number> }

      To finally deliver the datagram, the last router in its path must
      map the Host-number (or rest) part of an IP address into the
      physical address of a host connection to the constituent network.

      This simple notion has been extended by the concept of subnets,
      which were introduced in order to allow arbitrary complexity of
      interconnected LAN structures within an organization, while
      insulating the Internet system against explosive growth in network
      numbers and routing complexity.  Subnets essentially provide a
      multi-level hierarchical routing structure for the Internet
      system.  The subnet extension, described in [INTERNET:2], is now a
      required part of the Internet architecture.  The basic idea is to
      partition the <Host-number> field into two parts: a subnet number,
      and a true host number on that subnet:

         IP-address  ::=


Almquist & Kastenholz                                          [Page 18]

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           { <Network-number>, <Subnet-number>, <Host-number> }

      The interconnected physical networks within an organization will
      be given the same network number but different subnet numbers.
      The distinction between the subnets of such a subnetted network is
      normally not visible outside of that network.  Thus, routing in
      the rest of the Internet will be based only upon the <Network-
      number> part of the IP destination address; routers outside the
      network will combine <Subnet-number> and <Host-number> together to
      form an uninterpreted rest part of the 32-bit IP address.  Within
      the subnetted network, the routers must route on the basis of an
      extended network number:

         { <Network-number>, <Subnet-number> }

      Under certain circumstances, it may be desirable to support
      subnets of a particular network being interconnected only via a
      path which is not part of the subnetted network.  Even though many
      IGP's and no EGP's currently support this configuration
      effectively, routers need to be able to support this configuration
      of subnetting (see Section [4.2.3.4]).  In general, routers should
      not make assumptions about what are subnets and what are not, but
      simply ignore the concept of Class in networks, and treat each
      route as a { network, mask }-tuple.

      DISCUSSION:
         It is becoming clear that as the Internet grows larger and
         larger, the traditional uses of Class A, B, and C networks will
         be modified in order to achieve better use of IP's 32-bit
         address space.  Classless Interdomain Routing (CIDR)
         [INTERNET:15] is a method currently being deployed in the
         Internet backbones to achieve this added efficiency.  CIDR
         depends on the ability of assigning and routing to networks
         that are not based on Class A, B, or C networks.  Thus, routers
         should always treat a route as a network with a mask.

      Furthermore, for similar reasons, a subnetted network need not
      have a consistent subnet mask through all parts of the network.
      For example, one subnet may use an 8 bit subnet mask, another 10
      bit, and another 6 bit.  Routers need to be able to support this
      type of configuration (see Section [4.2.3.4]).

      The bit positions containing this extended network number are
      indicated by a 32-bit mask called the subnet mask; it is
      recommended but not required that the <Subnet-number> bits be
      contiguous and fall between the <Network-number> and the <Host-
      number> fields.  No subnet should be assigned the value zero or -1


Almquist & Kastenholz                                          [Page 19]

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      (all one bits).

      Although the inventors of the subnet mechanism probably expected
      that each piece of an organization's network would have only a
      single subnet number, in practice it has often proven necessary or
      useful to have several subnets share a single physical cable.

      There are special considerations for the router when a connected
      network provides a broadcast or multicast capability; these will
      be discussed later.

2.2.6  IP Multicasting

      IP multicasting is an extension of Link Layer multicast to IP
      internets.  Using IP multicasts, a single datagram can be
      addressed to multiple hosts. This collection of hosts is called a
      multicast group.  Each multicast group is represented as a Class D
      IP address.  An IP datagram sent to the group is to be delivered
      to each group member with the same best-effort delivery as that
      provided for unicast IP traffic.  The sender of the datagram does
      not itself need to be a member of the destination group.

      The semantics of IP multicast group membership are defined in
      [INTERNET:4].  That document describes how hosts and routers join
      and leave multicast groups.  It also defines a protocol, the
      Internet Group Management Protocol (IGMP), that monitors IP
      multicast group membership.

      Forwarding of IP multicast datagrams is accomplished either
      through static routing information or via a multicast routing
      protocol.  Devices that forward IP multicast datagrams are called
      multicast routers. They may or may not also forward IP unicasts.
      In general, multicast datagrams are forwarded on the basis of both
      their source and destination addresses.  Forwarding of IP
      multicast packets is described in more detail in Section [5.2.1].
      Appendix D discusses multicast routing protocols.

2.2.7  Unnumbered Lines and Networks and Subnets

      Traditionally, each network interface on an IP host or router has
      its own IP address.  Over the years, people have observed that
      this can cause inefficient use of the scarce IP address space,
      since it forces allocation of an IP network number, or at least a
      subnet number, to every point-to-point link.

      To solve this problem, a number of people have proposed and
      implemented the concept of unnumbered serial lines.  An unnumbered


Almquist & Kastenholz                                          [Page 20]

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      serial line does not have any IP network or subnet number
      associated with it.  As a consequence, the network interfaces
      connected to an unnumbered serial line do not have IP addresses.

      Because the IP architecture has traditionally assumed that all
      interfaces had IP addresses, these unnumbered interfaces cause
      some interesting dilemmas.  For example, some IP options (e.g.
      Record Route) specify that a router must insert the interface
      address into the option, but an unnumbered interface has no IP
      address.  Even more fundamental (as we shall see in chapter 5) is
      that routes contain the IP address of the next hop router.  A
      router expects that that IP address will be on an IP (sub)net that
      the router is connected to.  That assumption is of course violated
      if the only connection is an unnumbered serial line.

      To get around these difficulties, two schemes have been invented.
      The first scheme says that two routers connected by an unnumbered
      serial line aren't really two routers at all, but rather two
      half-routers which together make up a single (virtual) router.
      The unnumbered serial line is essentially considered to be an
      internal bus in the virtual router.  The two halves of the virtual
      router must coordinate their activities in such a way that they
      act exactly like a single router.

      This scheme fits in well with the IP architecture, but suffers
      from two important drawbacks.  The first is that, although it
      handles the common case of a single unnumbered serial line, it is
      not readily extensible to handle the case of a mesh of routers and
      unnumbered serial lines.  The second drawback is that the
      interactions between the half routers are necessarily complex and
      are not standardized, effectively precluding the connection of
      equipment from different vendors using unnumbered serial lines.

      Because of these drawbacks, this memo has adopted an alternative
      scheme, which has been invented multiple times but which is
      probably originally attributable to Phil Karn.  In this scheme, a
      router which has unnumbered serial lines also has a special IP
      address, called a router-id in this memo.  The router-id is one of
      the router's IP addresses (a router is required to have at least
      one IP address).  This router-id is used as if it is the IP
      address of all unnumbered interfaces.







Almquist & Kastenholz                                          [Page 21]

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2.2.8  Notable Oddities


2.2.8.1  Embedded Routers

         A router may be a stand-alone computer system, dedicated to its
         IP router functions.  Alternatively, it is possible to embed
         router functions within a host operating system which supports
         connections to two or more networks.  The best-known example of
         an operating system with embedded router code is the Berkeley
         BSD system.  The embedded router feature seems to make
         internetting easy, but it has a number of hidden pitfalls:

         (1)  If a host has only a single constituent-network interface,
              it should not act as a router.

              For example, hosts with embedded router code that
              gratuitously forward broadcast packets or datagrams on the
              same net often cause packet avalanches.

         (2)  If a (multihomed) host acts as a router, it must implement
              ALL the relevant router requirements contained in this
              document.

              For example, the routing protocol issues and the router
              control and monitoring problems are as hard and important
              for embedded routers as for stand-alone routers.

              Since Internet router requirements and specifications may
              change independently of operating system changes, an
              administration that operates an embedded router in the
              Internet is strongly advised to have the ability to
              maintain and update the router code (e.g., this might
              require router code source).

         (3)  Once a host runs embedded router code, it becomes part of
              the Internet system.  Thus, errors in software or
              configuration can hinder communication between other
              hosts.  As a consequence, the host administrator must lose
              some autonomy.

              In many circumstances, a host administrator will need to
              disable router code embedded in the operating system, and
              any embedded router code must be organized so that it can
              be easily disabled.

         (4)  If a host running embedded router code is concurrently


Almquist & Kastenholz                                          [Page 22]

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              used for other services, the O&M (Operation and
              Maintenance) requirements for the two modes of use may be
              in serious conflict.

              For example, router O&M will in many cases be performed
              remotely by an operations center; this may require
              privileged system access which the host administrator
              would not normally want to distribute.

2.2.8.2  Transparent Routers

         There are two basic models for interconnecting local-area
         networks and wide-area (or long-haul) networks in the Internet.
         In the first, the local-area network is assigned a network
         number and all routers in the Internet must know how to route
         to that network.  In the second, the local-area network shares
         (a small part of) the address space of the wide-area network.
         Routers that support this second model are called address
         sharing routers or transparent routers.  The focus of this memo
         is on routers that support the first model, but this is not
         intended to exclude the use of transparent routers.

         The basic idea of a transparent router is that the hosts on the
         local-area network behind such a router share the address space
         of the wide-area network in front of the router.  In certain
         situations this is a very useful approach and the limitations
         do not present significant drawbacks.

         The words in front and behind indicate one of the limitations
         of this approach: this model of interconnection is suitable
         only for a geographically (and topologically) limited stub
         environment.  It requires that there be some form of logical
         addressing in the network level addressing of the wide-area
         network.  All of the IP addresses in the local environment map
         to a few (usually one) physical address in the wide-area
         network.  This mapping occurs in a way consistent with the { IP
         address <-> network address } mapping used throughout the
         wide-area network.

         Multihoming is possible on one wide-area network, but may
         present routing problems if the interfaces are geographically
         or topologically separated.  Multihoming on two (or more)
         wide-area networks is a problem due to the confusion of
         addresses.

         The behavior that hosts see from other hosts in what is
         apparently the same network may differ if the transparent


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         router cannot fully emulate the normal wide-area network
         service.  For example, the ARPANET used a Link Layer protocol
         that provided a Destination Dead indication in response to an
         attempt to send to a host which was powered off.  However, if
         there were a transparent router between the ARPANET and an
         Ethernet, a host on the ARPANET would not receive a Destination
         Dead indication if it sent a datagram to a host that was
         powered off and was connected to the ARPANET via the
         transparent router instead of directly.

2.3  Router Characteristics

   An Internet router performs the following functions:

   (1)  Conforms to specific Internet protocols specified in this
        document, including the Internet Protocol (IP), Internet Control
        Message Protocol (ICMP), and others as necessary.

   (2)  Interfaces to two or more packet networks.  For each connected
        network the router must implement the functions required by that
        network.  These functions typically include:

        o  Encapsulating and decapsulating the IP datagrams with the
           connected network framing (e.g., an Ethernet header and
           checksum),

        o  Sending and receiving IP datagrams up to the maximum size
           supported by that network, this size is the network's Maximum
           Transmission Unit or MTU,

        o  Translating the IP destination address into an appropriate
           network-level address for the connected network (e.g., an
           Ethernet hardware address), if needed, and

        o  Responding to the network flow control and error indication,
           if any.

        See chapter 3 (Link Layer).

   (3)  Receives and forwards Internet datagrams.  Important issues in
        this process are buffer management, congestion control, and
        fairness.

        o  Recognizes various error conditions and generates ICMP error
           and information messages as required.

        o  Drops datagrams whose time-to-live fields have reached zero.


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        o  Fragments datagrams when necessary to fit into the MTU of the
           next network.

        See chapter 4 (Internet Layer - Protocols) and chapter 5
        (Internet Layer - Forwarding) for more information.

   (4)  Chooses a next-hop destination for each IP datagram, based on
        the information in its routing database.  See chapter 5
        (Internet Layer - Forwarding) for more information.

   (5)  (Usually) supports an interior gateway protocol (IGP) to carry
        out distributed routing and reachability algorithms with the
        other routers in the same autonomous system.  In addition, some
        routers will need to support an exterior gateway protocol (EGP)
        to exchange topological information with other autonomous
        systems.  See chapter 7 (Application Layer - Routing Protocols)
        for more information.

   (6)  Provides network management and system support facilities,
        including loading, debugging, status reporting, exception
        reporting and control.  See chapter 8 (Application Layer -
        Network Management Protocols) and chapter 10 (Operation and
        Maintenance) for more information.

   A router vendor will have many choices on power, complexity, and
   features for a particular router product.  It may be helpful to
   observe that the Internet system is neither homogeneous nor fully-
   connected.  For reasons of technology and geography it is growing
   into a global interconnect system plus a fringe of LANs around the
   edge. More and more these fringe LANs are becoming richly
   interconnected, thus making them less out on the fringe and more
   demanding on router requirements.

   o  The global interconnect system is comprised of a number of wide-
      area networks to which are attached routers of several Autonomous
      Systems (AS); there are relatively few hosts connected directly to
      the system.

   o  Most hosts are connected to LANs.  Many organizations have
      clusters of LANs interconnected by local routers.  Each such
      cluster is connected by routers at one or more points into the
      global interconnect system.  If it is connected at only one point,
      a LAN is known as a stub network.

   Routers in the global interconnect system generally require:

   o  Advanced Routing and Forwarding Algorithms


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      These routers need routing algorithms which are highly dynamic and
      also offer type-of-service routing.  Congestion is still not a
      completely resolved issue (see Section [5.3.6]).  Improvements in
      these areas are expected, as the research community is actively
      working on these issues.

   o  High Availability

      These routers need to be highly reliable, providing 24 hours a
      day, 7 days a week service.  Equipment and software faults can
      have a wide-spread (sometimes global) effect.  In case of failure,
      they must recover quickly.  In any environment, a router must be
      highly robust and able to operate, possibly in a degraded state,
      under conditions of extreme congestion or failure of network
      resources.

   o  Advanced O&M Features

      Internet routers normally operate in an unattended mode.  They
      will typically be operated remotely from a centralized monitoring
      center.  They need to provide sophisticated means for monitoring
      and measuring traffic and other events and for diagnosing faults.

   o  High Performance

      Long-haul lines in the Internet today are most frequently 56 Kbps,
      DS1 (1.4Mbps), and DS3 (45Mbps) speeds.  LANs are typically
      Ethernet (10Mbps) and, to a lesser degree, FDDI (100Mbps).
      However, network media technology is constantly advancing and even
      higher speeds are likely in the future.  Full-duplex operation is
      provided at all of these speeds.

   The requirements for routers used in the LAN fringe (e.g., campus
   networks) depend greatly on the demands of the local networks.  These
   may be high or medium-performance devices, probably competitively
   procured from several different vendors and operated by an internal
   organization (e.g., a campus computing center).  The design of these
   routers should emphasize low average latency and good burst
   performance, together with delay and type-of-service sensitive
   resource management. In this environment there may be less formal O&M
   but it will not be less important.  The need for the routing
   mechanism to be highly dynamic will become more important as networks
   become more complex and interconnected.  Users will demand more out
   of their local connections because of the speed of the global
   interconnects.

   As networks have grown, and as more networks have become old enough


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   that they are phasing out older equipment, it has become increasingly
   imperative that routers interoperate with routers from other vendors.

   Even though the Internet system is not fully interconnected, many
   parts of the system need to have redundant connectivity.  Rich
   connectivity allows reliable service despite failures of
   communication lines and routers, and it can also improve service by
   shortening Internet paths and by providing additional capacity.
   Unfortunately, this richer topology can make it much more difficult
   to choose the best path to a particular destination.

2.4  Architectural Assumptions

   The current Internet architecture is based on a set of assumptions
   about the communication system.  The assumptions most relevant to
   routers are as follows:

   o  The Internet is a network of networks.

      Each host is directly connected to some particular network(s); its
      connection to the Internet is only conceptual.  Two hosts on the
      same network communicate with each other using the same set of
      protocols that they would use to communicate with hosts on distant
      networks.

   o  Routers don't keep connection state information.

      To improve the robustness of the communication system, routers are
      designed to be stateless, forwarding each IP packet independently
      of other packets.  As a result, redundant paths can be exploited
      to provide robust service in spite of failures of intervening
      routers and networks.

      All state information required for end-to-end flow control and
      reliability is implemented in the hosts, in the transport layer or
      in application programs.  All connection control information is
      thus co-located with the end points of the communication, so it
      will be lost only if an end point fails.  Routers effect flow
      control only indirectly, by dropping packets or increasing network
      delay.

      Note that future protocol developments may well end up putting
      some more state into routers.  This is especially likely for
      resource reservation and flows.




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   o  Routing complexity should be in the routers.

      Routing is a complex and difficult problem, and ought to be
      performed by the routers, not the hosts.  An important objective
      is to insulate host software from changes caused by the inevitable
      evolution of the Internet routing architecture.

   o  The system must tolerate wide network variation.

      A basic objective of the Internet design is to tolerate a wide
      range of network characteristics - e.g., bandwidth, delay, packet
      loss, packet reordering, and maximum packet size.  Another
      objective is robustness against failure of individual networks,
      routers, and hosts, using whatever bandwidth is still available.
      Finally, the goal is full open system interconnection: an Internet
      router must be able to interoperate robustly and effectively with
      any other router or Internet host, across diverse Internet paths.

      Sometimes implementors have designed for less ambitious goals.
      For example, the LAN environment is typically much more benign
      than the Internet as a whole; LANs have low packet loss and delay
      and do not reorder packets.  Some vendors have fielded
      implementations that are adequate for a simple LAN environment,
      but work badly for general interoperation.  The vendor justifies
      such a product as being economical within the restricted LAN
      market.  However, isolated LANs seldom stay isolated for long;
      they are soon connected to each other, to organization-wide
      internets, and eventually to the global Internet system.  In the
      end, neither the customer nor the vendor is served by incomplete
      or substandard routers.

      The requirements spelled out in this document are designed for a
      full-function router.  It is intended that fully compliant routers
      will be usable in almost any part of the Internet.














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3.  LINK LAYER

Although  [INTRO:1] covers Link Layer standards (IP over foo, ARP,
etc.), this document anticipates that Link-Layer material will be
covered in a separate Link Layer Requirements document.  A Link-Layer
requirements document would be applicable to both hosts and routers.
Thus, this document will not obsolete the parts of [INTRO:1] that deal
with link-layer issues.

3.1  INTRODUCTION

   Routers have essentially the same Link Layer protocol requirements as
   other sorts of Internet systems.  These requirements are given in
   chapter 3 of Requirements for Internet Gateways [INTRO:1].  A router
   MUST comply with its requirements and SHOULD comply with its
   recommendations.  Since some of the material in that document has
   become somewhat dated, some additional requirements and explanations
   are included below.

   DISCUSSION:
      It is expected that the Internet community will produce a
      Requirements for Internet Link Layer standard which will supersede
      both this chapter and chapter 3 of [INTRO:1].


3.2  LINK/INTERNET LAYER INTERFACE

   Although this document does not attempt to specify the interface
   between the Link Layer and the upper layers, it is worth noting here
   that other parts of this document, particularly chapter 5, require
   various sorts of information to be passed across this layer boundary.

   This section uses the following definitions:

   o  Source physical address

      The source physical address is the Link Layer address of the host
      or router from which the packet was received.

   o  Destination physical address

      The destination physical address is the Link Layer address to
      which the packet was sent.

   The information that must pass from the Link Layer to the
   Internetwork Layer for each received packet is:


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   (1)  The IP packet [5.2.2],

   (2)  The length of the data portion (i.e., not including the Link-
        Layer framing) of the Link Layer frame [5.2.2],

   (3)  The identity of the physical interface from which the IP packet
        was received [5.2.3], and

   (4)  The classification of the packet's destination physical address
        as a Link Layer unicast, broadcast, or multicast [4.3.2],
        [5.3.4].

   In addition, the Link Layer also should provide:

   (5)  The source physical address.

   The information that must pass from the Internetwork Layer to the
   Link Layer for each transmitted packet is:

   (1)  The IP packet [5.2.1]

   (2)  The length of the IP packet [5.2.1]

   (3)  The destination physical interface [5.2.1]

   (4)  The next hop IP address [5.2.1]

   In addition, the Internetwork Layer also should provide:

   (5)  The Link Layer priority value [5.3.3.2]

   The Link Layer must also notify the Internetwork Layer if the packet
   to be transmitted causes a Link Layer precedence-related error
   [5.3.3.3].

3.3  SPECIFIC ISSUES


3.3.1  Trailer Encapsulation

      Routers which can connect to 10Mb Ethernets MAY be able to receive
      and forward Ethernet packets encapsulated using the trailer
      encapsulation described in [LINK:1].  However, a router SHOULD NOT
      originate trailer encapsulated packets.  A router MUST NOT
      originate trailer encapsulated packets without first verifying,
      using the mechanism described in section 2.3.1 of [INTRO:2], that
      the immediate destination of the packet is willing and able to


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      accept trailer-encapsulated packets.  A router SHOULD NOT agree
      (using these same mechanisms) to accept trailer-encapsulated
      packets.

3.3.2  Address Resolution Protocol - ARP

      Routers which implement ARP MUST be compliant and SHOULD be
      unconditionally compliant with the requirements in section 2.3.2
      of [INTRO:2].

      The link layer MUST NOT report a Destination Unreachable error to
      IP solely because there is no ARP cache entry for a destination.

      A router MUST not believe any ARP reply which claims that the Link
      Layer address of another host or router is a broadcast or
      multicast address.

3.3.3  Ethernet and 802.3 Coexistence

      Routers which can connect to 10Mb Ethernets MUST be compliant and
      SHOULD be unconditionally compliant with the requirements of
      Section [2.3.3] of [INTRO:2].

3.3.4  Maximum Transmission Unit - MTU

      The MTU of each logical interface MUST be configurable.

      Many Link Layer protocols define a maximum frame size that may be
      sent.  In such cases, a router MUST NOT allow an MTU to be set
      which would allow sending of frames larger than those allowed by
      the Link Layer protocol.  However, a router SHOULD be willing to
      receive a packet as large as the maximum frame size even if that
      is larger than the MTU.

      DISCUSSION:
         Note that this is a stricter requirement than imposed on hosts
         by [INTRO:2], which requires that the MTU of each physical
         interface be configurable.

         If a network is using an MTU smaller than the maximum frame
         size for the Link Layer, a router may receive packets larger
         than the MTU from hosts which are in the process of
         initializing themselves, or which have been misconfigured.

         In general, the Robustness Principle indicates that these
         packets should be successfully received, if at all possible.


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3.3.5  Point-to-Point Protocol - PPP

      Contrary to [INTRO:1], the Internet does have a standard serial
      line protocol: the Point-to-Point Protocol (PPP), defined in
      [LINK:2], [LINK:3], [LINK:4], and [LINK:5].

      A serial line interface is any interface which is designed to send
      data over a telephone, leased, dedicated or direct line (either 2
      or 4 wire) using a standardized modem or bit serial interface
      (such as RS-232, RS-449 or V.35), using either synchronous or
      asynchronous clocking.

      A general purpose serial interface is a serial line interface
      which is not solely for use as an access line to a network for
      which an alternative IP link layer specification exists (such as
      X.25 or Frame Relay).

      Routers which contain such general purpose serial interfaces MUST
      implement PPP.

      PPP MUST be supported on all general purpose serial interfaces on
      a router.  The router MAY allow the line to be configured to use
      serial line protocols other than PPP, all general purpose serial
      interfaces MUST default to using PPP.

3.3.5.1  Introduction

         This section provides guidelines to router implementors so that
         they can ensure interoperability with other routers using PPP
         over either synchronous or asynchronous links.

         It is critical that an implementor understand the semantics of
         the option negotiation mechanism.  Options are a means for a
         local device to indicate to a remote peer what the local device
         will *accept* from the remote peer, not what it wishes to send.
         It is up to the remote peer to decide what is most convenient
         to send within the confines of the set of options that the
         local device has stated that it can accept.  Therefore it is
         perfectly acceptable and normal for a remote peer to ACK all
         the options indicated in an LCP Configuration Request (CR) even
         if the remote peer does not support any of those options.
         Again, the options are simply a mechanism for either device to
         indicate to its peer what it will accept, not necessarily what
         it will send.




Almquist & Kastenholz                                          [Page 32]

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3.3.5.2  Link Control Protocol (LCP) Options

         The PPP Link Control Protocol (LCP) offers a number of options
         that may be negotiated.  These options include (among others)
         address and control field compression, protocol field
         compression, asynchronous character map, Maximum Receive Unit
         (MRU), Link Quality Monitoring (LQM), magic number (for
         loopback detection), Password Authentication Protocol (PAP),
         Challenge Handshake Authentication Protocol (CHAP), and the
         32-bit Frame Check Sequence (FCS).

         A router MAY do address/control field compression on either
         synchronous or asynchronous links.  A router MAY do protocol
         field compression on either synchronous or asynchronous links.
         A router MAY indicate that it can accept these compressions,
         but MUST be able to accept uncompressed PPP header information
         even if it has indicated a willingness to receive compressed
         PPP headers.

         DISCUSSION:
            These options control the appearance of the PPP header.
            Normally the PPP header consists of the address field (one
            byte containing the value 0xff), the control field (one byte
            containing the value 0x03), and the two-byte protocol field
            that identifies the contents of the data area of the frame.
            If a system negotiates address and control field compression
            it indicates to its peer that it will accept PPP frames that
            have or do not have these fields at the front of the header.
            It does not indicate that it will be sending frames with
            these fields removed.  The protocol field may also be
            compressed from two to one byte in most cases.


         IMPLEMENTATION:
            Some hardware does not deal well with variable length header
            information.  In those cases it makes most sense for the
            remote peer to send the full PPP header.  Implementations
            may ensure this by not sending the address/control field and
            protocol field compression options to the remote peer.  Even
            if the remote peer has indicated an ability to receive
            compressed headers there is no requirement for the local
            router to send compressed headers.

         A router MUST negotiate the Async Control Character Map (ACCM)
         for asynchronous PPP links, but SHOULD NOT negotiate the ACCM
         for synchronous links.  If a router receives an attempt to
         negotiate the ACCM over a synchronous link, it MUST ACKnowledge


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         the option and then ignore it.

         DISCUSSION:
            There are implementations that offer both sync and async
            modes of operation and may use the same code to implement
            the option negotiation.  In this situation it is possible
            that one end or the other may send the ACCM option on a
            synchronous link.

         A router SHOULD properly negotiate the maximum receive unit
         (MRU).  Even if a system negotiates an MRU smaller than 1,500
         bytes, it MUST be able to receive a 1,500 byte frame.

         A router SHOULD negotiate and enable the link quality
         monitoring (LQM) option.

         DISCUSSION:
            This memo does not specify a policy for deciding whether the
            link's quality is adequate.  However, it is important (see
            Section [3.3.6]) that a router disable failed links.

         A router SHOULD implement and negotiate the magic number option
         for loopback detection.

         A router MAY support the authentication options (PAP - password
         authentication protocol, and/or CHAP - challenge handshake
         authentication protocol).

         A router MUST support 16-bit CRC frame check sequence (FCS) and
         MAY support the 32-bit CRC.

3.3.5.3  IP Control Protocol (ICP) Options

         A router MAY offer to perform IP address negotiation.  A router
         MUST accept a refusal (REJect) to perform IP address
         negotiation from the peer.

         A router SHOULD NOT perform Van Jacobson header compression of
         TCP/IP packets if the link speed is in excess of 64 Kbps.
         Below that speed the router MAY perform Van Jacobson (VJ)
         header compression.  At link speeds of 19,200 bps or less the
         router SHOULD perform VJ header compression.






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3.3.6  Interface Testing

      A router MUST have a mechanism to allow routing software to
      determine whether a physical interface is available to send
      packets or not.  A router SHOULD have a mechanism to allow routing
      software to judge the quality of a physical interface.  A router
      MUST have a mechanism for informing the routing software when a
      physical interface becomes available or unavailable to send
      packets because of administrative action.  A router MUST have a
      mechanism for informing the routing software when it detects a
      Link level interface has become available or unavailable, for any
      reason.

      DISCUSSION:
         It is crucial that routers have workable mechanisms for
         determining that their network connections are functioning
         properly, since failure to do so (or failure to take the proper
         actions when a problem is detected) can lead to black holes.

         The mechanisms available for detecting problems with network
         connections vary considerably, depending on the Link Layer
         protocols in use and also in some cases on the interface
         hardware chosen by the router manufacturer.  The intent is to
         maximize the capability to detect failures within the Link-
         Layer constraints.























Almquist & Kastenholz                                          [Page 35]

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4.  INTERNET LAYER - PROTOCOLS


4.1  INTRODUCTION

   This chapter and chapter 5 discuss the protocols used at the Internet
   Layer: IP, ICMP, and IGMP.  Since forwarding is obviously a crucial
   topic in a document discussing routers, chapter 5 limits itself to
   the aspects of the protocols which directly relate to forwarding.
   The current chapter contains the remainder of the discussion of the
   Internet Layer protocols.

4.2  INTERNET PROTOCOL - IP


4.2.1  INTRODUCTION

      Routers MUST implement the IP protocol, as defined by
      [INTERNET:1].  They MUST also implement its mandatory extensions:
      subnets (defined in [INTERNET:2]), and IP broadcast (defined in
      [INTERNET:3]).

      A router  MUST be compliant, and SHOULD be unconditionally
      compliant, with the requirements of sections 3.2.1 and 3.3 of
      [INTRO:2], except that:

      o  Section 3.2.1.1 may be ignored, since it duplicates
         requirements found in this memo.

      o  Section 3.2.1.2 may be ignored, since it duplicates
         requirements found in this memo.

      o  Section 3.2.1.3 should be ignored, since it is superseded by
         Section [4.2.2.11] of this memo.

      o  Section 3.2.1.4 may be ignored, since it duplicates
         requirements found in this memo.

      o  Section 3.2.1.6 should be ignored, since it is superseded by
         Section [4.2.2.4] of this memo.

      o  Section 3.2.1.8 should be ignored, since it is superseded by
         Section [4.2.2.1] of this memo.

      In the following, the action specified in certain cases is to
      silently discard a received datagram.  This means that the
      datagram will be discarded without further processing and that the


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      router will not send any ICMP error message (see Section [4.3]) as
      a result.  However, for diagnosis of problems a router SHOULD
      provide the capability of logging the error (see Section [1.3.3]),
      including the contents of the silently-discarded datagram, and
      SHOULD record the event in a statistics counter.

4.2.2  PROTOCOL WALK-THROUGH

      RFC 791 is [INTERNET:1], the specification for the Internet
      Protocol.

4.2.2.1  Options: RFC-791 Section 3.2

         In datagrams received by the router itself, the IP layer MUST
         interpret those IP options that it understands and preserve the
         rest unchanged for use by higher layer protocols.

         Higher layer protocols may require the ability to set IP
         options in datagrams they send or examine IP options in
         datagrams they receive.  Later sections of this document
         discuss specific IP option support required by higher layer
         protocols.

         DISCUSSION:
            Neither this memo nor [INTRO:2] define the order in which a
            receiver must process multiple options in the same IP
            header.  Hosts and routers originating datagrams containing
            multiple options must be aware that this introduces an
            ambiguity in the meaning of certain options when combined
            with a source-route option.

         Here are the requirements for specific IP options:

         (a)  Security Option

              Some environments require the Security option in every
              packet originated or received.  Routers SHOULD IMPLEMENT
              the revised security option described in [INTERNET:5].

              DISCUSSION:
                 Note that the security options described in
                 [INTERNET:1] and RFC 1038 ([INTERNET:16]) are obsolete.

         (b)  Stream Identifier Option

              This option is obsolete; routers SHOULD NOT place this
              option in a datagram that the router originates.  This


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              option MUST be ignored in datagrams received by the
              router.

         (c)  Source Route Options

              A router MUST be able to act as the final destination of a
              source route.  If a router receives a packet containing a
              completed source route (i.e., the pointer points beyond
              the last field and the destination address in the IP
              header addresses the router), the packet has reached its
              final destination; the option as received (the recorded
              route) MUST be passed up to the transport layer (or to
              ICMP message processing).

              In order to respond correctly to source-routed datagrams
              it receives, a router MUST provide a means whereby
              transport protocols and applications can reverse the
              source route in a received datagram and insert the
              reversed source route into datagrams they originate (see
              Section 4 of [INTRO:2] for details).

              Some applications in the router MAY require that the user
              be able to enter a source route.

              A router MUST NOT originate a datagram containing multiple
              source route options.  What a router should do if asked to
              forward a packet containing multiple source route options
              is described in Section [5.2.4.1].

              When a source route option is created, it MUST be
              correctly formed even if it is being created by reversing
              a recorded route that erroneously includes the source host
              (see case (B) in the discussion below).

              DISCUSSION:
                 Suppose a source routed datagram is to be routed from
                 source S to destination D via routers G1, G2, ... Gn.
                 Source S constructs a datagram with G1's IP address as
                 its destination address, and a source route option to
                 get the datagram the rest of the way to its
                 destination.  However, there is an ambiguity in the
                 specification over whether the source route option in a
                 datagram sent out by S should be (A) or (B):

                 (A):  {>>G2, G3, ... Gn, D}     <--- CORRECT

                 (B):  {S, >>G2, G3, ... Gn, D}  <---- WRONG


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                 (where >> represents the pointer).  If (A) is sent, the
                 datagram received at D will contain the option: {G1,
                 G2, ... Gn >>}, with S and D as the IP source and
                 destination addresses.  If (B) were sent, the datagram
                 received at D would again contain S and D as the same
                 IP source and destination addresses, but the option
                 would be: {S, G1, ...Gn >>}; i.e., the originating host
                 would be the first hop in the route.

         (d)  Record Route Option

              Routers MAY support the Record Route option in datagrams
              originated by the router.

         (e)  Timestamp Option

              Routers MAY support the timestamp option in datagrams
              originated by the router.  The following rules apply:

              o  When originating a datagram containing a Timestamp
                 Option, a router MUST record a timestamp in the option
                 if

                 - Its Internet address fields are not pre-specified or
                 - Its first pre-specified address is the IP address of
                    the logical interface over which the datagram is
                    being sent (or the router's router-id if the
                    datagram is being sent over an unnumbered
                    interface).

              o  If the router itself receives a datagram containing a
                 Timestamp Option, the router MUST insert the current
                 timestamp into the Timestamp Option (if there is space
                 in the option to do so) before passing the option to
                 the transport layer or to ICMP for processing.

              o  A timestamp value MUST follow the rules given in
                 Section [3.2.2.8] of [INTRO:2].

              IMPLEMENTATION:
                 To maximize the utility of the timestamps contained in
                 the timestamp option, it is suggested that the
                 timestamp inserted be, as nearly as practical, the time
                 at which the packet arrived at the router.  For
                 datagrams originated by the router, the timestamp
                 inserted should be, as nearly as practical, the time at
                 which the datagram was passed to the Link Layer for


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                 transmission.


4.2.2.2  Addresses in Options: RFC-791 Section 3.1

         When a router inserts its address into a Record Route, Strict
         Source and Record Route, Loose Source and Record Route, or
         Timestamp, it MUST use the IP address of the logical interface
         on which the packet is being sent.  Where this rule cannot be
         obeyed because the output interface has no IP address (i.e., is
         an unnumbered interface), the router MUST instead insert its
         router-id.  The router's router-id is one of the router's IP
         addresses.  Which of the router's addresses is used as the
         router-id MUST NOT change (even across reboots) unless changed
         by the network manager or unless the configuration of the
         router is changed such that the IP address used as the router-
         id ceases to be one of the router's IP addresses.  Routers with
         multiple unnumbered interfaces MAY have multiple router-id's.
         Each unnumbered interface MUST be associated with a particular
         router-id.  This association MUST NOT change (even across
         reboots) without reconfiguration of the router.

         DISCUSSION:
            This specification does not allow for routers which do not
            have at least one IP address.  We do not view this as a
            serious limitation, since a router needs an IP address to
            meet the manageability requirements of Chapter [8] even if
            the router is connected only to point-to-point links.


         IMPLEMENTATION:
            One possible method of choosing the router-id that fulfills
            this requirement is to use the numerically smallest (or
            greatest) IP address (treating the address as a 32-bit
            integer) that is assigned to the router.


4.2.2.3  Unused IP Header Bits: RFC-791 Section 3.1

         The IP header contains two reserved bits: one in the Type of
         Service byte and the other in the Flags field.  A router MUST
         NOT set either of these bits to one in datagrams originated by
         the router.  A router MUST NOT drop (refuse to receive or
         forward) a packet merely because one or more of these reserved
         bits has a non-zero value.



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         DISCUSSION:
            Future revisions to the IP protocol may make use of these
            unused bits.  These rules are intended to ensure that these
            revisions can be deployed without having to simultaneously
            upgrade all routers in the Internet.


4.2.2.4  Type of Service: RFC-791 Section 3.1

         The Type-of-Service byte in the IP header is divided into three
         sections:  the Precedence field (high-order 3 bits), a field
         that is customarily called Type of Service or TOS (next 4
         bits), and a reserved bit (the low order bit).

         Rules governing the reserved bit were described in Section
         [4.2.2.3].

         A more extensive discussion of the TOS field and its use can be
         found in [ROUTE:11].

         The description of the IP Precedence field is superseded by
         Section [5.3.3].  RFC-795, Service Mappings, is obsolete and
         SHOULD NOT be implemented.

4.2.2.5  Header Checksum: RFC-791 Section 3.1

         As stated in Section [5.2.2], a router MUST verify the IP
         checksum of any packet which is received.  The router MUST NOT
         provide a means to disable this checksum verification.

         IMPLEMENTATION:
            A more extensive description of the IP checksum, including
            extensive implementation hints, can be found in [INTERNET:6]
            and [INTERNET:7].


4.2.2.6  Unrecognized Header Options: RFC-791 Section 3.1

         A router MUST ignore IP options which it does not recognize.  A
         corollary of this requirement is that a router MUST implement
         the End of Option List option and the No Operation option,
         since neither contains an explicit length.






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         DISCUSSION:
            All future IP options will include an explicit length.


4.2.2.7  Fragmentation: RFC-791 Section 3.2

         Fragmentation, as described in [INTERNET:1], MUST be supported
         by a router.

         When a router fragments an IP datagram, it SHOULD minimize the
         number of fragments.  When a router fragments an IP datagram,
         it MUST send the fragments in order.  A fragmentation method
         which may generate one IP fragment which is significantly
         smaller than the other MAY cause the first IP fragment to be
         the smaller one.

         DISCUSSION:
            There are several fragmentation techniques in common use in
            the Internet.  One involves splitting the IP datagram into
            IP fragments with the first being MTU sized, and the others
            being approximately the same size, smaller than the MTU.
            The reason for this is twofold.  The first IP fragment in
            the sequence will be the effective MTU of the current path
            between the hosts, and the following IP fragments are sized
            to hopefully minimize the further fragmentation of the IP
            datagram.  Another technique is to split the IP datagram
            into MTU sized IP fragments, with the last fragment being
            the only one smaller, as per page 26 of [INTERNET:1].

            A common trick used by some implementations of TCP/IP is to
            fragment an IP datagram into IP fragments that are no larger
            than 576 bytes when the IP datagram is to travel through a
            router.  In general, this allows the resulting IP fragments
            to pass the rest of the path without further fragmentation.
            This would, though, create more of a load on the destination
            host, since it would have a larger number of IP fragments to
            reassemble into one IP datagram.  It would also not be
            efficient on networks where the MTU only changes once, and
            stays much larger than 576 bytes (such as an 802.5 network
            with a MTU of 2048 or an Ethernet network with an MTU of
            1536).

            One other fragmentation technique discussed was splitting
            the IP datagram into approximately equal sized IP fragments,
            with the size being smaller than the next hop network's MTU.
            This is intended to minimize the number of fragments that
            would result from additional fragmentation further down the


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            path.

            In most cases, routers should try and create situations that
            will generate the lowest number of IP fragments possible.

            Work with slow machines leads us to believe that if it is
            necessary to send small packets in a fragmentation scheme,
            sending the small IP fragment first maximizes the chance of
            a host with a slow interface of receiving all the fragments.


4.2.2.8  Reassembly: RFC-791 Section 3.2

         As specified in Section 3.3.2 of [INTRO:2], a router MUST
         support reassembly of datagrams which it delivers to itself.

4.2.2.9  Time to Live: RFC-791 Section 3.2

         Time to Live (TTL) handling for packets originated or received
         by the router is governed by [INTRO:2].  Note in particular
         that a router MUST NOT check the TTL of a packet except when
         forwarding it.

4.2.2.10  Multi-subnet Broadcasts: RFC-922

         All-subnets broadcasts (called multi-subnet broadcasts in
         [INTERNET:3]) have been deprecated.  See Section [5.3.5.3].

4.2.2.11  Addressing: RFC-791 Section 3.2

         There are now five classes of IP addresses: Class A through
         Class E.  Class D addresses are used for IP multicasting
         [INTERNET:4], while Class E addresses are reserved for
         experimental use.

         A multicast (Class D) address is a 28-bit logical address that
         stands for a group of hosts, and may be either permanent or
         transient.  Permanent multicast addresses are allocated by the
         Internet Assigned Number Authority [INTRO:7], while transient
         addresses may be allocated dynamically to transient groups.
         Group membership is determined dynamically using IGMP
         [INTERNET:4].

         We now summarize the important special cases for Unicast (that
         is class A, B, and C) IP addresses, using the following
         notation for an IP address:


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            { <Network-number>, <Host-number> }

         or

            { <Network-number>, <Subnet-number>, <Host-number> }

         and the notation -1 for a field that contains all 1 bits and
         the notation 0 for a field that contains all 0 bits.  This
         notation is not intended to imply that the 1-bits in a subnet
         mask need be contiguous.

         (a)  { 0, 0 }

              This host on this network.  It MUST NOT be used as a
              source address by routers, except the router MAY use this
              as a source address as part of an initialization procedure
              (e.g., if the router is using BOOTP to load its
              configuration information).

              Incoming datagrams with a source address of { 0, 0 } which
              are received for local delivery (see Section [5.2.3]),
              MUST be accepted if the router implements the associated
              protocol and that protocol clearly defines appropriate
              action to be taken.  Otherwise, a router MUST silently
              discard any locally-delivered datagram whose source
              address is { 0, 0 }.

              DISCUSSION:
                 Some protocols define specific actions to take in
                 response to a received datagram whose source address is
                 { 0, 0 }.  Two examples are BOOTP and ICMP Mask
                 Request.  The proper operation of these protocols often
                 depends on the ability to receive datagrams whose
                 source address is { 0, 0 }.  For most protocols,
                 however, it is best to ignore datagrams having a source
                 address of { 0, 0 } since they were probably generated
                 by a misconfigured host or router.  Thus, if a router
                 knows how to deal with a given datagram having a { 0, 0
                 } source address, the router MUST accept it.
                 Otherwise, the router MUST discard it.

              See also Section [4.2.3.1] for a non-standard use of { 0,
              0 }.

         (b)  { 0, <Host-number> }

              Specified host on this network.  It MUST NOT be sent by


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              routers except that the router MAY uses this as a source
              address as part of an initialization procedure by which
              the it learns its own IP address.

         (c)  { -1, -1 }

              Limited broadcast.  It MUST NOT be used as a source
              address.

              A datagram with this destination address will be received
              by every host and router on the connected physical
              network, but will not be forwarded outside that network.

         (d)  { <Network-number>, -1 }

              Network Directed Broadcast - a broadcast directed to the
              specified network.  It MUST NOT be used as a source
              address.  A router MAY originate Network Directed
              Broadcast packets.  A router MUST receive Network Directed
              Broadcast packets; however a router MAY have a
              configuration option to prevent reception of these
              packets.  Such an option MUST default to allowing
              reception.

         (e)  { <Network-number>, <Subnet-number>, -1 }

              Subnetwork Directed Broadcast - a broadcast sent to the
              specified subnet.  It MUST NOT be used as a source
              address.  A router MAY originate Network Directed
              Broadcast packets.  A router MUST receive Network Directed
              Broadcast packets; however a router MAY have a
              configuration option to prevent reception of these
              packets.  Such an option MUST default to allowing
              reception.

         (f)  { <Network-number>, -1, -1 }

              All Subnets Directed Broadcast - a broadcast sent to all
              subnets of the specified subnetted network.  It MUST NOT
              be used as a source address.  A router MAY originate
              Network Directed Broadcast packets.  A router MUST receive
              Network Directed Broadcast packets; however a router MAY
              have a configuration option to prevent reception of these
              packets.  Such an option MUST default to allowing
              reception.



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         (g)  { 127, <any> }

              Internal host loopback address.  Addresses of this form
              MUST NOT appear outside a host.

         The <Network-number> is administratively assigned so that its
         value will be unique in the entire world.

         IP addresses are not permitted to have the value 0 or -1 for
         any of the <Host-number>, <Network-number>, or <Subnet-number>
         fields (except in the special cases listed above).  This
         implies that each of these fields will be at least two bits
         long.

         For further discussion of broadcast addresses, see Section
         [4.2.3.1].

         Since (as described in Section [4.2.1]) a router must support
         the subnet extensions to IP, there will be a subnet mask of the
         form: { -1, -1, 0 } associated with each of the host's local IP
         addresses; see Sections [4.3.3.9], [5.2.4.2], and [10.2.2].

         When a router originates any datagram, the IP source address
         MUST be one of its own IP addresses (but not a broadcast or
         multicast address).  The only exception is during
         initialization.

         For most purposes, a datagram addressed to a broadcast or
         multicast destination is processed as if it had been addressed
         to one of the router's IP addresses; that is to say:

         o  A router MUST receive and process normally any packets with
            a broadcast destination address.

         o  A router MUST receive and process normally any packets sent
            to a multicast destination address which the router is
            interested in.

         The term specific-destination address means the equivalent
         local IP address of the host.  The specific-destination address
         is defined to be the destination address in the IP header
         unless the header contains a broadcast or multicast address, in
         which case the specific-destination is an IP address assigned
         to the physical interface on which the datagram arrived.

         A router MUST silently discard any received datagram containing
         an IP source address that is invalid by the rules of this


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         section.  This validation could be done either by the IP layer
         or by each protocol in the transport layer.

         DISCUSSION:
            A misaddressed datagram might be caused by a Link Layer
            broadcast of a unicast datagram or by another router or host
            that is confused or misconfigured.


4.2.3  SPECIFIC ISSUES


4.2.3.1  IP Broadcast Addresses

         For historical reasons, there are a number of IP addresses
         (some standard and some not) which are used to indicate that an
         IP packet is an IP broadcast.  A router

         (1)  MUST treat as IP broadcasts packets addressed to
              255.255.255.255, { <Network-number>, -1 }, { <Network-
              number>, <Subnet-number>, -1 }, and { <Network-number>,
              -1, -1 }.

         (2)  SHOULD silently discard on receipt (i.e., don't even
              deliver to applications in the router) any packet
              addressed to 0.0.0.0, { <Network-number>, 0 }, {
              <Network-number>, <Subnet-number>, 0 }, or { <Network-
              number>, 0, 0 }; if these packets are not silently
              discarded, they MUST be treated as IP broadcasts (see
              Section [5.3.5]).  There MAY be a configuration option to
              allow receipt of these packets.  This option SHOULD
              default to discarding them.

         (3)  SHOULD (by default) use the limited broadcast address
              (255.255.255.255) when originating an IP broadcast
              destined for a connected network or subnet (except when
              sending an ICMP Address Mask Reply, as discussed in
              Section [4.3.3.9]).  A router MUST receive limited
              broadcasts.

         (4)  SHOULD NOT originate datagrams addressed to 0.0.0.0, {
              <Network-number>, 0 }, { <Network-number>, <Subnet-
              number>, 0 }, or { <Network-number>, 0, 0 }.  There MAY be
              a configuration option to allow generation of these
              packets (instead of using the relevant 1s format
              broadcast).  This option SHOULD default to not generating
              them.


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         DISCUSSION:
            In the second bullet, the router obviously cannot recognize
            addresses of the form { <Network-number>, <Subnet-number>, 0
            } if the router does not know how the particular network is
            subnetted.  In that case, the rules of the second bullet do
            not apply because, from the point of view of the router, the
            packet is not an IP broadcast packet.


4.2.3.2  IP Multicasting

         An IP router SHOULD satisfy the Host Requirements with respect
         to IP multicasting, as specified in Section 3.3.7 of [INTRO:2].
         An IP router SHOULD support local IP multicasting on all
         connected networks for which a mapping from Class D IP
         addresses to link-layer addresses has been specified (see the
         various IP-over-xxx specifications), and on all connected
         point-to-point links.  Support for local IP multicasting
         includes originating multicast datagrams, joining multicast
         groups and receiving multicast datagrams, and leaving multicast
         groups.  This implies support for all of [INTERNET:4] including
         IGMP (see Section [4.4]).

         DISCUSSION:
            Although [INTERNET:4] is entitled Host Extensions for IP
            Multicasting, it applies to all IP systems, both hosts and
            routers.  In particular, since routers may join multicast
            groups, it is correct for them to perform the host part of
            IGMP, reporting their group memberships to any multicast
            routers that may be present on their attached networks
            (whether or not they themselves are multicast routers).

            Some router protocols may specifically require support for
            IP multicasting (e.g., OSPF [ROUTE:1]), or may recommend it
            (e.g., ICMP Router Discovery [INTERNET:13]).


4.2.3.3  Path MTU Discovery

         In order to eliminate fragmentation or minimize it, it is
         desirable to know what is the path MTU along the path from the
         source to destination.  The path MTU is the minimum of the MTUs
         of each hop in the path.  [INTERNET:14] describes a technique
         for dynamically discovering the maximum transmission unit (MTU)
         of an arbitrary internet path.  For a path that passes through
         a router that does not support [INTERNET:14], this technique
         might not discover the correct Path MTU, but it will always


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         choose a Path MTU as accurate as, and in many cases more
         accurate than, the Path MTU that would be chosen by older
         techniques or the current practice.

         When a router is originating an IP datagram, it SHOULD use the
         scheme described in [INTERNET:14] to limit the datagram's size.
         If the router's route to the datagram's destination was learned
         from a routing protocol that provides Path MTU information, the
         scheme described in [INTERNET:14] is still used, but the Path
         MTU information from the routing protocol SHOULD be used as the
         initial guess as to the Path MTU and also as an upper bound on
         the Path MTU.

4.2.3.4  Subnetting

         Under certain circumstances, it may be desirable to support
         subnets of a particular network being interconnected only via a
         path which is not part of the subnetted network.  This is known
         as discontiguous subnetwork support.

         Routers MUST support discontiguous subnetworks.

         IMPLEMENTATION:
            In general, a router should not make assumptions about what
            are subnets and what are not, but simply ignore the concept
            of Class in networks, and treat each route as a { network,
            mask }-tuple.


         DISCUSSION:
            The Internet has been growing at a tremendous rate of late.
            This has been placing severe strains on the IP addressing
            technology.  A major factor in this strain is the strict IP
            Address class boundaries.  These make it difficult to
            efficiently size network numbers to their networks and
            aggregate several network numbers into a single route
            advertisement.  By eliminating the strict class boundaries
            of the IP address and treating each route as a {network
            number, mask}-tuple these strains may be greatly reduced.

            The technology for currently doing this is Classless
            Interdomain Routing (CIDR) [INTERNET:15].

         Furthermore, for similar reasons, a subnetted network need not
         have a consistent subnet mask through all parts of the network.
         For example, one subnet may use an 8 bit subnet mask, another
         10 bit, and another 6 bit.  This is known as variable subnet-


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         masks.

         Routers MUST support variable subnet-masks.

4.3  INTERNET CONTROL MESSAGE PROTOCOL - ICMP


4.3.1  INTRODUCTION

      ICMP is an auxiliary protocol, which provides routing, diagnostic
      and and error functionality for IP. It is described in
      [INTERNET:8].  A router MUST support ICMP.

      ICMP messages are grouped in two classes which are discussed in
      the following sections:

      ICMP error messages:

      Destination Unreachable     Section 4.3.3.1
      Redirect                    Section 4.3.3.2
      Source Quench               Section 4.3.3.3
      Time Exceeded               Section 4.3.3.4
      Parameter Problem           Section 4.3.3.5

      ICMP query messages:
      Echo                        Section 4.3.3.6
      Information                 Section 4.3.3.7
      Timestamp                   Section 4.3.3.8
      Address Mask                Section 4.3.3.9
      Router Discovery            Section 4.3.3.10


      General ICMP requirements and discussion are in the next section.

4.3.2  GENERAL ISSUES


4.3.2.1  Unknown Message Types

         If an ICMP message of unknown type is received, it MUST be
         passed to the ICMP user interface (if the router has one) or
         silently discarded (if the router doesn't have one).






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4.3.2.2  ICMP Message TTL

         When originating an ICMP message, the router MUST initialize
         the TTL.  The TTL for ICMP responses must not be taken from the
         packet which triggered the response.

4.3.2.3  Original Message Header

         Every ICMP error message includes the Internet header and at
         least the first 8 data bytes of the datagram that triggered the
         error.  More than 8 bytes MAY be sent, but the resulting ICMP
         datagram SHOULD have a length of less than or equal to 576
         bytes.  The returned IP header (and user data) MUST be
         identical to that which was received, except that the router is
         not required to undo any modifications to the IP header that
         are normally performed in forwarding that were performed before
         the error was detected (e.g., decrementing the TTL, updating
         options).  Note that the requirements of Section [4.3.3.5]
         supersede this requirement in some cases (i.e., for a Parameter
         Problem message, if the problem  is in a modified field, the
         router must undo the modification).  See Section [4.3.3.5])

4.3.2.4  ICMP Message Source Address

         Except where this document specifies otherwise, the IP source
         address in an ICMP message originated by the router MUST be one
         of the IP addresses associated with the physical interface over
         which the ICMP message is transmitted.  If the interface has no
         IP addresses associated with it, the router's router-id (see
         Section [5.2.5]) is used instead.

4.3.2.5  TOS and Precedence

         ICMP error messages SHOULD have their TOS bits set to the same
         value as the TOS bits in the packet which provoked the sending
         of the ICMP error message, unless setting them to that value
         would cause the ICMP error message to be immediately discarded
         because it could not be routed to its destination.  Otherwise,
         ICMP error messages MUST be sent with a normal (i.e. zero) TOS.
         An ICMP reply message SHOULD have its TOS bits set to the same
         value as the TOS bits in the ICMP request that provoked the
         reply.

         EDITOR'S COMMENTS:
            The following paragraph originally read:

               ICMP error messages MUST have their IP Precedence field


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               set to the same value as the IP Precedence field in the
               packet which provoked the sending of the ICMP error
               message, except that the precedence value MUST be 6
               (INTERNETWORK CONTROL) or 7 (NETWORK CONTROL), SHOULD be
               7, and MAY be settable for the following types of ICMP
               error messages: Unreachable, Redirect, Time Exceeded, and
               Parameter Problem.

            I believe that the following paragraph is equivalent and
            easier for humans to parse (Source Quench is the only other
            ICMP Error message).  Other interpretations of the original
            are sought.

         ICMP Source Quench error messages MUST have their IP Precedence
         field set to the same value as the IP Precedence field in the
         packet which provoked the sending of the ICMP Source Quench
         message.  All other ICMP error messages (Destination
         Unreachable, Redirect, Time Exceeded, and Parameter Problem)
         MUST have their precedence value set to 6 (INTERNETWORK
         CONTROL) or 7 (NETWORK CONTROL), SHOULD be 7.  The IP
         Precedence value for these error messages MAY be settable.

         An ICMP reply message MUST have its IP Precedence field set to
         the same value as the IP Precedence field in the ICMP request
         that provoked the reply.

4.3.2.6  Source Route

         If the packet which provokes the sending of an ICMP error
         message contains a source route option, the ICMP error message
         SHOULD also contain a source route option of the same type
         (strict or loose), created by reversing the portion before the
         pointer of the route recorded in the source route option of the
         original packet UNLESS the ICMP error message is an ICMP
         Parameter Problem complaining about a source route option in
         the original packet.

         DISCUSSION:
            In environments which use the U.S. Department of Defense
            security option (defined in [INTERNET:5]), ICMP messages may
            need to include a security option.  Detailed information on
            this topic should be available from the Defense
            Communications Agency.





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4.3.2.7  When Not to Send ICMP Errors

         An ICMP error message MUST NOT be sent as the result of
         receiving:

         o  An ICMP error message, or

         o  A packet which fails the IP header validation tests
            described in Section [5.2.2] (except where that section
            specifically permits the sending of an ICMP error message),
            or

         o  A packet destined to an IP broadcast or IP multicast
            address, or

         o  A packet sent as a Link Layer broadcast or multicast, or

         o  A packet whose source address has a network number of zero
            or is an invalid source address (as defined in Section
            [5.3.7]), or

         o  Any fragment of a datagram other then the first fragment
            (i.e., a packet for which the fragment offset in the IP
            header is nonzero).

         Furthermore, an ICMP error message MUST NOT be sent in any case
         where this memo states that a packet is to be silently
         discarded.

         NOTE:  THESE RESTRICTIONS TAKE PRECEDENCE OVER ANY REQUIREMENT
         ELSEWHERE IN THIS DOCUMENT FOR SENDING ICMP ERROR MESSAGES.

         DISCUSSION:
            These rules aim to prevent the broadcast storms that have
            resulted from routers or hosts returning ICMP error messages
            in response to broadcast packets.  For example, a broadcast
            UDP packet to a non-existent port could trigger a flood of
            ICMP Destination Unreachable datagrams from all devices that
            do not have a client for that destination port.  On a large
            Ethernet, the resulting collisions can render the network
            useless for a second or more.

            Every packet that is broadcast on the connected network
            should have a valid IP broadcast address as its IP
            destination (see Section [5.3.4] and [INTRO:2]).  However,
            some devices violate this rule.  To be certain to detect
            broadcast packets, therefore, routers are required to check


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            for a link-layer broadcast as well as an IP-layer address.


         IMPLEMENTATION:
            This requires that the link layer inform the IP layer when a
            link-layer broadcast packet has been received; see Section
            [3.1].


4.3.2.8  Rate Limiting

         A router which sends ICMP Source Quench messages MUST be able
         to limit the rate at which the messages can be generated.  A
         router SHOULD also be able to limit the rate at which it sends
         other sorts of ICMP error messages (Destination Unreachable,
         Redirect, Time Exceeded, Parameter Problem).  The rate limit
         parameters SHOULD be settable as part of the configuration of
         the router.  How the limits are applied (e.g., per router or
         per interface) is left to the implementor's discretion.

         DISCUSSION:
            Two problems for a router sending ICMP error message are:
            (1)  The consumption of bandwidth on the reverse path, and
            (2)  The use of router resources (e.g., memory, CPU time)

            To help solve these problems a router can limit the
            frequency with which it generates ICMP error messages.  For
            similar reasons, a router may limit the frequency at which
            some other sorts of messages, such as ICMP Echo Replies, are
            generated.


         IMPLEMENTATION:
            Various mechanisms have been used or proposed for limiting
            the rate at which ICMP messages are sent:

            (1)  Count-based - for example, send an ICMP error message
                 for every N dropped packets overall or per given source
                 host.  This mechanism might be appropriate for ICMP
                 Source Quench, but probably not for other types of ICMP
                 messages.

            (2)  Timer-based - for example, send an ICMP error message
                 to a given source host or overall at most once per T
                 milliseconds.

            (3)  Bandwidth-based - for example, limit the rate at which


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                 ICMP messages are sent over a particular interface to
                 some fraction of the attached network's bandwidth.


4.3.3  SPECIFIC ISSUES


4.3.3.1  Destination Unreachable

         If a route can not forward a packet because it has no routes at
         all to the destination network specified in the packet then the
         router MUST generate a Destination Unreachable, Code 0 (Network
         Unreachable) ICMP message.  If the router does have routes to
         the destination network specified in the packet but the TOS
         specified for the routes is neither the default TOS (0000) nor
         the TOS of the packet that the router is attempting to route,
         then the router MUST generate a Destination Unreachable, Code
         11 (Network Unreachable for TOS) ICMP message.

         If a packet is to be forwarded to a host on a network that is
         directly connected to the router (i.e., the router is the
         last-hop router) and the router has ascertained that there is
         no path to the destination host then the router MUST generate a
         Destination Unreachable, Code 1 (Host Unreachable) ICMP
         message.  If a packet is to be forwarded to a host that is on a
         network that is directly connected to the router and the router
         cannot forward the packet because because no route to the
         destination has a TOS that is either equal to the TOS requested
         in the packet or is the default TOS (0000) then the router MUST
         generate a Destination Unreachable, Code 12 (Host Unreachable
         for TOS) ICMP message.

         DISCUSSION:
            The intent is that a router generates the "generic"
            host/network unreachable if it has no path at all (including
            default routes) to the destination.  If the router has one
            or more paths to the destination, but none of those paths
            have an acceptable TOS, then the router generates the
            "unreachable for TOS" message.


4.3.3.2  Redirect

         The ICMP Redirect message is generated to inform a host on the
         same subnet that the router used by the host to route certain
         packets should be changed.


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         Contrary to section 3.2.2.2 of [INTRO:2], a router MAY ignore
         ICMP Redirects when choosing a path for a packet originated by
         the router if the router is running a routing protocol or if
         forwarding is enabled on the router and on the interface over
         which the packet is being sent.

4.3.3.3  Source Quench

         A router SHOULD NOT originate ICMP Source Quench messages.  As
         specified in Section [4.3.2], a router which does originate
         Source Quench messages MUST be able to limit the rate at which
         they are generated.

         DISCUSSION:
            Research seems to suggest that Source Quench consumes
            network bandwidth but is an ineffective (and unfair)
            antidote to congestion.  See, for example, [INTERNET:9] and
            [INTERNET:10].  Section [5.3.6] discusses the current
            thinking on how routers ought to deal with overload and
            network congestion.

         A router MAY ignore any ICMP Source Quench messages it
         receives.

         DISCUSSION:
            A router itself may receive a Source Quench as the result of
            originating a packet sent to another router or host.  Such
            datagrams might be, e.g., an EGP update sent to another
            router, or a telnet stream sent to a host.  A mechanism has
            been proposed ([INTERNET:11], [INTERNET:12]) to make the IP
            layer respond directly to Source Quench by controlling the
            rate at which packets are sent, however, this proposal is
            currently experimental and not currently recommended.


4.3.3.4  Time Exceeded

         When a router is forwarding a packet and the TTL field of the
         packet is reduced to 0, the requirements of section [5.2.3.8]
         apply.

         When the router is reassembling a packet that is destined for
         the router, it MUST fulfill requirements of [INTRO:2], section
         [3.3.2] apply.

         When the router receives (i.e., is destined for the router) a
         Time Exceeded message, it MUST comply with section 3.2.2.4 of


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         [INTRO:2].

4.3.3.5  Parameter Problem

         A router MUST generate a Parameter Problem message for any
         error not specifically covered by another ICMP message.  The IP
         header field or IP option including the byte indicated by the
         pointer field MUST be included unchanged in the IP header
         returned with this ICMP message.  Section [4.3.2] defines an
         exception to this requirement.

         A new variant of the Parameter Problem message was defined in
         [INTRO:2]:
              Code 1 = required option is missing.

         DISCUSSION:
            This variant is currently in use in the military community
            for a missing security option.


4.3.3.6  Echo Request/Reply

         A router MUST implement an ICMP Echo server function that
         receives Echo Requests and sends corresponding Echo Replies.  A
         router MUST be prepared to receive, reassemble and echo an ICMP
         Echo Request datagram at least as large as the maximum of 576
         and the MTUs of all the connected networks.

         The Echo server function MAY choose not to respond to ICMP echo
         requests addressed to IP broadcast or IP multicast addresses.

         A router SHOULD have a configuration option which, if enabled,
         causes the router to silently ignore all ICMP echo requests; if
         provided, this option MUST default to allowing responses.

         DISCUSSION:
            The neutral provision about responding to broadcast and
            multicast Echo Requests results from the conclusions reached
            in section [3.2.2.6] of [INTRO:2].

         As stated in Section [10.3.3], a router MUST also implement an
         user/application-layer interface for sending an Echo Request
         and receiving an Echo Reply, for diagnostic purposes.  All ICMP
         Echo Reply messages MUST be passed to this interface.

         The IP source address in an ICMP Echo Reply MUST be the same as
         the specific-destination address of the corresponding ICMP Echo


Almquist & Kastenholz                                          [Page 57]

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         Request message.

         Data received in an ICMP Echo Request MUST be entirely included
         in the resulting Echo Reply.

         If a Record Route and/or Timestamp option is received in an
         ICMP Echo Request, this option (these options) SHOULD be
         updated to include the current router and included in the IP
         header of the Echo Reply message, without truncation.  Thus,
         the recorded route will be for the entire round trip.

         If a Source Route option is received in an ICMP Echo Request,
         the return route MUST be reversed and used as a Source Route
         option for the Echo Reply message.

4.3.3.7  Information Request/Reply

         A router SHOULD NOT originate or respond to these messages.

         DISCUSSION:
            The Information Request/Reply pair was intended to support
            self-configuring systems such as diskless workstations, to
            allow them to discover their IP network numbers at boot
            time.  However, these messages are now obsolete.  The RARP
            and BOOTP protocols provide better mechanisms for a host to
            discover its own IP address.


4.3.3.8  Timestamp and Timestamp Reply

         A router MAY implement Timestamp and Timestamp Reply.  If they
         are implemented then:

         o  The ICMP Timestamp server function MUST return a Timestamp
            Reply to every Timestamp message that is received.  It
            SHOULD be designed for minimum variability in delay.

         o  An ICMP Timestamp Request message to an IP broadcast or IP
            multicast address MAY be silently discarded.

         o  The IP source address in an ICMP Timestamp Reply MUST be the
            same as the specific-destination address of the
            corresponding Timestamp Request message.

         o  If a Source Route option is received in an ICMP Timestamp
            Request, the return route MUST be reversed and used as a
            Source Route option for the Timestamp Reply message.


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         o  If a Record Route and/or Timestamp option is received in a
            Timestamp Request, this (these) option(s) SHOULD be updated
            to include the current router and included in the IP header
            of the Timestamp Reply message.

         o  If the router provides an application-layer interface for
            sending Timestamp Request messages then incoming Timestamp
            Reply messages MUST be passed up to the ICMP user interface.

         The preferred form for a timestamp value (the standard value)
         is milliseconds since midnight, Universal Time.  However, it
         may be difficult to provide this value with millisecond
         resolution. For example, many systems use clocks that update
         only at line frequency, 50 or 60 times per second.  Therefore,
         some latitude is allowed in a standard value:

         (a)  A standard value MUST be updated at least 16 times per
              second (i.e., at most the six low-order bits of the value
              may be undefined).

         (b)  The accuracy of a standard value MUST approximate that of
              operator-set CPU clocks, i.e., correct within a few
              minutes.

         IMPLEMENTATION:
            To meet the second condition, a router may need to query
            some time server when the router is booted or restarted. It
            is recommended that the UDP Time Server Protocol be used for
            this purpose. A more advanced implementation would use the
            Network Time Protocol (NTP) to achieve nearly millisecond
            clock synchronization; however, this is not required.


4.3.3.9  Address Mask Request/Reply

         A router MUST implement support for receiving ICMP Address Mask
         Request messages and responding with ICMP Address Mask Reply
         messages.  These messages are defined in [INTERNET:2].

         A router SHOULD have a configuration option for each logical
         interface specifying whether the router is allowed to answer
         Address Mask Requests for that interface; this option MUST
         default to allowing responses.  A router MUST NOT respond to an
         Address Mask Request before the router knows the correct subnet
         mask.

         A router MUST NOT respond to an Address Mask Request which has


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         a source address of 0.0.0.0 and which arrives on a physical
         interface which has associated with it multiple logical
         interfaces and the subnet masks for those interfaces are not
         all the same.

         A router SHOULD examine all ICMP Address Mask Replies which it
         receives to determine whether the information it contains
         matches the router's knowledge of the subnet mask.  If the ICMP
         Address Mask Reply appears to be in error, the router SHOULD
         log the subnet mask and the sender's IP address.  A router MUST
         NOT use the contents of an ICMP Address Mask Reply to determine
         the correct subnet mask.

         Because hosts may not be able to learn the subnet mask if a
         router is down when the host boots up, a router MAY broadcast a
         gratuitous ICMP Address Mask Reply on each of its logical
         interfaces after it has configured its own subnet masks.
         However, this feature can be dangerous in environments which
         use variable length subnet masks.  Therefore, if this feature
         is implemented, gratuitous Address Mask Replies MUST NOT be
         broadcast over any logical interface(s) which either:

         o  Are not configured to send gratuitous Address Mask Replies.
            Each logical interface MUST have a configuration parameter
            controlling this, and that parameter MUST default to not
            sending the gratuitous Address Mask Replies.

         o  Share the same IP network number and physical interface but
            have different subnet masks.

         The { <Network-number>, -1, -1 } form (on subnetted networks)
         or the { <Network-number>, -1 } form (on non-subnetted
         networks) of the IP broadcast address MUST be used for
         broadcast Address Mask Replies.

         DISCUSSION:
            The ability to disable sending Address Mask Replies by
            routers is required at a few sites which intentionally lie
            to their hosts about the subnet mask.  The need for this is
            expected to go away as more and more hosts become compliant
            with the Host Requirements standards.

            The reason for both the second bullet above and the
            requirement about which IP broadcast address to use is to
            prevent problems when multiple IP networks or subnets are in
            use on the same physical network.


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4.3.3.10  Router Advertisement and Solicitations

         An IP router MUST support the router part of the ICMP Router
         Discovery Protocol [INTERNET:13] on all connected networks on
         which the router supports either IP multicast or IP broadcast
         addressing.  The implementation MUST include all of the
         configuration variables specified for routers, with the
         specified defaults.

         DISCUSSION:
            Routers are not required to implement the host part of the
            ICMP Router Discovery Protocol, but might find it useful for
            operation while IP forwarding is disabled (i.e., when
            operating as a host).


         DISCUSSION:
            We note that it is quite common for hosts to use RIP as the
            router discovery protocol.  Such hosts listen to RIP traffic
            and use and use information extracted from that traffic to
            discover routers and to make decisions as to which router to
            use as a first-hop router for a given destination.  While
            this behavior is discouraged, it is still common and
            implementors should be aware of it.


4.4  INTERNET GROUP MANAGEMENT PROTOCOL - IGMP

   IGMP [INTERNET:4] is a protocol used between hosts and multicast
   routers on a single physical network to establish hosts' membership
   in particular multicast groups.  Multicast routers use this
   information, in conjunction with a multicast routing protocol, to
   support IP multicast forwarding across the Internet.

   A router SHOULD implement the host part of IGMP.













Almquist & Kastenholz                                          [Page 61]

RFC 1716          Towards Requirements for IP Routers      November 1994


5.  INTERNET LAYER - FORWARDING


5.1  INTRODUCTION

   This section describes the process of forwarding packets.

5.2  FORWARDING WALK-THROUGH

   There is no separate specification of the forwarding function in IP.
   Instead, forwarding is covered by the protocol specifications for the
   internet layer protocols ([INTERNET:1], [INTERNET:2], [INTERNET:3],
   [INTERNET:8], and [ROUTE:11]).

5.2.1  Forwarding Algorithm

      Since none of the primary protocol documents describe the
      forwarding algorithm in any detail, we present it here.  This is
      just a general outline, and omits important details, such as
      handling of congestion, that are dealt with in later sections.

      It is not required that an implementation follow exactly the
      algorithms given in sections [5.2.1.1], [5.2.1.2], and [5.2.1.3].
      Much of the challenge of writing router software is to maximize
      the rate at which the router can forward packets while still
      achieving the same effect of the algorithm.  Details of how to do
      that are beyond the scope of this document, in part because they
      are heavily dependent on the architecture of the router.  Instead,
      we merely point out the order dependencies among the steps:

      (1)  A router MUST verify the IP header, as described in section
           [5.2.2], before performing any actions based on the contents
           of the header.  This allows the router to detect and discard
           bad packets before the expenditure of other resources.

      (2)  Processing of certain IP options requires that the router
           insert its IP address into the option.  As noted in Section
           [5.2.4], the address inserted MUST be the address of the
           logical interface on which the packet is sent or the router's
           router-id if the packet is sent over an unnumbered interface.
           Thus, processing of these options cannot be completed until
           after the output interface is chosen.

      (3)  The router cannot check and decrement the TTL before checking
           whether the packet should be delivered to the router itself,
           for reasons mentioned in Section [4.2.2.9].


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      (4)  More generally, when a packet is delivered locally to the
           router, its IP header MUST NOT be modified in any way (except
           that a router may be required to insert a timestamp into any
           Timestamp options in the IP header).  Thus, before the router
           determines whether the packet is to be delivered locally to
           the router, it cannot update the IP header in any way that it
           is not prepared to undo.

5.2.1.1  General

         This section covers the general forwarding algorithm.  This
         algorithm applies to all forms of packets to be forwarded:
         unicast, multicast, and broadcast.


         (1)  The router receives the IP packet (plus additional
              information about it, as described in Section [3.1]) from
              the Link Layer.

         (2)  The router validates the IP header, as described in
              Section [5.2.2].  Note that IP reassembly is not done,
              except on IP fragments to be queued for local delivery in
              step (4).

         (3)  The router performs most of the processing of any IP
              options.  As described in Section [5.2.4], some IP options
              require additional processing after the routing decision
              has been made.

         (4)  The router examines the destination IP address of the IP
              datagram, as described in Section [5.2.3], to determine
              how it should continue to process the IP datagram.  There
              are three possibilities:

              o  The IP datagram is destined for the router, and should
                 be queued for local delivery, doing reassembly if
                 needed.

              o  The IP datagram is not destined for the router, and
                 should be queued for forwarding.

              o  The IP datagram should be queued for forwarding, but (a
                 copy) must also be queued for local delivery.





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5.2.1.2  Unicast

         Since the local delivery case is well-covered by [INTRO:2], the
         following assumes that the IP datagram was queued for
         forwarding.  If the destination is an IP unicast address:

         (5)  The forwarder determines the next hop IP address for the
              packet, usually by looking up the packet's destination in
              the router's routing table.  This procedure is described
              in more detail in Section [5.2.4].  This procedure also
              decides which network interface should be used to send the
              packet.

         (6)  The forwarder verifies that forwarding the packet is
              permitted.  The source and destination addresses should be
              valid, as described in Section [5.3.7] and Section [5.3.4]
              If the router supports administrative constraints on
              forwarding, such as those described in Section [5.3.9],
              those constraints must be satisfied.

         (7)  The forwarder decrements (by at least one) and checks the
              packet's TTL, as described in Section [5.3.1].

         (8)  The forwarder performs any IP option processing that could
              not be completed in step 3.

         (9)  The forwarder performs any necessary IP fragmentation, as
              described in Section [4.2.2.7].  Since this step occurs
              after outbound interface selection (step 5), all fragments
              of the same datagram will be transmitted out the same
              interface.

         (10) The forwarder determines the Link Layer address of the
              packet's next hop.  The mechanisms for doing this are Link
              Layer-dependent (see chapter 3).

         (11) The forwarder encapsulates the IP datagram (or each of the
              fragments thereof) in an appropriate Link Layer frame and
              queues it for output on the interface selected in step 5.

         (12) The forwarder sends an ICMP redirect if necessary, as
              described in Section [4.3.3.2].






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5.2.1.3  Multicast

         If the destination is an IP multicast, the following steps are
         taken.

         Note that the main differences between the forwarding of IP
         unicasts and the forwarding of IP multicasts are

         o  IP multicasts are usually forwarded based on both the
            datagram's source and destination IP addresses,

         o  IP multicast uses an expanding ring search,

         o  IP multicasts are forwarded as Link Level multicasts, and

         o  ICMP errors are never sent in response to IP multicast
            datagrams.

         Note that the forwarding of IP multicasts is still somewhat
         experimental. As a result, the algorithm presented below is not
         mandatory, and is provided as an example only.

         (5a) Based on the IP source and destination addresses found in
              the datagram header, the router determines whether the
              datagram has been received on the proper interface for
              forwarding. If not, the datagram is dropped silently.  The
              method for determining the proper receiving interface
              depends on the multicast routing algorithm(s) in use. In
              one of the simplest algorithms, reverse path forwarding
              (RPF), the proper interface is the one that would be used
              to forward unicasts back to the datagram source.

         (6a) Based on the IP source and destination addresses found in
              the datagram header, the router determines the datagram's
              outgoing interfaces. In order to implement IP multicast's
              expanding ring search (see [INTERNET:4]) a minimum TTL
              value is specified for each outgoing interface. A copy of
              the multicast datagram is forwarded out each outgoing
              interface whose minimum TTL value is less than or equal to
              the TTL value in the datagram header, by separately
              applying the remaining steps on each such interface.

         (7a) The router decrements the packet's TTL by one.

         (8a) The forwarder performs any IP option processing that could
              not be completed in step (3).


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         (9a) The forwarder performs any necessary IP fragmentation, as
              described in Section [4.2.2.7].

         (10a) The forwarder determines the Link Layer address to use in
              the Link Level encapsulation. The mechanisms for doing
              this are Link Layer-dependent. On LANs a Link Level
              multicast or broadcast is selected, as an algorithmic
              translation of the datagrams' class D destination address.
              See the various IP-over-xxx specifications for more
              details.

         (11a) The forwarder encapsulates the packet (or each of the
              fragments thereof) in an appropriate Link Layer frame and
              queues it for output on the appropriate interface.

5.2.2  IP Header Validation

      Before a router can process any IP packet, it MUST perform a the
      following basic validity checks on the packet's IP header to
      ensure that the header is meaningful.  If the packet fails any of
      the following tests, it MUST be silently discarded, and the error
      SHOULD be logged.

      (1)  The packet length reported by the Link Layer must be large
           enough to hold the minimum length legal IP datagram (20
           bytes).

      (2)  The IP checksum must be correct.

      (3)  The IP version number must be 4.  If the version number is
           not 4 then the packet may well be another version of IP, such
           as ST-II.

      (4)  The IP header length field must be at least 5.

      (5)  The IP total length field must be at least 4 * IP header
           length field.

      A router MUST NOT have a configuration option which allows
      disabling any of these tests.

      If the packet passes the second and third tests, the IP header
      length field is at least 4, and both the IP total length field and
      the packet length reported by the Link Layer are at least 16 then,
      despite the above rule, the router MAY respond with an ICMP
      Parameter Problem message, whose pointer points at the IP header
      length field (if it failed the fourth test) or the IP total length


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      field (if it failed the fifth test).  However, it still MUST
      discard the packet and still SHOULD log the error.

      These rules (and this entire document) apply only to version 4 of
      the Internet Protocol.  These rules should not be construed as
      prohibiting routers from supporting other versions of IP.
      Furthermore, if a router can truly classify a packet as being some
      other version of IP then it ought not treat that packet as an
      error packet within the context of this memo.

      IMPLEMENTATION:
         It is desirable for purposes of error reporting, though not
         always entirely possible, to determine why a header was
         invalid.  There are four possible reasons:

         o  The Link Layer truncated the IP header

         o  The datagram is using a version of IP other than the
            standard one (version 4).

         o  The IP header has been corrupted in transit.

         o  The sender generated an illegal IP header.

         It is probably desirable to perform the checks in the order
         listed, since we believe that this ordering is most likely to
         correctly categorize the cause of the error.  For purposes of
         error reporting, it may also be desirable to check if a packet
         which fails these tests has an IP version number equal to 6.
         If it does, the packet is probably an ST-II datagram and should
         be treated as such.  ST-II is described in [FORWARD:1].

      Additionally, the router SHOULD verify that the packet length
      reported by the Link Layer is at least as large as the IP total
      length recorded in the packet's IP header.  If it appears that the
      packet has been truncated, the packet MUST be discarded, the error
      SHOULD be logged, and the router SHOULD respond with an ICMP
      Parameter Problem message whose pointer points at the IP total
      length field.

      DISCUSSION:
         Because any higher layer protocol which concerns itself with
         data corruption will detect truncation of the packet data when
         it reaches its final destination, it is not absolutely
         necessary for routers to perform the check suggested above in
         order to maintain protocol correctness.  However, by making
         this check a router can simplify considerably the task of


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         determining which hop in the path is truncating the packets.
         It will also reduce the expenditure of resources down-stream
         from the router in that down-stream systems will not need to
         deal with the packet.

      Finally, if the destination address in the IP header is not one of
      the addresses of the router, the router SHOULD verify that the
      packet does not contain a Strict Source and Record Route option.
      If a packet fails this test, the router SHOULD log the error and
      SHOULD respond with an ICMP Parameter Problem error with the
      pointer pointing at the offending packet's IP destination address.

      DISCUSSION:
         Some people might suggest that the router should respond with a
         Bad Source Route message instead of a Parameter Problem
         message.  However, when a packet fails this test, it usually
         indicates a protocol error by the previous hop router, whereas
         Bad Source Route would suggest that the source host had
         requested a nonexistent or broken path through the network.


5.2.3  Local Delivery Decision

      When a router receives an IP packet, it must decide whether the
      packet is addressed to the router (and should be delivered
      locally) or the packet is addressed to another system (and should
      be handled by the forwarder).  There is also a hybrid case, where
      certain IP broadcasts and IP multicasts are both delivered locally
      and forwarded.  A router MUST determine which of the these three
      cases applies using the following rules:

      o  An unexpired source route option is one whose pointer value
         does not point past the last entry in the source route.  If the
         packet contains an unexpired source route option, the pointer
         in the option is advanced until either the pointer does point
         past the last address in the option or else the next address is
         not one of the router's own addresses.  In the latter (normal)
         case, the  packet is forwarded (and not delivered locally)
         regardless of the rules below.

      o  The packet is delivered locally and not considered for
         forwarding in the following cases:

         - The packet's destination address exactly matches one of the
            router's IP addresses,

         - The packet's destination address is a limited broadcast


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            address ({-1, -1}), and

         - The packet's destination is an IP multicast address which is
            limited to a single subnet (such as 224.0.0.1 or 224.0.0.2)
            and (at least) one of the logical interfaces associated with
            the physical interface on which the packet arrived is a
            member of the destination multicast group.

      o  The packet is passed to the forwarder AND delivered locally in
         the following cases:

         - The packet's destination address is an IP broadcast address
            that addresses at least one of the router's logical
            interfaces but does not address any of the logical
            interfaces associated with the physical interface on which
            the packet arrived

         - The packet's destination is an IP multicast address which is
            not limited to a single subnetwork (such as 224.0.0.1 and
            224.0.0.2 are) and (at least) one of the logical interfaces
            associated with the physical interface on which the packet
            arrived is a member of the destination multicast group.

      o  The packet is delivered locally if the packet's destination
         address is an IP broadcast address (other than a limited
         broadcast address) that addresses at least one of the logical
         interfaces associated with the physical interface on which the
         packet arrived.  The packet is ALSO passed to the forwarder
         unless the link on which the packet arrived uses an IP
         encapsulation that does not encapsulate broadcasts differently
         than unicasts (e.g. by using different Link Layer destination
         addresses).

      o  The packet is passed to the forwarder in all other cases.

      DISCUSSION:
         The purpose of the requirement in the last sentence of the
         fourth bullet is to deal with a directed broadcast to another
         net or subnet on the same physical cable.  Normally, this works
         as expected: the sender sends the broadcast to the router as a
         Link Layer unicast.  The router notes that it arrived as a
         unicast, and therefore must be destined for a different logical
         net (or subnet) than the sender sent it on.  Therefore, the
         router can safely send it as a Link Layer broadcast out the
         same (physical) interface over which it arrived.  However, if
         the router can't tell whether the packet was received as a Link
         Layer unicast, the sentence ensures that the router does the


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         safe but wrong thing rather than the unsafe but right thing.


      IMPLEMENTATION:
         As described in Section [5.3.4], packets received as Link Layer
         broadcasts are generally not forwarded.  It may be advantageous
         to avoid passing to the forwarder packets it would later
         discard because of the rules in that section.

         Some Link Layers (either because of the hardware or because of
         special code in the drivers) can deliver to the router copies
         of all Link Layer broadcasts and multicasts it transmits.  Use
         of this feature can simplify the implementation of cases where
         a packet has to both be passed to the forwarder and delivered
         locally, since forwarding the packet will automatically cause
         the router to receive a copy of the packet that it can then
         deliver locally.  One must use care in these circumstances in
         order to prevent treating a received loop-back packet as a
         normal packet that was received (and then being subject to the
         rules of forwarding, etc etc).

         Even in the absence of such a Link Layer, it is of course
         hardly necessary to make a copy of an entire packet in order to
         queue it both for forwarding and for local delivery, though
         care must be taken with fragments, since reassembly is
         performed on locally delivered packets but not on forwarded
         packets.  One simple scheme is to associate a flag with each
         packet on the router's output queue which indicates whether it
         should be queued for local delivery after it has been sent.

5.2.4  Determining the Next Hop Address

      When a router is going to forward a packet, it must determine
      whether it can send it directly to its destination, or whether it
      needs to pass it through another router.  If the latter, it needs
      to determine which router to use.  This section explains how these
      determinations are made.

      This section makes use of the following definitions:

      o  LSRR - IP Loose Source and Record Route option

      o  SSRR - IP Strict Source and Record Route option

      o  Source Route Option - an LSRR or an SSRR

      o  Ultimate Destination Address - where the packet is being sent


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         to: the last address in the source route of a source-routed
         packet, or the destination address in the IP header of a non-
         source-routed packet

      o  Adjacent - reachable without going through any IP routers

      o  Next Hop Address - the IP address of the adjacent host or
         router to which the packet should be sent next

      o  Immediate Destination Address - the ultimate destination
         address, except in source routed packets, where it is the next
         address specified in the source route

      o  Immediate Destination - the node, system, router, end-system,
         or whatever that is addressed by the Immediate Destination
         Address.

5.2.4.1  Immediate Destination Address

         If the destination address in the IP header is one of the
         addresses of the router and the packet contains a Source Route
         Option, the Immediate Destination Address is the address
         pointed at by the pointer in that option if the pointer does
         not point past the end of the option.  Otherwise, the Immediate
         Destination Address is the same as the IP destination address
         in the IP header.

         A router MUST use the Immediate Destination Address, not the
         Ultimate Destination Address, when determining how to handle a
         packet.

         It is an error for more than one source route option to appear
         in a datagram.  If it receives one, it SHOULD discard the
         packet and reply with an ICMP Parameter Problem message whose
         pointer points at the beginning of the second source route
         option.

5.2.4.2  Local/Remote Decision

         After it has been determined that the IP packet needs to be
         forwarded in accordance with the rules specified in Section
         [5.2.3], the following algorithm MUST be used to determine if
         the Immediate Destination is directly accessible (see
         [INTERNET:2]):

         (1)  For each network interface that has not been assigned any
              IP address (the unnumbered lines as described in Section


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              [2.2.7]), compare the router-id of the other end of the
              line to the Immediate Destination Address.  If they are
              exactly equal, the packet can be transmitted through this
              interface.

              DISCUSSION:
                 In other words, the router or host at the remote end of
                 the line is the destination of the packet or is the
                 next step in the source route of a source routed
                 packet.

         (2)  If no network interface has been selected in the first
              step, for each IP address assigned to the router:
              (a)  Apply the subnet mask associated with the address to
                   this IP address.

                   IMPLEMENTATION:
                      The result of this operation will usually have
                      been computed and saved during initialization.

              (b)  Apply the same subnet mask to the Immediate
                   Destination Address of the packet.
              (c)  Compare the resulting values. If they are equal to
                   each other, the packet can be transmitted through the
                   corresponding network interface.

         (3)  If an interface has still not been selected, the Immediate
              Destination is accessible only through some other router.
              The selection of the router and the next hop IP address is
              described in Section [5.2.4.3].

5.2.4.3  Next Hop Address


         EDITOR'S COMMENTS:
            Note that this section has been extensively rewritten.  The
            original document indicated that Phil Almquist wished to
            revise this section to conform to his "Ruminations on the
            Next Hop" document.  I am under the assumption that the
            working group generally agreed with this goal; there was an
            editor's note from Phil that remained in this document to
            that effect, and the RoNH document contains a "mandatory
            RRWG algorithm".

            So, I have taken said algorithm from RoNH and moved it into
            here.


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            Additional useful or interesting information from RoNH has
            been extracted and placed into an appendix to this note.

         The router applies the algorithm in the previous section to
         determine if the Immediate Destination Address is adjacent.  If
         so, the next hop address is the same as the Immediate
         Destination Address.  Otherwise, the packet must be forwarded
         through another router to reach its Immediate Destination.  The
         selection of this router is the topic of this section.

         If the packet contains an SSRR, the router MUST discard the
         packet and reply with an ICMP Bad Source Route error.
         Otherwise, the router looks up the Immediate Destination
         Address in its routing table to determine an appropriate next
         hop address.

         DISCUSSION:
            Per the IP specification, a Strict Source Route must specify
            a sequence of nodes through which the packet must traverse;
            the packet must go from one node of the source route to the
            next, traversing intermediate networks only.  Thus, if the
            router is not adjacent to the next step of the source route,
            the source route can not be fulfilled.  Therefore, the ICMP
            Bad Source Route error.

         The goal of the next-hop selection process is to examine the
         entries in the router's Forwarding Information Base (FIB) and
         select the best route (if there is one) for the packet from
         those available in the FIB.

         Conceptually, any route lookup algorithm starts out with a set
         of candidate routes which consists of the entire contents of
         the FIB.  The algorithm consists of a series of steps which
         discard routes from the set.  These steps are referred to as
         Pruning Rules.  Normally, when the algorithm terminates there
         is exactly one route remaining in the set.  If the set ever
         becomes empty, the packet is discarded because the destination
         is unreachable.  It is also possible for the algorithm to
         terminate when more than one route remains in the set.  In this
         case, the router may arbitrarily discard all but one of them,
         or may perform "load-splitting" by choosing whichever of the
         routes has been least recently used.

         With the exception of rule 3 (Weak TOS), a router MUST use the
         following Pruning Rules when selecting a next hop for a packet.
         If a router does consider TOS when making next-hop decisions,
         the Rule 3 must be applied in the order indicated below.  These


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         rules MUST be (conceptually) applied to the FIB in the order
         that they are presented.  (For some historical perspective,
         additional pruning rules, and other common algorithms in use,
         see Appendix E).

         DISCUSSION:
            Rule 3 is optional in that Section [5.3.2] says that a
            router only SHOULD consider TOS when making forwarding
            decisions.


         (1)  Basic Match
              This rule discards any routes to destinations other than
              the Immediate Destination Address of the packet.  For
              example, if a packet's Immediate Destination Address is
              36.144.2.5, this step would discard a route to net
              128.12.0.0 but would retain any routes to net 36.0.0.0,
              any routes to subnet 36.144.0.0, and any default routes.

              More precisely, we assume that each route has a
              destination attribute, called route.dest, and a
              corresponding mask, called route.mask, to specify which
              bits of route.dest are significant.  The Immediate
              Destination Address of the packet being forwarded is
              ip.dest.  This rule discards all routes from the set of
              candidate routes except those for which (route.dest &
              route.mask) = (ip.dest & route.mask).

         (2)  Longest Match
              Longest Match is a refinement of Basic Match, described
              above.  After Basic Match pruning is performed, the
              remaining routes are examined to determine the maximum
              number of bits set in any of their route.mask attributes.
              The step then discards from the set of candidate routes
              any routes which have fewer than that maximum number of
              bits set in their route.mask attributes.

              For example, if a packet's Immediate Destination Address
              is 36.144.2.5 and there are  {route.dest, route.mask}
              pairs of {36.144.2.0, 255.255.255.0}, {36.144.0.5,
              255.255.0.255}, {36.144.0.0, 255.255.0.0}, and {36.0.0.0,
              255.0.0.0}, then this rule would keep only the first two
              pairs; {36.144.2.0, 255.255.255.0} and {36.144.0.5,
              255.255.0.255}.




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         (3)  Weak TOS
              Each route has a type of service attribute, called
              route.tos, whose possible values are assumed to be
              identical to those used in the TOS field of the IP header.
              Routing protocols which distribute TOS information fill in
              route.tos appropriately in routes they add to the FIB;
              routes from other routing protocols are treated as if they
              have the default TOS (0000).  The TOS field in the IP
              header of the packet being routed is called ip.tos.

              The set of candidate routes is examined to determine if it
              contains any routes for which route.tos = ip.tos.  If so,
              all routes except those for which route.tos = ip.tos are
              discarded.  If not, all routes except those for which
              route.tos = 0000 are discarded from the set of candidate
              routes.

              Additional discussion of routing based on Weak TOS may be
              found in [ROUTE:11].

              DISCUSSION:
                 The effect of this rule is to select only those routes
                 which have a TOS that matches the TOS requested in the
                 packet.  If no such routes exist then routes with the
                 default TOS are considered.  Routes with a non-default
                 TOS that is not the TOS requested in the packet are
                 never used, even if such routes are the only available
                 routes that go to the packet's destination.

         (4)  Best Metric
              Each route has a metric attribute, called route.metric,
              and a routing domain identifier, called route.domain.
              Each member of the set of candidate routes is compared
              with each other member of the set.  If route.domain is
              equal for the two routes and route.metric is strictly
              inferior for one when compared with the other, then the
              one with the inferior metric is discarded from the set.
              The determination of inferior is usually by a simple
              arithmetic comparison, though some protocols may have
              structured metrics requiring more complex comparisons.

         (5)  Vendor Policy
              Vendor Policy is sort of a catch-all to make up for the
              fact that the previously listed rules are often inadequate
              to chose from among the possible routes.  Vendor Policy
              pruning rules are extremely vendor-specific.  See section
              [5.2.4.4].


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         This algorithm has two distinct disadvantages.  Presumably, a
         router implementor might develop techniques to deal with these
         disadvantages and make them a part of the Vendor Policy pruning
         rule.

         (1)  IS-IS and OSPF route classes are not directly handled.

         (2)  Path properties other than type of service (e.g. MTU) are
              ignored.

         It is also worth noting a deficiency in the way that TOS is
         supported: routing protocols which support TOS are implicitly
         preferred when forwarding packets which have non-zero TOS
         values.

         The Basic Match and Longest Match pruning rules generalize the
         treatment of a number of particular types of routes.  These
         routes are selected in the following, decreasing, order of
         preference:

         (1)  Host Route: This is a route to a specific end system.

         (2)  Subnetwork Route: This is a route to a particular subnet
              of a network.

         (3)  Default Subnetwork Route: This is a route to all subnets
              of a particular net for which there are not (explicit)
              subnet routes.

         (4)  Network Route: This is a route to a particular network.

         (5)  Default Network Route (also known as the default route):
              This is a route to all networks for which there are no
              explicit routes to the net or any of its subnets.

         If, after application of the pruning rules, the set of routes
         is empty (i.e., no routes were found), the packet MUST be
         discarded and an appropriate ICMP error generated (ICMP Bad
         Source Route if the Immediate Destination Address came from a
         source route option; otherwise, whichever of ICMP Destination
         Host Unreachable or Destination Network Unreachable is
         appropriate, as described in Section [4.3.3.1]).






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5.2.4.4  Administrative Preference

         One suggested mechanism for the Vendor Policy Pruning Rule is
         to use administrative preference.

         Each route has associated with it a preference value, based on
         various attributes of the route (specific mechanisms for
         assignment of preference values are suggested below).  This
         preference value is an integer in the range [0..255], with zero
         being the most preferred and 254 being the least preferred.
         255 is a special value that means that the route should never
         be used.  The first step in the Vendor Policy pruning rule
         discards all but the most preferable routes (and always
         discards routes whose preference value is 255).

         This policy is not safe in that it can easily be misused to
         create routing loops.  Since no protocol ensures that the
         preferences configured for a router are consistent with the
         preferences configured in its neighbors, network managers must
         exercise care in configuring preferences.

         o  Address Match
            It is useful to be able to assign a single preference value
            to all routes (learned from the same routing domain) to any
            of a specified set of destinations, where the set of
            destinations is all destinations that match a specified
            address/mask pair.

         o  Route Class
            For routing protocols which maintain the distinction, it is
            useful to be able to assign a single preference value to all
            routes (learned from the same routing domain) which have a
            particular route class (intra-area, inter-area, external
            with internal metrics, or external with external metrics).

         o  Interface
            It is useful to be able to assign a single preference value
            to all routes (learned from a particular routing domain)
            that would cause packets to be routed out a particular
            logical interface on the router (logical interfaces
            generally map one-to-one onto the router's network
            interfaces, except that any network interface which has
            multiple IP addresses will have multiple logical interfaces
            associated with it).

         o  Source router
            It is useful to be able to assign a single preference value


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            to all routes (learned from the same routing domain) which
            were learned from any of a set of routers, where the set of
            routers are those whose updates have a source address which
            match a specified address/mask pair.

         o  Originating AS
            For routing protocols which provide the information, it is
            useful to be able to assign a single preference value to all
            routes (learned from a particular routing domain) which
            originated in another particular routing domain.  For BGP
            routes, the originating AS is the first AS listed in the
            route's AS_PATH attribute.  For OSPF external routes, the
            originating AS may be considered to be the low order 16 bits
            of the route's external route tag if the tag's Automatic bit
            is set and the tag's PathLength is not equal to 3.

         o  External route tag
            It is useful to be able to assign a single preference value
            to all OSPF external routes (learned from the same routing
            domain) whose external route tags match any of a list of
            specified values.  Because the external route tag may
            contain a structured value, it may be useful to provide the
            ability to match particular subfields of the tag.

         o  AS path
            It may be useful to be able to assign a single preference
            value to all BGP routes (learned from the same routing
            domain) whose AS path "matches" any of a set of specified
            values.  It is not yet clear exactly what kinds of matches
            are most useful.  A simple option would be to allow matching
            of all routes for which a particular AS number appears (or
            alternatively, does not appear) anywhere in the route's
            AS_PATH attribute.  A more general but somewhat more
            difficult alternative would be to allow matching all routes
            for which the AS path matches a specified regular
            expression.

5.2.4.6  Load Splitting

         At the end of the Next-hop selection process, multiple routes
         may still remain.  A router has several options when this
         occurs.  It may arbitrarily discard some of the routes.  It may
         reduce the number of candidate routes by comparing metrics of
         routes from routing domains which are not considered
         equivalent.  It may retain more than one route and employ a
         load-splitting mechanism to divide traffic among them.  Perhaps
         the only thing that can be said about the relative merits of


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         the options is that load-splitting is useful in some situations
         but not in others, so a wise implementor who implements load-
         splitting will also provide a way for the network manager to
         disable it.

5.2.5  Unused IP Header Bits: RFC-791 Section 3.1

      The IP header contains several reserved bits, in the Type of
      Service field and in the Flags field.  Routers MUST NOT drop
      packets merely because one or more of these reserved bits has a
      non-zero value.

      Routers MUST ignore and MUST pass through unchanged the values of
      these reserved bits.  If a router fragments a packet, it MUST copy
      these bits into each fragment.

      DISCUSSION:
         Future revisions to the IP protocol may make use of these
         unused bits.  These rules are intended to ensure that these
         revisions can be deployed without having to simultaneously
         upgrade all routers in the Internet.


5.2.6  Fragmentation and Reassembly: RFC-791 Section 3.2

      As was discussed in Section [4.2.2.7], a router MUST support IP
      fragmentation.

      A router MUST NOT reassemble any datagram before forwarding it.

      DISCUSSION:
         A few people have suggested that there might be some topologies
         where reassembly of transit datagrams by routers might improve
         performance.  In general, however, the fact that fragments may
         take different paths to the destination precludes safe use of
         such a feature.

         Nothing in this section should be construed to control or limit
         fragmentation or reassembly performed as a link layer function
         by the router.








Almquist & Kastenholz                                          [Page 79]

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5.2.7  Internet Control Message Protocol - ICMP

      General requirements for ICMP were discussed in Section [4.3].
      This section discusses ICMP messages which are sent only by
      routers.

5.2.7.1  Destination Unreachable

         The ICMP Destination Unreachable message is sent by a router in
         response to a packet which it cannot forward because the
         destination (or next hop) is unreachable or a service is
         unavailable

         A router MUST be able to generate ICMP Destination Unreachable
         messages and SHOULD choose a response code that most closely
         matches the reason why the message is being generated.

         The following codes are defined in [INTERNET:8] and [INTRO:2]:

         0 =  Network Unreachable - generated by a router if a
              forwarding path (route) to the destination network is not
              available;

         1 =  Host Unreachable - generated by a router if a forwarding
              path (route) to the destination host on a directly
              connected network is not available;

         2 =  Protocol Unreachable - generated if the transport protocol
              designated in a datagram is not supported in the transport
              layer of the final destination;

         3 =  Port Unreachable -  generated if the designated transport
              protocol (e.g. UDP) is unable to demultiplex the datagram
              in the transport layer of the final destination but has no
              protocol mechanism to inform the sender;

         4 =  Fragmentation Needed and DF Set - generated if a router
              needs to fragment a datagram but cannot since the DF flag
              is set;

         5 =  Source Route Failed - generated if a router cannot forward
              a packet to the next hop in a source route option;

         6 =  Destination Network Unknown - This code SHOULD NOT be
              generated since it would imply on the part of the router
              that the destination network does not exist (net
              unreachable code 0 SHOULD be used in place of code 6);


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         7 =  Destination Host Unknown - generated only when a router
              can determine (from link layer advice) that the
              destination host does not exist;

         11 = Network Unreachable For Type Of Service - generated by a
              router if a forwarding path (route) to the destination
              network with the requested or default TOS is not
              available;

         12 = Host Unreachable For Type Of Service - generated if a
              router cannot forward a packet because its route(s) to the
              destination do not match either the TOS requested in the
              datagram or the default TOS (0).

         The following additional codes are hereby defined:

         13 = Communication Administratively Prohibited - generated if a
              router cannot forward a packet due to administrative
              filtering;

         14 = Host Precedence Violation.  Sent by the first hop router
              to a host to indicate that a requested precedence is not
              permitted for the particular combination of
              source/destination host or network, upper layer protocol,
              and source/destination port;

         15 = Precedence cutoff in effect.  The network operators have
              imposed a minimum level of precedence required for
              operation, the datagram was sent with a precedence below
              this level;

         NOTE: [INTRO:2] defined Code 8 for source host isolated.
         Routers SHOULD NOT generate Code 8; whichever of Codes 0
         (Network Unreachable) and 1 (Host Unreachable) is appropriate
         SHOULD be used instead.  [INTRO:2] also defined Code 9 for
         communication with destination network administratively
         prohibited and Code 10 for communication with destination host
         administratively prohibited.  These codes were intended for use
         by end-to-end encryption devices used by U.S military agencies.
         Routers SHOULD use the newly defined Code 13 (Communication
         Administratively Prohibited) if they administratively filter
         packets.

         Routers MAY have a configuration option that causes Code 13
         (Communication Administratively Prohibited) messages not to be
         generated.  When this option is enabled, no ICMP error message
         is sent in response to a packet which is dropped because its


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         forwarding is administratively prohibited.

         Similarly, routers MAY have a configuration option that causes
         Code 14 (Host Precedence Violation) and Code 15 (Precedence
         Cutoff in Effect) messages not to be generated.  When this
         option is enabled, no ICMP error message is sent in response to
         a packet which is dropped  because of a precedence violation.

         Routers MUST use Host Unreachable or Destination Host Unknown
         codes whenever other hosts on the same destination network
         might be reachable; otherwise, the source host may erroneously
         conclude that all hosts on the network are unreachable, and
         that may not be the case.

         [INTERNET:14] describes a slight modification the form of
         Destination Unreachable messages containing Code 4
         (Fragmentation needed and DF set).  A router MUST use this
         modified form when originating Code 4 Destination Unreachable
         messages.

5.2.7.2  Redirect

         The ICMP Redirect message is generated to inform a host on the
         same subnet that the router used by the host to route certain
         packets should be changed.

         Routers MUST NOT generate the Redirect for Network or Redirect
         for Network and Type of Service messages (Codes 0 and 2)
         specified in [INTERNET:8].  Routers MUST be able to generate
         the Redirect for Host message (Code 1) and SHOULD be able to
         generate the Redirect for Type of Service and Host message
         (Code 3) specified in [INTERNET:8].

         DISCUSSION:
            If the directly-connected network is not subnetted, a router
            can normally generate a network Redirect which applies to
            all hosts on a specified remote network.  Using a network
            rather than a host Redirect may economize slightly on
            network traffic and on host routing table storage.  However,
            the savings are not significant, and subnets create an
            ambiguity about the subnet mask to be used to interpret a
            network Redirect.  In a general subnet environment, it is
            difficult to specify precisely the cases in which network
            Redirects can be used.  Therefore, routers must send only
            host (or host and type of service) Redirects.

         A Code 3 (Redirect for Host and Type of Service) message is


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         generated when the packet provoking the redirect has a
         destination for which the path chosen by the router would
         depend (in part) on the TOS requested.

         Routers which can generate Code 3 redirects (Host and Type of
         Service) MUST have a configuration option (which defaults to
         on) to enable Code 1 (Host) redirects to be substituted for
         Code 3 redirects.  A router MUST send a Code 1 Redirect in
         place of a Code 3 Redirect if it has been configured to do so.

         If a router is not able to generate Code 3 Redirects then it
         MUST generate Code 1 Redirects in situations where a Code 3
         Redirect is called for.

         Routers MUST NOT generate a Redirect Message unless all of the
         following conditions are met:

         o  The packet is being forwarded out the same physical
            interface that it was received from,

         o  The IP source address in the packet is on the same Logical
            IP (sub)network as the next-hop IP address, and

         o  The packet does not contain an IP source route option.

         The source address used in the ICMP Redirect MUST belong to the
         same logical (sub)net as the destination address.

         A router using a routing protocol (other than static routes)
         MUST NOT consider paths learned from ICMP Redirects when
         forwarding a packet.  If a router is not using a routing
         protocol, a router MAY have a configuration which, if set,
         allows the router to consider routes learned via ICMP Redirects
         when forwarding packets.

         DISCUSSION:
            ICMP Redirect is a mechanism for routers to convey routing
            information to hosts.  Routers use other mechanisms to learn
            routing information, and therefore have no reason to obey
            redirects.  Believing a redirect which contradicted the
            router's other information would likely create routing
            loops.

            On the other hand, when a router is not acting as a router,
            it MUST comply with the behavior required of a host.



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5.2.7.3  Time Exceeded

         A router MUST generate a Time Exceeded message Code 0 (In
         Transit) when it discards a packet due to an expired TTL field.
         A router MAY have a per-interface option to disable origination
         of these messages on that interface, but that option MUST
         default to allowing the messages to be originated.

5.2.8  INTERNET GROUP MANAGEMENT PROTOCOL - IGMP

      IGMP [INTERNET:4] is a protocol used between hosts and multicast
      routers on a single physical network to establish hosts'
      membership in particular multicast groups.  Multicast routers use
      this information, in conjunction with a multicast routing
      protocol, to support IP multicast forwarding across the Internet.

      A router SHOULD implement the multicast router part of IGMP.

5.3  SPECIFIC ISSUES


5.3.1  Time to Live (TTL)

      The Time-to-Live (TTL) field of the IP header is defined to be a
      timer limiting the lifetime of a datagram.  It is an 8-bit field
      and the units are seconds.  Each router (or other module) that
      handles a packet MUST decrement the TTL by at least one, even if
      the elapsed time was much less than a second.  Since this is very
      often the case, the TTL is effectively a hop count limit on how
      far a datagram can propagate through the Internet.

      When a router forwards a packet, it MUST reduce the TTL by at
      least one.  If it holds a packet for more than one second, it MAY
      decrement the TTL by one for each second.

      If the TTL is reduced to zero (or less), the packet MUST be
      discarded, and if the destination is not a multicast address the
      router MUST send an ICMP Time Exceeded message, Code 0 (TTL
      Exceeded in Transit) message to the source.  Note that a router
      MUST NOT discard an IP unicast or broadcast packet with a non-zero
      TTL merely because it can predict that another router on the path
      to the packet's final destination will decrement the TTL to zero.
      However, a router MAY do so for IP multicasts, in order to more
      efficiently implement IP multicast's expanding ring search
      algorithm (see [INTERNET:4]).



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      DISCUSSION:
         The IP TTL is used, somewhat schizophrenically, as both a hop
         count limit and a time limit.  Its hop count function is
         critical to ensuring that routing problems can't melt down the
         network by causing packets to loop infinitely in the network.
         The time limit function is used by transport protocols such as
         TCP to ensure reliable data transfer.  Many current
         implementations treat TTL as a pure hop count, and in parts of
         the Internet community there is a strong sentiment that the
         time limit function should instead be performed by the
         transport protocols that need it.

         In this specification, we have reluctantly decided to follow
         the strong belief among the router vendors that the time limit
         function should be optional.  They argued that implementation
         of the time limit function is difficult enough that it is
         currently not generally done.  They further pointed to the lack
         of documented cases where this shortcut has caused TCP to
         corrupt data (of course, we would expect the problems created
         to be rare and difficult to reproduce, so the lack of
         documented cases provides little reassurance that there haven't
         been a number of undocumented cases).

         IP multicast notions such as the expanding ring search may not
         work as expected unless the TTL is treated as a pure hop count.
         The same thing is somewhat true of traceroute.

         ICMP Time Exceeded messages are required because the traceroute
         diagnostic tool depends on them.

         Thus, the tradeoff is between severely crippling, if not
         eliminating, two very useful tools vs. a very rare and
         transient data transport problem (which may not occur at all).


5.3.2  Type of Service (TOS)

      The Type-of-Service byte in the IP header is divided into three
      sections:  the Precedence field (high-order 3 bits), a field that
      is customarily called Type of Service or "TOS (next 4 bits), and a
      reserved bit (the low order bit).  Rules governing the reserved
      bit were described in Section [4.2.2.3].  The Precedence field
      will be discussed in Section [5.3.3].  A more extensive discussion
      of the TOS field and its use can be found in [ROUTE:11].

      A router SHOULD consider the TOS field in a packet's IP header
      when deciding how to forward it.  The remainder of this section


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      describes the rules that apply to routers that conform to this
      requirement.

      A router MUST maintain a TOS value for each route in its routing
      table.  Routes learned via a routing protocol which does not
      support TOS MUST be assigned a TOS of zero (the default TOS).

      To choose a route to a destination, a router MUST use an algorithm
      equivalent to the following:

      (1)  The router locates in its routing table all available routes
           to the destination (see Section [5.2.4]).

      (2)  If there are none, the router drops the packet because the
           destination is unreachable.  See section [5.2.4].

      (3)  If one or more of those routes have a TOS that exactly
           matches the TOS specified in the packet, the router chooses
           the route with the best metric.

      (4)  Otherwise, the router repeats the above step, except looking
           at routes whose TOS is zero.

      (5)  If no route was chosen above, the router drops the packet
           because the destination is unreachable.  The router returns
           an ICMP Destination Unreachable error specifying the
           appropriate code: either Network Unreachable with Type of
           Service (code 11) or Host Unreachable with Type of Service
           (code 12).

      DISCUSSION:
         Although TOS has been little used in the past, its use by hosts
         is now mandated by the Requirements for Internet Hosts RFCs
         ([INTRO:2] and [INTRO:3]).  Support for TOS in routers may
         become a MUST in the future, but is a SHOULD for now until we
         get more experience with it and can better judge both its
         benefits and its costs.

         Various people have proposed that TOS should affect other
         aspects of the forwarding function.  For example:

         (1)  A router could place packets which have the Low Delay bit
              set ahead of other packets in its output queues.

         (2)  a router is forced to discard packets, it could try to
              avoid discarding those which have the High Reliability bit
              set.


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         These ideas have been explored in more detail in [INTERNET:17]
         but we don't yet have enough experience with such schemes to
         make requirements in this area.


5.3.3  IP Precedence

      This section specifies requirements and guidelines for appropriate
      processing of the IP Precedence field in routers.  Precedence is a
      scheme for allocating resources in the network based on the
      relative importance of different traffic flows.  The IP
      specification defines specific values to be used in this field for
      various types of traffic.

      The basic mechanisms for precedence processing in a router are
      preferential resource allocation, including both precedence-
      ordered queue service and precedence-based congestion control, and
      selection of Link Layer priority features.  The router also
      selects the IP precedence for routing, management and control
      traffic it originates.  For a more extensive discussion of IP
      Precedence and its implementation see [FORWARD:6].

      Precedence-ordered queue service, as discussed in this section,
      includes but is not limited to the queue for the forwarding
      process and queues for outgoing links.  It is intended that a
      router supporting precedence should also use the precedence
      indication at whatever points in its processing are concerned with
      allocation of finite resources, such as packet buffers or Link
      Layer connections.  The set of such points is implementation-
      dependent.

      DISCUSSION:
         Although the Precedence field was originally provided for use
         in DOD systems where large traffic surges or major damage to
         the network are viewed as inherent threats, it has useful
         applications for many non-military IP networks.  Although the
         traffic handling capacity of networks has grown greatly in
         recent years, the traffic generating ability of the users has
         also grown, and network overload conditions still occur at
         times.  Since IP-based routing and management protocols have
         become more critical to the successful operation of the
         Internet, overloads present two additional risks to the
         network:

         (1)  High delays may result in routing protocol packets being
              lost.  This may cause the routing protocol to falsely
              deduce a topology change and propagate this false


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              information to other routers.  Not only can this cause
              routes to oscillate, but an extra processing burden may be
              placed on other routers.

         (2)  High delays may interfere with the use of network
              management tools to analyze and perhaps correct or relieve
              the problem in the network that caused the overload
              condition to occur.

         Implementation and appropriate use of the Precedence mechanism
         alleviates both of these problems.


5.3.3.1  Precedence-Ordered Queue Service

         Routers SHOULD implement precedence-ordered queue service.
         Precedence-ordered queue service means that when a packet is
         selected for output on a (logical) link, the packet of highest
         precedence that has been queued for that link is sent.  Routers
         that implement precedence-ordered queue service MUST also have
         a configuration option to suppress precedence-ordered queue
         service in the Internet Layer.

         Any router MAY implement other policy-based throughput
         management procedures that result in other than strict
         precedence ordering, but it MUST be configurable to suppress
         them (i.e., use strict ordering).

         As detailed in Section [5.3.6], routers that implement
         precedence-ordered queue service discard low precedence packets
         before discarding high precedence packets for congestion
         control purposes.

         Preemption (interruption of processing or transmission of a
         packet) is not envisioned as a function of the Internet Layer.
         Some protocols at other layers may provide preemption features.

5.3.3.2  Lower Layer Precedence Mappings

         Routers that implement precedence-ordered queueing MUST
         IMPLEMENT, and other routers SHOULD IMPLEMENT, Lower Layer
         Precedence Mapping.

         A router which implements Lower Layer Precedence Mapping:

         o  MUST be able to map IP Precedence to Link Layer priority
            mechanisms for link layers that have such a feature defined.


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         o  MUST have a configuration option to select the Link Layer's
            default priority treatment for all IP traffic

         o  SHOULD be able to configure specific nonstandard mappings of
            IP precedence values to Link Layer priority values for each
            interface.

         DISCUSSION:
            Some research questions the workability of the priority
            features of some Link Layer protocols, and some networks may
            have faulty implementations of the link layer priority
            mechanism.  It seems prudent to provide an escape mechanism
            in case such problems show up in a network.

            On the other hand, there are proposals to use novel queueing
            strategies to implement special services such as low-delay
            service.  Special services and queueing strategies to
            support them need further research and experimentation
            before they are put into widespread use in the Internet.
            Since these requirements are intended to encourage (but not
            force) the use of precedence features in the hope of
            providing better Internet service to all users, routers
            supporting precedence-ordered queue service should default
            to maintaining strict precedence ordering regardless of the
            type of service requested.

            Implementors may wish to consider that correct link layer
            mapping of IP precedence is required by DOD policy for
            TCP/IP systems used on DOD networks.


5.3.3.3  Precedence Handling For All Routers

         A router (whether or not it employs precedence-ordered queue
         service):

         (1)  MUST accept and process incoming traffic of all precedence
              levels normally, unless it has been administratively
              configured to do otherwise.

         (2)  MAY implement a validation filter to administratively
              restrict the use of precedence levels by particular
              traffic sources.  If provided, this filter MUST NOT filter
              out or cut off the following sorts of ICMP error messages:
              Destination Unreachable, Redirect, Time Exceeded, and
              Parameter Problem.  If this filter is provided, the
              procedures required for packet filtering by addresses are


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              required for this filter also.

              DISCUSSION:
                 Precedence filtering should be applicable to specific
                 source/destination IP Address pairs, specific
                 protocols, specific ports, and so on.

              An ICMP Destination Unreachable message with code 14
              SHOULD be sent when a packet is dropped by the validation
              filter, unless this has been suppressed by configuration
              choice.

         (3)  MAY implement a cutoff function which allows the router to
              be set to refuse or drop traffic with precedence below a
              specified level.  This function may be activated by
              management actions or by some implementation dependent
              heuristics, but there MUST be a configuration option to
              disable any heuristic mechanism that operates without
              human intervention.  An ICMP Destination Unreachable
              message with code 15 SHOULD be sent when a packet is
              dropped by the cutoff function, unless this has been
              suppressed by configuration choice.

              A router MUST NOT refuse to forward datagrams with IP
              precedence of 6 (Internetwork Control) or 7 (Network
              Control) solely due to precedence cutoff.  However, other
              criteria may be used in conjunction with precedence cutoff
              to filter high precedence traffic.

              DISCUSSION:
                 Unrestricted precedence cutoff could result in an
                 unintentional cutoff of routing and control traffic.
                 In general, host traffic should be restricted to a
                 value of 5 (CRITIC/ECP) or below although this is not a
                 requirement and may not be valid in certain systems.


         (4)  MUST NOT change precedence settings on packets it did not
              originate.

         (5)  SHOULD be able to configure distinct precedence values to
              be used for each routing or management protocol supported
              (except for those protocols, such as OSPF, which specify
              which precedence value must be used).

         (6)  MAY be able to configure routing or management traffic
              precedence values independently for each peer address.


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         (7)  MUST respond appropriately to Link Layer precedence-
              related error indications where provided.  An ICMP
              Destination Unreachable message with code 15 SHOULD be
              sent when a packet is dropped because a link cannot accept
              it due to a precedence-related condition, unless this has
              been suppressed by configuration choice.

              DISCUSSION:
                 The precedence cutoff mechanism described in (3) is
                 somewhat controversial.  Depending on the topological
                 location of the area affected by the cutoff, transit
                 traffic may be directed by routing protocols into the
                 area of the cutoff, where it will be dropped.  This is
                 only a problem if another path which is unaffected by
                 the cutoff exists between the communicating points.
                 Proposed ways of avoiding this problem include
                 providing some minimum bandwidth to all precedence
                 levels even under overload conditions, or propagating
                 cutoff information in routing protocols.  In the
                 absence of a widely accepted (and implemented) solution
                 to this problem, great caution is recommended in
                 activating cutoff mechanisms in transit networks.

                 A transport layer relay could legitimately provide the
                 function prohibited by (4) above.  Changing precedence
                 levels may cause subtle interactions with TCP and
                 perhaps other protocols; a correct design is a non-
                 trivial task.

                 The intent of (5) and (6) (and the discussion of IP
                 Precedence in ICMP messages in Section [4.3.2]) is that
                 the IP precedence bits should be appropriately set,
                 whether or not this router acts upon those bits in any
                 other way.  We expect that in the future specifications
                 for routing protocols and network management protocols
                 will specify how the IP Precedence should be set for
                 messages sent by those protocols.

                 The appropriate response for (7) depends on the link
                 layer protocol in use.  Typically, the router should
                 stop trying to send offensive traffic to that
                 destination for some period of time, and should return
                 an ICMP Destination Unreachable message with code 15
                 (service not available for precedence requested) to the
                 traffic source.  It also should not try to reestablish
                 a preempted Link Layer connection for some period of
                 time.


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RFC 1716          Towards Requirements for IP Routers      November 1994


5.3.4  Forwarding of Link Layer Broadcasts

      The encapsulation of IP packets in most Link Layer protocols
      (except PPP) allows a receiver to distinguish broadcasts and
      multicasts from unicasts simply by examining the Link Layer
      protocol headers (most commonly, the Link Layer destination
      address).  The rules in this section which refer to Link Layer
      broadcasts apply only to Link Layer protocols which allow
      broadcasts to be distinguished; likewise, the rules which refer to
      Link Layer multicasts apply only to Link Layer protocols which
      allow multicasts to be distinguished.

      A router MUST NOT forward any packet which the router received as
      a Link Layer broadcast (even if the IP destination address is also
      some form of broadcast address) unless the packet is an all-
      subnets-directed broadcast being forwarded as specified in
      [INTERNET:3].

      DISCUSSION:
         As noted in Section [5.3.5.3], forwarding of all-subnets-
         directed broadcasts in accordance with [INTERNET:3] is optional
         and is not something that routers do by default.

      A router MUST NOT forward any packet which the router received as
      a Link Layer multicast unless the packet's destination address is
      an IP multicast address.

      A router SHOULD silently discard a packet that is received via a
      Link Layer broadcast but does not specify an IP multicast or IP
      broadcast destination address.

      When a router sends a packet as a Link Layer broadcast, the IP
      destination address MUST be a legal IP broadcast or IP multicast
      address.

5.3.5  Forwarding of Internet Layer Broadcasts

      There are two major types of IP broadcast addresses; limited
      broadcast and directed broadcast.  In addition, there are three
      subtypes of directed broadcast; a broadcast directed to a
      specified network, a broadcast directed to a specified subnetwork,
      and a broadcast directed to all subnets of a specified network.
      Classification by a router of a broadcast into one of these
      categories depends on the broadcast address and on the router's
      understanding (if any) of the subnet structure of the destination
      network.  The same broadcast will be classified differently by
      different routers.


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      A limited IP broadcast address is defined to be all-ones: { -1, -1
      } or 255.255.255.255.

      A net-directed broadcast is composed of the network portion of the
      IP address with a local part of all-ones, { <Network-number>, -1
      }.  For example, a Class A net broadcast address is
      net.255.255.255, a Class B net broadcast address is
      net.net.255.255 and a Class C net broadcast address is
      net.net.net.255 where net is a byte of the network address.

      An all-subnets-directed broadcast is composed of the network part
      of the IP address with a subnet and a host part of all-ones, {
      <Network-number>, -1, -1 }.  For example, an all-subnets broadcast
      on a subnetted class B network is net.net.255.255.  A network must
      be known to be subnetted and the subnet part must be all-ones
      before a broadcast can be classified as all-subnets-directed.

      A subnet-directed broadcast address is composed of the network and
      subnet part of the IP address with a host part of all-ones, {
      <Network-number>, <Subnet-number>, -1 }.  For example, a subnet-
      directed broadcast to subnet 2 of a class B network might be
      net.net.2.255 (if the subnet mask was 255.255.255.0) or
      net.net.1.127 (if the subnet mask was 255.255.255.128).  A network
      must be known to be subnetted and the net and subnet part must not
      be all-ones before an IP broadcast can be classified as subnet-
      directed.

      As was described in Section [4.2.3.1], a router may encounter
      certain non-standard IP broadcast addresses:

      o  0.0.0.0 is an obsolete form of the limited broadcast address

      o  { broadcast address.

      o  { broadcast address.

      o  { form of a subnet-directed broadcast address.

      As was described in that section, packets addressed to any of
      these addresses SHOULD be silently discarded, but if they are not,
      they MUST be treated in accordance with the same rules that apply
      to packets addressed to the non-obsolete forms of the broadcast
      addresses described above.  These rules are described in the next
      few sections.




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5.3.5.1  Limited Broadcasts

         Limited broadcasts MUST NOT be forwarded.  Limited broadcasts
         MUST NOT be discarded.  Limited broadcasts MAY be sent and
         SHOULD be sent instead of directed broadcasts where limited
         broadcasts will suffice.

         DISCUSSION:
            Some routers contain UDP servers which function by resending
            the requests (as unicasts or directed broadcasts) to other
            servers.  This requirement should not be interpreted as
            prohibiting such servers.  Note, however, that such servers
            can easily cause packet looping if misconfigured.  Thus,
            providers of such servers would probably be well-advised to
            document their setup carefully and to consider carefully the
            TTL on packets which are sent.


5.3.5.2  Net-directed Broadcasts

         A router MUST classify as net-directed broadcasts all valid,
         directed broadcasts destined for a remote network or an
         attached nonsubnetted network.  A router MUST forward net-
         directed broadcasts.  Net-directed broadcasts MAY be sent.

         A router MAY have an option to disable receiving net-directed
         broadcasts on an interface and MUST have an option to disable
         forwarding net-directed broadcasts.  These options MUST default
         to permit receiving and forwarding net-directed broadcasts.

         DISCUSSION:
            There has been some debate about forwarding or not
            forwarding directed broadcasts.  In this memo we have made
            the forwarding decision depend on the router's knowledge of
            the subnet mask for the destination network.  Forwarding
            decisions for subnetted networks should be made by routers
            with an understanding of the subnet structure.  Therefore,
            in general, routers must forward directed broadcasts for
            networks they are not attached to and for which they do not
            understand the subnet structure.  One router may interpret
            and handle the same IP broadcast packet differently than
            another, depending on its own understanding of the structure
            of the destination (sub)network.





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5.3.5.3  All-subnets-directed Broadcasts

         A router MUST classify as all-subnets-directed broadcasts all
         valid directed broadcasts destined for a directly attached
         subnetted network which have all-ones in the subnet part of the
         address.  If the destination network is not subnetted, the
         broadcast MUST be treated as a net-directed broadcast.

         A router MUST forward an all-subnets-directed broadcast as a
         link level broadcast out all physical interfaces connected to
         the IP network addressed by the broadcast, except that:

         o  A router MUST NOT forward an all-subnet-directed broadcast
            that was received by the router as a Link Layer broadcast,
            unless the router is forwarding the broadcast in accordance
            with [INTERNET:3] (see below).

         o  If a router receives an all-subnets-directed broadcast over
            a network which does not indicate via Link Layer framing
            whether the frame is a broadcast or a unicast, the packet
            MUST NOT be forwarded to any network which likewise does not
            indicate whether a frame is a broadcast.

         o  A router MUST NOT forward an all-subnets-directed broadcast
            if the router is configured not to forward such broadcasts.
            A router MUST have a configuration option to deny forwarding
            of all-subnets-directed broadcasts.  The configuration
            option MUST default to permit forwarding of all-subnets-
            directed broadcasts.

         EDITOR'S COMMENTS:
            The algorithm presented here is broken.  The working group
            explicitly desired this algorithm, knowing its failures.

            The second bullet, above, prevents All Subnets Directed
            Broadcasts from traversing more than one PPP (or other
            serial) link in a row.  Such a topology is easily conceived.
            Suppose that some corporation builds its corporate backbone
            out of PPP links, connecting routers at geographically
            dispersed locations.  Suppose that this corporation has 3
            sites (S1, S2, and S3) and there is a router at each site
            (R1, R2, and R3).  At each site there are also several LANs
            connected to the local router.  Let there be a PPP link
            connecting S1 to S2 and one connecting S2 to S3 (i.e. the
            links are R1-R2 and R2-R3).  So, if a host on a LAN at S1
            sends a All Subnets Directed Broadcast, R1 will forward the
            broadcast over the R1-R2 link to R2.  R2 will forward the


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            broadcast to the LAN(s) connected to R2.  Since the PPP does
            not differentiate broadcast from non-broadcast frames, R2
            will NOT forward the broadcast onto the R2-R3 link.
            Therefore, the broadcast will not reach S3.

         [INTERNET:3] describes an alternative set of rules for
         forwarding of all-subnets-directed broadcasts (called multi-
         subnet-broadcasts in that document).  A router MAY IMPLEMENT
         that alternative set of rules, but MUST use the set of rules
         described above unless explicitly configured to use the
         [INTERNET:3] rules.  If routers will do [INTERNET:3]-style
         forwarding, then the router MUST have a configuration option
         which MUST default to doing the rules presented in this
         document.

         DISCUSSION:
            As far as we know, the rules for multi-subnet broadcasts
            described in [INTERNET:3] have never been implemented,
            suggesting that either they are too complex or the utility
            of multi-subnet broadcasts is low.  The rules described in
            this section match current practice.  In the future, we
            expect that IP multicast (see [INTERNET:4]) will be used to
            better solve the sorts of problems that multi-subnets
            broadcasts were intended to address.

            We were also concerned that hosts whose system managers
            neglected to configure with a subnet mask could
            unintentionally send multi-subnet broadcasts.

         A router SHOULD NOT originate all-subnets broadcasts, except as
         required by Section [4.3.3.9] when sending ICMP Address Mask
         Replies on subnetted networks.

         DISCUSSION:
            The current intention is to decree that (like 0-filled IP
            broadcasts) the notion of the all-subnets broadcast is
            obsolete.  It should be treated as a directed broadcast to
            the first subnet of the net in question that it appears on.

            Routers may implement a switch (default off) which if turned
            on enables the [INTERNET:3] behavior for all-subnets
            broadcasts.

            If a router has a configuration option to allow for
            forwarding all-subnet broadcasts, it should use a spanning
            tree, RPF, or other multicast forwarding algorithm (which
            may be computed for other purposes such as bridging or OSPF)


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            to distribute the all-subnets broadcast efficiently.  In
            general, it is better to use an IP multicast address rather
            than an all-subnets broadcast.


5.3.5.4  Subnet-directed Broadcasts

         A router MUST classify as subnet-directed broadcasts all valid
         directed broadcasts destined for a directly attached subnetted
         network in which the subnet part is not all-ones.  If the
         destination network is not subnetted, the broadcast MUST be
         treated as a net-directed broadcast.

         A router MUST forward subnet-directed broadcasts.

         A router MUST have a configuration option to prohibit
         forwarding of subnet-directed broadcasts.  Its default setting
         MUST permit forwarding of subnet-directed broadcasts.

         A router MAY have a configuration option to prohibit forwarding
         of subnet-directed broadcasts from a source on a network on
         which the router has an interface.  If such an option is
         provided, its default setting MUST permit forwarding of
         subnet-directed broadcasts.

5.3.6  Congestion Control

      Congestion in a network is loosely defined as a condition where
      demand for resources (usually bandwidth or CPU time) exceeds
      capacity.  Congestion avoidance tries to prevent demand from
      exceeding capacity, while congestion recovery tries to restore an
      operative state.  It is possible for a router to contribute to
      both of these mechanisms.  A great deal of effort has been spent
      studying the problem.  The reader is encouraged to read
      [FORWARD:2] for a survey of the work.  Important papers on the
      subject include [FORWARD:3], [FORWARD:4], [FORWARD:5], and
      [INTERNET:10], among others.

      The amount of storage that router should have available to handle
      peak instantaneous demand when hosts use reasonable congestion
      policies, such as described in [FORWARD:5], is a function of the
      product of the bandwidth of the link times the path delay of the
      flows using the link, and therefore storage should increase as
      this Bandwidth*Delay product increases.  The exact function
      relating storage capacity to probability of discard is not known.

      When a router receives a packet beyond its storage capacity it


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      must (by definition, not by decree) discard it or some other
      packet or packets.  Which packet to discard is the subject of much
      study but, unfortunately, little agreement so far.

      A router MAY discard the packet it has just received; this is the
      simplest but not the best policy.  It is considered better policy
      to randomly pick some transit packet on the queue and discard it
      (see [FORWARD:2]).  A router MAY use this Random Drop algorithm to
      determine which packet to discard.

      If a router implements a discard policy (such as Random Drop)
      under which it chooses a packet to discard from among a pool of
      eligible packets:

      o  If precedence-ordered queue service (described in Section
         [5.3.3.1]) is implemented and enabled, the router MUST NOT
         discard a packet whose IP precedence is higher than that of a
         packet which is not discarded.

      o  A router MAY protect packets whose IP headers request the
         maximize reliability TOS, except where doing so would be in
         violation of the previous rule.

      o  A router MAY protect fragmented IP packets, on the theory that
         dropping a fragment of a datagram may increase congestion by
         causing all fragments of the datagram to be retransmitted by
         the source.

      o  To help prevent routing perturbations or disruption of
         management functions, the router MAY protect packets used for
         routing control, link control, or network management from being
         discarded.  Dedicated routers (i.e.. routers which are not also
         general purpose hosts, terminal servers, etc.) can achieve an
         approximation of this rule by protecting packets whose source
         or destination is the router itself.

      Advanced methods of congestion control include a notion of
      fairness, so that the 'user' that is penalized by losing a packet
      is the one that contributed the most to the congestion.  No matter
      what mechanism is implemented to deal with bandwidth congestion
      control, it is important that the CPU effort expended be
      sufficiently small that the router is not driven into CPU
      congestion also.

      As described in Section [4.3.3.3], this document recommends that a
      router should not send a Source Quench to the sender of the packet
      that it is discarding.  ICMP Source Quench is a very weak


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      mechanism, so it is not necessary for a router to send it, and
      host software should not use it exclusively as an indicator of
      congestion.

5.3.7  Martian Address Filtering

      An IP source address is invalid if it is an IP broadcast address
      or is not a class A, B, or C address.

      An IP destination address is invalid if it is not a class A, B, C,
      or D address.

      A router SHOULD NOT forward any packet which has an invalid IP
      source address or a source address on network 0.  A router SHOULD
      NOT forward, except over a loopback interface, any packet which
      has a source address on network 127.  A router MAY have a switch
      which allows the network manager to disable these checks.  If such
      a switch is provided, it MUST default to performing the checks.

      A router SHOULD NOT forward any packet which has an invalid IP
      destination address or a destination address on network 0.  A
      router SHOULD NOT forward, except over a loopback interface, any
      packet which has a destination address on network 127.  A router
      MAY have a switch which allows the network manager to disable
      these checks.  If such a switch is provided, it MUST default to
      performing the checks.

      If a router discards a packet because of these rules, it SHOULD
      log at least the IP source address, the IP destination address,
      and, if the problem was with the source address, the physical
      interface on which the packet was received and the Link Layer
      address of the host or router from which the packet was received.

5.3.8  Source Address Validation

      A router SHOULD IMPLEMENT the ability to filter traffic based on a
      comparison of the source address of a packet and the forwarding
      table for a logical interface on which the packet was received.
      If this filtering is enabled, the router MUST silently discard a
      packet if the interface on which the packet was received is not
      the interface on which a packet would be forwarded to reach the
      address contained in the source address.  In simpler terms, if a
      router wouldn't route a packet containing this address through a
      particular interface, it shouldn't believe the address if it
      appears as a source address in a packet read from this interface.

      If this feature is implemented, it MUST be disabled by default.


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      DISCUSSION:
         This feature can provide useful security improvements in some
         situations, but can erroneously discard valid packets in
         situations where paths are asymmetric.


5.3.9  Packet Filtering and Access Lists

      As a means of providing security and/or limiting traffic through
      portions of a network a router SHOULD provide the ability to
      selectively forward (or filter) packets.  If this capability is
      provided, filtering of packets MUST be configurable either to
      forward all packets or to selectively forward them based upon the
      source and destination addresses.  Each source and destination
      address SHOULD allow specification of an arbitrary mask.

      If supported, a router MUST be configurable to allow one of an

      o  Include list -  specification of a list of address pairs to be
         forwarded, or an

      o  Exclude list -  specification of a list of address pairs NOT to
         be forwarded.

      A router MAY provide a configuration switch which allows a choice
      between specifying an include or an exclude list.

      A value matching any address (e.g. a keyword any or an address
      with a mask of all 0's) MUST be allowed as a source and/or
      destination address.

      In addition to address pairs, the router MAY allow any combination
      of transport and/or application protocol and source and
      destination ports to be specified.

      The router MUST allow packets to be silently discarded (i.e..
      discarded without an ICMP error message being sent).

      The router SHOULD allow an appropriate ICMP unreachable message to
      be sent when a packet is discarded. The ICMP message SHOULD
      specify Communication Administratively Prohibited (code 13) as the
      reason for the destination being unreachable.

      The router SHOULD allow the sending of ICMP destination
      unreachable messages (code 13) to be configured for each
      combination of address pairs, protocol types, and ports it allows
      to be specified.


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      The router SHOULD count and SHOULD allow selective logging of
      packets not forwarded.

5.3.10  Multicast Routing

      An IP router SHOULD support forwarding of IP multicast packets,
      based either on static multicast routes or on routes dynamically
      determined by a multicast routing protocol (e.g., DVMRP
      [ROUTE:9]).  A router that forwards IP multicast packets is called
      a multicast router.

5.3.11  Controls on Forwarding

      For each physical interface, a router SHOULD have a configuration
      option which specifies whether forwarding is enabled on that
      interface.  When forwarding on an interface is disabled, the
      router:

      o  MUST silently discard any packets which are received on that
         interface but are not addressed to the router

      o  MUST NOT send packets out that interface, except for datagrams
         originated by the router

      o  MUST NOT announce via any routing protocols the availability of
         paths through the interface

      DISCUSSION:
         This feature allows the network manager to essentially turn off
         an interface but leaves it accessible for network management.

         Ideally, this control would apply to logical rather than
         physical interfaces, but cannot because there is no known way
         for a router to determine which logical interface a packet
         arrived on when there is not a one-to-one correspondence
         between logical and physical interfaces.


5.3.12  State Changes

      During the course of router operation, interfaces may fail or be
      manually disabled, or may become available for use by the router.
      Similarly, forwarding may be disabled for a particular interface
      or for the entire router or may be (re)enabled.  While such
      transitions are (usually) uncommon, it is important that routers
      handle them correctly.


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5.3.12.1  When a Router Ceases Forwarding

         When a router ceases forwarding it MUST stop advertising all
         routes, except for third party routes.  It MAY continue to
         receive and use routes from other routers in its routing
         domains.  If the forwarding database is retained, the router
         MUST NOT cease timing the routes in the forwarding database.
         If routes that have been received from other routers are
         remembered, the router MUST NOT cease timing the routes which
         it has remembered.  It MUST discard any routes whose timers
         expire while forwarding is disabled, just as it would do if
         forwarding were enabled.

         DISCUSSION:
            When a router ceases forwarding, it essentially ceases being
            a router.  It is still a host, and must follow all of the
            requirements of Host Requirements [INTRO: 2].  The router
            may still be a passive member of one or more routing
            domains, however.  As such, it is allowed to maintain its
            forwarding database by listening to other routers in its
            routing domain.  It may not, however, advertise any of the
            routes in its forwarding database, since it itself is doing
            no forwarding.  The only exception to this rule is when the
            router is advertising a route which uses only some other
            router, but which this router has been asked to advertise.

         A router MAY send ICMP destination unreachable (host
         unreachable) messages to the senders of packets that it is
         unable to forward. It SHOULD NOT send ICMP redirect messages.

         DISCUSSION:
            Note that sending an ICMP destination unreachable (host
            unreachable) is a router action.  This message should not be
            sent by hosts.   This exception to the rules for hosts is
            allowed so that packets may be rerouted in the shortest
            possible time, and so that black holes are avoided.


5.3.12.2  When a Router Starts Forwarding

         When a router begins forwarding, it SHOULD expedite the sending
         of new routing information to all routers with which it
         normally exchanges routing information.





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5.3.12.3  When an Interface Fails or is Disabled

         If an interface fails or is disabled a router MUST remove and
         stop advertising all routes in its forwarding database which
         make use of that interface.  It MUST disable all static routes
         which make use of that interface.  If other routes to the same
         destination and TOS are learned or remembered by the router,
         the router MUST choose the best alternate, and add it to its
         forwarding database.  The router SHOULD send ICMP destination
         unreachable or ICMP redirect messages, as appropriate, in reply
         to all packets which it is unable to forward due to the
         interface being unavailable.

5.3.12.4  When an Interface is Enabled

         If an interface which had not been available becomes available,
         a router MUST reenable any static routes which use that
         interface.  If routes which would use that interface are
         learned by the router,  then these routes MUST be evaluated
         along with all of the other learned routes, and the router MUST
         make a decision as to which routes should be placed in the
         forwarding database.  The implementor is referred to Chapter
         [7], Application Layer - Routing Protocols for further
         information on how this decision is made.

         A router SHOULD expedite the sending of new routing information
         to all routers with which it normally exchanges routing
         information.

5.3.13  IP Options

      Several options, such as Record Route and Timestamp, contain slots
      into which a router inserts its address when forwarding the
      packet.  However, each such option has a finite number of slots,
      and therefore a router may find that there is not free slot into
      which it can insert its address.  No requirement listed below
      should be construed as requiring a router to insert its address
      into an option that has no remaining slot to insert it into.
      Section [5.2.5] discusses how a router must choose which of its
      addresses to insert into an option.

5.3.13.1  Unrecognized Options

         Unrecognized IP options in forwarded packets MUST be passed
         through unchanged.



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5.3.13.2  Security Option

         Some environments require the Security option in every packet;
         such a requirement is outside the scope of this document and
         the IP standard specification.  Note, however, that the
         security options described in [INTERNET:1] and [INTERNET:16]
         are obsolete.  Routers SHOULD IMPLEMENT the revised security
         option described in [INTERNET:5].

5.3.13.3  Stream Identifier Option

         This option is obsolete.  If the Stream Identifier option is
         present in a packet forwarded by the router, the option MUST be
         ignored and passed through unchanged.

5.3.13.4  Source Route Options

         A router MUST implement support for source route options in
         forwarded packets.  A router MAY implement a configuration
         option which, when enabled, causes all source-routed packets to
         be discarded.  However, such an option MUST NOT be enabled by
         default.

         DISCUSSION:
            The ability to source route datagrams through the Internet
            is important to various network diagnostic tools.  However,
            in a few rare cases, source routing may be used to bypass
            administrative and security controls within a network.
            Specifically, those cases where manipulation of routing
            tables is used to provide administrative separation in lieu
            of other methods such as packet filtering may be vulnerable
            through source routed packets.


5.3.13.5  Record Route Option

         Routers MUST support the Record Route option in forwarded
         packets.

         A router MAY provide a configuration option which, if enabled,
         will cause the router to ignore (i.e. pass through unchanged)
         Record Route options in forwarded packets.  If provided, such
         an option MUST default to enabling the record-route.  This
         option does not affect the processing of Record Route options
         in datagrams received by the router itself (in particular,
         Record Route options in ICMP echo requests will still be
         processed in accordance with Section [4.3.3.6]).


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         DISCUSSION:
            There are some people who believe that Record Route is a
            security problem because it discloses information about the
            topology of the network.  Thus, this document allows it to
            be disabled.


5.3.13.6  Timestamp Option

         Routers MUST support the timestamp option in forwarded packets.
         A timestamp value MUST follow the rules given in Section
         [3.2.2.8] of [INTRO:2].

         If the flags field = 3 (timestamp and prespecified address),
         the router MUST add its timestamp if the next prespecified
         address matches any of the router's IP addresses.  It is not
         necessary that the prespecified address be either the address
         of the interface on which the packet arrived or the address of
         the interface over which it will be sent.

         IMPLEMENTATION:
            To maximize the utility of the timestamps contained in the
            timestamp option, it is suggested that the timestamp
            inserted be, as nearly as practical, the time at which the
            packet arrived at the router.  For datagrams originated by
            the router, the timestamp inserted should be, as nearly as
            practical, the time at which the datagram was passed to the
            network layer for transmission.

         A router MAY provide a configuration option which, if enabled,
         will cause the router to ignore (i.e. pass through unchanged)
         Timestamp options in forwarded datagrams when the flag word is
         set to zero (timestamps only) or one (timestamp and registering
         IP address).  If provided, such an option MUST default to off
         (that is, the router does not ignore the timestamp).  This
         option does not affect the processing of Timestamp options in
         datagrams received by the router itself (in particular, a
         router will insert timestamps into Timestamp options in
         datagrams received by the router, and Timestamp options in ICMP
         echo requests will still be processed in accordance with
         Section [4.3.3.6]).

         DISCUSSION:
            Like the Record Route option, the Timestamp option can
            reveal information about a network's topology.  Some people
            consider this to be a security concern.


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6.  TRANSPORT LAYER

A router is not required to implement any Transport Layer protocols
except those required to support Application Layer protocols supported
by the router.  In practice, this means that most routers implement both
the Transmission Control Protocol (TCP) and the User Datagram Protocol
(UDP).

6.1  USER DATAGRAM PROTOCOL - UDP

   The User Datagram Protocol (UDP) is specified in [TRANS:1].

   A router which implements UDP MUST be compliant, and SHOULD be
   unconditionally compliant, with the requirements of section 4.1.3 of
   [INTRO:2], except that:

   o  This specification does not specify the interfaces between the
      various protocol layers.  Thus, a router need not comply with
      sections 4.1.3.2, 4.1.3.3, and 4.1.3.5 of [INTRO:2] (except of
      course where compliance is required for proper functioning of
      Application Layer protocols supported by the router).

   o  Contrary to section 4.1.3.4 of [INTRO:2], an application MUST NOT
      be able to disable to generation of UDP checksums.


   DISCUSSION:
      Although a particular application protocol may require that UDP
      datagrams it receives must contain a UDP checksum, there is no
      general requirement that received UDP datagrams contain UDP
      checksums.  Of course, if a UDP checksum is present in a received
      datagram, the checksum must be verified and the datagram discarded
      if the checksum is incorrect.


6.2  TRANSMISSION CONTROL PROTOCOL - TCP

   The Transmission Control Protocol (TCP) is specified in [TRANS:2].

   A router which implements TCP MUST be compliant, and SHOULD be
   unconditionally compliant, with the requirements of section 4.2 of
   [INTRO:2], except that:

   o  This specification does not specify the interfaces between the
      various protocol layers.  Thus, a router need not comply with the
      following requirements of [INTRO:2] (except of course where
      compliance is required for proper functioning of Application Layer


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      protocols supported by the router):

      Section 4.2.2.2:
           Passing a received PSH flag to the application layer is now
           OPTIONAL.

      Section 4.2.2.4:
           A TCP MUST inform the application layer asynchronously
           whenever it receives an Urgent pointer and there was
           previously no pending urgent data, or whenever the Urgent
           pointer advances in the data stream.  There MUST be a way for
           the application to learn how much urgent data remains to be
           read from the connection, or at least to determine whether or
           not more urgent data remains to be read.

      Section 4.2.3.5:
           An application MUST be able to set the value for R2 for a
           particular connection.  For example, an interactive
           application might set R2 to ``infinity,'' giving the user
           control over when to disconnect.

      Section 4.2.3.7:
           If an application on a multihomed host does not specify the
           local IP address when actively opening a TCP connection, then
           the TCP MUST ask the IP layer to select a local IP address
           before sending the (first) SYN.  See the function
           GET_SRCADDR() in Section 3.4.

      Section 4.2.3.8:
           An application MUST be able to specify a source route when it
           actively opens a TCP connection, and this MUST take
           precedence over a source route received in a datagram.

   o  For similar reasons, a router need not comply with any of the
      requirements of section 4.2.4 of [INTRO:2].

   o  The requirements of section 4.2.2.6 of [INTRO:2] are amended as
      follows: a router which implements the host portion of MTU
      discovery (discussed in Section [4.2.3.3] of this memo) uses 536
      as the default value of SendMSS only if the path MTU is unknown;
      if the path MTU is known, the default value for SendMSS is the
      path MTU - 40.

   o  The requirements of section 4.2.2.6 of [INTRO:2] are amended as
      follows: ICMP Destination Unreachable codes 11 and 12 are
      additional soft error conditions.  Therefore, these message MUST
      NOT cause TCP to abort a connection.


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   DISCUSSION:
      It should particularly be noted that a TCP implementation in a
      router must conform to the following requirements of [INTRO:2]:

      o  Providing a configurable TTL. [4.2.2.1]

      o  Providing an interface to configure keep-alive behavior, if
         keep-alives are used at all. [4.2.3.6]

      o  Providing an error reporting mechanism, and the ability to
         manage it.  [4.2.4.1]

      o  Specifying type of service. [4.2.4.2]

      The general paradigm applied is that if a particular interface is
      visible outside the router, then all requirements for the
      interface must be followed.  For example, if a router provides a
      telnet function, then it will be generating traffic, likely to be
      routed in the external networks.  Therefore, it must be able to
      set the type of service correctly or else the telnet traffic may
      not get through.



























Almquist & Kastenholz                                         [Page 108]

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7.  APPLICATION LAYER - ROUTING PROTOCOLS


7.1  INTRODUCTION

   An Autonomous System (AS) is defined as a set of routers all
   belonging under the same authority and all subject to a consistent
   set of routing policies.  Interior gateway protocols (IGPs) are used
   to distribute routing information inside of an AS (i.e.  intra-AS
   routing). Exterior gateway protocols are used to exchange routing
   information between ASs (i.e. inter-AS routing).

7.1.1  Routing Security Considerations

      Routing is one of the few places where the Robustness Principle
      (be liberal in what you accept) does not apply.  Routers should be
      relatively suspicious in accepting routing data from other routing
      systems.

      A router SHOULD provide the ability to rank routing information
      sources from most trustworthy to least trustworthy and to accept
      routing information about any particular destination from the most
      trustworthy sources first.  This was implicit in the original
      core/stub autonomous system routing model using EGP and various
      interior routing protocols.  It is even more important with the
      demise of a central, trusted core.

      A router SHOULD provide a mechanism to filter out obviously
      invalid routes (such as those for net 127).

      Routers MUST NOT by default redistribute routing data they do not
      themselves use, trust or otherwise consider invalid.  In rare
      cases, it may be necessary to redistribute suspicious information,
      but this should only happen under direct intercession by some
      human agency.

      In general, routers must be at least a little paranoid about
      accepting routing data from anyone, and must be especially careful
      when they distribute routing information provided to them by
      another party.  See below for specific guidelines.

      Routers SHOULD IMPLEMENT peer-to-peer authentication for those
      routing protocols that support them.





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7.1.2  Precedence

      Except where the specification for a particular routing protocol
      specifies otherwise, a router SHOULD set the IP Precedence value
      for IP datagrams carrying routing traffic it originates to 6
      (INTERNETWORK CONTROL).

      DISCUSSION:
         Routing traffic with VERY FEW exceptions should be the highest
         precedence traffic on any network.  If a system's routing
         traffic can't get through, chances are nothing else will.


7.2  INTERIOR GATEWAY PROTOCOLS


7.2.1  INTRODUCTION

      An Interior Gateway Protocol (IGP) is used to distribute routing
      information between the various routers in a particular AS.
      Independent of the algorithm used to implement a particular IGP,
      it should perform the following functions:

      (1)  Respond quickly to changes in the internal topology of an AS

      (2)  Provide a mechanism such that circuit flapping does not cause
           continuous routing updates

      (3)  Provide quick convergence to loop-free routing

      (4)  Utilize minimal bandwidth

      (5)  Provide equal cost routes to enable load-splitting

      (6)  Provide a means for authentication of routing updates

      Current IGPs used in the internet today are characterized as
      either being being based on a distance-vector or a link-state
      algorithm.

      Several IGPs are detailed in this section, including those most
      commonly used and some recently developed protocols which may be
      widely used in the future.  Numerous other protocols intended for
      use in intra-AS routing exist in the Internet community.

      A router which implements any routing protocol (other than static
      routes) MUST IMPLEMENT OSPF (see Section [7.2.2]) and MUST


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      IMPLEMENT RIP (see Section [7.2.4]).  A router MAY implement
      additional IGPs.

7.2.2  OPEN SHORTEST PATH FIRST - OSPF


7.2.2.1  Introduction

         Shortest Path First (SPF) based routing protocols are a class
         of link-state algorithms which are based on the shortest-path
         algorithm of Dijkstra.  Although SPF based algorithms have been
         around since the inception of the ARPANet, it is only recently
         that they have achieved popularity both inside both the IP and
         the OSI communities.  In an SPF based system, each router
         obtains an exact replica of the entire topology database via a
         process known as flooding.  Flooding insures a reliable
         transfer of the information. Each individual router then runs
         the SPF algorithm on its database to build the IP routing
         table.  The OSPF routing protocol is an implementation of an
         SPF algorithm.  The current version, OSPF version 2, is
         specified in [ROUTE:1].  Note that RFC-1131, which describes
         OSPF version 1, is obsolete.

         Note that to comply with Section [8.3] of this memo, a router
         which implements OSPF MUST implement the OSPF MIB [MGT:14].

7.2.2.2  Specific Issues


         Virtual Links

              There is a minor error in the specification that can cause
              routing loops when all of the following conditions are
              simultaneously true:

              (1)  A virtual link is configured through a transit area,

              (2)  Two separate paths exist, each having the same
                   endpoints, but one utilizing only non-virtual
                   backbone links, and the other using links in the
                   transit area, and

              (3)  The latter path is part of the (underlying physical
                   representation of the) configured virtual link,
                   routing loops may occur.

              To prevent this, an implementation of OSPF SHOULD invoke


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              the calculation in Section 16.3 of [ROUTE:1] whenever any
              part of the path to the destination is a virtual link (the
              specification only says this is necessary when the first
              hop is a virtual link).

7.2.2.3  New Version of OSPF

         As of this writing (4/4/94) there is a new version of the OSPF
         specification that is winding its way through the Internet
         standardization process.  A prudent implementor will be aware
         of this and develop an implementation accordingly.

         The new version fixes several errors in the current
         specification [ROUTE:1].  For this reason, implementors and
         vendors ought to expect to upgrade to the new version
         relatively soon.  In particular, the following problems exist
         in [ROUTE:1] that the new version fixes:

         o  In [ROUTE:1], certain configurations of virtual links can
            lead to incorrect routing and/or routing loops. A fix for
            this is specified in the new specification.

         o  In [ROUTE:1], OSPF external routes to For example, a router
            cannot import into an OSPF domain external routes both for
            192.2.0.0, 255.255.0.0 and 192.2.0.0, 255.255.255.0.  Routes
            such as these may become common with the deployment of CIDR
            [INTERNET:15].  This has been addressed in the new OSPF
            specification.

         o  In [ROUTE:1], OSPF Network-LSAs originated before a router
            changes its OSPF Router ID can confuse the Dijkstra
            calculation if the router again becomes Designated Router
            for the network. This has been fixed.

7.2.3  INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM - DUAL IS-IS

      The American National Standards Institute (ANSI) X3S3.3 committee
      has defined an intra-domain routing protocol.  This protocol is
      titled Intermediate System to Intermediate System Routeing
      Exchange Protocol.

      Its application to an IP network has been defined in [ROUTE:2],
      and is referred to as Dual IS-IS (or sometimes as Integrated IS-
      IS).  IS-IS is based on a link-state (SPF) routing algorithm and
      shares all the advantages for this class of protocols.



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7.2.4  ROUTING INFORMATION PROTOCOL - RIP


7.2.4.1  Introduction

         RIP is specified in [ROUTE:3].  Although RIP is still quite
         important in the Internet, it is being replaced in
         sophisticated applications by more modern IGPs such as the ones
         described above.

         Another common use for RIP is as a router discovery protocol.
         Section [4.3.3.10] briefly touches upon this subject.

7.2.4.2  Protocol Walk-Through


         Dealing with changes in topology: [ROUTE:3], pp. 11

              An implementation of RIP MUST provide a means for timing
              out routes.  Since messages are occasionally lost,
              implementations MUST NOT invalidate a route based on a
              single missed update.

              Implementations MUST by default wait six times the update
              interval before invalidating a route.  A router MAY have
              configuration options to alter this value.

              DISCUSSION:
                 It is important to routing stability that all routers
                 in a RIP autonomous system use similar timeout value
                 for invalidating routes, and therefore it is important
                 that an implementation default to the timeout value
                 specified in the RIP specification.  However, that
                 timeout value is overly conservative in environments
                 where packet loss is reasonably rare.  In such an
                 environment, a network manager may wish to be able to
                 decrease the timeout period in order to promote faster
                 recovery from failures.


              IMPLEMENTATION:
                 There is a very simple mechanism which a router may use
                 to meet the requirement to invalidate routes promptly
                 after they time out.  Whenever the router scans the
                 routing table to see if any routes have timed out, it
                 also notes the age of the least recently updated route
                 which has not yet timed out.  Subtracting this age from


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                 the timeout period gives the amount of time until the
                 router again needs to scan the table for timed out
                 routes.


         Split Horizon: [ROUTE:3], pp. 14-15

              An implementation of RIP MUST implement split horizon, a
              scheme used for avoiding problems caused by including
              routes in updates sent to the router from which they were
              learned.

              An implementation of RIP SHOULD implement Split horizon
              with poisoned reverse, a variant of split horizon which
              includes routes learned from a router sent to that router,
              but sets their metric to infinity.  Because of the routing
              overhead which may be incurred by implementing split
              horizon with poisoned reverse, implementations MAY include
              an option to select whether poisoned reverse is in effect.
              An implementation SHOULD limit the period of time in which
              it sends reverse routes at an infinite metric.

              IMPLEMENTATION:
                 Each of the following algorithms can be used to limit
                 the period of time for which poisoned reverse is
                 applied to a route.  The first algorithm is more
                 complex but does a more complete job of limiting
                 poisoned reverse to only those cases where it is
                 necessary.

                 The goal of both algorithms is to ensure that poison
                 reverse is done for any destination whose route has
                 changed in the last Route Lifetime (typically 180
                 seconds), unless it can be sure that the previous route
                 used the same output interface.  The Route Lifetime is
                 used because that is the amount of time RIP will keep
                 around an old route before declaring it stale.

                 The time intervals (and derived variables) used in the
                 following algorithms are as follows:

                 Tu   The Update Timer; the number of seconds between
                      RIP updates.  This typically defaults to 30
                      seconds.

                 Rl   The Route Lifetime, in seconds.  This is the
                      amount of time that a route is presumed to be


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                      good, without requiring an update.  This typically
                      defaults to 180 seconds.

                 Ul   The Update Loss; the number of consecutive updates
                      that have to be lost or fail to mention a route
                      before RIP deletes the route.  Ul is calculated to
                      be (Rl/Tu)+1.  The +1 is to account for the fact
                      that the first time the ifcounter is decremented
                      will be less than Tu seconds after it is
                      initialized.  Typically, Ul will be 7: (180/30)+1.


                 In   The value to set ifcounter to when a destination
                      is newly learned.  This value is Ul-4, where the 4
                      is RIP's garbage collection timer/30

                 The first algorithm is:

                 - Associated with each destination is a counter, called
                    the ifcounter below.  Poison reverse is done for any
                    route whose destination's ifcounter is greater than
                    zero.

                 - After a regular (not triggered or in response to a
                    request) update is sent, all of the non-zero
                    ifcounters are decremented by one.

                 - When a route to a destination is created, its
                    ifcounter is set as follows:

                    - If the new route is superseding a valid route, and
                       the old route used a different (logical) output
                       interface, then the ifcounter is set to Ul.

                    - If the new route is superseding a stale route, and
                       the old route used a different (logical) output
                       interface, then the ifcounter is set to MAX(0, Ul
                       - INT(seconds that the route has been stale/Ut).

                    - If there was no previous route to the destination,
                       the ifcounter is set to In.

                    - Otherwise, the ifcounter is set to zero

                 - RIP also maintains a timer, called the resettimer
                    below.  Poison reverse is done on all routes
                    whenever resettimer has not expired (regardless of


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                    the ifcounter values).

                 - When RIP is started, restarted, reset, or otherwise
                    has its routing table cleared, it sets the
                    resettimer to go off in Rl seconds.

                 The second algorithm is identical to the first except
                 that:

                 - The rules which set the ifcounter to non-zero values
                    are changed to always set it to Rl/Tu, and

                 - The resettimer is eliminated.

            Triggered updates: [ROUTE:3], pp. 15-16; pp. 29

                 Triggered updates (also called flash updates) are a
                 mechanism for immediately notifying a router's
                 neighbors when the router adds or deletes routes or
                 changes their metrics.  A router MUST send a triggered
                 update when routes are deleted or their metrics are
                 increased.  A router MAY send a triggered update when
                 routes are added or their metrics decreased.

                 Since triggered updates can cause excessive routing
                 overhead, implementations MUST use the following
                 mechanism to limit the frequency of triggered updates:

                 (1)  When a router sends a triggered update, it sets a
                      timer to a random time between one and five
                      seconds in the future.  The router must not
                      generate additional triggered updates before this
                      timer expires.

                 (2)  If the router would generate a triggered update
                      during this interval it sets a flag indicating
                      that a triggered update is desired.  The router
                      also logs the desired triggered update.

                 (3)  When the triggered update timer expires, the
                      router checks the triggered update flag. If the
                      flag is set then the router sends a single
                      triggered update which includes all of the changes
                      that were logged.  The router then clears the flag
                      and, since a triggered update was sent, restarts
                      this algorithm.


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                 (4)  The flag is also cleared whenever a regular update
                      is sent.

                 Triggered updates SHOULD include all routes that have
                 changed since the most recent regular (non-triggered)
                 update.  Triggered updates MUST NOT include routes that
                 have not changed since the most recent regular update.

                 DISCUSSION:
                    Sending all routes, whether they have changed
                    recently or not, is unacceptable in triggered
                    updates because the tremendous size of many Internet
                    routing tables could otherwise result in
                    considerable bandwidth being wasted on triggered
                    updates.

            Use of UDP: [ROUTE:3], pp. 18-19.

                 RIP packets sent to an IP broadcast address SHOULD have
                 their initial TTL set to one.

                 Note that to comply with Section [6.1] of this memo, a
                 router MUST use UDP checksums in RIP packets which it
                 originates, MUST discard RIP packets received with
                 invalid UDP checksums, but MUST not discard received
                 RIP packets simply because they do not contain UDP
                 checksums.

            Addressing Considerations: [ROUTE:3], pp. 22

                 A RIP implementation SHOULD support host routes.  If it
                 does not, it MUST (as described on page 27 of
                 [ROUTE:3]) ignore host routes in received updates.  A
                 router MAY log ignored hosts routes.

                 The special address 0.0.0.0 is used to describe a
                 default route. A default route is used as the route of
                 last resort (i.e. when a route to the specific net does
                 not exist in the routing table). The router MUST be
                 able to create a RIP entry for the address 0.0.0.0.

            Input Processing - Response: [ROUTE:3], pp. 26

                 When processing an update, the following validity
                 checks MUST be performed:

                 o  The response MUST be from UDP port 520.


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                 o  The source address MUST be on a directly connected
                    subnet (or on a directly connected, non-subnetted
                    network) to be considered valid.

                 o  The source address MUST NOT be one of the router's
                    addresses.

                    DISCUSSION:
                       Some networks, media, and interfaces allow a
                       sending node to receive packets that it
                       broadcasts.  A router must not accept its own
                       packets as valid routing updates and process
                       them.  The last requirement prevents a router
                       from accepting its own routing updates and
                       processing them (on the assumption that they were
                       sent by some other router on the network).

                 An implementation MUST NOT replace an existing route if
                 the metric received is equal to the existing metric
                 except in accordance with the following heuristic.

                 An implementation MAY choose to implement the following
                 heuristic to deal with the above situation. Normally,
                 it is useless to change the route to a network from one
                 router to another if both are advertised at the same
                 metric. However, the route being advertised by one of
                 the routers may be in the process of timing out.
                 Instead of waiting for the route to timeout, the new
                 route can be used after a specified amount of time has
                 elapsed. If this heuristic is implemented, it MUST wait
                 at least halfway to the expiration point before the new
                 route is installed.

7.2.4.3  Specific Issues


         RIP Shutdown

              An implementation of RIP SHOULD provide for a graceful
              shutdown using the following steps:

              (1)  Input processing is terminated,

              (2)  Four updates are generated at random intervals of
                   between two and four seconds, These updates contain
                   all routes that were previously announced, but with
                   some metric changes.  Routes that were being


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                   announced at a metric of infinity should continue to
                   use this metric.  Routes that had been announced with
                   a non-infinite metric should be announced with a
                   metric of 15 (infinity - 1).

                   DISCUSSION:
                      The metric used for the above really ought to be
                      16 (infinity); setting it to 15 is a kludge to
                      avoid breaking certain old hosts which wiretap the
                      RIP protocol.  Such a host will (erroneously)
                      abort a TCP connection if it tries to send a
                      datagram on the connection while the host has no
                      route to the destination (even if the period when
                      the host has no route lasts only a few seconds
                      while RIP chooses an alternate path to the
                      destination).

         RIP Split Horizon and Static Routes

              Split horizon SHOULD be applied to static routes by
              default.  An implementation SHOULD provide a way to
              specify, per static route, that split horizon should not
              be applied to this route.

7.2.5  GATEWAY TO GATEWAY PROTOCOL - GGP

      The Gateway to Gateway protocol is considered obsolete and SHOULD
      NOT be implemented.

7.3  EXTERIOR GATEWAY PROTOCOLS


7.3.1  INTRODUCTION

      Exterior Gateway Protocols are utilized for inter-Autonomous
      System routing to exchange reachability information for a set of
      networks internal to a particular autonomous system to a
      neighboring autonomous system.

      The area of inter-AS routing is a current topic of research inside
      the Internet Engineering Task Force.  The Exterior Gateway
      Protocol (EGP) described in Section [7.3.3] has traditionally been
      the inter-AS protocol of choice.  The Border Gateway Protocol
      (BGP) eliminates many of the restrictions and limitations of EGP,
      and is therefore growing rapidly in popularity.  A router is not
      required to implement any inter-AS routing protocol.  However, if
      a router does implement EGP it also MUST IMPLEMENT BGP.


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      Although it was not designed as an exterior gateway protocol, RIP
      (described in Section [7.2.4]) is sometimes used for inter-AS
      routing.

7.3.2  BORDER GATEWAY PROTOCOL - BGP


7.3.2.1  Introduction

         The Border Gateway Protocol (BGP) is an inter-AS routing
         protocol which exchanges network reachability information with
         other BGP speakers. The information for a network includes the
         complete list of ASs that traffic must transit to reach that
         network. This information can then be used to insure loop-free
         paths.  This information is sufficient to construct a graph of
         AS connectivity from which routing loops may be pruned and some
         policy decisions at the AS level may be enforced.

         BGP is defined by [ROUTE:4].  [ROUTE:5] specifies the proper
         usage of BGP in the Internet, and provides some useful
         implementation hints and guidelines.  [ROUTE:12] and [ROUTE:13]
         provide additional useful information.

         To comply with Section [8.3] of this memo, a router which
         implements BGP MUST also implement the BGP MIB [MGT:15].

         To characterize the set of policy decisions that can be
         enforced using BGP, one must focus on the rule that an AS
         advertises to its neighbor ASs only those routes that it itself
         uses.  This rule reflects the hop-by-hop routing paradigm
         generally used throughout the current Internet.  Note that some
         policies cannot be supported by the hop-by-hop routing paradigm
         and thus require techniques such as source routing to enforce.
         For example, BGP does not enable one AS to send traffic to a
         neighbor AS intending that that traffic take a different route
         from that taken by traffic originating in the neighbor AS.  On
         the other hand, BGP can support any policy conforming to the
         hop-by-hop routing paradigm.

         Implementors of BGP are strongly encouraged to follow the
         recommendations outlined in Section 6 of [ROUTE:5].

7.3.2.2  Protocol Walk-through

         While BGP provides support for quite complex routing policies
         (as an example see Section 4.2 in [ROUTE:5]), it is not
         required for all BGP implementors to support such policies.  At


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         a minimum, however, a BGP implementation:

         (1)  SHOULD allow an AS to control announcements of the BGP
              learned routes to adjacent AS's. Implementations SHOULD
              support such control with at least the granularity of a
              single network. Implementations SHOULD also support such
              control with the granularity of an autonomous system,
              where the autonomous system may be either the autonomous
              system that originated the route, or the autonomous system
              that advertised the route to the local system (adjacent
              autonomous system).

         (2)  SHOULD allow an AS to prefer a particular path to a
              destination (when more than one path is available).  Such
              function SHOULD be implemented by allowing system
              administrator to assign weights to Autonomous Systems, and
              making route selection process to select a route with the
              lowest weight (where weight of a route is defined as a sum
              of weights of all AS's in the AS_PATH path attribute
              associated with that route).

         (3)  SHOULD allow an AS to ignore routes with certain AS's in
              the AS_PATH path attribute. Such function can be
              implemented by using technique outlined in (2), and by
              assigning infinity as weights for such AS's. The route
              selection process must ignore routes that have weight
              equal to infinity.

7.3.3  EXTERIOR GATEWAY PROTOCOL - EGP


7.3.3.1  Introduction

         The Exterior Gateway Protocol (EGP) specifies an EGP which is
         used to exchange reachability information between routers of
         the same or differing autonomous systems. EGP is not considered
         a routing protocol since there is no standard interpretation
         (i.e. metric) for the distance fields in the EGP update
         message, so distances are comparable only among routers of the
         same AS.  It is however designed to provide high-quality
         reachability information, both about neighbor routers and about
         routes to non-neighbor routers.

         EGP is defined by [ROUTE:6].  An implementor almost certainly
         wants to read [ROUTE:7] and [ROUTE:8] as well, for they contain
         useful explanations and background material.


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         DISCUSSION:
            The present EGP specification has serious limitations, most
            importantly a restriction which limits routers to
            advertising only those networks which are reachable from
            within the router's autonomous system.  This restriction
            against propagating third party EGP information is to
            prevent long-lived routing loops.  This effectively limits
            EGP to a two-level hierarchy.

            RFC-975 is not a part of the EGP specification, and should
            be ignored.


7.3.3.2  Protocol Walk-through


         Indirect Neighbors: RFC-888, pp. 26

            An implementation of EGP MUST include indirect neighbor
            support.

         Polling Intervals: RFC-904, pp. 10

            The interval between Hello command retransmissions and the
            interval between Poll retransmissions SHOULD be configurable
            but there MUST be a minimum value defined.

            The interval at which an implementation will respond to
            Hello commands and Poll commands SHOULD be configurable but
            there MUST be a minimum value defined.

         Network Reachability: RFC-904, pp. 15

            An implementation MUST default to not providing the external
            list of routers in other autonomous systems; only the
            internal list of routers together with the nets which are
            reachable via those routers should be included in an Update
            Response/Indication packet.  However, an implementation MAY
            elect to provide a configuration option enabling the
            external list to be provided.  An implementation MUST NOT
            include in the external list routers which were learned via
            the external list provided by a router in another autonomous
            system. An implementation MUST NOT send a network back to
            the autonomous system from which it is learned, i.e. it MUST
            do split-horizon on an autonomous system level.

            If more than 255 internal or 255 external routers need to be


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