<pre>Network Working Group                                      D. Piscitello
Request for Comments: 1561                               Core Competence
Category: Experimental                                     December 1993


                  <span class="h1">Use of ISO CLNP in TUBA Environments</span>

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Abstract

   This memo specifies a profile of the ISO/IEC 8473 Connectionless-mode
   Network Layer Protocol (CLNP, [<a href="#ref-1">1</a>]) for use in conjunction with <a href="./rfc1347">RFC</a>
   <a href="./rfc1347">1347</a>, TCP/UDP over Bigger Addresses (TUBA, [<a href="#ref-2" title="&quot;TCP/UDP over Bigger Addresses (TUBA)&quot;">2</a>]).  It describes the
   use of CLNP to provide the lower-level service expected by
   Transmission Control Protocol (TCP, [<a href="#ref-3" title="&quot;Transmission Control Protocol (TCP)&quot;">3</a>]) and User Datagram Protocol
   (UDP, [<a href="#ref-4" title="&quot;User Datagram Protocol (UDP)&quot;">4</a>]).  CLNP provides essentially the same datagram service as
   Internet Protocol (IP, [<a href="#ref-5" title="&quot;Internet Protocol (IP)&quot;">5</a>]), but offers a means of conveying bigger
   network addresses (with additional structure, to aid routing).

   While the protocols offer nearly the same services, IP and CLNP are
   not identical. This document describes a means of preserving the
   semantics of IP information that is absent from CLNP while preserving
   consistency between the use of CLNP in Internet and OSI environments.
   This maximizes the use of already-deployed CLNP implementations.

Acknowledgments

   Many thanks to Ross Callon (Wellfleet Communications), John Curran
   (BBN), Cyndi Jung (3Com), Paul Brooks (UNSW), Brian Carpenter (CERN),
   Keith Sklower (Cal Berkeley), Dino Farinacci and Dave Katz (Cisco
   Systems), Rich Colella (NIST/CSL) and David Oran (DEC) for their
   assistance in composing this text.













<span class="grey">Piscitello                                                      [Page 1]</span></pre>
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Conventions

   The following language conventions are used in the items of
   specification in this document:

         * MUST, SHALL, or MANDATORY -- the item is an absolute
           requirement of the specification.

         * SHOULD or RECOMMENDED -- the item should generally be
           followed for all but exceptional circumstances.

         * MAY or OPTIONAL -- the item is truly optional and may be
           followed or ignored according to the needs of the
           implementor.

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Terminology</span>

   To the extent possible, this document is written in the language of
   the Internet. For example, packet is used rather than "protocol data
   unit", and "fragment" is used rather than "segment".  There are some
   terms that carry over from OSI; these are, for the most part, used so
   that cross-reference between this document and <a href="./rfc994">RFC 994</a> [<a href="#ref-6" title="&quot;ISO DIS 8473, Protocol for Providing the Connectionless Network Service&quot;">6</a>] or ISO/IEC
   8473 is not entirely painful.  OSI acronyms are for the most part
   avoided.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Introduction</span>

   The goal of this specification is to allow compatible and
   interoperable implementations to encapsulate TCP and UDP packets in
   CLNP data units. In a sense, it is more of a "hosts requirements"
   document for the network layer of TUBA implementations than a
   protocol specification. It is assumed that readers are familiar with
   STD 5, <a href="./rfc791">RFC 791</a>, STD 5, <a href="./rfc792">RFC 792</a> [<a href="#ref-7" title="&quot;Internet Control Message Protocol (ICMP)&quot;">7</a>], STD 3, <a href="./rfc1122">RFC 1122</a> [<a href="#ref-8" title="&quot;Requirements for Internet Hosts - Communication Layers&quot;">8</a>], and, to a
   lesser extent, <a href="./rfc994">RFC 994</a> and ISO/IEC 8473.  This document is compatible
   with (although more restrictive than) ISO/IEC 8473; specifically, the
   order, semantics, and processing of CLNP header fields is consistent
   between this and ISO/IEC 8473.

   [Note: <a href="./rfc994">RFC 994</a> contains the Draft International Standard version of
   ISO CLNP, in ASCII text. This is not the final version of the ISO/IEC
   protocol specification; however, it should provide sufficient
   background for the purpose of understanding the relationship of CLNP
   to IP, and the means whereby IP information is to be encoded in CLNP
   header fields. Postscript versions of ISO CLNP and associated routing
   protocols are available via anonymous FTP from merit.edu, and may be
   found in the directory /pub/ISO/IEC.





<span class="grey">Piscitello                                                      [Page 2]</span></pre>
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<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Overview of CLNP</span>

   ISO CLNP is a datagram network protocol. It provides fundamentally
   the same underlying service to a transport layer as IP. CLNP provides
   essentially the same maximum datagram size, and for those
   circumstances where datagrams may need to traverse a network whose
   maximum packet size is smaller than the size of the datagram, CLNP
   provides mechanisms for fragmentation (data unit identification,
   fragment/total length and offset). Like IP, a checksum computed on
   the CLNP header provides a verification that the information used in
   processing the CLNP datagram has been transmitted correctly, and a
   lifetime control mechanism ("Time to Live") imposes a limit on the
   amount of time a datagram is allowed to remain in the internet
   system. As is the case in IP, a set of options provides control
   functions needed or useful in some situations but unnecessary for the
   most common communications.

   Note that the encoding of options differs between the two protocols,
   as do the means of higher level protocol identification. Note also
   that CLNP and IP differ in the way header and fragment lengths are
   represented, and that the granularity of lifetime control (time-to-
   live) is finer in CLNP.

   Some of these differences are not considered "issues", as CLNP
   provides flexibility in the way that certain options may be specified
   and encoded (this will facilitate the use and encoding of certain IP
   options without change in syntax); others, e.g., higher level
   protocol identification and timestamp, must be accommodated in a
   transparent manner in this profile for correct operation of TCP and
   UDP, and continued interoperability with OSI implementations. <a href="#section-4">Section</a>
   <a href="#section-4">4</a> describes how header fields of CLNP must be populated to satisfy
   the needs of TCP and UDP.

   Errors detected during the processing of a CLNP datagram MAY be
   reported using CLNP Error Reports. Implementations of CLNP for TUBA
   environments MUST be capable of processing Error Reports (this is
   consistent with the 1992 edition (2)  of the ISO/IEC 8473 standard).
   Control messages (e.g., echo request/reply and redirect) are
   similarly handled in CLNP, i.e., identified as separate network layer
   packet types.  The relationship between CLNP Error and Control
   messages and Internet Control Message Protocol (ICMP, [<a href="#ref-7" title="&quot;Internet Control Message Protocol (ICMP)&quot;">7</a>]), and
   issues relating to the handling of these messages is described in
   <a href="#section-5">Section 5</a>.








<span class="grey">Piscitello                                                      [Page 3]</span></pre>
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   Table 1 provides a high-level comparison of CLNP to IP:

 Function              | ISO CLNP               | DOD IP
 ----------------------|------------------------|-----------------------
 Header Length         | indicated in octets    | in 32-bit words
 Version Identifier    | 1 octet                | 4 bits
 Lifetime (TTL)        | 500 msec units         | 1 sec units
 Flags                 | Fragmentation allowed, | Don't Fragment,
                       | More Fragments         | More Fragments,
                       | Suppress Error Reports | &lt;not defined&gt;
 Packet Type           | 5 bits                 | &lt;not defined&gt;
 Fragment Length       | 16 bits, in octets     | 16 bits, in octets
 Header Checksum       | 16-bit (Fletcher)      | 16-bit
 Total Length          | 16 bits, in octets     | &lt;not defined&gt;
 Addressing            | Variable length        | 32-bit fixed
 Data Unit Identifier  | 16 bits                | 16 bits
 Fragment offset       | 16 bits, in octets     | 13 bits, 8-octet units
 Higher Layer Protocol | Selector in address    | Protocol
 Options               | Security               | Security
                       | Priority               | TOS Precedence bits
                       | Complete Source Route  | Strict Source Route
                       | Quality of Service     | Type of Service
                       | Partial Source Route   | Loose Source Route
                       | Record Route           | Record Route
                       | Padding                | Padding
                       | &lt;defined herein&gt;       | Timestamp

                 Table 1. Comparison of IP to CLNP

   The composition and processing of a TCP pseudo-header when CLNP is
   used to provide the lower-level service expected by TCP and UDP is
   described in <a href="#section-6">Section 6</a>.

   [Note: This experimental RFC does not discuss multicasting.
   Presently, there are proposals for multicast extensions for CLNP in
   ISO/IEC/JTC1/SC6, and a parallel effort within TUBA. A future
   revision to this RFC will incorporate any extensions to CLNP that may
   be introduced as a result of the adoption of one of these
   alternatives.]












<span class="grey">Piscitello                                                      [Page 4]</span></pre>
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<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Proposed Internet Header using CLNP</span>

   A summary of the contents of the CLNP header, as it is proposed for
   use in TUBA environments, is illustrated in Figure 4-1:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |        ........Data Link Header........       | NLP ID        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |Header Length  |     Version   | Lifetime (TTL)|Flags|  Type   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |        Fragment Length        |           Checksum            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Dest Addr Len |               Destination Address...          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |               ... Destination Address...                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |               ... Destination Address...                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |               ... Destination Address...                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |               ... Destination Address...                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | PROTO field   | Src  Addr Len |  Source  Address...           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |               ... Source Address...                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |               ... Source Address...                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |               ... Source Address...                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |               ... Source Address...                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |Source Address |   Reserved    |       Data Unit Identifier    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |         Fragment Offset       |   Total Length of packet      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                   Options  (see Table 1)                      |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                               Data                            |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Note that each tick mark represents one bit position.

                     Figure 4-1. CLNP for TUBA



<span class="grey">Piscitello                                                      [Page 5]</span></pre>
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  Note 1: For illustrative purposes, Figure 4-1 shows Destination
          and Source Addresses having a length of 19 octets,
          including the PROTO/reserved field. In general, addresses
          can be variable length, up to a maximum of 20 octets,
          including the PROTO/reserved field.

  Note 2: Due to differences in link layer protocols, it is not
          possible to ensure that the packet starts on an even
          alignment. Note, however, that many link level protocols
          over which CLNP is operated use a odd length link
          (e.g., IEEE 802.2). (In Figure 4-1, the rest of the CLNP
          packet is even-aligned.)

   The encoding of CLNP fields for use in TUBA environments is as
   follows.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>  Network Layer Protocol Identification (NLP ID)</span>

   This one-octet field identifies this as the ISO/IEC 8473 protocol; it
   MUST set to binary 1000 0001.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>  Header Length Indication (Header Length)</span>

   Header Length is the length of the CLNP header in octets, and thus
   points to the beginning of the data. The value 255 is reserved. The
   header length is the same for all fragments of the same (original)
   CLNP packet.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>  Version</span>

   This one-octet field identifies the version of the protocol; it MUST
   be set to a binary value 0000 0001.

<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>  Lifetime (TTL)</span>

   Like the TTL field of IP, this field indicates the maximum time the
   datagram is allowed to remain in the internet system.  If this field
   contains the value zero, then the datagram MUST be destroyed; a host,
   however, MUST NOT send a datagram with a lifetime value of zero.
   This field is modified in internet header processing.  The time is
   measured in units of 500 milliseconds, but since every module that
   processes a datagram MUST decrease the TTL by at least one even if it
   process the datagram in less than 500 millisecond, the TTL must be
   thought of only as an upper bound on the time a datagram may exist.
   The intention is to cause undeliverable datagrams to be discarded,
   and to bound the maximum CLNP datagram lifetime. [Like IP, the
   colloquial usage of TTL in CLNP is as a coarse hop-count.]




<span class="grey">Piscitello                                                      [Page 6]</span></pre>
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   Unless otherwise directed, a host SHOULD use a value of 255 as the
   initial lifetime value.

<span class="h3"><a class="selflink" id="section-4.5" href="#section-4.5">4.5</a>  Flags</span>

   Three flags are defined. These occupy bits 0, 1, and 2 of the
   Flags/Type octet:

                          0   1   2
                        +---+---+---+
                        | F | M | E |
                        | P | F | R |
                        +---+---+---+

   The Fragmentation Permitted (FP) flag, when set to a value of one
   (1), is semantically equivalent to the "may fragment" value of the
   Don't Fragment field of IP; similarly, when set to zero (0), the
   Fragmentation Permitted flag is semantically equivalent to the "Don't
   Fragment" value of the Don't Fragment Flag of IP.

   [Note: If the Fragmentation Permitted field is set to the value 0,
   then the Data Unit Identifier, Fragment Offset, and Total Length
   fields are not present. This denotes a single fragment datagram. In
   such datagrams, the Fragment Length field contains the total length
   of the datagram.]

   The More Fragments flag of CLNP is semantically and syntactically the
   same as the More Fragments flag of IP; a value of one (1) indicates
   that more segments/fragments are forthcoming; a value of zero (0)
   indicates that the last octet of the original packet is present in
   this segment.

   The Error Report (ER) flag is used to suppress the generation of an
   error message by a host/router that detects an error during the
   processing of a CLNP datagram; a value of one (1) indicates that the
   host that originated this datagram thinks error reports are useful,
   and would dearly love to receive one if a host/router finds it
   necessary to discard its datagram(s).













<span class="grey">Piscitello                                                      [Page 7]</span></pre>
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<span class="h3"><a class="selflink" id="section-4.6" href="#section-4.6">4.6</a>  Type field</span>

   The type field distinguishes data CLNP packets from Error Reports
   from Echo packets. The following values of the type field apply:

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   |   flags   | 1 | 1 | 1 | 0 | 0 |  =&gt; Encoding of Type = data packet
   +---+---+---+---+---+---+---+---+
   |   flags   | 0 | 0 | 0 | 0 | 1 |  =&gt; Encoding of Type = error report
   +---+---+---+---+---+---+---+---+
   |   flags   | 1 | 1 | 1 | 1 | 0 |  =&gt; Encoding of Type = echo request
   +---+---+---+---+---+---+---+---+
   |   flags   | 1 | 1 | 1 | 1 | 1 |  =&gt; Encoding of Type = echo reply
   +---+---+---+---+---+---+---+---+

   Error Report packets are described in <a href="#section-5">Section 5</a>.

   Echo packets and their use are described in <a href="./rfc1139">RFC 1139</a> [<a href="#ref-9" title="&quot;An Echo Function for ISO 8473&quot;">9</a>].

<span class="h3"><a class="selflink" id="section-4.7" href="#section-4.7">4.7</a>  Fragment Length</span>

   Like the Total Length of the IP header, the Fragment length field
   contains the length in octets of the fragment (i.e., this datagram)
   including both header and data.

   [Note: CLNP also may also have a Total Length field, that contains
   the length of the original datagram; i.e., the sum of the length of
   the CLNP header plus the length of the data submitted by the higher
   level protocol, e.g., TCP or UDP. See <a href="#section-4.12">Section 4.12</a>.]

<span class="h3"><a class="selflink" id="section-4.8" href="#section-4.8">4.8</a>  Checksum</span>

   A checksum is computed on the header only. It MUST be verified at
   each host/router that processes the packet; if header fields are
   changed during processing (e.g., the Lifetime), the checksum is
   modified. If the checksum is not used, this field MUST be coded with
   a value of zero (0). See <a href="#appendix-A">Appendix A</a> for algorithms used in the
   computation and adjustment of the checksum. Readers are encouraged to
   see [<a href="#ref-10" title="&quot;Improving the Efficiency of the ISO Checksum Calculation&quot;">10</a>] for a description of an efficient implementation of the
   checksum algorithm.

<span class="h3"><a class="selflink" id="section-4.9" href="#section-4.9">4.9</a>  Addressing</span>

   CLNP uses OSI network service access point addresses (NSAPAs); NSAPAs
   serve the same identification and location functions as an IP
   address, plus the protocol selector value encoded in the IPv4
   datagram header, and  with additional hierarchy.  General purpose



<span class="grey">Piscitello                                                      [Page 8]</span></pre>
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   CLNP implementations MUST handle NSAP addresses of variable length up
   to 20 octets, as defined in ISO/IEC 8348 [<a href="#ref-11">11</a>]. TUBA implementations,
   especially routers, MUST accommodate these as well. Thus, for
   compatibility and interoperability with OSI use of CLNP, the initial
   octet of the Destination Address is assumed to be an Authority and
   Format Indicator, as defined in ISO/IEC 8348. NSAP addresses may be
   between 8 and 20 octets long (inclusive).

   TUBA implementations MUST support both ANSI and GOSIP style
   addresses; these are described in <a href="./rfc1237">RFC 1237</a> [<a href="#ref-12" title="&quot;Guidelines for OSI NSAP Allocation in the Internet&quot;">12</a>], and illustrated in
   Figure 4-2.  <a href="./rfc1237">RFC 1237</a> describes the ANSI/GOSIP initial domain parts
   as well as the format and composition of the domain specific part. It
   is further recommended that TUBA implementations support the
   assignment of system identifiers for TUBA/CLNP hosts defined in [<a href="#ref-13" title="&quot;Assignment of System Identifiers for TUBA/CLNP Hosts&quot;">13</a>]
   for the purposes of host address autoconfiguration as described in
   [<a href="#ref-14">14</a>]. Additional considerations specific to the interpretation and
   encoding of the selector part are described in sections <a href="#section-4.9.2">4.9.2</a> and
   4.9.4.

            +-------------+
            | &lt;-- IDP --&gt; |
            +----+--------+----------------------------------+
            |AFI |  IDI   |           &lt;-- DSP --&gt;            |
            +----+--------+----+---+-----+----+-----+---+----+
            | 47 |  0005  |DFI |AA |Rsvd | RD |Area |ID |Sel |
            +----+--------+----+---+-----+----+-----+---+----+
     octets | 1  |   2    | 1  | 3 |  2  | 2  | 2   | 6 | 1  |
            +----+--------+----+---+-----+----+-----+---+----+

                 Figure 4-2 (a): GOSIP Version 2 NSAP structure.

            +-------------+
            |&lt;-- IDP --&gt;  |
            +----+--------+----------------------------------+
            |AFI |  IDI   |          &lt;-- DSP --&gt;             |
            +----+--------+----+---+-----+----+-----+---+----+
            | 39 |  840   |DFI |ORG|Rsvd | RD |Area |ID |Sel |
            +----+--------+----+---+-----+----+-----+---+----+
     octets | 1  |   2    | 1  | 3 |  2  | 2  |  2  | 6 | 1  |
            +----+--------+----+---+-----+----+-----+---+----+

             Figure 4-2 (b): ANSI NSAP address format for DCC=840









<span class="grey">Piscitello                                                      [Page 9]</span></pre>
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        Definitions:
                     IDP   Initial Domain Part
                     AFI   Authority and Format Identifier
                     IDI   Initial Domain Identifier
                     DSP   Domain Specific Part
                     DFI   DSP Format Identifier
                     AA    Administration Authority
                     ORG   Organization Name (numeric form)
                     Rsvd  Reserved
                     RD    Routing Domain Identifier
                     Area  Area Identifier
                     ID    System Identifier
                     Sel   NSAP Selector

<span class="h4"><a class="selflink" id="section-4.9.1" href="#section-4.9.1">4.9.1</a>  Destination Address Length Indicator</span>

   This field indicates the length, in octets, of the Destination
   Address.

<span class="h4"><a class="selflink" id="section-4.9.2" href="#section-4.9.2">4.9.2</a>  Destination Address</span>

   This field contains an OSI NSAP address, as described in <a href="#section-4.9">Section 4.9</a>.
   It MUST always contain the address of the final destination. (This is
   true even for packets containing a source route option, see <a href="#section-4.13.4">Section</a>
   <a href="#section-4.13.4">4.13.4</a>).

   The final octet of the destination address MUST always contain the
   value of the PROTO field, as defined in IP.  The 8-bit PROTO field
   indicates the next level protocol used in the data portion of the
   CLNP datagram.  The values for various protocols are specified in
   "Assigned Numbers" [<a href="#ref-15" title="&quot;Assigned Numbers&quot;">15</a>]. For the PROTO field, the value of zero (0)
   is reserved.

   TUBA implementations that support TCP/UDP as well as OSI MUST use the
   protocol value (1Dh, Internet decimal 29) reserved for ISO transport
   protocol class 4.

<span class="h4"><a class="selflink" id="section-4.9.3" href="#section-4.9.3">4.9.3</a>  Source Address Length Indicator</span>

   This field indicates the length, in octets, of the Source Address.

<span class="h4"><a class="selflink" id="section-4.9.4" href="#section-4.9.4">4.9.4</a>  Source Address</span>

   This field contains an OSI NSAP address, as described in <a href="#section-4.9">Section 4.9</a>.

   The final octet of the source address is reserved. It MAY be set to
   the protocol field value on transmission, and shall be ignored on
   reception (the value of zero MUST not be used).



<span class="grey">Piscitello                                                     [Page 10]</span></pre>
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<span class="h3"><a class="selflink" id="section-4.10" href="#section-4.10">4.10</a>  Data Unit Identifier</span>

   Like the Identification field of IP, this 16-bit field is used to
   distinguish segments of the same (original) packet for the purposes
   of reassembly. This field is present when the fragmentation permitted
   flag is set to one.

<span class="h3"><a class="selflink" id="section-4.11" href="#section-4.11">4.11</a>  Fragment Offset</span>

   Like the Fragment Offset of IP, this 16-bit is used to identify the
   relative octet position of the data in this fragment with respect to
   the start of the data submitted to CLNP; i.e., it indicates where in
   the original datagram this fragment belongs.  The offset is measured
   in octets; the value of this field shall always be a multiple of
   eight (8). This field is present when the fragmentation permitted
   flag is set to one.

<span class="h3"><a class="selflink" id="section-4.12" href="#section-4.12">4.12</a>  Total Length</span>

   The total length of the CLNP packet in octets is determined by the
   originator and placed in the Total Length field of the header. The
   Total Length field specifies the entire length of the original
   datagram, including both the header and data. This field MUST NOT be
   changed in any fragment of the original packet for the duration of
   the packet lifetime. This field is present when the fragmentation
   permitted flag is set to one.

<span class="h3"><a class="selflink" id="section-4.13" href="#section-4.13">4.13</a>  Options</span>

   All CLNP options are "triplets" of the form &lt;parameter code&gt;,
   &lt;parameter length&gt;, and &lt;parameter value&gt;.  Both the parameter code
   and length fields are always one octet long; the length parameter
   value, in octets, is indicated in the parameter length field. The
   following options are defined for CLNP for TUBA.

<span class="h4"><a class="selflink" id="section-4.13.1" href="#section-4.13.1">4.13.1</a>  Security</span>

   The value of the parameter code field is binary 1100 0101. The length
   field MUST be set to the length of a Basic (and Extended) Security IP
   option(s) as identified in <a href="./rfc1108">RFC 1108</a> [<a href="#ref-16" title="&quot;Security Option for IP&quot;">16</a>], plus 1.  Octet 1 of the
   security parameter value field -- the CLNP Security Format Code -- is
   set to a binary value 0100 0000, indicating that the remaining octets
   of the security field contain either the Basic or Basic and Extended
   Security options as identified in <a href="./rfc1108">RFC 1108</a>. This encoding points to
   the administration of the source address (e.g., ISOC) as the
   administration of the security option; it is thus distinguished from
   the globally unique format whose definition is reserved for OSI use.
   Implementations wishing to use a security option MUST examine the



<span class="grey">Piscitello                                                     [Page 11]</span></pre>
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   PROTO field in the source address; if the value of PROTO indicates
   the CLNP client is TCP or UDP, the security option described in <a href="./rfc1108">RFC</a>
   <a href="./rfc1108">1108</a> is used.

   [Note: If IP options change, TUBA implementations MUST follow the new
   recommendations. This RFC, or revisions thereof, must document the
   new recommendations to assure compatibility.]

   The formats of the Security option, encoded as a CLNP option, is as
   follows. The CLNP option will be used to convey the Basic and
   Extended Security options as sub-options; i.e., the exact encoding of
   the Basic/Extended Security IP Option is carried in a single CLNP
   Security Option, with the length of the CLNP Security option
   reflecting the sum of the lengths of the Basic and Extended Security
   IP Option.

   +--------+--------+--------+--------+--------+---//----+-
   |11000100|XXXXXXXX|01000000|10000010|YYYYYYYY|         |      ...
   +--------+--------+--------+--------+--------+---//----+----
    CLNP       CLNP     CLNP     BASIC   BASIC    BASIC
    OPTION    OPTION   FORMAT  SECURITY  OPTION   OPTION
    TYPE      LENGTH    CODE    TYPE     LENGTH   VALUE
    (197)                       (130)

                          ---+------------+------------+----//-------+
                     ...     |  10000101  |  000LLLLL  |             |
                        -----+------------+------------+----//-------+
                                EXTENDED     EXTENDED    EXTENDED OPTION
                                OPTION       OPTION          VALUE
                               TYPE (133)    LENGTH

   The syntax, semantics and  processing of the Basic and Extended IP
   Security Options are defined in <a href="./rfc1108">RFC 1108</a>.

<span class="h4"><a class="selflink" id="section-4.13.2" href="#section-4.13.2">4.13.2</a>  Type of Service</span>

   [Note: Early drafts recommended the use of IP Type of Service as
   specified in <a href="./rfc1349">RFC 1349</a>. There now appears to be a broad consensus that
   this encoding is insufficient, and there is renewed interest in
   exploring the utility of the "congestion experienced" flag available
   in the CLNP QOS Maintenance option. This RFC thus recommends the use
   of the QOS Maintenance option native to CLNP.]

   The Quality of Service Maintenance option allows the originator of a
   CLNP datagram to convey information about the quality of service
   requested by the originating upper layer process. Routers MAY use
   this information as an aid in selecting a route when more than one
   route satisfying other routing criteria is available and the



<span class="grey">Piscitello                                                     [Page 12]</span></pre>
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   available routes are know to differ with respect to the following
   qualities of service: ability to preserve sequence, transit delay,
   cost, residual error probability. Through this option, a router may
   also indicate that it is experiencing congestion.

   The encoding of this option is as follows:

      +-----------+-----------+----------+
      | 1100 0011 | 0000 0001 | 110ABCDE |
      +-----------+-----------+----------+
       CLNP QOS     OPTION      QOS FLAGS
       TYPE (195)   LENGTH

   The value of the parameter code field MUST be set to a value of
   binary 1100 0011 (the CLNP Quality of Service Option Code point).
   The length field MUST be set to one (1).

   Bits 8-6 MUST be set as indicated in the figure. The flags "ABCDE"
   are interpreted as follows:

         A=1  choose path that maintains sequence over
              one that minimizes transit delay
         A=0  choose path that minimizes transit delay over
              one that maintains sequence
         B=1  congestion experienced
         B=0  no congestion to report
         C=1  choose path that minimizes transit delay over
              over low cost
         C=0  choose low cost over path that
              minimizes transit delay
         D=1  choose pathe with low residual error probability over
              one that minimizes transit delay
         D=0  choose path that minimizes transit delay over
              one with low residual error probability
         E=1  choose path with low residual error probability over
              low cost
         E=0  choose path with low cost over one with low
              residual error probability

<span class="h4"><a class="selflink" id="section-4.13.3" href="#section-4.13.3">4.13.3</a>  Padding</span>

   The padding field is used to lengthen the packet header to a
   convenient size. The parameter code field MUST be set to a value of
   binary 1100 1100. The value of the  parameter length field is
   variable. The parameter value MAY contain any value; the contents of
   padding fields MUST be ignored by the receiver.





<span class="grey">Piscitello                                                     [Page 13]</span></pre>
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      +----------+----------+-----------+
      | 11001100 | LLLLLLLL | VVVV VVVV |
      +----------+----------+-----------+

<span class="h4"><a class="selflink" id="section-4.13.4" href="#section-4.13.4">4.13.4</a>  Source Routing</span>

   Like the strict source route option of IP, the Complete Source Route
   option of CLNP is used to specify the exact and entire route an
   internet datagram MUST take. Similarly, the Partial Source Route
   option of CLNP provides the equivalent of the loose source route
   option of IP; i.e., a means for the source of an internet datagram to
   supply (some) routing information to be used by gateways in
   forwarding the internet datagram towards its destination. The
   identifiers encoded in this option are network entity titles, which
   are semantically and syntactically the same as NSAPAs and which can
   be used to unambiguously identify a network entity in an intermediate
   system (router).

   The parameter code for Source Routing is binary 1100 1000. The length
   of the source routing parameter value is variable.

   The first octet of the parameter value is a type code, indicating
   Complete Source Routing (binary 0000 0001) or Partial Source Routing
   (binary 0000 0000). The second octet identifies the offset of the
   next network entity title to be processed in the list, relative to
   the start of the parameter (i.e., a value of 3 is used to identify
   the first address in the list). The offset value is modified by each
   router using a complete source route or by each listed router using a
   partial source route to point to the next NET.

   The third octet begins the list of network entity titles. Only the
   NETs of intermediate systems are included in the list; the source and
   destination addresses shall not be included.  The list consists of
   variable length network entity title entries; the first octet of each
   entry gives the length of the network entity title that comprises the
   remainder of the entry.

<span class="h4"><a class="selflink" id="section-4.13.5" href="#section-4.13.5">4.13.5</a>  Record Route</span>

   Like the IP record route option, the Record route option of CLNP is
   used to trace the route a CLNP datagram takes.  A recorded route
   consists of a list of network entity titles (see Source Routing). The
   list is constructed as the CLNP datagram is forwarded along a path
   towards its final destination. Only titles of intermediate systems
   (routers) that processed the datagram are included in the recorded
   route; the network entity title of the originator of the datagram
   SHALL NOT be recorded in the list.




<span class="grey">Piscitello                                                     [Page 14]</span></pre>
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   The parameter code for Record Route is binary 1100 1011. The length
   of the record route parameter value is variable.

   The first octet of the parameter value is a type code, indicating
   Complete Recording of Route (0000 0001) or Partial Recording of Route
   (0000 0000). When complete recording of route is selected, reassembly
   at intermediate systems MAY be performed only when all fragments of a
   given datagram followed the same route; partial recording of route
   eliminates or "loosens" this constraint.

   The second octet identifies the offset where the next network entity
   title entry (see Source Routing) MAY be recorded (i.e., the end of
   the current list), relative to the start of the parameter.  A value
   of 3 is used to identify the initial recording position. The process
   of recording a network entity title entry is as follows. A router
   adds the length of its network entity title entry to the value of
   record route offset and compares this new value to the record route
   list length indicator; if the value does not exceed the length of the
   list, entity title entry is recorded, and the offset value is
   incremented by the value of the length of the network entity title
   entry. Otherwise, the recording of route is terminated, and the
   router MUST not record its network entity title in the option. If
   recording of route has been terminated, this (second) octet has a
   value 255.

   The third octet begins the list of network entity titles.

<span class="h4"><a class="selflink" id="section-4.13.6" href="#section-4.13.6">4.13.6</a>  Timestamp</span>

   [Note: There is no timestamp option in edition 1 of ISO/IEC 8473, but
   the option has been proposed and submitted to ISO/IEC JTC1/SC6.]

   The parameter code value 1110 1110 is used to identify the Timestamp
   option; the syntax and semantics of Timestamp are identical to that
   defined in IP.

   The Timestamp Option is defined in STD 5, <a href="./rfc791">RFC 791</a>. The CLNP parameter
   code 1110 1110 is used rather than the option type code 68 to
   identify the Timestamp option, and  the parameter value conveys the
   option length. Octet 1 of the Timestamp parameter value shall be
   encoded as the pointer (octet 3 of IP Timestamp); octet 2 of the
   parameter value shall be encoded as the overflow/format octet (octet
   4 of IP Timestamp); the remaining octets shall be used to encode the
   timestamp list. The size is fixed by the source, and cannot be
   changed to accommodate additional timestamp information.






<span class="grey">Piscitello                                                     [Page 15]</span></pre>
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        +--------+--------+--------+--------+
        |11101110| length | pointer|oflw|flg|
        +--------+--------+--------+--------+
        |         network entity title      |
        +--------+--------+--------+--------+
        |             timestamp             |
        +--------+--------+--------+--------+
        |                 .                 |
                          .

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Error Reporting and Control Message Handling</span>

   CLNP and IP  differ in the way in which errors are reported to hosts.
   In IP environments, the Internet Control Message Protocol (ICMP, [<a href="#ref-7" title="&quot;Internet Control Message Protocol (ICMP)&quot;">7</a>])
   is used to return (error) messages to hosts that originate packets
   that cannot be processed. ICMP messages are transmitted as user data
   in IP datagrams. Unreachable destinations, incorrectly composed IP
   datagram headers, IP datagram discards due to congestion, and
   lifetime/reassembly time exceeded are reported; the complete internet
   header that caused the error plus (at least) 8 octets of the segment
   contained in that IP datagram are returned to the sender as part of
   the ICMP error message. For certain errors, e.g., incorrectly
   composed IP datagram headers, the specific octet which caused the
   problem is identified.

   In CLNP environments, an unique message type, the Error Report type,
   is used in the network layer protocol header to distinguish Error
   Reports from CLNP datagrams. CLNP Error Reports are generated on
   detection of the same types of errors as with ICMP.  Like ICMP error
   messages, the complete CLNP header that caused the error is returned
   to the sender in the data portion of the Error Report.
   Implementations SHOULD return at least 8 octets of the datagram
   contained in the CLNP datagram to the sender of the original CLNP
   datagram. Here too, for certain errors, the specific octet which
   caused the problem is identified.

   A summary of the contents of the CLNP Error Report, as it is proposed
   for use in TUBA environments, is illustrated in Figure 5-1:













<span class="grey">Piscitello                                                     [Page 16]</span></pre>
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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        ........Data Link Header........       | NLP ID        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Header Length  |     Version   | Lifetime (TTL)| 000 | Type=ER |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  TOTAL Length of Error Report |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Dest Addr Len |               Destination Address...          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Destination Address...                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Destination Address...                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Destination Address...                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Destination Address...                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | PROTO field   | Src  Addr Len |  Source  Address...           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Source Address...                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Source Address...                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Source Address...                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Source Address...                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       ... Source Address      | Reason for Discard (type/len) |
   |                               |   1100 0001   | 0000 0010     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Reason for Discard        |    Options...                 |
   |   code        |   pointer     |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Options                             |
   :                                                               :
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Data                               |
   :                                                               :
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Note that each tick mark represents one bit position.

                      Figure 5-1. Error Report Format




<span class="grey">Piscitello                                                     [Page 17]</span></pre>
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<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>  Rules for processing an Error Report</span>

   The following is a summary of the rules for processing an Error
   Report:

         * An Error Report is not generated to report a problem
           encountered while processing an Error Report.

         * Error Reports MAY NOT be fragmented (hence, the
           fragmentation part is absent).

         * The Reason for Discard Code field is populated with one of
           the values from Table 5-1.

         * The Pointer field is populated with number of the first
           octet of the field that caused the Error Report to be
           generated. If it is not possible to identify the offending
           octet, this field MUST be zeroed.

         * If the Priority or Type of Service option is present in the
           errored datagram, the Error Report MUST specify the same
           option, using the value specified in the original datagram.

         * If the Security option is present in the errored datagram,
           the Error Report MUST specify the same option, using the
           value specified in the original datagram; if the Security
           option is not supported by the intermediate system, no Error
           Report is to be generated (i.e., "silently discard" the
           received datagram).

         * If the Complete Source Route option is specified in the
           errored datagram, the Error Report MUST compose a reverse of
           that route, and return the datagram along the same path.


















<span class="grey">Piscitello                                                     [Page 18]</span></pre>
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<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>  Comparison of ICMP and CLNP Error Messages</span>

   Table 5-1 provides a loose comparison of ICMP message types and codes
   to CLNP Error Type Codes (values in Internet decimal):

 CLNP Error Type  Codes            | ICMP Message           (Type, Code)
 ----------------------------------|------------------------------------
 Reason not specified          (0) | Parameter Problem           (12, 0)
 Protocol Procedure Error      (1) | Parameter Problem           (12, 0)
 Incorrect Checksum            (2) | Parameter Problem           (12, 0)
 PDU Discarded--Congestion     (3) | Source Quench                (4, 0)
 Header Syntax Error           (4) | Parameter problem           (12, 0)
 Need to Fragment could not    (5) | Frag needed, DF set          (3, 4)
 Incomplete PDU received       (6) | Parameter Problem           (12, 0)
 Duplicate Option              (7) | Parameter Problem           (12, 0)
 Destination Unreachable     (128) | Dest Unreachable,Net unknown (3, 0)
 Destination Unknown         (129) | Dest Unreachable,host unknown(3, 1)
 Source Routing Error        (144) | Source Route failed          (3, 5)
 Source Route Syntax Error   (145) | Source Route failed          (3, 5)
 Unknown Address in Src Route(146) | Source Route failed          (3, 5)
 Path not acceptable         (147) | Source Route failed          (3, 5)
 Lifetime expired            (160) | TTL exceeded                (11, 0)
 Reassembly Lifetime Expired (161) | Reassembly time exceeded    (11, 1)
 Unsupported Option          (176) | Parameter Problem           (12, 0)
 Unsupported Protocol Version(177) | Parameter problem           (12, 0)
 Unsupported Security Option (178) | Parameter problem           (12, 0)
 Unsupported Src Rte Option  (179) | Parameter problem           (12, 0)
 Unsupported Rcrd Rte        (180) | Parameter problem           (12, 0)
 Reassembly interference     (192) | Reassembly time exceeded    (11, 1)

    Table 5-1. Comparison of CLNP Error Reports to ICMP Error Messages

 Note 1: The current accepted practice for IP is that source quench
         should not be used; if it is used, implementations MUST
         not return a source quench packet for every relevant packet.
         TUBA/CLNP implementations are encouraged to adhere to these
         guidelines.

 Note 2: There are no corresponding CLNP Error Report Codes for the
         following ICMP error message types:
         - Protocol Unreachable  (3, 2)
         - Port Unreachable      (3, 3)
         [Note: Additional error code points available in the ER type
              code block can be used to identify these message types.]







<span class="grey">Piscitello                                                     [Page 19]</span></pre>
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<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Pseudo-Header Considerations</span>

   A checksum is computed on UDP and TCP segments to verify the
   integrity of the UDP/TCP segment. To further verify that the UDP/TCP
   segment has arrived at its correct destination, a pseudo-header
   consisting of information used in the delivery of the UDP/TCP segment
   is composed and included in the checksum computation.

   To compute the checksum on a UDP or TCP segment prior to
   transmission, implementations MUST compose a pseudo-header to the
   UDP/TCP segment consisting of the following information that will be
   used when composing the CLNP datagram:

         * Destination Address Length Indicator

         * Destination Address (including PROTO field)

         * Source Address Length Indicator

         * Source Address (including Reserved field)

         * A two-octet encoding of the Protocol value

         * TCP/UDP segment length

   If the length of the {source address length field + source address +
   destination address field + destination address } is not an integral
   number of octets, a trailing 0x00 nibble is padded. If GOSIP
   compliant NSAP addresses are used, this never happens (this is known
   as the Farinacci uncertainty principle).  The last byte in the
   Destination Address has the value 0x06 for TCP and 0x11 for UDP, and
   the Protocol field is encoded 0x0006 for TCP and 0x0011 for UDP.  If
   needed, an octet of zero is added to the end of the UDP/TCP segment
   to pad the datagram to a length that is a multiple of 16 bits.

   [Note: the pseudoheader is encoded in this manner to expedite
   processing, as it allows implementations to grab a contiguous stream
   of octets beginning at the destination address length indicator and
   terminating at the final octet of the source address; the PROTOCOL
   field is present to have a consistent representation across IPv4 and
   CLNP/TUBA implementations.]










<span class="grey">Piscitello                                                     [Page 20]</span></pre>
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   Figure 6-1 illustrates the resulting pseudo-header when both source
   and destination addresses are maximum length.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Dest Addr Len |               Destination Address...          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Destination Address...                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Destination Address...                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Destination Address...                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Destination Address...                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    (PROTO)    | Src  Addr Len |  Source  Address...           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Source Address...                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Source Address...                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Source Address...                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ... Source Address...                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | ...           | (Reserved)    |    Protocol                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   UDP/TCP segment length      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 6-1. Pseudo-header

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Security Considerations</span>

   ISO CLNP is an unreliable network datagram protocol, and is subject
   to the same security considerations as Internet Protocol ([<a href="#ref-5" title="&quot;Internet Protocol (IP)&quot;">5</a>], [<a href="#ref-8" title="&quot;Requirements for Internet Hosts - Communication Layers&quot;">8</a>]);
   methods for conveying the same security handling information
   recommended for IP are described in <a href="#section-4.13.1">Section 4.13.1</a>, Security Option.












<span class="grey">Piscitello                                                     [Page 21]</span></pre>
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<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Author's Address</span>

   David M. Piscitello
   Core Competence
   1620 Tuckerstown Road
   Dresher, PA 19025

   Phone: 215-830-0692
   EMail: wk04464@worldlink.com

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  References</span>

   [<a id="ref-1">1</a>] ISO/IEC 8473-1992. International Standards Organization -- Data
       Communications -- Protocol for Providing the Connectionless
       Network Service, Edition 2.

   [<a id="ref-2">2</a>] Callon, R., "TCP/UDP over Bigger Addresses (TUBA)", <a href="./rfc1347">RFC 1347</a>,
       Internet Architecture Board, May 1992.

   [<a id="ref-3">3</a>] Postel, J., "Transmission Control Protocol (TCP)", STD 7, <a href="./rfc793">RFC</a>
       <a href="./rfc793">793</a>, USC/Information Sciences Institute, September 1981.

   [<a id="ref-4">4</a>] Postel, J., "User Datagram Protocol (UDP)", STD 6, <a href="./rfc768">RFC 768</a>,
       USC/Information Sciences Institute, September 1981.

   [<a id="ref-5">5</a>] Postel, J., "Internet Protocol (IP)", STD 5, <a href="./rfc791">RFC 791</a>,
       USC/Information Sciences Institute, September 1981.

   [<a id="ref-6">6</a>] Chapin, L., "ISO DIS 8473, Protocol for Providing the
       Connectionless Network Service", <a href="./rfc994">RFC 994</a>, March 1986.

   [<a id="ref-7">7</a>] Postel, J., "Internet Control Message Protocol (ICMP)", STD 5,
       <a href="./rfc792">RFC 792</a>, USC/Information Sciences Institute, September 1981.

   [<a id="ref-8">8</a>] Braden, R., Editor, "Requirements for Internet Hosts -
       Communication Layers", STD 3, <a href="./rfc1122">RFC 1122</a>, Internet Engineering Task
       Force, October 1989.

   [<a id="ref-9">9</a>] Hagens, R., "An Echo Function for ISO 8473", <a href="./rfc1139">RFC 1139</a>, IETF-OSI
       Working Group, May 1993.

  [<a id="ref-10">10</a>] Sklower, K., "Improving the Efficiency of the ISO Checksum
       Calculation" ACM SIGCOMM CCR 18, no. 5 (October 1989):32-43.

  [<a id="ref-11">11</a>] ISO/IEC 8348-1992. International Standards Organization--Data
       Communications--OSI Network Layer Service and Addressing.





<span class="grey">Piscitello                                                     [Page 22]</span></pre>
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  [<a id="ref-12">12</a>] Callon, R., Gardner, E., and R. Hagens, "Guidelines for OSI NSAP
       Allocation in the Internet", <a href="./rfc1237">RFC 1237</a>, NIST, Mitre, DEC, July
       1991.

  [<a id="ref-13">13</a>] Piscitello, D., "Assignment of System Identifiers for TUBA/CLNP
       Hosts", <a href="./rfc1526">RFC 1526</a>, Bellcore, September 1993.

  [<a id="ref-14">14</a>] ISO/IEC 9542:1988/PDAM 1. Information Processing Systems -- Data
       Communications -- ES/IS Routeing Protocol for use with ISO CLNP
       -- Amendment 1: Dynamic Discovery of OSI NSAP Addresses by End
       Systems.

  [<a id="ref-15">15</a>] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, <a href="./rfc1340">RFC 1340</a>
       USC/Information Sciences Institute, July 1992.

  [<a id="ref-16">16</a>] Kent, S., "Security Option for IP", <a href="./rfc1108">RFC 1108</a>, BBN Communications,
       November 1991.


































<span class="grey">Piscitello                                                     [Page 23]</span></pre>
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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>. Checksum Algorithms (from ISO/IEC 8473)</span>

       Symbols used in algorithms:

        c0, c1          variables used in the algorithms
        i               position of octet in header (first
                        octet is i=1)
        Bi              value of octet i in the header
        n               position of first octet of checksum (n=8)
        L               Length of header in octets
        X               Value of octet one of the checksum parameter
        Y               Value of octet two of the checksum parameter

   Addition is performed in one of the two following modes:

         * modulo 255 arithmetic;

         * eight-bit one's complement arithmetic;

   The algorithm for Generating the Checksum Parameter Value is as
   follows:

  A.  Construct the complete header with the value of the
      checksum parameter field set to zero; i.e., c0 &lt;- c1 &lt;- 0;

  B.  Process each octet of the header sequentially from i=1 to L
      by:

         * c0 &lt;- c0 + Bi

         * c1 &lt;- c1 + c0

  C.  Calculate X, Y as follows:

         * X &lt;- (L - 8)(c0 - c1) modulo 255

         * Y &lt;- (L - 7)(-C0) + c1

  D.  If X = 0, then X &lt;- 255

  E.  If Y = 0, then Y &lt;- 255

  F.  place the values of X and Y in octets 8 and 9 of the
      header, respectively

   The algorithm for checking the value of the checksum parameter is as
   follows:




<span class="grey">Piscitello                                                     [Page 24]</span></pre>
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  A.  If octets 8 and 9 of the header both contain zero, then the
      checksum calculation has succeeded; else if either but not
      both of these octets contains the value zero then the
      checksum is incorrect; otherwise, initialize: c0 &lt;- c1 &lt;- 0

  B.  Process each octet of the header sequentially from i = 1 to
      L by:

         * c0 &lt;- c0 + Bi

         * c1 &lt;- c1 + c0

  C.  When all the octets have been processed, if c0 = c1 = 0,
      then the checksum calculation has succeeded, else it has
      failed.

   There is a separate algorithm to adjust the checksum parameter value
   when a octet has been modified (such as the TTL). Suppose the value
   in octet k is changed by Z = newvalue - oldvalue. If X and Y denote
   the checksum values held in octets n and n+1 respectively, then
   adjust X and Y as follows:

   If X = 0 and Y = 0 then do nothing, else if X = 0 or Y = 0 then the
   checksum is incorrect, else:

   X &lt;- (k - n - 1)Z + X   modulo 255

   Y &lt;- (n - k)Z + Y       modulo 255

   If X = 0, then X &lt;- 255; if Y = 0, then Y &lt;- 255.

   In the example, n = 89; if the octet altered is the TTL (octet 4),
   then k = 4. For the case where the lifetime is decreased by one unit
   (Z = -1), the assignment statements for the new values of X and Y in
   the immediately preceeding algorithm simplify to:

   X &lt;- X + 5      Modulo 255

   Y &lt;- Y - 4      Modulo 255












Piscitello                                                     [Page 25]
</pre>