<pre>Internet Engineering Task Force (IETF)                    J. Macker, Ed.
Request for Comments: 6621                                           NRL
Category: Experimental                                          May 2012
ISSN: 2070-1721


                    <span class="h1">Simplified Multicast Forwarding</span>

Abstract

   This document describes a Simplified Multicast Forwarding (SMF)
   mechanism that provides basic Internet Protocol (IP) multicast
   forwarding suitable for limited wireless mesh and mobile ad hoc
   network (MANET) use.  It is mainly applicable in situations where
   efficient flooding represents an acceptable engineering design trade-
   off.  It defines techniques for multicast duplicate packet detection
   (DPD), to be applied in the forwarding process, for both IPv4 and
   IPv6 protocol use.  This document also specifies optional mechanisms
   for using reduced relay sets to achieve more efficient multicast data
   distribution within a mesh topology as compared to Classic Flooding.
   Interactions with other protocols, such as use of information
   provided by concurrently running unicast routing protocols or
   interaction with other multicast protocols, as well as multiple
   deployment approaches are also described.  Distributed algorithms for
   selecting reduced relay sets and related discussion are provided in
   the appendices.  Basic issues relating to the operation of multicast
   MANET border routers are discussed, but ongoing work remains in this
   area and is beyond the scope of this document.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc6621">http://www.rfc-editor.org/info/rfc6621</a>.





<span class="grey">Macker                        Experimental                      [Page 1]</span></pre>
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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
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   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   <a href="#section-1">1</a>. Introduction and Scope ..........................................<a href="#page-4">4</a>
   <a href="#section-2">2</a>. Terminology .....................................................<a href="#page-4">4</a>
   <a href="#section-3">3</a>. Applicability Statement .........................................<a href="#page-5">5</a>
   <a href="#section-4">4</a>. Overview and Functioning ........................................<a href="#page-6">6</a>
   <a href="#section-5">5</a>. SMF Packet Processing and Forwarding ............................<a href="#page-8">8</a>
   <a href="#section-6">6</a>. SMF Duplicate Packet Detection .................................<a href="#page-10">10</a>
      <a href="#section-6.1">6.1</a>. IPv6 Duplicate Packet Detection ...........................<a href="#page-11">11</a>
           <a href="#section-6.1.1">6.1.1</a>. IPv6 SMF_DPD Option Header .........................<a href="#page-12">12</a>
           <a href="#section-6.1.2">6.1.2</a>. IPv6 Identification-Based DPD ......................<a href="#page-14">14</a>
           <a href="#section-6.1.3">6.1.3</a>. IPv6 Hash-Based DPD ................................<a href="#page-16">16</a>
      <a href="#section-6.2">6.2</a>. IPv4 Duplicate Packet Detection ...........................<a href="#page-17">17</a>
           <a href="#section-6.2.1">6.2.1</a>. IPv4 Identification-Based DPD ......................<a href="#page-18">18</a>
           <a href="#section-6.2.2">6.2.2</a>. IPv4 Hash-Based DPD ................................<a href="#page-19">19</a>
   <a href="#section-7">7</a>. Relay Set Selection ............................................<a href="#page-20">20</a>
      <a href="#section-7.1">7.1</a>. Non-Reduced Relay Set Forwarding ..........................<a href="#page-20">20</a>
      <a href="#section-7.2">7.2</a>. Reduced Relay Set Forwarding ..............................<a href="#page-20">20</a>
   <a href="#section-8">8</a>. SMF Neighborhood Discovery Requirements ........................<a href="#page-23">23</a>
      <a href="#section-8.1">8.1</a>. SMF Relay Algorithm TLV Types .............................<a href="#page-24">24</a>
           <a href="#section-8.1.1">8.1.1</a>. SMF Message TLV Type ...............................<a href="#page-24">24</a>



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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


           <a href="#section-8.1.2">8.1.2</a>. SMF Address Block TLV Type .........................<a href="#page-25">25</a>
   <a href="#section-9">9</a>. SMF Border Gateway Considerations ..............................<a href="#page-26">26</a>
      <a href="#section-9.1">9.1</a>. Forwarded Multicast Groups ................................<a href="#page-27">27</a>
      <a href="#section-9.2">9.2</a>. Multicast Group Scoping ...................................<a href="#page-28">28</a>
      <a href="#section-9.3">9.3</a>. Interface with Exterior Multicast Routing Protocols .......<a href="#page-29">29</a>
      <a href="#section-9.4">9.4</a>. Multiple Border Routers ...................................<a href="#page-29">29</a>
   <a href="#section-10">10</a>. Security Considerations .......................................<a href="#page-31">31</a>
   <a href="#section-11">11</a>. IANA Considerations ...........................................<a href="#page-32">32</a>
      <a href="#section-11.1">11.1</a>. IPv6 SMF_DPD Header Extension Option Type ................<a href="#page-33">33</a>
      <a href="#section-11.2">11.2</a>. TaggerId Types (TidTy) ...................................<a href="#page-33">33</a>
      <a href="#section-11.3">11.3</a>. Well-Known Multicast Address .............................<a href="#page-34">34</a>
      <a href="#section-11.4">11.4</a>. SMF TLVs .................................................<a href="#page-34">34</a>
           11.4.1. Expert Review for Created Type Extension
                   Registries ........................................<a href="#page-34">34</a>
           <a href="#section-11.4.2">11.4.2</a>. SMF Message TLV Type (SMF_TYPE) ...................<a href="#page-34">34</a>
           <a href="#section-11.4.3">11.4.3</a>. SMF Address Block TLV Type (SMF_NBR_TYPE) .........<a href="#page-35">35</a>
           <a href="#section-11.4.4">11.4.4</a>. SMF Relay Algorithm ID Registry ...................<a href="#page-35">35</a>
   <a href="#section-12">12</a>. Acknowledgments ...............................................<a href="#page-36">36</a>
   <a href="#section-13">13</a>. References ....................................................<a href="#page-37">37</a>
      <a href="#section-13.1">13.1</a>. Normative References .....................................<a href="#page-37">37</a>
      <a href="#section-13.2">13.2</a>. Informative References ...................................<a href="#page-38">38</a>
   <a href="#appendix-A">Appendix A</a>.  Essential Connecting Dominating Set (E-CDS)
                Algorithm ............................................<a href="#page-40">40</a>
     <a href="#appendix-A.1">A.1</a>.  E-CDS Relay Set Selection Overview ........................<a href="#page-40">40</a>
     <a href="#appendix-A.2">A.2</a>.  E-CDS Forwarding Rules ....................................<a href="#page-41">41</a>
     <a href="#appendix-A.3">A.3</a>.  E-CDS Neighborhood Discovery Requirements .................<a href="#page-41">41</a>
     <a href="#appendix-A.4">A.4</a>.  E-CDS Selection Algorithm .................................<a href="#page-44">44</a>
   <a href="#appendix-B">Appendix B</a>.  Source-Based Multipoint Relay (S-MPR) Algorithm ......<a href="#page-46">46</a>
     <a href="#appendix-B.1">B.1</a>.  S-MPR Relay Set Selection Overview ........................<a href="#page-46">46</a>
     <a href="#appendix-B.2">B.2</a>.  S-MPR Forwarding Rules ....................................<a href="#page-47">47</a>
     <a href="#appendix-B.3">B.3</a>.  S-MPR Neighborhood Discovery Requirements .................<a href="#page-48">48</a>
     <a href="#appendix-B.4">B.4</a>.  S-MPR Selection Algorithm .................................<a href="#page-50">50</a>
   <a href="#appendix-C">Appendix C</a>.  Multipoint Relay Connected Dominating Set
                (MPR-CDS) Algorithm ..................................<a href="#page-52">52</a>
     <a href="#appendix-C.1">C.1</a>.  MPR-CDS Relay Set Selection Overview ......................<a href="#page-52">52</a>
     <a href="#appendix-C.2">C.2</a>.  MPR-CDS Forwarding Rules ..................................<a href="#page-53">53</a>
     <a href="#appendix-C.3">C.3</a>.  MPR-CDS Neighborhood Discovery Requirements ...............<a href="#page-53">53</a>
     <a href="#appendix-C.4">C.4</a>.  MPR-CDS Selection Algorithm ...............................<a href="#page-54">54</a>













<span class="grey">Macker                        Experimental                      [Page 3]</span></pre>
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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


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

   Unicast routing protocols, designed for MANET and wireless mesh use,
   often apply distributed algorithms to flood routing control plane
   messages within a MANET routing domain.  For example, algorithms
   specified within [<a href="./rfc3626" title="&quot;Optimized Link State Routing Protocol (OLSR)&quot;">RFC3626</a>] and [<a href="./rfc3684" title="&quot;Topology Dissemination Based on Reverse-Path Forwarding (TBRPF)&quot;">RFC3684</a>] provide distributed methods
   of dynamically electing reduced relay sets that attempt to
   efficiently flood routing control messages while maintaining a
   connected set under dynamic topological conditions.

   Simplified Multicast Forwarding (SMF) extends the efficient flooding
   concept to the data forwarding plane, providing an appropriate
   multicast forwarding capability for use cases where localized,
   efficient flooding is considered an effective design approach.  The
   baseline design is intended to provide a basic, best-effort multicast
   forwarding capability that is constrained to operate within a single
   MANET routing domain.

   An SMF routing domain is an instance of an SMF routing protocol with
   common policies, under a single network administration authority.
   The main design goals of this document are to:

   o  adapt efficient relay sets in MANET environments [<a href="./rfc2501" title="&quot;Mobile Ad hoc Networking (MANET): Routing Protocol Performance Issues and Evaluation Considerations&quot;">RFC2501</a>], and

   o  define the needed IPv4 and IPv6 multicast duplicate packet
      detection (DPD) mechanisms to support multi-hop packet forwarding.

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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

   The terms introduced in [<a href="./rfc5444" title="&quot;Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format&quot;">RFC5444</a>], including "packet", "message",
   "TLV Block", "TLV", and "address", are to be interpreted as described
   therein.














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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   The following abbreviations are used throughout this document:

   +--------------+----------------------------------------------------+
   | Abbreviation | Definition                                         |
   +--------------+----------------------------------------------------+
   | MANET        | Mobile Ad Hoc Network                              |
   | SMF          | Simplified Multicast Forwarding                    |
   | CF           | Classic Flooding                                   |
   | CDS          | Connected Dominating Set                           |
   | MPR          | Multipoint Relay                                   |
   | S-MPR        | Source-based MPR                                   |
   | MPR-CDS      | MPR-based CDS                                      |
   | E-CDS        | Essential CDS                                      |
   | NHDP         | Neighborhood Discovery Protocol                    |
   | DPD          | Duplicate Packet Detection                         |
   | I-DPD        | Identification-based DPD                           |
   | H-DPD        | Hash-based DPD                                     |
   | HAV          | Hash assist value                                  |
   | FIB          | Forwarding Information Base                        |
   | TLV          | type-length-value encoding                         |
   | DoS          | Denial of Service                                  |
   | SMF Router   | A MANET Router implementing the protocol specified |
   |              | in this document                                   |
   | SMF Routing  | A MANET Routing Domain wherein the protocol        |
   | Domain       | specified in this document is operating            |
   +--------------+----------------------------------------------------+

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Applicability Statement</span>

   Within dynamic wireless routing topologies, maintaining traditional
   forwarding trees to support a multicast routing protocol is often not
   as effective as in wired networks due to the reduced reliability and
   increased dynamics of mesh topologies [<a href="#ref-MGL04" title="&quot;Group Communications in Mobile Ad hoc Networks&quot;">MGL04</a>][GM99].  A basic packet
   forwarding service reaching all connected routers running the SMF
   protocol within a MANET routing domain may provide a useful group
   communication paradigm for various classes of applications, for
   example, multimedia streaming, interactive group-based messaging and
   applications, peer-to-peer middleware multicasting, and multi-hop
   mobile discovery or registration services.  SMF is likely only
   appropriate for deployment in limited dynamic MANET routing domains
   (further defined as administratively scoped multicast forwarding
   domains in <a href="#section-9.2">Section 9.2</a>) so that the flooding process can be
   contained.

   A design goal is that hosts may also participate in multicast traffic
   transmission and reception with standard IP network-layer semantics
   (e.g., special or unnecessary encapsulation of IP packets should be
   avoided in this case).  SMF deployments are able to connect and



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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   interoperate with existing standard multicast protocols operating
   within more conventional Internet infrastructures.  To this end, a
   multicast border router or proxy mechanism MUST be used when deployed
   alongside more fixed-infrastructure IP multicast routing such
   Protocol Independent Multicast (PIM) variants [<a href="./rfc3973" title="&quot;Protocol Independent Multicast - Dense Mode (PIM-DM): Protocol Specification (Revised)&quot;">RFC3973</a>][RFC4601].
   Experimental SMF implementations and deployments have demonstrated
   gateway functionality at MANET border routers operating with existing
   external IP multicast routing protocols [<a href="#ref-CDHM07" title="&quot;Connecting MANET Multicast&quot;">CDHM07</a>][DHS08][<a href="#ref-DHG09" title="&quot;Experiment and field demonstration of a 802.11-based ground-UAV mobile ad-hoc network&quot;">DHG09</a>].  SMF
   may be extended or combined with other mechanisms to provide
   increased reliability and group-specific filtering; the details for
   this are out of the scope of this document.

   This document does not presently support forwarding of packets with
   directed broadcast addresses as a destination [<a href="./rfc2644" title="&quot;Changing the Default for Directed Broadcasts in Routers&quot;">RFC2644</a>].

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Overview and Functioning</span>

   Figure 1 provides an overview of the logical SMF router architecture,
   consisting of "Neighborhood Discovery", "Relay Set Selection", and
   "Forwarding Process" components.  Typically, relay set selection (or
   self-election) occurs based on dynamic input from a neighborhood
   discovery process.  SMF supports the case where neighborhood
   discovery and/or relay set selection information is obtained from a
   coexistent process (e.g., a lower-layer mechanism or a unicast
   routing protocol using relay sets).  In some algorithm designs, the
   forwarding decision for a packet can also depend on previous hop or
   incoming interface information.  The asterisks (*) in Figure 1 mark
   the primitives and relationships needed by relay set algorithms
   requiring previous-hop packet-forwarding knowledge.
                ______________                _____________
               |              |              |             |
               | Neighborhood |              |  Relay Set  |
               |  Discovery   |-------------&gt;|  Selection  |
               |              |   neighbor   |             |
               |______________|     info     |_____________|
                      \                              /
                       \                            /
                neighbor\                          /forwarding
                  info*  \      ____________      /  status
                          \    |            |    /
                           `--&gt;| Forwarding |&lt;--'
                               |  Process   |
             ~~~~~~~~~~~~~~~~~&gt;|____________|~~~~~~~~~~~~~~~~~&gt;
             incoming packet,                 forwarded packets
             interface id*, and
             previous hop*

                     Figure 1: SMF Router Architecture



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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   Certain IP multicast packets, defined in Sections <a href="#section-9.2">9.2</a> and <a href="#section-5">5</a>, are
   "non-forwardable".  These multicast packets MUST be ignored by the
   SMF forwarding engine.  The SMF forwarding engine MAY also work with
   policies and management interfaces to allow additional filtering
   control over which multicast packets are considered for potential SMF
   forwarding.  This interface would allow more refined dynamic
   forwarding control once such techniques are matured for MANET
   operation.  At present, further discussion of dynamic control is left
   to future work.

   Interoperable SMF implementations MUST use compatible DPD approaches
   and be able to process the header options defined in this document
   for IPv6 operation.

   Classic Flooding (CF) is defined as the simplest case of SMF
   multicast forwarding.  With CF, all SMF routers forward each received
   multicast packet exactly once.  In this case, the need for any relay
   set selection or neighborhood topology information is eliminated, at
   the expense of additional network overhead incurred from unnecessary
   packet retransmissions.  With CF, the SMF DPD functionality is still
   required.  While SMF supports CF as a mode of operation, the use of
   more efficient relay set modes is RECOMMENDED in order to reduce
   contention and congestion caused by unnecessary packet
   retransmissions [<a href="#ref-NTSC99" title="&quot;The Broadcast Storm Problem in a Mobile Ad Hoc Network&quot;">NTSC99</a>].

   An efficient reduced relay set is constructed by selecting and
   updating, as needed, a subset of all possible routers in a MANET
   routing domain to act as SMF forwarders.  Known distributed relay set
   selection algorithms have demonstrated the ability to provide and
   maintain a dynamic connected set for forwarding multicast IP packets
   [<a href="#ref-MDC04" title="&quot;Simplified Multicast Forwarding in Mobile Ad hoc Networks&quot;">MDC04</a>].  A few such relay set selection algorithms are described in
   the appendices of this document, and the basic designs borrow
   directly from previously documented IETF work.  SMF relay set
   configuration is extensible, and additional relay set algorithms
   beyond those specified here can be accommodated in future work.

   Determining and maintaining an optimized set of relays generally
   requires dynamic neighborhood topology information.  Neighborhood
   topology discovery functions MAY be provided by a MANET unicast
   routing protocol or by using the MANET Neighborhood Discovery
   Protocol (NHDP) [<a href="./rfc6130" title="&quot;Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)&quot;">RFC6130</a>], operating concurrently with SMF.  This
   specification also allows alternative lower-layer interfaces (e.g.,
   radio router interfaces) to provide the necessary neighborhood
   information to aid in supporting more effective relay set selection.
   An SMF implementation SHOULD provide the ability for multicast
   forwarding state to be dynamically managed per operating network
   interface.  The relay state maintenance options and interactions are
   outlined in <a href="#section-7">Section 7</a>.  This document states specific requirements



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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   for neighborhood discovery with respect to the forwarding process and
   the relay set selection algorithms described herein.  For determining
   dynamic relay sets in the absence of other control protocols, SMF
   relies on NHDP to assist in IP-layer 2-hop neighborhood discovery and
   maintenance for relay set selection.  "SMF_TYPE" and "SMF_NBR_TYPE"
   Message and Address Block TLV structures (per [<a href="./rfc5444" title="&quot;Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format&quot;">RFC5444</a>]) are defined
   by this document for use with NHDP to carry SMF-specific information.
   It is RECOMMENDED that all routers performing SMF operation in
   conjunction with NHDP include these TLV types in any NHDP HELLO
   messages generated.  This capability allows for routers participating
   in SMF to be explicitly identified along with their respective
   dynamic relay set algorithm configuration.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  SMF Packet Processing and Forwarding</span>

   The SMF packet processing and forwarding actions are conducted with
   the following packet handling activities:

   1.  Processing of outbound, locally generated multicast packets.

   2.  Reception and processing of inbound packets on specific network
       interfaces.

   The purpose of intercepting outbound, locally generated multicast
   packets is to apply any added packet marking needed to satisfy the
   DPD requirements so that proper forwarding may be conducted.  Note
   that for some system configurations, the interception of outbound
   packets for this purpose is not necessary.

   Inbound multicast packets are received by the SMF implementation and
   processed for possible forwarding.  SMF implementations MUST be
   capable of forwarding IP multicast packets with destination addresses
   that are not router-local and link-local for IPv6, as defined in
   [<a href="./rfc4291" title="&quot;IP Version 6 Addressing Architecture&quot;">RFC4291</a>], and that are not within the local network control block as
   defined by [<a href="./rfc5771" title="&quot;IANA Guidelines for IPv4 Multicast Address Assignments&quot;">RFC5771</a>].

   This will support generic multi-hop multicast application needs or
   distribute designated multicast traffic ingressing the SMF routing
   domain via border routers.  The multicast addresses to be forwarded
   should be maintained by an a priori list or a dynamic forwarding
   information base (FIB) that MAY interact with future MANET dynamic
   group membership extensions or management functions.

   The SL-MANET-ROUTERS multicast group is defined to contain all
   routers within an SMF routing domain, so that packets transmitted to
   the multicast address associated with the group will be attempted to
   be delivered to all connected routers running SMF.  Due to the mobile
   nature of a MANET, routers running SMF may not be topologically



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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   connected at particular times.  For IPv6, SL-MANET-ROUTERS is
   specified to be "site-local".  Minimally, SMF MUST forward, as
   instructed by the relay set selection algorithm, unique (non-
   duplicate) packets received for SL-MANET-ROUTERS when the Time to
   Live (TTL) / hop limit or hop limit value in the IP header is greater
   than 1.  SMF MUST forward all additional global-scope multicast
   addresses specified within the dynamic FIB or configured list as
   well.  In all cases, the following rules MUST be observed for SMF
   multicast forwarding:

   1.  Any IP packets not addressed to an IP multicast address MUST NOT
       be forwarded by the SMF forwarding engine.

   2.  IP multicast packets with TTL/hop limit &lt;= 1 MUST NOT be
       forwarded.

   3.  Link local IP multicast packets MUST NOT be forwarded.

   4.  Incoming IP multicast packets with an IP source address matching
       one of those of the local SMF router interface(s) MUST NOT be
       forwarded.

   5.  Received frames with the Media Access Control (MAC) source
       address matching any MAC address of the router's interfaces MUST
       NOT be forwarded.

   6.  Received packets for which SMF cannot reasonably ensure temporal
       DPD uniqueness MUST NOT be forwarded.

   7.  Prior to being forwarded, the TTL/hop limit of the forwarded
       packet MUST be decremented by one.

   Note that rule #4 is important because over some types of wireless
   interfaces, the originating SMF router may receive retransmissions of
   its own packets when they are forwarded by adjacent routers.  This
   rule avoids unnecessary retransmission of locally generated packets
   even when other forwarding decision rules would apply.

   An additional processing rule also needs to be considered based upon
   a potential security threat.  As discussed in <a href="#section-10">Section 10</a>, there is a
   potential DoS attack that can be conducted by remotely "previewing"
   (e.g., via a directional receive antenna) packets that an SMF router
   would be forwarding and conducting a "pre-play" attack by
   transmitting the packet before the SMF router would otherwise receive
   it, but with a reduced TTL/hop limit field value.  This form of
   attack can cause an SMF router to create a DPD entry that would block
   the proper forwarding of the valid packet (with correct TTL/hop
   limit) through the SMF routing domain.  A RECOMMENDED approach to



<span class="grey">Macker                        Experimental                      [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   prevent this attack, when it is a concern, would be to cache temporal
   packet TTL/hop limit values along with the per-packet DPD state (hash
   value(s) and/or identifier as described in <a href="#section-6">Section 6</a>).  Then, if a
   subsequent matching (with respect to DPD) packet arrives with a
   larger TTL/hop limit value than the packet that was previously
   forwarded, SMF should forward the new packet and update the TTL/hop
   limit value cached with corresponding DPD state to the new, larger
   TTL/hop limit value.  There may be temporal cases where SMF would
   unnecessarily forward some duplicate packets using this approach, but
   those cases are expected to be minimal and acceptable when compared
   with the potential threat of denied service.

   Once the SMF multicast forwarding rules have been applied, an SMF
   implementation MUST make a forwarding decision dependent upon the
   relay set selection algorithm in use.  If the SMF implementation is
   using Classic Flooding (CF), the forwarding decision is implicit once
   DPD uniqueness is determined.  Otherwise, a forwarding decision
   depends upon the current interface-specific relay set state.  The
   descriptions of the relay set selection algorithms in the appendices
   to this document specify the respective heuristics for multicast
   packet forwarding and specific DPD or other processing required to
   achieve correct SMF behavior in each case.  For example, one class of
   forwarding is based upon relay set selection status and the packet's
   previous hop, while other classes designate the local SMF router as a
   forwarder for all neighboring routers.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  SMF Duplicate Packet Detection</span>

   Duplicate packet detection (DPD) is often a requirement in MANET or
   wireless mesh packet forwarding mechanisms because packets may be
   transmitted out via the same physical interface as the one over which
   they were received.  Routers may also receive multiple copies of the
   same packets from different neighbors or interfaces.  SMF operation
   requires DPD, and implementations MUST provide mechanisms to detect
   and reduce the likelihood of forwarding duplicate multicast packets
   using temporal packet identification.  It is RECOMMENDED this be
   implemented by keeping a history of recently processed multicast
   packets for comparison with incoming packets.  A DPD packet cache
   history SHOULD be kept long enough so as to span the maximum network
   traversal lifetime, MAX_PACKET_LIFETIME, of multicast packets being
   forwarded within an SMF routing domain.  The DPD mechanism SHOULD
   avoid keeping unnecessary state for packet flows such as those that
   are locally generated or link-local destinations that would not be
   considered for forwarding, as presented in <a href="#section-5">Section 5</a>.

   For both IPv4 and IPv6, this document describes two basic multicast
   duplicate packet detection mechanisms: header content identification-
   based (I-DPD) and hash-based (H-DPD) duplicate packet detection.



<span class="grey">Macker                        Experimental                     [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   I-DPD is a mechanism using specific packet headers, and option
   headers in the case of IPv6, in combination with flow state to
   estimate the temporal uniqueness of a packet.  H-DPD uses hashing
   over header fields and payload of a multicast packet to provide an
   estimation of temporal uniqueness.

   Trade-offs of the two approaches to DPD merit different
   considerations dependent upon the specific SMF deployment scenario.

   Because of the potential addition of a hop-by-hop option header with
   IPv6, all SMF routers in the same SMF deployments MUST be configured
   so as to use a common mechanism and DPD algorithm.  The main
   difference between IPv4 and IPv6 SMF DPD specifications is the
   avoidance of any additional header options for IPv4.

   For each network interface, SMF implementations MUST maintain DPD
   packet state as needed to support the forwarding heuristics of the
   relay set algorithm used.  In general, this involves keeping track of
   previously forwarded packets so that duplicates are not forwarded,
   but some relay techniques have additional considerations, such as
   those discussed in <a href="#appendix-B.2">Appendix B.2</a>.

   Additional details of I-DPD and H-DPD processing and maintenance for
   different classes of packets are described in the following
   subsections.

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  IPv6 Duplicate Packet Detection</span>

   This section describes the mechanisms and options for SMF IPv6 DPD.
   The base IPv6 packet header does not provide an explicit packet
   identification header field that can be exploited for I-DPD.  The
   following options are therefore described to support IPv6 DPD:

   1.  a hop-by-hop SMF_DPD option header, defined in this document
       (<a href="#section-6.1.1">Section 6.1.1</a>),

   2.  the use of IPv6 fragment header fields for I-DPD, if one is
       present (<a href="#section-6.1.2">Section 6.1.2</a>),

   3.  the use of IPsec sequencing for I-DPD when a non-fragmented,
       IPsec header is detected (<a href="#section-6.1.2">Section 6.1.2</a>), and

   4.  an H-DPD approach assisted, as needed, by the SMF_DPD option
       header (<a href="#section-6.1.3">Section 6.1.3</a>).

   SMF MUST provide a DPD marking module that can insert the hop-by-hop
   IPv6 header option, defined in <a href="#section-6.1.1">Section 6.1.1</a>.  This module MUST be
   invoked after any source-based fragmentation that may occur with



<span class="grey">Macker                        Experimental                     [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   IPv6, so as to ensure that all fragments are suitably marked.  SMF
   IPv6 DPD is presently specified to allow either a packet hash or
   header identification method for DPD.  An SMF implementation MUST be
   configured to operate either in I-DPD or H-DPD mode and perform the
   corresponding tasks, outlined in Sections <a href="#section-6.1.2">6.1.2</a> and <a href="#section-6.1.3">6.1.3</a>.

<span class="h4"><a class="selflink" id="section-6.1.1" href="#section-6.1.1">6.1.1</a>.  IPv6 SMF_DPD Option Header</span>

   This section defines an IPv6 Hop-by-Hop Option [<a href="./rfc2460" title="&quot;Internet Protocol, Version 6 (IPv6) Specification&quot;">RFC2460</a>], SMF_DPD, to
   serve the purpose of unique packet identification for IPv6 I-DPD.
   Additionally, the SMF_DPD option header provides a mechanism to
   guarantee non-collision of hash values for different packets when
   H-DPD is used.

   If this is the only hop-by-hop option present, the optional TaggerId
   field (see below) is not included, and the size of the DPD packet
   identifier (sequence number) or hash token is 24 bits or less, this
   will result in the addition of 8 bytes to the IPv6 packet header
   including the "Next Header", "Header Extension Length", SMF_DPD
   option fields, and padding.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     ...              |0|0|0|  01000  | Opt. Data Len |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |H|  DPD Identifier Option Fields or Hash Assist Value  ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 2: IPv6 SMF_DPD Hop-by-Hop Option Header

   "Option Type" = 00001000.  The highest order three bits are 000
   because this specification requires that routers not recognizing this
   option type skip over this option and continue processing the header
   and that the option must not change en route [<a href="./rfc2460" title="&quot;Internet Protocol, Version 6 (IPv6) Specification&quot;">RFC2460</a>].

   "Opt. Data Len" = Length of option content (i.e., 1 + (&lt;IdType&gt; ?
   (&lt;IdLen&gt; + 1): 0) + Length(DPD ID)).

   "H-bit" = a hash indicator bit value identifying DPD marking type. 0
   == sequence-based approach with optional TaggerId and a tuple-based
   sequence number. 1 == indicates a hash assist value (HAV) field
   follows to aid in avoiding hash-based DPD collisions.

   When the "H-bit" is cleared (zero value), the SMF_DPD format to
   support I-DPD operation is specified as shown in Figure 3 and defines
   the extension header in accordance with [<a href="./rfc2460" title="&quot;Internet Protocol, Version 6 (IPv6) Specification&quot;">RFC2460</a>].




<span class="grey">Macker                        Experimental                     [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      ...              |0|0|0|  01000  | Opt. Data Len |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0|TidTy| TidLen|             TaggerId (optional) ...           |
       +-+-+-+-+-+-+-+-+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |            Identifier  ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 3: IPv6 SMF_DPD Option Header in I-DPD mode

   "TidTy" = a 3-bit field indicating the presence and type of the
   optional TaggerId field.

   "TidLen" = a 4-bit field indicating the length (in octets) of the
   following TaggerId field.

   "TaggerId" = a field, is used to differentiate multiple ingressing
   border gateways that may commonly apply the SMF_DPD option header to
   packets from a particular source.  Table 1 lists the TaggerId types
   used in this document:

   +---------+---------------------------------------------------------+
   | Name    | Purpose                                                 |
   +---------+---------------------------------------------------------+
   | NULL    | Indicates no TaggerId field is present. "TidLen" MUST   |
   |         | also be set to ZERO.                                    |
   | DEFAULT | A TaggerId of non-specific context is present. "TidLen  |
   |         | + 1" defines the length of the TaggerId field in bytes. |
   | IPv4    | A TaggerId representing an IPv4 address is present. The |
   |         | "TidLen" MUST be set to 3.                              |
   | IPv6    | A TaggerId representing an IPv6 address is present. The |
   |         | "TidLen" MUST be set to 15.                             |
   +---------+---------------------------------------------------------+

                          Table 1: TaggerId Types

   This format allows a quick check of the "TidTy" field to determine if
   a TaggerId field is present.  If "TidTy" is NULL, then the length of
   the DPD packet &lt;Identifier&gt; field corresponds to (&lt;Opt. Data Len&gt; -
   1).  If the &lt;TidTy&gt; is non-NULL, then the length of the TaggerId
   field is equal to (&lt;TidLen&gt; - 1), and the remainder of the option
   data comprises the DPD packet &lt;Identifier&gt; field.  When the TaggerId
   field is present, the &lt;Identifier&gt; field can be considered a unique
   packet identifier in the context of the &lt;TaggerId:srcAddr:dstAddr&gt;
   tuple.  When the TaggerId field is not present, then it is assumed
   that the source applied the SMF_DPD option and the &lt;Identifier&gt; can



<span class="grey">Macker                        Experimental                     [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   be considered unique in the context of the IPv6 packet header
   &lt;srcAddr:dstAddr&gt; tuple.  IPv6 I-DPD operation details are in
   <a href="#section-6.1.2">Section 6.1.2</a>.

   When the "H-bit" in the SMF_DPD option data is set, the data content
   value is interpreted as a hash assist value (HAV) used to facilitate
   H-DPD operation.  In this case, the source or ingressing gateways
   apply the SMF_DPD with an HAV only when required to differentiate the
   hash value of a new packet with respect to hash values in the DPD
   cache.  This situation can be detected locally on the router by
   running the hash algorithm and checking the DPD cache, prior to
   ingressing a previously unmarked packet or a locally sourced packet.
   This helps to guarantee the uniqueness of generated hash values when
   H-DPD is used.  Additionally, this avoids the added overhead of
   applying the SMF_DPD option header to every packet.  For many hash
   algorithms, it is expected that only sparse use of the SMF_DPD option
   may be required.  The format of the SMF_DPD option header for H-DPD
   operation is given in Figure 4.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     ...              |0|0|0| OptType | Opt. Data Len |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|    Hash Assist Value (HAV) ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 4: IPv6 SMF_DPD Option Header in H-DPD Mode

   The SMF_DPD option should be applied with an HAV to produce a unique
   hash digest for packets within the context of the IPv6 packet header
   &lt;srcAddr&gt;.  The size of the HAV field is implied by "Opt. Data Len".
   The appropriate size of the field depends upon the collision
   properties of the specific hash algorithm used.  More details on IPv6
   H-DPD operation are provided in <a href="#section-6.1.3">Section 6.1.3</a>.

<span class="h4"><a class="selflink" id="section-6.1.2" href="#section-6.1.2">6.1.2</a>.  IPv6 Identification-Based DPD</span>

   Table 2 summarizes the IPv6 I-DPD processing and forwarding decision
   approach.  Within the table, '*' indicates an ignore field condition.











<span class="grey">Macker                        Experimental                     [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   +-------------+-----------+-----------+-----------------------------+
   | IPv6        | IPv6      | IPv6      | SMF IPv6 I-DPD Mode Action  |
   | Fragment    | IPsec     | I-DPD     |                             |
   | Header      | Header    | Header    |                             |
   +-------------+-----------+-----------+-----------------------------+
   | Present     | *         | Not       | Use Fragment Header I-DPD   |
   |             |           | Present   | Check and Process for       |
   |             |           |           | Forwarding                  |
   | Not Present | Present   | Not       | Use IPsec Header I-DPD      |
   |             |           | Present   | Check and Process for       |
   |             |           |           | Forwarding                  |
   | Present     | *         | Present   | Invalid; do not forward.    |
   | Not Present | Present   | Present   | Invalid; do not forward.    |
   | Not Present | Not       | Not       | Add I-DPD Header, and       |
   |             | Present   | Present   | Process for Forwarding      |
   | Not Present | Not       | Present   | Use I-DPD Header Check and  |
   |             | Present   |           | Process for Forwarding      |
   +-------------+-----------+-----------+-----------------------------+

                   Table 2: IPv6 I-DPD Processing Rules

   1.  If a received IPv6 multicast packet is an IPv6 fragment, SMF MUST
       use the fragment extension header fields for packet
       identification.  This identifier can be considered unique in the
       context of the &lt;srcAddr:dstAddr&gt; of the IP packet.

   2.  If the packet is an unfragmented IPv6 IPsec packet, SMF MUST use
       IPsec fields for packet identification.  The IPsec header
       &lt;sequence&gt; field can be considered a unique identifier in the
       context of the &lt;IPsecType:srcAddr:dstAddr:SPI&gt; where "IPsecType"
       is either Authentication Header (AH) or Encapsulating Security
       Payload (ESP) [<a href="./rfc4302" title="&quot;IP Authentication Header&quot;">RFC4302</a>].

   3.  For unfragmented, non-IPsec IPv6 packets, the use of the SMF_DPD
       option header is necessary to support I-DPD operation.  The
       SMF_DPD option header is applied in the context of the &lt;srcAddr&gt;
       of the IP packet.  Hosts or ingressing SMF gateways are
       responsible for applying this option to support DPD.  Table 3
       summarizes these packet identification types:












<span class="grey">Macker                        Experimental                     [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   +-----------+---------------------------------+---------------------+
   | IPv6      | Packet DPD ID Context           | Packet DPD ID       |
   | Packet    |                                 |                     |
   | Type      |                                 |                     |
   +-----------+---------------------------------+---------------------+
   | Fragment  | &lt;srcAddr:dstAddr&gt;               | &lt;fragmentOffset:id&gt; |
   | IPsec     | &lt;IPsecType:srcAddr:dstAddr:SPI&gt; | &lt;sequence&gt;          |
   | Packet    |                                 |                     |
   | Regular   | &lt;[TaggerId:]srcAddr:dstAddr&gt;    | &lt;SMF_DPD option     |
   | Packet    |                                 | header id&gt;          |
   +-----------+---------------------------------+---------------------+

              Table 3: IPv6 I-DPD Packet Identification Types

   "IPsecType" is either Authentication Header (AH) or Encapsulating
   Security Payload (ESP).

   The "TaggerId" is an optional field of the IPv6 SMF_DPD option
   header.

<span class="h4"><a class="selflink" id="section-6.1.3" href="#section-6.1.3">6.1.3</a>.  IPv6 Hash-Based DPD</span>

   A default hash-based DPD approach (H-DPD) for use by SMF is specified
   as follows.  An SHA-1 [<a href="./rfc3174" title="&quot;US Secure Hash Algorithm 1 (SHA1)&quot;">RFC3174</a>] hash of the non-mutable header
   fields, options fields, and data content of the IPv6 multicast packet
   is used to produce a 160-bit digest.  The approach for calculating
   this hash value SHOULD follow the same guidelines described for
   calculating the Integrity Check Value (ICV) described in [<a href="./rfc4302" title="&quot;IP Authentication Header&quot;">RFC4302</a>]
   with respect to non-mutable fields.  This approach should have a
   reasonably low probability of digest collision when packet headers
   and content are varying.  SHA-1 is being applied in SMF only to
   provide a low probability of collision and is not being used for
   cryptographic or authentication purposes.  A history of the packet
   hash values SHOULD be maintained within the context of the IPv6
   packet header &lt;srcAddr&gt;.  SMF ingress points (i.e., source hosts or
   gateways) use this history to confirm that new packets are unique
   with respect to their hash value.  The hash assist value (HAV) field
   described in <a href="#section-6.1.1">Section 6.1.1</a> is provided as a differentiating field
   when a digest collision would otherwise occur.  Note that the HAV is
   an immutable option field, and SMF MUST process any included HAV
   values (see <a href="#section-6.1.1">Section 6.1.1</a>) in its hash calculation.

   If a packet results in a digest collision (i.e., by checking the
   H-DPD digest history) within the DPD cache kept by SMF forwarders,
   the packet SHOULD be silently dropped.  If a digest collision is
   detected at an SMF ingress point, the H-DPD option header is
   constructed with a randomly generated HAV.  An HAV is recalculated as
   needed to produce a non-colliding hash value prior to forwarding.



<span class="grey">Macker                        Experimental                     [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   The multicast packet is then forwarded with the added IPv6 SMF_DPD
   option header.  A common hash approach MUST be used by SMF routers
   for the applied HAV to consistently avoid hash collision and thus
   inadvertent packet drops.

   The SHA-1 indexing and IPv6 HAV approaches are specified at present
   for consistency and robustness to suit experimental uses.  Future
   approaches and experimentation may discover design trade-offs in hash
   robustness and efficiency worth considering.  Enhancements MAY
   include reducing the maximum payload length that is processed,
   determining shorter indexes, or applying more efficient hashing
   algorithms.  Use of the HAV functionality may allow for application
   of "lighter-weight" hashing techniques that might not have been
   initially considered otherwise due to poor collision properties.
   Such techniques could reduce packet-processing overhead and memory
   requirements.

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  IPv4 Duplicate Packet Detection</span>

   This section describes the mechanisms and options for IPv4 DPD.  The
   following areas are described to support IPv4 DPD:

   1.  the use of IPv4 fragment header fields for I-DPD when they exist
       (<a href="#section-6.2.1">Section 6.2.1</a>),

   2.  the use of IPsec sequencing for I-DPD when a non-fragmented IPv4
       IPsec packet is detected (<a href="#section-6.2.1">Section 6.2.1</a>), and

   3.  an H-DPD approach(<a href="#section-6.2.2">Section 6.2.2</a>) when neither of the above cases
       can be applied.

   Although the IPv4 datagram has a 16-bit Identification (ID) field as
   specified in [<a href="./rfc0791" title="&quot;Internet Protocol&quot;">RFC0791</a>], it cannot be relied upon for DPD purposes due
   to common computer operating system implementation practices and the
   reasons described in the updated specification of the IPv4 ID Field
   [<a href="#ref-IPV4-ID-UPDATE">IPV4-ID-UPDATE</a>].  An SMF IPv4 DPD marking option like the IPv6
   SMF_DPD option header is not specified since IPv4 header options are
   not as tractable for hosts as they are for IPv6.  However, when IPsec
   is applied or IPv4 packets have been fragmented, the I-DPD approach
   can be applied reliably using the corresponding packet identifier
   fields described in <a href="#section-6.2.1">Section 6.2.1</a>.  For the general IPv4 case (non-
   IPsec and non-fragmented packets), the H-DPD approach of
   <a href="#section-6.2.2">Section 6.2.2</a> is RECOMMENDED.








<span class="grey">Macker                        Experimental                     [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   Since IPv4 SMF does not specify an option header, the
   interoperability constraints are looser than in the IPv6 version, and
   forwarders may operate with mixed H-DPD and I-DPD modes as long as
   they consistently perform the appropriate DPD routines outlined in
   the following sections.  However, it is RECOMMENDED that a deployment
   be configured with a common mode for operational consistency.

<span class="h4"><a class="selflink" id="section-6.2.1" href="#section-6.2.1">6.2.1</a>.  IPv4 Identification-Based DPD</span>

   Table 4 summarizes the IPv4 I-DPD processing approach once a packet
   has passed the basic forwardable criteria described in <a href="#section-5">Section 5</a>.  To
   summarize, for IPv4, I-DPD is applicable only for packets that have
   been fragmented or have IPsec applied.  In Table 4, '*' indicates an
   ignore field condition.  DF, MF, and Fragment offset correspond to
   related fields and flags defined in [<a href="./rfc0791" title="&quot;Internet Protocol&quot;">RFC0791</a>].

   +------+------+----------+---------+--------------------------------+
   | DF   | MF   | Fragment | IPsec   | IPv4 I-DPD Action              |
   | flag | flag | offset   |         |                                |
   +------+------+----------+---------+--------------------------------+
   | 1    | 1    | *        | *       | Invalid; do not forward.       |
   | 1    | 0    | nonzero  | *       | Invalid; do not forward.       |
   | *    | 0    | zero     | not     | Use H-DPD check instead        |
   |      |      |          | Present |                                |
   | *    | 0    | zero     | Present | IPsec enhanced Tuple I-DPD     |
   |      |      |          |         | Check and Process for          |
   |      |      |          |         | Forwarding                     |
   | 0    | 0    | nonzero  | *       | Extended Fragment Offset Tuple |
   |      |      |          |         | I-DPD Check and Process for    |
   |      |      |          |         | Forwarding                     |
   | 0    | 1    | zero or  | *       | Extended Fragment Offset Tuple |
   |      |      | nonzero  |         | I-DPD Check and Process for    |
   |      |      |          |         | Forwarding                     |
   +------+------+----------+---------+--------------------------------+

                   Table 4: IPv4 I-DPD Processing Rules

   For performance reasons, IPv4 network fragmentation and reassembly of
   multicast packets within wireless MANET networks should be minimized,
   yet SMF provides the forwarding of fragments when they occur.  If the
   IPv4 multicast packet is a fragment, SMF MUST use the fragmentation
   header fields for packet identification.  This identification can be
   considered temporally unique in the context of the &lt;protocol:srcAddr:
   dstAddr&gt; of the IPv4 packet.  If the packet is an unfragmented IPv4
   IPsec packet, SMF MUST use IPsec fields for packet identification.
   The IPsec header &lt;sequence&gt; field can be considered a unique





<span class="grey">Macker                        Experimental                     [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   identifier in the context of the &lt;IPsecType:srcAddr:dstAddr:SPI&gt;
   where "IPsecType" is either AH or ESP [<a href="./rfc4302" title="&quot;IP Authentication Header&quot;">RFC4302</a>].  Table 5 summarizes
   these packet identification types:

   +-----------+---------------------------------+---------------------+
   | IPv4      | Packet Identification Context   | Packet Identifier   |
   | Packet    |                                 |                     |
   | Type      |                                 |                     |
   +-----------+---------------------------------+---------------------+
   | Fragment  | &lt;protocol:srcAddr:dstAddr&gt;      | &lt;fragmentOffset:id&gt; |
   | IPsec     | &lt;IPsecType:srcAddr:dstAddr:SPI&gt; | &lt;sequence&gt;          |
   | Packet    |                                 |                     |
   +-----------+---------------------------------+---------------------+

              Table 5: IPv4 I-DPD Packet Identification Types

   "IPsecType" is either Authentication Header (AH) or Encapsulating
   Security Payload (ESP).

<span class="h4"><a class="selflink" id="section-6.2.2" href="#section-6.2.2">6.2.2</a>.  IPv4 Hash-Based DPD</span>

   The hashing technique here is similar to that specified for IPv6 in
   <a href="#section-6.1.3">Section 6.1.3</a>, but the H-DPD header option with HAV is not
   considered.  To ensure consistent IPv4 H-DPD operation among SMF
   routers, a default hashing approach is specified.  A common DPD
   hashing algorithm for an SMF routing area is RECOMMENDED because
   colliding hash values for different packets result in "false
   positive" duplicate packet detection, and there is small probability
   that valid packets may be dropped based on the hashing technique
   used.  Since the "hash assist value" approach is not available for
   IPv4, use of a common hashing approach minimizes the probability of
   hash collision packet drops over multiple hops of forwarding.

   SMF MUST perform a SHA-1 [<a href="./rfc3174" title="&quot;US Secure Hash Algorithm 1 (SHA1)&quot;">RFC3174</a>] hash of the immutable header
   fields, option fields, and data content of the IPv4 multicast packet
   resulting in a 160-bit digest.  The approach for calculating the hash
   value SHOULD follow the same guidelines described for calculating the
   Integrity Check Value (ICV) described in [<a href="./rfc4302" title="&quot;IP Authentication Header&quot;">RFC4302</a>] with respect to
   non-mutable fields.  A history of the packet hash values SHOULD be
   maintained in the context of &lt;protocol:srcAddr:dstAddr&gt;.  The context
   for IPv4 is more specific than that of IPv6 since the SMF_DPD HAV
   cannot be employed to mitigate hash collisions.  A RECOMMENDED
   implementation detail for IPv4 H-DPD is to concatenate the 16-bit
   IPv4 ID value with the computed hash value as an extended DPD hash
   value that may provide reduced hash collisions in the cases where the
   IPv4 ID field is being set by host operating systems or gateways.





<span class="grey">Macker                        Experimental                     [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   When this approach is taken, the use of the supplemental "internal
   hash" technique as described in <a href="#section-10">Section 10</a> is RECOMMENDED as a
   security measure.

   The SHA-1 hash is specified at present for consistency and
   robustness.  Future approaches and experimentation may discover
   design trade-offs in hash robustness and efficiency worth considering
   for future revisions of SMF.  This MAY include reducing the packet
   payload length that is processed, determining shorter indexes, or
   applying a more efficient hashing algorithm.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Relay Set Selection</span>

   SMF is flexible in its support of different reduced relay set
   mechanisms for efficient flooding, the constraints imposed herein
   being detailed in this section.

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  Non-Reduced Relay Set Forwarding</span>

   SMF implementations MUST support CF as a basic forwarding mechanism
   when reduced relay set information is not available or not selected
   for operation.  In CF mode, each router transmits a packet once that
   has passed the SMF forwarding rules.  The DPD techniques described in
   <a href="#section-6">Section 6</a> are critical to proper operation and prevention of
   duplicate packet retransmissions by the same relays.

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  Reduced Relay Set Forwarding</span>

   MANET reduced relay sets are often achieved by distributed algorithms
   that can dynamically calculate a topological connected dominating set
   (CDS).

   A goal of SMF is to apply reduced relay sets for more efficient
   multicast dissemination within dynamic topologies.  To accomplish
   this, an SMF implementation MUST support the ability to modify its
   multicast packet forwarding rules based upon relay set state received
   dynamically during operation.  In this way, SMF operates effectively
   as neighbor adjacencies or multicast forwarding policies within the
   topology change.

   In early SMF experimental prototyping, the relay set information was
   derived from coexistent unicast routing control plane traffic
   flooding processes [<a href="#ref-MDC04" title="&quot;Simplified Multicast Forwarding in Mobile Ad hoc Networks&quot;">MDC04</a>].  From this experience, extra pruning
   considerations were sometimes required when utilizing a relay set
   from a separate routing protocol process.  As an example, relay sets
   formed for the unicast control plane flooding MAY include additional
   redundancy that may not be desired for multicast forwarding use
   (e.g., biconnected relay set).



<span class="grey">Macker                        Experimental                     [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   Here is a recommended criteria list for SMF relay set selection
   algorithm candidates:

   1.  Robustness to topological dynamics and mobility

   2.  Localized election or coordination of any relay sets

   3.  Reasonable minimization of CDS relay set size given the above
       constraints

   4.  Heuristic support for preference or election metrics

   Some relay set algorithms meeting these criteria are described in the
   appendices of this document.  Additional relay set selection
   algorithms may be specified in separate specifications in the future.
   Each appendix subsection in this document can serve as a template for
   specifying additional relay algorithms.

   Figure 5 depicts an information flow diagram of possible relay set
   control options.  The SMF Relay Set State represents the information
   base that is used by SMF in the forwarding decision process.  The
   diagram demonstrates that the SMF Relay Set State may be determined
   by three fundamentally different methods:

   o  Independent operation with NHDP [<a href="./rfc6130" title="&quot;Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)&quot;">RFC6130</a>] input providing dynamic
      network neighborhood adjacency information, used by a particular
      relay set selection algorithm.

   o  Slave operation with an existing unicast MANET routing protocol,
      capable of providing CDS election information for use by SMF.

   o  Cross-layer operation that may involve L2 triggers or information
      describing neighbors or links.

   Other heuristics to influence and control election can come from
   network management or other interfaces as shown on the right of
   Figure 5.  CF mode simplifies the control and does not require other
   input but relies solely on DPD.













<span class="grey">Macker                        Experimental                     [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


                       Possible L2 Trigger/Information
                                      |
                                      |
    ______________              ______v_____         __________________
   |    MANET     |            |            |       |                  |
   | Neighborhood |            | Relay Set  |       | Other Heuristics |
   |  Discovery   |-----------&gt;| Selection  |&lt;------|(Preference, etc.)|
   |   Protocol   | neighbor   | Algorithm  |       |  Net Management  |
   |______________|   info     |____________|       |__________________|
          \                              /
           \                            /
    neighbor\                          / Dynamic Relay
      info*  \      ____________      /    Set Status
              \    |    SMF     |    / (State, {neighbor info})
               `--&gt;| Relay Set  |&lt;--'
                   |   State    |
                --&gt;|____________|
               /
              /
    ______________
   |  Coexistent  |
   |    MANET     |
   |   Unicast    |
   |   Process    |
   |______________|

             Figure 5: SMF Reduced Relay Set Information Flow

   Following is further discussion of the three styles of SMF operation
   with reduced relay sets as illustrated in Figure 5:

   1.  Independent operation: In this case, SMF operates independently
       from any unicast routing protocols.  To support reduced relay
       sets, SMF MUST perform its own relay set selection using
       information gathered from signaling.  It is RECOMMENDED that an
       associated NHDP process be used for this signaling.  NHDP
       messaging SHOULD be appended with additional [<a href="./rfc5444" title="&quot;Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format&quot;">RFC5444</a>] type-
       length-value (TLV) content as to support SMF-specific
       requirements as discussed in [<a href="./rfc6130" title="&quot;Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)&quot;">RFC6130</a>] and to support specific
       relay set operation as described in the appendices of this
       document or future specifications.  Unicast routing protocols may
       coexist, even using the same NHDP process, but signaling that
       supports reduced relay set selection for SMF is independent of
       these protocols.







<span class="grey">Macker                        Experimental                     [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   2.  Operation with CDS-aware unicast routing protocol: In this case,
       a coexistent unicast routing protocol provides dynamic relay set
       state based upon its own control plane CDS or neighborhood
       discovery information.

   3.  Cross-layer operation: In this case, SMF operates using
       neighborhood status and triggers from a cross-layer information
       base for dynamic relay set selection and maintenance (e.g.,
       lower-link layer).

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  SMF Neighborhood Discovery Requirements</span>

   This section defines the requirements for use of the MANET
   Neighborhood Discovery Protocol (NHDP) [<a href="./rfc6130" title="&quot;Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)&quot;">RFC6130</a>] to support SMF
   operation.  Note that basic CF forwarding requires no neighborhood
   topology knowledge since in this configured mode, every SMF router
   relays all traffic.  Supporting more reduced SMF relay set operation
   requires the discovery and maintenance of dynamic neighborhood
   topology information.  NHDP can be used to provide this necessary
   information; however, there are SMF-specific requirements for NHDP
   use.  This is the case for both "independent" SMF operation where
   NHDP is being used specifically to support SMF or when one NHDP
   instance is used for both SMF and a coexistent MANET unicast routing
   protocol.

   NHDP HELLO messages and the resultant neighborhood information base
   are described separately within the NHDP specification.  To
   summarize, NHDP provides the following basic functions:

   1.  1-hop neighbor link sensing and bidirectionality checks of
       neighbor links,

   2.  2-hop neighborhood discovery including collection of 2-hop
       neighbors and connectivity information,

   3.  Collection and maintenance of the above information across
       multiple interfaces, and

   4.  A method for signaling SMF information throughout the 2-hop
       neighborhood through the use of TLV extensions.

   Appendices A-C of this document describe CDS-based relay set
   selection algorithms that can achieve efficient SMF operation, even
   in dynamic, mobile networks and each of the algorithms has been
   initially experimented with in a working SMF prototype [<a href="#ref-MDDA07" title="&quot;Evaluation of Distributed Cover Set Algorithms in Mobile Ad hoc Network for Simplified Multicast Forwarding&quot;">MDDA07</a>].
   When using these algorithms in conjunction with NHDP, a method
   verifying neighbor SMF operation is required in order to ensure
   correct relay set selection.  NHDP, along with SMF operation



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   verification, provides the necessary information required by these
   algorithms to conduct relay set selection.  Verification of SMF
   operation may be done administratively or through the use of the SMF
   relay algorithms TLVs defined in the following subsections.  Use of
   the SMF relay algorithm TLVs is RECOMMENDED when using NHDP for SMF
   neighborhood discovery.

   <a href="#section-8.1">Section 8.1</a> specifies SMF-specific TLV types, supporting general SMF
   operation or supporting the algorithms described in the appendices.
   The appendices describing several relay set algorithms also specify
   any additional requirements for use with NHDP and reference the
   applicable TLV types as needed.

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  SMF Relay Algorithm TLV Types</span>

   This section specifies TLV types to be used within NHDP messages to
   identify the CDS relay set selection algorithm(s) in use.  Two TLV
   types are defined: one Message TLV type and one Address Block TLV
   type.

<span class="h4"><a class="selflink" id="section-8.1.1" href="#section-8.1.1">8.1.1</a>.  SMF Message TLV Type</span>

   The Message TLV type denoted SMF_TYPE is used to identify the
   existence of an SMF instance operating in conjunction with NHDP.
   This Message TLV type makes use of the extended type field as defined
   by [<a href="./rfc5444" title="&quot;Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format&quot;">RFC5444</a>] to convey the CDS relay set selection algorithm
   currently in use by the SMF message originator.  When NHDP is used to
   support SMF operation, the SMF_TYPE TLV, containing the extended type
   field with the appropriate value, SHOULD be included in NHDP_HELLO
   messages (HELLO messages as defined in [<a href="./rfc6130" title="&quot;Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)&quot;">RFC6130</a>]).  This allows SMF
   routers to learn when neighbors are configured to use NHDP for
   information exchange including algorithm type and related algorithm
   information.  This information can be used to take action, such as
   ignoring neighbor information using incompatible algorithms.  It is
   possible that SMF neighbors MAY be configured differently and still
   operate cooperatively, but these cases will vary dependent upon the
   algorithm types designated.

   This document defines a Message TLV type as specified in Table 6
   conforming to [<a href="./rfc5444" title="&quot;Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format&quot;">RFC5444</a>].  The TLV extended type field is used to
   contain the sender's "Relay Algorithm Type".  The interpretation of
   the "value" content of these TLVs is defined per "Relay Algorithm
   Type" and may contain algorithm-specific information.








<span class="grey">Macker                        Experimental                     [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


          +---------------+----------------+--------------------+
          |               | TLV Syntax     | Field Values       |
          +---------------+----------------+--------------------+
          | type          | &lt;tlv-type&gt;     | SMF_TYPE           |
          | extended type | &lt;tlv-type-ext&gt; | &lt;relayAlgorithmId&gt; |
          | length        | &lt;length&gt;       | variable           |
          | value         | &lt;value&gt;        | variable           |
          +---------------+----------------+--------------------+

                       Table 6: SMF Type Message TLV

   In Table 6, &lt;relayAlgorithmId&gt; is an 8-bit field containing a number
   0-255 representing the "Relay Algorithm Type" of the originator
   address of the corresponding NHDP message.

   Values for the &lt;relayAlgorithmId&gt; are defined in Table 7.  The table
   provides value assignments, future IANA assignment spaces, and an
   experimental space.  The experimental space use MUST NOT assume
   uniqueness; thus, it SHOULD NOT be used for general interoperable
   deployment prior to official IANA assignment.

   +-------------+--------------------+--------------------------------+
   |  Type Value |    Extended Type   |            Algorithm           |
   |             |        Value       |                                |
   +-------------+--------------------+--------------------------------+
   |   SMF_TYPE  |          0         |               CF               |
   |   SMF_TYPE  |          1         |              S-MPR             |
   |   SMF_TYPE  |          2         |              E-CDS             |
   |   SMF_TYPE  |          3         |             MPR-CDS            |
   |   SMF_TYPE  |        4-127       |  Future Assignment STD action  |
   |   SMF_TYPE  |       128-239      |     No STD action required     |
   |   SMF_TYPE  |       240-255      |       Experimental Space       |
   +-------------+--------------------+--------------------------------+

                 Table 7: SMF Relay Algorithm Type Values

   Acceptable &lt;length&gt; and &lt;value&gt; fields of an SMF_TYPE TLV are
   dependent on the extended type value (i.e., relay algorithm type).
   The appropriate algorithm type, as conveyed in the &lt;tlv-type-ext&gt;
   field, defines the meaning and format of its TLV &lt;value&gt; field.  For
   the algorithms defined by this document, see the appropriate appendix
   for the &lt;value&gt; field format.

<span class="h4"><a class="selflink" id="section-8.1.2" href="#section-8.1.2">8.1.2</a>.  SMF Address Block TLV Type</span>

   An Address Block TLV type, denoted SMF_NBR_TYPE (i.e., SMF neighbor
   relay algorithm) is specified in Table 8.  This TLV enables CDS relay
   algorithm operation and configuration to be shared among 2-hop



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   neighborhoods.  Some relay algorithms require 2-hop neighbor
   configuration in order to correctly select relay sets.  It is also
   useful when mixed relay algorithm operation is possible.  Some
   examples of mixed use are outlined in the appendices.

   The message SMF_TYPE TLV and Address Block SMF_NBR_TYPE TLV types
   share a common format.

          +---------------+----------------+--------------------+
          |               | TLV syntax     | Field Values       |
          +---------------+----------------+--------------------+
          | type          | &lt;tlv-type&gt;     | SMF_NBR_TYPE       |
          | extended type | &lt;tlv-type-ext&gt; | &lt;relayAlgorithmId&gt; |
          | length        | &lt;length&gt;       | variable           |
          | value         | &lt;value&gt;        | variable           |
          +---------------+----------------+--------------------+

                    Table 8: SMF Type Address Block TLV

   &lt;relayAlgorithmId&gt; in Table 8 is an 8-bit unsigned integer field
   containing a number 0-255 representing the "Relay Algorithm Type"
   value that corresponds to any associated address in the address
   block.  Note that "Relay Algorithm Type" values for 2-hop neighbors
   can be conveyed in a single TLV or multiple value TLVs as described
   in [<a href="./rfc5444" title="&quot;Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format&quot;">RFC5444</a>].  It is expected that SMF routers using NHDP construct
   address blocks with SMF_NBR_TYPE TLVs to advertise "Relay Algorithm
   Type" and to advertise neighbor algorithm values received in SMF_TYPE
   TLVs from those neighbors.

   Again, values for the &lt;relayAlgorithmId&gt; are defined in Table 7.

   The interpretation of the "value" field of SMF_NBR_TYPE TLVs is
   defined per "Relay Algorithm Type" and may contain algorithm-specific
   information.  See the appropriate appendix for definitions of value
   fields for the algorithms defined by this document.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  SMF Border Gateway Considerations</span>

   It is expected that SMF will be used to provide simple forwarding of
   multicast traffic within a MANET or mesh routing topology.  A border
   router gateway approach should be used to allow interconnection of
   SMF routing domains with networks using other multicast routing
   protocols, such as PIM.  It is important to note that there are many
   scenario-specific issues that should be addressed when discussing
   border multicast routers.  At the present time, experimental
   deployments of SMF and PIM border router approaches have been
   demonstrated [<a href="#ref-DHS08" title="&quot;MANET Multicast with Multiple Gateways&quot;">DHS08</a>].  Some of the functionality border routers may
   need to address includes the following:



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   1.  Determination of which multicast group traffic transits the
       border router whether entering or exiting the attached SMF
       routing domain.

   2.  Enforcement of TTL/hop limit threshold or other scoping policies.

   3.  Any marking or labeling to enable DPD on ingressing packets.

   4.  Interface with exterior multicast routing protocols.

   5.  Possible operation with multiple border routers (presently beyond
       the scope of this document).

   6.  Provisions for participating non-SMF devices (routers or hosts).

   Each of these areas is discussed in more detail in the following
   subsections.  Note the behavior of SMF border routers is the same as
   that of non-border SMF routers when forwarding packets on interfaces
   within the SMF routing domain.  Packets that are passed outbound to
   interfaces operating fixed-infrastructure multicast routing protocols
   SHOULD be evaluated for duplicate packet status since present
   standard multicast forwarding mechanisms do not usually perform this
   function.

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Forwarded Multicast Groups</span>

   Mechanisms for dynamically determining groups for forwarding into a
   MANET SMF routing domain is an evolving technology area.  Ideally,
   only traffic for which there is active group membership should be
   injected into the SMF domain.  This can be accomplished by providing
   an IPv4 Internet Group Membership Protocol (IGMP) or IPv6 Multicast
   Listener Discovery (MLD) proxy protocol so that MANET SMF routers can
   inform attached border routers (and hence multicast networks) of
   their current group membership status.  For specific systems and
   services, it may be possible to statically configure group membership
   joins in border routers, but it is RECOMMENDED that some form of
   IGMP/MLD proxy or other explicit, dynamic control of membership be
   provided.  Specification of such an IGMP/MLD proxy protocol is beyond
   the scope of this document.

   For outbound traffic, SMF border routers perform duplicate packet
   detection and forward non-duplicate traffic that meets TTL/hop limit
   and scoping criteria to interfaces external to the SMF routing
   domain.  Appropriate IP multicast routing (e.g., PIM-based solutions)
   on those interfaces can make further forwarding decisions with
   respect to the multicast packet.  Note that the presence of multiple





<span class="grey">Macker                        Experimental                     [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   border routers associated with a MANET routing domain raises
   additional issues.  This is further discussed in <a href="#section-9.4">Section 9.4</a> but
   further work is expected to be needed here.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  Multicast Group Scoping</span>

   Multicast scoping is used by network administrators to control the
   network routing domains reachable by multicast packets.  This is
   usually done by configuring external interfaces of border routers in
   the border of a routing domain to not forward multicast packets that
   must be kept within the SMF routing domain.  This is commonly done
   based on TTL/hop limit of messages or by using administratively
   scoped group addresses.  These schemes are known respectively as:

   1.  TTL scoping.

   2.  Administrative scoping.

   For IPv4, network administrators can configure border routers with
   the appropriate TTL/hop limit thresholds or administratively scoped
   multicast groups for the router interfaces as with any traditional
   multicast router.  However, for the case of TTL/hop limit scoping, it
   SHOULD be taken into account that the packet could traverse multiple
   hops within the MANET SMF routing domain before reaching the border
   router.  Thus, TTL thresholds SHOULD be selected carefully.

   For IPv6, multicast address spaces include information about the
   scope of the group.  Thus, border routers of an SMF routing domain
   know if they must forward a packet based on the IPv6 multicast group
   address.  For the case of IPv6, it is RECOMMENDED that a MANET SMF
   routing domain be designated a site-scoped multicast domain.  Thus,
   all IPv6 site-scoped multicast packets in the range FF05::/16 SHOULD
   be kept within the MANET SMF routing domain by border routers.  IPv6
   packets in any other wider range scopes (i.e., FF08::/16, FF0B::/16,
   and FF0E::16) MAY traverse border routers unless other restrictions
   different from the scope applies.

   Given that scoping of multicast packets is performed at the border
   routers and given that existing scoping mechanisms are not designed
   to work with mobile routers, it is assumed that non-border routers
   running SMF will not stop forwarding multicast data packets of an
   appropriate site scoping.  That is, it is assumed that an SMF routing
   domain is a site-scoped multicast area.








<span class="grey">Macker                        Experimental                     [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


<span class="h3"><a class="selflink" id="section-9.3" href="#section-9.3">9.3</a>.  Interface with Exterior Multicast Routing Protocols</span>

   The traditional operation of multicast routing protocols is tightly
   integrated with the group membership function.  Leaf routers are
   configured to periodically gather group membership information, while
   intermediate routers conspire to create multicast trees connecting
   routers with directly connected multicast sources and routers with
   active multicast receivers.  In the concrete case of SMF, border
   routers can be considered leaf routers.  Mechanisms for multicast
   sources and receivers to interoperate with border routers over the
   multi-hop MANET SMF routing domain as if they were directly connected
   to the router need to be defined.  The following issues need to be
   addressed:

   1.  A mechanism by which border routers gather membership information

   2.  A mechanism by which multicast sources are known by the border
       router

   3.  A mechanism for exchange of exterior routing protocol messages
       across the SMF routing domain if the SMF routing domain is to
       provide transit connectivity for multicast traffic.

   It is beyond the scope of this document to address implementation
   solutions to these issues.  As described in <a href="#section-9.1">Section 9.1</a>, IGMP/MLD
   proxy mechanisms can address some of these issues.  Similarly,
   exterior routing protocol messages could be tunneled or conveyed
   across an SMF routing domain but doing this robustly in a distributed
   wireless environment likely requires additional considerations
   outside the scope of this document.

   The need for the border router to receive traffic from recognized
   multicast sources within the SMF routing domain is important to
   achieve interoperability with some existing routing protocols.  For
   instance, PIM-S requires routers with locally attached multicast
   sources to register them to the Rendezvous Point (RP) so that routers
   can join the multicast tree.  In addition, if those sources are not
   advertised to other autonomous systems (ASes) using Multicast Source
   Discovery Protocol (MSDP), receivers in those external networks are
   not able to join the multicast tree for that source.

<span class="h3"><a class="selflink" id="section-9.4" href="#section-9.4">9.4</a>.  Multiple Border Routers</span>

   An SMF routing domain might be deployed with multiple participating
   routers having connectivity to external, fixed-infrastructure
   networks.  Allowing multiple routers to forward multicast traffic to/
   from the SMF routing domain can be beneficial since it can increase
   reliability and provide better service.  For example, if the SMF



<span class="grey">Macker                        Experimental                     [Page 29]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   routing domain were to fragment with different SMF routers
   maintaining connectivity to different border routers, multicast
   service could still continue successfully.  But, the case of multiple
   border routers connecting an SMF routing domain to external networks
   presents several challenges for SMF:

   1.  Handling duplicate unmarked IPv4 or IPv6 (without IPsec
       encapsulation or DPD option) packets possibly injected by
       multiple border routers.

   2.  Handling of duplicate traffic injected by multiple border routers
       by source-based relay algorithms.

   3.  Determining which border router(s) will forward outbound
       multicast traffic.

   4.  Additional challenges with interfaces to exterior multicast
       routing protocols.

   When multiple border routers are present, they may be alternatively
   (due to route changes) or simultaneously injecting common traffic
   into the SMF routing domain that has not been previously marked for
   IPv6 SMF_DPD.  Different border routers would not be able to
   implicitly synchronize sequencing of injected traffic since they may
   not receive exactly the same messages due to packet losses.  For IPv6
   I-DPD operation, the optional TaggerId field described for the
   SMF_DPD option header can be used to mitigate this issue.  When
   multiple border routers are injecting a flow into an SMF routing
   domain, there are two forwarding policies that SMF routers running
   I-DPD may implement:

   1.  Redundantly forward the multicast flows (identified by &lt;srcAddr:
       dstAddr&gt;) from each border router, performing DPD processing on a
       &lt;TaggerID:dstAddr&gt; or &lt;TaggerID:srcAddr:dstAddr&gt; basis, or

   2.  Use some basis to select the flow of one tagger (border router)
       over the others and forward packets for applicable flows
       (identified by &lt;sourceAddress:dstAddr&gt;) only for the selected
       TaggerId until timeout or some other criteria to favor another
       tagger occurs.

   It is RECOMMENDED that the first approach be used in the case of
   I-DPD operation.  Additional specification may be required to
   describe an interoperable forwarding policy based on this second
   option.  Note that the implementation of the second option requires
   that per-flow (i.e., &lt;srcAddr::dstAddr&gt;) state be maintained for the
   selected TaggerId.




<span class="grey">Macker                        Experimental                     [Page 30]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   The deployment of H-DPD operation may alleviate DPD resolution when
   ingressing traffic comes from multiple border routers.  Non-colliding
   hash indexes (those not requiring the H-DPD options header in IPv6)
   should be resolved effectively.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Security Considerations</span>

   Gratuitous use of option headers can cause problems in routers.
   Other IP routers external to an SMF routing domain that might receive
   forwarded multicast SHOULD ignore SMF-specific IPv6 header options
   when encountered.  The header option types are encoded appropriately
   to allow for this behavior.

   This section briefly discusses several SMF denial-of-service (DoS)
   attack scenarios and provides some initial recommended mitigation
   strategies.

   A potential denial-of-service attack against SMF forwarding is
   possible when a malicious router has a form of wormhole access to
   non-adjacent parts of a network topology.  In the wireless ad hoc
   case, a directional antenna is one way to provide such a wormhole
   physically.  If such a router can preview forwarded packets in a non-
   adjacent part of the network and forward modified versions to another
   part of the network, it can perform the following attack.  The
   malicious router could reduce the TTL/hop limit or hop limit of the
   packet and transmit it to the SMF router causing it to forward the
   packet with a limited TTL/hop limit (or even drop it) and make a DPD
   entry that could block or limit the subsequent forwarding of later-
   arriving valid packets with correct TTL/hop limit values.  This would
   be a relatively low-cost, high-payoff attack that would be hard to
   detect and thus attractive to potential attackers.  An approach of
   caching TTL/hop limit information with DPD state and taking
   appropriate forwarding actions is identified in <a href="#section-5">Section 5</a> to mitigate
   this form of attack.

   Sequence-based packet identifiers are predictable and thus provide an
   opportunity for a DoS attack against forwarding.  Forwarding
   protocols that use DPD techniques, such as SMF, may be vulnerable to
   DoS attacks based on spoofing packets with apparently valid packet
   identifier fields.  In wireless environments, where SMF will most
   likely be used, the opportunity for such attacks may be more
   prevalent than in wired networks.  In the case of IPv4 packets,
   fragmented IP packets, or packets with IPsec headers applied, the DPD
   "identifier portions" of potential future packets that might be
   forwarded is highly predictable and easily subject to DoS attacks
   against forwarding.  A RECOMMENDED technique to counter this concern
   is for SMF implementations to generate an "internal" hash value that
   is concatenated with the explicit I-DPD packet identifier to form a



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-32" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   unique identifier that is a function of the packet content as well as
   the visible identifier.  SMF implementations could seed their hash
   generation with a random value to make it unlikely that an external
   observer could guess how to spoof packets used in a denial-of-service
   attack against forwarding.  Since the hash computation and state is
   kept completely internal to SMF routers, the cryptographic properties
   of this hashing would not need to be extensive and thus possibly of
   low complexity.  Experimental implementations may determine that even
   a lightweight hash of only portions of packets may suffice to serve
   this purpose.

   While H-DPD is not as readily susceptible to this form of DoS attack,
   it is possible that a sophisticated adversary could use side
   information to construct spoofing packets to mislead forwarders using
   a well-known hash algorithm.  Thus, similarly, a separate "internal"
   hash value could be concatenated with the well-known hash value to
   alleviate this security concern.

   The support of forwarding IPsec packets without further modification
   for both IPv4 and IPv6 is supported by this specification.

   Authentication mechanisms to identify the source of IPv6 option
   headers should be considered to reduce vulnerability to a variety of
   attacks.

   Furthermore, when the MANET Neighborhood Discovery Protocol [<a href="./rfc6130" title="&quot;Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)&quot;">RFC6130</a>]
   is used, the security considerations described in [<a href="./rfc6130" title="&quot;Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)&quot;">RFC6130</a>] also
   apply.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  IANA Considerations</span>

   This document defines one IPv6 Hop-by-Hop Option, a type for which
   has been allocated from the IPv6 "Destination Options and Hop-by-Hop
   Options" registry of [<a href="./rfc2780" title="&quot;IANA Allocation Guidelines For Values In the Internet Protocol and Related Headers&quot;">RFC2780</a>].

   This document creates one registry called "TaggerId Types" for
   recording TaggerId types, (TidTy), as a sub-registry in the "IPv6
   Parameters" registry.

   This document registers one well-known multicast address from each of
   the IPv4 and IPv6 multicast address spaces.

   This document defines one Message TLV, a type for which has been
   allocated from the "Message TLV Types" registry of [<a href="./rfc5444" title="&quot;Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format&quot;">RFC5444</a>].

   Finally, this document defines one Address Block TLV, a type for
   which has been allocated from the "Address Block TLV Types" registry
   of [<a href="./rfc5444" title="&quot;Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format&quot;">RFC5444</a>].



<span class="grey">Macker                        Experimental                     [Page 32]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-33" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


<span class="h3"><a class="selflink" id="section-11.1" href="#section-11.1">11.1</a>.  IPv6 SMF_DPD Header Extension Option Type</span>

   IANA has allocated an IPv6 Option Type from the IPv6 "Destination
   Options and Hop-by-Hop Options" registry of [<a href="./rfc2780" title="&quot;IANA Allocation Guidelines For Values In the Internet Protocol and Related Headers&quot;">RFC2780</a>], as specified
   in Table 9.

   +-----------+-------------------------+-------------+---------------+
   | Hex Value |       Binary Value      | Description | Reference     |
   |           |    act | chg | rest     |             |               |
   +-----------+-------------------------+-------------+---------------+
   |     8     |     00 |  0  | 01000    | SMF_DPD     | This Document |
   +-----------+-------------------------+-------------+---------------+

                   Table 9: IPv6 Option Type Allocation

<span class="h3"><a class="selflink" id="section-11.2" href="#section-11.2">11.2</a>.  TaggerId Types (TidTy)</span>

   A portion of the option data content in the SMF_DPD is the Tagger
   Identifier Type (TidTy), which provides a context for the optionally
   included TaggerId.

   IANA has created a registry for recording TaggerId Types (TidTy),
   with initial assignments and allocation policies, as specified in
   Table 10.

   +------+----------+------------------------------------+------------+
   | Type | Mnemonic | Description                        | Reference  |
   +------+----------+------------------------------------+------------+
   |   0  |   NULL   | No TaggerId field is present       | This       |
   |      |          |                                    | document   |
   |   1  |  DEFAULT | A TaggerId of non-specific context | This       |
   |      |          | is present                         | document   |
   |   2  |   IPv4   | A TaggerId representing an IPv4    | This       |
   |      |          | address is present                 | document   |
   |   3  |   IPv6   | A TaggerId representing an IPv6    | This       |
   |      |          | address is present                 | document   |
   |  4-7 |          | Unassigned                         |            |
   +------+----------+------------------------------------+------------+

                         Table 10: TaggerId Types

   For allocation of unassigned values 4-7, IETF Review [<a href="./rfc5226" title="">RFC5226</a>] is
   required.








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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-34" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


<span class="h3"><a class="selflink" id="section-11.3" href="#section-11.3">11.3</a>.  Well-Known Multicast Address</span>

   IANA has allocated an IPv4 multicast address "SL-MANET-ROUTERS"
   (224.0.1.186) from the "Internetwork Control Block (224.0.1.0-
   224.0.1.255 (224.0.1/24))" sub-registry of the "IPv4 Multicast
   Address" registry.

   IANA has allocated an IPv6 multicast address "SL-MANET-ROUTERS" from
   the "Site-Local Scope Multicast Addresses" sub-sub-registry of the
   "Fixed Scope Multicast Addresses" sub-registry of the "INTERNET
   PROTOCOL VERSION 6 MULTICAST ADDRESSES" registry.

<span class="h3"><a class="selflink" id="section-11.4" href="#section-11.4">11.4</a>.  SMF TLVs</span>

<span class="h4"><a class="selflink" id="section-11.4.1" href="#section-11.4.1">11.4.1</a>.  Expert Review for Created Type Extension Registries</span>

   Creation of Address Block TLV Types and Message TLV Types in
   registries of [<a href="./rfc5444" title="&quot;Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format&quot;">RFC5444</a>], and hence in the HELLO-message-specific
   registries of [<a href="./rfc6130" title="&quot;Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)&quot;">RFC6130</a>], entails creation of corresponding Type
   Extension registries for each such type.  For such Type Extension
   registries, where an Expert Review is required, the designated expert
   SHOULD take the same general recommendations into consideration as
   those specified by [<a href="./rfc5444" title="&quot;Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format&quot;">RFC5444</a>].

<span class="h4"><a class="selflink" id="section-11.4.2" href="#section-11.4.2">11.4.2</a>.  SMF Message TLV Type (SMF_TYPE)</span>

   This document defines one Message TLV Type, "SMF_TYPE", which has
   been allocated from the "HELLO Message-Type-specific Message TLV
   Types" registry, defined in [<a href="./rfc6130" title="&quot;Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)&quot;">RFC6130</a>].

   This created a new Type Extension registry, with initial assignments
   as specified in Table 11.

   +----------+------+-----------+--------------------+----------------+
   |   Name   | Type |    Type   | Description        | Allocation     |
   |          |      | Extension |                    | Policy         |
   +----------+------+-----------+--------------------+----------------+
   | SMF_TYPE |  128 |   0-255   | Specifies relay    | <a href="#section-11.4.4">Section 11.4.4</a> |
   |          |      |           | algorithm          |                |
   |          |      |           | supported by the   |                |
   |          |      |           | SMF router,        |                |
   |          |      |           | originating the    |                |
   |          |      |           | HELLO message,     |                |
   |          |      |           | according to       |                |
   |          |      |           | <a href="#section-11.4.4">Section 11.4.4</a>.    |                |
   +----------+------+-----------+--------------------+----------------+

          Table 11: SMF_TYPE Message TLV Type Extension Registry



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-35" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


<span class="h4"><a class="selflink" id="section-11.4.3" href="#section-11.4.3">11.4.3</a>.  SMF Address Block TLV Type (SMF_NBR_TYPE)</span>

   This document defines one Address Block TLV Type, "SMF_NBR_TYPE",
   which has been allocated from the "HELLO Message-Type-specific
   Address Block TLV Types" registry, defined in [<a href="./rfc6130" title="&quot;Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)&quot;">RFC6130</a>].

   This has created a new Type Extension registry, with initial
   assignments as specified in Table 12.

   +--------------+--------+-----------+-----------------+-------------+
   |     Name     |  Type  |    Type   | Description     | Allocation  |
   |              |        | Extension |                 | Policy      |
   +--------------+--------+-----------+-----------------+-------------+
   | SMF_NBR_TYPE |   128  |   0-255   | Specifies relay | Section     |
   |              |        |           | algorithm       | 11.4.4      |
   |              |        |           | supported by    |             |
   |              |        |           | the SMF router  |             |
   |              |        |           | corresponding   |             |
   |              |        |           | to the          |             |
   |              |        |           | advertised      |             |
   |              |        |           | address,        |             |
   |              |        |           | according to    |             |
   |              |        |           | <a href="#section-11.4.4">Section 11.4.4</a>. |             |
   +--------------+--------+-----------+-----------------+-------------+

     Table 12: SMF_NBR_TYPE Address Block TLV Type Extension Registry

<span class="h4"><a class="selflink" id="section-11.4.4" href="#section-11.4.4">11.4.4</a>.  SMF Relay Algorithm ID Registry</span>

   Types for the Type Extension Registries for the SMF_TYPE Message TLV
   and the SMF_NBR_TYPE Address Block TLV are unified in this single SMF
   Relay Algorithm ID Registry, defined in this section.

   IANA has created a registry for recording Relay Algorithm
   Identifiers, with initial assignments and allocation policies as
   specified in Table 13.















<span class="grey">Macker                        Experimental                     [Page 35]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-36" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


          +---------+---------+-------------+-------------------+
          | Value   | Name    | Description | Allocation Policy |
          +---------+---------+-------------+-------------------+
          | 0       | CF      | <a href="#section-4">Section 4</a>   |                   |
          | 1       | S-MPR   | <a href="#appendix-B">Appendix B</a>  |                   |
          | 2       | E-CDS   | <a href="#appendix-A">Appendix A</a>  |                   |
          | 3       | MPR-CDS | <a href="#appendix-C">Appendix C</a>  |                   |
          | 4-127   |         | Unassigned  | Expert Review     |
          | 128-255 |         | Unassigned  | Experimental Use  |
          +---------+---------+-------------+-------------------+

                 Table 13: Relay Set Algorithm Type Values

   A specification requesting an allocation from the 4-127 range from
   the SMF Relay Algorithm ID Registry MUST specify the interpretation
   of the &lt;value&gt; field (if any).

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  Acknowledgments</span>

   Many of the concepts and mechanisms used and adopted by SMF resulted
   over several years of discussion and related work within the MANET
   working group since the late 1990s.  There are obviously many
   contributors to past discussions and related draft documents within
   the working group that have influenced the development of SMF
   concepts, and they deserve acknowledgment.  In particular, this
   document is largely a direct product of the earlier SMF design team
   within the IETF MANET working group and borrows text and
   implementation ideas from the related individuals and activities.
   Some of the direct contributors who have been involved in design,
   content editing, prototype implementation, major commenting, and core
   discussions are listed below in alphabetical order.  We appreciate
   all the input and feedback from the many community members and early
   implementation users we have heard from that are not on this list as
   well.

      Brian Adamson
      Teco Boot
      Ian Chakeres
      Thomas Clausen
      Justin Dean
      Brian Haberman
      Ulrich Herberg
      Charles Perkins
      Pedro Ruiz
      Fred Templin
      Maoyu Wang





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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-37" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


<span class="h2"><a class="selflink" id="section-13" href="#section-13">13</a>.  References</span>

<span class="h3"><a class="selflink" id="section-13.1" href="#section-13.1">13.1</a>.  Normative References</span>

   [<a id="ref-MPR-CDS">MPR-CDS</a>]  Adjih, C., Jacquet, P., and L. Viennot, "Computing
              Connected Dominating Sets with Multipoint Relays", Ad Hoc
              and Sensor Wireless Networks, January 2005.

   [<a id="ref-RFC0791">RFC0791</a>]  Postel, J., "Internet Protocol", STD 5, <a href="./rfc791">RFC 791</a>,
              September 1981.

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-RFC2460">RFC2460</a>]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", <a href="./rfc2460">RFC 2460</a>, December 1998.

   [<a id="ref-RFC2644">RFC2644</a>]  Senie, D., "Changing the Default for Directed Broadcasts
              in Routers", <a href="https://www.rfc-editor.org/bcp/bcp34">BCP 34</a>, <a href="./rfc2644">RFC 2644</a>, August 1999.

   [<a id="ref-RFC2780">RFC2780</a>]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
              Values In the Internet Protocol and Related Headers",
              <a href="https://www.rfc-editor.org/bcp/bcp37">BCP 37</a>, <a href="./rfc2780">RFC 2780</a>, March 2000.

   [<a id="ref-RFC3174">RFC3174</a>]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", <a href="./rfc3174">RFC 3174</a>, September 2001.

   [<a id="ref-RFC3626">RFC3626</a>]  Clausen, T. and P. Jacquet, "Optimized Link State Routing
              Protocol (OLSR)", <a href="./rfc3626">RFC 3626</a>, October 2003.

   [<a id="ref-RFC4291">RFC4291</a>]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", <a href="./rfc4291">RFC 4291</a>, February 2006.

   [<a id="ref-RFC4302">RFC4302</a>]  Kent, S., "IP Authentication Header", <a href="./rfc4302">RFC 4302</a>,
              December 2005.

   [<a id="ref-RFC5226">RFC5226</a>]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc5226">RFC 5226</a>,
              May 2008.

   [<a id="ref-RFC5444">RFC5444</a>]  Clausen, T., Dearlove, C., Dean, J., and C. Adjih,
              "Generalized Mobile Ad Hoc Network (MANET) Packet/Message
              Format", <a href="./rfc5444">RFC 5444</a>, February 2009.

   [<a id="ref-RFC5614">RFC5614</a>]  Ogier, R. and P. Spagnolo, "Mobile Ad Hoc Network (MANET)
              Extension of OSPF Using Connected Dominating Set (CDS)
              Flooding", <a href="./rfc5614">RFC 5614</a>, August 2009.




<span class="grey">Macker                        Experimental                     [Page 37]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-38" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   [<a id="ref-RFC5771">RFC5771</a>]  Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
              IPv4 Multicast Address Assignments", <a href="https://www.rfc-editor.org/bcp/bcp51">BCP 51</a>, <a href="./rfc5771">RFC 5771</a>,
              March 2010.

   [<a id="ref-RFC6130">RFC6130</a>]  Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc
              Network (MANET) Neighborhood Discovery Protocol (NHDP)",
              <a href="./rfc6130">RFC 6130</a>, April 2011.

<span class="h3"><a class="selflink" id="section-13.2" href="#section-13.2">13.2</a>.  Informative References</span>

   [<a id="ref-CDHM07">CDHM07</a>]   Chakeres, I., Danilov, C., Henderson, T., and J. Macker,
              "Connecting MANET Multicast", IEEE MILCOM
              2007 Proceedings, 2007.

   [<a id="ref-DHG09">DHG09</a>]    Danilov, C., Henderson, T., Goff, T., Kim, J., Macker, J.,
              Weston, J., Neogi, N., Ortiz, A., and D. Uhlig,
              "Experiment and field demonstration of a 802.11-based
              ground-UAV mobile ad-hoc network", Proceedings of the 28th
              IEEE conference on Military Communications, 2009.

   [<a id="ref-DHS08">DHS08</a>]    Danilov, C., Henderson, T., Spagnolo, T., Goff, T., and J.
              Kim, "MANET Multicast with Multiple Gateways", IEEE MILCOM
              2008 Proceedings, 2008.

   [<a id="ref-GM99">GM99</a>]     Garcia-Luna-Aceves, JJ. and E. Madruga, "The Core-Assisted
              Mesh Protocol", Selected Areas in Communications, IEEE
              Journal,  Volume 17, Issue 8, August 1999.

   [<a id="ref-IPV4-ID-UPDATE">IPV4-ID-UPDATE</a>]
              Touch, J., <a style="text-decoration: none" href='https://www.google.com/search?sitesearch=datatracker.ietf.org%2Fdoc%2Fhtml%2F&amp;q=inurl:draft-+%22Updated+Specification+of+the+IPv4+ID+Field%22'>"Updated Specification of the IPv4 ID Field"</a>,
              Work in Progress, September 2011.

   [<a id="ref-JLMV02">JLMV02</a>]   Jacquet, P., Laouiti, V., Minet, P., and L. Viennot,
              "Performance of Multipoint Relaying in Ad Hoc Mobile
              Routing Protocols", Networking , 2002.

   [<a id="ref-MDC04">MDC04</a>]    Macker, J., Dean, J., and W. Chao, "Simplified Multicast
              Forwarding in Mobile Ad hoc Networks", IEEE MILCOM 2004
              Proceedings, 2004.

   [<a id="ref-MDDA07">MDDA07</a>]   Macker, J., Downard, I., Dean, J., and R. Adamson,
              "Evaluation of Distributed Cover Set Algorithms in Mobile
              Ad hoc Network for Simplified Multicast Forwarding", ACM
              SIGMOBILE Mobile Computing and Communications
              Review, Volume 11, Issue 3, July 2007.






<span class="grey">Macker                        Experimental                     [Page 38]</span></pre>
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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   [<a id="ref-MGL04">MGL04</a>]    Mohapatra, P., Gui, C., and J. Li, "Group Communications
              in Mobile Ad hoc Networks", IEEE Computer, Vol. 37, No. 2,
              February 2004.

   [<a id="ref-NTSC99">NTSC99</a>]   Ni, S., Tseng, Y., Chen, Y., and J. Sheu, "The Broadcast
              Storm Problem in a Mobile Ad Hoc Network", Proceedings of
              ACM Mobicom 99, 1999.

   [<a id="ref-RFC2501">RFC2501</a>]  Corson, M. and J. Macker, "Mobile Ad hoc Networking
              (MANET): Routing Protocol Performance Issues and
              Evaluation Considerations", <a href="./rfc2501">RFC 2501</a>, January 1999.

   [<a id="ref-RFC3684">RFC3684</a>]  Ogier, R., Templin, F., and M. Lewis, "Topology
              Dissemination Based on Reverse-Path Forwarding (TBRPF)",
              <a href="./rfc3684">RFC 3684</a>, February 2004.

   [<a id="ref-RFC3973">RFC3973</a>]  Adams, A., Nicholas, J., and W. Siadak, "Protocol
              Independent Multicast - Dense Mode (PIM-DM): Protocol
              Specification (Revised)", <a href="./rfc3973">RFC 3973</a>, January 2005.

   [<a id="ref-RFC4601">RFC4601</a>]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", <a href="./rfc4601">RFC 4601</a>, August 2006.




























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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Essential Connecting Dominating Set (E-CDS) Algorithm</span>

   The "Essential Connected Dominating Set" (E-CDS) algorithm [<a href="./rfc5614" title="&quot;Mobile Ad Hoc Network (MANET) Extension of OSPF Using Connected Dominating Set (CDS) Flooding&quot;">RFC5614</a>]
   forms a single CDS mesh for the SMF operating region.  It allows
   routers to use 2-hop neighborhood topology information to dynamically
   perform relay self-election to form a CDS.  Its packet-forwarding
   rules are not dependent upon previous hop knowledge.  Additionally,
   E-CDS SMF forwarders can be easily mixed without problems with CF SMF
   forwarders, even those not participating in NHDP.  Another benefit is
   that packets opportunistically received from non-symmetric neighbors
   may be forwarded without compromising flooding efficiency or
   correctness.  Furthermore, multicast sources not participating in
   NHDP may freely inject their traffic, and any neighboring E-CDS
   relays will properly forward the traffic.  The E-CDS-based relay set
   selection algorithm is based upon [<a href="./rfc5614" title="&quot;Mobile Ad Hoc Network (MANET) Extension of OSPF Using Connected Dominating Set (CDS) Flooding&quot;">RFC5614</a>].  E-CDS was originally
   discussed in the context of forming partial adjacencies and efficient
   flooding for MANET OSPF extensions work, and the core algorithm is
   applied here for SMF.

   It is RECOMMENDED that the SMF_TYPE:E-CDS Message TLV be included in
   NHDP_HELLO messages that are generated by routers conducting E-CDS
   SMF operation.  It is also RECOMMENDED that the SMF_NBR_TYPE:E-CDS
   Address Block TLV be used to advertise neighbor routers that are also
   conducting E-CDS SMF operation.

<span class="h3"><a class="selflink" id="appendix-A.1" href="#appendix-A.1">A.1</a>.  E-CDS Relay Set Selection Overview</span>

   The E-CDS relay set selection requires 2-hop neighborhood information
   collected through NHDP or another process.  Relay routers, in E-CDS
   SMF selection, are "self-elected" using a Router Identifier (Router
   ID) and an optional nodal metric, referred to here as Router Priority
   for all 1-hop and 2-hop neighbors.  To ensure proper relay set self-
   election, the Router ID and Router Priority MUST be consistent among
   participating routers.  It is RECOMMENDED that NHDP be used to share
   Router ID and Router Priority through the use of SMF_TYPE:E-CDS TLVs
   as described in this appendix.  The Router ID is a logical
   identification that MUST be consistent across interoperating SMF
   neighborhoods, and it is RECOMMENDED to be chosen as the numerically
   largest address contained in a router's "Neighbor Address List" as
   defined in NHDP.  The E-CDS self-election process can be summarized
   as follows:

   1.  If an SMF router has a higher ordinal (Router Priority, Router
       ID) than all of its symmetric neighbors, it elects itself to act
       as a forwarder for all received multicast packets.






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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   2.  Else, if there does not exist a path from the neighbor with
       largest (Router Priority, Router ID) to any other neighbor, via
       neighbors with larger values of (Router Priority, Router ID),
       then it elects itself to the relay set.

   The basic form of E-CDS described and applied within this
   specification does not provide for redundant relay set selection
   (e.g., bi-connected), but such capability is supported by the basic
   E-CDS design.

<span class="h3"><a class="selflink" id="appendix-A.2" href="#appendix-A.2">A.2</a>.  E-CDS Forwarding Rules</span>

   With E-CDS, any SMF router that has selected itself as a relay
   performs DPD and forwards all non-duplicative multicast traffic
   allowed by the present forwarding policy.  Packet previous-hop
   knowledge is not needed for forwarding decisions when using E-CDS.

   1.  Upon packet reception, DPD is performed.  Note E-CDS requires a
       single duplicate table for the set of interfaces associated with
       the relay set selection.

   2.  If the packet is a duplicate, no further action is taken.

   3.  If the packet is non-duplicative:

       A.  A DPD entry is made for the packet identifier.

       B.  The packet is forwarded out to all interfaces associated with
           the relay set selection.

   As previously mentioned, even packets sourced (or relayed) by routers
   not participating in NHDP and/or the E-CDS relay set selection may be
   forwarded by E-CDS forwarders without problem.  A particular
   deployment MAY choose to not forward packets from previous hop
   routers that have been not explicitly identified via NHDP or other
   means as operating as part of a different relay set algorithm (e.g.,
   S-MPR) to allow coexistent deployments to operate correctly.  Also,
   E-CDS relay set selection may be configured to be influenced by
   statically configured CF relays that are identified via NHDP or other
   means.

<span class="h3"><a class="selflink" id="appendix-A.3" href="#appendix-A.3">A.3</a>.  E-CDS Neighborhood Discovery Requirements</span>

   It is possible to perform E-CDS relay set selection without
   modification of NHDP, basing the self-election process exclusively on
   the "Neighbor Address List" of participating SMF routers, for
   example, by setting the Router Priority to a default value and
   selecting the Router ID as the numerically largest address contained



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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   in the "Neighbor Address List".  However, steps MUST be taken to
   ensure that all NHDP-enabled routers not using SMF_TYPE:E-CDS full
   type Message TLVs are, in fact, running SMF E-CDS with the same
   methods for selecting Router Priority and Router ID; otherwise,
   incorrect forwarding may occur.  Note that SMF routers with higher
   Router Priority values will be favored as relays over routers with
   lower Router Priority.  Thus, preferred relays MAY be
   administratively configured to be selected when possible.
   Additionally, other metrics (e.g., nodal degree, energy capacity,
   etc.) may also be taken into account in constructing a Router
   Priority value.  When using Router Priority with multiple interfaces,
   all interfaces on a router MUST use and advertise a common Router
   Priority value.  A router's Router Priority value may be
   administratively or algorithmically selected.  The method of
   selection does not need to be the same among different routers.

   E-CDS relay set selection may be configured to be influenced by
   statically configured CF relays that are identified via NHDP or other
   means.  Nodes advertising CF through NHDP may be considered E-CDS SMF
   routers with maximal Router Priority.

   To share a router's Router Priority with its 1-hop neighbors, the
   SMF_TYPE:E-CDS Message TLV's &lt;value&gt; field is defined as shown in
   Table 14.

              +----------------+---------+-----------------+
              | Length (bytes) | Value   | Router Priority |
              +----------------+---------+-----------------+
              | 0              | N/A     | 64              |
              | 1              | &lt;value&gt; | 0-127           |
              +----------------+---------+-----------------+

                    Table 14: E-CDS Message TLV Values

   Where &lt;value&gt; is a one-octet-long bit field that is defined as:

   bit 0: the leftmost bit is reserved and SHOULD be set to 0.

   bits 1-7: contain the unsigned Router Priority value, 0-127, which is
   associated with the "Neighbor Address List".

   Combinations of value field lengths and values other than specified
   here are NOT permitted and SHOULD be ignored.  Figure 6 shows an
   example SMF_TYPE:E-CDS Message TLV.







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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     ...              |   SMF_TYPE    |1|0|0|1|0|0|   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     E-CDS     |0|0|0|0|0|0|0|1|R|  priority   |     ...       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 6: E-CDS Message TLV Example

   To convey Router Priority values among 2-hop neighborhoods, the
   SMF_NBR_TYPE:E-CDS Address Block TLV's &lt;value&gt; field is used.  Multi-
   index and multivalue TLV layouts as defined in [<a href="./rfc5444" title="&quot;Generalized Mobile Ad Hoc Network (MANET) Packet/Message Format&quot;">RFC5444</a>] are
   supported.  SMF_NBR_TYPE:E-CDS value fields are defined thus:

   +---------------+--------+----------+-------------------------------+
   | Length(bytes) | # Addr | Value    | Router Priority               |
   +---------------+--------+----------+-------------------------------+
   | 0             | Any    | N/A      | 64                            |
   | 1             | Any    | &lt;value&gt;  | &lt;value&gt; is for all addresses  |
   | N             | N      | &lt;value&gt;* | Each address gets its own     |
   |               |        |          | &lt;value&gt;                       |
   +---------------+--------+----------+-------------------------------+

                 Table 15: E-CDS Address Block TLV Values

   Where &lt;value&gt; is a one-byte bit field that is defined as:

   bit 0: the leftmost bit is reserved and SHOULD be set to 0.

   bits 1-7: contain the unsigned Router Priority value, 0-127, which is
   associated with the appropriate address(es).

   Combinations of value field lengths and # of addresses other than
   specified here are NOT permitted and SHOULD be ignored.  A default
   technique of using nodal degree (i.e., count of 1-hop neighbors) is
   RECOMMENDED for the value field of these TLV types.  Below are two
   example SMF_NBR_TYPE:E-CDS Address Block TLVs.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     ...              | SMF_NBR_TYPE  |1|0|0|1|0|0|   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     E-CDS     |0|0|0|0|0|0|0|1|R|  priority   |     ...       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 7: E-CDS Address Block TLV Example 1



<span class="grey">Macker                        Experimental                     [Page 43]</span></pre>
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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   The single value example TLV, depicted in Figure 7, specifies that
   all address(es) contained in the address block are running SMF using
   the E-CDS algorithm and all address(es) share the value field and
   therefore the same Router Priority.
       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     ...              | SMF_NBR_TYPE  |1|0|1|1|0|1|   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     E-CDS     |  index-start  |   index-end   |    length     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |R|  priority0  |R|  priority1  |      ...      |R|  priorityN  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 8: E-CDS Address Block TLV Example 2

   The example multivalued TLV, depicted in Figure 8, specifies that
   address(es) contained in the address block from index-start to index-
   end inclusive are running SMF using the E-CDS algorithm.  Each
   address is associated with its own value byte and therefore its own
   Router Priority.

<span class="h3"><a class="selflink" id="appendix-A.4" href="#appendix-A.4">A.4</a>.  E-CDS Selection Algorithm</span>

   This section describes an algorithm for E-CDS relay selection (self-
   election).  The algorithm described uses 2-hop information.  Note
   that it is possible to extend this algorithm to use k-hop information
   with added computational complexity and mechanisms for sharing k-hop
   topology information that are not described in this document or
   within the NHDP specification.  It should also be noted that this
   algorithm does not impose the hop limit bound described in [<a href="./rfc5614" title="&quot;Mobile Ad Hoc Network (MANET) Extension of OSPF Using Connected Dominating Set (CDS) Flooding&quot;">RFC5614</a>]
   when performing the path search that is used for relay selection.
   However, the algorithm below could be easily augmented to accommodate
   this additional criterion.  It is not expected that the hop limit
   bound will provide significant benefit to the algorithm defined in
   this appendix.

   The tuple of Router Priority and Router ID is used in E-CDS relay set
   selection.  Precedence is given to the Router Priority portion, and
   the Router ID value is used as a tiebreaker.  The evaluation of this
   tuple is referred to as "RtrPri(n)" in the description below where
   "n" references a specific router.  Note that it is possible that the
   Router Priority portion may be optional and the evaluation of
   "RtrPri()" be solely based upon the unique Router ID.  Since there
   MUST NOT be any duplicate Router ID values among SMF routers, a
   comparison of "RtrPri(n)" between any two routers will always be an
   inequality.  The use of nodal degree for calculating Router Priority
   is RECOMMENDED as default, and the largest IP address in the



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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   "Neighbor Address List" as advertised by NHDP MUST be used as the
   Router ID.  NHDP provides all interface addresses throughout the
   2-hop neighborhood through HELLO messages, so explicitly conveying a
   Router ID is not necessary.  The following steps describe a basic
   algorithm for conducting E-CDS relay selection for a router "n0":

   1.  Initialize the set "N1" with tuples ("Router Priority", "Router
       ID", "Neighbor Address List") for each 1-hop neighbor of "n0".

   2.  If "N1" has less than 2 tuples, then "n0" does not elect itself
       as a relay, and no further steps are taken.

   3.  Initialize the set "N2" with tuples ("Router Priority", "Router
       ID", "2-hop address") for each "2-hop address" of "n0", where
       "2-hop address" is defined in NHDP.

   4.  If "RtrPri(n0)" is greater than that of all tuples in the union
       of "N1" and "N2", then "n0" selects itself as a relay, and no
       further steps are taken.

   5.  Initialize all tuples in the union of "N1" and "N2" as
       "unvisited".

   6.  Find the tuple "n1_Max" that has the largest "RtrPri()" of all
       tuples in "N1".

   7.  Initialize queue "Q" to contain "n1_Max", marking "n1_Max" as
       "visited".

   8.  While router queue "Q" is not empty, remove router "x" from the
       head of "Q", and for each 1-hop neighbor "n" of router "x"
       (excluding "n0") that is not marked "visited".

       A.  Mark router "n" as "visited".

       B.  If "RtrPri(n)" is greater than "RtrPri(n0)", append "n" to
           "Q".

   9.  If any tuple in "N1" remains "unvisited", then "n0" selects
       itself as a relay.  Otherwise, "n0" does not act as a relay.

   Note these steps are re-evaluated upon neighborhood status changes.
   Steps 5 through 8 of this procedure describe an approach to a path
   search.  The purpose of this path search is to determine if paths
   exist from the 1-hop neighbor with maximum "RtrPri()" to all other
   1-hop neighbors without traversing an intermediate router with a
   "RtrPri()" value less than "RtrPri(n0)".  These steps comprise a
   breadth-first traversal that evaluates only paths that meet that



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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   criteria.  If all 1-hop neighbors of "n0" are "visited" during this
   traversal, then the path search has succeeded, and router "n0" does
   not need to provide relay.  It can be assumed that other routers will
   provide relay operation to ensure SMF connectivity.

   It is possible to extend this algorithm to consider neighboring SMF
   routers that are known to be statically configured for CF (always
   relaying).  The modification to the above algorithm is to process
   such routers as having a maximum possible Router Priority value.  It
   is expected that routers configured for CF and participating in NHDP
   would indicate this with use of the SMF_TYPE:CF and SMF_NBR_TYPE:CF
   TLV types in their NHDP_HELLO message and address blocks,
   respectively.

<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  Source-Based Multipoint Relay (S-MPR) Algorithm</span>

   The source-based multipoint relay (S-MPR) set selection algorithm
   enables individual routers, using 2-hop topology information, to
   select relays from their set of neighboring routers.  Relays are
   selected so that forwarding to the router's complete 2-hop neighbor
   set is covered.  This distributed relay set selection technique has
   been shown to approximate a minimal connected dominating set (MCDS)
   in [<a href="#ref-JLMV02" title="&quot;Performance of Multipoint Relaying in Ad Hoc Mobile Routing Protocols&quot;">JLMV02</a>].  Individual routers must collect 2-hop neighborhood
   information from neighbors, determine an appropriate current relay
   set, and inform selected neighbors of their relay status.  Note that
   since each router picks its neighboring relays independently, S-MPR
   forwarders depend upon previous hop information (e.g., source MAC
   address) to operate correctly.  The Optimized Link State Routing
   (OLSR) protocol has used this algorithm and protocol for relay of
   link state updates and other control information [<a href="./rfc3626" title="&quot;Optimized Link State Routing Protocol (OLSR)&quot;">RFC3626</a>], and it
   has been demonstrated operationally in dynamic network environments.

   It is RECOMMENDED that the SMF_TYPE:S-MPR Message TLV be included in
   NHDP_HELLO messages that are generated by routers conducting S-MPR
   SMF operation.  It is also RECOMMENDED that the SMF_NBR_TYPE:S-MPR
   Address Block TLV be used to specify which neighbor routers are
   conducting S-MPR SMF operation.

<span class="h3"><a class="selflink" id="appendix-B.1" href="#appendix-B.1">B.1</a>.  S-MPR Relay Set Selection Overview</span>

   The S-MPR algorithm uses bi-directional 1-hop and 2-hop neighborhood
   information collected via NHDP to select, from a router's 1-hop
   neighbors, a set of relays that will cover the router's entire 2-hop
   neighbor set upon forwarding.  The algorithm described uses a
   "greedy" heuristic of first picking the 1-hop neighbor who will cover
   the most 2-hop neighbors.  Then, excluding those 2-hop neighbors that
   have been covered, additional relays from its 1-hop neighbor set are




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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   iteratively selected until the entire 2-hop neighborhood is covered.
   Note that 1-hop neighbors also identified as 2-hop neighbors are
   considered as 1-hop neighbors only.

   NHDP HELLO messages supporting S-MPR forwarding operation SHOULD use
   the TLVs defined in <a href="#section-8.1">Section 8.1</a> using the S-MPR extended type.  The
   value field of an Address Block TLV that has a full type value of
   SMF_NBR_TYPE:S-MPR is defined in Table 17 such that signaling of MPR
   selections to 1-hop neighbors is possible.  The value field of a
   message block TLV that has a full type value of SMF_TYPE:S-MPR is
   defined in Table 16 such that signaling of Router Priority (described
   as "WILLINGNESS" in [<a href="./rfc3626" title="&quot;Optimized Link State Routing Protocol (OLSR)&quot;">RFC3626</a>]) to 1-hop neighbors is possible.  It is
   important to note that S-MPR forwarding is dependent upon the
   previous hop of an incoming packet.  An S-MPR router MUST forward
   packets only for neighbors that have explicitly selected it as a
   multipoint relay (i.e., its "selectors").  There are also some
   additional requirements for duplicate packet detection to support
   S-MPR SMF operation that are described below.

   For multiple interface operation, MPR selection SHOULD be conducted
   on a per-interface basis.  However, it is possible to economize MPR
   selection among multiple interfaces by selecting common MPRs to the
   extent possible.

<span class="h3"><a class="selflink" id="appendix-B.2" href="#appendix-B.2">B.2</a>.  S-MPR Forwarding Rules</span>

   An S-MPR SMF router MUST only forward packets for neighbors that have
   explicitly selected it as an MPR.  The source-based forwarding
   technique also stipulates some additional duplicate packet detection
   operations.  For multiple network interfaces, independent DPD state
   MUST be maintained for each separate interface.  The following
   provides the procedure for S-MPR packet forwarding given the arrival
   of a packet on a given interface, denoted &lt;srcIface&gt;.  There are
   three possible actions, depending upon the previous-hop transmitter:

   1.  If the previous-hop transmitter has selected the current router
       as an MPR,

       A.  The packet identifier is checked against the DPD state for
           each possible outbound interface, including the &lt;srcIface&gt;.

       B.  If the packet is not a duplicate for an outbound interface,
           the packet is forwarded on that interface and a DPD entry is
           made for the given packet identifier for the interface.

       C.  If the packet is a duplicate, no action is taken for that
           interface.




<span class="grey">Macker                        Experimental                     [Page 47]</span></pre>
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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   2.  Else, if the previous-hop transmitter is a 1-hop symmetric
       neighbor, a DPD entry is added for that packet for the
       &lt;srcIface&gt;, but the packet is not forwarded.

   3.  Otherwise, no action is taken.

   Action number two in the list above is non-intuitive but important to
   ensure correctness of S-MPR SMF operation.  The selection of source-
   based relays does not result in a common set among neighboring
   routers, so relays MUST mark, in their DPD state, packets received
   from non-selector, symmetric, 1-hop neighbors (for a given interface)
   and not forward subsequent duplicates of that packet if received on
   that interface.  Deviation here can result in unnecessary, repeated
   packet forwarding throughout the network or incomplete flooding.

   Nodes not participating in neighborhood discovery and relay set
   selection will not be able to source multicast packets into the area
   and have SMF forward them, unlike E-CDS or MPR-CDS where forwarding
   may occur dependent on topology.  Correct S-MPR relay behavior will
   occur with the introduction of repeaters (non-NHDP/SMF participants
   that relay multicast packets using duplicate detection and CF), but
   the repeaters will not efficiently contribute to S-MPR forwarding as
   these routers will not be identified as neighbors (symmetric or
   otherwise) in the S-MPR forwarding process.  NHDP/SMF participants
   MUST NOT forward packets that are not selected by the algorithm, as
   this can disrupt network-wide S-MPR flooding, resulting in incomplete
   or inefficient flooding.  The result is that non-S-MPR SMF routers
   will be unable to source multicast packets and have them forwarded by
   other S-MPR SMF routers.

<span class="h3"><a class="selflink" id="appendix-B.3" href="#appendix-B.3">B.3</a>.  S-MPR Neighborhood Discovery Requirements</span>

   Nodes may optionally signal a Router Priority value to their 1-hop
   neighbors by using the SMF_TYPE:S-MPR message block TLV value field.
   If the value field is omitted, a default Router Priority value of 64
   is to be assumed.  This is summarized here:

               +---------------+---------+-----------------+
               | Length(bytes) | Value   | Router Priority |
               +---------------+---------+-----------------+
               | 0             | N/A     | 64              |
               | 1             | &lt;value&gt; | 0-127           |
               +---------------+---------+-----------------+

                    Table 16: S-MPR Message TLV Values






<span class="grey">Macker                        Experimental                     [Page 48]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-49" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   Where &lt;value&gt; is a one-octet-long bit field defined as:

   bit 0: the leftmost bit is reserved and SHOULD be set to 0.

   bits 1-7: contain the Router Priority value, 0-127, which is
   associated with the "Neighbor Address List".

   Router Priority values for S-MPR are interpreted in the same fashion
   as "WILLINGNESS" ([<a href="./rfc3626" title="&quot;Optimized Link State Routing Protocol (OLSR)&quot;">RFC3626</a>]), with the value 0 indicating a router
   will NEVER forward and value 127 indicating a router will ALWAYS
   forward.  Values 1-126 indicate how likely a S-MPR SMF router will be
   selected as an MPR by a neighboring SMF router, with higher values
   increasing the likelihood.  Combinations of value field lengths and
   values other than those specified here are NOT permitted and SHOULD
   be ignored.  Below is an example SMF_TYPE:S-MPR Message TLV.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     ...              |   SMF_TYPE    |1|0|0|1|0|0|   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     S-MPR     |0|0|0|0|0|0|0|1|R|  priority   |     ...       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 9: S-MPR Message TLV Example

   S-MPR election operation requires 2-hop neighbor knowledge as
   provided by NHDP [<a href="./rfc6130" title="&quot;Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)&quot;">RFC6130</a>] or from external sources.  MPRs are
   dynamically selected by each router, and selections MUST be
   advertised and dynamically updated within NHDP or an equivalent
   protocol or mechanism.  For NHDP use, the SMF_NBR_TYPE:S-MPR Address
   Block TLV value field is defined as such:

   +---------------+--------+----------+-------------------------------+
   | Length(bytes) | # Addr | Value    | Meaning                       |
   +---------------+--------+----------+-------------------------------+
   | 0             | Any    | N/A      | NOT MPRs                      |
   | 1             | Any    | &lt;value&gt;  | &lt;value&gt; is for all addresses  |
   | N             | N      | &lt;value&gt;* | Each address gets its own     |
   |               |        |          | &lt;value&gt;                       |
   +---------------+--------+----------+-------------------------------+

                 Table 17: S-MPR Address Block TLV Values








<span class="grey">Macker                        Experimental                     [Page 49]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-50" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   Where &lt;value&gt;, if present, is a one-octet bit field defined as:

   bit 0: The leftmost bit is the M bit that, when set, indicates MPR
   selection of the relevant interface, represented by the associated
   address(es), by the originator router of the NHDP HELLO message.
   When unset, it indicates the originator router of the NHDP HELLO
   message has not selected the relevant interfaces, represented by the
   associated address(es), as its MPR.

   bits 1-7: These bits are reserved and SHOULD be set to 0.

   Combinations of value field lengths and number of addresses other
   than specified here are NOT permitted and SHOULD be ignored.  All
   bits, excepting the leftmost bit, are RESERVED and SHOULD be set to
   0.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     ...              | SMF_NBR_TYPE  |1|1|0|1|0|0|   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     S-MPR     |  start-index  |0|0|0|0|0|0|0|1|M|  reserved   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 10: S-MPR Address Block TLV Example

   The single index TLV example, depicted in Figure 10, indicates that
   the address specified by the &lt;start-index&gt; field is running SMF using
   S-MPR and has been selected by the originator of the NHDP HELLO
   message as an MPR forwarder if the M bit is set.  Multivalued TLVs
   may also be used to specify MPR selection status of multiple
   addresses using only one TLV.  See Figure 8 for a similar example on
   how this may be done.

<span class="h3"><a class="selflink" id="appendix-B.4" href="#appendix-B.4">B.4</a>.  S-MPR Selection Algorithm</span>

   This section describes a basic algorithm for the S-MPR selection
   process.  Note that the selection is with respect to a specific
   interface of the router performing selection, and other router
   interfaces referenced are reachable from this reference router
   interface.  This is consistent with the S-MPR forwarding rules
   described above.  When multiple interfaces per router are used, it is
   possible to enhance the overall selection process across multiple
   interfaces such that common routers are selected as MPRs for each
   interface to avoid unnecessary inefficiencies in flooding.  The
   following steps describe a basic algorithm for conducting S-MPR
   selection for a router interface "n0":




<span class="grey">Macker                        Experimental                     [Page 50]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-51" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   1.  Initialize the set "MPR" to empty.

   2.  Initialize the set "N1" to include all 1-hop neighbors of "n0".

   3.  Initialize the set "N2" to include all 2-hop neighbors, excluding
       "n0" and any routers in "N1".  Nodes that are only reachable via
       "N1" routers with router priority values of NEVER are also
       excluded.

   4.  For each interface "y" in "N1", initialize a set "N2(y)" to
       include any interfaces in "N2" that are 1-hop neighbors of "y".

   5.  For each interface "x" in "N1" with a router priority value of
       "ALWAYS" (or using the CF relay algorithm), select "x" as an MPR:

       A.  Add "x" to the set "MPR" and remove "x" from "N1".

       B.  For each interface "z" in "N2(x)", remove "z" from "N2".

       C.  For each interface "y" in "N1", remove any interfaces in
           "N2(x)" from "N2(y)".

   6.  For each interface "z" in "N2", initialize the set "N1(z)" to
       include any interfaces in "N1" that are 1-hop neighbors of "z".

   7.  For each interface "x" in "N2" where "N1(x)" has only one member,
       select "x" as an MPR:

       A.  Add "x" to the set "MPR" and remove "x" from "N1".

       B.  For each interface "z" in "N2(x)", remove "z" from "N2" and
           delete "N1(z)".

       C.  For each interface "y" in "N1", remove any interfaces in
           "N2(x)" from "N2(y)".

   8.  While "N2" is not empty, select the interface "x" in "N1" with
       the largest router priority that has the number of members in
       "N_2(x)" as an MPR:

       A.  Add "x" to the set "MPR" and remove "x" from "N1".

       B.  For each interface "z" in "N2(x)", remove "z" from "N2".

       C.  For each interface "y" in "N1", remove any interfaces in
           "N2(x)" from "N2(y)".





<span class="grey">Macker                        Experimental                     [Page 51]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-52" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   After the set of routers "MPR" is selected, router "n_0" must signal
   its selections to its neighbors.  With NHDP, this is done by using
   the MPR Address Block TLV to mark selected neighbor addresses in
   NHDP_HELLO messages.  Neighbors MUST record their MPR selection
   status and the previous hop address (e.g., link or MAC layer) of the
   selector.  Note these steps are re-evaluated upon neighborhood status
   changes.

<span class="h2"><a class="selflink" id="appendix-C" href="#appendix-C">Appendix C</a>.  Multipoint Relay Connected Dominating Set (MPR-CDS)</span>
             Algorithm

   The MPR-CDS algorithm is an extension to the basic S-MPR election
   algorithm that results in a shared (non-source-specific) SMF CDS.
   Thus, its forwarding rules are not dependent upon previous hop
   information, similar to E-CDS.  An overview of the MPR-CDS selection
   algorithm is provided in [<a href="#ref-MPR-CDS" title="&quot;Computing Connected Dominating Sets with Multipoint Relays&quot;">MPR-CDS</a>].

   It is RECOMMENDED that the SMF_TYPE Message TLV be included in
   NHDP_HELLO messages that are generated by routers conducting MPR-CDS
   SMF operation.

<span class="h3"><a class="selflink" id="appendix-C.1" href="#appendix-C.1">C.1</a>.  MPR-CDS Relay Set Selection Overview</span>

   The MPR-CDS relay set selection process is based upon the MPR
   selection process of the S-MPR algorithm with the added refinement of
   a distributed technique for subsequently down-selecting to a common
   reduced, shared relay set.  A router ordering (or "prioritization")
   metric is used as part of this down-selection process; like the E-CDS
   algorithm, this metric can be based upon router address(es) or some
   other unique router identifier (e.g., Router ID based on largest
   address contained within the "Neighbor Address List") as well as an
   additional Router Priority measure, if desired.  The process for MPR-
   CDS relay selection is as follows:

   1.  First, MPR selection per the S-MPR algorithm is conducted, with
       selectors informing their MPRs (via NHDP) of their selection.

   2.  Then, the following rules are used on a distributed basis by
       selected routers to possibly deselect themselves and thus jointly
       establish a common set of shared SMF relays:

       A.  If a selected router has a larger "RtrPri()" than all of its
           1-hop symmetric neighbors, then it acts as a relay for all
           multicast traffic, regardless of the previous hop.







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<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


       B.  Else, if the 1-hop symmetric neighbor with the largest
           "RtrPri()" value has selected the router, then it also acts
           as a relay for all multicast traffic, regardless of the
           previous hop.

       C.  Otherwise, it deselects itself as a relay and does not
           forward any traffic unless changes occur that require re-
           evaluation of the above steps.

   This technique shares many of the desirable properties of the E-CDS
   technique with regards to compatibility with multicast sources not
   participating in NHDP and the opportunity for statically configured
   CF routers to be present, regardless of their participation in NHDP.

<span class="h3"><a class="selflink" id="appendix-C.2" href="#appendix-C.2">C.2</a>.  MPR-CDS Forwarding Rules</span>

   The forwarding rules for MPR-CDS are similar to those for E-CDS.  Any
   SMF router that has selected itself as a relay performs DPD and
   forwards all non-duplicative multicast traffic allowed by the present
   forwarding policy.  Packet previous hop knowledge is not needed for
   forwarding decisions when using MPR-CDS.

   1.  Upon packet reception, DPD is performed.  Note that MPR-CDS
       requires one duplicate table for the set of interfaces associated
       with the relay set selection.

   2.  If the packet is a duplicate, no further action is taken.

   3.  If the packet is non-duplicative:

       A.  A DPD entry is added for the packet identifier

       B.  The packet is forwarded out to all interfaces associated with
           the relay set selection.

   As previously mentioned, even packets sourced (or relayed) by routers
   not participating in NHDP and/or the MPR-CDS relay set selection may
   be forwarded by MPR-CDS forwarders without problem.  A particular
   deployment MAY choose to not forward packets from sources or relays
   that have been explicitly identified via NHDP or other means as
   operating as part of a different relay set algorithm (e.g., S-MPR) to
   allow coexistent deployments to operate correctly.

<span class="h3"><a class="selflink" id="appendix-C.3" href="#appendix-C.3">C.3</a>.  MPR-CDS Neighborhood Discovery Requirements</span>

   The neighborhood discovery requirements for MPR-CDS have commonality
   with both the S-MPR and E-CDS algorithms.  MPR-CDS selection
   operation requires 2-hop neighbor knowledge as provided by NHDP



<span class="grey">Macker                        Experimental                     [Page 53]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-54" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


   [<a id="ref-RFC6130">RFC6130</a>] or from external sources.  Unlike S-MPR operation, there is
   no need for associating link-layer address information with 1-hop
   neighbors since MPR-CDS forwarding is independent of the previous hop
   similar to E-CDS forwarding.

   To advertise an optional Router Priority value or "WILLINGNESS", an
   originating router may use the Message TLV of type SMF_TYPE:MPR-CDS,
   which shares a common &lt;value&gt; format with both SMF_TYPE:E-CDS
   (Table 14) and SMF_TYPE:S-MPR (Table 16).

   MPR-CDS only requires 1-hop knowledge of Router Priority for correct
   operation.  In the S-MPR phase of MPR-CDS selection, MPRs are
   dynamically determined by each router, and selections MUST be
   advertised and dynamically updated using NHDP or an equivalent
   protocol or mechanism.  The &lt;value&gt; field of the SMF_NBR_TYPE:MPR-CDS
   type TLV shares a common format with SMF_NBR_TYPE:S-MPR (Table 17) to
   convey MPR selection.

<span class="h3"><a class="selflink" id="appendix-C.4" href="#appendix-C.4">C.4</a>.  MPR-CDS Selection Algorithm</span>

   This section describes an algorithm for the MPR-CDS selection
   process.  Note that the selection described is with respect to a
   specific interface of the router performing selection, and other
   router interfaces referenced are reachable from this reference router
   interface.  An ordered tuple of Router Priority and Router ID is used
   in MPR-CDS relay set selection.  The Router ID value should be set to
   the largest advertised address of a given router; this information is
   provided to one-hop neighbors via NHDP by default.  Precedence is
   given to the Router Priority portion, and the Router ID value is used
   as a tiebreaker.  The evaluation of this tuple is referred to as
   "RtrPri(n)" in the description below where "n" references a specific
   router.  Note that it is possible that the Router Priority portion
   may be optional and the evaluation of "RtrPri()" be solely based upon
   the unique Router ID.  Since there MUST NOT be any duplicate address
   values among SMF routers, a comparison of "RtrPri(n)" between any two
   routers will always be an inequality.  The following steps, repeated
   upon any changes detected within the 1-hop and 2-hop neighborhood,
   describe a basic algorithm for conducting MPR-CDS selection for a
   router interface "n0":

   1.  Perform steps 1-8 of <a href="#appendix-B.4">Appendix B.4</a> to select MPRs from the set of
       1-hop neighbors of "n0" and notify/update neighbors of
       selections.

   2.  Upon being selected as an MPR (or any change in the set of
       routers selecting "n0" as an MPR):





<span class="grey">Macker                        Experimental                     [Page 54]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-55" ></span>
<span class="grey"><a href="./rfc6621">RFC 6621</a>                           SMF                          May 2012</span>


       A.  If no neighbors have selected "n0" as an MPR, "n0" does not
           act as a relay, and no further steps are taken until a change
           in neighborhood topology or selection status occurs.

       B.  Determine the router "n1_max" that has the maximum "RtrPri()"
           of all 1-hop neighbors.

       C.  If "RtrPri(n0)" is greater than "RtrPri(n1_max)", then "n0"
           selects itself as a relay for all multicast packets.

       D.  Else, if "n1_max" has selected "n0" as an MPR, then "0"
           selects itself as a relay for all multicast packets.

       E.  Otherwise, "n0" does not act as a relay.

   It is possible to extend this algorithm to consider neighboring SMF
   routers that are known to be statically configured for CF (always
   relaying).  The modification to the above algorithm is to process
   such routers as having a maximum possible Router Priority value.
   This is the same as the case for participating routers that have been
   configured with a S-MPR "WILLINGNESS" value of "WILL_ALWAYS".  It is
   expected that routers configured for CF and participating in NHDP
   would indicate their status with use of the SMF_TYPE TLV type in
   their NHDP_HELLO message TLV block.  It is important to note,
   however, that CF routers will not select MPR routers and therefore
   cannot guarantee connectedness.

Author's Address

   Joseph Macker (editor)
   NRL
   Washington, DC  20375
   USA

   EMail: macker@itd.nrl.navy.mil
















Macker                        Experimental                     [Page 55]
</pre>