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rfc:rfc6621

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

                  Simplified Multicast Forwarding

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 Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6621.

Macker Experimental [Page 1] RFC 6621 SMF May 2012

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 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
 the copyright in such materials, this document may not be modified
 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

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

Macker Experimental [Page 2] RFC 6621 SMF May 2012

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

Macker Experimental [Page 3] RFC 6621 SMF May 2012

1. Introduction and Scope

 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 [RFC3626] and [RFC3684] 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 [RFC2501], and
 o  define the needed IPv4 and IPv6 multicast duplicate packet
    detection (DPD) mechanisms to support multi-hop packet forwarding.

2. Terminology

 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
 [RFC2119].
 The terms introduced in [RFC5444], including "packet", "message",
 "TLV Block", "TLV", and "address", are to be interpreted as described
 therein.

Macker Experimental [Page 4] RFC 6621 SMF May 2012

 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            |
 +--------------+----------------------------------------------------+

3. Applicability Statement

 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 [MGL04][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 Section 9.2) 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

Macker Experimental [Page 5] RFC 6621 SMF May 2012

 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 [RFC3973][RFC4601].
 Experimental SMF implementations and deployments have demonstrated
 gateway functionality at MANET border routers operating with existing
 external IP multicast routing protocols [CDHM07][DHS08][DHG09].  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 [RFC2644].

4. Overview and Functioning

 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   |------------->|  Selection  |
             |              |   neighbor   |             |
             |______________|     info     |_____________|
                    \                              /
                     \                            /
              neighbor\                          /forwarding
                info*  \      ____________      /  status
                        \    |            |    /
                         `-->| Forwarding |<--'
                             |  Process   |
           ~~~~~~~~~~~~~~~~~>|____________|~~~~~~~~~~~~~~~~~>
           incoming packet,                 forwarded packets
           interface id*, and
           previous hop*
                   Figure 1: SMF Router Architecture

Macker Experimental [Page 6] RFC 6621 SMF May 2012

 Certain IP multicast packets, defined in Sections 9.2 and 5, 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 [NTSC99].
 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
 [MDC04].  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) [RFC6130], 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 Section 7.  This document states specific requirements

Macker Experimental [Page 7] RFC 6621 SMF May 2012

 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 [RFC5444]) 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.

5. SMF Packet Processing and Forwarding

 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
 [RFC4291], and that are not within the local network control block as
 defined by [RFC5771].
 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

Macker Experimental [Page 8] RFC 6621 SMF May 2012

 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 <= 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 Section 10, 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

Macker Experimental [Page 9] RFC 6621 SMF May 2012

 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 Section 6).  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.

6. SMF Duplicate Packet Detection

 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 Section 5.
 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.

Macker Experimental [Page 10] RFC 6621 SMF May 2012

 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 Appendix B.2.
 Additional details of I-DPD and H-DPD processing and maintenance for
 different classes of packets are described in the following
 subsections.

6.1. IPv6 Duplicate Packet Detection

 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
     (Section 6.1.1),
 2.  the use of IPv6 fragment header fields for I-DPD, if one is
     present (Section 6.1.2),
 3.  the use of IPsec sequencing for I-DPD when a non-fragmented,
     IPsec header is detected (Section 6.1.2), and
 4.  an H-DPD approach assisted, as needed, by the SMF_DPD option
     header (Section 6.1.3).
 SMF MUST provide a DPD marking module that can insert the hop-by-hop
 IPv6 header option, defined in Section 6.1.1.  This module MUST be
 invoked after any source-based fragmentation that may occur with

Macker Experimental [Page 11] RFC 6621 SMF May 2012

 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 6.1.2 and 6.1.3.

6.1.1. IPv6 SMF_DPD Option Header

 This section defines an IPv6 Hop-by-Hop Option [RFC2460], 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 [RFC2460].
 "Opt. Data Len" = Length of option content (i.e., 1 + (<IdType> ?
 (<IdLen> + 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 [RFC2460].

Macker Experimental [Page 12] RFC 6621 SMF May 2012

      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 <Identifier> field corresponds to (<Opt. Data Len> -
 1).  If the <TidTy> is non-NULL, then the length of the TaggerId
 field is equal to (<TidLen> - 1), and the remainder of the option
 data comprises the DPD packet <Identifier> field.  When the TaggerId
 field is present, the <Identifier> field can be considered a unique
 packet identifier in the context of the <TaggerId:srcAddr:dstAddr>
 tuple.  When the TaggerId field is not present, then it is assumed
 that the source applied the SMF_DPD option and the <Identifier> can

Macker Experimental [Page 13] RFC 6621 SMF May 2012

 be considered unique in the context of the IPv6 packet header
 <srcAddr:dstAddr> tuple.  IPv6 I-DPD operation details are in
 Section 6.1.2.
 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
 <srcAddr>.  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 Section 6.1.3.

6.1.2. IPv6 Identification-Based DPD

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

Macker Experimental [Page 14] RFC 6621 SMF May 2012

 +-------------+-----------+-----------+-----------------------------+
 | 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 <srcAddr:dstAddr> 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
     <sequence> field can be considered a unique identifier in the
     context of the <IPsecType:srcAddr:dstAddr:SPI> where "IPsecType"
     is either Authentication Header (AH) or Encapsulating Security
     Payload (ESP) [RFC4302].
 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 <srcAddr>
     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:

Macker Experimental [Page 15] RFC 6621 SMF May 2012

 +-----------+---------------------------------+---------------------+
 | IPv6      | Packet DPD ID Context           | Packet DPD ID       |
 | Packet    |                                 |                     |
 | Type      |                                 |                     |
 +-----------+---------------------------------+---------------------+
 | Fragment  | <srcAddr:dstAddr>               | <fragmentOffset:id> |
 | IPsec     | <IPsecType:srcAddr:dstAddr:SPI> | <sequence>          |
 | Packet    |                                 |                     |
 | Regular   | <[TaggerId:]srcAddr:dstAddr>    | <SMF_DPD option     |
 | Packet    |                                 | header id>          |
 +-----------+---------------------------------+---------------------+
            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.

6.1.3. IPv6 Hash-Based DPD

 A default hash-based DPD approach (H-DPD) for use by SMF is specified
 as follows.  An SHA-1 [RFC3174] 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 [RFC4302]
 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 <srcAddr>.  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 Section 6.1.1 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 Section 6.1.1) 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.

Macker Experimental [Page 16] RFC 6621 SMF May 2012

 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.

6.2. IPv4 Duplicate Packet Detection

 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
     (Section 6.2.1),
 2.  the use of IPsec sequencing for I-DPD when a non-fragmented IPv4
     IPsec packet is detected (Section 6.2.1), and
 3.  an H-DPD approach(Section 6.2.2) when neither of the above cases
     can be applied.
 Although the IPv4 datagram has a 16-bit Identification (ID) field as
 specified in [RFC0791], 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
 [IPV4-ID-UPDATE].  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 Section 6.2.1.  For the general IPv4 case (non-
 IPsec and non-fragmented packets), the H-DPD approach of
 Section 6.2.2 is RECOMMENDED.

Macker Experimental [Page 17] RFC 6621 SMF May 2012

 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.

6.2.1. IPv4 Identification-Based DPD

 Table 4 summarizes the IPv4 I-DPD processing approach once a packet
 has passed the basic forwardable criteria described in Section 5.  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 [RFC0791].
 +------+------+----------+---------+--------------------------------+
 | 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 <protocol:srcAddr:
 dstAddr> of the IPv4 packet.  If the packet is an unfragmented IPv4
 IPsec packet, SMF MUST use IPsec fields for packet identification.
 The IPsec header <sequence> field can be considered a unique

Macker Experimental [Page 18] RFC 6621 SMF May 2012

 identifier in the context of the <IPsecType:srcAddr:dstAddr:SPI>
 where "IPsecType" is either AH or ESP [RFC4302].  Table 5 summarizes
 these packet identification types:
 +-----------+---------------------------------+---------------------+
 | IPv4      | Packet Identification Context   | Packet Identifier   |
 | Packet    |                                 |                     |
 | Type      |                                 |                     |
 +-----------+---------------------------------+---------------------+
 | Fragment  | <protocol:srcAddr:dstAddr>      | <fragmentOffset:id> |
 | IPsec     | <IPsecType:srcAddr:dstAddr:SPI> | <sequence>          |
 | Packet    |                                 |                     |
 +-----------+---------------------------------+---------------------+
            Table 5: IPv4 I-DPD Packet Identification Types
 "IPsecType" is either Authentication Header (AH) or Encapsulating
 Security Payload (ESP).

6.2.2. IPv4 Hash-Based DPD

 The hashing technique here is similar to that specified for IPv6 in
 Section 6.1.3, 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 [RFC3174] 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 [RFC4302] with respect to
 non-mutable fields.  A history of the packet hash values SHOULD be
 maintained in the context of <protocol:srcAddr:dstAddr>.  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.

Macker Experimental [Page 19] RFC 6621 SMF May 2012

 When this approach is taken, the use of the supplemental "internal
 hash" technique as described in Section 10 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.

7. Relay Set Selection

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

7.1. Non-Reduced Relay Set Forwarding

 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
 Section 6 are critical to proper operation and prevention of
 duplicate packet retransmissions by the same relays.

7.2. Reduced Relay Set Forwarding

 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 [MDC04].  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).

Macker Experimental [Page 20] RFC 6621 SMF May 2012

 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 [RFC6130] 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.

Macker Experimental [Page 21] RFC 6621 SMF May 2012

                     Possible L2 Trigger/Information
                                    |
                                    |
  ______________              ______v_____         __________________
 |    MANET     |            |            |       |                  |
 | Neighborhood |            | Relay Set  |       | Other Heuristics |
 |  Discovery   |----------->| Selection  |<------|(Preference, etc.)|
 |   Protocol   | neighbor   | Algorithm  |       |  Net Management  |
 |______________|   info     |____________|       |__________________|
        \                              /
         \                            /
  neighbor\                          / Dynamic Relay
    info*  \      ____________      /    Set Status
            \    |    SMF     |    / (State, {neighbor info})
             `-->| Relay Set  |<--'
                 |   State    |
              -->|____________|
             /
            /
  ______________
 |  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 [RFC5444] type-
     length-value (TLV) content as to support SMF-specific
     requirements as discussed in [RFC6130] 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.

Macker Experimental [Page 22] RFC 6621 SMF May 2012

 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).

8. SMF Neighborhood Discovery Requirements

 This section defines the requirements for use of the MANET
 Neighborhood Discovery Protocol (NHDP) [RFC6130] 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 [MDDA07].
 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

Macker Experimental [Page 23] RFC 6621 SMF May 2012

 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.
 Section 8.1 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.

8.1. SMF Relay Algorithm TLV Types

 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.

8.1.1. SMF Message TLV Type

 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 [RFC5444] 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 [RFC6130]).  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 [RFC5444].  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.

Macker Experimental [Page 24] RFC 6621 SMF May 2012

        +---------------+----------------+--------------------+
        |               | TLV Syntax     | Field Values       |
        +---------------+----------------+--------------------+
        | type          | <tlv-type>     | SMF_TYPE           |
        | extended type | <tlv-type-ext> | <relayAlgorithmId> |
        | length        | <length>       | variable           |
        | value         | <value>        | variable           |
        +---------------+----------------+--------------------+
                     Table 6: SMF Type Message TLV
 In Table 6, <relayAlgorithmId> 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 <relayAlgorithmId> 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 <length> and <value> 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 <tlv-type-ext>
 field, defines the meaning and format of its TLV <value> field.  For
 the algorithms defined by this document, see the appropriate appendix
 for the <value> field format.

8.1.2. SMF Address Block TLV Type

 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

Macker Experimental [Page 25] RFC 6621 SMF May 2012

 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          | <tlv-type>     | SMF_NBR_TYPE       |
        | extended type | <tlv-type-ext> | <relayAlgorithmId> |
        | length        | <length>       | variable           |
        | value         | <value>        | variable           |
        +---------------+----------------+--------------------+
                  Table 8: SMF Type Address Block TLV
 <relayAlgorithmId> 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 [RFC5444].  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 <relayAlgorithmId> 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.

9. SMF Border Gateway Considerations

 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 [DHS08].  Some of the functionality border routers may
 need to address includes the following:

Macker Experimental [Page 26] RFC 6621 SMF May 2012

 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.

9.1. Forwarded Multicast Groups

 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

Macker Experimental [Page 27] RFC 6621 SMF May 2012

 border routers associated with a MANET routing domain raises
 additional issues.  This is further discussed in Section 9.4 but
 further work is expected to be needed here.

9.2. Multicast Group Scoping

 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.

Macker Experimental [Page 28] RFC 6621 SMF May 2012

9.3. Interface with Exterior Multicast Routing Protocols

 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 Section 9.1, 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.

9.4. Multiple Border Routers

 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

Macker Experimental [Page 29] RFC 6621 SMF May 2012

 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 <srcAddr:
     dstAddr>) from each border router, performing DPD processing on a
     <TaggerID:dstAddr> or <TaggerID:srcAddr:dstAddr> 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 <sourceAddress:dstAddr>) 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., <srcAddr::dstAddr>) state be maintained for the
 selected TaggerId.

Macker Experimental [Page 30] RFC 6621 SMF May 2012

 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.

10. Security Considerations

 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 Section 5 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

Macker Experimental [Page 31] RFC 6621 SMF May 2012

 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 [RFC6130]
 is used, the security considerations described in [RFC6130] also
 apply.

11. IANA Considerations

 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 [RFC2780].
 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 [RFC5444].
 Finally, this document defines one Address Block TLV, a type for
 which has been allocated from the "Address Block TLV Types" registry
 of [RFC5444].

Macker Experimental [Page 32] RFC 6621 SMF May 2012

11.1. IPv6 SMF_DPD Header Extension Option Type

 IANA has allocated an IPv6 Option Type from the IPv6 "Destination
 Options and Hop-by-Hop Options" registry of [RFC2780], 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

11.2. TaggerId Types (TidTy)

 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 [RFC5226] is
 required.

Macker Experimental [Page 33] RFC 6621 SMF May 2012

11.3. Well-Known Multicast Address

 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.

11.4. SMF TLVs

11.4.1. Expert Review for Created Type Extension Registries

 Creation of Address Block TLV Types and Message TLV Types in
 registries of [RFC5444], and hence in the HELLO-message-specific
 registries of [RFC6130], 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 [RFC5444].

11.4.2. SMF Message TLV Type (SMF_TYPE)

 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 [RFC6130].
 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    | Section 11.4.4 |
 |          |      |           | algorithm          |                |
 |          |      |           | supported by the   |                |
 |          |      |           | SMF router,        |                |
 |          |      |           | originating the    |                |
 |          |      |           | HELLO message,     |                |
 |          |      |           | according to       |                |
 |          |      |           | Section 11.4.4.    |                |
 +----------+------+-----------+--------------------+----------------+
        Table 11: SMF_TYPE Message TLV Type Extension Registry

Macker Experimental [Page 34] RFC 6621 SMF May 2012

11.4.3. SMF Address Block TLV Type (SMF_NBR_TYPE)

 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 [RFC6130].
 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    |             |
 |              |        |           | Section 11.4.4. |             |
 +--------------+--------+-----------+-----------------+-------------+
   Table 12: SMF_NBR_TYPE Address Block TLV Type Extension Registry

11.4.4. SMF Relay Algorithm ID Registry

 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.

Macker Experimental [Page 35] RFC 6621 SMF May 2012

        +---------+---------+-------------+-------------------+
        | Value   | Name    | Description | Allocation Policy |
        +---------+---------+-------------+-------------------+
        | 0       | CF      | Section 4   |                   |
        | 1       | S-MPR   | Appendix B  |                   |
        | 2       | E-CDS   | Appendix A  |                   |
        | 3       | MPR-CDS | Appendix C  |                   |
        | 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 <value> field (if any).

12. Acknowledgments

 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

Macker Experimental [Page 36] RFC 6621 SMF May 2012

13. References

13.1. Normative References

 [MPR-CDS]  Adjih, C., Jacquet, P., and L. Viennot, "Computing
            Connected Dominating Sets with Multipoint Relays", Ad Hoc
            and Sensor Wireless Networks, January 2005.
 [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
            September 1981.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", RFC 2460, December 1998.
 [RFC2644]  Senie, D., "Changing the Default for Directed Broadcasts
            in Routers", BCP 34, RFC 2644, August 1999.
 [RFC2780]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
            Values In the Internet Protocol and Related Headers",
            BCP 37, RFC 2780, March 2000.
 [RFC3174]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
            (SHA1)", RFC 3174, September 2001.
 [RFC3626]  Clausen, T. and P. Jacquet, "Optimized Link State Routing
            Protocol (OLSR)", RFC 3626, October 2003.
 [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
            Architecture", RFC 4291, February 2006.
 [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
            December 2005.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [RFC5444]  Clausen, T., Dearlove, C., Dean, J., and C. Adjih,
            "Generalized Mobile Ad Hoc Network (MANET) Packet/Message
            Format", RFC 5444, February 2009.
 [RFC5614]  Ogier, R. and P. Spagnolo, "Mobile Ad Hoc Network (MANET)
            Extension of OSPF Using Connected Dominating Set (CDS)
            Flooding", RFC 5614, August 2009.

Macker Experimental [Page 37] RFC 6621 SMF May 2012

 [RFC5771]  Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
            IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
            March 2010.
 [RFC6130]  Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc
            Network (MANET) Neighborhood Discovery Protocol (NHDP)",
            RFC 6130, April 2011.

13.2. Informative References

 [CDHM07]   Chakeres, I., Danilov, C., Henderson, T., and J. Macker,
            "Connecting MANET Multicast", IEEE MILCOM
            2007 Proceedings, 2007.
 [DHG09]    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.
 [DHS08]    Danilov, C., Henderson, T., Spagnolo, T., Goff, T., and J.
            Kim, "MANET Multicast with Multiple Gateways", IEEE MILCOM
            2008 Proceedings, 2008.
 [GM99]     Garcia-Luna-Aceves, JJ. and E. Madruga, "The Core-Assisted
            Mesh Protocol", Selected Areas in Communications, IEEE
            Journal,  Volume 17, Issue 8, August 1999.
 [IPV4-ID-UPDATE]
            Touch, J., "Updated Specification of the IPv4 ID Field",
            Work in Progress, September 2011.
 [JLMV02]   Jacquet, P., Laouiti, V., Minet, P., and L. Viennot,
            "Performance of Multipoint Relaying in Ad Hoc Mobile
            Routing Protocols", Networking , 2002.
 [MDC04]    Macker, J., Dean, J., and W. Chao, "Simplified Multicast
            Forwarding in Mobile Ad hoc Networks", IEEE MILCOM 2004
            Proceedings, 2004.
 [MDDA07]   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.

Macker Experimental [Page 38] RFC 6621 SMF May 2012

 [MGL04]    Mohapatra, P., Gui, C., and J. Li, "Group Communications
            in Mobile Ad hoc Networks", IEEE Computer, Vol. 37, No. 2,
            February 2004.
 [NTSC99]   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.
 [RFC2501]  Corson, M. and J. Macker, "Mobile Ad hoc Networking
            (MANET): Routing Protocol Performance Issues and
            Evaluation Considerations", RFC 2501, January 1999.
 [RFC3684]  Ogier, R., Templin, F., and M. Lewis, "Topology
            Dissemination Based on Reverse-Path Forwarding (TBRPF)",
            RFC 3684, February 2004.
 [RFC3973]  Adams, A., Nicholas, J., and W. Siadak, "Protocol
            Independent Multicast - Dense Mode (PIM-DM): Protocol
            Specification (Revised)", RFC 3973, January 2005.
 [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
            "Protocol Independent Multicast - Sparse Mode (PIM-SM):
            Protocol Specification (Revised)", RFC 4601, August 2006.

Macker Experimental [Page 39] RFC 6621 SMF May 2012

Appendix A. Essential Connecting Dominating Set (E-CDS) Algorithm

 The "Essential Connected Dominating Set" (E-CDS) algorithm [RFC5614]
 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 [RFC5614].  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.

A.1. E-CDS Relay Set Selection Overview

 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.

Macker Experimental [Page 40] RFC 6621 SMF May 2012

 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.

A.2. E-CDS Forwarding Rules

 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.

A.3. E-CDS Neighborhood Discovery Requirements

 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

Macker Experimental [Page 41] RFC 6621 SMF May 2012

 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 <value> field is defined as shown in
 Table 14.
            +----------------+---------+-----------------+
            | Length (bytes) | Value   | Router Priority |
            +----------------+---------+-----------------+
            | 0              | N/A     | 64              |
            | 1              | <value> | 0-127           |
            +----------------+---------+-----------------+
                  Table 14: E-CDS Message TLV Values
 Where <value> 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.

Macker Experimental [Page 42] RFC 6621 SMF May 2012

     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 <value> field is used.  Multi-
 index and multivalue TLV layouts as defined in [RFC5444] 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    | <value>  | <value> is for all addresses  |
 | N             | N      | <value>* | Each address gets its own     |
 |               |        |          | <value>                       |
 +---------------+--------+----------+-------------------------------+
               Table 15: E-CDS Address Block TLV Values
 Where <value> 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

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 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.

A.4. E-CDS Selection Algorithm

 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 [RFC5614]
 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

Macker Experimental [Page 44] RFC 6621 SMF May 2012

 "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

Macker Experimental [Page 45] RFC 6621 SMF May 2012

 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.

Appendix B. Source-Based Multipoint Relay (S-MPR) Algorithm

 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 [JLMV02].  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 [RFC3626], 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.

B.1. S-MPR Relay Set Selection Overview

 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

Macker Experimental [Page 46] RFC 6621 SMF May 2012

 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 Section 8.1 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 [RFC3626]) 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.

B.2. S-MPR Forwarding Rules

 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 <srcIface>.  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 <srcIface>.
     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.

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 2.  Else, if the previous-hop transmitter is a 1-hop symmetric
     neighbor, a DPD entry is added for that packet for the
     <srcIface>, 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.

B.3. S-MPR Neighborhood Discovery Requirements

 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             | <value> | 0-127           |
             +---------------+---------+-----------------+
                  Table 16: S-MPR Message TLV Values

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 Where <value> 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" ([RFC3626]), 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 [RFC6130] 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    | <value>  | <value> is for all addresses  |
 | N             | N      | <value>* | Each address gets its own     |
 |               |        |          | <value>                       |
 +---------------+--------+----------+-------------------------------+
               Table 17: S-MPR Address Block TLV Values

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 Where <value>, 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 <start-index> 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.

B.4. S-MPR Selection Algorithm

 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":

Macker Experimental [Page 50] RFC 6621 SMF May 2012

 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)".

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 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.

Appendix C. Multipoint Relay Connected Dominating Set (MPR-CDS)

           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 [MPR-CDS].
 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.

C.1. MPR-CDS Relay Set Selection Overview

 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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     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.

C.2. MPR-CDS Forwarding Rules

 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.

C.3. MPR-CDS Neighborhood Discovery Requirements

 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

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 [RFC6130] 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 <value> 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 <value> 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.

C.4. MPR-CDS Selection Algorithm

 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 Appendix B.4 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):

Macker Experimental [Page 54] RFC 6621 SMF May 2012

     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]

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