IPv4 Multicast over an IPv6 Multicast in Softwire Mesh Network
draft-ietf-softwire-mesh-multicast-19

Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12                        
          13 14 15 16 17 18 19                                          
Softwire WG                                                        M. Xu
Internet-Draft                                                    Y. Cui
Intended status: Standards Track                                   J. Wu
Expires: March 28, 2018                                          S. Yang
                                                     Tsinghua University
                                                                 C. Metz
                                                             G. Shepherd
                                                           Cisco Systems
                                                      September 24, 2017


                        Softwire Mesh Multicast
                 draft-ietf-softwire-mesh-multicast-18

Abstract

   The Internet needs to support IPv4 and IPv6 packets.  Both address
   families and their related protocol suites support multicast of the
   single-source and any-source varieties.  During IPv6 transition,
   there will be scenarios where a backbone network running one IP
   address family internally (referred to as internal IP or I-IP), while
   the attached client networks running another IP address family
   (referred to as external IP or E-IP).  The I-IP backbone should offer
   both unicast and multicast transit services to the client E-IP
   networks.

   Softwire Mesh is a solution providing E-IP unicast and multicast
   support across an I-IP backbone.  This document describes the
   mechanism for supporting Internet-style multicast across a set of
   E-IP and I-IP networks supporting softwire mesh.  We focus on IPv4-
   over-IPv6 scenario in this document, due to lack of real-world use
   cases for IPv6-over-IPv4 scenario.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on March 28, 2018.

Copyright Notice

   Copyright (c) 2017 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
   (https://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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Scenarios of Interest . . . . . . . . . . . . . . . . . . . .   6
   4.  IPv4-over-IPv6 Mechanism  . . . . . . . . . . . . . . . . . .   7
     4.1.  Mechanism Overview  . . . . . . . . . . . . . . . . . . .   8
     4.2.  Group Address Mapping . . . . . . . . . . . . . . . . . .   8
     4.3.  Source Address Mapping  . . . . . . . . . . . . . . . . .   9
     4.4.  Routing Mechanism . . . . . . . . . . . . . . . . . . . .  10
   5.  Control Plane Functions of AFBR . . . . . . . . . . . . . . .  11
     5.1.  E-IP (*,G) and (S,G) State Maintenance  . . . . . . . . .  11
     5.2.  I-IP (S',G') State Maintenance  . . . . . . . . . . . . .  11
     5.3.  E-IP (S,G,rpt) State Maintenance  . . . . . . . . . . . .  11
     5.4.  Inter-AFBR Signaling  . . . . . . . . . . . . . . . . . .  11
     5.5.  SPT Switchover  . . . . . . . . . . . . . . . . . . . . .  14
     5.6.  Other PIM Message Types . . . . . . . . . . . . . . . . .  14
     5.7.  Other PIM States Maintenance  . . . . . . . . . . . . . .  14
   6.  Data Plane Functions of the AFBR  . . . . . . . . . . . . . .  14
     6.1.  Process and Forward Multicast Data  . . . . . . . . . . .  14
     6.2.  TTL . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     6.3.  Fragmentation . . . . . . . . . . . . . . . . . . . . . .  15
   7.  Packet Format and Translation . . . . . . . . . . . . . . . .  15
   8.  Softwire Mesh Multicast Encapsulation . . . . . . . . . . . .  16
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     11.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  18



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The Internet needs to support IPv4 and IPv6 packets.  Both address
   families and their related protocol suites support multicast of the
   single-source and any-source varieties.  During IPv6 transition,
   there will be scenarios where a backbone network running one IP
   address family internally (referred to as internal IP or I-IP) will
   provide transit services to attached client networks running another
   IP address family (referred to as external IP or E-IP).

   One solution is to leverage the multicast functions inherent in the
   I-IP backbone, to efficiently forward client E-IP multicast packets
   inside an I-IP core tree, which is rooted at one or more ingress
   Address Family Border Routers (AFBR) and branches out to one or more
   egress Address Family Border Routers (AFBR).

   [RFC4925] outlines the requirements for the softwires mesh scenario
   and includes support for multicast traffic.  It is likely that client
   E-IP multicast sources and receivers will reside in different client
   E-IP networks connected to an I-IP backbone network.  This requires
   the client E-IP source-rooted or shared tree to traverse the I-IP
   backbone network.

   One method of accomplishing this is to re-use the multicast VPN
   approach outlined in [RFC6513].  MVPN-like schemes can support the
   softwire mesh scenario and achieve a "many-to-one" mapping between
   the E-IP client multicast trees and the transit core multicast trees.
   The advantage of this approach is that the number of trees in the
   I-IP backbone network scales less than linearly with the number of
   E-IP client trees.  Corporate enterprise networks and by extension
   multicast VPNs have been known to run applications that create too
   many (S,G) states.  Aggregation at the edge contains the (S,G) states
   for customer's VPNs and these need to be maintained by the network
   operator.  The disadvantage of this approach is the possibility of
   inefficient bandwidth and resource utilization when multicast packets
   are delivered to a receiving AFBR with no attached E-IP receivers.

   Internet-style multicast is somewhat different in that the trees are
   source-rooted and relatively sparse.  The need for multicast
   aggregation at the edge (where many customer multicast trees are
   mapped into one or more backbone multicast trees) does not exist and
   to date has not been identified.  Thus the need for a basic or closer
   alignment with E-IP and I-IP multicast procedures emerges.






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   [RFC5565] describes the "Softwire Mesh Framework".  This document
   provides a more detailed description of how one-to-one mapping
   schemes ([RFC5565], Section 11.1) for IPv4 over IPv6 can be achieved.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Terminology

   Figure 1 shows an example of how a softwire mesh network can support
   multicast traffic.  A multicast source S is located in one E-IP
   client network, while candidate E-IP group receivers are located in
   the same or different E-IP client networks that all share a common
   I-IP transit network.  When E-IP sources and receivers are not local
   to each other, they can only communicate with each other through the
   I-IP core.  There may be several E-IP sources for a single multicast
   group residing in different client E-IP networks.  In the case of
   shared trees, the E-IP sources, receivers and RPs might be located in
   different client E-IP networks.  In the simplest case, a single
   operator manages the resources of the I-IP core, although the inter-
   operator case is also possible and so not precluded.



























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                 ._._._._.            ._._._._.
                |         |          |         |   --------
                |  E-IP   |          |  E-IP   |--|Source S|
                | network |          | network |   --------
                 ._._._._.            ._._._._.
                    |                    |
                   AFBR             upstream AFBR
                    |                    |
                  __+____________________+__
                 /   :   :           :   :  \
                |    :      :      :     :   |  E-IP Multicast
                |    : I-IP transit core :   |  packets are forwarded
                |    :     :       :     :   |  across the I-IP
                |    :   :            :  :   |  transit core
                 \_._._._._._._._._._._._._./
                     +                   +
                downstream AFBR    downstream AFBR
                     |                   |
                  ._._._._            ._._._._
     --------    |        |          |        |   --------
    |Receiver|-- |  E-IP  |          |  E-IP  |--|Receiver|
     --------    |network |          |network |   --------
                  ._._._._            ._._._._


                Figure 1: Softwire Mesh Multicast Framework

   Terminology used in this document:

   o Address Family Border Router (AFBR) - A router interconnecting two
   or more networks using different IP address families.  In the context
   of softwire mesh multicast, the AFBR runs E-IP and I-IP control
   planes to maintain E-IP and I-IP multicast states respectively and
   performs the appropriate encapsulation/decapsulation of client E-IP
   multicast packets for transport across the I-IP core.  An AFBR will
   act as a source and/or receiver in an I-IP multicast tree.

   o Upstream AFBR: An AFBR router that is located on the upper reaches
   of a multicast data flow.

   o Downstream AFBR: An AFBR router that is located on the lower
   reaches of a multicast data flow.

   o I-IP (Internal IP): This refers to IP address family (i.e., either
   IPv4 or IPv6) that is supported by the core (or backbone) network.






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   o E-IP (External IP): This refers to the IP address family (i.e.
   either IPv4 or IPv6) that is supported by the client network(s)
   attached to the I-IP transit core.

   o I-IP core tree: A distribution tree rooted at one or more AFBR
   source nodes and branched out to one or more AFBR leaf nodes.  An
   I-IP core tree is built using standard IP or MPLS multicast signaling
   protocols operating exclusively inside the I-IP core network.  An
   I-IP core tree is used to forward E-IP multicast packets belonging to
   E-IP trees across the I-IP core.  Another name for an I-IP core tree
   is multicast or multipoint softwire.

   o E-IP client tree: A distribution tree rooted at one or more hosts
   or routers located inside a client E-IP network and branched out to
   one or more leaf nodes located in the same or different client E-IP
   networks.

   o uPrefix46: The /96 unicast IPv6 prefix for constructing an
   IPv4-embedded IPv6 source address in IPv4-over-IPv6 scenario.

   o mPrefix46: The /96 multicast IPv6 prefix for constructing an
   IPv4-embedded IPv6 multicast address in IPv4-over-IPv6 scenario.

   o Inter-AFBR signaling: A mechanism used by downstream AFBRs to send
   PIM messages to the upstream AFBR.

3.  Scenarios of Interest

   This document focus on IPv4-over-IPv6 scenario, the following diagram
   shows the scenario.





















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                   ._._._._.            ._._._._.
                  |  IPv4   |          |  IPv4   |   --------
                  | Client  |          | Client  |--|Source S|
                  | network |          | network |   --------
                   ._._._._.            ._._._._.
                      |                    |
                     AFBR             upstream AFBR
                      |                    |
                    __+____________________+__
                   /   :   :           :   :  \
                  |    :      :      :     :   |
                  |    : IPv6 transit core :   |
                  |    :     :       :     :   |
                  |    :   :            :  :   |
                   \_._._._._._._._._._._._._./
                       +                   +
                  downstream AFBR     downstream AFBR
                       |                   |
                    ._._._._            ._._._._
       --------    |  IPv4  |          |  IPv4  |   --------
      |Receiver|-- | Client |          | Client |--|Receiver|
       --------    | network|          | network|   --------
                    ._._._._            ._._._._


                     Figure 2: IPv4-over-IPv6 Scenario

   In Figure 2, the E-IP client networks run IPv4 and the I-IP core runs
   IPv6.

   Because of the much larger IPv6 group address space, the client
   E-IPv4 tree can be mapped to a specific I-IPv6 core tree.  This
   simplifies operations on the AFBR because it becomes possible to
   algorithmically map an IPv4 group/source address to an IPv6 group/
   source address and vice-versa.

   The IPv4-over-IPv6 scenario is an emerging requirement as network
   operators build out native IPv6 backbone networks.  These networks
   support native IPv6 services and applications but in many cases,
   support for legacy IPv4 unicast and multicast services will also need
   to be accomodated.

4.  IPv4-over-IPv6 Mechanism








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4.1.  Mechanism Overview

   Routers in the client E-IPv4 networks have routes to all other client
   E-IPv4 networks.  Through PIM messages, E-IPv4 hosts and routers have
   discovered or learnt of (S,G) or (*,G) IPv4 addresses.  Any I-IPv6
   multicast state instantiated in the core is referred to as (S',G') or
   (*,G') and is certainly separated from E-IPv4 multicast state.

   Suppose a downstream AFBR receives an E-IPv4 PIM Join/Prune message
   from the E-IPv4 network for either an (S,G) tree or a (*,G) tree.
   The AFBR can translate the E-IPv4 PIM message into an I-IPv6 PIM
   message with the latter being directed towards the I-IP IPv6 address
   of the upstream AFBR.  When the I-IPv6 PIM message arrives at the
   upstream AFBR, it is translated back into an E-IPv4 PIM message.  The
   result of these actions is the construction of E-IPv4 trees and a
   corresponding I-IP tree in the I-IP network.  An example of the
   packet format and traslation is provided in Section 8.

   In this case, it is incumbent upon the AFBR routers to perform PIM
   message conversions in the control plane and IP group address
   conversions or mappings in the data plane.  The AFBRs perform an
   algorithmic, one-to-one mapping of IPv4-to-IPv6.

4.2.  Group Address Mapping

   For the IPv4-over-IPv6 scenario, a simple algorithmic mapping between
   IPv4 multicast group addresses and IPv6 group addresses is performed.
   Figure 4 shows the reminder of the format:


     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
     | 0-------------32--40--48--56--64--72--80--88--96-----------127|
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
     |                    mPrefix46                  |group  address |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


           Figure 4: IPv4-Embedded IPv6 Multicast Address Format

   An IPv6 multicast prefix (mPrefix46) is assigned to each AFBR.  AFBRs
   will prepend the prefix to an IPv4 multicast group address when
   translating it to an IPv6 multicast group address.

   The construction of the mPrefix46 for SSM is the same as the
   construction of the mPrefix64 described in Section 5 of [RFC8114].

   With this scheme, each IPv4 multicast address can be mapped into an
   IPv6 multicast address (with the assigned prefix), and each IPv6



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   multicast address with the assigned prefix can be mapped into an IPv4
   multicast address.  The group address translation algorithm can be
   reffered in Section 5.2 of [RFC8114].

4.3.  Source Address Mapping

   There are two kinds of multicast: ASM and SSM.  Considering that the
   I-IP network and E-IP network may support different kinds of
   multicast, the source address translation rules needed to support all
   possible scenarios may become very complex.  But since SSM can be
   implemented with a strict subset of the PIM-SM protocol mechanisms
   [RFC7761], we can treat the I-IP core as SSM-only to make it as
   simple as possible.  There then remain only two scenarios to be
   discussed in detail:

   o  E-IP network supports SSM

      One possible way to make sure that the translated I-IPv6 PIM
      message reaches upstream AFBR is to set S' to a virtual IPv6
      address that leads to the upstream AFBR.  Figure 5 is the
      recommended address format based on [RFC6052]:




      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      | 0-------------32--40--48--56--64--72--80--88--96-----------127|
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |     prefix    |v4(32)         | u | suffix    |source address |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |<------------------uPrefix46------------------>|


        Figure 5: IPv4-Embedded IPv6 Virtual Source Address Format

      In this address format,

      *  The "prefix" field contains a "Well-Known" prefix or an ISP-
         defined prefix.  An existing "Well-Known" prefix is 64:ff9b,
         which is defined in [RFC6052];

      *  The "v4" field is the IP address of one of upstream AFBR's
         E-IPv4 interfaces;

      *  The "u" field is defined in [RFC4291], and MUST be set to zero;

      *  The "suffix" field is reserved for future extensions and SHOULD
         be set to zero;



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      *  The "source address" field stores the original source address.

      We call the overall /96 prefix ("prefix" field and "v4" field and
      "u" field and "suffix" field altogether) "uPrefix46".



   o  E-IP network supports ASM

      The (S,G) source list entry and the (*,G) source list entry only
      differ in that the latter has both the WC and RPT bits of the
      Encoded-Source-Address set, while the former is all cleared (See
      Section 4.9.5.1 of [RFC7761]).  So we can translate source list
      entries in (*,G) messages into source list entries in (S'G')
      messages by applying the format specified in Figure 5 and clearing
      both the WC and RPT bits at downstream AFBRs, and vice-versa for
      the reverse translation at upstream AFBRs.



4.4.  Routing Mechanism

   In the mesh multicast scenario, routing information is REQUIRED to be
   distributed among AFBRs to make sure that the PIM messages that a
   downstream AFBR propagates reach the right upstream AFBR.

   Every AFBR MUST know the /32 prefix in "IPv4-Embedded IPv6 Virtual
   Source Address Format".  To achieve this, every AFBR should announce
   one of its E-IPv4 interfaces in the "v4" field, and the corresponding
   uPrefix46.  The announcement SHOULD be sent to the other AFBRs
   through MBGP [RFC4760].  Since every IP address of upstream AFBR's
   E-IPv4 interface is different from each other, every uPrefix46 that
   AFBR announces MUST be different, and uniquely identifies each AFBR.
   "uPrefix46" is an IPv6 prefix, and the distribution mechanism is the
   same as the traditional mesh unicast scenario.  But "v4" field is an
   E-IPv4 address, and BGP messages are NOT tunneled through softwires
   or any other mechanism specified in [RFC5565], AFBRs MUST be able to
   transport and encode/decode BGP messages that are carried over
   I-IPv6, whose NLRI and NH are of E-IPv4 address family.

   In this way, when a downstream AFBR receives an E-IPv4 PIM (S,G)
   message, it can translate this message into (S',G') by looking up the
   IP address of the corresponding AFBR's E-IPv4 interface.  Since the
   uPrefix46 of S' is unique, and is known to every router in the I-IPv6
   network, the translated message will be forwarded to the
   corresponding upstream AFBR, and the upstream AFBR can translate the
   message back to (S,G).  When a downstream AFBR receives an E-IPv4 PIM
   (*,G) message, S' can be generated according to the format specified



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   in Figure 4, with "source address" field set to *(the IPv4 address of
   RP).  The translated message will be forwarded to the corresponding
   upstream AFBR.  Since every PIM router within a PIM domain MUST be
   able to map a particular multicast group address to the same RP (see
   Section 4.7 of [RFC7761]), when the upstream AFBR checks the "source
   address" field of the message, it finds the IPv4 address of the RP,
   and assertains that this is originally a (*,G) message.  This is then
   translated back to the (*,G) message and processed.

5.  Control Plane Functions of AFBR

   AFBRs are responsible for the following functions:

5.1.  E-IP (*,G) and (S,G) State Maintenance

   E-IP (*,G) and (S,G) state maintenance on AFBR is the same as E-IP
   (*,G) and (S,G) state maintenance on mAFTR described in Section 7.2
   of [RFC8114]

5.2.  I-IP (S',G') State Maintenance

   It is possible that the I-IP transit core runs another non-transit
   I-IP PIM-SSM instance.  Since the translated source address starts
   with the unique "Well-Known" prefix or the ISP-defined prefix that
   SHOULD NOT be used by other service provider, mesh multicast will not
   influence non-transit PIM-SSM multicast at all.  When an AFBR
   receives an I-IP (S',G') message, it MUST check S'.  If S' starts
   with the unique prefix, then the message is actually a translated
   E-IP (S,G) or (*,G) message, and the AFBR MUST translate this message
   back to E-IP PIM message and process it.

5.3.  E-IP (S,G,rpt) State Maintenance

   When an AFBR wishes to propagate a Join/Prune(S,G,rpt) message to an
   I-IP upstream router, the AFBR MUST operate as specified in
   Section 6.5 and Section 6.6.

5.4.  Inter-AFBR Signaling

   Assume that one downstream AFBR has joined a RPT of (*,G) and a SPT
   of (S,G), and decide to perform an SPT switchover.  According to
   [RFC7761], it SHOULD propagate a Prune(S,G,rpt) message along with
   the periodical Join(*,G) message upstream towards RP.  However,
   routers in the I-IP transit core do not process (S,G,rpt) messages
   since the I-IP transit core is treated as SSM-only.  As a result, the
   downstream AFBR is unable to prune S from this RPT, so it will
   receive two copies of the same data of (S,G).  In order to solve this




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   problem, we introduce a new mechanism for downstream AFBRs to inform
   upstream AFBRs of pruning any given S from an RPT.

   When a downstream AFBR wishes to propagate a (S,G,rpt) message
   upstream, it SHOULD encapsulate the (S,G,rpt) message, then send the
   encapsulated unicast message to the corresponding upstream AFBR,
   which we call "RP'".

   When RP' receives this encapsulated message, it SHOULD decapsulate
   the message as in the unicast scenario, and retrieve the original
   (S,G,rpt) message.  The incoming interface of this message may be
   different to the outgoing interface which propagates multicast data
   to the corresponding downstream AFBR, and there may be other
   downstream AFBRs that need to receive multicast data of (S,G) from
   this incoming interface, so RP' SHOULD NOT simply process this
   message as specified in [RFC7761] on the incoming interface.

   To solve this problem, we introduce an "interface agent" to process
   all the encapsulated (S,G,rpt) messages the upstream AFBR receives.
   The interface agent's RP' SHOULD prune S from the RPT of group G when
   no downstream AFBR is subscribed to receive multicast data of (S,G)
   along the RPT.

   In this way, we ensure that downstream AFBRs will not miss any
   multicast data that they need.  The cost of this is that multicast
   data of (S,G) will be duplicated along the RPT received by SPT-
   switched-over downstream AFBRs, if at least one downstream AFBR
   exists that has not yet sent Prune(S,G,rpt) messages to the upstream
   AFBR.

   In certain deployment scenarios (e.g. if there is only a single
   downstream router), the interface agent function is not required.

   The mechanism used to achieve this is left to the implementation.
   The following diagram provides one possible solution for an
   "interface agent" implementation:















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          +----------------------------------------+
          |                                        |
          |       +-----------+----------+         |
          |       |  PIM-SM   |    UDP   |         |
          |       +-----------+----------+         |
          |          ^                |            |
          |          |                |            |
          |          |                v            |
          |       +----------------------+         |
          |       |       I/F Agent      |         |
          |       +----------------------+         |
          |   PIM    ^                | multicast  |
          | messages |                |   data     |
          |          |  +-------------+---+        |
          |       +--+--|-----------+     |        |
          |       |     v           |     v        |
          |     +--------- +     +----------+      |
          |     | I-IP I/F |     | I-IP I/F |      |
          |     +----------+     +----------+      |
          |        ^     |          ^     |        |
          |        |     |          |     |        |
          +--------|-----|----------|-----|--------+
                   |     v          |     v



             Figure 7: Interface Agent Implementation Example

   Figure 7 shows an example of interface agent implementation using UDP
   encapsulation.  The interface agent has two responsibilities: In the
   control plane, it SHOULD work as a real interface that has joined
   (*,G), representing of all the I-IP interfaces which are outgoing
   interfaces of the (*,G) state machine, and process the (S,G,rpt)
   messages received from all the I-IP interfaces.

   The interface agent maintains downstream (S,G,rpt) state machines of
   every downstream AFBR, and submits Prune (S,G,rpt) messages to the
   PIM-SM module only when every (S,G,rpt) state machine is at Prune(P)
   or PruneTmp(P') state, which means that no downstream AFBR is
   subscribed to receive multicast data of (S,G) along the RPT of G.
   Once a (S,G,rpt) state machine changes to NoInfo(NI) state, which
   means that the corresponding downstream AFBR has switched to receive
   multicast data of (S,G) along the RPT again, the interface agent
   SHOULD send a Join (S,G,rpt) to the PIM-SM module immediately.

   In the data plane, upon receiving a multicast data packet, the
   interface agent SHOULD encapsulate it at first, then propagate the
   encapsulated packet from every I-IP interface.



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   NOTICE: It is possible that an E-IP neighbor of RP' that has joined
   the RPT of G, so the per-interface state machine for receiving E-IP
   Join/Prune (S,G,rpt) messages SHOULD be preserved.

5.5.  SPT Switchover

   After a new AFBR requests the receipt of traffic destined for a
   multicast group, it will receive all the data from the RPT at first.
   At this time, every downstream AFBR will receive multicast data from
   any source from this RPT, in spite of whether they have switched over
   to an SPT of some source(s) or not.

   To minimize this redundancy, it is recommended that every AFBR's
   SwitchToSptDesired(S,G) function employs the "switch on first packet"
   policy.  In this way, the delay in switchover to SPT is kept as small
   as possible, and after the moment that every AFBR has performed the
   SPT switchover for every S of group G, no data will be forwarded in
   the RPT of G, thus no more unnecessary duplication will be produced.

5.6.  Other PIM Message Types

   In addition to Join or Prune, other message types exist, including
   Register, Register-Stop, Hello and Assert.  Register and Register-
   Stop messages are sent by unicast, while Hello and Assert messages
   are only used between directly linked routers to negotiate with each
   other.  It is not necessary to translate these for forwarding, thus
   the processing of these messages is out of scope for this document.

5.7.  Other PIM States Maintenance

   In addition to states mentioned above, other states exist, including
   (*,*,RP) and I-IP (*,G') state.  Since we treat the I-IP core as SSM-
   only, the maintenance of these states is out of scope for this
   document.

6.  Data Plane Functions of the AFBR

6.1.  Process and Forward Multicast Data

   The data plane behavior on AFBR is similar with the data plane
   behavior on mAFTR described in Section 7.4 of [RFC8114].  If there is
   at least one outgoing interface whose IP address family is different
   from the incoming interface, the AFBR MUST encapsulate this packet
   with mPrefix46-derived and uPrefix46-derived IPv6 address to form an
   IPv6 multicast packet.






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6.2.  TTL

   Processing of TTL information in protocol headers depends on the
   tunneling technology, and it is out of scope of this document.

6.3.  Fragmentation

   The encapsulation performed by an upstream AFBR will increase the
   size of packets.  As a result, the outgoing I-IP link MTU may not
   accommodate the larger packet size.  As it is not always possible for
   core operators to increase the MTU of every link.  Fragmentation
   after encapsulation and reassembling of encapsulated packets MUST be
   supported by AFBRs [RFC5565].

7.  Packet Format and Translation

   Because the PIM-SM Specification is independent of the underlying
   unicast routing protocol, the packet format in Section 4.9 of
   [RFC7761] remains the same, except that the group address and source
   address MUST be translated when traversing AFBR.

   For example, Figure 8 shows the register-stop message format in IPv4
   and IPv6 address family.




























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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |PIM Ver| Type  |   Reserved    |           Checksum            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             IPv4 Group Address (Encoded-Group format)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            IPv4 Source Address (Encoded-Unicast format)       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    (1). IPv4 Register-Stop Message Format

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |PIM Ver| Type  |   Reserved    |           Checksum            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             IPv6 Group Address (Encoded-Group format)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            IPv6 Source Address (Encoded-Unicast format)       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    (2). IPv6 Register-Stop Message Format


                  Figure 8: Register-Stop Message Format

   In Figure 8, the semantics of fields "PIM Ver", "Type", "Reserved",
   and "Checksum" remain the same.

   IPv4 Group Address (Encoded-Group format): The encoded-group format
   of the IPv4 group address described in Section 4.2.

   IPv4 Source Address (Encoded-Group format): The encoded-unicast
   format of the IPv4 source address described in Section 4.3.

   IPv6 Group Address (Encoded-Group format): The encoded-group format
   of the IPv6 group address described in Section 4.2.

   IPv6 Source Address (Encoded-Group format): The encoded-unicast
   format of the IPv6 source address described in Section 4.3.

8.  Softwire Mesh Multicast Encapsulation

   Softwire mesh multicast encapsulation does not require the use of any
   one particular encapsulation mechanism.  Rather, it MUST accommodate
   a variety of different encapsulation mechanisms, and allow the use of
   encapsulation mechanisms mentioned in [RFC4925].  Additionally, all
   of the AFBRs attached to the I-IP network MUST implement the same
   encapsulation mechanism.



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9.  Security Considerations

   The security concerns raised in [RFC4925] and [RFC7761] are
   applicable here.  In addition, the additional workload associated
   with some schemes could be exploited by an attacker to perform a out
   DDoS attack.  Compared with [RFC4925], the security concerns SHOULD
   be considered more carefully: an attacker could potentially set up
   many multicast trees in the edge networks, causing too many multicast
   states in the core network.

10.  IANA Considerations

   This document includes no request to IANA.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <https://www.rfc-editor.org/info/rfc4760>.

   [RFC4925]  Li, X., Ed., Dawkins, S., Ed., Ward, D., Ed., and A.
              Durand, Ed., "Softwire Problem Statement", RFC 4925,
              DOI 10.17487/RFC4925, July 2007,
              <https://www.rfc-editor.org/info/rfc4925>.

   [RFC5565]  Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
              Framework", RFC 5565, DOI 10.17487/RFC5565, June 2009,
              <https://www.rfc-editor.org/info/rfc5565>.







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   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              DOI 10.17487/RFC6052, October 2010,
              <https://www.rfc-editor.org/info/rfc6052>.

   [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
              BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
              2012, <https://www.rfc-editor.org/info/rfc6513>.

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <https://www.rfc-editor.org/info/rfc7761>.

   [RFC8114]  Boucadair, M., Qin, C., Jacquenet, C., Lee, Y., and Q.
              Wang, "Delivery of IPv4 Multicast Services to IPv4 Clients
              over an IPv6 Multicast Network", RFC 8114,
              DOI 10.17487/RFC8114, March 2017,
              <https://www.rfc-editor.org/info/rfc8114>.

11.2.  Informative References

   [RFC7371]  Boucadair, M. and S. Venaas, "Updates to the IPv6
              Multicast Addressing Architecture", RFC 7371,
              DOI 10.17487/RFC7371, September 2014,
              <https://www.rfc-editor.org/info/rfc7371>.

Appendix A.  Acknowledgements

   Wenlong Chen, Xuan Chen, Alain Durand, Yiu Lee, Jacni Qin and Stig
   Venaas provided useful input into this document.

Authors' Addresses

   Mingwei Xu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084
   P.R. China

   Phone: +86-10-6278-5822
   Email: xumw@tsinghua.edu.cn








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   Yong Cui
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084
   P.R. China

   Phone: +86-10-6278-5822
   Email: cuiyong@tsinghua.edu.cn


   Jianping Wu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084
   P.R. China

   Phone: +86-10-6278-5983
   Email: jianping@cernet.edu.cn


   Shu Yang
   Tsinghua University
   Graduate School at Shenzhen
   Shenzhen  518055
   P.R. China

   Phone: +86-10-6278-5822
   Email: yangshu@csnet1.cs.tsinghua.edu.cn


   Chris Metz
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Phone: +1-408-525-3275
   Email: chmetz@cisco.com


   Greg Shepherd
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Phone: +1-541-912-9758
   Email: shep@cisco.com



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