Softwire WG                                                        M. Xu
Internet-Draft                                                    Y. Cui
Intended status: Standards Track                                   J. Wu
Expires: November 23, 2016 May 17, 2017                                            S. Yang
                                                     Tsinghua University
                                                                 C. Metz
                                                             G. Shepherd
                                                           Cisco Systems
                                                            May 22,
                                                       November 13, 2016

                        Softwire Mesh Multicast
                 draft-ietf-softwire-mesh-multicast-13
                 draft-ietf-softwire-mesh-multicast-14

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) will
   provide transit services to attached client networks running another
   IP address family (referred to as external IP or E-IP).  It is
   expected that the I-IP backbone will offer unicast and multicast
   transit services to the client E-IP networks.

   Softwire Mesh is a solution to 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.

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
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   This Internet-Draft will expire on November 23, 2016. May 17, 2017.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Scenarios of Interest . . . . . . . . . . . . . . . . . . . .   6
     3.1.  IPv4-over-IPv6  . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  IPv6-over-IPv4  . . . . . . . . . . . . . . . . . . . . .   7
   4.  IPv4-over-IPv6 Mechanism  . . . . . . . . . . . . . . . . . .   9
     4.1.  Mechanism Overview  . . . . . . . . . . . . . . . . . . .   9
     4.2.  Group Address Mapping . . . . . . . . . . . . . . . . . .   9
     4.3.  Source Address Mapping  . . . . . . . . . . . . . . . . .  10
     4.4.  Routing Mechanism . . . . . . . . . . . . . . . . . . . .  11
   5.  IPv6-over-IPv4 Mechanism  . . . . . . . . . . . . . . . . . .  12
     5.1.  Mechanism Overview  . . . . . . . . . . . . . . . . . . .  12
     5.2.  Group Address Mapping . . . . . . . . . . . . . . . . . .  12
     5.3.  Source Address Mapping  . . . . . . . . . . . . . . . . .  12
     5.4.  Routing Mechanism . . . . . . . . . . . . . . . . . . . .  13  14
   6.  Control Plane Functions of AFBR . . . . . . . . . . . . . . .  14
     6.1.  E-IP (*,G) State Maintenance  . . . . . . . . . . . . . .  14
     6.2.  E-IP (S,G) State Maintenance  . . . . . . . . . . . . . .  14
     6.3.  I-IP (S',G') State Maintenance  . . . . . . . . . . . . .  14  15
     6.4.  E-IP (S,G,rpt) State Maintenance  . . . . . . . . . . . .  15
     6.5.  Inter-AFBR Signaling  . . . . . . . . . . . . . . . . . .  15
     6.6.  SPT Switchover  . . . . . . . . . . . . . . . . . . . . .  17
     6.7.  Other PIM Message Types . . . . . . . . . . . . . . . . .  17
     6.8.  Other PIM States Maintenance  . . . . . . . . . . . . . .  17
   7.  Data Plane Functions of the AFBR  . . . . . . . . . . . . . . . .  17  18
     7.1.  Process and Forward Multicast Data  . . . . . . . . . . .  17  18
     7.2.  Selecting a Tunneling Technology  . . . . . . . . . . . .  18
     7.3.  TTL . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.4.  Fragmentation . . . . . . . . . . . . . . . . . . . . . .  18
   8.  Packet Format and Translation . . . . . . . . . . . . . . . .  18
   9.  Softwire Mesh Multicast Encapsulation . . . . . . . . . . . .  19
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  20
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     12.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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).

   The preferred

   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 roots is rooted at one or more ingress AFBR
   nodes and branches out to one or more egress AFBR leaf nodes.

   [RFC4925] outlines the requirements for the softwires mesh scenario
   including the multicast.
   and includes support for multicast traffic.  It is straightforward to envisage 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 that
   the client E-IP source-rooted or shared tree should to traverse the I-IP
   backbone network.

   One method to accomplish 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
   that
   for customer's VPNs and these need to be maintained by the network operator supporting the
   customer VPNs.
   operator.  The disadvantage of this approach is the possible possibility of
   inefficient bandwidth and resource utilization when multicast packets
   are delivered to a receiver receiving AFBR with no attached E-IP receivers.

   Internet-style multicast is somewhat different in that the trees are
   relatively sparse
   source-rooted and source-rooted. relatively sparse.  The need for multicast
   aggregation at the edge (where many customer multicast trees are
   mapped into a few or 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.

   A framework on how to support such methods is described in [RFC5565].
   In this document,

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

   An

   Figure 1 shows an example of how a softwire mesh network supporting can support
   multicast is
   illustrated in Figure 1. 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 some 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 simple case single
   operator manages the resources of the I-IP core are managed by a single operator core, although the inter-
   provider
   operator case is also possible and so not precluded.

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

                Figure 1: Softwire Mesh Multicast Framework

   Terminologies

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

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

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

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

   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 uPrefix64: The /96 unicast IPv6 prefix for constructing an
   IPv4-embedded IPv6 source address in IPv6-over-IPv4 scenario.

   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 section describes the two different scenarios where that softwires
   mesh multicast will apply. is appliacable to.

3.1.  IPv4-over-IPv6
                   ._._._._.            ._._._._.
                  |  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 this scenario, Figure 2, the E-IP client networks run IPv4 and the I-IP core runs
   IPv6.  This scenario is illustrated in Figure 2.

   Because of the much larger IPv6 group address space, it will not be a
   problem to map individual 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 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
   naturally
   support native IPv6 services and applications but it is
   with near 100% certainty that in many cases,
   support for legacy IPv4 networks handling unicast and multicast MUST services will also need
   to be accommodated. accomodated.

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

                     Figure 3: IPv6-over-IPv4 Scenario

   In this scenario, Figure 3, the E-IP Client Networks run IPv6 while the I-IP core
   runs IPv4.  This scenario is illustrated in Figure 3.

   IPv6 multicast group addresses are longer than IPv4 multicast group
   addresses.  It will
   addresses so it is not be possible to perform an algorithmic IPv6 - to -
   IPv4 address mapping without the risk of multiple IPv6 group
   addresses mapped to the same IPv4 address address, resulting in unnecessary
   bandwidth and resource consumption.  Therefore consumption.Therefore, additional efforts will
   be REQUIRED required to ensure that client E-IPv6 multicast packets can be
   injected into the correct I-IPv4 multicast trees at the AFBRs.  This
   clear mismatch in IPv6 and IPv4 group address lengths means that it
   will not be possible to perform a one-to-one mapping between IPv6 and
   IPv4 group addresses unless the IPv6 group address is scoped. scoped, such as
   applying a "Well-Known" prefix or an ISP-defined prefix.

   As mentioned earlier, this scenario is common in the MVPN
   environment.  As native IPv6 deployments and multicast applications
   emerge from the outer reaches of the greater public IPv4 Internet, it
   is envisaged that the IPv6 over IPv4 softwire mesh multicast scenario
   will be a necessary feature supported by network operators.

4.  IPv4-over-IPv6 Mechanism

4.1.  Mechanism Overview

   Routers in the client E-IPv4 networks contain have routes to all other client
   E-IPv4 networks.  Through the set of known and deployed
   mechanisms, 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 MUST be 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.  It becomes possible to
   devise  The AFBRs perform an algorithmic
   algorithmic, one-to-one IPv4-to-IPv6 address mapping at
   AFBRs. 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 supported.
   [RFC7371] has already defined an applicable format. performed.
   Figure 4 is 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

   The mPrefix46 for SSM mode is also

   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 mPrefix46 for SSM mode is also defined in Section 4.1 of
   [RFC7371] :

   o  ff3x:0:8000::/96 ('x' is any valid scope)
   With this scheme, each IPv4 multicast address can be mapped into an
   IPv6 multicast address (with the assigned prefix), and each IPv6
   multicast address with the assigned prefix can be mapped into an IPv4
   multicast address.

4.3.  Source Address Mapping

   There are two kinds of multicast --- multicast: ASM and SSM.  Considering that I-IP
   network and E-IP network may support different kind kinds of multicast,
   the source address translation rules could be very complex needed to support all possible scenarios.
   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, possible.
   There then there remains 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

      *  The "prefix" field contains a "Well-Known" prefix or an ISP-defined 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;

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

      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 translate them
      back vice-versa for
      the reverse translation at upstream AFBRs vice-versa. 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.

   To make it feasible,

   Every AFBR MUST know the /32 prefix in "IPv4-Embedded IPv6 Virtual
   Source Address Format" MUST be known to every AFBR, and Format".  To achieve this, every AFBR should not only announce the IP address of
   one of its E-IPv4 interfaces presented in the "v4" field to other AFBRs by MPBGP, but
   also announce field, and the corresponding uPrefix46
   uPrefix46.  The announcement SHOULD be sent to the I-IPv6 network. other AFBRs
   through MBGP.  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 of it mechanism is the
   same as the
   distribution in the traditional mesh unicast scenario.  But since "v4" field is an
   E-IPv4 address, and BGP messages are NOT tunneled through softwires
   or through any other mechanism as 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 eventually arrive at 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
   in Figure 4, with "source address" field set to *(the IPv4 address of
   RP).  The translated message will eventually arrive at 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 this the upstream AFBR checks the "source
   address" field of the message, it will find finds the IPv4 address of the RP, so this upstream AFBR judges
   and assertains that this is originally a (*,G) message, message.  This is then it translates the message
   translated back to the (*,G) message and processes it. processed.

5.  IPv6-over-IPv4 Mechanism

5.1.  Mechanism Overview

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

   This particular scenario introduces unique challenges.  Unlike the
   IPv4-over-IPv6 scenario, it is impossible to map all of the IPv6
   multicast address space into the IPv4 address space to address the
   one-to-one Softwire Multicast requirement.  To coordinate with the
   "IPv4-over-IPv6" scenario and keep the solution as simple as
   possible, one possible solution to this problem is to limit the scope
   of the E-IPv6 source addresses for mapping, such as applying a "Well-
   Known" prefix or an ISP-defined prefix.

5.2.  Group Address Mapping

   To keep one-to-one group address mapping simple, the group address
   range of E-IP IPv6 can be reduced in a number of ways to limit the
   scope of addresses that need to be mapped into the I-IP IPv4 space.

   A recommended multicast address format is defined in [RFC7371].  The

   For example, the high order bits of the E-IPv6 address range will be
   fixed for mapping purposes.  With this scheme, each IPv4 multicast
   address can be mapped into an IPv6 multicast address (with the
   assigned prefix), and each IPv6 multicast address with the assigned
   prefix can be mapped into an IPv4 multicast address.

5.3.  Source Address Mapping

   There are two kinds of multicast --- ASM and SSM.  Considering that
   I-IP network and E-IP network may support different kind of
   multicast, the source address translation rules could be very complex needed to support all
   possible scenarios. 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, possible.  There then there remains remain only two scenarios to be
   discussed in detail:

   o  E-IP network supports SSM

      To make sure that the translated I-IPv4 PIM message reaches the
      upstream AFBR, we need to set S' to an IPv4 address that leads to
      the upstream AFBR.  But due to the non-"one-to-one" mapping of
      E-IPv6 to I-IPv4 unicast address, the upstream AFBR is unable to
      remap the I-IPv4 source address to the original E-IPv6 source
      address without any constraints.

      We apply a fixed IPv6 prefix and static mapping to solve this
      problem.  A recommended source address format is defined in
      [RFC6052].  Figure 6 is the reminder of the format:

      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      | 0-------------32--40--48--56--64--72--80--88--96-----------127|
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                     uPrefix64                 |source address |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

            Figure 6: IPv4-Embedded IPv6 Source Address Format

      In this address format, the "uPrefix64" field starts with a "Well-
      Known" prefix or an ISP-defined prefix.  An existing "Well-Known"
      prefix is 64:ff9b/32, which is defined in [RFC6052]; The "source
      address" field is the corresponding I-IPv4 source address.

   o  The 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 setting
      both the WC and RPT bits at downstream AFBRs, and translate them
      back vice-versa for
      the reverse translation at upstream AFBRs vice-versa. AFBRs.  Here, the E-IPv6
      address of RP MUST follow the format specified in Figure 6.  RP'
      is the upstream AFBR that locates between RP and the downstream
      AFBR.

5.4.  Routing Mechanism

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

   To make it feasible, the /96 uPrefix64 MUST be known to every AFBR,
   every E-IPv6 address of sources that support mesh multicast MUST
   follow the format specified in Figure 6, and the corresponding
   upstream AFBR of this source MUST announce the I-IPv4 address in
   "source address" field of this source's IPv6 address to the I-IPv4
   network.  Since uPrefix64 is static and unique in IPv6-over-IPv4
   scenario, there is no need to distribute it using BGP.  The
   distribution of "source address" field of multicast source addresses
   is a pure I-IPv4 process and no more specification is needed.

   In this way, when a downstream AFBR receives a (S,G) message, it can
   translate the message into (S',G') by simply taking off the prefix in
   S.  Since S' is known to every router in I-IPv4 network, the
   translated message will eventually arrive at be forwarded to the corresponding upstream
   AFBR, and the upstream AFBR can translate the message back to (S,G)
   by appending the prefix to S'.  When a downstream AFBR receives a
   (*,G) message, it can translate it into (S',G') by simply taking off
   the prefix in *(the E-IPv6 address of RP).  Since S' is known to
   every router in I-IPv4 network, the translated message will
   eventually arrive at be
   forwarded to RP'.  And 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]), RP' knows that S' is the mapped
   I-IPv4 address of RP, so RP' will translate the message back to (*,G)
   by appending the prefix to S' and propagate it towards RP.

6.  Control Plane Functions of AFBR

   The

   AFBRs are responsible for the following functions:

6.1.  E-IP (*,G) State Maintenance

   When an AFBR wishes to propagate a Join/Prune(*,G) message to an I-IP
   upstream router, the AFBR MUST translate Join/Prune(*,G) messages
   into Join/Prune(S',G') messages following the rules specified above,
   then send the latter.

6.2.  E-IP (S,G) State Maintenance

   When an AFBR wishes to propagate a Join/Prune(S,G) message to an I-IP
   upstream router, the AFBR MUST translate Join/Prune(S,G) messages
   into Join/Prune(S',G') messages following the rules specified above,
   then send the latter.

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

   It is possible that the I-IP transit core runs other 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 otherwise, by other service provider, mesh multicast will not
   influence non-transit PIM-SSM multicast at all.  When one an AFBR
   receives an I-IP (S',G') message, it MUST check S'.  If S' starts
   with the unique prefix, it
   means that this then the message is actually a translated
   E-IP (S,G) or (*,G) message, then and the AFBR MUST translate this message
   back to E-IP PIM message and process it.

6.4.  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 do operate as specified in
   Section 6.5 and Section 6.6.

6.5.  Inter-AFBR Signaling

   Assume that one downstream AFBR has joined a RPT of (*,G) and a SPT
   of (S,G), and decide to perform a SPT switchover.  According to
   [RFC7761], it SHOULD propagate a Prune(S,G,rpt) message along with
   the periodical Join(*,G) message upstream towards RP.  Unfortunately,  However,
   routers in the I-IP transit core are do not supposed to understand process (S,G,rpt) messages
   since the I-IP transit core is treated as SSM-only.  As a result, this the
   downstream AFBR is unable to prune S from this RPT, then so it will
   receive two copies of the same data of (S,G).  In order to solve this
   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
   this
   the message as what it does in the unicast scenario, and get retrieve the original
   (S,G,rpt) message.  The incoming interface of this message may be
   different from 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 and keep the solution as simple simply as possible, we introduce an
   "interface agent" to process all the encapsulated (S,G,rpt) messages
   the upstream AFBR receives, and prune S from the RPT of group G when
   no downstream AFBR wants is subscribed to receive multicast data of (S,G)
   along the RPT.  In this way, we do insure ensure that downstream AFBRs will not
   miss any multicast data that they need, at the cost of duplicated
   multicast data of (S,G) along the RPT received by SPT-switched-over
   downstream AFBRs, if there exists at least one downstream AFBR exists that has not
   yet sent Prune(S,G,rpt) messages to the upstream AFBR.  The following
   diagram shows an example of how an "interface agent" MAY be
   implemented:

          +----------------------------------------+
          |                                        |
          |       +-----------+----------+         |
          |       |  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 where we
   choose using UDP
   encapsulation.  The interface agent has two responsibilities: In the
   control plane, it SHOULD work as a real interface that has joined (*,G) in representative
   (*,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) 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 wants 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 changed its mind switched to receive multicast data
   of (S,G) along the RPT again, the interface agent SHOULD send a Join(S,G,rpt) 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 onto from
   every I-IP interface.

   NOTICE: There may exist 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)
   Join/Prune (S,G,rpt) messages SHOULD still take effect. keep alive.

6.6.  SPT Switchover

   After a new AFBR expresses its interest in receiving 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 of in switchover to SPT is kept as
   little 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 redundancy will be produced.

6.7.  Other PIM Message Types

   Apart from Join or Prune, there exists 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 them these for forwarding, thus
   the process processing of these messages is out of scope for this document.

6.8.  Other PIM States Maintenance

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

7.  Data Plane Functions of the AFBR

7.1.  Process and Forward Multicast Data

   On receiving multicast data from upstream routers, the AFBR looks up checks
   its forwarding table to check find the IP address of each outgoing
   interface.  If there exists is at least one outgoing interface whose IP
   address family is different from the incoming interface, the AFBR
   MUST encapsulate/decapsulate this packet and forward it to such via the
   outgoing interface(s), then forward the data to via other outgoing
   interfaces without encapsulation/decapsulation.

   When a downstream AFBR that has already switched over to the SPT of S
   receives an encapsulated multicast data packet of (S,G) along the
   RPT, it SHOULD silently drop this packet.

7.2.  Selecting a Tunneling Technology

   Choosing tunneling technology depends on the policies configured at on
   AFBRs.  It is recommended REQUIRED that all AFBRs use the same technology,
   otherwise some AFBRs may SHALL not be able to decapsulate encapsulated
   packets from other AFBRs that use a different tunneling technology.

7.3.  TTL

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

7.4.  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 extra 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. AFBRs [RFC5565].

8.  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.

       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 mentioned described in Section 4.2 and 5.2.

   IPv4 Source Address (Encoded-Group format): The encoded-unicast
   format of the IPv4 source address mentioned described in Section 4.3 and 5.3.

   IPv6 Group Address (Encoded-Group format): The encoded-group format
   of the IPv6 group address mentioned described in Section 4.2 and 5.2.

   IPv6 Source Address (Encoded-Group format): The encoded-unicast
   format of the IPv6 source address mentioned described in Section 4.3 and 5.3.

9.  Softwire Mesh Multicast Encapsulation

   Softwire mesh multicast encapsulation does not require the use of any
   one particular encapsulation mechanism.  Rather, it MUST must accommodate
   a variety of different encapsulation mechanisms, and MUST 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.

10.  Security Considerations

   Some

   The security concerns raised in [RFC4925] and [RFC7761] are
   applicable here.  In addition, the additional workload associated
   with some schemes will place heavy burden on routers, which can could be used exploited by
   attackers as an attacker to perform a tool when they carry out
   DDoS attack.  Compared with [RFC4925], the security concerns SHOULD
   be considered more carefully.
   The attackers can carefully: an attacker could potentially set up
   many multicast trees in the edge networks, causing too many multicast
   states in the core network.

   Besides, this document does not introduce any new security concern in
   addition to what is discussed in [RFC4925] and [RFC7761].

11.  IANA Considerations

   When AFBRs perform address mapping, they follow some predefined
   rules, especially the IPv6 prefix for source address mapping should
   be predefined, such that ingress AFBRs and egress AFBRs can complete
   the mapping procedure correctly.  The IPv6 prefix for translation can
   be unified within only the transit core, or within global area.  In
   the later condition, the prefix MUST be assigned by

   This document includes no request to IANA.

12.  References

12.1.  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,
              <http://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, <http://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, <http://www.rfc-editor.org/info/rfc4301>.

   [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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc5565>.

   [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,
              <http://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, <http://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, <http://www.rfc-editor.org/info/rfc7761>.

12.2.  Informative References

   [RFC7371]  Boucadair, M. and S. Venaas, "Updates to the IPv6
              Multicast Addressing Architecture", RFC 7371,
              DOI 10.17487/RFC7371, September 2014,
              <http://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: xmw@cernet.edu.cn

   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