Network Working Group                                              M. Xu
Internet-Draft                                                    Y. Cui
Expires: May 1, October 20, 2012                                        S. Yang
                                                                   J. Wu
                                                     Tsinghua University
                                                                 C. Metz
                                                             G. Shepherd
                                                           Cisco Systems
                                                        October 29, 2011
                                                          April 18, 2012

                        Softwire Mesh Multicast
                 draft-ietf-softwire-mesh-multicast-01
                 draft-ietf-softwire-mesh-multicast-02

Abstract

   The Internet needs to support IPv4 and IPv6 packets.  Both address
   families and their attendant protocol suites support multicast of the
   single-source and any-source varieties.  As part of the transition to
   IPv6, 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.

   Softwires Mesh is a solution to E-IP unicast and multicast support
   across an I-IP backbone.  This document describes the mechanisms for
   supporting Internet-style multicast across a set of E-IP and I-IP
   networks supporting softwires 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://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."

   This Internet-Draft will expire on May 1, October 20, 2012.

Copyright Notice

   Copyright (c) 2011 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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Scenarios of Interest  . . . . . . . . . . . . . . . . . . . .  7
     3.1.  IPv4-over-IPv6 . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  IPv6-over-IPv4 . . . . . . . . . . . . . . . . . . . . . .  8
   4.  IPv4-over-IPv6 . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  Mechanism  . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.2.  Source  Group Address Mapping  . . . . . . . . . . . . . . . . . . 10
     4.3.  Group  Source Address Mapping . . . . . . . . . . . . . . . . . . 12 11
     4.4.  Actions performed by AFBR  Routing Mechanism  . . . . . . . . . . . . . . . . . . . . 12
     4.5.  Distribution of Routing Information among AFBRs  Actions performed by AFBR  . . . . . . . . . 13 . . . . . . . 12
   5.  IPv6-over-IPv4 . . . . . . . . . . . . . . . . . . . . . . . . 14 16
     5.1.  Mechanism  . . . . . . . . . . . . . . . . . . . . . . . . 14 16
     5.2.  Source  Group Address Mapping  . . . . . . . . . . . . . . . . . . 14 16
     5.3.  Group  Source Address Mapping . . . . . . . . . . . . . . . . . . 15 16
     5.4.  Actions performed by AFBR  Routing Mechanism  . . . . . . . . . . . . . . . . 16 . . . . 17
     5.5.  Distribution of Routing Information among AFBRs  Actions performed by AFBR  . . . . . 16 . . . . . . . . . . . 18
   6.  Other Consideration  . . . . . . . . . . . . . . . . . . . . . 17 21
     6.1.  Selecting a Tunneling Technology . . . . . . . . . . . . . 17 21
     6.2.  Fragmentation  . . . . . . . . . . . . . . . . . . . . . . 17 21
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18 22
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19 23
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 24
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 20 24
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 20 24
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 21 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 26

1.  Introduction

   The Internet needs to support IPv4 and IPv6 packets.  Both address
   families and their attendant protocol suites support multicast of the
   single-source and any-source varieties.  As part of the transition to
   IPv6, 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 solution is to leverage the multicast functions,
   inherent in the I-IP backbone, to efficiently and scalably tunnel
   encapsulated client E-IP multicast packets inside an I-IP core tree,
   which roots at one or more ingress AFBR nodes and branches out to one
   or more egress AFBR leaf nodes.

   [6] outlines the requirements for the softwires mesh scenario
   including the multicast.  It is straightforward to envisage 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
   traverse the I-IP backbone network.

   One method to accomplish this is to re-use the multicast VPN approach
   outlined in [10].  MVPN-like schemes can support the softwire mesh
   scenario and achieve a "many-to-one" mapping between the E-IP client
   multicast trees and 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 a large amount of
   (S,G) states.  Aggregation at the edge contains the (S,G) states that
   need to be maintained by the network operator supporting the customer
   VPNs.  The disadvantage of this approach is the possible inefficient
   bandwidth and resource utilization when multicast packets are
   delivered to a receiver AFBR with no attached E-IP receiver.

   Internet-style multicast is somewhat different in that the trees
   tends to be relatively sparse and source-rooted.  The need for
   multicast aggregation at the edge (where many customer multicast
   trees are mapped into a few or one 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 [8].  In
   this document, a more detailed discussion supporting the "one-to-one"
   mapping schemes for the IPv6 over IPv4 and IPv4 over IPv6 scenarios
   will be discussed.

2.  Terminology

   An example of a softwire mesh network supporting multicast is
   illustrated in Figure 1.  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 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 simple case the resources of
   the I-IP core are managed by a single operator although the inter-
   provider case is not precluded.

                 ._._._._.            ._._._._.
                |         |          |         |   --------
                |  E-IP   |          |  E-IP   |--|Source S|
                | network |          | network |   --------
                 ._._._._.            ._._._._.
                    |                    |
                   AFBR             upstream AFBR
                    |                    |
                  __+____________________+__
                 /   :   :           :   :  \
                |    :      :      :     :   |  E-IP Multicast
                |    : I-IP transit core :   |  message should
                |    :     :       :     :   |  get 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 dual-stack 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 (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 (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 tunnel 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 IPv4-
   embedded IPv6 source address.

3.  Scenarios of Interest

   This section describes the two different scenarios where softwires
   mesh multicast will apply.

3.1.  IPv4-over-IPv6

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

                     Figure 2: IPv4-over-IPv6 Scenario

   In this scenario, the E-IP client networks run IPv4 and 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 client E-IPv4 tree 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
   naturally support native IPv6 services and applications but it is
   with near 100% certainty that legacy IPv4 networks handling unicast
   and multicast should be accommodated.

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, 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 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 resulting in unnecessary
   bandwidth and resource consumption.  Therefore additional efforts
   will be 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.

   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

4.1.  Mechanism

   Routers in the client E-IPv4 networks contain routes to all other
   client E-IPv4 networks.  Through the set of known and deployed
   mechanisms, E-IPv4 hosts and routers have discovered or learned of
   (S,G) or (*,G) IPv4 addresses.  Any I-IP I-IPv6 multicast state
   instantiated in the core is referred to as (S',G') or (*,G') and is
   certainly separated from E-IP 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 I-IP IPv6 address of
   the upstream AFBR.  When the I-IPv6 PIM message arrives at the
   upstream AFBR, it should 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.

   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 an algorithmic one-to-one IPv4-to-IPv6 address mapping at
   AFBRs.

4.2.  Group Address Mapping

   For IPv4-over-IPv6 scenario, a simple algorithmic mapping between
   IPv4 multicast group addresses and IPv6 group addresses is supported.
   [11] has already defined an applicable format.  Figure 4 is the
   reminder of the format:

   |   8    |  4 |  4 |    16     |  4 |       60         |    32    |
   +--------+----+----+-----------+----+------------------+----------+
   |11111111|0011|scop|00.......00|64IX|   sub-group-id   |v4 address|
   +--------+----+----+-----------+----+------------------+----------+
                                                 +-+-+-+-+
   IPv4-IPv6 Interconnection bits (64IX):        |M|r|r|r|
                                                 +-+-+-+-+

      Figure 4: IPv4-Embedded IPv6 Multicast Address Format: SSM Mode

   The high order bits of the I-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 IPv4 multicast address.

4.3.  Source Address Mapping

   There are two kinds of multicast --- ASM and SSM.  It's possible for  Considering that
   I-IP network and E-IP network to may support different kinds kind of
   multicast, and the source address translation rules may vary a lot.
   There are four scenarios could be very complex
   to support all possible scenarios.  But since SSM can be discussed in detail:

   o  E-IP network supports SSM, implemented
   with a strict subset of the PIM-SM protocol mechanisms [5], we can
   treat I-IP network core as SSM-only to make it as simple as possible, then
   there remains 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 4 5 is the
      recommended address format based on [9]:

      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      | 0-------------32--40--48--56--64--72--80--88--96-----------127|
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |     prefix    |v4(32)         | u | suffix    |source address |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |<------------------uPrefix64------------------>|

          Figure 4: 5: IPv4-Embedded IPv6 Virtual Source Address Format

      In this address format, the "prefix" field contains a "Well-Known"
      prefix or a an ISP-defined prefix.  An existing "Well-Known" prefix
      is 64:ff9b, which is defined in [9]; "v4" field is the IP address
      of one of upstream AFBR's E-IPv4 interface; interfaces; "u" field is defined
      in [4], and MUST be set to zero; "suffix" field is reserved for
      future extensions and SHOULD be set to zero; "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) "uPrefix64".

   o  E-IP network supports ASM

      ASM and SSM have simalar PIM message format.  The main differences
      between ASM and SSM are RP and (*,G) messages.  To make this
      scenario feasible, we must be able to translate (*,G) messages
      into (S',G') messages at downstream AFBRs, and translate it back
      at upstream AFBRs.

4.4.  Routing Mechanism

   In the mesh multicast scenario, routing information is required to
   distribute among AFBRs to make sure that PIM messages a downstream
   AFBR send reach the right upstream AFBR.

   To make it feasible, the /32 prefix in "IPv4-Embedded IPv6 Virtual
   Source Address Format" must be known to every AFBR, and every AFBR
   should not only announce the IP address of one of its E-IPv4 interface
   interfaces presented in the "v4" field to other AFBRs by MPBGP, but
   also announce the corresponding uPrefix64 to the I-IPv6 network.
   Since every IP address of upstream AFBR's E-IPv4 interface is
   different from each other, every uPrefix64 that AFBR announces shoud should
   be different either, and uniquely identifies each AFBR.  As uPrefix64
   is an IPv6 prefix, the distribution of uPrefix64 is the same as the
   distribution in 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 [8], 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 a an E-IPv4 PIM (S,G)
   message, it can translate it into (S',G') by looking up the IP
   address of the corresponding AFBR's E-IPv4 interface.  Since the
   uPrefix64 of S' is unique, and is known to every router in the I-IPv6
   network, the translated message will eventually arrive at the
   corresponding upstream AFBR, and the upstream AFBR can translate the
   message back to (S,G).

   o  E-IP network supports SSM, I-IP network supports ASM
      Since any network that supports ASM can also support SSM, we can
      construct  When a SSM tree downstream AFBR receives an E-IPv4 PIM
   (*,G) message, S' can be generated according to the format specified
   in I-IP network. Figure 4, with "source address" field setting to *(the IPv4
   address of RP).  The operation in this
      scenario is the same as that in the first scenario.

   o  E-IP network supports ASM, I-IP network supports SSM
      ASM and SSM have the same PIM message format.  The main
      differences between ASM and SSM are RP and (*,G) messages.  To
      make this scenario feasible, we must be able to translate (*,G)
      messages into (S',G') messages at downstream AFBRs, and translate
      it back at upstream AFBRs.  Assume RP' is translated message will eventually arrive at the
   corresponding upstream AFBR that
      locates between RP and the downstream AFBR.  When  Since every PIM router within a downstream
      AFBR receives an E-IPv4 PIM (*,G) message, S' can
   domain must be generated
      according to the format specified in Figure 4, with "v4" field
      setting able to the IP map a particular multicast group address of one of RP's E-IPv4 interface and
      "source address" field setting to *(the IPv4 address
   the same RP (see Section 4.7 of RP).  The
      translated message will eventually arrive at RP'.  RP' [5]), when this upstream AFBR checks
   the "source address" field and finds of the message, it'll find the IPv4
   address of RP, so RP' this upstream AFBR judges that this is originally a
   (*,G) message, then it translates the message back to the (*,G)
   message and forwards it to RP. processes it.

4.5.  Actions performed by AFBR

   The following actions are performed by AFBRs:

   o  E-IP network supports ASM, I-IP network supports ASM
      To keep it as simple as possible, we treat I-IP network as SSM and  E-IPv4 (*,G) state maintenance

      When an AFBR wishes to propagate a (*,G) Join/Prune message to an
      I-IPv6 upstream router, the solution is AFBR MUST translate (*,G) Join/Prune
      messages into (S',G') Join/Prune messages following the same as rules
      specified above, then send the third scenario.

4.3.  Group Address Mapping

   For IPv4-over-IPv6 scenario, latter.

   o  E-IPv4 (S,G) state maintenance

      When an AFBR wishes to propagate a simple algorithmic mapping between
   IPv4 multicast group addresses and IPv6 group addresses is supported.
   [11] has already defined (S,G) Join/Prune message to an applicable format.  Figure 5 is
      I-IPv6 upstream router, the
   reminder of AFBR MUST translate (S,G) Join/Prune
      messages into (S',G') Join/Prune messages following the format:

   |   8    |  4 |  4 |    16     |  4 |       60         |    32    |
   +--------+----+----+-----------+----+------------------+----------+
   |11111111|0011|scop|00.......00|64IX|   sub-group-id   |v4 address|
   +--------+----+----+-----------+----+------------------+----------+
                                                 +-+-+-+-+
   IPv4-IPv6 Interconnection bits (64IX):        |M|r|r|r|
                                                 +-+-+-+-+

   Figure 5: IPv4-Embedded IPv6 Multicast Address Format: SSM Mode

   The high order bits of rules
      specified above, then send the latter.

   o  I-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 (S',G') state maintenance

      It is possible that there runs a pure I-IPv6 PIM-SSM in the assigned prefix),
   and each IPv6 multicast I-IPv6
      transit core.  Since the translated souce address starts with the assigned
      unique "Well-Known" prefix can or the ISP-defined prefix that should
      not be
   mapped into IPv4 used otherwise, mash multicast address.

4.4.  Actions performed by AFBR

   The following actions are performed by AFBRs:

   o  Receive E-IPv4 PIM messages won't influnce pure PIM-SM
      multicast at all.  When a downstream one AFBR receives an E-IPv4 PIM a I-IPv6 (S',G')
      message, it should check the address family of the next-hop towards the destination. S'.  If S' starts with the address family unique prefix,
      it means that this message is IPv4, actually a translated E-IPv4 (S,G)
      or (*,G) message, then the AFBR should forward the translate this message
      without any translation; otherwise it should take the following
      operation.

   o  Translate E-IPv4 PIM messages into I-IPv6 PIM messages back
      to E-IPv4 PIM message with S(or *) and G is translated into I-IPv6
      PIM message with S' and G' following the rules specified above. process it.

   o  Transmit I-IPv6 PIM messages
      The downstream  E-IPv4 (S,G,rpt) state maintenance

      When an AFBR sends the I-IPv6 PIM wishes to propagate a (S,G,rpt) Join/Prune message to the
      an I-IPv6 upstream
      AFBR.  When router, the upstream AFBR receives this MUST do as follows.

   o  Inter-AFBR signaling

      (S,G,rpt) messages are not supported by I-IPv6 PIM transit core since
      I-IPv6 transit core only works in SSM.  As a result, we're unable
      to stop receiving data from any given S along the RP tree even if
      downstream AFBR has already switched over to the SPT, which may
      bring about a lot of redundancy.  In order to solve this problem,
      we introduce a new mechanism for downstream AFBR to inform
      upstream AFBR to prune a given S from RPT, in order to reduce
      redundancy.

      When a downstream AFBR wishes to propagate a (S,G,rpt) message to
      I-IPv6 upstream router, it should encapsulate the (S,G,rpt)
      message, then unicast the encapsulated message to the
      corresponding upstream AFBR, which we call it
      checks "RP'".

      The encapsulated message will evevtually arrive at RP', but the prefix
      incoming interface of it may be different from the source address and judges outcoming
      interface along the RP tree to the corresponding downstream AFBR
      that send this message, so RP' is unable to determine the
      message
      (S,G,rpt) state of each I-IPv6 outgoing interface.  To solve this
      problem, and keep the solution as simple as possible, we
      conceptually treat all the I-IPv6 outgoing interfaces as equal,
      and introduce a "virtual interface" as the representative of all
      the I-IPv6 outgoing interfaces, which is specified in Figure 6.

          +----------------------------------------+
          |                                        |
          |       +-----------+----------+         |
          |       |  PIM-SSM  |    UDP   |         |
          |       +-----------+----------+         |
          |          ^                |            |
          |          |                |            |
          |          |                v            |
          |       +----------------------+         |
          |       |      Virtual I/F     |         |
          |       +----------------------+         |
          |   PIM    ^                | multicast  |
          | messages |                |   data     |
          |          |  +-------------+---+        |
          |       +--+--|-----------+     |        |
          |       |     v           |     v        |
          |     +--------- +     +----------+      |
          |     | I-IP I/F |     | I-IP I/F |      |
          |     +----------+     +----------+      |
          |        ^     |          ^     |        |
          |        |     |          |     |        |
          +--------|-----|----------|-----|--------+
                   |     v          |     v

                  Figure 6: upstream AFBR virtual interface

      The virtual interface has two responsibilities: On control plane,
      it should process the encapsulated (S,G,rpt) messages received
      from all the I-IPv6 interfaces, and work as a translated message, then translates real interface that
      has joint (*,G).  Since all the message back I-IPv6 interfaces are treated
      equal, the virtual interface only send (S,G,rpt) Prune messages to E-IPv4 PIM
      PIM-SSM module when every received encapsulated message and sends it towards has a
      (S,G,rpt) Prune inside, which means that no downstream AFBR want
      to receive data from source or RP. S of group G along the RPT; On data
      plane, upon receiving a multicast data packet, the virtual
      interface should encapsulate it at first, then send to every
      I-IPv6 interface a copy of the encapsulated data.In this way,
      downstream AFBRS may receive some redundant data, but avoid black
      holes.

      NOTICE: There may exist an E-IPv4 neighbor of RP' that has joint
      the RP tree, so the per-interface state machine for receiving
      E-IPv4 (S,G,rpt) Join/Prune messages should still take effect.

   o  Process and forward multicast data

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

4.5.  Distribution of Routing Information among AFBRs

   It is described in [8] that AFBRs take advantage

      Since all I-IP interfaces of BGP to distribute upstream AFBR are treated equal, a
      AFBR may receive encapsulated data from S along the E-IP routing information RP tree even
      if it has already switched over to each other by I-IP transport.  In
   IPv4-over-IPv6 scenario SPT of softwire mesh multicast in addition, every S. At this time, the
      AFBR should not only announce the IP address of one of silently drop this data.

   o  SPT switchover

      When a new AFBR expresses its E-IPv4
   interface presented interest in the "v4" field receiving traffic
      destined for a multicast group, it needs to other AFBRs, but also
   announce receive all the data
      along the RP tree at first.  But since downstream AFBRs in fact
      receive the corresponding uPrefix64 union set of data needed by every downstream AFBR, RP'
      has to forward all the I-IPv6 network data from RP to ensure all the softwire mesh multicast mechanism functions properly.  This
   should also be done by BGP. downstream AFBRs.
      As uPrefix64 is an IPv6 prefix, a result, the distribution downstream AFBRs that have already switched to
      the shortest-path tree will receive two copies of uPrefix64 is the same as data,
      namely redundancy.

      To reduce the redundancy, we recommend every AFBR's
      SwitchToSptDesired(S,G) function employ the distribution in unicast scenario.  But since "v4"
   field "switch on first
      packet" policy.  In this way, the delay of switchover to SPT is an E-IPv4 address, and BGP messages are NOT tunneled through
   softwires or through any other mechanism
      kept as specified in [8], 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. little as possible, so is the redundancy.

5.  IPv6-over-IPv4

5.1.  Mechanism

   Routers in the client E-IPv6 networks contain routes to all other
   client E-IPv6 networks.  Through the set of known and deployed
   mechanisms, E-IPv6 hosts and routers have discovered or learned 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's impossible to map all of the IPv6
   multicast address space into 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 [11].  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 IPv4 address space to assigned prefix), and
   each IPv6 multicast 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" assigned prefix or a ISP-defined prefix.

5.2. can be mapped
   into IPv4 multicast address.

5.3.  Source Address Mapping

   There are two kinds of multicast --- ASM and SSM.  It's possible for  Considering that
   I-IP network and E-IP network to may support different kind of
   multicast,
   and the source address translation rules may vary could be very complex
   to support all possible scenarios.  But since SSM can be implemented
   with a lot.  There are
   four strict subset of the PIM-SM protocol mechanisms [5], we can
   treat I-IP core as SSM-only to make it as simple as possible, then
   there remains only two scenarios to be discussed in detail:

   o  E-IP network supports SSM, I-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 [9].
      Figure 6 7 is the reminder of the format:

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

              Figure 6: 7: IPv4-Embedded IPv6 Source Address Format

      In this address format, the "uPrefix64" field starts with a "Well-
      Known" prefix or a an ISP-defined prefix.  An existing "Well-Known"
      prefix is 64:ff9b/32, which is defined in [9]; "source address"
      field is the corresponding I-IPv4 source address.

   o  E-IP network supports ASM

      ASM and SSM have similar PIM message format.  The main differences
      between ASM and SSM are RP and (*,G) messages.  To make this
      scenario feasible, we must be able to translate (*,G) messages
      into (S',G') messages at downstream AFBRs and translate it back at
      upstream AFBRs.  Here, the E-IPv6 address of RP MUST follow the
      format specified in Figure 7.  Assume 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
   distribute among AFBRs to make sure that PIM messages a downstream
   AFBR send 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, 7, and the corresponding
   upstream AFBR should announce the I-IPv4 address in "source address"
   field to the 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 network. 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 the corresponding
   upstream AFBR, and the upstream AFBR can translate the message back
   to (S,G) by appending the prefix to S'.

   o  E-IP network supports SSM, I-IP network supports ASM
      Since any network that supports ASM can also support SSM, we can
      construct  When a SSM tree downstream AFBR
   receives a (*,G) message, it can translate it into (S',G') by simply
   taking off the prefix in I-IP network. *(the E-IPv6 address of RP).  Since S' is
   known to every router in I-IPv4 network, the translated message will
   eventually arrive at 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 [5]), 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.

5.5.  Actions performed by AFBR

   The operation following actions are performed by AFBRs:

   o  E-IPv6 (*,G) state maintenance

      When an AFBR wishes to propagate a (*,G) Join/Prune message to an
      I-IPv4 upstream router, the AFBR MUST translate (*,G) Join/Prune
      messages into (S',G') Join/Prune messages following the rules
      specified above, then send the latter.

   o  E-IPv6 (S,G) state maintenance

      When an AFBR wishes to propagate a (S,G) Join/Prune message to an
      I-IPv4 upstream router, the AFBR MUST translate (S,G) Join/Prune
      messages into (S',G') Join/Prune messages following the rules
      specified above, then send the latter.

   o  I-IPv4 (S',G') state maintenance

      It is possible that there runs a pure I-IPv4 PIM-SSM in this
      scenario the I-IPv4
      transit core.  Since the translated souce address is known to the
      corresponding upstream AFBR, mash multicast won't influnce pure
      PIM-SM multicast at all.  When one AFBR receives a (S',G') message
      whose S' is the same as "source address" field of an E-IPv6 source, which
      means that in the first scenario.

   o  E-IP network supports ASM, I-IP network supports SSM
      ASM and SSM have the same PIM this message format.  The main
      differences between ASM and SSM are RP and is actually a translated E-IPv6 (S,G) or
      (*,G) messages.  To
      make message, it should translate this scenario feasible, we must be able message back to translate (*,G)
      messages into (S',G') messages at downstream AFBRs E-IPv6 PIM
      message and translate
      it back at process it.

   o  E-IPv6 (S,G,rpt) state maintenance

      When an AFBR wishes to propagate a (S,G,rpt) Join/Prune message to
      an I-IPv4 upstream AFBRs.  Here, router, the E-IPv6 address of RP AFBR MUST
      follow the format specified do as follows.

   o  Inter-AFBR signaling

      (S,G,rpt) messages are not supported by I-IPv4 transit core since
      I-IPv4 transit core only works in Figure 6.  Assume RP' is SSM.  As a result, we're unable
      to stop receiving data from any given S along the
      upstream AFBR that locates between RP and tree even if
      downstream AFBR has already switched over to the SPT, which may
      bring about a lot of redundancy.  In order to solve this problem,
      we introduce a new mechanism for downstream AFBR. AFBR to inform
      upstream AFBR to prune a given S from RPT, in order to reduce
      redundancy.

      When a downstream AFBR receives wishes to propagate a (*,G) message, it can translate (S,G,rpt) message to
      I-IPv4 upstream router, it into (S',G') by simply taking off should encapsulate the prefix in *(the E-IPv6
      address of RP).  Since S' is known (S,G,rpt)
      message, then unicast the encapsulated message to every router in I-IPv4
      network, the translated
      corresponding upstream AFBR, which we call it "RP'".

      The encapsulated message will eventually evevtually arrive at RP'.
      RP' knows that S' is RP', but the mapped I-IPv4 address
      incoming interface of RP, it may be different from the outcoming
      interface along the RP tree to the corresponding downstream AFBR
      that send this message, so RP' will
      translate the message back is unable to (*,G) by appending determine the prefix to S'
      and forward it to RP.

   o  E-IP network supports ASM, I-IP network supports ASM
      (S,G,rpt) state of each I-IPv4 outgoing interface.  To solve this
      problem, and keep it the solution as simple as possible, we
      conceptually treat I-IP network as SSM and
      the solution is all the same I-IPv4 outgoing interfaces as the third scenario.

5.3.  Group Address Mapping

   To keep one-to-one group address mapping simple, the group address
   range of E-IP IPv6 can be reduced in equal,
      and introduce a number of ways to limit "virtual interface" as the
   scope representative of addresses that need to be mapped into all
      the I-IP IPv4 space.

   A recommended multicast address format I-IPv4 outgoing interfaces, which is defined specified in [11].  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 IPv4 multicast address.

5.4.  Actions performed by AFBR Figure 6.

      The following actions are performed by AFBRs

   o  Receive E-IPv6 PIM messages
      When a downstream AFBR receives an E-IPv6 PIM message, it should
      check the address family of the upstream router.  If the address
      family is IPv6, the AFBR should not translate this message;
      otherwise virtual interface has two responsibilities: On control plane,
      it should take process the following operation.

   o  Translate E-IPv6 PIM messages into I-IPv4 PIM encapsulated (S,G,rpt) messages
      E-IPv6 PIM message with S(or *) and G is translated into received
      from all the I-IPv4
      PIM message with S' interfaces, and G' following work as a real interface that
      has joint (*,G).  Since all the rules specified above.

   o  Transmit I-IPv4 PIM interfaces are treated
      equal, the virtual interface only send (S,G,rpt) Prune messages
      The to
      PIM-SSM module when every received encapsulated message has a
      (S,G,rpt) Prune inside, which means that no downstream AFBR sends the I-IPv4 PIM message want
      to receive data from source S of group G along the upstream
      AFBR.  When RPT; On data
      plane, upon receiving a multicast data packet, the upstream AFBR receives this I-IPv4 PIM message, virtual
      interface should encapsulate it
      checks at first, then send to every
      I-IPv4 interface a copy of the source address and judges encapsulated data.In this way,
      downstream AFBRS may receive some redundant data, but avoid black
      holes.

      NOTICE: There may exist an E-IPv6 neighbor of RP' that has joint
      the message is a
      translated message, then translates RP tree, so the message back to per-interface state machine for receiving
      E-IPv6 PIM
      message and sends it towards source or RP. (S,G,rpt) Join/Prune messages should still take effect.

   o  Process and forward multicast data

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

5.5.  Distribution

      Since all I-IP interfaces of Routing Information among upstream AFBR are treated equal, a
      AFBR may receive encapsulated data from S along the RP tree even
      if it has already switched over to SPT of S. At this time, the
      AFBR should silently drop this data.

   o  SPT switchover

      When a new AFBR expresses its interest in receiving traffic
      destined for a multicast group, it needs to receive all the data
      along the RP tree at first.  But since downstream AFBRs

   Since uPrefix64 is static and unique in IPv6-over-IPv4 scenario,
   there is no need fact
      receive the union set of data needed by every downstream AFBR, RP'
      has to distribute it using BGP.  The distribution forward all the data from RP to all the downstream AFBRs.
      As a result, the downstream AFBRs that have already switched to
      the shortest-path tree will receive two copies of
   "source address" field the same data,
      namely redundancy.

      To reduce the redundancy, we recommend every AFBR's
      SwitchToSptDesired(S,G) function employ the "switch on first
      packet" policy.  In this way, the delay of multicast source addresses switchover to SPT is a pure I-IPv4
   process and no more specification
      kept as little as possible, so is needed. the redundancy.

6.  Other Consideration

6.1.  Selecting a Tunneling Technology

   The choice of tunneling technology is a matter of policy configured
   at AFBRs.

   In most cases, the policy of choosing tunneling technologies will be
   very simple, such as all AFBRs use the same technology.  But it's
   possible that there doesn't exist one technique that all AFBRs
   support.  A recommanded recommended solution is described in [8], which divides
   AFBRs into one or more classes, and each of these classes is assigned
   a technology that every AFBR in this class supports.  In this way,
   all the AFBRs in the same class can choose the right technology to
   communicate with each other.

6.2.  Fragmentation

   The encapsulation performed by upstream AFBR will increase the size
   of packets.  As a result, the outgoing I-IP link MTU may not
   accommodate the extra size.  As it's not always possible for core
   operators to increase every link's MTU, fragmentation and
   reassembling of encapsulated packets MUST be supported by AFBRs.

7.  Security Considerations

   The AFBR routers could maintain secure communications through the use
   of Security Architecture for the Internet Protocol as described
   in[RFC4301].  But when adopting some schemes that will cause heavy
   burden on routers, some attacker may use it as a tool for DDoS
   attack.

8.  IANA Considerations

   When AFBRs perform address mapping, they should follow some
   predefined rules, especially the IPv6 prefix for source address
   mapping should be predefined, so that ingress AFBR and egress AFBR
   can finish 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 should be assigned
   by IANA.

9.  References

9.1.  Normative References

   [1]   Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
         "Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.

   [2]   Foster, B. and F. Andreasen, "Media Gateway Control Protocol
         (MGCP) Redirect and Reset Package", RFC 3991, February 2005.

   [3]   Hinden, R. and S. Deering, "IP Version 6 Addressing
         Architecture", RFC 2373, July 1998.

   [4]   Hinden, R. and S. Deering, "IP Version 6 Addressing
         Architecture", RFC 4291, February 2006.

   [5]   Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
         "Protocol Independent Multicast - Sparse Mode (PIM-SM):
         Protocol Specification (Revised)", RFC 4601, August 2006.

   [6]   Li, X., Dawkins, S., Ward, D., and A. Durand, "Softwire Problem
         Statement", RFC 4925, July 2007.

   [7]   Wijnands, IJ., Boers, A., and E. Rosen, "The Reverse Path
         Forwarding (RPF) Vector TLV", RFC 5496, March 2009.

   [8]   Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
         Framework", RFC 5565, June 2009.

   [9]   Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li,
         "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
         October 2010.

9.2.  Informative References

   [10]  Aggarwal, R., Bandi, S., Cai, Y., Morin, T., Rekhter, Y.,
         Rosen, E., Wijnands, I., R. and S. Yasukawa, E. Rosen, "Multicast in MPLS/
         BGP MPLS/BGP IP VPNs",
         draft-ietf-l3vpn-2547bis-mcast-10 (work in progress),
         January 2010.

   [11]  Boucadair, M., Qin, J., Lee, Y., Venaas, S., Li, X., and M. Xu,
         "IPv4-Embedded IPv6 Multicast Address Format",
         draft-boucadair-behave-64-multicast-address-format-02
         draft-ietf-mboned-64-multicast-address-format-01 (work in
         progress), June 2011. February 2012.

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

   Shu Yang
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084
   P.R. China

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