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Versions: 00 01 02 draft-ietf-softwire-mesh-multicast

Network Working Group                                              M. Xu
Internet-Draft                                                    Y. Cui
Expires: January 10, 2012                                        S. Yang
                                                     Tsinghua University
                                                                 C. Metz
                                                             G. Shepherd
                                                           Cisco Systems
                                                            July 9, 2011


                        Softwire Mesh Multicast
                  draft-xu-softwire-mesh-multicast-02

Abstract

   The Internet needs 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 for supporting E-IP unicast and
   multicast 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 January 10, 2012.




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Copyright Notice

   Copyright (c) 2011 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
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   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
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   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.

























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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 Address Mapping . . . . . . . . . . . . . . . . . . 10
     4.3.  Group Address Mapping  . . . . . . . . . . . . . . . . . . 12
     4.4.  Actions performed by AFBR  . . . . . . . . . . . . . . . . 12
   5.  IPv6-over-IPv4 . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Mechanism  . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.2.  Source Address Mapping . . . . . . . . . . . . . . . . . . 14
     5.3.  Group Address Mapping  . . . . . . . . . . . . . . . . . . 16
     5.4.  Actions performed by AFBR  . . . . . . . . . . . . . . . . 16
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 19
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21



























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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
   rooted at one or more ingress AFBR nodes and branching out to one or
   more egress AFBR leaf nodes.

   [6] outlines the requirements for the softwires mesh scenario
   including 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 will need to
   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 possible inefficient
   bandwidth and resource utilization if 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.



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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



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   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 at the upstream of a
   multicast data flow.

   o Downstream AFBR: The AFBR router that is located at the downstream
   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 single-source or multi-source 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 single-source or multi-source 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.














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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



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   naturally support native IPv6 services and applications but it is
   with near 100% certainty that legacy IPv4 networks handling unicast
   and multicast will need to 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 and 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



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   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.












































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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 multicast state instantiated
   in the core is referred to as (S',G') or (*,G') and is of course
   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.  Source Address Mapping

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

   o  E-IP network supports SSM, I-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 is the
      recommended address format based on [9]:


       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
       | 0-------------32--40--48--56--64--72--80--88--96--104---------|
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
       |     prefix    |v4(32)         | u | suffix    |source address |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


      Figure 4: IPv4-Embedded IPv6 Virtual Source Address Format



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      In this address format, the "prefix" field contains a "Well-Known"
      prefix or a 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; "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.
      To make it feasible, the /32 prefix must be known to every AFBR,
      and AFBRs should not only announce the /96 prefixes of S' to the
      I-IPv6 network, but also announce the IP addresses of upstream
      AFBRs' E-IPv4 interface presented in the "v4" field to other AFBRs
      by MPBGP.  In this way, when a downstream AFBR receives a (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 S' is
      globally unique and the /96 prefix of S' is known to every router
      in 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 should also support SSM, we
      can construct a SSM tree in I-IP network.  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 the upstream AFBR that
      locates between RP and the downstream AFBR.  When downstream AFBR
      receives an E-IPv4 PIM (*,G) message, S' can be generated
      according to the format specified in Figure 4, with "v4" field
      setting to the IP address of one of RP's E-IPv4 interface and
      "source address" field setting to *(the IPv4 address of RP).  The
      translated message will eventually arrive at RP'.  RP' checks the
      "source address" field and find the IPv4 address of RP, so RP'
      judges that this is originally a (*,G) message, then it translates
      the message back to (*,G) message and forward it to RP.
      Traveling all the way from sources to the RP, and then back down
      the shared tree may result in the multicast data packets passing
      through RP' twice, which brings about undesirable increased
      latency or bandwidth consumption.  For this reason, RP' MAY
      perform a "cut-through", namely when RP' receives multicast data
      packets sent from sources to RP, it not only forwards them to RP,
      but also forwards them directly onto the multicast tree built in
      the I-IPv6 network.  (S,G,rpt) messages should be sent towards RP
      to avoid reduplication.



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   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
      the solution is the same as the third scenario.

4.3.  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 5 is a
   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 5: 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.

4.4.  Actions performed by AFBR

   The following actions are performed by AFBRs:

   o  Receive E-IPv4 PIM messages
      When a downstream AFBR receives an E-IPv4 PIM message, it should
      check the address family of the next-hop towards the destination.
      If the address family is IPv4, the AFBR should forward the message
      without any translation; otherwise it should take the following
      operation.

   o  Translate E-IPv4 PIM messages into I-IPv6 PIM messages
      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.

   o  Transmit I-IPv6 PIM messages
      The downstream AFBR sends the I-IPv6 PIM message to the upstream
      AFBR.  When the upstream AFBR receives this I-IPv6 PIM message, it



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      checks the prefix of the source address and judges that the
      message is a translated message, then translates the message back
      to E-IPv4 PIM message and sends it towards source or RP.

   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.







































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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 of course
   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 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 a ISP-defined prefix.

5.2.  Source Address Mapping

   There are two kinds of multicast --- ASM and SSM.  It's possible for
   I-IP network and E-IP network to support different kind of multicast,
   and the source address translation rules may vary a lot.  There are
   four 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 is a reminder of the format:


       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
       | 0-------------32--40--48--56--64--72--80--88--96--104---------|
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
       |                   prefix(96)                  |    v4(32)     |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


      Figure 6: IPv4-Embedded IPv6 Source Address Format




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      In this address format, the "prefix" field contains a "Well-Known"
      prefix or a ISP-defined prefix.  An existing "Well-Known" prefix
      is 64:ff9b, which is defined in [9]; "v4" field is the
      corresponding I-IPv4 source address.
      To make it feasible, the /96 prefix 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 should announce the I-IPv4 address in "v4" field to
      the I-IPv4 network.  In this way, when a downstream AFBR receives
      a (S,G) message, it can translate it into (S',G') by simply take
      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 should also support SSM, we
      can construct a SSM tree in I-IP network.  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.  Here, the E-IPv6 address of RP MUST
      follow the format specified in Figure 6.  Assume RP' is the
      upstream AFBR that locates between RP and the downstream AFBR.
      When a downstream AFBR receives a (*,G) message, it can translate
      it into (S',G') by simply take 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 RP'.
      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 forward it to RP.
      Traveling all the way from sources to the RP, and then back down
      the shared tree may result in the multicast data packets passing
      through RP' twice, which brings about undesirable increased
      latency or bandwidth consumption.  For this reason, RP' MAY
      perform a "cut-through", namely when RP' receives multicast data
      packets sent from sources to RP, it not only forwards them to RP,
      but also forwards them directly onto the multicast tree built in
      the I-IPv6 network.  (S,G,rpt) messages should be sent towards RP
      to avoid reduplication.

   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
      the solution is the same as the third scenario.



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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 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 assigned prefix), and
   each IPv6 multicast address with the assigned prefix can be mapped
   into IPv4 multicast address.

5.4.  Actions performed by AFBR

   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 it should take the following operation.

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

   o  Transmit I-IPv4 PIM messages
      The downstream AFBR sends the I-IPv4 PIM message to the upstream
      AFBR.  When the upstream AFBR receives this I-IPv4 PIM message, it
      checks the source address and judges that the message is a
      translated message, then translates the message back to E-IPv6 PIM
      message and sends it towards source or RP.

   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.









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6.  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.












































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7.  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.










































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8.  References

8.1.  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.

8.2.  Informative References

   [10]  Aggarwal, R., Bandi, S., Cai, Y., Morin, T., Rekhter, Y.,
         Rosen, E., Wijnands, I., and S. Yasukawa, "Multicast in 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 (work in
         progress), June 2011.







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Appendix A.  Acknowledgements

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















































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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


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

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











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

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











































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