Softwire WG                                                        M. Xu
Internet-Draft                                                    Y. Cui
Intended status: Standards Track                                   J. Wu
Expires: December 20, 2018 March 19, 2019                              Tsinghua University
                                                                 S. Yang
                                                             Oudmon Tech
                                                     Shenzhen University
                                                                 C. Metz
                                                           Cisco Systems
                                                           June 18,
                                                      September 15, 2018

     IPv4 Multicast over an IPv6 Multicast in Softwire Mesh Network


   During the transition to IPv6, there will be scenarios where a
   backbone network internally running one IP address family (referred
   to as the internal IP or I-IP family), connects client networks
   running another IP address family (referred to as the external IP or
   E-IP family).  In such cases, the I-IP backbone needs to offer both
   unicast and multicast transit services to the client E-IP networks.

   This document describes a mechanism for supporting multicast across
   backbone networks where the I-IP and E-IP protocol families differ.
   The document focuses on IPv4-over-IPv6 scenario, due to lack of real-
   world use cases for IPv6-over-IPv4 scenario.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

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   This Internet-Draft will expire on December 20, 2018. March 19, 2019.

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   document authors.  All rights reserved.

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   ( in effect on the date of
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Mesh Multicast Mechanism  . . . . . . . . . . . . . . . . . .   7
     5.1.  Mechanism Overview  . . . . . . . . . . . . . . . . . . .   7   8
     5.2.  Group Address Mapping . . . . . . . . . . . . . . . . . .   7   8
     5.3.  Source Address Mapping  . . . . . . . . . . . . . . . . .   8   9
     5.4.  Routing Mechanism . . . . . . . . . . . . . . . . . . . .   9
   6.  Control Plane Functions of AFBR . . . . . . . . . . . . . . .  10
     6.1.  E-IP (*,G) and (S,G) State Maintenance  . . . . . . . . .  10
     6.2.  I-IP (S',G') State Maintenance  . . . . . . . . . . . . .  10
     6.3.  E-IP (S,G,rpt) State Maintenance  . . . . . . . . . . . .  10  11
     6.4.  Inter-AFBR Signaling  . . . . . . . . . . . . . . . . . .  10  11
     6.5.  SPT Switchover  . . . . . . . . . . . . . . . . . . . . .  13
     6.6.  Other PIM Message Types . . . . . . . . . . . . . . . . .  13
     6.7.  Other PIM States Maintenance  . . . . . . . . . . . . . .  13
   7.  Data Plane Functions of the AFBR  . . . . . . . . . . . . . .  13
     7.1.  Process and Forward Multicast Data  . . . . . . . . . . .  13  14
     7.2.  TTL . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.3.  Fragmentation . . . . . . . . . . . . . . . . . . . . . .  14
   8.  Packet Format and Translation . . . . . . . . . . . . . . . .  14
   9.  Softwire Mesh Multicast Encapsulation . . . . . . . . . . . .  15
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     12.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   During the transition to IPv6, there will be scenarios where a
   backbone network internally running one IP address family (referred
   to as the internal IP or I-IP family), connects client networks
   running another IP address family (referred to as the external IP or
   E-IP family).

   One solution is to leverage the multicast functions inherent in the
   I-IP backbone to efficiently forward client E-IP multicast packets
   inside an I-IP core tree.  The I-IP tree is rooted at one or more
   ingress Address Family Border Routers (AFBRs) [RFC5565] and branches
   out to one or more egress AFBRs.

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

   This could be accomplished by re-using the multicast VPN approach
   outlined in [RFC6513].  MVPN-like schemes can support the softwire
   mesh scenario and achieve a "many-to-one" mapping between the E-IP
   client multicast trees and the transit core multicast trees.  The
   advantage of this approach is that the number of trees in the I-IP
   backbone network scales less than linearly with the number of E-IP
   client trees.  Corporate enterprise networks, and by extension
   multicast VPNs, have been known to run applications that create too
   many (S,G) states [RFC7899]. [RFC7761][RFC7899].  Aggregation at the edge
   contains the (S,G) states for customer's VPNs and these need to be
   maintained by the network operator.  The disadvantage of this
   approach is the possibility of inefficient bandwidth and resource
   utilization when multicast packets are delivered to a receiving AFBR
   with no attached E-IP receivers.

   [RFC8114] provides a solution for delivering IPv4 multicast services
   over an IPv6 network.  But it mainly focuses on the DS-lite [RFC6333]
   scenario, where IPv4 addresses assigned by a broadband service
   provider are shared among customers.  This document describes a
   detailed solution for the IPv4-
   over-IPv6 IPv4-over-IPv6 softwire mesh scenario,
   where client networks run IPv4 and the backbone network runs IPv6.

   Internet-style multicast is somewhat different to the [RFC8114]
   scenario in that the trees are source-rooted and relatively sparse.
   The need for multicast aggregation at the edge (where many customer
   multicast trees are mapped into one or more backbone multicast trees)
   does not exist and to date has not been identified.  Thus the need
   for alignment between the E-IP and I-IP multicast mechanisms emerges.

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

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

                   +---------+          +---------+
                   |         |          |         |  +--------+
                   |  E-IP   |          |  E-IP   +--+Source S|
                   | network |          | network |  +--------+
                   +---+-----+          +--+------+
                       |                   |
                     +-+--------+  +-------+--+
                     |          |  | upstream |
                   +-|   AFBR   +--+   AFBR   |-+
                   | +----------+  +----------+ |
                   |                            |  E-IP Multicast
                   |      I-IP transit core     |  packets are forwarded
                   |                            |  across the I-IP
                   | +----------+  +----------+ |  transit core
                   +-|dowstream |  |downstream|-+
                     |   AFBR   |--|   AFBR   |
                     +--+-------+  +--------+-+
                        |                   |
                    +---+----+          +---+----+
       +--------+   |        |          |        |  +--------+
       |Receiver+---+  E-IP  |          |  E-IP  +--+Receiver|
       +--------+   |network |          |network |  +--------+
                    +--------+          +--------+

                Figure 1: Softwire Mesh Multicast Framework

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Terminology

   Terminology used in this document:

   o Address Family Border Router (AFBR) - A router interconnecting two
   or more networks using different IP address families.  Besides, 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

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

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

   o I-IP (Internal IP): This refers to IP address family that is
   supported by the core network.  In this document, the I-IP is IPv6.

   o E-IP (External IP): This refers to the IP address family that is
   supported by the client network(s) attached to the I-IP transit core.
   In this document, the I-IP E-IP is IPv6. 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 (in this document, we focus on IP multicast) operating
   exclusively inside the I-IP core network.  An I-IP core tree is used
   to forward E-IP multicast packets belonging to E-IP trees across the
   I-IP core.  Another name for an I-IP core tree is multicast or
   multipoint softwire.

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

   o uPrefix46: The /96 unicast IPv6 prefix for constructing an
   IPv4-embedded IPv6 unicast address [RFC6052].

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

   o PIMv4, PIMv6: refer to [RFC8114].

   o Inter-AFBR signaling: A mechanism used by downstream AFBRs to send
   PIMv6 messages to the upstream AFBR.

4.  Scope

   This document focuses on the IPv4-over-IPv6 scenario, as shown in the
   following diagram:

                   +---------+        +---------+
                   |  IPv4   |        |  IPv4   |  +--------+
                   | Client  |        | Client  |--+Source S|
                   | Network |        | Network |  +--------+
                   +----+----+        +----+----+
                        |                  |
                     +--+-------+  +-------+--+
                     |          |  | Upstream |
                   +-+   AFBR   +--+   AFBR   |-+
                   | +----------+  +----------+ |
                   |                            |
                   |      IPv6 transit core     |
                   |                            |
                   | +----------+  +----------+ |
                     |   AFBR   |  |   AFBR   |
                     +--+-------+  +-------+--+
                        |                  |
                   +----+----+        +----+----+
       +--------+  |  IPv4   |        |  IPv4   |  +--------+
       |Receiver+--+ Client  |        | Client  +--+Receiver|
       +--------+  | Network |        | Network |  +--------+
                   +---------+        +---------+

                     Figure 2: IPv4-over-IPv6 Scenario

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

   Because of the much larger IPv6 group address space, the client E-IP
   tree can be mapped to a specific I-IP 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

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

5.  Mesh Multicast Mechanism
5.1.  Mechanism Overview

   Routers in the client E-IP networks have routes to all other client
   E-IP networks.  Through PIMv4 messages, E-IP hosts and routers have
   discovered or learnt of (S,G) or (*,G) (*,G)[RFC7761] IPv4 addresses.  Any
   I-IP multicast state instantiated in the core is referred to as
   (S',G') or (*,G') and is separated from E-IP multicast state.

   Suppose a downstream AFBR receives an E-IP PIM Join/Prune message
   from the E-IP network for either an (S,G) tree or a (*,G) tree.  The
   AFBR translates the PIMv4 message into an PIMv6 message with the
   latter being directed towards the I-IP IPv6 address of the upstream
   AFBR.  When the PIMv6 message arrives at the upstream AFBR, it is
   translated back into an PIMv4 message.  The result of these actions
   is the construction of E-IP trees and a corresponding I-IP tree in
   the I-IP network.  An example of the packet format and translation is
   provided in Section 8.

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

5.2.  Group Address Mapping

   A simple algorithmic mapping between IPv4 multicast group addresses
   and IPv6 group addresses is performed.  Figure 3 is provided as a
   reminder of the format:

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

           Figure 3: IPv4-Embedded IPv6 Multicast Address Format

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

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

   With this scheme, each IPv4 multicast address can be mapped into an
   IPv6 multicast address (with the assigned prefix), and each IPv6
   multicast address with the assigned prefix can be mapped into an IPv4
   multicast address.  The group address translation algorithm can be
   referred in Section 5.2 of [RFC8114].

5.3.  Source Address Mapping

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

   o  E-IP network supports SSM

      One possible way to make sure that the translated PIMv6 message
      reaches upstream AFBR is to set S' to a virtual IPv6 address that
      leads to the upstream AFBR.  The unicast adddress translation
      should be achieved according to [RFC6052]

   o  E-IP network supports ASM

      The (S,G) source list entry and the (*,G) source list entry differ
      only in that the latter has both the WildCard (WC) and RPT bits of
      the Encoded-Source-Address set, while with the former, the bits
      are cleared (See Section of [RFC7761]).  As a result, the
      source list entries in (*,G) messages can be translated into
      source list entries in (S',G') messages by clearing both the WC
      and RPT bits at downstream AFBRs, and vice-versa for the reverse
      translation at upstream AFBRs.

5.4.  Routing Mechanism

   With mesh multicast, PIMv6 messages originating from a downstream
   AFBR need to be propogated to the correct upstream AFBR, and every
   AFBR needs the /96 prefix in "IPv4-Embedded IPv6 Virtual Source
   Address Format".

   To achieve this, every AFBR MUST announce the address of one of its
   E-IPv4 interfaces in the "v4" field alongside the corresponding
   uPreifx64.  The announcement MUST be sent to the other AFBRs through
   MBGP [RFC4760].  Every uPrefix46 that an AFBR announces MUST be
   unique.  "uPrefix46" is an IPv6 prefix, and the distribution
   mechanism is the same as the traditional mesh unicast scenario.

   As the "v4" field is an E-IP address, and BGP messages are not
   tunneled through softwires or any other mechanism specified in
   [RFC5565], AFBRs MUST be able to transport and encode/decode BGP
   messages that are carried over the I-IP, and whose NLRI and NH are of
   the E-IP address family.

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

   When a downstream AFBR receives an E-IP PIM (*,G) message, S' can be
   generated according to the format specified in Figure 3, with the
   "source address" field set to * (wildcard value).  The translated
   message will be forwarded to the corresponding upstream AFBR.  Since
   every PIM router within a PIM domain MUST be able to map a particular
   multicast group address to the same RP (see Section 4.7 of
   [RFC7761]), when the upstream AFBR checks the "source address" field
   of the message, it finds the IPv4 address of the RP, and ascertains
   that this is originally a (*,G) message.  This is then translated
   back to the (*,G) message and processed.

6.  Control Plane Functions of AFBR

   AFBRs are responsible for the following functions:

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

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

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

   It is possible that the I-IP transit core runs another, non-transit,
   I-IP PIM-SSM instance.  Since the translated source address starts
   with the unique "Well-Known" prefix or the ISP-defined prefix that
   MUST NOT be used by another service provider, mesh multicast will not
   influence non-transit PIM-SSM multicast at all.  When an AFBR
   receives an I-IP (S',G') message, it MUST check S'.  If S' starts
   with the unique prefix, then the message is actually a translated
   E-IP (S,G) or (*,G) message, and the AFBR translate this message back
   to a PIMv4 message and process it.

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

   When an AFBR wishes to propagate a Join/Prune(S,G,rpt) Join/Prune(S,G,rpt)[RFC7761]
   message to an I-IP upstream router, the AFBR MUST operate as
   specified in Section 6.5 and Section 6.6.

6.4.  Inter-AFBR Signaling

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

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

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

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

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

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

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

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

             Figure 4: Interface Agent Implementation Example

   Figure 4 shows an example of an interface agent implementation using
   UDP encapsulation.  The interface agent has two responsibilities: In
   the control plane, it SHOULD work as a real interface that has joined
   (*,G), representing of all the I-IP interfaces which are outgoing
   interfaces of the (*,G) state machine, and process the (S,G,rpt)
   messages received from all the I-IP interfaces.

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

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

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

6.5.  SPT Switchover

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

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

6.6.  Other PIM Message Types

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

6.7.  Other PIM States Maintenance

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

7.  Data Plane Functions of the AFBR
7.1.  Process and Forward Multicast Data

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

7.2.  TTL

   Processing of TTL information in protocol headers depends on the
   tunneling technology, technology [I-D.ietf-intarea-tunnels], and it is out of
   scope of this document.

7.3.  Fragmentation

   The encapsulation performed by an upstream AFBR will increase the
   size of packets.  As a result, the outgoing I-IP link MTU may not
   accommodate the larger packet size.  As it  It is not always possible for
   core operators to increase the MTU of every link.  Fragmentation link, thus fragmentation
   after encapsulation and reassembling of encapsulated packets MUST be
   supported by AFBRs [RFC5565].  The specific requirements for
   fragmentation and tunnel configuration COULD be referred to in
   [I-D.ietf-intarea-tunnels], which is under revision currently.

8.  Packet Format and Translation

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

   For example, Figure 5 shows the register-stop message format in the
   IPv4 and IPv6 address families.

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |PIM Ver| Type  |   Reserved    |           Checksum            |
      |             IPv6 Group Address (Encoded-Group format)         |
      |            IPv6 Source Address (Encoded-Unicast format)       |
                    (2). IPv6 Register-Stop Message Format

                  Figure 5: Register-Stop Message Format

   In Figure 5, the semantics of fields "PIM Ver", "Type", "Reserved",
   and "Checksum" can be referred in Section 4.9 of [RFC7761].

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

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

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

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

9.  Softwire Mesh Multicast Encapsulation

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

10.  Security Considerations

   The security concerns raised in [RFC4925] and [RFC7761] are
   applicable here.

   The additional workload associated with some schemes could be
   exploited by an attacker to perform a DDoS attack.

   Compared with [RFC4925], the security concerns SHOULD be considered
   more carefully: an attacker could potentially set up many multicast
   trees in the edge networks, causing too many multicast states in the
   core network.  To defend against these attacks, BGP policies SHOULD
   be carefully configured, e.g., AFBRs only accept Well-Known prefix
   advertisements from trusted peers.

11.  IANA Considerations

   This document includes no request to IANA.

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,

   [RFC5565]  Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
              Framework", RFC 5565, DOI 10.17487/RFC5565, June 2009,

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              DOI 10.17487/RFC6052, October 2010,

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,

   [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
              BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
              2012, <>.

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

   [RFC7899]  Morin, T., Ed., Litkowski, S., Patel, K., Zhang, Z.,
              Kebler, R., and J. Haas, "Multicast VPN State Damping",
              RFC 7899, DOI 10.17487/RFC7899, June 2016,

   [RFC8114]  Boucadair, M., Qin, C., Jacquenet, C., Lee, Y., and Q.
              Wang, "Delivery of IPv4 Multicast Services to IPv4 Clients
              over an IPv6 Multicast Network", RFC 8114,
              DOI 10.17487/RFC8114, March 2017,

12.2.  Informative References

              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-09 (work in
              progress), July 2018.

   [RFC4925]  Li, X., Ed., Dawkins, S., Ed., Ward, D., Ed., and A.
              Durand, Ed., "Softwire Problem Statement", RFC 4925,
              DOI 10.17487/RFC4925, July 2007,

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

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

   Phone: +86-10-6278-5822

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

   Phone: +86-10-6278-5983

   Shu Yang
   Oudmon Tech
   OUDMON Technology Co.,ltd
   Shenzhen  518057 University
   South Campus, Shenzhen University
   Shenzhen  518060
   P.R. China

   Phone: +86-755-2601-3697 +86-755-2653-4078

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

   Phone: +1-408-525-3275