    Internet Engineering Task Force (IETF)                             M. Xu
    Request for Comments: 8638                                        Y. Cui
    Category: Standards Track                                          J. Wu
    ISSN: 2070-1721                                      Tsinghua University
                                                                     S. Yang
                                                         Shenzhen University
                                                                     C. Metz
                                                               Cisco Systems
                                                              September 2019

        IPv4 Multicast over an IPv6 Multicast in Softwire Mesh Networks

    Abstract

       During the transition to IPv6, there are 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 the IPv4-over-IPv6 scenario, due to lack of
       real-world use cases for the IPv6-over-IPv4 scenario.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8638.

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    Copyright Notice

       Copyright (c) 2019 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (https://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
       2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
       3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
       4.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
       5.  Mesh Multicast Mechanism  . . . . . . . . . . . . . . . . . .   7
         5.1.  Mechanism Overview  . . . . . . . . . . . . . . . . . . .   7
         5.2.  Group Address Mapping . . . . . . . . . . . . . . . . . .   7
         5.3.  Source Address Mapping  . . . . . . . . . . . . . . . . .   8
         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
         6.4.  Inter-AFBR Signaling  . . . . . . . . . . . . . . . . . .  10
         6.5.  SPT Switchover  . . . . . . . . . . . . . . . . . . . . .  13
         6.6.  Other PIM Message Types . . . . . . . . . . . . . . . . .  13
         6.7.  Maintenance of Other PIM States . . . . . . . . . . . . .  13
       7.  Data-Plane Functions of the AFBR  . . . . . . . . . . . . . .  13
         7.1.  Process and Forward Multicast Data  . . . . . . . . . . .  13
         7.2.  TTL or Hop Count  . . . . . . . . . . . . . . . . . . . .  14
         7.3.  Fragmentation . . . . . . . . . . . . . . . . . . . . . .  14
       8.  Packet Format and Translation . . . . . . . . . . . . . . . .  14
       9.  Softwire Mesh Multicast Encapsulation . . . . . . . . . . . .  16
       10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
       11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
       12. Normative References  . . . . . . . . . . . . . . . . . . . .  16
       Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  18
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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    1.  Introduction

       During the transition to IPv6, there are 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 source-rooted or shared tree of the client E-IP to traverse the
       I-IP backbone network.

       This could be accomplished by reusing the multicast VPN (MVPN)
       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, which are source-specific states related to a
       specified multicast group [RFC7761] [RFC7899].  Aggregation at the
       edge contains the (S,G) states for customers' 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 scenario
       [RFC6333], where IPv4 addresses assigned by a broadband service
       provider are shared among customers.  This document describes a
       detailed solution for the IPv4-over-IPv6 softwire mesh scenario,
       where client networks run IPv4 and the backbone network runs IPv6.

       Internet-style multicast is somewhat different from the scenario in
       [RFC8114] in that the trees are source-rooted and relatively sparse.
       The need for multicast aggregation at the edge (where many customer

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       multicast trees are mapped to 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
       achieved.

       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
       precluded.

                       +---------+          +---------+
                       |         |          |         |  +--------+
                       |  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
                       +-|downstream|  |downstream|-+
                         |   AFBR   |--|   AFBR   |
                         +--+-------+  +--------+-+
                            |                   |
                        +---+----+          +---+----+
           +--------+   |        |          |        |  +--------+
           |Receiver+---+  E-IP  |          |  E-IP  +--+Receiver|
           +--------+   |network |          |network |  +--------+
                        +--------+          +--------+

                    Figure 1: Softwire Mesh Multicast Framework

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    2.  Requirements Language

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
       capitals, as shown here.

    3.  Terminology

       The following terminology is used in this document.

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

       o  Upstream AFBR: An AFBR that is closer to the source of a multicast
          data flow.

       o  Downstream AFBR: An AFBR that is closer to a receiver of a
          multicast data flow.

       o  I-IP (Internal IP): This refers to the 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 E-IP is 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 networks.

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       o  uPrefix64: The /96 unicast IPv6 prefix for constructing an
          IPv4-embedded IPv6 unicast address [RFC8114].

       o  mPrefix64: The /96 multicast IPv6 prefix for constructing an
          IPv4-embedded IPv6 multicast address [RFC8114].

       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     |
                       |                            |
                       | +----------+  +----------+ |
                       +-+Downstream+--+Downstream+-+
                         |   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 IPv6.

       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

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       map an IPv4 group/source address to an IPv6 group/source address and
       vice versa.

       The IPv4-over-IPv6 scenario is an emerging requirement as network
       operators build out native IPv6 backbone networks.  These networks
       support native IPv6 services and applications, but, in many cases,
       support for legacy IPv4 unicast and multicast services will also need
       to be 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 IPv4 addresses that are in (S,G) or (*,G)
       state [RFC7761].  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 a 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 a 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|
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
       |                    mPrefix64                  | group address |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

               Figure 3: IPv4-Embedded IPv6 Multicast Address Format

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       An IPv6 multicast prefix (mPrefix64) 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 mPrefix64 for Source-Specific Multicast (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 to an
       IPv6 multicast address (with the assigned prefix), and each IPv6
       multicast address with the assigned prefix can be mapped to an IPv4
       multicast address.  The group address translation algorithm is
       specified in Section 5.2 of [RFC8114].

    5.3.  Source Address Mapping

       There are two kinds of multicast: Any-Source 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 the upstream AFBR is to set S' to a virtual IPv6 address
          that leads to the upstream AFBR.  The unicast address 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 4.9.5.1 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.

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    5.4.  Routing Mechanism

       With mesh multicast, PIMv6 messages originating from a downstream
       AFBR need to be propagated to the correct upstream AFBR, and every
       AFBR needs the /96 prefix in the IPv4-embedded IPv6 source address
       format [RFC6052].

       To achieve this, every AFBR MUST announce the address of one of its
       E-IPv4 interfaces in the "v4" field [RFC6052] alongside the
       corresponding uPrefix46.  The announcement MUST be sent to the other
       AFBRs through Multiprotocol BGP (MBGP) [RFC4760].  Every uPrefix64
       that an AFBR announces MUST be unique.  "uPrefix64" 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 Network Layer
       Reachability Information (NLRI) and next hop (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
       uPrefix64 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 with the "source address" field set to * (the wildcard
       value).  The translated message will be forwarded to the
       corresponding upstream AFBR.  Every PIM router within a PIM domain
       MUST be able to map a particular multicast group address to the same
       RP when the source address is set to the wildcard value.  (See
       Section 4.7 of [RFC7761].)  So, when the upstream AFBR checks the
       "source address" field of the message, it finds the IPv4 address of
       the RP and ascertains that this was originally a (*,G) message.  This
       is then translated back to the (*,G) message and processed.

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    6.  Control-Plane Functions of AFBR

       AFBRs are responsible for the functions detailed in the subsections
       that follow.

    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 a multicast AFTR (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 translates this message
       back to a PIMv4 message and processes it.

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

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

    6.4.  Inter-AFBR Signaling

       Assume that one downstream AFBR has joined an RPT of (*,G) and a
       shortest path tree (SPT) of (S,G) and decided to perform an SPT
       switchover.  (See Section 4.2.1 of [RFC7761].)  According to
       [RFC7761], it should propagate a Prune(S,G,rpt) message along with
       the periodic 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'".

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       When RP' receives this encapsulated message, it MUST 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 from the outgoing interface that propagates multicast data
       to the corresponding downstream AFBR, and there may be other
       downstream AFBRs that need to receive multicast data for (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 for (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 switchover, if at least one downstream AFBR
       exists that has not yet sent Prune(S,G,rpt) messages to the upstream
       AFBR.

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

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       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 all the I-IP interfaces that are outgoing
       interfaces of the (*,G) state machine, and it should 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 it 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 for (S,G) along the RPT again, the interface agent
       MUST send a Join(S,G,rpt) to the PIM-SM module immediately.

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    RFC 8638                 Softwire Mesh Multicast          September 2019

       In the data plane, upon receiving a multicast data packet, the
       interface agent MUST 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.  Maintenance of Other PIM States

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

    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
       mPrefix64-derived and uPrefix64-derived IPv6 addresses to form an
       IPv6 multicast packet.

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    7.2.  TTL or Hop Count

       Upon encapsulation, the TTL and hop count in the outer header SHOULD
       be set by policy.  Upon decapsulation, the TTL and hop count in the
       inner header SHOULD be modified by policy; it MUST NOT be incremented
       and it MAY be decremented to reflect the cost of tunnel forwarding.
       Besides, processing of TTL and hop count information in protocol
       headers depends on the tunneling technology, which 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.  It is not always possible for
       core operators to increase the MTU of every link, thus source
       fragmentation after encapsulation and reassembling of encapsulated
       packets MUST be supported by AFBRs [RFC5565].  Path MTU Discovery
       (PMTUD) [RFC8201] SHOULD be enabled, and ICMPv6 packets MUST NOT be
       filtered in the I-IP network.  Fragmentation and tunnel configuration
       considerations are provided in Section 8 of [RFC5565].  The detailed
       procedure can be referred in Section 7.2 of [RFC2473].

    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.

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       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)       |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        (a) 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)       |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        (b) IPv6 Register-Stop Message Format

                      Figure 5: Register-Stop Message Format

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

       IPv4 Group Address (Encoded-Group format): The encoded-group format
       of the IPv4 group address described in Section 4.9.1 of [RFC7761].

       IPv4 Source Address (Encoded-Group format): The encoded-unicast
       format of the IPv4 source address described in Section 4.9.1 of
       [RFC7761].

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

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

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    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 and follow the requirements mentioned in
       Section 8 of [RFC5565].

    10.  Security Considerations

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

       The additional workload associated with some schemes, such as
       interface agents, 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.  Besides, cryptographic methods
       for authenticating BGP sessions [RFC7454] could be used.

    11.  IANA Considerations

       This document has no IANA actions.

    12.  Normative References

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

       [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
                  IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
                  December 1998, <https://www.rfc-editor.org/info/rfc2473>.

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

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    RFC 8638                 Softwire Mesh Multicast          September 2019

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

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

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

       [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,
                  <https://www.rfc-editor.org/info/rfc6333>.

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

       [RFC7454]  Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
                  and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
                  February 2015, <https://www.rfc-editor.org/info/rfc7454>.

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

       [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,
                  <https://www.rfc-editor.org/info/rfc7899>.

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

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                  2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                  May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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    RFC 8638                 Softwire Mesh Multicast          September 2019

       [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
                  "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
                  DOI 10.17487/RFC8201, July 2017,
                  <https://www.rfc-editor.org/info/rfc8201>.

    Acknowledgements

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

    Authors' Addresses

       Mingwei Xu
       Tsinghua University
       Department of Computer Science
       Beijing  100084
       China

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

       Yong Cui
       Tsinghua University
       Department of Computer Science
       Beijing  100084
       China

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

       Jianping Wu
       Tsinghua University
       Department of Computer Science
       Beijing  100084
       China

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

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    RFC 8638                 Softwire Mesh Multicast          September 2019

       Shu Yang
       Shenzhen University
       South Campus
       Shenzhen  518060
       China

       Phone: +86-755-2653-4078
       Email: yang.shu@szu.edu.cn

       Chris Metz
       Cisco Systems
       170 West Tasman Drive
       San Jose, CA  95134
       United States of America

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

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