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<pre>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
<span class="h1">IPv4 Multicast over an IPv6 Multicast in Softwire Mesh Networks</span>
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 <a href="./rfc7841#section-2">Section 2 of RFC 7841</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="https://www.rfc-editor.org/info/rfc8638">https://www.rfc-editor.org/info/rfc8638</a>.
<span class="grey">Xu, et al. Standards Track [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc8638">RFC 8638</a> Softwire Mesh Multicast September 2019</span>
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a>) 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
<a href="#section-1">1</a>. Introduction . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-2">2</a>. Requirements Language . . . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-3">3</a>. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-4">4</a>. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
<a href="#section-5">5</a>. Mesh Multicast Mechanism . . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-5.1">5.1</a>. Mechanism Overview . . . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-5.2">5.2</a>. Group Address Mapping . . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-5.3">5.3</a>. Source Address Mapping . . . . . . . . . . . . . . . . . <a href="#page-8">8</a>
<a href="#section-5.4">5.4</a>. Routing Mechanism . . . . . . . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-6">6</a>. Control-Plane Functions of AFBR . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-6.1">6.1</a>. E-IP (*,G) and (S,G) State Maintenance . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-6.2">6.2</a>. I-IP (S',G') State Maintenance . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-6.3">6.3</a>. E-IP (S,G,rpt) State Maintenance . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-6.4">6.4</a>. Inter-AFBR Signaling . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-6.5">6.5</a>. SPT Switchover . . . . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-6.6">6.6</a>. Other PIM Message Types . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-6.7">6.7</a>. Maintenance of Other PIM States . . . . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-7">7</a>. Data-Plane Functions of the AFBR . . . . . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-7.1">7.1</a>. Process and Forward Multicast Data . . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-7.2">7.2</a>. TTL or Hop Count . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-7.3">7.3</a>. Fragmentation . . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-8">8</a>. Packet Format and Translation . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-9">9</a>. Softwire Mesh Multicast Encapsulation . . . . . . . . . . . . <a href="#page-16">16</a>
<a href="#section-10">10</a>. Security Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
<a href="#section-11">11</a>. IANA Considerations . . . . . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
<a href="#section-12">12</a>. Normative References . . . . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-18">18</a>
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-18">18</a>
<span class="grey">Xu, et al. Standards Track [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc8638">RFC 8638</a> Softwire Mesh Multicast September 2019</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
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) [<a href="./rfc5565" title=""Softwire Mesh Framework"">RFC5565</a>] and branches
out to one or more egress AFBRs.
[<a id="ref-RFC4925">RFC4925</a>] 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 [<a href="./rfc6513" title=""Multicast in MPLS/ BGP IP VPNs"">RFC6513</a>]. 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 [<a href="./rfc7761" title=""Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)"">RFC7761</a>] [<a href="./rfc7899" title=""Multicast VPN State Damping"">RFC7899</a>]. 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.
[<a id="ref-RFC8114">RFC8114</a>] provides a solution for delivering IPv4 multicast services
over an IPv6 network, but it mainly focuses on the DS-Lite scenario
[<a href="./rfc6333" title=""Dual- Stack Lite Broadband Deployments Following IPv4 Exhaustion"">RFC6333</a>], 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
[<a href="./rfc8114" title=""Delivery of IPv4 Multicast Services to IPv4 Clients over an IPv6 Multicast Network"">RFC8114</a>] in that the trees are source-rooted and relatively sparse.
The need for multicast aggregation at the edge (where many customer
<span class="grey">Xu, et al. Standards Track [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc8638">RFC 8638</a> Softwire Mesh Multicast September 2019</span>
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.
[<a id="ref-RFC5565">RFC5565</a>] describes the "Softwire Mesh Framework". This document
provides a more detailed description of how one-to-one mapping
schemes (<a href="./rfc5565#section-11.1">[RFC5565], Section 11.1</a>) 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
<span class="grey">Xu, et al. Standards Track [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc8638">RFC 8638</a> Softwire Mesh Multicast September 2019</span>
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Requirements Language</span>
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
<a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a> [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>] [<a href="./rfc8174" title=""Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"">RFC8174</a>] when, and only when, they appear in all
capitals, as shown here.
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Terminology</span>
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.
<span class="grey">Xu, et al. Standards Track [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc8638">RFC 8638</a> Softwire Mesh Multicast September 2019</span>
o uPrefix64: The /96 unicast IPv6 prefix for constructing an
IPv4-embedded IPv6 unicast address [<a href="./rfc8114" title=""Delivery of IPv4 Multicast Services to IPv4 Clients over an IPv6 Multicast Network"">RFC8114</a>].
o mPrefix64: The /96 multicast IPv6 prefix for constructing an
IPv4-embedded IPv6 multicast address [<a href="./rfc8114" title=""Delivery of IPv4 Multicast Services to IPv4 Clients over an IPv6 Multicast Network"">RFC8114</a>].
o PIMv4, PIMv6: Refer to [<a href="./rfc8114" title=""Delivery of IPv4 Multicast Services to IPv4 Clients over an IPv6 Multicast Network"">RFC8114</a>].
o Inter-AFBR signaling: A mechanism used by downstream AFBRs to send
PIMv6 messages to the upstream AFBR.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Scope</span>
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.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Mesh Multicast Mechanism</span>
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. Mechanism Overview</span>
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 [<a href="./rfc7761" title=""Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)"">RFC7761</a>]. 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 <a href="#section-8">Section 8</a>.
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.
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. Group Address Mapping</span>
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
<a href="./rfc8114#section-5">Section 5 of [RFC8114]</a>.
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 <a href="./rfc8114#section-5.2">Section 5.2 of [RFC8114]</a>.
<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>. Source Address Mapping</span>
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 [<a href="./rfc7761" title=""Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)"">RFC7761</a>], 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 [<a href="./rfc6052" title=""IPv6 Addressing of IPv4/IPv6 Translators"">RFC6052</a>].
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 <a href="./rfc7761#section-4.9.5.1">Section 4.9.5.1 of [RFC7761]</a>.) 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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<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>. Routing Mechanism</span>
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 [<a href="./rfc6052" title=""IPv6 Addressing of IPv4/IPv6 Translators"">RFC6052</a>].
To achieve this, every AFBR MUST announce the address of one of its
E-IPv4 interfaces in the "v4" field [<a href="./rfc6052" title=""IPv6 Addressing of IPv4/IPv6 Translators"">RFC6052</a>] alongside the
corresponding uPrefix46. The announcement MUST be sent to the other
AFBRs through Multiprotocol BGP (MBGP) [<a href="./rfc4760" title=""Multiprotocol Extensions for BGP-4"">RFC4760</a>]. 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
[<a href="./rfc5565" title=""Softwire Mesh Framework"">RFC5565</a>], 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
<a href="./rfc7761#section-4.7">Section 4.7 of [RFC7761]</a>.) 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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<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Control-Plane Functions of AFBR</span>
AFBRs are responsible for the functions detailed in the subsections
that follow.
<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>. E-IP (*,G) and (S,G) State Maintenance</span>
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 <a href="./rfc8114#section-7.2">Section 7.2 of [RFC8114]</a>.
<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>. I-IP (S',G') State Maintenance</span>
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.
<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>. E-IP (S,G,rpt) State Maintenance</span>
When an AFBR wishes to propagate a Join/Prune(S,G,rpt) message
[<a href="./rfc7761" title=""Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)"">RFC7761</a>] to an I-IP upstream router, the AFBR MUST operate as
specified in Sections <a href="#section-6.5">6.5</a> and <a href="#section-6.6">6.6</a>.
<span class="h3"><a class="selflink" id="section-6.4" href="#section-6.4">6.4</a>. Inter-AFBR Signaling</span>
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 <a href="./rfc7761#section-4.2.1">Section 4.2.1 of [RFC7761]</a>.) According to
[<a href="./rfc7761" title=""Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)"">RFC7761</a>], 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 [<a href="./rfc7761" title=""Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)"">RFC7761</a>] 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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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.
<span class="h3"><a class="selflink" id="section-6.5" href="#section-6.5">6.5</a>. SPT Switchover</span>
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.
<span class="h3"><a class="selflink" id="section-6.6" href="#section-6.6">6.6</a>. Other PIM Message Types</span>
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.
<span class="h3"><a class="selflink" id="section-6.7" href="#section-6.7">6.7</a>. Maintenance of Other PIM States</span>
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.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Data-Plane Functions of the AFBR</span>
<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>. Process and Forward Multicast Data</span>
Refer to <a href="./rfc8114#section-7.4">Section 7.4 of [RFC8114]</a>. 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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<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>. TTL or Hop Count</span>
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.
<span class="h3"><a class="selflink" id="section-7.3" href="#section-7.3">7.3</a>. Fragmentation</span>
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 [<a href="./rfc5565" title=""Softwire Mesh Framework"">RFC5565</a>]. Path MTU Discovery
(PMTUD) [<a href="./rfc8201" title=""Path MTU Discovery for IP version 6"">RFC8201</a>] SHOULD be enabled, and ICMPv6 packets MUST NOT be
filtered in the I-IP network. Fragmentation and tunnel configuration
considerations are provided in <a href="./rfc5565#section-8">Section 8 of [RFC5565]</a>. The detailed
procedure can be referred in <a href="./rfc2473#section-7.2">Section 7.2 of [RFC2473]</a>.
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Packet Format and Translation</span>
Because the PIM-SM specification is independent of the underlying
unicast routing protocol, the packet format in <a href="./rfc7761#section-4.9">Section 4.9 of
[RFC7761]</a> 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 <a href="./rfc7761#section-4.9">Section 4.9 of [RFC7761]</a>.
IPv4 Group Address (Encoded-Group format): The encoded-group format
of the IPv4 group address described in <a href="./rfc7761#section-4.9.1">Section 4.9.1 of [RFC7761]</a>.
IPv4 Source Address (Encoded-Group format): The encoded-unicast
format of the IPv4 source address described in <a href="./rfc7761#section-4.9.1">Section 4.9.1 of
[RFC7761]</a>.
IPv6 Group Address (Encoded-Group format): The encoded-group format
of the IPv6 group address described in <a href="#section-5.2">Section 5.2</a>.
IPv6 Source Address (Encoded-Group format): The encoded-unicast
format of the IPv6 source address described in <a href="#section-5.3">Section 5.3</a>.
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<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. Softwire Mesh Multicast Encapsulation</span>
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 [<a href="./rfc4925" title=""Softwire Problem Statement"">RFC4925</a>]. Additionally, all
of the AFBRs attached to the I-IP network MUST implement the same
encapsulation mechanism and follow the requirements mentioned in
<a href="./rfc5565#section-8">Section 8 of [RFC5565]</a>.
<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>. Security Considerations</span>
The security concerns raised in [<a href="./rfc4925" title=""Softwire Problem Statement"">RFC4925</a>] and [<a href="./rfc7761" title=""Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)"">RFC7761</a>] 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 [<a href="./rfc4925" title=""Softwire Problem Statement"">RFC4925</a>], 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 [<a href="./rfc7454" title=""BGP Operations and Security"">RFC7454</a>] could be used.
<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>. IANA Considerations</span>
This document has no IANA actions.
<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>. Normative References</span>
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
DOI 10.17487/RFC2119, March 1997,
<<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>>.
[<a id="ref-RFC2473">RFC2473</a>] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", <a href="./rfc2473">RFC 2473</a>, DOI 10.17487/RFC2473,
December 1998, <<a href="https://www.rfc-editor.org/info/rfc2473">https://www.rfc-editor.org/info/rfc2473</a>>.
[<a id="ref-RFC4760">RFC4760</a>] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", <a href="./rfc4760">RFC 4760</a>,
DOI 10.17487/RFC4760, January 2007,
<<a href="https://www.rfc-editor.org/info/rfc4760">https://www.rfc-editor.org/info/rfc4760</a>>.
<span class="grey">Xu, et al. Standards Track [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc8638">RFC 8638</a> Softwire Mesh Multicast September 2019</span>
[<a id="ref-RFC4925">RFC4925</a>] Li, X., Ed., Dawkins, S., Ed., Ward, D., Ed., and A.
Durand, Ed., "Softwire Problem Statement", <a href="./rfc4925">RFC 4925</a>,
DOI 10.17487/RFC4925, July 2007,
<<a href="https://www.rfc-editor.org/info/rfc4925">https://www.rfc-editor.org/info/rfc4925</a>>.
[<a id="ref-RFC5565">RFC5565</a>] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
Framework", <a href="./rfc5565">RFC 5565</a>, DOI 10.17487/RFC5565, June 2009,
<<a href="https://www.rfc-editor.org/info/rfc5565">https://www.rfc-editor.org/info/rfc5565</a>>.
[<a id="ref-RFC6052">RFC6052</a>] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", <a href="./rfc6052">RFC 6052</a>,
DOI 10.17487/RFC6052, October 2010,
<<a href="https://www.rfc-editor.org/info/rfc6052">https://www.rfc-editor.org/info/rfc6052</a>>.
[<a id="ref-RFC6333">RFC6333</a>] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", <a href="./rfc6333">RFC 6333</a>, DOI 10.17487/RFC6333, August 2011,
<<a href="https://www.rfc-editor.org/info/rfc6333">https://www.rfc-editor.org/info/rfc6333</a>>.
[<a id="ref-RFC6513">RFC6513</a>] Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
BGP IP VPNs", <a href="./rfc6513">RFC 6513</a>, DOI 10.17487/RFC6513, February
2012, <<a href="https://www.rfc-editor.org/info/rfc6513">https://www.rfc-editor.org/info/rfc6513</a>>.
[<a id="ref-RFC7454">RFC7454</a>] Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
and Security", <a href="https://www.rfc-editor.org/bcp/bcp194">BCP 194</a>, <a href="./rfc7454">RFC 7454</a>, DOI 10.17487/RFC7454,
February 2015, <<a href="https://www.rfc-editor.org/info/rfc7454">https://www.rfc-editor.org/info/rfc7454</a>>.
[<a id="ref-RFC7761">RFC7761</a>] 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, <a href="./rfc7761">RFC 7761</a>, DOI 10.17487/RFC7761, March
2016, <<a href="https://www.rfc-editor.org/info/rfc7761">https://www.rfc-editor.org/info/rfc7761</a>>.
[<a id="ref-RFC7899">RFC7899</a>] Morin, T., Ed., Litkowski, S., Patel, K., Zhang, Z.,
Kebler, R., and J. Haas, "Multicast VPN State Damping",
<a href="./rfc7899">RFC 7899</a>, DOI 10.17487/RFC7899, June 2016,
<<a href="https://www.rfc-editor.org/info/rfc7899">https://www.rfc-editor.org/info/rfc7899</a>>.
[<a id="ref-RFC8114">RFC8114</a>] Boucadair, M., Qin, C., Jacquenet, C., Lee, Y., and Q.
Wang, "Delivery of IPv4 Multicast Services to IPv4 Clients
over an IPv6 Multicast Network", <a href="./rfc8114">RFC 8114</a>,
DOI 10.17487/RFC8114, March 2017,
<<a href="https://www.rfc-editor.org/info/rfc8114">https://www.rfc-editor.org/info/rfc8114</a>>.
[<a id="ref-RFC8174">RFC8174</a>] Leiba, B., "Ambiguity of Uppercase vs Lowercase in <a href="./rfc2119">RFC</a>
<a href="./rfc2119">2119</a> Key Words", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc8174">RFC 8174</a>, DOI 10.17487/RFC8174,
May 2017, <<a href="https://www.rfc-editor.org/info/rfc8174">https://www.rfc-editor.org/info/rfc8174</a>>.
<span class="grey">Xu, et al. Standards Track [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc8638">RFC 8638</a> Softwire Mesh Multicast September 2019</span>
[<a id="ref-RFC8201">RFC8201</a>] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, <a href="./rfc8201">RFC 8201</a>,
DOI 10.17487/RFC8201, July 2017,
<<a href="https://www.rfc-editor.org/info/rfc8201">https://www.rfc-editor.org/info/rfc8201</a>>.
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
<span class="grey">Xu, et al. Standards Track [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc8638">RFC 8638</a> Softwire Mesh Multicast September 2019</span>
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
Xu, et al. Standards Track [Page 19]
</pre>
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