Softwire WG M. Xu
Internet-Draft Y. Cui
Intended status: Standards Track J. Wu
Expires: May 17, 2017 S. Yang
Tsinghua University
C. Metz
G. Shepherd
Cisco Systems
November 13, 2016
Softwire Mesh Multicast
draft-ietf-softwire-mesh-multicast-14
Abstract
The Internet needs to support IPv4 and IPv6 packets. Both address
families and their related protocol suites support multicast of the
single-source and any-source varieties. During IPv6 transition,
there will be scenarios where a backbone network running one IP
address family internally (referred to as internal IP or I-IP) will
provide transit services to attached client networks running another
IP address family (referred to as external IP or E-IP). It is
expected that the I-IP backbone will offer unicast and multicast
transit services to the client E-IP networks.
Softwire Mesh is a solution providing E-IP unicast and multicast
support across an I-IP backbone. This document describes the
mechanism for supporting Internet-style multicast across a set of
E-IP and I-IP networks supporting softwire mesh.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 17, 2017.
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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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
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Scenarios of Interest . . . . . . . . . . . . . . . . . . . . 6
3.1. IPv4-over-IPv6 . . . . . . . . . . . . . . . . . . . . . 6
3.2. IPv6-over-IPv4 . . . . . . . . . . . . . . . . . . . . . 7
4. IPv4-over-IPv6 Mechanism . . . . . . . . . . . . . . . . . . 9
4.1. Mechanism Overview . . . . . . . . . . . . . . . . . . . 9
4.2. Group Address Mapping . . . . . . . . . . . . . . . . . . 9
4.3. Source Address Mapping . . . . . . . . . . . . . . . . . 10
4.4. Routing Mechanism . . . . . . . . . . . . . . . . . . . . 11
5. IPv6-over-IPv4 Mechanism . . . . . . . . . . . . . . . . . . 12
5.1. Mechanism Overview . . . . . . . . . . . . . . . . . . . 12
5.2. Group Address Mapping . . . . . . . . . . . . . . . . . . 12
5.3. Source Address Mapping . . . . . . . . . . . . . . . . . 12
5.4. Routing Mechanism . . . . . . . . . . . . . . . . . . . . 14
6. Control Plane Functions of AFBR . . . . . . . . . . . . . . . 14
6.1. E-IP (*,G) State Maintenance . . . . . . . . . . . . . . 14
6.2. E-IP (S,G) State Maintenance . . . . . . . . . . . . . . 14
6.3. I-IP (S',G') State Maintenance . . . . . . . . . . . . . 15
6.4. E-IP (S,G,rpt) State Maintenance . . . . . . . . . . . . 15
6.5. Inter-AFBR Signaling . . . . . . . . . . . . . . . . . . 15
6.6. SPT Switchover . . . . . . . . . . . . . . . . . . . . . 17
6.7. Other PIM Message Types . . . . . . . . . . . . . . . . . 17
6.8. Other PIM States Maintenance . . . . . . . . . . . . . . 17
7. Data Plane Functions of the AFBR . . . . . . . . . . . . . . 18
7.1. Process and Forward Multicast Data . . . . . . . . . . . 18
7.2. Selecting a Tunneling Technology . . . . . . . . . . . . 18
7.3. TTL . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.4. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 18
8. Packet Format and Translation . . . . . . . . . . . . . . . . 18
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9. Softwire Mesh Multicast Encapsulation . . . . . . . . . . . . 19
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
12.1. Normative References . . . . . . . . . . . . . . . . . . 20
12.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
The Internet needs to support IPv4 and IPv6 packets. Both address
families and their related protocol suites support multicast of the
single-source and any-source varieties. During IPv6 transition,
there will be scenarios where a backbone network running one IP
address family internally (referred to as internal IP or I-IP) will
provide transit services to attached client networks running another
IP address family (referred to as external IP or E-IP).
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, which is rooted at one or more ingress AFBR
nodes and branches out to one or more egress AFBR leaf nodes.
[RFC4925] outlines the requirements for the softwires 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.
One method of accomplishing this is to re-use 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. 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.
Internet-style multicast is somewhat different in that the trees are
source-rooted and relatively sparse. The need for multicast
aggregation at the edge (where many customer multicast trees are
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mapped into one or more backbone multicast trees) does not exist and
to date has not been identified. Thus the need for a basic or closer
alignment with E-IP and I-IP multicast procedures emerges.
[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 IPv6 over IPv4 and IPv4 over
IPv6 can be achieved.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Terminology
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 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.
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._._._._. ._._._._.
| | | | --------
| E-IP | | E-IP |--|Source S|
| network | | network | --------
._._._._. ._._._._.
| |
AFBR upstream AFBR
| |
__+____________________+__
/ : : : : \
| : : : : | E-IP Multicast
| : I-IP transit core : | packets are forwarded
| : : : : | across the I-IP
| : : : : | transit core
\_._._._._._._._._._._._._./
+ +
downstream AFBR downstream AFBR
| |
._._._._ ._._._._
-------- | | | | --------
|Receiver|-- | E-IP | | E-IP |--|Receiver|
-------- |network | |network | --------
._._._._ ._._._._
Figure 1: Softwire Mesh Multicast Framework
Terminology used in this document:
o Address Family Border Router (AFBR) - A router interconnecting two
or more networks using different IP address families. In the context
of softwire mesh multicast, the AFBR runs E-IP and I-IP control
planes to maintain E-IP and I-IP multicast states respectively and
performs the appropriate encapsulation/decapsulation of client E-IP
multicast packets for transport across the I-IP core. An AFBR will
act as a source and/or receiver in an I-IP multicast tree.
o Upstream AFBR: The AFBR router that is located on the upper reaches
of a multicast data flow.
o Downstream AFBR: The AFBR router that is located on the lower
reaches of a multicast data flow.
o I-IP (Internal IP): This refers to IP address family (i.e., either
IPv4 or IPv6) that is supported by the core (or backbone) network.
An I-IPv6 core network runs IPv6 and an I-IPv4 core network runs
IPv4.
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o E-IP (External IP): This refers to the IP address family (i.e.
either IPv4 or IPv6) that is supported by the client network(s)
attached to the I-IP transit core. An E-IPv6 client network runs
IPv6 and an E-IPv4 client network runs IPv4.
o I-IP core tree: A distribution tree rooted at one or more AFBR
source nodes and branched out to one or more AFBR leaf nodes. An
I-IP core tree is built using standard IP or MPLS multicast signaling
protocols operating exclusively inside the I-IP core network. An
I-IP core tree is used to 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.
o uPrefix64: The /96 unicast IPv6 prefix for constructing an
IPv4-embedded IPv6 source address in IPv6-over-IPv4 scenario.
o uPrefix46: The /96 unicast IPv6 prefix for constructing an
IPv4-embedded IPv6 source address in IPv4-over-IPv6 scenario.
o mPrefix46: The /96 multicast IPv6 prefix for constructing an
IPv4-embedded IPv6 multicast address in IPv4-over-IPv6 scenario.
o Inter-AFBR signaling: A mechanism used by downstream AFBRs to send
PIM messages to the upstream AFBR.
3. Scenarios of Interest
This section describes the two different scenarios that softwires
mesh multicast is appliacable to.
3.1. IPv4-over-IPv6
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._._._._. ._._._._.
| IPv4 | | IPv4 | --------
| Client | | Client |--|Source S|
| network | | network | --------
._._._._. ._._._._.
| |
AFBR upstream AFBR
| |
__+____________________+__
/ : : : : \
| : : : : |
| : IPv6 transit core : |
| : : : : |
| : : : : |
\_._._._._._._._._._._._._./
+ +
downstream AFBR downstream 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-IPv4 tree can be mapped to a specific I-IPv6 core tree. This
simplifies operations on the AFBR because it becomes possible to
algorithmically map an IPv4 group/source address to an IPv6 group/
source address and vice-versa.
The IPv4-over-IPv6 scenario is an emerging requirement as network
operators build out native IPv6 backbone networks. These networks
support native IPv6 services and applications but in many cases,
support for legacy IPv4 unicast and multicast services will also need
to be accomodated.
3.2. IPv6-over-IPv4
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._._._._. ._._._._.
| IPv6 | | IPv6 | --------
| Client | | Client |--|Source S|
| network | | network | --------
._._._._. ._._._._.
| |
AFBR upstream AFBR
| |
__+____________________+__
/ : : : : \
| : : : : |
| : IPv4 transit core : |
| : : : : |
| : : : : |
\_._._._._._._._._._._._._./
+ +
downstream AFBR downstream AFBR
| |
._._._._ ._._._._
-------- | IPv6 | | IPv6 | --------
|Receiver|-- | Client | | Client |--|Receiver|
-------- | network| | network| --------
._._._._ ._._._._
Figure 3: IPv6-over-IPv4 Scenario
In Figure 3, the E-IP Client Networks run IPv6 while the I-IP core
runs IPv4.
IPv6 multicast group addresses are longer than IPv4 multicast group
addresses so it is not possible to perform an algorithmic IPv6 to
IPv4 address mapping without the risk of multiple IPv6 group
addresses mapped to the same IPv4 address, resulting in unnecessary
bandwidth and resource consumption.Therefore, additional efforts will
be required to ensure that client E-IPv6 multicast packets can be
injected into the correct I-IPv4 multicast trees at the AFBRs. This
clear mismatch in IPv6 and IPv4 group address lengths means that it
will not be possible to perform a one-to-one mapping between IPv6 and
IPv4 group addresses unless the IPv6 group address is scoped, such as
applying a "Well-Known" prefix or an ISP-defined prefix.
As mentioned earlier, this scenario is common in the MVPN
environment. As native IPv6 deployments and multicast applications
emerge from the outer reaches of the greater public IPv4 Internet, it
is envisaged that the IPv6 over IPv4 softwire mesh multicast scenario
will be a necessary feature supported by network operators.
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4. IPv4-over-IPv6 Mechanism
4.1. Mechanism Overview
Routers in the client E-IPv4 networks have routes to all other client
E-IPv4 networks. Through PIM messages, E-IPv4 hosts and routers have
discovered or learnt of (S,G) or (*,G) IPv4 addresses. Any I-IPv6
multicast state instantiated in the core is referred to as (S',G') or
(*,G') and is certainly separated from E-IPv4 multicast state.
Suppose a downstream AFBR receives an E-IPv4 PIM Join/Prune message
from the E-IPv4 network for either an (S,G) tree or a (*,G) tree.
The AFBR can translate the E-IPv4 PIM message into an I-IPv6 PIM
message with the latter being directed towards the I-IP IPv6 address
of the upstream AFBR. When the I-IPv6 PIM message arrives at the
upstream AFBR, it MUST be translated back into an E-IPv4 PIM message.
The result of these actions is the construction of E-IPv4 trees and a
corresponding I-IP tree in the I-IP network. An example of the
packet format and traslation is provided in Section 8.
In this case, it is incumbent upon the AFBR routers to perform PIM
message conversions in the control plane and IP group address
conversions or mappings in the data plane. The AFBRs perform an
algorithmic, one-to-one mapping of IPv4-to-IPv6.
4.2. Group Address Mapping
For the IPv4-over-IPv6 scenario, a simple algorithmic mapping between
IPv4 multicast group addresses and IPv6 group addresses is performed.
Figure 4 shows the reminder of the format:
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 0-------------32--40--48--56--64--72--80--88--96-----------127|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| mPrefix46 |group address |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 4: IPv4-Embedded IPv6 Multicast Address Format
An IPv6 multicast prefix (mPrefix46) is assigned to each AFBR. AFBRs
will prepend the prefix to an IPv4 multicast group address when
translating it to an IPv6 multicast group address.
The mPrefix46 for SSM mode is also defined in Section 4.1 of
[RFC7371]
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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.
4.3. Source Address Mapping
There are two kinds of multicast: ASM and SSM. Considering that 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 I-IPv6 PIM
message reaches upstream AFBR is to set S' to a virtual IPv6
address that leads to the upstream AFBR. Figure 5 is the
recommended address format based on [RFC6052]:
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 0-------------32--40--48--56--64--72--80--88--96-----------127|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| prefix |v4(32) | u | suffix |source address |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|<------------------uPrefix46------------------>|
Figure 5: IPv4-Embedded IPv6 Virtual Source Address Format
In this address format,
* The "prefix" field contains a "Well-Known" prefix or an ISP-
defined prefix. An existing "Well-Known" prefix is 64:ff9b,
which is defined in [RFC6052];
* The "v4" field is the IP address of one of upstream AFBR's
E-IPv4 interfaces;
* The "u" field is defined in [RFC4291], and MUST be set to zero;
* The "suffix" field is reserved for future extensions and SHOULD
be set to zero;
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* The "source address" field stores the original S.
We call the overall /96 prefix ("prefix" field and "v4" field and
"u" field and "suffix" field altogether) "uPrefix46".
o E-IP network supports ASM
The (S,G) source list entry and the (*,G) source list entry only
differ in that the latter has both the WC and RPT bits of the
Encoded-Source-Address set, while the former is all cleared (See
Section 4.9.5.1 of [RFC7761]). So we can translate source list
entries in (*,G) messages into source list entries in (S'G')
messages by applying the format specified in Figure 5 and clearing
both the WC and RPT bits at downstream AFBRs, and vice-versa for
the reverse translation at upstream AFBRs.
4.4. Routing Mechanism
In the mesh multicast scenario, routing information is REQUIRED to be
distributed among AFBRs to make sure that the PIM messages that a
downstream AFBR propagates reach the right upstream AFBR.
Every AFBR MUST know the /32 prefix in "IPv4-Embedded IPv6 Virtual
Source Address Format". To achieve this, every AFBR should announce
one of its E-IPv4 interfaces in the "v4" field, and the corresponding
uPrefix46. The announcement SHOULD be sent to the other AFBRs
through MBGP. Since every IP address of upstream AFBR's E-IPv4
interface is different from each other, every uPrefix46 that AFBR
announces MUST be different, and uniquely identifies each AFBR.
"uPrefix46" is an IPv6 prefix, and the distribution mechanism is the
same as the traditional mesh unicast scenario. But "v4" field is an
E-IPv4 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
I-IPv6, whose NLRI and NH are of E-IPv4 address family.
In this way, when a downstream AFBR receives an E-IPv4 PIM (S,G)
message, it can translate this message into (S',G') by looking up the
IP address of the corresponding AFBR's E-IPv4 interface. Since the
uPrefix46 of S' is unique, and is known to every router in the I-IPv6
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-IPv4 PIM
(*,G) message, S' can be generated according to the format specified
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in Figure 4, with "source address" field set to *(the IPv4 address of
RP). 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 assertains that this is originally a (*,G) message. This is then
translated back to the (*,G) message and processed.
5. IPv6-over-IPv4 Mechanism
5.1. Mechanism Overview
Routers in the client E-IPv6 networks contain routes to all other
client E-IPv6 networks. Through PIM messages, E-IPv6 hosts and
routers have discovered or learnt of (S,G) or (*,G) IPv6 addresses.
Any I-IP multicast state instantiated in the core is referred to as
(S',G') or (*,G') and is separated from E-IP multicast state.
This particular scenario introduces unique challenges. Unlike the
IPv4-over-IPv6 scenario, it is impossible to map all of the IPv6
multicast address space into the IPv4 address space to address the
one-to-one Softwire Multicast requirement. To coordinate with the
"IPv4-over-IPv6" scenario and keep the solution as simple as
possible, one possible solution to this problem is to limit the scope
of the E-IPv6 source addresses for mapping, such as applying a "Well-
Known" prefix or an ISP-defined prefix.
5.2. Group Address Mapping
To keep one-to-one group address mapping simple, the group address
range of E-IP IPv6 can be reduced in a number of ways to limit the
scope of addresses that need to be mapped into the I-IP IPv4 space.
For example, the high order bits of the E-IPv6 address range will be
fixed for mapping purposes. With this scheme, each IPv4 multicast
address can be mapped into an IPv6 multicast address (with the
assigned prefix), and each IPv6 multicast address with the assigned
prefix can be mapped into an IPv4 multicast address.
5.3. Source Address Mapping
There are two kinds of multicast --- ASM and SSM. Considering that
I-IP network and E-IP network may support different kind 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
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simple as possible. There then remain only two scenarios to be
discussed in detail:
o E-IP network supports SSM
To make sure that the translated I-IPv4 PIM message reaches the
upstream AFBR, we need to set S' to an IPv4 address that leads to
the upstream AFBR. But due to the non-"one-to-one" mapping of
E-IPv6 to I-IPv4 unicast address, the upstream AFBR is unable to
remap the I-IPv4 source address to the original E-IPv6 source
address without any constraints.
We apply a fixed IPv6 prefix and static mapping to solve this
problem. A recommended source address format is defined in
[RFC6052]. Figure 6 is the reminder of the format:
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 0-------------32--40--48--56--64--72--80--88--96-----------127|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| uPrefix64 |source address |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 6: IPv4-Embedded IPv6 Source Address Format
In this address format, the "uPrefix64" field starts with a "Well-
Known" prefix or an ISP-defined prefix. An existing "Well-Known"
prefix is 64:ff9b/32, which is defined in [RFC6052]; The "source
address" field is the corresponding I-IPv4 source address.
o The E-IP network supports ASM
The (S,G) source list entry and the (*,G) source list entry only
differ in that the latter has both the WC and RPT bits of the
Encoded-Source-Address set, while the former is all cleared (See
Section 4.9.5.1 of [RFC7761]). So we can translate source list
entries in (*,G) messages into source list entries in (S',G')
messages by applying the format specified in Figure 5 and setting
both the WC and RPT bits at downstream AFBRs, and vice-versa for
the reverse translation at upstream AFBRs. Here, the E-IPv6
address of RP MUST follow the format specified in Figure 6. RP'
is the upstream AFBR that locates between RP and the downstream
AFBR.
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5.4. Routing Mechanism
In the mesh multicast scenario, routing information is REQUIRED to be
distributed among AFBRs to make sure that PIM messages that a
downstream AFBR propagates reach the right upstream AFBR.
To make it feasible, the /96 uPrefix64 MUST be known to every AFBR,
every E-IPv6 address of sources that support mesh multicast MUST
follow the format specified in Figure 6, and the corresponding
upstream AFBR of this source MUST announce the I-IPv4 address in
"source address" field of this source's IPv6 address to the I-IPv4
network. Since uPrefix64 is static and unique in IPv6-over-IPv4
scenario, there is no need to distribute it using BGP. The
distribution of "source address" field of multicast source addresses
is a pure I-IPv4 process and no more specification is needed.
In this way, when a downstream AFBR receives a (S,G) message, it can
translate the message into (S',G') by simply taking off the prefix in
S. Since S' is known to every router in I-IPv4 network, the
translated message will be forwarded to the corresponding upstream
AFBR, and the upstream AFBR can translate the message back to (S,G)
by appending the prefix to S'. When a downstream AFBR receives a
(*,G) message, it can translate it into (S',G') by simply taking off
the prefix in *(the E-IPv6 address of RP). Since S' is known to
every router in I-IPv4 network, the translated message will be
forwarded to RP'. And since every PIM router within a PIM domain
MUST be able to map a particular multicast group address to the same
RP (see Section 4.7 of [RFC7761]), RP' knows that S' is the mapped
I-IPv4 address of RP, so RP' will translate the message back to (*,G)
by appending the prefix to S' and propagate it towards RP.
6. Control Plane Functions of AFBR
AFBRs are responsible for the following functions:
6.1. E-IP (*,G) State Maintenance
When an AFBR wishes to propagate a Join/Prune(*,G) message to an I-IP
upstream router, the AFBR MUST translate Join/Prune(*,G) messages
into Join/Prune(S',G') messages following the rules specified above,
then send the latter.
6.2. E-IP (S,G) State Maintenance
When an AFBR wishes to propagate a Join/Prune(S,G) message to an I-IP
upstream router, the AFBR MUST translate Join/Prune(S,G) messages
into Join/Prune(S',G') messages following the rules specified above,
then send the latter.
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6.3. 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
SHOULD NOT be used by other 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 MUST translate this message
back to E-IP PIM message and process it.
6.4. E-IP (S,G,rpt) State Maintenance
When an AFBR wishes to propagate a Join/Prune(S,G,rpt) message to an
I-IP upstream router, the AFBR MUST operate as specified in
Section 6.5 and Section 6.6.
6.5. Inter-AFBR Signaling
Assume that one downstream AFBR has joined a RPT of (*,G) and a SPT
of (S,G), and decide to perform a SPT switchover. According to
[RFC7761], it SHOULD propagate a Prune(S,G,rpt) message along with
the periodical Join(*,G) message upstream towards 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 of (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 a (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 as simply as possible, we introduce an
"interface agent" to process all the encapsulated (S,G,rpt) messages
the upstream AFBR receives, and prune S from the RPT of group G when
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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, at the cost of duplicated
multicast data of (S,G) along the RPT received by SPT-switched-over
downstream AFBRs, if at least one downstream AFBR exists that has not
yet sent Prune(S,G,rpt) messages to the upstream AFBR. The following
diagram shows an example of how an "interface agent" MAY be
implemented:
+----------------------------------------+
| |
| +-----------+----------+ |
| | PIM-SM | UDP | |
| +-----------+----------+ |
| ^ | |
| | | |
| | v |
| +----------------------+ |
| | I/F Agent | |
| +----------------------+ |
| PIM ^ | multicast |
| messages | | data |
| | +-------------+---+ |
| +--+--|-----------+ | |
| | v | v |
| +--------- + +----------+ |
| | I-IP I/F | | I-IP I/F | |
| +----------+ +----------+ |
| ^ | ^ | |
| | | | | |
+--------|-----|----------|-----|--------+
| v | v
Figure 7: Interface Agent Implementation Example
Figure 7 shows an example of 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 of every downstream
AFBR, and submits Prune (S,G,rpt) messages to the PIM-SM module only
when every (S,G,rpt) state machine is at Prune(P) or PruneTmp(P')
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state, which means that no downstream AFBR is subscribed to receive
multicast data of (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' that has joined
the RPT of G, so the per-interface state machine for receiving E-IP
Join/Prune (S,G,rpt) messages SHOULD keep alive.
6.6. SPT Switchover
After a new AFBR expresses its interest in receiving 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 of some source(s) 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 redundancy will be produced.
6.7. Other PIM Message Types
Apart from 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.
6.8. Other PIM States Maintenance
Apart from 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.
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7. Data Plane Functions of the AFBR
7.1. Process and Forward Multicast Data
On receiving multicast data from upstream routers, the AFBR checks
its forwarding table to find the IP address of each outgoing
interface. If there is at least one outgoing interface whose IP
address family is different from the incoming interface, the AFBR
MUST encapsulate/decapsulate this packet and forward it via the
outgoing interface(s), then forward the data via other outgoing
interfaces without encapsulation/decapsulation.
When a downstream AFBR that has already switched over to the SPT of S
receives an encapsulated multicast data packet of (S,G) along the
RPT, it SHOULD silently drop this packet.
7.2. Selecting a Tunneling Technology
Choosing tunneling technology depends on the policies configured on
AFBRs. It is REQUIRED that all AFBRs use the same technology,
otherwise some AFBRs SHALL not be able to decapsulate encapsulated
packets from other AFBRs that use a different tunneling technology.
7.3. TTL
Processing of TTL depends on the tunneling technology, and it is out
of scope of this document.
7.4. 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 is not always possible for
core operators to increase the MTU of every link. Fragmentation
after encapsulation and reassembling of encapsulated packets MUST be
supported by AFBRs [RFC5565].
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 AFBR.
For example, Figure 8 shows the register-stop message format in IPv4
and IPv6 address family.
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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 8: Register-Stop Message Format
In Figure 8, the semantics of fields "PIM Ver", "Type", "Reserved",
and "Checksum" remain the same.
IPv4 Group Address (Encoded-Group format): The encoded-group format
of the IPv4 group address described in Section 4.2 and 5.2.
IPv4 Source Address (Encoded-Group format): The encoded-unicast
format of the IPv4 source address described in Section 4.3 and 5.3.
IPv6 Group Address (Encoded-Group format): The encoded-group format
of the IPv6 group address described in Section 4.2 and 5.2.
IPv6 Source Address (Encoded-Group format): The encoded-unicast
format of the IPv6 source address described in Section 4.3 and 5.3.
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.
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10. Security Considerations
The security concerns raised in [RFC4925] and [RFC7761] are
applicable here. In addition, the additional workload associated
with some schemes could be exploited by an attacker to perform a out
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.
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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <http://www.rfc-editor.org/info/rfc4301>.
[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,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc6052>.
[RFC6513] Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
2012, <http://www.rfc-editor.org/info/rfc6513>.
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[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, <http://www.rfc-editor.org/info/rfc7761>.
12.2. Informative References
[RFC7371] Boucadair, M. and S. Venaas, "Updates to the IPv6
Multicast Addressing Architecture", RFC 7371,
DOI 10.17487/RFC7371, September 2014,
<http://www.rfc-editor.org/info/rfc7371>.
Appendix A. Acknowledgements
Wenlong Chen, Xuan Chen, Alain Durand, Yiu Lee, Jacni Qin and Stig
Venaas provided useful input into this document.
Authors' Addresses
Mingwei Xu
Tsinghua University
Department of Computer Science, Tsinghua University
Beijing 100084
P.R. China
Phone: +86-10-6278-5822
Email: xmw@cernet.edu.cn
Yong Cui
Tsinghua University
Department of Computer Science, Tsinghua University
Beijing 100084
P.R. China
Phone: +86-10-6278-5822
Email: cuiyong@tsinghua.edu.cn
Jianping Wu
Tsinghua University
Department of Computer Science, Tsinghua University
Beijing 100084
P.R. China
Phone: +86-10-6278-5983
Email: jianping@cernet.edu.cn
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Shu Yang
Tsinghua University
Graduate School at Shenzhen
Shenzhen 518055
P.R. China
Phone: +86-10-6278-5822
Email: yangshu@csnet1.cs.tsinghua.edu.cn
Chris Metz
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Phone: +1-408-525-3275
Email: chmetz@cisco.com
Greg Shepherd
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Phone: +1-541-912-9758
Email: shep@cisco.com
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