Network Working Group M. Xu
Internet-Draft Y. Cui
Expires: January 15, 2013 J. Wu
S. Yang
Tsinghua University
C. Metz
G. Shepherd
Cisco Systems
July 14, 2012
Softwire Mesh Multicast
draft-ietf-softwire-mesh-multicast-03
Abstract
The Internet needs to support IPv4 and IPv6 packets. Both address
families and their attendant protocol suites support multicast of the
single-source and any-source varieties. As part of the transition to
IPv6, there will be scenarios where a backbone network running one IP
address family internally (referred to as internal IP or I-IP) will
provide transit services to attached client networks running another
IP address family (referred to as external IP or E-IP). 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 to E-IP unicast and multicast support
across an I-IP backbone. This document describes the mechanisms for
supporting Internet-style multicast across a set of E-IP and I-IP
networks supporting 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 January 15, 2013.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Scenarios of Interest . . . . . . . . . . . . . . . . . . . . 7
3.1. IPv4-over-IPv6 . . . . . . . . . . . . . . . . . . . . . . 7
3.2. IPv6-over-IPv4 . . . . . . . . . . . . . . . . . . . . . . 8
4. IPv4-over-IPv6 Mechanism . . . . . . . . . . . . . . . . . . . 10
4.1. Mechanism Overview . . . . . . . . . . . . . . . . . . . . 10
4.2. Group Address Mapping . . . . . . . . . . . . . . . . . . 10
4.3. Source Address Mapping . . . . . . . . . . . . . . . . . . 11
4.4. Routing Mechanism . . . . . . . . . . . . . . . . . . . . 12
5. IPv6-over-IPv4 Mechanism . . . . . . . . . . . . . . . . . . . 14
5.1. Mechanism Overview . . . . . . . . . . . . . . . . . . . . 14
5.2. Group Address Mapping . . . . . . . . . . . . . . . . . . 14
5.3. Source Address Mapping . . . . . . . . . . . . . . . . . . 14
5.4. Routing Mechanism . . . . . . . . . . . . . . . . . . . . 15
6. Actions performed by AFBR . . . . . . . . . . . . . . . . . . 17
6.1. E-IP (*,G) state maintenance . . . . . . . . . . . . . . . 17
6.2. E-IP (S,G) state maintenance . . . . . . . . . . . . . . . 17
6.3. I-IP (S',G') state maintenance . . . . . . . . . . . . . . 17
6.4. E-IP (S,G,rpt) state maintenance . . . . . . . . . . . . . 17
6.5. Inter-AFBR signaling . . . . . . . . . . . . . . . . . . . 17
6.6. Process and forward multicast data . . . . . . . . . . . . 19
6.7. SPT switchover . . . . . . . . . . . . . . . . . . . . . . 19
7. Other Considerations . . . . . . . . . . . . . . . . . . . . . 21
7.1. Other PIM Message Types . . . . . . . . . . . . . . . . . 21
7.2. Selecting a Tunneling Technology . . . . . . . . . . . . . 21
7.3. TTL . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.4. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.1. Normative References . . . . . . . . . . . . . . . . . . . 24
10.2. Informative References . . . . . . . . . . . . . . . . . . 24
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
The Internet needs to support IPv4 and IPv6 packets. Both address
families and their attendant protocol suites support multicast of the
single-source and any-source varieties. As part of the transition to
IPv6, there will be scenarios where a backbone network running one IP
address family internally (referred to as internal IP or I-IP) will
provide transit services to attached client networks running another
IP address family (referred to as external IP or E-IP).
The preferred solution is to leverage the multicast functions
inherent in the I-IP backbone, to efficiently and scalably forward
client E-IP multicast packets inside an I-IP core tree, which roots
at one or more ingress AFBR nodes and branches out to one or more
egress AFBR leaf nodes.
[6] outlines the requirements for the softwires mesh scenario
including the multicast. It is straightforward to envisage that
client E-IP multicast sources and receivers will reside in different
client E-IP networks connected to an I-IP backbone network. This
requires that the client E-IP source-rooted or shared tree should
traverse the I-IP backbone network.
One method to accomplish this is to re-use the multicast VPN approach
outlined in [10]. MVPN-like schemes can support the softwire mesh
scenario and achieve a "many-to-one" mapping between the E-IP client
multicast trees and 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 a large amount of
(S,G) states. Aggregation at the edge contains the (S,G) states that
need to be maintained by the network operator supporting the customer
VPNs. The disadvantage of this approach is the possible inefficient
bandwidth and resource utilization when multicast packets are
delivered to a receiver AFBR with no attached E-IP receivers.
Internet-style multicast is somewhat different in that the trees tend
to be relatively sparse and source-rooted. The need for multicast
aggregation at the edge (where many customer multicast trees are
mapped into a few or one backbone multicast trees) does not exist and
to date has not been identified. Thus the need for a basic or closer
alignment with E-IP and I-IP multicast procedures emerges.
A framework on how to support such methods is described in [8]. In
this document, a more detailed discussion supporting the "one-to-one"
mapping schemes for the IPv6 over IPv4 and IPv4 over IPv6 scenarios
will be discussed.
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2. Terminology
An example of a softwire mesh network supporting multicast is
illustrated in Figure 1. A multicast source S is located in one E-IP
client network, while candidate E-IP group receivers are located in
the same or different E-IP client networks that all share a common
I-IP transit network. When E-IP sources and receivers are not local
to each other, they can only communicate with each other through the
I-IP core. There may be several E-IP sources for some multicast
group residing in different client E-IP networks. In the case of
shared trees, the E-IP sources, receivers and RPs might be located in
different client E-IP networks. In a simple case the resources of
the I-IP core are managed by a single operator although the inter-
provider case is not precluded.
._._._._. ._._._._.
| | | | --------
| E-IP | | E-IP |--|Source S|
| network | | network | --------
._._._._. ._._._._.
| |
AFBR upstream AFBR
| |
__+____________________+__
/ : : : : \
| : : : : | E-IP Multicast
| : I-IP transit core : | packets should
| : : : : | get across the
| : : : : | I-IP transit core
\_._._._._._._._._._._._._./
+ +
downstream AFBR downstream AFBR
| |
._._._._ ._._._._
-------- | | | | --------
|Receiver|-- | E-IP | | E-IP |--|Receiver|
-------- |network | |network | --------
._._._._ ._._._._
Figure 1: Softwire Mesh Multicast Framework
Terminology used in this document:
o Address Family Border Router (AFBR) - A dual-stack router
interconnecting two or more networks using different IP address
families. In the context of softwire mesh multicast, the AFBR runs
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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 the form of IP (i.e., either
IPv4 or IPv6) that is supported by the core (or backbone) network.
An I-IPv6 core network runs IPv6 and an I-IPv4 core network runs
IPv4.
o E-IP (External IP): This refers to the form of IP (i.e. either IPv4
or IPv6) that is supported by the client network(s) attached to the
I-IP transit core. An E-IPv6 client network runs IPv6 and an E-IPv4
client network runs IPv4.
o I-IP core tree: A 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 IPv4-
embedded IPv6 source address.
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3. Scenarios of Interest
This section describes the two different scenarios where softwires
mesh multicast will apply.
3.1. IPv4-over-IPv6
._._._._. ._._._._.
| IPv4 | | IPv4 | --------
| Client | | Client |--|Source S|
| network | | network | --------
._._._._. ._._._._.
| |
AFBR upstream AFBR
| |
__+____________________+__
/ : : : : \
| : : : : |
| : IPv6 transit core : |
| : : : : |
| : : : : |
\_._._._._._._._._._._._._./
+ +
downstream AFBR downstream AFBR
| |
._._._._ ._._._._
-------- | IPv4 | | IPv4 | --------
|Receiver|-- | Client | | Client |--|Receiver|
-------- | network| | network| --------
._._._._ ._._._._
Figure 2: IPv4-over-IPv6 Scenario
In this scenario, the E-IP client networks run IPv4 and I-IP core
runs IPv6. This scenario is illustrated in Figure 2.
Because of the much larger IPv6 group address space, it will not be a
problem to map individual client E-IPv4 tree to a specific I-IPv6
core tree. This simplifies operations on the AFBR because it becomes
possible to algorithmically map an IPv4 group/source address to an
IPv6 group/source address and vice-versa.
The IPv4-over-IPv6 scenario is an emerging requirement as network
operators build out native IPv6 backbone networks. These networks
naturally support native IPv6 services and applications but it is
with near 100% certainty that legacy IPv4 networks handling unicast
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and multicast should be accommodated.
3.2. IPv6-over-IPv4
._._._._. ._._._._.
| IPv6 | | IPv6 | --------
| Client | | Client |--|Source S|
| network | | network | --------
._._._._. ._._._._.
| |
AFBR upstream AFBR
| |
__+____________________+__
/ : : : : \
| : : : : |
| : IPv4 transit core : |
| : : : : |
| : : : : |
\_._._._._._._._._._._._._./
+ +
downstream AFBR downstream AFBR
| |
._._._._ ._._._._
-------- | IPv6 | | IPv6 | --------
|Receiver|-- | Client | | Client |--|Receiver|
-------- | network| | network| --------
._._._._ ._._._._
Figure 3: IPv6-over-IPv4 Scenario
In this scenario, the E-IP Client Networks run IPv6 while the I-IP
core runs IPv4. This scenario is illustrated in Figure 3.
IPv6 multicast group addresses are longer than IPv4 multicast group
addresses. It will not be possible to perform an algorithmic IPv6 -
to - IPv4 address mapping without the risk of multiple IPv6 group
addresses mapped to the same IPv4 address resulting in unnecessary
bandwidth and resource consumption. Therefore additional efforts
will be required to ensure that client E-IPv6 multicast packets can
be injected into the correct I-IPv4 multicast trees at the AFBRs.
This clear mismatch in IPv6 and IPv4 group address lengths means that
it will not be possible to perform a one-to-one mapping between IPv6
and IPv4 group addresses unless the IPv6 group address is scoped.
As mentioned earlier, this scenario is common in the MVPN
environment. As native IPv6 deployments and multicast applications
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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 contain routes to all other
client E-IPv4 networks. Through the set of known and deployed
mechanisms, E-IPv4 hosts and routers have discovered or 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 I-IP IPv6 address of
the upstream AFBR. When the I-IPv6 PIM message arrives at the
upstream AFBR, it should be translated back into an E-IPv4 PIM
message. The result of these actions is the construction of E-IPv4
trees and a corresponding I-IP tree in the I-IP network.
In this case it is incumbent upon the AFBR routers to perform PIM
message conversions in the control plane and IP group address
conversions or mappings in the data plane. It becomes possible to
devise an algorithmic one-to-one IPv4-to-IPv6 address mapping at
AFBRs.
4.2. Group Address Mapping
For IPv4-over-IPv6 scenario, a simple algorithmic mapping between
IPv4 multicast group addresses and IPv6 group addresses is supported.
[11] has already defined an applicable format. Figure 4 is the
reminder of the format:
| 8 | 4 | 4 | 16 | 4 | 60 | 32 |
+--------+----+----+-----------+----+------------------+----------+
|11111111|0011|scop|00.......00|64IX| sub-group-id |v4 address|
+--------+----+----+-----------+----+------------------+----------+
+-+-+-+-+
IPv4-IPv6 Interconnection bits (64IX): |M|resvd|
+-+-+-+-+
"resvd" are reserved bits.
Figure 4: IPv4-Embedded IPv6 Multicast Address Format: SSM Mode
The high order bits of the I-IPv6 address range will be fixed for
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mapping purposes. With this scheme, each IPv4 multicast address can
be mapped into an IPv6 multicast address(with the assigned prefix),
and each IPv6 multicast address with the assigned prefix can be
mapped into IPv4 multicast address.
4.3. 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 could be very complex
to support all possible scenarios. But since SSM can be implemented
with a strict subset of the PIM-SM protocol mechanisms [5], we can
treat I-IP core as SSM-only to make it as simple as possible, then
there remains 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 [9]:
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 0-------------32--40--48--56--64--72--80--88--96-----------127|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| prefix |v4(32) | u | suffix |source address |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|<------------------uPrefix64------------------>|
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 [9]; "v4" field is the IP address
of one of upstream AFBR's E-IPv4 interfaces; "u" field is defined
in [4], and MUST be set to zero; "suffix" field is reserved for
future extensions and SHOULD be set to zero; "source address"
field stores the original S. We call the overall /96 prefix
("prefix" field and "v4" field and "u" field and "suffix" field
altogether) "uPrefix64".
o E-IP network supports ASM
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The (S,G) source list entry and the (*,G) source list entry only
differ in that the latter have both the WC and RPT bits of the
Encoded-Source-Address set, while the former all cleared (See
Section 4.9.5.1 of [5]). 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 upstream AFBRs, and translate them back at
upstream AFBRs vice-versa.
4.4. 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 /32 prefix in "IPv4-Embedded IPv6 Virtual
Source Address Format" must be known to every AFBR, and every AFBR
should not only announce the IP address of one of its E-IPv4
interfaces presented in the "v4" field to other AFBRs by MPBGP, but
also announce the corresponding uPrefix64 to the I-IPv6 network.
Since every IP address of upstream AFBR's E-IPv4 interface is
different from each other, every uPrefix64 that AFBR announces should
be different either, and uniquely identifies each AFBR. "uPrefix64"
is an IPv6 prefix, and the distribution of it is the same as the
distribution in the traditional mesh unicast scenario. But since
"v4" field is an E-IPv4 address, and BGP messages are NOT tunneled
through softwires or through any other mechanism as specified in [8],
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
uPrefix64 of S' is unique, and is known to every router in the I-IPv6
network, the translated message will eventually arrive at the
corresponding upstream AFBR, and the upstream AFBR can translate the
message back to (S,G). When a downstream AFBR receives an E-IPv4 PIM
(*,G) message, S' can be generated according to the format specified
in Figure 4, with "source address" field set to *(the IPv4 address of
RP). The translated message will eventually arrive at 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 [5]), when this upstream AFBR checks
the "source address" field of the message, it'll find the IPv4
address of RP, so this upstream AFBR judges that this is originally a
(*,G) message, then it translates the message back to the (*,G)
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message and processes it.
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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 the set of known and deployed
mechanisms, 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 certainly
separated from E-IP multicast state.
This particular scenario introduces unique challenges. Unlike the
IPv4-over-IPv6 scenario, it's impossible to map all of the IPv6
multicast address space into the IPv4 address space to address the
one-to-one Softwire Multicast requirement. To coordinate with the
"IPv4-over-IPv6" scenario and keep the solution as simple as
possible, one possible solution to this problem is to limit the scope
of the E-IPv6 source addresses for mapping, such as applying a "Well-
Known" prefix or 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.
A recommended multicast address format is defined in [11]. The high
order bits of the E-IPv6 address range will be fixed for mapping
purposes. With this scheme, each IPv4 multicast address can be
mapped into an IPv6 multicast address(with the assigned prefix), and
each IPv6 multicast address with the assigned prefix can be mapped
into IPv4 multicast address.
5.3. 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 could be very complex
to support all possible scenarios. But since SSM can be implemented
with a strict subset of the PIM-SM protocol mechanisms [5], we can
treat I-IP core as SSM-only to make it as simple as possible, then
there remains 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
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E-IPv6 to I-IPv4 unicast address, the upstream AFBR is unable to
remap the I-IPv4 source address to the original E-IPv6 source
address without any constraints.
We apply a fixed IPv6 prefix and static mapping to solve this
problem. A recommended source address format is defined in [9].
Figure 6 is 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 [9]; "source address"
field is the corresponding I-IPv4 source address.
o E-IP network supports ASM
The (S,G) source list entry and the (*,G) source list entry only
differ in that the latter have both the WC and RPT bits of the
Encoded-Source-Address set, while the former all cleared (See
Section 4.9.5.1 of [5]). 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 upstream AFBRs, and translate them back at
upstream AFBRs vice-versa. 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.
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
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upstream AFBR of this source should 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 eventually arrive at the corresponding
upstream AFBR, and the upstream AFBR can translate the message back
to (S,G) by appending the prefix to S'. 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
eventually arrive at 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 [5]), 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.
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6. Actions performed by AFBR
The following actions are performed by AFBRs:
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.
6.3. I-IP (S',G') state maintenance
It is possible that there runs a non-transit I-IP PIM-SSM in the I-IP
transit core. Since the translated source address starts with the
unique "Well-Known" prefix or the ISP-defined prefix that should not
be used otherwise, mesh multicast won't influence non-transit PIM-SM
multicast at all. When one AFBR receives an I-IP (S',G') message, it
should check S'. If S' starts with the unique prefix, it means that
this message is actually a translated E-IP (S,G) or (*,G) message,
then the AFBR should 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 do 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 [5],
it should propagate a Prune(S,G,rpt) message along with the
periodical Join(*,G) message upstream towards RP. Unfortunately,
routers in I-IP transit core are not supposed to understand (S,G,rpt)
messages since I-IP transit core is treated as SSM-only. As a
result, this downstream AFBR is unable to prune S from this RPT, then
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 RPT.
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When a downstream AFBR wishes to propagate a (S,G,rpt) message
upstream router, it should encapsulate the (S,G,rpt) message, then
unicast the encapsulated message to the corresponding upstream AFBR,
which we call "RP'".
When RP' receives this encapsulated message, it should decapsulate
this message as what it does in the unicast scenario, and get the
original (S,G,rpt) message. The incoming interface of this message
may be different from 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 [5] on the incoming interface.
To solve this problem, and keep the solution as simple 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 no downstream AFBR wants to receive multicast
data of (S,G) along the RPT. In this way, we do insure that
downstream AFBRs won't miss any multicast data that they needs, at
the cost of duplicated multicast data of (S,G) along the RPT received
by SPT-switched-over downstream AFBRs, if there exists at least one
downstream AFBR that hasn't 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 | |
| +----------+ +----------+ |
| ^ | ^ | |
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| | | | | |
+--------|-----|----------|-----|--------+
| v | v
Figure 7: Interface Agent Implementation Example
In this example, the interface agent has two responsibilities: In the
control plane, it should work as a real interface that has joined
(*,G) in representative of all the I-IP interfaces who should have
been outgoing interfaces of (*,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') state, which means that no downstream AFBR wants 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 changed it mind to receive
multicast data of (S,G) along the RPT again, the interface agent
should send a Join(S,G,rpt) to 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 onto every I-IP interface.
NOTICE: There may exist 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 still take effect.
6.6. Process and forward multicast data
On receiving multicast data from upstream routers, the AFBR looks up
its forwarding table to check the IP address of each outgoing
interface. If there exists at least one outgoing interface whose IP
address family is different from the incoming interface, the AFBR
should encapsulate/decapsulate this packet and forward it to such
outgoing interface(s), then forward the data to other outgoing
interfaces without encapsulation/decapsulation.
When a downstream AFBR that has already switched over to SPT of S
receives an encapsulated multicast data packet of (S,G) along the
RPT, it should silently drop this packet.
6.7. 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
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first. At this time, every downstream AFBR will receive multicast
data from any source from this RPT, in spit of whether they have
switched over to SPT of some source(s) or not.
To minimize this redundancy, it's recommended that every AFBR's
SwitchToSptDesired(S,G) function employs the "switch on first packet"
policy. In this way, the delay of switchover to SPT is kept as
little 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.
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7. Other Considerations
7.1. Other PIM Message Types
Apart from Join or Prune, there exists other message types 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 routers on a link to negotiate with each other.
They don't need to be translated for forwarding, thus the process of
these messages is out of scope for this document.
7.2. Selecting a Tunneling Technology
The choice of tunneling technology is a matter of policy configured
at AFBRs. It's recommended that all AFBRs use the same technology,
otherwise some AFBRs may not be able to decapsulate encapsulated
packets from other AFBRs that use a different tunneling technology.
7.3. TTL
The process of TTL depends on the tunneling technology, and is out of
scope for this document.
7.4. Fragmentation
The encapsulation performed by upstream AFBR will increase the size
of packets. As a result, the outgoing I-IP link MTU may not
accommodate the extra size. As it's not always possible for core
operators to increase every link's MTU, fragmentation and
reassembling of encapsulated packets MUST be supported by AFBRs.
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8. Security Considerations
The AFBR routers could maintain secure communications through the use
of Security Architecture for the Internet Protocol as described in
[RFC4301]. But when adopting some schemes that will cause heavy
burden on routers, some attacker may use it as a tool for DDoS
attack.
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9. IANA Considerations
When AFBRs perform address mapping, they should follow some
predefined rules, especially the IPv6 prefix for source address
mapping should be predefined, so that ingress AFBR and egress AFBR
can finish the mapping procedure correctly. The IPv6 prefix for
translation can be unified within only the transit core, or within
global area. In the later condition, the prefix should be assigned
by IANA.
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10. References
10.1. Normative References
[1] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
"Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.
[2] Foster, B. and F. Andreasen, "Media Gateway Control Protocol
(MGCP) Redirect and Reset Package", RFC 3991, February 2005.
[3] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[4] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[5] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[6] Li, X., Dawkins, S., Ward, D., and A. Durand, "Softwire Problem
Statement", RFC 4925, July 2007.
[7] Wijnands, IJ., Boers, A., and E. Rosen, "The Reverse Path
Forwarding (RPF) Vector TLV", RFC 5496, March 2009.
[8] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
Framework", RFC 5565, June 2009.
[9] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li,
"IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
October 2010.
[10] Rosen, E. and R. Aggarwal, "Multicast in MPLS/BGP IP VPNs",
RFC 6513, February 2012.
10.2. Informative References
[11] Boucadair, M., Qin, J., Lee, Y., Venaas, S., Li, X., and M. Xu,
"IPv6 Multicast Address Format With Embedded IPv4 Multicast
Address", draft-ietf-mboned-64-multicast-address-format-02
(work in progress), May 2012.
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Appendix A. Acknowledgements
Wenlong Chen, Xuan Chen, Alain Durand, Yiu Lee, Jacni Qin and Stig
Venaas provided useful input into this document.
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Authors' Addresses
Mingwei Xu
Tsinghua University
Department of Computer Science, Tsinghua University
Beijing 100084
P.R. China
Phone: +86-10-6278-5822
Email: xmw@cernet.edu.cn
Yong Cui
Tsinghua University
Department of Computer Science, Tsinghua University
Beijing 100084
P.R. China
Phone: +86-10-6278-5822
Email: cuiyong@tsinghua.edu.cn
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
Shu Yang
Tsinghua University
Department of Computer Science, Tsinghua University
Beijing 100084
P.R. China
Phone: +86-10-6278-5822
Email: yangshu@csnet1.cs.tsinghua.edu.cn
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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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