Softwires Working Group C. Metz, Ed.
Internet-Draft Cisco Systems
Intended status: Informational Y. Cui, Ed.
Expires: August 18, 2008 M. Xu, Ed.
Tsinghua University
February 15, 2008
Softwires Mesh Multicast Problem Statement
draft-metz-softwires-multicast-problem-statement-00.txt
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Abstract
This document defines a problem statemet for Softwires Mesh
Multicast.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Scenarios of Interest . . . . . . . . . . . . . . . . . . 4
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Considerations . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Optimality vs Scalability . . . . . . . . . . . . . . . . 6
3.2. Single-Source Multicast vs Any-Source Multicast . . . . . 7
3.3. E-IP Client Networks and MPLS Multicast . . . . . . . . . 7
3.4. Client E-IP Multicast Signaling between AFBR Nodes . . . . 7
3.5. I-IP Unicast Core . . . . . . . . . . . . . . . . . . . . 7
4. Problem Examples . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Client E-IP = IPv6 and I-IP Backbone = IPv4 . . . . . . . 9
4.2. Client E-IP = IPv4 and I-IP Backbone = IPv6 . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. Introduction
The Internet will need to support IPv4 and IPv6 packets. Both
address families and their attendent 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.
The Softwires Working Group has defined a framework by which E-IP
unicast and multicast packets can be tunneled across an I-IP backbone
network [I-D.draft-ietf-softwire-mesh-framework-03]. The tunnels are
referred to as Softwires. The Softwires Problem Statement [RFC4925]
calls out multicast as a requirement. The charter for the Softwires
working group explicitly mentions multicast and at the Vancouver IETF
meeting, a healthy discussion on Softwire Multicast ensued. It was
suggested and agreed to at the time that a problem statement for
softwire mesh multicast be created and discussed at the next IETF
meeting in Philadelphia.
This document describes the softwires mesh multicast problem
statement.
1.1. Terminology
The following terminology will be used in this document.
o Softwire (SW) - A "tunnel" that is created on the basis of a
control protocol setup between softwire endpoints with a shared
point-to- point, multipoint-to-point, point-to-multipoint or
multipoint-to-multipoing state. Softwires are generally dynamic
in nature (they may be initiated and terminated on demand), but
may be very long-lived.
o Address Family Border Router (AFBR) - The dual-stack router that
interconnects two networks that use different address families.
In the context of softwires multicast, the AFBR runs E-IP and I-IP
control planes to maintain E-IP and I-IP multicast state
respectively and performs the appropriate encapsulation/
decapsultion client E-IP multicast packets for transport across
the I-IP backbone.
o I-IP ("Internal IP"). This refers to the form of IP (i.e., either
IPv4 or IPv6) that is supported by the transit (or backbone)
network.
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o E-IP ("External IP") This refers to the form of IP (i.e. either
IPv4 or IPv6) that is supported by the client networks.
o I-IP Core Tree. A single-source or multi-source distribution tree
rooted at one or more AFBR source nodes and branching 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 backbone network. An I-IP core tree
is used to tunnel E-IP multicast packets belonging to E-IP trees
across the I-IP backbone. Another name for an I-IP core tree is
multicast or multipoint softwire.
o E-IP client tree. A single-source or multi-source distribution
tree rooted at one or more hosts or routers located inside a
client E-IP network and branching out to one or more leaf nodes
located in the same or different client E-IP networks.
1.2. Scenarios of Interest
The scenarios of interest are the following:
o IPv6-over-IPv4. This is the case when the I-IP backbone is IPv4
and the client E-IP networks are global IPv6
o IPv4-over-IPv6. This is the case where the I-IP backbone is IPv6
and the client E-IP networks are global IPv4
While the focus on Softwires is IPv6 transition, it should be noted
that the mechanisms defined so far in the softwire mesh framework and
any additional protocol machinery required for softwire mesh
multicast can and should work in the cases where E-IP and I-IP
networks support the same IP address family.
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2. Motivation
The fundamental objective of any I-IP backbone, in particular as it
relates to Softwires Mesh and IPv6 transition is to provide transit
connectivity between constituent client E-IP networks. The transit
connectivity between client E-IP networks must support both IP
unicast and multicast packets. With respect to the latter it is
possible, although highly undesirable for provisioning and
scalability reasons, to accomodate multicast connectivity across the
I-IP backbone via a series of inter-AFBR point-to-point tunnels using
mechanisms such as GRE [RFC2784] or L2TPv3 [RFC3931]
The preferred solution is to leverage the multicast functions,
inherent in the I-IP backbone, to efficiently and scalably tunnel
encapsulated client E-IP multicast packets inside an I-IP core tree
rooted at one or more ingress AFBR nodes and branching out to one or
more egress AFBR leaf nodes.
Thus we require protocol machinery capable of dynamically building
single-source or multi-source I-IP core trees (aka multipoint
softwires) capable of providing client E-IP multicast connectivity.
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3. Considerations
This section lays out several areas to consider when addressing the
softwires mesh multicast problem.
3.1. Optimality vs Scalability
The issues regarding optimality versus scalability have been
discussed ad nauseum. [I-D.draft-ietf-l3vpn-2547bis-mcast-06]At one
end of the spectrum each E-IP client tree maps to one I-IP core tree
on a 1:1 basis so the amount of multicast state inside the core is
O(# of E-IP client trees). 1:1 mapping infers E-IP client and I-IP
core branch and/or leaf routers will be contiguous on the AFBR nodes
thus contributing to an optimal delivery. In addition 1:1 mapping is
consistent with Internet-style multicast in that there is no
aggregation of E-IP client state performed by routers.
At the other end of the spectrum we have aggregation where a set of
AFBR-rooted single-source trees or even an individual any-source tree
could serve to deliver all client E-IP multicast packets to all AFBR
leaf nodes where a decision to forward to downstream client E-IP
routers will be determined by E-IP multicast state. This N:1 mapping
of N number of E-IP client trees mapped to one or a few I-IP core
trees is scalable from a state perspective. However it could be sub-
optimal because some AFBR nodes might have to discard E-IP multicast
packets received through an I-IP core tree for which there are no
downstream receivers.
Aggregation is appropriate for the L3VPN cases where the backbone
needs to handle potentially a large number of VPN-specific, receiver-
dense, low-volume multicast trees. The advantages of state
containment in the backbone at the expense of low-volume packet drops
on uninterested leaf routers is acceptable. This is not the case for
Internet multicast where receiver populations are sparse and per-tree
traffic volumes tend towards higher-bandwidth consuming multimedia
streams.
The mission for softwires is to support global E-IP connectivity
across an I-IP backbone network. We extend this notion naturally in
support of multicast which leads to the conclusion that only
Internet-style multicast (i.e 1:1 mapping with no aggregation )
should be addressed in Softwires.
If the IETF community under the auspices of another working group
(e.g. MBONED) deems that aggregation of Internet multicast is a
problem needing a solution,then those requirements should be
documented and disseminated to other working groups for
consideration.
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It should also be noted that a solution for multicast state
aggregation exists today in the form of multicast VPNs.
3.2. Single-Source Multicast vs Any-Source Multicast
Noting again that softwires must address Internet multicast, it is
required that singles-source multicast (e.g. PIM-SSM) be supported.
Any-source multicast (ASM) could be considered but we observe that
there is a decided lack of security in the current schemes. One
could have unauthorized sources blasting multicast packets into a
shared tree in a benign or malicious denial-of-service attack.
The use embedded RP [RFC3956]in which the address of the rendezvous
point (RP) for the shared tree is embedded in the group address is
one way that ASM could scale for the Internet but the security issue
still remains.
3.3. E-IP Client Networks and MPLS Multicast
Client E-IP networks will run native IPv4 or native IPv6 multicast to
build E-IP client trees and to replicate and forward client E-IP
multicast packets. No requirement or statement has been put forth to
date suggesting that the client E-IP networks will run MPLS
multicast.
It is noted that MPLS multicast is permitted to run in I-IP backbone
networks.
3.4. Client E-IP Multicast Signaling between AFBR Nodes
AFBR nodes might need to establish some form of control plane
interaction to exchange client E-IP multicast configuration and/or
routing information. Configuring and running a full mesh of inter-
AFBR E-IP PIM adjacencies in an overlay fashion is one possible
solution. Another solution is to extend MP-BGP to distribute E-IP
multicast routing information between AFBR nodes.
[I-D.draft-ietf-l3vpn-2547bis-mcast-bgp-04]
3.5. I-IP Unicast Core
It is possible that the I-IP backbone only supports softwire mesh
unicast routing and forwarding. In such a scenario, the ingess AFBR
node(s) will need to replicate, map and then encapsulate E-IP client
multicast packets in softwire unicast tunnel headers for transport to
the appropriate egress AFBR node(s). There the softwire tunnel
header is removed and E-IP client multicast packet process occurs.
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A method for mapping E-IP client multicast group addresses to the
I-IP unicast addresses of the one or more egress AFBR nodes will be
required.
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4. Problem Examples
This section outlines several examples of where softwire mesh
multicast would apply.
4.1. Client E-IP = IPv6 and I-IP Backbone = IPv4
This first example will likely be the most common one encountered.
The client E-IP networks are running global IPv6 routing and are
attached via single-homed or multi-homed connections to dual-stack
AFBR nodes positioned at the edges of a backbone I-IP network running
IPv4.
The client E-IP IPv6 networks are capable of running single-source
and any-source multicast applications and support PIM-SM and PIM-SSM
for building and maintaining E-IP client trees. Multicast sources
(hosts or routers) are rooted in one or more client E-IP networks and
leafs or receivers are located in the same or remote client E-IP
networks on the other side of the I-IP IPv4 backbone network.
The I-IP IPv4 backbone runs PIM or mLDP to build and maintain I-IP
core trees. The encapsulations applied to client E-IP IPv6 multicast
packets tunneled inside the I-IP core trees are IPv4 multicast or
MPLS labels.
The core routers do not hold client E-IP routes so support for the
RPF Vector is needed. This enables core routers to forward I-IP PIM
IPv4 join/prune messages towards the AFBR leading to the E-IP source
or RP.
The dual-stack AFBR nodes run E-IP PIM to exchange client E-IP
multicast routing information with attached client E-IP routers. The
AFBR nodes must also exchange client E-IP routing information with
other AFBR nodes. And finally an AFBR is expected to participate in
the signaling (I-IP PIM or I-IP mLDP) necessary to establis the I-IP
core tree.
The encapsulation and decapsulation of client E-IP multicast packets
in I-IP multipoint softwire packets is performed by the AFBR.
4.2. Client E-IP = IPv4 and I-IP Backbone = IPv6
This second example will be encountered as well. The client E-IP
networks are running global IPv4 routing and are attached via single-
homed or multi-homed connections to dual-stack AFBR nodes positioned
at the edges of a backbone I-IP network running native IPv6.
Now client E-IP IPv4 networks will have multicast connectivity
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extended across an I-IP IPv6 backbone network with the same general
interactions in place as described in the previous scenario. The one
exception is the strong likelihood that MPLS multicast will not
operate in the I-IP IPv6 backbone network.
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5. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
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6. Security Considerations
TBA ..
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7. Acknowledgements
Toerless Eckert, Greg Shephard, Eric Rosen and Vasu Kengeri provided
useful input.
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8. References
8.1. Normative References
[I-D.draft-ietf-l3vpn-2547bis-mcast-06]
Rosen, E., Ed., "Multicast in MPLS/BGP IP VPNs",
<draft-ietf-l3vpn-2547bis-mcast-06.txt>.
[I-D.draft-ietf-l3vpn-2547bis-mcast-bgp-04]
Rosen, E. and R. Aggarwal, "BGP Encodings and Procedures
for Multicast in MPLS/BGP IP VPNs", November 2007,
<draft-ietf-l3vpn-2547bis-mcast-bgp-04.txt>.
[I-D.draft-ietf-softwire-mesh-framework-03]
Rosen, E., Ed., "Softwire Mesh Framework", January 2008,
<draft-ietf-softwire-mesh-framework-03.txt>.
[RFC2784] Farinacci, D., "Generic Router Encapsulation", March 2000.
[RFC3931] Townsley, M., Ed., "Layer-2 Tunneling Protocol - Version
3", March 2005.
[RFC3956] Savola, P., "Embedding the Rendezvous Point (RP) Address
in an IPv6 Multicast Address", November 2004.
[RFC4925] Li, X., Ed., "Softwire Problem Statement", July 2007.
8.2. Informative References
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Authors' Addresses
Chris Metz (editor)
Cisco Systems
170 West Tasman Drive
San Jose, California 95134-1706
USA
Phone: +1-408-525-3275
Email: chmetz@cisco.com
Yong Cui (editor)
Tsinghua University
Department of Computer Science, Tsinghua University
Beijing 100084
P.R.China
Phone: +86-10-6278-5822
Email: cuiyong@tsinghua.edu.cn
Mingwei Xu (editor)
Tsinghua University
Department of Computer Science, Tsinghua University
Beijing 100084
P.R.China
Phone: +86-10-6278-5822
Email: xmw@csnet1.cs.tsinghua.edu.cn
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