MBONED Working Group Dorian Kim
Internet Draft Verio
David Meyer
Cisco Systems
Henry Kilmer
Dino Farinacci
Procket Networks
Category Informational
December, 1999
Anycast RP mechanism using PIM and MSDP
<draft-ietf-mboned-anycast-rp-04.txt>
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC Internet-Drafts.
2026 are working documents of the Internet Engineering Task Force
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The list of current Internet-Drafts can be accessed at
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2. Abstract
This document describes a mechanism to allow for an arbitrary number
of RPs per group in a single shared-tree PIM-SM domain.
This memo is a product of the MBONE Deployment Working Group (MBONED)
in the Operations and Management Area of the Internet Engineering
Task Force. Submit comments to <mboned@ns.uoregon.edu> or the
authors.
3. Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
4. Introduction
PIM-SM as defined in RFC 2352 allows for only a single active RP per
group, and as such the decision of optimal RP placement can become
problematic for a multi-regional network deploying PIM-SM.
Anycast RP relaxes an important constraint in PIM-SM, namely, that
there can be only one group to RP mapping active at any time. The
single mapping property has several implications, including traffic
concentration, lack of scalable register decapsulation (when using
the shared tree), slow convergence when an active RP fails, possible
sub-optimal forwarding of multicast packets, and distant RP
dependencies. These properties of PIM-SM have been demonstrated in
native continental or inter-continental scale multicast deployments.
As a result, it is clear that ISP backbones require a mechanism that
allows definition of multiple active RPs per group in single PIM-SM
domain. Further, any such mechanism should also address the issues
addressed above.
The mechanism described here is intended to address the need for
better fail-over (convergence time) and sharing of the register
decapsulation load (again, when using the shared-tree) among RPs in a
domain. It is primarily intended for applications within those
networks which are using MBGP, Multicast Source Discovery Protocol
[MSDP] and PIM-SM protocols for native multicast deployment, although
it not limited to those protocols. In particular, Anycast RP is
applicable in any PIM-SM network that also supports MSDP (MSDP is
required so that the various RPs in the domain maintain a consistent
view of the sources that are active). Note however, a domain
deploying Anycast RP is not required to run MBGP.
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5. Problem Definition
The anycast RP solution provides a solution for both fast fail-over
and shared-tree load balancing among any number of active RPs in a
domain.
5.1. Traffic Concentration and Distributing Decapsulation Load Among RPs
While PIM-SM allows for multiple RPs to be defined for a given group,
only one group to RP mapping can active at a given time. A
traditional deployment mechanism for balancing register decapsulation
load between multiple RPs covering the multicast group space is to
split up the 224.0.0.0/4 space between multiple defined RPs. This is
an acceptable solution as long as multicast traffic remains low, but
has problems as multicast traffic increases, especially because the
network operator defining group space split between RPs does not
alway have a priori knowledge of traffic distribution between groups.
This can be overcome via periodic reconfigurations, but operational
considerations cause this type of solution to scale poorly.
5.2. Sub-optimal Forwarding of Multicast Packets
When a single RP serves a given multicast group, all joins to that
group will be sent to that RP regardless of the topological distance
between the RP and the sources and receivers. Initial data will be
sent towards the RP also until configured shortest path tree switch
threshold is reached, or the data will always be sent towards the RP
if the network is configured to always use RP rooted shared tree.
This holds true even if all the sources and the receivers are in any
given single region, and RP is topologically distant from the sources
and the receivers. This is an artifact of the dynamic nature of
multicast group members, and of the fact that operators may not
always have a priori knowledge of the topological placement of the
group members.
Taken together, these effects can mean that (for example) although
all the sources and receivers of a given group are in Europe, they
are joining towards the RP in USA and the data will be traversing
relatively expensive pipe(s) twice, once to get to RP, and back down
the RP rooted tree again, creating inefficient use of expensive
resources.
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5.3. Distant RP Dependencies
As outlined above, a single active RP per group may cause local
sources and receivers to become dependent on a topologically distant
RP. In addition, when multiple RPs are configured, there can be
considerable convergence delay involved in switching to the backup
RP. This delay may exist independent of the toplogical location of
the primary and backup RPs.
6. Solution
Given the problem set outlined above, a good solution would allow an
operator to configure multiple RPs per group, and distribute those
RPs in a topologically significant manner to the sources and
receivers.
6.1. Mechanisms
All the RPs serving a given group or set of groups are configured
with identical unicast address, using a numbered interface on the RPs
(frequently a logical interface such as a loopback is used). RPs then
advertise group to RP mappings using this interface address. This
will cause group members (senders) to join (register) towards the
topologically closest RP. RPs MSDP peer with each other using an
address unique to each RP. Note that if the router implementation
chooses the anycast address as the router ID, then peerings and/or
adjacencies may not be established.
In summary then, the following steps are required:
6.1.1. Create the set of group-to-anycast-RP-address mappings
The first step is to create the set of group-to-anycast-RP-address
mappings to be used in the domain. Each RP participating in a anycast
RP set must be configured with a consistent set of group to RP
address mappings. This mapping will be used by the non-RP routers in
the domain.
6.1.2. Configure each RP for the group range with the anycast RP address
The next step is to configure each RP for the group range with the
anycast RP address. If a dynamic mechanism such as auto-RP or the
PIMv2 bootstrap mechanism is being used to advertise group to RP
mappings, the anycast IP address should be used for the RP address.
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6.1.3. Configure MSDP peerings between each of the anycast RPs in the
set
Unlike the group to RP mapping advertisements, MSDP peerings must use
an IP address that is unique to the endpoints. A general guideline is
to follow the addressing of the BGP peerings, e.g., loopbacks for
iBGP peering, physical interface addresses for eBGP peering. Note
that the anycast address MUST NOT be used as the RP address in SA
messages (as this would case the peer-RPF check to fail).
6.1.4. Configure the non-RP's with the group-to-anycast-RP-address
mappings
Finally, each non-RP router must learn the set of group to RP
mappings. This could be done via static configuration, auto-RP, or by
PIMv2 bootstrap mechanism.
6.1.5. Ensure that the anycast IP address is reachable by all routers in
the domain
This is typically accomplished by causing each RP to inject the /32
into the domain's IGP.
6.2. Interaction with MSDP Peer-RPF check
Each MSDP peer receives and forwards the message away from the RP
address in a "peer-RPF flooding" fashion. The notion of peer-RPF
flooding is with respect to forwarding SA messages [MSDP]. The BGP
routing tables are examined to determine which peer is the next hop
towards the originating RP of the SA message. Such a peer is called
an "RPF peer". See [MSDP] for details of the Peer-RPF check. Not
finally that the anycast address MUST NOT be used as the RP address
in the RP's SA messages.
6.3. State Implications
It should be noted that using MSDP in this way forces the creation of
(S,G) state along the path from the receiver to the source. This
state may not be present if a single RP was used and receivers were
forced to stay on the shared tree.
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7. Security considerations
Since the solution described here makes heavy use of anycast
addressing, care must be taken to avoid spoofing. In particular
unicast routing and PIM RPs must be protected.
7.1. Unicast Routing
Both internal and external unicast routing can be weakly protected
with keyed MD5 [RFC1828], as implemented in an internal protocol such
as OSPF [RFC2382] or in BGP [RFC2385]. More generally, IPSEC
[RFC1825] could be used to provide protocol integrity for the unicast
routing system.
7.1.1. Effects of Unicast Routing Instability
While not a security issue, it is worth noting that if unicast
routing is unstable, then the actual RP that source or receiver is
using will be subject to the same instability.
7.2. Multicast Protocol Integrity
The mechanisms described in [PIMAUTH] should be used to provide
protocol message integrity protection and group-wise message origin
authentication.
7.3. MSDP Peer Integrity
As is the the case for BGP, MSDP peers can be protected using keyed
MD5 [RFC1828].
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8. Acknowledgments
John Meylor, Bill Fenner, Dave Thaler and Tom Pusateri provided
insightful comments on earlier versions for this idea.
9. References
[MSDP] D. Farinacci, et. al., "Multicast Source Discovery
Protocol (MSDP)", draft-ietf-msdp-spec-02.txt,
November, 1999.
[PIMAUTH] L. Wei, et al., "Authenticating PIM version 2 messages",
draft-ietf-pim-v2-auth-00.txt, November, 1998.
[RFC1825] Atkinson, R., "IP Security Architecture", August 1995.
[RFC1828] P. Metzger and W. Simpson, "IP Authentication using Keyed
MD5", RFC 1828, August, 1995.
[RFC2362] D. Estrin, et. al., "Protocol Independent Multicast-
Sparse Mode (PIM-SM): Protocol Specification", RFC
2362, June, 1998.
[RFC2382] Moy, J., "OSPF Version 2", RFC 2382, April 1998.
[RFC2385] Herrernan, A., "Protection of BGP Sessions via the TCP
MD5 Signature Option", RFC 2385, August, 1998.
[RFC2403] C. Madson and R. Glenn, "The Use of HMAC-MD5-96 within
ESP and AH", RFC 2403, November, 1998.
10. Author's Address
Dorian Kim
Verio, Inc.
2361 Lancashire Dr. #2A
Ann Arbor, MI 48015
Email: dorian@blackrose.org
Hank Kilmer
Email: hank@rem.com
Dino Farinacci
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Procket Networks
Email: dino@procket.com
David Meyer
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
Email: dmm@cisco.com
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