MBONED Working Group Dorian Kim
Internet Draft Verio
Henry Kilmer
Intermedia/DIGEX
Dino Farinacci
Cisco Systems
David Meyer
Cisco Systems
Category Informational
draft-ietf-mboned-logical-rp-00.txt March, 1999
Using MSDP to create Logical RPs
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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2. Abstract
This document describes a mechanism to allow for an arbitrary number
of RPs per group in a single share-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 currently defined 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.
The single active RP, or flat RP space design of PIM-SM has several
implications, including traffic concentration, lack of scalable load
balancing and redundancy between RPs, sub-optimal forwarding of
multicast packets, and distant RP dependencies. These properties of
PIM-SM have been demonstrated in recent native continental or inter-
continental scale multicast deployments. As a result, it became 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 addresses the issues addressed above.
The mechanism described here is intended to address the need for
redundancy and load sharing among RPs in a domain. It is primarily
intended for application within those networks which are using MBGP,
MSDP and PIM-SM protocols for native multicast deployment, although
it not limited to those protocols. In particular, the logical RP
solution 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 this logical RP solution is not required to run
MBGP.
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5. Problem Definition
The logical RP solution described here provides a solution for both
redundancy and load balancing among any number of active RPs in a
domain.
5.1. Traffic Concentration and Load Balancing Between 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 load balancing 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. The other alternative to periodic
reconfiguration is to split 224.0.0.0/4 space more finely between
more RPs, but this solution can have the disadvantage of creating
more complex RP configurations, along with the attendant operational
problems when RPs are configured [CLUSTERS].
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 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, single active RP per group may cause local sources
and receivers to become dependent on a topologically distant RP. In
case of a scenario where there are backup RPs configured, distant RP
dependence can be created due to the failure of the primary RP, which
is topologically closer, and may become exacerbated by switching to
the backup RP, which may be even more distant topologically, which
may lead to inferior performance, if not outright loss of
connectivity to an RP serving the group, depending on the network
condition at the given moment.
6. Solution
Given the problem set outlined above, a good solution would allow an
operator to define 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 the
unique shared addresses. Note that if the router implementation
chooses the shared address for the BGP router ID, then BGP peerings
will not be established. As a result, care should be taken to avoid
the ambiguity of the BGP router ID with the RP address(for example,
if the logical address chosen is the highest IP address configured on
the router, and the router implementation that automatically chooses
a router ID based upon highest IP address assigned to interfaces).
Finally, the solution described here can be implemented without any
modification to existing protocols or their implementations.
6.2. Further Applications of the Logical RP mechanism
The solution described above can also be applied to external MSDP
peers that are used to join two PIM-SM domains together. This can
provide redundancy to the MSDP peering session, ease operational
complexity as well as simplify configuration management. A side
effect to be aware of with this design is that which of the
configured MSDP sessions comes up will be determined via the unicast
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topology between two providers, and can be some what unpredictable.
If any of the backup peering sessions resets, the active session will
also reset.
7. Multicast State Scaling
Let k = m + r, where
r = resistering to an RP
m = number internal sources learned through MSDP
p = number of internal MSDP peers
For p = 1, m = 0
0 receivers ==> 1 (*,G) + 0 SAs
Greater than 1 receiver ==> k (S,G) + 0 SAs
For p > 1, m != 0
0 receivers ==> 1 (*,G) + m SAs
Greater than 1 receiver ==> k (S,G) + m SAs
Importantly, the multicast state growth is O(k), where k is not a
function of p, the number of internal (logical) RP peers.
8. Security considerations
Since the solution described here makes heavy use of logical
addressing, care must be taken to avoid spoofing. In particular
unicast routing and PIM RPs must be protected.
8.1. 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.
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8.2. Multicast Protocol Integrity
The mechanisms described in [PIMAUTH] should be used to provide
protocol message integrity protection and group-wise message origin
authentication.
9. Acknowledgments
John Meylor, Dave Thaler and Tom Pusateri provided insightful
comments on earlier versions for this idea.
10. References
[CLUSTERS] D. Farinacci, et. al., "Use of Anycast Clusters for
Inter-Domain Multicast Routing",
draft-ietf-farinacci-anycast-clusters-01.txt, March,
1998. ftp://ftpeng.cisco.com/ipmulticast/internet-drafts
[MSDP] D. Farinacci, et. al., "Multicast Source Discovery
Protocol (MSDP)", draft-farinacci-msdp-00.txt,
June, 1998.
[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.
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11. Author's Address
Dorian Kim
Verio, Inc.
2361 Lancashire Dr. #2A
Ann Arbor, MI 48015
Email: dorian@blackrose.org
Hank Kilmer
Intermedia/DIGEX
One DIGEX Plaza
Beltsville, Maryland 20705
Email: hank@digex.net
Dino Farinacci
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
Email: dino@cisco.com
David Meyer
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
Email: dmm@cisco.com
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