Network Working Group Dino Farinacci
INTERNET-DRAFT Procket Networks
Expiration Date: May 2004 Yiqun Cai
cisco Systems
November 9, 2003
Anycast-RP using PIM
<draft-ietf-pim-anycast-rp-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This proposal allows Anycast-RP to be used inside a domain which runs
PIM only. There are no other multicast protocols required to support
Anycast-RP, such as MSDP, which has been used traditionally to solve
this problem.
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1. Introduction
Anycast-RP as described in [2] is a mechanism ISP-based backbones
have used to get fast convergence when a PIM Rendezvous Point (RP)
router fails. To allow receivers and sources to Rendezvous to the
closest RP, the packets from a source needs to get to all RPs to find
joined receivers.
This notion of receivers finding sources is the fundamental problem
of source discovery which MSDP was intended to solve. However, if one
would like to retain the Anycast-RP benefits from [2] with less
protocol machinery, removing MSDP from the solution space is an
option.
This draft extends the Register mechanism in PIM so Anycast-RP
functionality can be retained without using MSDP.
2. Requirements
o Each router acting as an RP MUST be configured with a loopback
interface using the same (shared) IP address. This address is used to
tell other routers in the PIM domain, what IP address to use for the
RP address.
o The RP address or a prefix that covers the RP address is injected
into the unicast routing system inside of the domain.
o Each RP configures all other RPs used in the Anycast-RP set. This
must be consistently configured in all RPs in the set.
3. Mechanism
The following diagram illustrates a domain using 3 RPs where
receivers are joining to the closest RP according to where unicast
routing metrics take them and 2 sources sending packets to their
respective RPs.
S1-----RP1 RP2 RP3------S3
/ \ |
/ \ |
R1 R1' R2
Assume the above scenario is completely connected where R1, R1', and
R2 are receivers for a group, and S1 and S2 send to that group.
Assume RP1, RP2 and RP3 are all assigned the same IP address which is
used as the Anycast-RP address (let's say the IP address is RPA).
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The following procedure is used when S1 starts sourcing traffic:
o S1 sends a multicast packet.
o The DR directly attached to S1 will form a PIM Register message to
send to RP1. The IP address to use is the Anycast-RP address.
o RP1 will receive the PIM Register message, decapsulate it, send the
packet down the shared-tree to get the packet to receivers R1 and R1'.
o RP1 is configured with RP2 and RP3's IP address. It will forward the
Register message from S1's DR to both of them. RP1 will include
it's own IP address as the source address for the PIM Register
message.
o RP1 sends a Register-Stop back to the DR.
o RP1 MAY join back to the source-tree by triggering a (S1,G) Join
message toward S1. However, RP1 MUST create (S1,G) state.
o RP2 receives the Register message from RP1, decapsulates it, and
also sends the packet down the shared-tree to get the packet to
receiver R2.
o RP2 sends a Register-Stop back to the RP1.
o RP2 MAY join back to the source-tree by triggering a (S1,G) Join
message toward S1. However, RP2 MUST create (S1,G) state.
o RP3 receives the Register message from RP1, decapsulates it, but
since there is no receivers joined for the group, it can discard
the packet.
o RP3 sends a Register-Stop back to the RP1.
o RP3 creates (S1,G) state so when a receiver joins after S1 starts
sending, RP3 can join quickly to the source-tree for S1.
The procedure for S3 sending follows the same as above but it is RP3
which forwards the Register originated by S3's DR to RP1 and RP2.
Therefore, this example shows how sources anywhere in the domain,
associated with different RPs, can reach all receivers, also
associated with different RPs, in the same domain.
4. Observations and Guidelines about this Proposal
o An RP will forward a Register only if the Register is received
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from an IP address not in the Anycast-RP list (i.e. the Register
came from a DR and not another RP).
o Each DR that PIM registers for a source will send the message to it's
closest RP address. Therefore there are no changes to the DR logic.
o Packets flow to all receivers no matter what RP they have joined to.
o The source gets Registered to a single RP by the DR, it's the
responsibility of the RP, the DR selects, to get the packet to all
other RPs in the Anycast-RP set.
o Logic is changed only in the RPs. The logic change is for forwarding
Register messages. Register-Stop processing is unchanged. However, an
implementation MAY suppress sending Register-Stop messages in response
to a Register received from an RP.
o The rate-limiting of Register and Register-Stop messages are done
end-to-end. That is from DR -> RP1 -> {RP2 and RP3}. There is no need for
specific rate-limiting logic between the RPs.
o When topology changes occur, the existing source-tree adjusts as it
does today according to [1]. The existing shared-trees, as well,
adjust as it does today according to [1].
o Physical RP changes are as fast as unicast route convergence.
Retaining the benefit of [2].
o An RP that doesn't support this draft can be mixed with RPs that do
support this draft. However, the non-supporter RPs should not have
sources registering to it but may have receivers joined to it.
o If Null Registers are sent (Registers with an IP header and no IP
payload), they MUST be replicated to all of the RPs in the Anycast-RP
set so that source state remains alive for active sources.
o The number of RPs in the Anycast-RP set should remain small so the
amount of non-native replication is kept to a minimum.
5. Possible Configuration Language
A possible set of commands to be used could be:
ip pim anycast-rp <anycast-rp-addr> <rp-addr>
Where:
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<anycast-rp-addr> describes the Anycast-RP set for the RP which
is assigned to the group range. This IP address is the address
that first-hop and last-hop PIM routers use to register and join
to.
<rp-addr> describes the IP address where Register messages are
forwarded to. This IP address is any address assigned to the RP
router not including the <anycast-rp-addr>.
Example:
From the illustration above, the configuration commands would be:
ip pim anycast-rp RPA RP1
ip pim anycast-rp RPA RP2
ip pim anycast-rp RPA RP3
Comment:
It may be useful to include the local router's IP address in the
command set so the above lines can be cut-and-pasted or scripted
into all the RPs in the Anycast-RP set.
But the implementation would have to be aware of it's own address
and not inadvertently send a Register to itself.
6. Interaction with MSDP running in an Anycast-PIM Router
The objective of this Anycast-PIM proposal is to remove the
dependence on using MSDP. This can be achieved by removing MSDP
peering between the Anycast RPs. However, to advertise internal
sources to routers outside of a PIM routing domain and to learn
external sources from other routing domains, MSDP may still be
required.
In this capacity, when there are internal sources that need to be
advertised externally, an Anycast-RP which receives a Register
message, either from a DR or an Anycast-RP, should process it as
described in this specification as well as how to process a Register
message as described in [2]. That means an SA for the same internal
source could be originated by multiple Anycast-RPs doing the MSDP
peering. If this is not desirable, configuration of one or more
(rather than all) Anycast-RP MSDP routers would be the only ones to
originate SAs for internal sources. And in some situations, there is
a good possibility not all Anycast-RPs in the set will have MSDP
peering sessions so this issue can be mitigated to a certain extent.
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From an Anycast-RP perspective, a source should be considered
internal to a domain, when it is discovered by an Anycast-RP through
a received Register message. Regardless, if the Register message was
sent by a DR, another Anycast-RP member, or the router itself.
For learning sources external to a domain, the MSDP SA messages could
arrive to multiple MSDP-peering Anycast-RPs. If data does not
accompany the SA, follow the rules documented in [2]. That is, have
PIM send a (S,G) join towards the source. If data accompanies the SA,
duplicate packets could be delivered to receivers (since each RP
would deliver the packet down each of their respective shared-trees).
To avoid this, only the highest IP address RP in the Anycast-RP set
will deliver the SA-data-encapsulated packets down it's shared tree.
To reach other share-trees rooted at the other Anycast-RPs, the
highest IP addressed router will create a Register message and
forward packet to the set of RPs in the Anycast-RP set as it would as
a functioning DR receiving packets from a directly connected source.
If not all routers in the Anycast-RP set are MSDP peering routers,
then configuration is required to identify which members are MSDP
peers, and the highest IP address of this subset will be used to
create and originate the Register message.
Within a routing domain, it is recommended that an Anycast-RP set
defined in this specification should not be mixed with MSDP peering
amoung the members. In some cases, the source discovery will work but
it may not be obvious to the implementations what sources are local
to the domain and which are not. This may affect external MSDP
advertisement of internal sources.
Having said that, this draft makes no attempt to connect MSDP peering
domains together by using Anycast-PIM inside a transit domain.
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7. Acknowledgments
The authors would like to thank Yiqun Cai and Dino Farinacci for
prototyping this draft in the cisco IOS implementation and Procket
implementation, respectively.
The authors would like to thank John Zwiebel for doing
interoperability testing of the two prototype implementations.
And finally, the authors would like to thank Greg Shepard and Lenny
Giuliano for comments on the draft.
8. Author Information
Dino Farinacci
Procket Networks
dino@procket.com
Yiqun Cai
cisco Systems
ycai@cisco.com
9. References
[1] Estrin, et al., "Protocol Independent Multicast-Sparse Mode (PIM-
SM): Protocol Specification", RFC 2362, June 1998.
[2] Kim, Meyer, Kilmer, Farinacci, "Anycast RP mechanism using PIM
and MSDP", Internet Draft draft-ietf-mboned-anycast-rp-08.txt,
May 2001.
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