Network Working Group D. Farinacci
Request for Comments: 4610 Y. Cai
Category: Standards Track Cisco Systems
August 2006
Anycast-RP Using Protocol Independent Multicast (PIM)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This specification allows Anycast-RP (Rendezvous Point) to be used
inside a domain that runs Protocol Independent Multicast (PIM) only.
Other multicast protocols (such as Multicast Source Discovery
Protocol (MSDP), which has been used traditionally to solve this
problem) are not required to support Anycast-RP.
1. Introduction
Anycast-RP as described in [I1] is a mechanism that 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 need to get
to all RPs to find joined receivers.
This notion of receivers finding sources is the fundamental problem
of source discovery that MSDP was intended to solve. However, if one
would like to retain the Anycast-RP benefits from [I1] with less
protocol machinery, removing MSDP from the solution space is an
option.
This memo extends the Register mechanism in PIM so Anycast-RP
functionality can be retained without using MSDP.
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1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [N2].
2. Overview
o A unicast IP address is chosen to use as the RP address. This
address is statically configured, or distributed using a dynamic
protocol, to all PIM routers throughout the domain.
o A set of routers in the domain is chosen to act as RPs for this RP
address. These routers are called the Anycast-RP set.
o Each router in the Anycast-RP set is configured with a loopback
interface using the RP address.
o Each router in the Anycast-RP set also needs a separate IP address,
to be used for communication between the RPs.
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 router in the Anycast-RP set is configured with the addresses
of all other routers 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.
The rules described in this section do not override the rules in
[N1]. They are intended to blend with the rules in [N1]. If there
is any question on the interpretation, precedent is given to [N1].
S1-----RP1 RP2 RP3------S3
/ \ |
/ \ |
R1 R1' R2
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Assume the above scenario is completely connected where R1, R1', and
R2 are receivers for a group, and S1 and S3 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).
Note, the address used for the RP address in the domain (the
Anycast-RP address) needs to be different than the addresses used by
the Anycast-RP routers to communicate with each other.
The following procedure is used when S1 starts sourcing traffic:
o S1 sends a multicast packet.
o The designated router (DR) directly attached to S1 will form a PIM
Register message to send to the Anycast-RP address (RPA). The
unicast routing system will deliver the PIM Register message to the
nearest RP, in this case RP1.
o RP1 will receive the PIM Register message, decapsulate it, and 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. Since the
Register message did not come from one of the RPs in the anycast-RP
set, RP1 assumes the packet came from a DR. If the Register is not
addressed to the Anycast-RP address, an error has occurred and it
should be rate-limited logged.
o RP1 will then send a copy of the Register message from S1's DR to
both RP2 and RP3. RP1 will use its own IP address as the source
address for the PIM Register message.
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 RP1 sends a Register-Stop back to the DR. If, for some reason, the
Register messages to RP2 and RP3 are lost, then when the Register
suppression timer expires in the DR, it will resend Registers to
allow another chance for all RPs in the Anycast-RP set to obtain
the (S,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 RP1. RP2 MAY wait to send the
Register-Stop if it decides to join the source-tree. RP2 should
wait until it has received data from the source on the source-tree
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before sending the Register-Stop. If RP2 decides to wait, the
Register-Stop will be sent when the next Register is received. If
RP2 decides not to wait, the Register-Stop is sent now.
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 are no receivers joined for the group, it can discard
the packet.
o RP3 sends a Register-Stop back to 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.
o RP1 processes the Register-Stop from each of RP2 and RP3. There is
no specific action taken when processing Register-Stop messages.
The procedure for S3 sending follows the same as above but it is RP3
that sends a copy of 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 send a copy of a Register only if the Register is
received from an IP address not in the Anycast-RP list (i.e., the
Register came from a DR and not another RP). An implementation
MUST safeguard against inconsistently configured Anycast-RP sets in
each RP by copying the Time to Live (TTL) from a Register message
to the Register messages it copies and sends to other RPs.
o Each DR that PIM registers for a source will send the message to
the Anycast-RP address (which results in the packet getting to the
closest physical RP). 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 that receives the PIM Register messages
from the DR (the closest RP to the DR based on routing metrics) to
get the packet to all other RPs in the Anycast-RP set.
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o Logic is changed only in the RPs. The logic change is for sending
copies of 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 [N1]. The existing shared-trees, as well,
adjust as they do today according to [N1].
o Physical RP changes are as fast as unicast route convergence,
retaining the benefit of [I1].
o An RP that doesn't support this specification can be mixed with RPs
that do support this specification. However, the non-supporter RP
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.
o Since the RP, who receives a Register from the DR, will send copies
of the Register to the other RPs at the same time it sends a
Register-Stop to the DR, there could be packet loss and lost state
in the other RPs until the time the DR sends Register messages
again.
5. 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.
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5.1. Anycast-PIM Stub Domain Functionality
In this capacity, when there are internal sources that need to be
advertised externally, an Anycast-RP that 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 [N1]. That means a Source-Active (SA) for
the same internal source could be originated by multiple Anycast-RPs
doing the MSDP peering. There is nothing inherently wrong with this
other than that the source is being advertised into the MSDP
infrastructure from multiple places from the source domain. However,
if this is not desirable, configuration of one or more (rather than
all) Anycast-RP MSDP routers would allow only those routers to
originate SAs for the internal source. 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.
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 of whether 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 at multiple MSDP-peering Anycast-RPs. The rules for
processing an SA, according to [I1], should be followed. That is, if
G is joined in the domain, an (S,G) join is sent towards the source.
And if data accompanies the SA, each Anycast-PIM RP doing MSDP
peering will forward the data down each of its respective shared-
trees.
The above assumes each Anycast-RP has external MSDP peering
connections. If this is not the case, the Anycast-PIM routers with
the MSDP peering connections would follow the same procedure as if a
Data-Register or Null-Register was received from either a DR or
another Anycast-RP. That is, they would send Registers to the other
members of the Anycast-RP set.
If there is a mix of Anycast-RPs that do and do not have external
MSDP peering connections, then the ones that do must be configured
with the set that do not. So Register messages are sent only to the
members of the Anycast-RP set that do not have external MSDP peering
connections.
The amount of Register traffic generated by this MSDP-peering RP
would be equal to the number of active sources external to the
domain. The Source-Active state would have to be conveyed to all
other RPs in the Anycast-RP set since the MSDP-peering RP would not
know about the group membership associated with the other RPs. To
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RFC 4610 Anycast-RP using PIM August 2006
avoid this periodic control traffic, it is recommended that all
Anycast-RPs be configured with external MSDP peering sessions so no
RP in the Anycast-RP set will have to originate Register messages on
behalf of external sources.
5.2. Anycast-PIM Transit Domain Functionality
Within a routing domain, it is recommended that an Anycast-RP set
defined in this specification should not be mixed with MSDP peering
among the members. In some cases, the source discovery will work but
it may not be obvious to the implementations which sources are local
to the domain and which are not. This may affect external MSDP
advertisement of internal sources.
Having said that, this document makes no attempt to connect MSDP
peering domains together by using Anycast-PIM inside a transit
domain.
6. Security Consideration
This section describes the security consideration for Register and
Register-Stop messages between Anycast-RPs. For PIM messages between
DR and RP, please see [N1].
6.1. Attack Based On Forged Messages
An attacker may forge a Register message using one of the addresses
in the Anycast-RP list in order to achieve one or more of the
following effects:
1. Overwhelm the target RP in a denial-of-service (DoS) attack
2. Inject unauthorized data to receivers served by the RP
3. Inject unauthorized data and create bogus SA entries in other
PIM domains if the target RP has external MSDP peerings
An attacker may also forge a Register-Stop message using one of the
addresses in the Anycast-RP list. However, besides denial-of-
service, the effect of such an attack is limited because an RP
usually ignores Register-Stop messages.
6.2. Protect Register and Register-Stop Messages
The DoS attack using forged Register or Register-Stop messages cannot
be prevented. But the RP can still be protected. For example, the
RP can rate-limit incoming messages. It can also choose to refuse to
process any Register-Stop messages. The actual protection mechanism
is implementation specific.
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The distribution of unauthorized data and bogus Register messages can
be prevented using the method described in section 6.3.2 of [N1].
When RP1 sends a copy of a register to RP2, RP1 acts as [N1]
describes the DR and RP2 acts as [N1] describes the RP.
As described in [N1], an RP can be configured using a unique SA and
Security Parameter Index (SPI) for traffic (Registers or Register-
Stops) to each member of Anycast-RPs in the list, but this results in
a key management problem; therefore, it may be preferable in PIM
domains where all Rendezvous Points are under a single administrative
control to use the same authentication algorithm parameters
(including the key) for all Registered packets in a domain.
7. Acknowledgements
The authors prototyped this document in the cisco IOS and Procket
implementations, respectively.
The authors would like to thank John Zwiebel for doing
interoperability testing of the two prototype implementations.
The authors would like to thank Thomas Morin from France Telecom for
having an extensive discussion on Multicast the Registers to an SSM-
based full mesh among the Anycast-RP set. This idea may come in a
subsequent document.
And finally, the authors would like to thank the following for their
comments on earlier drafts:
Greg Shepherd (Procket Networks (now Cisco Systems))
Lenny Giuliano (Juniper Networks)
Prashant Jhingran (Huawei Technologies)
Pekka Savola (CSC/FUNET)
Bill Fenner (AT&T)
James Lingard (Data Connection)
Amit Shukla (Juniper Networks)
Tom Pusateri (Juniper Networks)
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8. References
8.1. Normative References
[N1] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[N2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[I1] Kim, D., Meyer, D., Kilmer, H., and D. Farinacci, "Anycast
Rendevous Point (RP) mechanism using Protocol Independent
Multicast (PIM) and Multicast Source Discovery Protocol (MSDP)",
RFC 3446, January 2003.
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Appendix A: Possible Configuration Language
A possible set of commands to be used could be:
ip pim anycast-rp <anycast-rp-addr> <rp-addr>
Where:
<anycast-rp-addr> describes the Anycast-RP set for the RP that 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 copies
are sent 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 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 its own address
and not inadvertently send a Register to itself.
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Authors' Addresses
Dino Farinacci
Cisco Systems
EMail: dino@cisco.com
Yiqun Cai
Cisco Systems
EMail: ycai@cisco.com
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RFC 4610 Anycast-RP using PIM August 2006
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