Internet Engineering Task Force (IETF) B. Joshi

Internet Engineering Task Force (IETF) B. Joshi
Request for Comments: 6226 Infosys Technologies Ltd.
Updates: 4601 A. Kessler
Category: Standards Track Cisco Systems, Inc.
ISSN: 2070-1721 D. McWalter
May 2011

PIM Group-to-Rendezvous-Point Mapping

Abstract

Each Protocol Independent Multicast - Sparse Mode (PIM-SM) router in
a PIM domain that supports Any Source Multicast (ASM) maintains
Group-to-RP mappings that are used to identify a Rendezvous Point
(RP) for a specific multicast group. PIM-SM has defined an algorithm
to choose a RP from the Group-to-RP mappings learned using various
mechanisms. This algorithm does not consider the PIM mode and the
mechanism through which a Group-to-RP mapping was learned.

This document defines a standard algorithm to deterministically
choose between several Group-to-RP mappings for a specific group.
This document first explains the requirements to extend the Group-to-
RP mapping algorithm and then proposes the new algorithm.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6226.

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RFC 6226 PIM Group-to-RP Mapping May 2011

Copyright Notice

Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction ....................................................2
2. Terminology .....................................................3
3. Existing Algorithm ..............................................4
4. Assumptions .....................................................5
5. Common Use Cases ................................................5
6. Proposed Algorithm ..............................................6
7. Interpretation of MIB Objects ...................................8
8. Clarification for MIB Objects ...................................8
9. Use of Dynamic Group-to-RP Mapping Protocols ....................9
10. Considerations for Bidirectional-PIM and BSR Hash ..............9
11. Filtering Group-to-RP Mappings at Domain Boundaries ............9
12. Security Considerations .......................................10
13. Acknowledgements ..............................................10
14. Normative References ..........................................10

1. Introduction

Multiple mechanisms exist today to create and distribute Group-to-RP
mappings. Each PIM-SM router may learn Group-to-RP mappings through
various mechanisms, as described in Section 4.

It is critical that each router select the same 'RP' for a specific
multicast group address; otherwise, full multicast connectivity will
not be established. This is true even when using an Anycast RP to
provide redundancy. This RP address may correspond to a different
physical router, but it is one logical RP address and must be
consistent across the PIM domain. This is usually achieved by using
the same algorithm to select the RP in all the PIM routers in a
domain.

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PIM-SM [RFC4601] has defined an algorithm to select a 'RP' for a
given multicast group address, but it is not flexible enough for an
administrator to apply various policies. Please refer to Section 3
for more details.

The PIM-STD-MIB [RFC5060] includes a number of objects to allow an
administrator to set the precedence for Group-to-RP mappings that are
learned statically or dynamically and stored in the
'pimGroupMappingTable'. The Management Information Base (MIB) module
also defines an algorithm that can be applied to the data contained
in the 'pimGroupMappingTable' to determine Group-to-RP mappings.
However, this algorithm is not completely deterministic, because it
includes an implementation-specific 'precedence' value.

Network management stations will be able to deduce which RPs will be
selected by applying the algorithm from this document to the list of
Group-to-RP mappings from the 'pimGroupMappingTable'. The algorithm
provides MIB visibility into how routers will apply Group-to-RP
mappings and also fixes the inconsistency introduced by the way that
different vendors implement the selection of the Group-to-RP mappings
to create multicast forwarding state.

Embedded-RP, as defined in Section 7.1 of "Embedding the Rendezvous
Point (RP) Address in an IPv6 Multicast Address" [RFC3956], specifies
the following: "To avoid loops and inconsistencies, for addresses in
the range ff70::/12, the Embedded-RP mapping MUST be considered the
longest possible match and higher priority than any other mechanism".

2. 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 RFC 2119 [RFC2119].

This document also uses the following terms:

o PIM Mode

PIM Mode is the mode of operation for which a particular multicast
group is used. Wherever this term is used in this document, it
refers to either Sparse Mode or Bidirectional (BIDIR) Mode.

o Dynamic Group-to-RP Mapping Mechanisms

The term "dynamic Group-to-RP mapping mechanisms" in this document
refers to Bootstrap Router (BSR) [RFC5059] and Auto-RP.

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o Dynamic Mappings and Dynamically Learned Mappings

The terms "dynamic mappings" and "dynamically learned mappings"
refer to Group-to-RP mappings that have been learned by either BSR
or Auto-RP. Group-to-RP mappings that have been learned by
Embedded-RP are referred to as Embedded Group-to-RP mappings.

o Filtering

Filtering is the selective discarding of dynamic Group-to-RP
mapping information, based on the group address, the type of
Group-to-RP mapping message, and the interface on which the
mapping message was received.

o Multicast Domain and Boundaries

The term "multicast domain" used in this document refers to a
network topology that has a consistent set of Group-to-RP
mappings. The interface between two or more multicast domains is
a multicast domain boundary. The multicast boundaries are usually
enforced by filtering the dynamic mapping messages and/or
configuring different static RP mappings.

3. Existing Algorithm

The existing algorithm defined in PIM-SM (Section 4.7.1 of [RFC4601])
does not consider the following constraints:

o It does not consider the origin of a Group-to-RP mapping and
therefore will treat all of them equally.

o It does not provide the flexibility to give higher priority to a
specific PIM mode. For example, an entry learned for the PIM-
BIDIR Mode is treated with the same priority as an entry learned
for PIM-SM.

The algorithm defined in this document updates the algorithm defined
in PIM-SM (Section 4.7.1 of [RFC4601]). The new algorithm is
backward compatible and will produce the same result only if the
Group-to-RP mappings are learned from a single mapping source. The
full benefits of the new algorithm will not be realized until it is
widely deployed.

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4. Assumptions

We have made the following assumptions in defining this algorithm:

o A Group-to-RP mapping can be learned from various mechanisms. We
assume that the following list is ordered by decreasing preference
for these mechanisms:

* Embedded Group-to-RP mappings

* Dynamically learned mappings

* Static configuration

* Other mapping method

o Embedded Group-to-RP mappings are special and always have the
highest priority. They cannot be overridden by static
configuration or by dynamic Group-to-RP mappings.

o Dynamic mappings will override a static RP configuration if they
have overlapping ranges. However, it is possible to override
dynamic Group-to-RP mappings with static configurations, either by
filtering, or by configuring longer static group addresses that
override dynamic mappings when longest prefix matching is applied.

o A Group-to-RP mapping learned for PIM-BIDIR Mode is preferred to
an entry learned for PIM-SM Mode as stipulated in Section 3.3 of
[RFC5059].

o Dynamic Group-to-RP mapping mechanisms are filtered at domain
boundaries or for policy enforcement inside a domain.

5. Common Use Cases

A network operator deploying IP Multicast will require a
deterministic way to select the precedence for Group-to-RP mappings
in the following use cases:

o Default static Group-to-RP mappings with dynamically learned
entries

Many network operators will have a dedicated infrastructure for
the standard multicast group range (224/4) and so might be using
statically configured Group-to-RP mappings for this range. In
this case, to support some specific applications, they might want
to learn Group-to-RP mappings dynamically using either the BSR or
Auto-RP mechanism. In this case, to select Group-to-RP mappings

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for these specific applications, a longer prefix match should be
given preference over statically configured Group-to-RP mappings.
For example, 239.100.0.0/16, an administratively scoped multicast
address range, could be learned for a corporate communications
application. Network operators may change the Group-to-RP
mappings for these applications more often, and the mappings would
need to be learned dynamically. This is not an issue for IPv6
Multicast address ranges.

o Migration situations

Network operators occasionally go through a migration due to an
acquisition or a change in their network design. In order to
facilitate this migration, there is a need to have a deterministic
behavior of Group-to-RP mapping selection for entries learned
using the BSR and Auto-RP mechanisms. This will help in avoiding
any unforeseen interoperability issues between different vendors'
network elements.

o Use by management systems

A network management station can determine the RP for a specific
group in a specific router by running this algorithm on the Group-
to-RP mapping table fetched using MIB objects.

6. Proposed Algorithm

The following algorithm deterministically chooses between several
Group-to-RP mappings for a specific group. It also addresses the
above-mentioned shortcomings in the existing mechanism.

1. If the multicast group address being looked up contains an
embedded RP, the RP address extracted from the group address is
selected as the Group-to-RP mapping.

2. If the multicast group address being looked up is in the Source
Specific Multicast (SSM) range or is configured for Dense Mode,
no Group-to-RP mapping is selected, and this algorithm
terminates. The fact that no Group-to-RP mapping has been
selected can be represented in the PIM-STD-MIB module [RFC5060]
by setting the address type of the RP to 'unknown', as described
in Section 8.

3. From the set of all Group-to-RP mapping entries, the subset
whose group prefix contains the multicast group that is being
looked up is selected.

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4. If there are no entries available, then the Group-to-RP mapping
is undefined, and this algorithm terminates.

5. A longest prefix match is performed on the subset of Group-to-RP
mappings.

* If there is only one entry available, then that entry is
selected as the Group-to-RP mapping.

* If there are multiple entries available, the algorithm
continues with this smaller set of Group-to-RP mappings.

6. From the remaining set of Group-to-RP mappings, we select the
subset of entries based on the preference for the PIM modes to
which the multicast group addresses are assigned. A Group-to-RP
mapping entry with PIM Mode 'BIDIR' will be preferred to an
entry with PIM Mode 'PIM-SM'.

* If there is only one entry available, then that entry is
selected as the Group-to-RP mapping.

* If there are multiple entries available, the algorithm
continues with this smaller set of Group-to-RP mappings.

7. From the remaining set of Group-to-RP mappings, we select the
subset of the entries based on the origin. Group-to-RP mappings
learned dynamically are preferred over static mappings. If the
remaining dynamic Group-to-RP mappings are from BSR and Auto-RP,
then the mappings from BSR are preferred.

* If there is only one entry available, then that entry is
selected as the Group-to-RP mapping.

* If there are multiple entries available, the algorithm
continues with this smaller set of Group-to-RP mappings.

8. If the remaining Group-to-RP mappings were learned through BSR,
then the RP will be selected by comparing the RP Priority values
in the Candidate-RP-Advertisement messages. The RP mapping with
the lowest value indicates the highest priority [RFC5059].

* If more than one RP has the same highest priority (i.e., the
same lowest value), the algorithm continues with those Group-
to-RP mappings.

* If the remaining Group-to-RP mappings were NOT learned from
BSR, the algorithm continues with the next step.

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9. If the remaining Group-to-RP mappings were learned through BSR
and the PIM Mode of the group is 'PIM-SM', then the hash
function as defined in Section 4.7.2 of [RFC4601] will be used
to choose the RP. The RP with the highest resulting hash value
will be selected. Please see Section 10 for consideration of
hash for BIDIR-PIM and BSR.

* If more than one RP has the same highest hash value, the
algorithm continues with those Group-to-RP mappings.

* If the remaining Group-to-RP mappings were NOT learned from
BSR, the algorithm continues with the next step.

10. From the remaining set of Group-to-RP mappings, the RP with the
highest IP address (numerically greater) will be selected. This
will serve as a final tiebreaker.

7. Interpretation of MIB Objects

As described in [RFC5060], the Group-to-RP mapping information is
summarized in the pimGroupMappingTable. The precedence value is
stored in the 'pimGroupMappingPrecedence' object, which covers both
the dynamically learned Group-to-RP mapping information and the
static configuration. For static configurations, the
'pimGroupMappingPrecedence' object uses the value of the
'pimStaticRPPrecedence' object from the pimStaticRPTable.

The algorithm defined in this document does not use the concept of
precedence, and therefore the values configured in the
'pimGroupMappingPrecedence' and 'pimStaticRPPrecedence' objects in
the PIM-STD-MIB module [RFC5060] are not applicable to the new
algorithm. The objects still retain their meaning for 'legacy'
implementations, but since the algorithm defined in this document is
to be used in preference to those found in PIM-SM [RFC4601] and the
PIM-STD-MIB [RFC5060], the values of these objects will be ignored on
implementations that support the new algorithm.

8. Clarification for MIB Objects

An implementation of this specification can continue to be managed
using the PIM-STD-MIB [RFC5060]. Group-to-RP mapping entries are
created in the pimGroupMappingTable for group ranges that are SSM or
Dense mode. In these cases, the pimGroupMappingRPAddressType object
is set to unknown(0), and the PIM Mode in the pimGroupMappingPimMode
object is set to either ssm(2) or dm(5) to reflect the type of the
group range.

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Also, all the entries that are already included in the SSM Range
table in the IP Multicast MIB [RFC5132] are copied to the
pimGroupMappingTable. Such entries have their type in the
pimGroupMappingOrigin object set to configSsm(3) and the RP address
type in the pimGroupMappingRPAddressType object set to unknown(0), as
described above.

9. Use of Dynamic Group-to-RP Mapping Protocols

It is not usually necessary to run several dynamic Group-to-RP
mapping mechanisms in one administrative domain. Specifically,
interoperation of BSR and Auto-RP is OPTIONAL.

However, if a router does receive two overlapping sets of Group-to-RP
mappings, for example from Auto-RP and BSR, then some algorithm is
needed to deterministically resolve the situation. The algorithm in
this document MUST be used on all routers in the domain. This can be
important at domain border routers, and is likely to avoid conflicts
caused by misconfiguration (when routers receive overlapping sets of
Group-to-RP mappings) and when configuration is changing.

An implementation of PIM that supports only one mechanism for
learning Group-to-RP mappings MUST also use this algorithm. The
algorithm has been chosen so that existing standard implementations
are already compliant.

10. Considerations for Bidirectional-PIM and BSR Hash

BIDIR-PIM [RFC5015] is designed to avoid any data-driven events.
This is especially true in the case of a source-only branch. The RP
mapping is determined based on a group mask when the mapping is
received through a dynamic mapping protocol or statically configured.

Therefore, based on the algorithm defined in this document, the hash
in BSR is ignored for PIM-BIDIR RP mappings. It is RECOMMENDED that
network operators configure only one PIM-BIDIR RP for each RP
Priority.

11. Filtering Group-to-RP Mappings at Domain Boundaries

An implementation of PIM SHOULD support configuration to filter
specific dynamic mechanisms for a valid group prefix range. For
example, it should be possible to allow an administratively scoped
address range, such as 239/8, for the Auto-RP protocol, but to filter
out the BSR advertisement for the same range. Similarly, it should
be possible to filter out all Group-to-RP mappings learned from BSR
or the Auto-RP protocol.

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12. Security Considerations

This document enhances an existing algorithm to deterministically
choose between several Group-to-RP mappings for a specific group.
Different routers may select a different Group-to-RP mapping for the
same group if the Group-to-RP mappings learned in these routers are
not consistent. For example, let us assume that BSR is not enabled
in one of the routers, and so it does not learn any Group-to-RP
mappings from BSR. Now the Group-to-RP mappings learned in this
router may not be consistent with other routers in the network; it
may select a different RP or may not select any RP for a given group.
Such situations can be avoided if the mechanisms used to learn Group-
to-RP mappings are secure and consistent across the network. Secure
transport of the mapping protocols can be accomplished by using
authentication with IPsec, as described in Section 6.3 of [RFC4601].

13. Acknowledgements

This document is created based on discussion that occurred during
work on the PIM-STD-MIB [RFC5060]. Many thanks to Stig Venaas, Yiqun
Cai, and Toerless Eckert for providing useful comments.

14. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3956] Savola, P. and B. Haberman, "Embedding the Rendezvous
Point (RP) Address in an IPv6 Multicast Address",
RFC 3956, November 2004.

[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.

[RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-
PIM)", RFC 5015, October 2007.

[RFC5059] Bhaskar, N., Gall, A., Lingard, J., and S. Venaas,
"Bootstrap Router (BSR) Mechanism for Protocol Independent
Multicast (PIM)", RFC 5059, January 2008.

[RFC5060] Sivaramu, R., Lingard, J., McWalter, D., Joshi, B., and A.
Kessler, "Protocol Independent Multicast MIB", RFC 5060,
January 2008.

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RFC 6226 PIM Group-to-RP Mapping May 2011

[RFC5132] McWalter, D., Thaler, D., and A. Kessler, "IP Multicast
MIB", RFC 5132, December 2007.

Authors' Addresses

Bharat Joshi
Infosys Technologies Ltd.
44 Electronics City, Hosur Road
Bangalore 560 100
India

EMail: [email protected]
URI: http://www.infosys.com/

Andy Kessler
Cisco Systems, Inc.
425 E. Tasman Drive
San Jose, CA 95134
USA

EMail: [email protected]
URI: http://www.cisco.com/

David McWalter

EMail: [email protected]

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