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rfc:rfc6226

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.

Joshi, et al. Standards Track [Page 1] 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
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 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.

Joshi, et al. Standards Track [Page 2] RFC 6226 PIM Group-to-RP Mapping May 2011

 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.

Joshi, et al. Standards Track [Page 3] RFC 6226 PIM Group-to-RP Mapping May 2011

 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.

Joshi, et al. Standards Track [Page 4] RFC 6226 PIM Group-to-RP Mapping May 2011

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

Joshi, et al. Standards Track [Page 5] RFC 6226 PIM Group-to-RP Mapping May 2011

    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.

Joshi, et al. Standards Track [Page 6] RFC 6226 PIM Group-to-RP Mapping May 2011

 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.

Joshi, et al. Standards Track [Page 7] RFC 6226 PIM Group-to-RP Mapping May 2011

 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.

Joshi, et al. Standards Track [Page 8] RFC 6226 PIM Group-to-RP Mapping May 2011

 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.

Joshi, et al. Standards Track [Page 9] RFC 6226 PIM Group-to-RP Mapping May 2011

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.

Joshi, et al. Standards Track [Page 10] 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: bharat_joshi@infosys.com
 URI:   http://www.infosys.com/
 Andy Kessler
 Cisco Systems, Inc.
 425 E. Tasman Drive
 San Jose, CA  95134
 USA
 EMail: kessler@cisco.com
 URI:   http://www.cisco.com/
 David McWalter
 EMail: david@mcwalter.eu

Joshi, et al. Standards Track [Page 11]

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