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Versions: (draft-joshi-pim-group-rp-mapping) 00 01 02 03 04 05 06 07 08 09 10 RFC 6226

PIM Working Group                                               B. Joshi
Internet-Draft                                 Infosys Technologies Ltd.
Updates: 4601 (if approved)                                   A. Kessler
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: August 1, 2011                                      D. McWalter
                                                        January 28, 2011


                        PIM Group-to-RP Mapping
                 draft-ietf-pim-group-rp-mapping-10.txt

Abstract

   Each PIM-SM router in a Protocol Independent Multicast (PIM) Domain
   which supports Any Source Multicast (ASM) maintains Group-to-RP
   mappings which 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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 1, 2011.

Copyright Notice

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




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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Existing algorithm . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Common use cases . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Proposed algorithm . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Interpretation of MIB Objects  . . . . . . . . . . . . . . . . 10
   8.  Clarification for MIB Objects  . . . . . . . . . . . . . . . . 11
   9.  Use of dynamic Group-to-RP mapping protocols . . . . . . . . . 12
   10. Consideration for Bidirectional-PIM and BSR hash . . . . . . . 13
   11. Filtering Group-to-RP mappings at domain boundaries  . . . . . 14
   12. Security Consideration . . . . . . . . . . . . . . . . . . . . 15
   13. IANA Consideration . . . . . . . . . . . . . . . . . . . . . . 16
   14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   15. Normative References . . . . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19






















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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 even true in the case of Anycast RP for
   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.

   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.

   PIM-STD-MIB [RFC5060] includes a number of objects to allow an
   administrator to set the precedence for Group-to-RP mappings which
   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 protocol inconsistency with how different
   vendors select the Group-to-RP mappings to create multicast
   forwarding state.

   Embedded-RP as defined in section-7.1 of Embedded-RP address in IPv6
   Multicast address [RFC3956], mentions that 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.










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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 following terms:

   o  PIM Mode

   PIM Mode is the mode of operation a particular multicast group is
   used for.  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.

   o  Dynamic mappings or Dynamically learned mappings

   The terms Dynamic mappings or Dynamically learned mappings refer to
   group-to-RP mappings that have been learned by 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 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.











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3.  Existing algorithm

   The existing algorithm defined in PIM-SM (Section 4.7.1 in [RFC4601])
   does not consider 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 PIM-BIDIR
      mode is treated with same priority as an entry learned for PIM-SM.

   The algorithm defined in this document updates algorithm defined in
   PIM-SM ( Section 4.7.1 in [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 following assumptions in defining this algorithm:

   o  A Group-to-RP mapping can be learned from various mechanisms.  We
      assume that following list is in the decreasing preferences of
      these mechanism:

      *  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 either by static
      configuration or by dynamic Group-to-RP mappings.

   o  Dynamic mappings will override a static RP config 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 by section 3.3 of
      [RFC5059].

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



















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5.  Common 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 like to learn
   Group-to-RP mappings dynamically using either BSR or Auto-RP
   mechanism.  In this case to select Group-to-RP mappings 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 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
   behaviour of Group-to-RP mapping selection for entries learned using
   BSR and Auto-RP mechanism.  This will help in avoiding any unforeseen
   interoperability issues between different vendor's 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.


















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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 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.

   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, we continue with the
           algorithm 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
        which they 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, we continue with the
           algorithm with this smaller set of Group-to-RP Mappings






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   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 is 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, we continue with the
           algorithm 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 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 value we
           continue with the algorithm with those Group-to-RP mappings.

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

   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 look at section 10 for consideration of hash
        for BIDIR-PIM and BSR.

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

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

   10.  From the remaining set of Group-to-RP Mappings we will select
        the RP with the highest IP address (numerically greater).  This
        will serve as a final tiebreaker.












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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 as well as
   the static configuration.  For static configurations, the
   'pimGroupMappingPrecedence' object uses the value of the
   'pimStaticRPPrecedence' object from the pimStaticRPTable table.

   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 that found in PIM-SM [RFC4601] and PIM-
   STD-MIB [RFC5060], the values of these objects will be ignored on
   implementations that support the new algorithm.
































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8.  Clarification for MIB Objects

   An implementation of this specification can continue to be managed
   using the PIM-STD-MIB [RFC5060].  When a Group-to-RP mapping entry is
   created in the pimGroupMappingTable with RP address type in 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 represent group ranges for SSM or Dense mode.

   Also, all the entries which are already included in the SSM Range
   table in the IP Mcast 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.




































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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
   under 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.

































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








































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11.  Filtering Group-to-RP mappings at domain boundaries

   An implementation of PIM SHOULD support configuration to filter
   specific dynamic mechanism for a valid group prefix range.  For
   example, it should be possible to allow an administratively scoped
   address range, such as 239/8 range, for Auto-RP protocol but 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
   Auto-RP protocol.










































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12.  Security Consideration

   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].




































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13.  IANA Consideration

   This draft does not create any namespace for IANA to manage.
















































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14.  Acknowledgements

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














































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15.  Normative References

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

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

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

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

   [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.

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
























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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


























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