INTERNET-DRAFT                                   David Meyer
draft-ietf-mboned-ssm232-08.txt                  Rob Rockell
                                               Greg Shepherd
Category                               Best Current Practice
Expires: September 2004                           March 2004

        Source-Specific Protocol Independent Multicast in 232/8

Status of this Document

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   This document is a product of the MBONED WG.  Comments should be
   addressed to the authors, or the mailing list at

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.

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   IP Multicast group addresses in the 232/8 ( to range are designated as source-specific multicast
   destination addresses and are reserved for use by source-specific
   multicast applications and protocols. This document defines
   operational recommendations to ensure source-specific behavior within
   the 232/8 range.

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                           Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   3
    1.1. BCP, Experimental Protocols and Normative References. . . .   4
   2. Operational practices in 232/8 . . . . . . . . . . . . . . . .   5
    2.1. Preventing local sources from sending to shared tree. . . .   5
    2.2. Preventing remote sources from being learned/joined via MSDP. 6
    2.3. Preventing receivers from joining the shared tree . . . . .   6
    2.4. Preventing RP's as candidates for 232/8 . . . . . . . . . .   7
   3. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .   7
   4. Security Considerations. . . . . . . . . . . . . . . . . . . .   7
   5. IANA Considerations. . . . . . . . . . . . . . . . . . . . . .   8
   6. References . . . . . . . . . . . . . . . . . . . . . . . . . .   9
    6.1. Normative References. . . . . . . . . . . . . . . . . . . .   9
    6.2. Informative References. . . . . . . . . . . . . . . . . . .   9
   7. Author's Addresses . . . . . . . . . . . . . . . . . . . . . .  10
   8. Full Copyright Statement . . . . . . . . . . . . . . . . . . .  10
   9. Intellectual Property. . . . . . . . . . . . . . . . . . . . .  11
   10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Current PIM Sparse Mode (PIM-SM) [PIM-SM] relies on the shared
   Rendezvous Point (RP) tree to learn about active sources for a group
   and to support group-generic (not source specific) data distribution.
   The IP Multicast group address range 232/8 has been designated for
   Source-Specific PIM [RFC3569] applications and protocols [IANA] and
   SHOULD support source-only trees only, precluding the requirement of
   an RP and a shared tree; active sources in the 232/8 range will be
   discovered out of band. PIM Sparse Mode Designated Routers (DR), with
   local membership, are capable of joining the shortest path tree for
   the source directly using Source-Specific PIM (also known as PIM-SSM
   or simply SSM).

   Operational best common practices in the 232/8 group address range
   are necessary to ensure shortest path source-only trees across
   multiple domains in the Internet [RFC3569], and to prevent data from
   sources sending to groups in the 232/8 range from arriving via shared
   trees. This avoids unwanted data arrival, and allows several sources
   to use the same group address without conflict at the receivers.

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   The operational practices SHOULD:

    o Prevent local sources from sending to shared tree

    o Prevent receivers from joining the shared tree

    o Prevent RP's as candidates for 232/8

    o Prevent remote sources from being learned/joined via MSDP

1.1.  BCP, Experimental Protocols and Normative References

   This document describes the best current practice for a widely
   deployed Experimental protocol, MSDP. There is no plan to advance the
   MSDP's status (for example, to Proposed Standard). The reasons for
   this include:

    o MSDP was originally envisioned as a temporary protocol to be
      supplanted by whatever the IDMR working group produced as an
      inter-domain protocol. However, the IDMR WG (or subsequently,
      the BGMP WG) never produced a protocol that could be deployed
      to replace MSDP.

    o One of the primary reasons given for MSDP to be classified as
      Experimental was that the MSDP Working Group came up with
      modifications to the protocol that the WG thought made it
      better but that implementors didn't see any reasons to
      deploy. Without these modifications (e.g., UDP or GRE
      encapsulation), MSDP can have negative consequences to initial
      packets in datagram streams.

    o Scalability: Although we don't know what the hard limits might
      be, readvertising everything you know every 60 seconds clearly
      limits the amount of state you can advertise.

    o MSDP reached near ubiquitous deployment as the de-facto
      standard inter-domain multicast protocol in the IPv4 Internet.

    o No consensus could be reached regarding the reworking of MSDP
      to address the many concerns of various constituencies within
      the IETF. As a result, a decision was taken to document what is
      (ubiquitously) deployed and move that document to Experimental.
      While advancement of MSDP to Proposed Standard was considered,
      for the reasons mentioned above, it was immediately discarded.

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    o The advent of protocols such as source specific multicast and
      bi-directional PIM, as well as embedded RP techniques for
      IPv6, have further reduced consensus that a replacement
      protocol for MSDP for the IPv4 Internet is required.

   The RFC Editor's policy regarding references is that they be split
   into two categories known as "normative" and "informative". Normative
   references specify those documents which must be read to understand
   or implement the technology in an RFC (or whose technology must be
   present for the technology in the new RFC to work) [RFCED]. In order
   to understand this document, one must also understand both the PIM
   and MSDP documents. As a result, references to these documents are

   The IETF has adopted the policy that BCPs must not have normative
   references to Experimental protocols.  However, this document is a
   special case in that the underlying Experimental document (MSDP) is
   not planned to be advanced to Proposed Standard.

   The MBONED Working Group requests approval under the Variance
   Procedure as documented in RFC 2026 [RFC2026].

   Note to RFC-Editor: If IETF/IESG approves this, please change the
   above sentence into: The MBONED Working Group has requested approval
   under the Variance Procedure as documented in RFC 2026 [RFC2026].
   The IESG followed the Variance Procedure, and after an additional 4
   week IETF Last Call evaluated the comments and status and has
   approved this document.

   The key words "MUST"", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC 2119].

2.  Operational practices in 232/8

2.1.  Preventing local sources from sending to shared tree

   Eliminating the use of shared trees for groups in 232/8, while
   maintaining coexistence with PIM-SM, behavior of the RP and/or the DR
   needs to be modified. This can be accomplished by

    - preventing data for 232/8 groups from being sent encapsulated to

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      the RP by the DR

    - preventing the RP from accepting registers for 232/8 groups from
      the DR

    - preventing the RP from forwarding accepted data down (*,G)
      tree for 232/8 groups

2.2.  Preventing remote sources from being learned/joined via MSDP

   PIM-SSM does not require active source announcements via MSDP. All
   source announcements are received out of band, the the last hop
   router being responsible for sending (S,G) joins directly to the
   source. To prevent propagation of SAs in the 232/8 range, an RP

    - never originate an SA for any 232/8 groups

    - never accept or forward an SA for any 232/8 groups.

2.3.  Preventing receivers from joining the shared tree

   Local PIM domain practices need to be enforced to prevent local
   receivers from joining the shared tree for 232/8 groups. This can be
   accomplished by

    - preventing DR from sending (*,G) joins for 232/8 groups

    - preventing RP from accepting (*,G) join for 232/8 groups

   However, within a local PIM domain, any last-hop router NOT
   preventing (*,G) joins may trigger unwanted (*,G) state toward the RP
   which intersects an existing (S,G) tree, allowing the receiver on the
   shared tree to receive the data, breaking the source-specific
   [RFC3569] service model. It is therefore recommended that ALL routers
   in the domain MUST reject AND never originate (*,G) joins for 232/8

   In those cases in which an ISP is offering its customers (or others)
   the use of the ISP's RP, the ISP SHOULD NOT allow (*,G) joins in the

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   232/8 range.

2.4.  Preventing RP's as candidates for 232/8

   Because PIM-SSM does not require an RP, all RPs SHOULD NOT offer
   themselves as candidates in the 232/8 range. This can be accomplished

    - preventing RP/BSR from announcing in the 232/8 range

    - preventing ALL routers from accepting RP delegations in the
      232/8 range

    - precluding RP functionality on RP for the 232/8 range

   Note that in typical practice, RP's announce themselves as candidates
   for the 224/4 (which obviously includes 232/8). It is still
   acceptable to allow the advertisement of 224/4 (or any other superset
   of 232/8); however, this approach relies on the second point, above,
   namely, that routers silently just ignore the RP delegation in the
   232/8 range, and prevent sending or receiving using the shared tree,
   as described previously.  Finally, an RP SHOULD NOT be configured as
   a candidate RP for 232/8 (or more specific range).

3.  Acknowledgments

   This document is the work of many people in the multicast community,
   including (but not limited to) Dino Farinacci, John Meylor, John
   Zwiebel, Tom Pusateri, Dave Thaler, Toerless Eckert, Leonard
   Giuliano, Mike McBride, and Pekka Savola.

4.  Security Considerations

   This document describes operational practices that introduce no new
   security issues to either PIM-SM or PIM-SSM.

   However, in the event that the operational practices described in
   this document are not adhered to, some problems may surface.  In
   particular, section 2.3 describes the effects of non-compliance of

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   last-hop routers (or to some degree, rogue hosts sending PIM messages
   themselves) on the source-specific service model; creating the (*,G)
   state for source-specific (S,G) could enable a receiver to receive
   data it should not get. This can be mitigated by host-side multicast
   source filtering.

5.  IANA Considerations

   This document creates no new requirements on IANA namespaces

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6.  References

6.1.  Normative References

   [PIM-SM]        Fenner, B., et. al, "Protocol Independent Multicast -
                   Sparse Mode (PIM-SM): Protocol Specification
                   (Revised)", draft-ietf-pim-sm-v2-new-09.txt. Work
                   in progress.

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

   [RFC2026]       Bradner, S., "The Internet Standards Process --
                   Revision 3", RFC 2026/BCP 9, October, 1996.

   [RFC2028]       Hovey, R. and S. Bradner, "The Organizations
                   Involved in the IETF Standards Process", RFC
                   2028/BCP 11, October, 1996.

   [RFC2434]       Narten, T., and H. Alvestrand, "Guidelines for
                   Writing an IANA Considerations Section in RFCs",
                   RFC 2434/BCP 26, October 1998.

   [RFC3569]       Bhattacharyya, S. Editor, "An Overview of
                   Source-Specific Multicast (SSM)" RFC 3569, July,

   [RFC3618]       Meyer, D. and B. Fenner (Editors), "The Multicast
                   Source Discovery Protocol (MSDP)", RFC 3618,
                   October, 2003.

6.2.  Informative References


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7.  Author's Addresses

   David Meyer

   Robert Rockell

   Greg Shepherd

8.  Full Copyright Statement

   Copyright (C) The Internet Society (2004). This document is subject
   to the rights, licenses and restrictions contained in BCP 78 and
   except as set forth therein, the authors retain all their rights.

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   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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9.  Intellectual Property

   The IETF takes no position regarding the validity or scope of any
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10.  Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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