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rfc:rfc4608

Network Working Group D. Meyer Request for Comments: 4608 R. Rockell BCP: 120 G. Shepherd Category: Best Current Practice August 2006

      Source-Specific Protocol Independent Multicast in 232/8

Status of This Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 IP Multicast group addresses in the 232/8 (232.0.0.0 to
 232.255.255.255) 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.

Table of Contents

 1. Introduction ....................................................2
    1.1. BCP, Experimental Protocols, and Normative References ......2
 2. Operational practices in 232/8 ..................................4
    2.1. Preventing Local Sources from Sending to Shared Tree .......4
    2.2. Preventing Remote Sources from Being Learned/Joined
         via MSDP ...................................................4
    2.3. Preventing Receivers from Joining the Shared Tree ..........4
    2.4. Preventing RPs as Candidates for 232/8 .....................5
 3. Acknowledgements ................................................5
 4. Security Considerations .........................................5
 5. References ......................................................6
    5.1. Normative References .......................................6
    5.2. Informative References .....................................6

Meyer, et al. Best Current Practice [Page 1] RFC 4608 Source-Specific PIM in 232/8 August 2006

1. Introduction

 Current Protocol Independent Multicast - Sparse Mode (PIM-SM)
 [RFC4601] relies on the shared Rendezvous Point (RP) tree to learn
 about active sources for a group and to support group-generic (Any
 Source Multicast or ASM) data distribution.  The IP Multicast group
 address range 232/8 has been designated for Source-Specific Multicast
 [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 SSM functionality of PIM-SM.
 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.
 The operational practices SHOULD:
    o  Prevent local sources from sending to shared tree
    o  Prevent receivers from joining the shared tree
    o  Prevent RPs as candidates for 232/8
    o  Prevent remote sources from being learned/joined via Multicast
       Source Discovery Protocol (MSDP) [RFC3618]

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
 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 Inter-Domain Multicast Routing
       (IDMR) working group produced as an inter-domain protocol.
       However, the IDMR WG (or subsequently, the Border Gateway
       Multicast Protocol (BGMP) WG) never produced a protocol that
       could be deployed to replace MSDP.

Meyer, et al. Best Current Practice [Page 2] RFC 4608 Source-Specific PIM in 232/8 August 2006

    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 nearly 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 to move that document to
       Experimental.  Although advancement of MSDP to Proposed
       Standard was considered, for the reasons mentioned above, it
       was immediately discarded.
    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 that must be read for
 one 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 [RFC4601] and MSDP [RFC3618] documents.  As a
 result, references to these documents are normative.
 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].  The IESG followed the
 Variance Procedure and, after an additional 4-week IETF Last Call,
 evaluated the comments and status and has approved the document.

Meyer, et al. Best Current Practice [Page 3] RFC 4608 Source-Specific PIM in 232/8 August 2006

 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].

2. Operational practices in 232/8

2.1. Preventing Local Sources from Sending to Shared Tree

 In order to eliminate the use of shared trees for groups in 232/8,
 while maintaining coexistence with ASM in PIM-SM, the behavior of the
 RP and/or the DR needs to be modified.  This can be accomplished by
  1. preventing data for 232/8 groups from being sent encapsulated

to the RP by the DR,

  1. preventing the RP from accepting registers for 232/8 groups

from the DR, and

  1. 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

 SSM does not require active source announcements via MSDP.  All
 source announcements are received out of band, and the last hop
 router is responsible for sending (S,G) joins directly to the source.
 To prevent propagation of SAs in the 232/8 range, an RP SHOULD
  1. never originate an SA for any 232/8 groups, and
  1. 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
  1. preventing DR from sending (*,G) joins for 232/8 groups, and
  1. 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
 that intersects an existing (S,G) tree, allowing the receiver on the
 shared tree to receive the data, which breaks the source-specific

Meyer, et al. Best Current Practice [Page 4] RFC 4608 Source-Specific PIM in 232/8 August 2006

 [RFC3569] service model.  It is therefore recommended that ALL
 routers in the domain MUST reject AND never originate (*,G) joins for
 232/8 groups.
 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
 232/8 range.

2.4. Preventing RPs as Candidates for 232/8

 Because SSM does not require an RP, all RPs SHOULD NOT offer
 themselves as candidates in the 232/8 range.  This can be
 accomplished by
  1. preventing RP/BSR from announcing in the 232/8 range,
  1. preventing ALL routers from accepting RP delegations in the

232/8 range, and

  1. precluding RP functionality on RP for the 232/8 range.
 Note that in typical practice, RPs 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 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 for a more specific range).

3. Acknowledgements

 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 PIM-SM [RFC4601] in either or SSM [RFC3569] or ASM
 operation.
 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
 last-hop routers (or, to some degree, rogue hosts sending PIM
 messages themselves) on the source-specific service model.  Creating

Meyer, et al. Best Current Practice [Page 5] RFC 4608 Source-Specific PIM in 232/8 August 2006

 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. References

5.1. Normative References

 [RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
           "Protocol Independent Multicast - Sparse Mode (PIM-SM):
           Protocol Specification (Revised)", RFC 4601, August 2006.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
           3", BCP 9, RFC 2026, October 1996.
 [RFC3569] Bhattacharyya, S., "An Overview of Source-Specific
           Multicast (SSM)", RFC 3569, July 2003.
 [RFC3618] Fenner, B. and D. Meyer, "Multicast Source Discovery
           Protocol (MSDP)", RFC 3618, October 2003.

5.2. Informative References

 [IANA]    http://www.iana.org
 [RFCED]   http://www.rfc-editor.org/policy.html

Authors' Addresses

 David Meyer
 EMail: dmm@1-4-5.net
 Robert Rockell
 Sprint
 EMail: rrockell@sprint.net
 Greg Shepherd
 Cisco
 EMail: gjshep@gmail.com

Meyer, et al. Best Current Practice [Page 6] RFC 4608 Source-Specific PIM in 232/8 August 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 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.
 This document and the information contained herein are provided on an
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Meyer, et al. Best Current Practice [Page 7]

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