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IETF RFC 4608
Source-Specific Protocol Independent Multicast in 232/8
Last modified on Thursday, August 3rd, 2006
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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 © 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)
[RFC 4601] 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
[RFC 3569] 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 [RFC 3569], 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) [RFC 3618]
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 [RFC 4601] and MSDP [RFC 3618] 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 [RFC 2026]. 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 [RFC 2119].
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
- preventing data for 232/8 groups from being sent encapsulated
to the RP by the DR,
- preventing the RP from accepting registers for 232/8 groups
from the DR, and
- 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
- never originate an SA for any 232/8 groups, and
- 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, and
- 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
[RFC 3569] 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
- preventing RP/BSR from announcing in the 232/8 range,
- preventing ALL routers from accepting RP delegations in the
232/8 range, and
- 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 [RFC 4601] in either or SSM [RFC 3569] 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
[RFC 4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC 3569] Bhattacharyya, S., "An Overview of Source-Specific
Multicast (SSM)", RFC 3569, July 2003.
[RFC 3618] 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 © 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.
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Acknowledgement
Funding for the RFC Editor function is provided by the IETF
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Meyer, et al. Best Current Practice PAGE 7
Source-Specific Protocol Independent Multicast in 232/8
RFC TOTAL SIZE: 15030 bytes
PUBLICATION DATE: Thursday, August 3rd, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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