INTERNET-DRAFT IGMPv3 for SSM H. Holbrook
Expires January 14, 2001 Cisco Systems
B. Cain
Mirror Image Internet
14 July 2000
Using IGMPv3 For Source-Specific Multicast
<draft-holbrook-idmr-igmpv3-ssm-00.txt>
Status of this Memo
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 other groups
may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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].
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Abstract
This document describes changes to the Internet Group Management
Protocol Version 3 (IGMPv3) [IGMPv3] to support source-specific
multicast (SSM) [SSM].
1. Overview and Rationale
The Internet Group Management Protocol (IGMP) [RFC1112,IGMPv2,IGMPv3],
is the standard mechanism for communicating IP multicast group
membership requests from a host to its locally attached routers. IGMP
version 3 (IGMPv3) [IGMPv3] provides the ability for a host to
selectively request or filter traffic from individual sources within a
multicast group. The IGMPv3 algorithms and message processing rules
require small changes to support the source-specific multicast model.
This document defines the modifications required to the host and router
portions of IGMPv3 to support source-specific multicast.
2. IGMP Host Requirements for Source-Specific Multicast
This document does not require that the IP layer or the IGMP module of
an IGMPv3-enabled host treat SSM destination addresses specially. This
document does require host applications to:
- know the range of destination addresses that have SSM semantics
- use ONLY the source-specific APIs to request delivery of packets
sent to SSM destination addresses
The 232/8 address range is currently allocated for SSM, however routers
may be configured to force SSM semantics for other addresses, also. The
mechanism for discovering which addresses have SSM semantics is not
defined in this document.
Given that the host IP module is not required to know which addresses
are SSM and which addresses are not, the ISM APIs generally will not
return an error when applied to SSM destination addresses. However,
hosts that mistakenly use the ISM (e.g., "join(G)") or the full IGMPv3
APIs (e.g., "IPMulticastListen(G,EXCLUDE(S1))") to request delivery of
packets sent to an SSM address will not receive the requested service,
as routers will refuse to process any such request, as per section 10.2.
This section documents the behavior of hosts with respect to sending and
receiving the following IGMP message types:
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- IGMPv1/v2 Reports
- IGMPv3 Reports
- IGMPv1/v2 Queries
- IGMPv2 Leave
- IGMPv2 Group Specific Query
- IGMPv3 Group Specific Query
- IGMPv3 Group-and-Source Specific Query
2.1. IGMPv1/v2 Reports
IGMPv1 or IGMPv2 host report are not processed for SSM addresses. They
will not ever be seen, if other hosts on the LAN agree about which
addresses have SSM semantics. As long as hosts use the SSM APIs for SSM
addresses. IGMPv1 and IGMPv2 host reports will not be sent for SSM
addresses.
2.2. IGMPv3 Reports
Source-specific multicast destination-and-source pairs (channels) are
reported using IGMPv3 with the IGMPv3 INCLUDE report. A host
implementation MAY report either one or multiple channels in a single
IGMPv3 report.
When source-specific channels are reported in an IGMPv3 Report, the
report may contain one or more group records of the following types:
- MODE_IS_INCLUDE as part of a Current-State Record
- ALLOW_NEW_SOURCES as part of a State-Change Record
- BLOCK_OLD_SOURCES as part of a State-Change Record
The source list for any individual Group Record MAY be of length one or
more than one. If a host implementation so chooses, it MAY report both
SSM destination addresses and ISM destination addresses in the same
message.
If all applications use the SSM APIs for SSM addresses, then a host
would not normally send any of the following group record types for
addresses in the source-specific range:
- MODE_IS_EXCLUDE as part of a Current-State Record
- CHANGE_TO_INCLUDE_MODE as part of a Filter-Mode-Change Record
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- CHANGE_TO_EXCLUDE_MODE as part of a Filter-Mode-Change Record
EXCLUDE mode does not apply to SSM addresses, and the filter mode used
for a SSM address should never change to or from EXCLUDE mode under
correct application behavior. [Note: please see Section 4, Outstanding
Issues.]
2.3. IGMPv1/IGMPv2 Queries
If an IGMPv1 or IGMPv2 query is received, the IGMPv3 protocol
specification requires the host to revert to the older (IGMPv1 or
IGMPv2) mode of operation for that destination address. If this occurs,
the host will stop reporting source-specific subscriptions for that
destination address and start using either IGMPv1 or IGMPv2 to report
interest in the SSM destination address, unqualified by a source
address. If this occurs, SSM semantics will no longer be applied for G.
However, a router compliant with this document would never generate an
IGMPv1 or IGMPv2 query for an address in the SSM range, so this
situation would only occur if some router is not compliant with this
document for an address that the host believes to have SSM semantics.
If a host does revert to an older version of operation for some
destination address, it will no longer be able to send source-specific
IGMPv3 messages and it will not be able to subscribe to SSM channels
using that destination address. A host MAY have a configuration option
that allows it continue to send source-specific reception requests and
to refuse to revert to the older (IGMPv1 or IGMPv2) mode of operation
for addresses in the source-specific range, even if an IGMPv1 or IGMPv2
query is heard.
These problems only arise on a shared-medium link that has both SSM-
aware and non-SSM-aware routers present. Therefore, it SHOULD be
administratively assured that all routers on a given shared-medium
network are compliant with this document.
2.4. IGMPv2 Leave
IGMP Leave messages are not processed by hosts. IGMPv2 Leave messages
are not sent for SSM addresses.
2.5. IGMPv2 Group Specific Query
If a host receives an IGMPv2 Group Specific Query for an address in the
source-specific range, it MUST silently discard the query, even if the
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group listed matches the source-specific destination address of some
locally subscribed source-specific group. The transmission of such a
query indicates that the sender is not compliant with this document.
2.6. IGMPv3 Group Specific Query
If a host receives an IGMPv3 Group-Specific Query in the source-specific
range, it MUST respond with a report if the group matches the source-
specific destination address of any of its subscribed source-specific
groups.
Although in the current IGMPv3 protocol specification, routers would
have no reason to send one, the semantics of such a query are well-
defined in this range and future implementations may have reason to send
such a query. Be liberal in what you accept.
2.7. IGMPv3 Group-and-Source Specific Query
An IGMPv3 router will query a source-specific channel that a host has
requested to leave (via a BLOCK_OLD_SOURCES record) with a group-and-
source specific query. A host MUST respond to a group-and-source
specific query for which the group and source in the query match match
any channel for which the host has a subscription.
Hosts MUST be able to process a query with multiple sources listed per
group.
3. IGMP Router Requirements for Support Source-Specific Multicast
This section documents the behavior of routers with respect to the
following types of IGMP messages for source-specific destination
addresses:
- IGMPv3 Reports
- IGMPv3 General Query
- IGMPv3 Group-Specific Query
- IGMPv3 Group-and-Source Specific Query
- IGMPv1/v2 Reports
- IGMPv1/v2 Queries
- IGMPv2 Leave
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3.1. IGMPv3 Reports
IGMPv3 Reports are used to report source-specific subscriptions in the
SSM address range. If a router receives an IGMPv3 report that contains
a group record for a destination address in the source-specific range
that matches one of the types listed below, then it MUST ignore that
group record, however, it MUST process other group records within that
same report.
- Any Current-State Record with MODE_IS_EXCLUDE
- A CHANGE_TO_INCLUDE_MODE Filter-Mode-Change Record
- A CHANGE_TO_EXCLUDE_MODE Filter-Mode-Change Record
3.2. IGMPv3 General Queries
IGMPv3 General Queries are used to periodically build the total desired
membership state on a subnet. These queries are used for the same
purpose in the source-specific address range -- no change in behavior is
required. A router that supports the source-specific multicast address
range sends periodic IGMPv3 General Queries as per the IGMPv3
specification.
3.3. IGMPv3 Group Specific Queries
IGMPv3 routers that support source-specific multicast MAY send group-
specific queries for addresses in the source-specific range, although,
in the current IGMPv3 protocol spec, there is no scenario under which
this would occur.
3.4. IGMPv3 Group-and-Source Specific Queries
IGMPv3 Group-and-Source Specific Queries are used to verify that there
are no locally attached listeners when a receiver has indicated that it
is no longer interested in receiving traffic from a particular (S,G)
pair. Group-and-Source Specific Queries are used within the source-
specific address range when a router receives a BLOCK_OLD_SOURCES Record
for one or more source-specific groups.
3.5. IGMPv1/v2 Reports
An IGMPv1/v2 report for an address in the source-specific range could be
sent by a host that does not support the source-specific model. A
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router MUST ignore all IGMPv1 and IGMPv2 reports in the source-specific
address range and specifically MUST NOT use them to establish IP
forwarding state.
3.6. IGMPv1/v2 Queries
The IGMP querier on a shared-medium network is elected to be the one
with lowest source IP address. Therefore, an IGMPv3 router will yield
to an IGMPv1 or v2 querier with a lower IP address. IGMPv3 routers that
lose the querier election to a lower version router MUST log an error,
as per the IGMPv3 specification. However, IGMPv3 routers MUST NOT
revert into previous version compatibility mode for the source-specific
address range. An IGMPv3 router that loses the querier election to an
IGMPv1 or v2 querier SHOULD continue to process source-specific reports
in the source-specific address range.
3.7. IGMPv2 Leave
An IGMPv2 Leave may be received for a source-specific address from a
host that does not support the source-specific model. A router MUST
ignore all IGMPv2 leaves in the source-specific address range.
4. Outstanding Issues
EXCLUDE mode does not apply to SSM addresses, and the filter mode used
for a SSM address should never change to or from EXCLUDE mode under
correct application behavior. The IGMPv3 specification indicates that a
host should convert to EXCLUDE mode operation when it no longer has
enough memory to record INCLUDE mode requests. For SSM, it would be
preferable to return an error indication.
However, as specified in this document, a host does not have any
knowledge about the range of addresses that have SSM semantics -- only a
router has this information. One possible solution is to allow the
application layer to provide this information. In this solution, the
IGMPv3 IPMulticastListen API is extended to allow a host to request an
INCLUDE-mode subscription that must not be converted to an EXCLUDE mode
subscription if the router runs out of memory.
5. Acknowledgments
The authors would like to thank Vince Laviano, Nidhi Bhaskar, and Steve
Deering and for their input and careful review.
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6. References
[IGMPv3] Cain, B., Deering, S., and A. Thyagarajan, "Internet Group
Management Protocol, Version 3," Work in Progress.
[RFC1112] Deering, S., "Host Extensions for IP Multicasting," RFC 1112,
August 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," RFC 2119, March 1997.
[IGMPv2] Fenner, W., "Internet Group Management Protocol, Version 2,"
RFC 2236, November 1997.
[SSM] Holbrook, H., and Cain, B., "Source-Specific Multicast for IP",
Work in Progress.
7. Author's Address
Hugh Holbrook
Cisco Systems
holbrook@cisco.com
Brad Cain
Nortel Networks
bcain@nortelnetworks.com
This document expires January 14, 2001.
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