INTERNET-DRAFT IGMPv3/MLDv2 for SSM H. Holbrook
Expires Apr 26, 2003 Cisco Systems
B. Cain
Storigen Systems
B. Haberman
Caspian Networks
Oct 26, 2003
Using IGMPv3 and MLDv2 For Source-Specific Multicast
<draft-holbrook-idmr-igmpv3-ssm-05.txt>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions of
Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task
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and may be updated, replaced, or obsoleted by other documents at any
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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
The Internet Group Management Protocol Version 3 (IGMPv3) and the
Multicast Listener Discovery Protocol Version 2 (MLDv2) are protocols
that allow a host to inform its neighboring routers of its desire to
receive IPv4 and IPv6 multicast transmissions, respectively. Source-
Specific Multicast (SSM) is a form of multicast in which a receiver is
required to specify both the network-layer address of the source and the
multicast destination address in order to receive multicast traffic.
This document defines the notion of an "SSM-aware" host and describes
clarifications of and, in some cases, modifications to the behavior of
IGMPv3 and MLDv2 on routers and on SSM-aware hosts to accomodate source-
specific multicast.
1. Introduction
The Internet Group Management Protocol (IGMP) [RFC1112,IGMPv2,IGMPv3],
allows an IPv4 host to communicate IP multicast group membership
information to its neighboring 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 Multicast Listener Discovery Protocol (MLD) [RFC2710, MLDv2] offers
similar functionality for IPv6 hosts. MLD version 2 (MLDv2) provides
the analagous "source filtering" functionality of IGMPv3.
Due to the commonality of function, the term "Group Management Protocol"
or "GMP" will be used to refer to both IGMP and MLD. The term "Source
Filtering GMP", or "SFGMP" will be used to reference the IGMPv3 and
MLDv2 group management protocols.
The use of source-specific multicast is facilitated by small changes to
the SFGMP protocols on both hosts and routers. This document defines
modifications to the host and router portions of IGMPv3 and MLDv2 to
better support SSM. This document also provides a number of
clarifications to the behavior of IGMPv3 and MLDv2 in the presence of
SSM.
In order to emphasize the parts of this document that are modifications
of existing protocol specifications, as opposed to those parts that are
merely clarifications, the modifications are identified with the tag
"(MODIFICATION)".
2. Host Requirements for Source-Specific Multicast
This section defines the notion of an "SSM-aware" host and then goes on
to describe the API requirements and the SFGMP protocol requirements of
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an SSM-aware host. It is important to note that SSM can be used by any
host that supports source filtering APIs and whose operating system
supports the appropriate SFGMP. The SFGMP modifications described in
this section make SSM work better on an SSM-aware host, but they are not
strict prerequisites for the use of SSM.
The 232/8 IPv4 address range is currently allocated for SSM by IANA
[IANA-ALLOCATION]. In IPv6, the range is FF3x::/32 (where 'x' is a
valid IPv6 multicast scope value) [RFC3306,SSM]. A host or router may
be configured to apply SSM semantics to other addresses, as well. The
GMP module on a host or router SHOULD have a configuration option to set
the SSM address range(s). If this configuration option exists, it MUST
default to the IANA-allocated SSM range. The mechanism for setting this
configuration option MUST at least allow for manual configuration.
Protocol mechanisms to set this option may be defined in the future.
A host with such a configuration option is described as an "SSM-aware"
host.
2.1. API requirements
If the host IP module of an SSM-aware host receives a non source-
specific request to receive multicast traffic sent to an SSM destination
address, it SHOULD return an error to the application, as specified in
[MSFAPI] (MODIFICATION). On a non-SSM-aware host, an application that
uses the wrong API (e.g., "join(G)", "IPMulticastListen(G,EXCLUDE(S1))"
for IGMPv3, or IPv6MulticastListen(G,EXCLUDE(S2))" for MLDv2) to request
delivery of packets sent to an SSM address will not receive the
requested service (because a properly configured router will refuse to
process the request) and the application will receive no indication
other than a failure to receive the requested traffic.
2.2. GMP requirements
This section defines the behavior of the SFGMP protocol module on an
SSM-aware host, including two modifications to the protocols as
described in [IGMPv3,MLDv2]. It also includes a number of
clarifications of protocol operations. In doing so, it documents the
behavior of an SSM-aware host with respect to sending and receiving the
following GMP message types:
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- IGMPv1/v2 and MLDv1 Reports (2.2.1)
- IGMPv3 and MLDv2 Reports (2.2.2)
- IGMPv1/v2 and MLDv1 Queries (2.2.3)
- IGMPv2 Leave and MLDv1 Done (2.2.4)
- IGMPv2 and MLDv1 Group Specific Query (2.2.5)
- IGMPv3 and MLDv2 Group Specific Query (2.2.6)
- IGMPv3 and MLDv2 Group-and-Source Specific Query (2.2.7)
2.2.1. IGMPv1/v2 and MLDv1 Reports
An SSM-aware host SHOULD NOT send an IGMPv1, IGMPv2, or MLDv1 report for
an SSM address, except if the SSM-aware host is operating in "older-
version compatibility mode." This is an exceptional (error) condition,
indicating that the router(s) can not provide the SFGMP support needed
for SSM, and an error should be logged when the host enters
compatibility mode, as described below.
[IGMPv3] and [MLDv2] specify that a host MAY allow an older-version
report to suppress its own IGMPv3 or MLDv2 Membership Record. An SSM-
aware host, however, MUST NOT allow its report to be suppressed in this
situation (MODIFICATION). Suppressing reports in this scenario would
provide an avenue for an attacker to deny SSM service to other hosts on
the link.
2.2.2. IGMPv3 and MLDv2 Reports
A host implementation may report more than one SSM channel in a single
report, by including either multiple sources within a group record or by
including multiple group records.
A Group Record for a source-specific destination address may (under
normal operation) be one 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
A report may include both SSM destination addresses and non-source-
specific, i.e., Any-Source Multicast (ASM) destination addresses, in the
same message.
A CHANGE_TO_INCLUDE_MODE record may be sent by a host, for instance,
when the SSM address range is changed through configuration. A router
processes such a record normally.
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An SSM-aware host SHOULD NOT send any of the following record types for
an SSM address.
- MODE_IS_EXCLUDE as part of a Current-State Record
- CHANGE_TO_EXCLUDE_MODE as part of a Filter-Mode-Change Record
EXCLUDE mode does not apply to SSM addresses, and routers ignore
MODE_IS_EXCLUDE and CHANGE_TO_EXCLUDE_MODE requests in the SSM range, as
described below.
2.2.3. IGMPv1/IGMPv2 and MLDv1 Queries
If an IGMPv1, IGMPv2, or MLDv1 query is received, the SFGMP protocol
specifications require the host to revert to the older (IGMPv1, IGMPv2,
or MLDv1) mode of operation for that destination address. If this
occurs, the host will stop reporting source-specific subscriptions for
that destination address and will start using IGMPv1, IGMPv2, or MLDv1
to report interest in the SSM destination address, unqualified by a
source address. As a result, SSM semantics will no longer be applied to
G by the router.
A router compliant with this document would never generate an IGMPv1,
IGMPv2, or MLDv1 query for an address in the SSM range, thus this
situation only occurs if either the router is not compliant with this
document or if the host and the router disagree about the SSM address
range.
A host SHOULD log an error if it receives an IGMPv1, IGMPv2, or MLDv1
query for an SSM address (MODIFICATION).
In order to mitigate this problem, it must be administratively assured
that all routers on a given shared-medium network are compliant with
this document and are in agreement about the SSM address range.
2.2.4. IGMPv2 Leave and MLDv1 Done
IGMP Leave/Done messages are not processed by hosts. IGMPv2 Leave and
MLDv1 Done messages are not sent for SSM addresses.
2.2.5. IGMPv2 and MLDv1 Group Specific Query
If a host receives an IGMPv2 or MLDv1 Group Specific Query for an
address in any configured source-specific range, it SHOULD process the
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query normally according to [IGMPv3][MLDv2], even if the group queried
is a source-specific destination address. The transmission of such a
query likely indicates that the sending router is either not compliant
with this document or that it is not configured with the same SSM
address range(s) as the receiving host. A host SHOULD log an error in
this case.
2.2.6. IGMPv3 and MLDv1 Group Specific Query
If an SSM-aware host receives an SFGMP Group-Specific Query for an SSM
address, it MUST respond with a report if the group matches the source-
specific destination address of any of its subscribed source-specific
channels.
Although in the current IGMPv3 and MLDv2 protocol specifications, a
router 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.
2.2.7. IGMPv3 and MLDv2 Group-and-Source Specific Query
An SFGMP router typically uses a group-and-source specific query to
query an SSM channel that a host has requested to leave via a
BLOCK_OLD_SOURCES record. 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.
A host MUST be able to process a query with multiple sources listed per
group.
3. Router Requirements for Source-Specific Multicast
Routers must be aware of the SSM address range. The 232/8 and FF3x::/32
address ranges are currently allocated for SSM by IANA [IANA-
ALLOCATION][RFC3306]. An SSM router may have a configuration option to
apply SSM semantics to other addresses, as well. If this configuration
option exists, it MUST default to the IANA-allocated range.
This section documents the behavior of routers with respect to the
following types of SFGMP messages for source-specific destination
addresses:
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- IGMPv3 and MLDv2 Reports (3.1)
- IGMPv3 and MLDv2 General Query (3.2)
- IGMPv3 and MLDv2 Group-Specific Query (3.3)
- IGMPv3 and MLDv2 Group-and-Source Specific Query (3.4)
- IGMPv1/v2 and MLDv1 Reports (3.5)
- IGMPv1/v2 and MLDv1 Queries (3.6)
- IGMPv2 Leave and MLDv1 Done (3.7)
3.1. IGMPv3 and MLDv2 Reports
SFGMP Reports are used to report source-specific subscriptions in the
SSM address range. A router SHOULD ignore a group record of either of
the following types if it refers to an SSM destination address:
- MODE_IS_EXCLUDE Current-State Record
- CHANGE_TO_EXCLUDE_MODE Filter-Mode-Change Record
A router MAY choose to log an error in this case, and it MUST process
any other group records within the same report. A
CHANGE_TO_INCLUDE_MODE Filter-Mode-Change Record MUST be processed as
normal.
3.2. IGMPv3 and MLDv2 General Queries
An SSM router sends periodic SFGMP General Queries as per the IGMPv3 and
MLDv2 specifications. No change in behavior is required.
3.3. IGMPv3 and MLDv2 Group Specific Queries
SFGMP routers that support source-specific multicast MAY send group-
specific queries for addresses in the source-specific range. This
specification does not explicitly prohibit such a message, although, at
the time of writing, a router conformant to [IGMPv3][MLDv2] would never
send one.
3.4. IGMPv3 and MLDv2 Group-and-Source Specific Queries
SFGMP Group-and-Source Specific Queries are used when a receiver has
indicated that it is no longer interested in receiving traffic from a
particular (S,G) pair to determine if there are any remaining directly-
attached hosts with interest in that (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. These queries are sent normally, as per
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[IGMPv3][MLDv2].
3.5. IGMPv1/v2 and MLDv1 Reports
An IGMPv1/v2 or MLDv1 report for an address in the source-specific range
could be sent by a non-SSM-aware host. A router SHOULD ignore all such
reports and specifically SHOULD NOT use them to establish IP forwarding
state (MODIFICATION). A router MAY log an error if it receives such a
report.
3.6. IGMPv1/v2 and MLDv1 Queries
An SFGMP router that loses the querier election to a lower version
router MUST log an error, as per the IGMPv3 and MLDv2 specifications.
3.7. IGMPv2 Leave and MLDv1 Done
An IGMPv2 Leave or MLDv1 Done message may be sent by a non-SSM-aware
host. A router SHOULD ignore all such messages in the source-specific
address range and MAY log an error (MODIFICATION).
4. Security Considerations
The specific protocol modifications described in this document are not
known to create any security concerns that are not already present when
IGMPv3 or MLDv2 is used with ASM-style multicast. The reader is
referred to [SSM] for an analysis of SSM-specific security issues.
It is important that a router not accept non-source-specific reception
requests for an SSM destination address. The rules of [IGMPv3] and
[MLDv2] require a router, upon receiving such a membership report, to
revert to earlier version compatibility mode for the group in question.
If the router were to revert in this situation, it would prevent an
IGMPv3-capable host from receiving SSM service for that destination
address, thus creating a potential for an attacker to deny SSM service
to other hosts on the same link.
5. Notes
[To be removed before going to the IESG.]
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5.1. Change history
Changes in the -05 version: Additional wordsmithing, editing,
eliminating redundancy. Clean up the text describe SSM-awareness. Call
out what is actually a protocol modification vs. what is a clarification
of existing protocol behavior. Clarify that an ssm-aware host may send
IGMPv1,2/MLDv1 reports when in compatibility mode. Clean up the
abstract, add notion of SSM-aware to the abstract. Add reference to
[MSFAPI].
Changes in the -04 version: Updated to reflect ID-nits. Rewrote
Abstract, added Security Considerations, IPR, and Copyright statements.
Clarified the notion of an "SSM-aware" host as being a host with an
address range configuration option. Clarified that
CHANGE_TO_INCLUDE_MODE may be sent when the address range configuration
changes. Eliminated the mention of the option to ignore older version
queriers -- this conflicts the IGMPv3 spec and creates a bunch of bad
scenarios for ASM. Updated and split references to
normative/informative.
Changes in the -03 version: There were two major changes in this
revision (-03): the inclusion of IPv6, and the removal of the explicit
text stating that a host MAY choose to ignore IGMPv1/v2 (MLDv1) queries
in the SSM address range. The document was internally inconsistent on
this point (it said two different things in two different sections), and
the statement conflicted with the existing IGMPv3 specifications. This
situation indicates a serious configuration error and if the querying
router has already reverted to IGMPv2 mode for a group, then a host can
not receive proper SSM service for that group in any case. Hence the
authors decided that it was better to remove this statement.
5.2. A note on EXCLUDE mode
The IGMPv3 and MLDv2 protocol specifications explicitly state that a
host MUST NOT transition to EXCLUDE mode as a result of running out of
resources -- hence when a host runs out of resources, it will not fail
to provide SSM service.
6. Acknowledgments
The authors would like to thank Vince Laviano, Nidhi Bhaskar, Steve
Deering, and Toerless Eckert and for their input and careful review.
7. Normative References
[IGMPv2] Fenner, W., "Internet Group Management Protocol, Version 2,"
RFC 2236, November 1997.
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[IGMPv3] Cain, B., Deering, S., and A. Thyagarajan, "Internet Group
Management Protocol, Version 3," RFC 3376, October 2002.
[MSFAPI] Thaler, D., Fenner, B., and Quinn, B. "Socket Interface
Extensions for Multicast Source Filters," draft-ietf-magma-msf-
api-05.txt. 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.
[SSM] Holbrook, H., and Cain, B., "Source-Specific Multicast for IP",
draft-holbrook-ssm-arch-04.txt. Work in Progress.
[MLDv2] Vida, R., and Costa, L., "Multicast Listener Discovery Version 2
(MLDv2) for IPv6", draft-vida-mld-v2-07.txt. Work in progress.
[RFC2710] Deering, S., Fenner, B., and Haberman, B., "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
8. Informative References
[IANA-ALLOCATION] Internet Assigned Numbers Authority,
http://www.isi.edu/in-notes/iana/assignments/multicast-addresses.
[RFC3306] Haberman, B., and Thaler, D., "Unicast-prefix-based IPv6
Multicast Addresses", RFC 3306, August 2002.
Authors' Addresses
Hugh Holbrook
Cisco Systems
170 W. Tasman Drive.
San Jose, CA 95134
holbrook@cisco.com
Brad Cain
Storigen Systems
650 Suffolk St.
Lowell, MA 01854
bcain99@storigen.com
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Brian Haberman
Caspian Networks
bkhabs@nc.rr.com
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