PIM Working Group H. Asaeda
Internet-Draft NICT
Intended status: Standards Track July 15, 2013
Expires: January 16, 2014
IGMP/MLD-Based Explicit Membership Tracking Function for Multicast
Routers
draft-ietf-pim-explicit-tracking-06
Abstract
This document describes the IGMP/MLD-based explicit membership
tracking function for multicast routers supporting IGMPv3/MLDv2. The
explicit membership tracking function contributes to saving network
resources and shortening leave latency, and enables operators to see
which downstream routers(s) on a LAN are joined to which multicast
tree.
Status of This Memo
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This Internet-Draft will expire on January 16, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Membership State Information . . . . . . . . . . . . . . . . 4
4. Specific Query Suppression . . . . . . . . . . . . . . . . . 4
5. Shortening Leave Latency . . . . . . . . . . . . . . . . . . 5
6. Risk of Wrong Membership State . . . . . . . . . . . . . . . 6
7. All-Zero and Unspecified Source Addresses . . . . . . . . . . 7
8. Compatibility with Older Version Protocols . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. Normative References . . . . . . . . . . . . . . . . . . 9
12.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The Internet Group Management Protocol (IGMP) version 3 [2] for IPv4
and the Multicast Listener Discovery Protocol (MLD) version 2 [3] for
IPv6 are the standard protocols used by member hosts and multicast
routers. Lightweight IGMPv3 and Lightweight MLDv2 (or LW-IGMPv3 and
LW-MLDv2) [4] are subsets of the standard IGMPv3 and MLDv2.
When a host starts/finishes listening to particular multicast
channels, it sends IGMP/MLD State-Change Report messages specifying
the corresponding channel information as the join/leave request to
its upstream router (i.e., an adjacent multicast router or IGMP/MLD
proxy device [8]). The "unsolicited" report messages are sent only
when the host joins/leaves the channels. Since IGMP/MLD are non-
reliable protocols, unsolicited report messages may be lost or may
not reach upstream routers. To alleviate the problem, unsolicited
report messages are transmitted the [Robustness Variable] times
(defined in [2][3]).
Also, a querier router periodically sends IGMP/MLD General Query
messages within the General Query timer interval (i.e. [Query
Interval] value defined in [2][3]). Upon receiving the query
messages, the member hosts reply with "solicited" report messages.
Routers then keep their membership state information up to date.
However, this approach still does not guarantee that the membership
state is always perfectly synchronized. To minimize the possibility
of having outdated membership information, routers may shorten the
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periodic General Query timer interval. Unfortunately, this increases
the number of transmitted solicited report messages and induces
network congestion. And the greater the amount of network
congestion, the greater the potential for IGMP/MLD report messages
being lost and the membership state information being outdated in the
router.
IGMPv3 [2], MLDv2 [3], and these lightweight protocols [4] can
provide the ability to keep track of the downstream (adjacent)
multicast membership state to multicast routers, yet the
specifications are not clearly given. This document describes the
"IGMP/MLD-based explicit member tracking function" for multicast
routers and a way for routers to implement the function. By enabling
this explicit tracking function, routers can keep track of the
downstream multicast membership state. This function enables the
following things:
o Reducing the number of transmitted query and report messages
o Shortening leave latencies
o Per-host accounting
o Maintaining multicast channel characteristics (or statistics)
where this document mainly focuses on the above first and second
bullets in the following sections.
Note that the explicit tracking function does not change the
reliability of the message transmission. The list of tracked member
hosts may be outdated in the router because of host departure from
the network without sending State-Change Report messages or loss of
such messages due to network congestion. Therefore, a router even
enabling the function may need to send periodic IGMPv3/MLDv2 General
Query messages and solicit IGMPv3/MLDv2 report messages from
downstream member hosts to maintain an up-to-date membership state,
while the function contributes to saving network resources by tuning
query timers or values.
The explicit tracking function potentially requires a large amount of
memory so that routers keep all membership states. Particularly when
a router needs to maintain a large number of member hosts, this
resource requirement could have an impact. Operators may decide to
disable this function when their routers have insufficient memory
resources, despite the benefits mentioned above.
The explicit tracking function does not change message formats used
by the standard IGMPv3 [2] and MLDv2 [3], and their lightweight
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version protocols [4]; nor does it change a multicast data sender's
and receiver's behavior.
2. Terminology
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 [1].
3. Membership State Information
A router enabling the explicit tracking function maintains the
"membership state information". When a multicast router receives a
Current-State or State-Change Report message, it creates or modifies
this membership state information to maintain the membership state up
to date.
The membership state information is also known as the following
"interface state" defined by the standard IGMPv3 and MLDv2
specifications [2][3].
(multicast address, filter mode, source list)
The membership state information must be identified properly even
though a receiver (i.e., IGMP/MLD Report sender) sends the identical
report messages multiple times.
4. Specific Query Suppression
In accordance with [2] and [3], when a router receives the State-
Change Report and needs to confirm whether any hosts are still
interested in a channel or not, the router sends the corresponding
Group-Specific or Group-and-Source Specific Query messages along with
the definition of Section 6.4.2 of [2] and Section 7.4.2 of [3]. The
queries sent by actions defined in these sections need to be
transmitted [Last Member Query Count] (LMQC) or [Last Listener Query
Count] (LLQC) times, once every [Last Member Query Interval] (LMQI)
or [Last Listener Query Interval] (LLQI), in order to confirm the
sole member. (The default values for LMQI/LLQI defined in [2][3] are
1 second. The default values for LMQC/LLQC are the [Robustness
Variable] value whose default value is 2.) All member hosts joining
the identical channel then reply their own states after acquiring
these query messages. However, transmitting a large number of IGMP/
MLD Report messages consumes network resources, and this may pose a
particular problem especially when many hosts joining the identical
channel send these reports simultaneously.
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The explicit tracking function provides a method called "specific
query suppression" that reduces the number of Group-Specific or
Group-and-Source Specific Query messages transmitted from a router.
This in turn reduces the number of Current-State Report messages
transmitted from member hosts.
With the specific query suppression, regardless of the LMQC/LLQC
values, if the router receives one or more replies from the
downstream member(s), it can stop (i.e., cancel) retransmitting the
specific query message(s) and lower the corresponding source/group
timers. This contributes to saving network resources.
The specific query suppression in "hard state" may also be
implemented with the explicit tracking function; a router enabling
the specific query suppression in hard state does not send any
specific query message(s) and immediately leave the group or sources
when the sole member has left according to its membership state
information. The specific query suppression in hard state hence does
not rely on LMQC/LLQC and LMQI/LLQI values. This contributes to
shortening leave latency described in Section 5. However, this
behavior requires that the router perfectly tracks all member hosts.
(See a risk of wrong membership expectation described in Section 6.)
Note that the default behavior of the router that supports the
explicit tracking function SHOULD disable this specific query
suppression, in order to avoid the risk caused by the wrong
membership expectation or by the case in which multiple multicast
routers exist on a LAN and the querier router is not the forwarder
router. The former case is described in Section 6. For the latter
case, when the querier suppresses the specific query message
transmission, and expects that the State-Change Report sender is not
the sole member of the channel, it does not send the specific query
and none of the routers on the same LAN receive a Current-State
Report message from the corresponding member hosts. The forwarder in
this case may prune the routing path though there are other member
hosts subscribing to the channel on the LAN.
5. Shortening Leave Latency
A router enabling the explicit tracking function can shorten leave
latencies by tuning the following values; [Last Member Query Count]
(LMQC), [Last Listener Query Count] (LLQC), [Last Member Query
Interval] (LMQI), [Last Listener Query Interval] (LLQI), and
[Robustness Variable] values.
The [Last Member Query Interval] (LMQI) and [Last Listener Query
Interval] (LLQI) values defined in the standard specifications [2][3]
specify the maximum time allowed for a member host to send a
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responding Report. The [Last Member Query Count] (LMQC) and [Last
Listener Query Count] (LLQC) are the number of Group-Specific Queries
or Group-and-Source Specific Queries sent before the router assumes
there are no local members. The [Last Member Query Time] (LMQT) and
[Last Listener Query Time] (LLQT) values are the total time the
router should wait for a report after the Querier has sent the first
query.
The default values for LMQI/LLQI defined in [2][3] are 1 second, yet,
for a router enabling the explicit tracking function, the LMQI/LLQI
may be set to 1 second or shorter. As well, the default values for
LMQC/LLQC are the [Robustness Variable] value whose default value is
2, yet the LMQC/LLQC may be set to 1 for the router. Smaller LMQC/
LLQC values give shorter LMQT/LLQT, which shorten the leave
latencies.
Furthermore, if operators believe that their link is fairly robust or
that they can configure the [Robustness Variable] value appropriately
so that the chances of unsolicited messages being lost are
sufficiently low, and if the querier router is always the forwarder
router in their link, they will set smaller LMQC/LLQC and shorter
LMQI/LLQI (and hence shorter LMQT/LLQT) with the specific query
suppression or enable the specific query suppression in hard state
(Section 4) for their routers.
Note that setting smaller LMQC/LLQC values or adopting the specific
query suppression in hard state poses the risk of wrong membership
state described in Section 6. Operators setting smaller LMQC/LLQC
values must recognize this tradeoff.
6. Risk of Wrong Membership State
There are possibilities that a router's membership expectation is
inconsistent due to an outdated membership state. For example, (1) a
router expects that more than one corresponding member host exists on
its LAN, but in fact no member host exists for that multicast
channel, or (2) a router expects that no corresponding member host
exists on its LAN, but in fact one or more than one member host
exists for that multicast channel.
The first case may occur in an environment where the sole member host
departs the network without sending a State-Change Report message.
The router later detects that there is no member host for the
corresponding channels when it does not receive a Current-State
Report within the timeout of the response for the periodic General
Query (and then the group or source timers are expired). However,
this situation prolongs leave latency and wastes network resources
since the router forwards unneeded traffic for a while.
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The second case occurs when a router sends a specific query but does
not receive a Current-State Report from a downstream host within an
LMQT or LLQT period. It recognizes that no member host exists on the
LAN and might prune the routing path. The router reestablishes the
routing path when it receives the solicited report message for the
channels. However, the downstream hosts may loose the data packets
until the routing path is reestablished and the data forwarding is
restarted.
If operators do not believe that their link is fairly robust or that
they can configure the [Robustness Variable] value appropriately,
they may configure the LMQC/LLQC value to 2 (the default value of the
[Robustness Variable] value) or bigger value for their routers. In
this case, the routers would enable the explicit tracking function
but may want to disable the specific query suppression specified in
Section 4. Such configurations will not contribute to saving network
resources, but reduce the risk of the incorrect membership
expectation.
7. All-Zero and Unspecified Source Addresses
The IGMPv3 specification [2] mentions that an IGMPv3 report is
usually sent with a valid IP source address, yet it permits a host to
use the 0.0.0.0 source address (since the host has not yet acquired
an IP address), and routers must accept a report with this source
address.
When a router enabling the explicit tracking function receives IGMP
report messages with an all-zero source address, it deals with the
IGMP report messages correctly as defined in [2] and continuously
keeps track of the membership state. However, the router SHOULD NOT
maintain the host specifying all-zero source address in its
membership state information. The router will maintain its
membership state information by checking Current-State reports as
ordinary routers do.
On the other hand, the MLDv2 specification [3] mentions that routers
silently discard a message that is sent with an invalid link-local
address or sent with the unspecified address (::), without taking any
action, because of security considerations. According to this
specification, whether the explicit tracking function is used or not,
a router does not deal with a member hosts sending an MLD report
message with the unspecified source address.
8. Compatibility with Older Version Protocols
The explicit tracking function does not work with older versions of
IGMP or MLD, IGMPv1 [5], IGMPv2 [6], or MLDv1 [7], because a member
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host using these protocols enables "membership report suppression" by
which the host will cancel sending pending membership reports if a
similar report is observed from another member on the network.
To preserve compatibility with older versions of IGMP/MLD, routers
supporting IGMPv3/MLDv2 enable the host compatibility mode defined in
[2][3]. The host compatibility mode of an interface changes the
operational protocol version on the LAN whenever an older version
query (than the current compatibility mode) is heard or when certain
timer conditions occur. The routers can hence support downstream
hosts that are not upgraded to the latest versions and run membership
report suppression.
Therefore, if a multicast router supporting IGMPv3/MLDv2 and enabling
the explicit tracking function changes its compatibility mode to the
older versions, the router SHOULD disable the explicit tracking
function while it acts as the older version router.
9. IANA Considerations
This document has no actions for IANA.
10. Security Considerations
The explicit tracking function potentially requires a large amount of
memory so that routers keep all membership states. It gives some
impact in the cases where (1) a router attaches to a link or an IGMP/
MLD proxy device [8] that has a large number of member hosts, and a
router has insufficient memory resources to maintain a large number
of member hosts, or (2) a malicious host sends a large number of
invalid IGMP/MLD report messages.
For the first case, operators may disable the explicit tracking
function, despite the benefits mentioned above. For the second case,
some serious threats may be induced. In order to prevent abuse, a
router enabling the explicit tracking function may need to limit a
total amount of membership information the router can store and an
amount of membership information the router can store per host.
A router may rate-limit State-Change Report messages per host. When
the router enables rate-limiting per host, the router MAY ignore the
received State-Change Report messages to minimize the processing
overhead or prevent DoS attacks. The rate limit is left to the
router's implementation.
11. Acknowledgements
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Luis M. Contreras, Toerless Eckert, Sergio Figueiredo, Bharat Joshi,
Nicolai Leymann, Stig Venaas, and others provided many constructive
and insightful comments.
12. References
12.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to indicate
requirement levels", RFC 2119, March 1997.
[2] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 2002.
[3] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[4] Liu, H., Cao, W., and H. Asaeda, "Lightweight Internet
Group Management Protocol Version 3 (IGMPv3) and Multicast
Listener Discovery Version 2 (MLDv2) Protocols", RFC 5790,
February 2010.
12.2. Informative References
[5] Deering, S., "Host Extensions for IP Multicasting", RFC
1112, August 1989.
[6] Fenner, W., "Internet Group Management Protocol, Version
2", RFC 2373, July 1997.
[7] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710, October
1999.
[8] Fenner, B., He, H., Haberman, B., and H. Sandick,
"Internet Group Management Protocol (IGMP) / Multicast
Listener Discovery (MLD)-Based Multicast Forwarding ("IGMP
/MLD Proxying")", RFC 4605, August 2006.
Author's Address
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Hitoshi Asaeda
National Institute of Information and Communications Technology (NICT)
Network Research Headquarters
4-2-1 Nukui-Kitamachi
Koganei, Tokyo 184-8795
Japan
Email: asaeda@nict.go.jp
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