PIM Working Group                                              H. Asaeda
Internet-Draft                                                      NICT
Intended status: Standards Track                        January 29,                       February 25, 2013
Expires: August 2, 29, 2013

   IGMP/MLD-Based Explicit Membership Tracking Function for Multicast


   This document describes the IGMP/MLD-based explicit membership
   tracking function for multicast routers supporting IGMPv3/MLDv2.  The
   explicit tracking function contributes to saving network resources
   and fast leaves (i.e. shortening leave latency).

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Explicit Tracking Function . . . . . . . . . . . . . . . . . .  4
     3.1.  Membership State Information . . . . . . . . . . . . . . .  4
     3.2.  Specific Query Suppression . . . . . . . . . . . . . . . .  5
     3.3.  Shortening Leave Latency . . . . . . . . . . . . . . . . .  6
   4.  Lowering the Possibility of Outdated Membership State  . . . .  7
   5.  All-Zero and Unspecified Source Addresses  . . . . . . . . . .  8
   6.  Compatibility with Older Version Protocols . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     10.1. Normative References . . . . . . . . . . . . . . . . . . .  9
     10.2. Informative References . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.  Introduction

   The Internet Group Management Protocol (IGMP) version 3 [1] for IPv4
   and the Multicast Listener Discovery Protocol (MLD) version 2 [2] for
   IPv6 are the standard protocols used by member hosts and multicast
   routers.  Lightweight IGMPv3 and Lightweight MLDv2 (or LW-IGMPv3 and
   LW-MLDv2) [3] 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 [5]).  The "unsolicited" report messages are sent only when
   the host joins/leaves the channels.

   IGMP/MLD are non-reliable protocols; the unsolicited report messages
   may be lost or may not reach upstream routers.  To alleviate the
   problem, routers need to update membership information by
   periodically sending IGMP/MLD General Query messages.  Member hosts
   then reply with "solicited" report messages whenever they receive the
   query messages.

   Multicast routers are capable of periodically maintaining the
   multicast membership state of downstream hosts attached to the same
   link by acquiring unsolicited report messages and synchronizing the
   actual membership state within the General Query timer interval
   (i.e., [Query Interval] value defined in [1][2].)  However, this
   approach does not guarantee that the membership state is always
   perfectly synchronized.  To minimize the possibility of having
   outdated membership information, routers may shorten the 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.

   The IGMPv3 [1] and MLDv2 [2] protocols and these lightweight
   protocols [3] 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 details 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

   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)

   In addition, the processing of IGMP membership or MLD listener
   messages consumes CPU resources on individual IGMP/MLD querier and
   report sender devices.  The explicit tracking function therefore
   reduces not only the network load but also the CPU load on these

   The explicit tracking function does not guarantee that the membership
   state will always be perfectly synchronized; 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
   enabling the function ought 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.

   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 [1] and MLDv2 [2], and their lightweight
   version protocols [3]; nor does it change a multicast data sender's
   and receiver's behavior.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   this document are to be interpreted as described in RFC 2119 [4].

3.  Explicit Tracking Function

3.1.  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 this
   membership state information or adds or deletes the receiver IP
   address to or from it.  If there are no more receivers maintained,
   the router may keep the membership state information with an empty
   receiver list.

   The membership state information consists of the following

      (S, G, number of receivers, (receiver records))

   where each receiver record is of the form:

      (IGMP/MLD membership/listener report sender's address)

   This state information must work properly when a receiver (i.e.,
   report sender) sends the identical report messages multiple times.

   In the state information, each S and G indicates a single IPv4/IPv6
   address.  S is set to "Null" for Any-Source Multicast (ASM)
   communication (i.e., (*,G) join reception).  In order to simplify the
   implementation, the explicit tracking function MAY NOT keep the state
   of (S,G) joined with EXCLUDE filter mode.  In that case, if a router
   receives an (S,G) join/leave request with EXCLUDE filter mode from
   the downstream hosts, the router translates the request to a (*,G)
   join state/leave request and records the state and the receivers'
   addresses in the maintained membership state information.  Note that
   this membership state translation does not change the routing
   protocol behavior.  The routing protocol must deal with the original
   join/leave request and translate the request only for the membership
   state information.

3.2.  Specific Query Suppression

   In accordance with [1] and [2], whenever a router receives the State-
   Change Report, it sends the corresponding Group-Specific or Group-
   and-Source Specific Query messages to confirm whether or not the
   report sender is the sole member host.  All member hosts joining the
   identical channel send their own Current-State Report messages after
   acquiring these query messages.  Transmitting a large number of
   Current-State Report messages consumes network resources, and this
   may pose a particular problem when many hosts joining the identical
   channel send these reports simultaneously.

   The explicit tracking function can reduce the number of Group-
   Specific or Group-and-Source Specific Query messages transmitted from
   a router, and reduce the number of Current-State Report messages
   transmitted from member hosts.  If a router enables the explicit
   tracking function with "specific query suppression", it suppresses
   specific query transmission and transmits specific query messages
   only when the router expects that the State-Change Report sender is
   the sole member of the channel, based on its membership state
   information (expressed in Section 3.1).

   As standard behavior for [1] and [2], a router also sends a Group-
   Specific or Group-and-Source Specific Query multiple times when it
   receives a State Change Report message (e.g., leave request) from a
   member host.  This is in order to confirm whether or not the host is
   the sole member.  However, if the router enabling the explicit
   tracking function runs specific query suppression and receives one or
   more replies for the specific query retransmission from the
   downstream member(s), the router can cancel resending of the
   identical specific query message(s).

   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 situation in
   which multiple multicast routers exist on a LAN and the querier
   router is not the forwarder router.  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.

3.3.  Shortening Leave Latency

   A router enabling the explicit tracking function can shorten leave
   latencies by tuning several timers and values to what it expects
   whether or not the State-Change Report sender is the channel's sole

   The [Last Member Query Interval] (LMQI) and [Last Listener Query
   Interval] (LLQI) values specify the maximum time allowed for a member
   host to send a responding Report before the router prunes the channel
   from the network.  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

   The default value for LMQI/LLQI defined in the standard
   specifications [1][2] is 1 second.  For a router enabling the
   explicit tracking function, the LMQI/LLQI MAY be set to 1 second or
   shorter.  The LMQC/LLQC values MAY be set to 1 for the router,
   whereas their default values are the [Robustness Variable] value
   whose default value is 2.  Smaller LMQC/LLQC values give smaller
   LMQT/LLQT, which shortens the leave latencies.  On the other hand,
   setting smaller LMQC/LLQC values poses the risk described in
   Section 4.  Operators setting smaller LMQC/LLQC values must recognize
   this tradeoff.

4.  Lowering the Possibility of Outdated Membership State

   There are possibilities that a router enables the explicit tracking
   function but its membership expectation will be 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 more than one member host exists for that multicast
   channel.  These cases are particularly likely for a router that
   enables specific query suppression (as in Section 3.2) and configures
   small LMQC/LLQC for shortening leave latency (as in Section 3.3).

   The first of these cases may occur in an environment where the sole
   member host departs the network without sending a State-Change Report
   message.  This is because a router enabling specific query
   suppression does not send a specific query if it believes the report
   sender is not the sole member host.  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.  However, this situation prolongs
   leave latency and wastes network resources since the router forwards
   unneeded traffic until that point.

   The second case occurs when a router sends a specific query but does
   not receive a State-Change 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

   In order to reduce the possibility of the incorrect membership
   expectation and keep the up-to-date membership state information,
   when a router enabling the explicit tracking function enables
   specific query suppression, the router SHOULD configure the LMQC/LLQC
   value to 2 (the default value of the [Robustness Variable] value) or
   higher; or, when a router enabling the explicit tracking function
   configures a small LMQC/LLQC, the router SHOULD NOT enable specific
   query suppression.

5.  All-Zero and Unspecified Source Addresses

   The IGMPv3 specification [1] mentions that an IGMPv3 report is
   usually sent with a valid IP source address, yet it permits a host to
   use the source address (since the host has not yet acquired
   an IP address), and routers must accept a report with this source
   address.  The MLDv2 specification [2] mentions that an MLDv2 report
   must be sent with a valid IPv6 link-local source address, yet an
   MLDv2 report may be sent with the unspecified address (::) if the
   sending interface has not acquired a valid link-local address. [2]
   also mentions that routers silently discard a message that is not
   sent with a valid link-local address or sent with the unspecified
   address, without taking any action, because of security

   When a router enabling the explicit tracking function receives IGMP/
   MLD report messages with an all-zero or unspecified source address,
   it deals with the IGMP/MLD report messages correctly as defined in
   [1][2] and continuously keeps track of the membership state, but
   SHOULD NOT maintain the host specifying all-zero or an unspecified
   source address in its membership state information.

6.  Compatibility with Older Version Protocols

   The explicit tracking function does not work with older versions of
   IGMP or MLD, IGMPv1 [6], IGMPv2 [7], or MLDv1 [8], because a member
   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
   need to support downstream hosts that are not upgraded to the latest
   versions and run membership report suppression.  Therefore, if a
   multicast router enabling the explicit tracking function changes its
   compatibility mode to the older versions, the router SHOULD disable
   the explicit tracking function.  On the other hand, the router MAY
   NOT flush the maintained membership state information.  When the
   router changes back to IGMPv3 or MLDv2 mode, it resumes the function
   with the old membership state information even if the state
   information is outdated.  This provides "smooth state transition"
   that does not initiate the membership state information from scratch
   and synchronizes the actual membership state smoothly.

7.  IANA Considerations

   This document has no actions for IANA.

8.  Security Considerations


   The explicit tracking function potentially requires a large amount of
   memory so that routers keep all membership states.  Especially when
   malicious hosts send a large number of invalid IGMP/MLD report
   messages, some serious threats may be induced.  In order to prevent
   abuse, a router enabling the explicit tracking function MAY need to
   rate-limit a total amount of membership information the router can
   store and an amount of membership information the router can store
   per host.  The rate limit is no additional security consideration for [1][2][3] provided. left to the router's implementation.

9.  Acknowledgements

   Toerless Eckert, Sergio Figueiredo, Nicolai Leymann, Stig Venaas, and
   others provided many constructive and insightful comments.

10.  References

10.1.  Normative References

   [1]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
        Thyagarajan, "Internet Group Management Protocol, Version 3",
        RFC 3376, October 2002.

   [2]  Vida, R. and L. Costa, "Multicast Listener Discovery Version 2
        (MLDv2) for IPv6", RFC 3810, June 2004.

   [3]  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.

   [4]  Bradner, S., "Key words for use in RFCs to indicate requirement
        levels", RFC 2119, March 1997.

10.2.  Informative References

   [5]  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.

   [6]  Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
        August 1989.

   [7]  Fenner, W., "Internet Group Management Protocol, Version 2",
        RFC 2373, July 1997.

   [8]  Deering, S., Fenner, W., and B. Haberman, "Multicast Listener
        Discovery (MLD) for IPv6", RFC 2710, October 1999.

Author's Address

   Hitoshi Asaeda
   National Institute of Information and Communications Technology (NICT)
   Network Architecture Laboratory
   4-2-1 Nukui-Kitamachi
   Koganei, Tokyo  184-8795

   Email: asaeda@nict.go.jp