PIM Working Group                                              H. Asaeda
Internet-Draft                                                      NICT
Intended status: Standards Track                         August 31, 2014
Expires:                           March 4, 9, 2015
Expires: September 10, 2015

   IGMP/MLD-Based Explicit Membership Tracking Function for Multicast


   This document describes the IGMP/MLD-based explicit membership
   tracking function for multicast routers and IGMP/MLD proxy devices
   supporting IGMPv3/MLDv2.  The explicit membership tracking function
   contributes to saving network resources and shortening leave latency.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Membership State Information  . . . . . . . . . . . . . . . .   4
   4.  Specific Query Suppression  . . . . . . . . . . . . . . . . .   5
   5.  Shortening Leave Latency  . . . . . . . . . . . . . . . . . .   6
   6.  Risk of Wrong Membership State  . . . . . . . . . . . . . . .   7
   7.  All-Zero and Unspecified Source Addresses . . . . . . . . . .   7
   8.  Compatibility with Older Version Protocols  . . . . . . . . .   8
   9.  Interoperability  . . . . . . . . . . . . . . . . . . . . . .   8
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   11. Security Considerations . . . . . . . . . . . . . . . . . . .   9
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     13.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

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 this problem, unsolicited
   report messages are retransmitted a number of times according to the
   value of the [Robustness Variable] defined in [2][3].

   In addition, a querier router periodically sends IGMP/MLD General
   Query messages every 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
   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

   IGMPv3 [2], MLDv2 [3], and these lightweight protocols [4] can
   provide the ability to keep track of the downstream (adjacent)
   multicast membership state in 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.

   The explicit tracking function is important for the scalability of
   multicast networks, and might be widely implemented in modern
   multicast routers.  However, it could seriously break IGMP/MLD
   communications if not implemented or configured correctly.  For
   example, the explicit tracking function is useful for shortening
   leave latency, while wrong implementations or configurations in
   routers on a LAN may not work properly that the operators expect.

   This document aims to get the explicit tracking function correctly.
   Regarding the way for shortening leave latency, it specifies the way
   to do it by tuning the values used by IGMPv3 [2], MLDv2 [3], and
   their lightweight version protocols [4], or by enabling the mechanism
   called "specific query suppression" with a robust link state.  The
   latter mechanism does not make the router 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.

   This document describes the risk of having wrong membership state as
   well.  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.  This document guides for setting up
   appropriate values or mechanisms used with the explicit tracking
   function in routers.

   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 might be sensitive.  As the security
   consideration, this document describes that operators may decide to
   disable this function when their routers have insufficient memory
   resources, despite the benefits.

   The explicit tracking function does not change message formats used
   by IGMPv3 [2], MLDv2 [3], and their lightweight version protocols
   [4]; nor does it change a multicast data sender's and receiver's

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 consists of the following

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

   where "S" denotes source address, "G" denotes group or multicast
   address, and each receiver record is of the form:

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

   In the state information, each S and G indicates a single IPv4/IPv6
   address.  S is set to "All sources" for Any-Source Multicast (ASM)
   communication (i.e., (*,G) join reception).  In order to simplify the
   implementation, Lightweight-IGMPv3/MLDv2 [4] do not keep the state of
   (S,G) joined with EXCLUDE filter mode; 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.

   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.  And the maintained membership state
   information will be flushed when the router reboots or restarts the
   multicast routing processes.

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 as defined
   in 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 with 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.

   The explicit tracking function provides a mechanism called "specific
   query suppression".  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 SHOULD stop (i.e., cancel)
   retransmitting the specific query message(s) for the specified source
   and/or group.  It reduces the number of Group-Specific or Group-and-
   Source Specific Query messages transmitted from a router, and in turn
   reduces the number of Current-State Report messages transmitted from
   member hosts.  This contributes to saving network resources.

   The specific query suppression MAY define an option called "robust
   link state".  A router enabling the specific query suppression with a
   robust link 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 with a robust link 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.
   Then the routers (including the forwarder) on the same LAN do not
   receive a Current-State Report message from the corresponding member
   hosts.  The forwarder in this case may prune the routing path,
   although 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
   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

   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

   Furthermore, if operators are confident that their link is fairly
   robust (e.g., the [Robustness Variable] value is appropriately
   configured so that the chances of unsolicited messages being lost are
   sufficiently low), and if the querier router always acts as the
   forwarder router for all multicast channels in the LAN, 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 with a robust link state (Section 4) for their routers.

   Note that setting smaller LMQC/LLQC and shorter LMQI/LLQI values or
   adopting the specific query suppression with a robust link state
   poses the risk of wrong membership state described in Section 6.

   Operators setting these values or enabling that mechanism 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.

   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

   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

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 source address (since the host has not yet acquired
   an IP address), and routers must accept a report with this source

   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
   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.  Interoperability

   There might be various ways to implement the explicit tracking
   function.  Some existing implementations may not implement the
   mechanisms such as specific query suppression described in this
   document.  Yet, the explicit tracking function does not change on-
   wire behavior, and the function or mechanisms described in this
   document do not break the interoperability between the existing
   implementations and the implementation based on this specification.

   On the other hand, for the future implementation for the explicit
   tracking function, since this document specifies the minimum but
   effective sets of the explicit tracking function, it is RECOMMENDED
   to refer and follow this specification as the standard implementation
   for that function.

10.  IANA Considerations

   This document has no actions for IANA.

11.  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 State-Change Report messages without any intent to
   join the specified channels.

   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.  For instance;

   1.  Transmitting a large number of invalid IGMP/MLD report messages
       consumes network resources.

   2.  Keeping a large number of invalid membership states on a router
       consumes the router's memory resources.

   3.  Dealing with a large number of invalid membership states on a
       router consumes the router's CPU resources.

   In order to mitigate such threats, a router enabling the explicit
   tracking function may limit a total amount of membership information
   the router can store, or 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.

12.  Acknowledgements

   Luis M.  Contreras, Toerless Eckert, Adrian Farrel, Sergio
   Figueiredo, Bharat Joshi, Nicolai Leymann, Magnus Nystrom, Stig
   Venaas, and others provided many constructive and insightful

13.  References

13.1.  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.

13.2.  Informative References

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

   [6]        Fenner, W., "Internet Group Management Protocol, Version
              2", RFC 2236, November 1997.

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

   [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

   Hitoshi Asaeda
   National Institute of Information and Communications Technology
   4-2-1 Nukui-Kitamachi
   Koganei, Tokyo  184-8795

   Email: asaeda@nict.go.jp