rfc2710.txt

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RFC 2710         Multicast Listener Discovery for IPv6      October 1999


                                     --------------------------------
                             _______|________  gen. query timer      |
 ---------                  |                |        expired        |
| Initial |---------------->|                | (send general query,  |
 ---------  (send gen. q.,  |                |  start gen. q. timer) |
     start initial gen. q.  |                |<----------------------
             timer)         |    Querier     |
                            |                |
                       -----|                |<---
                      |     |                |    |
                      |     |________________|    |
query received from a |                           | other querier
router with a lower   |                           | present timer
IP address            |                           | expired
(start other querier  |      ________________     | (send gen. query,
 present timer)       |     |                |    | start gen. q. timer)
                      |     |                |    |
                      |     |                |    |
                       ---->|      Non       |----
                            |    Querier     |
                            |                |
                            |                |
                       ---->|                |----
                      |     |________________|    |
                      | query received from a     |
                      | router with a lower IP    |
                      | address                   |
                      | (start other querier      |
                      |  present timer)           |
                       ---------------------------

   A router starts in the Initial state on all attached links, and
   immediately transitions to Querier state.

   In addition, to keep track of which multicast addresses have
   listeners, a router may be in one of three possible states with
   respect to any single IPv6 multicast address on any single attached
   link:

   - "No Listeners Present" state, when there are no nodes on the link
      that have sent a Report for this multicast address.  This is the
      initial state for all multicast addresses on the router; it
      requires no storage in the router.

   - "Listeners Present" state, when there is a node on the link that
      has sent a Report for this multicast address.





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RFC 2710         Multicast Listener Discovery for IPv6      October 1999


   - "Checking Listeners" state, when the router has received a Done
      message but has not yet heard a Report for the identified address.

   There are five significant events that can cause router state
   transitions:

   - "report received" occurs when the router receives a Report for the
      address from the link.  To be valid, the Report message MUST come
      from a link-local IPv6 Source Address, be at least 24 octets long,
      and have a correct MLD checksum.

   - "done received" occurs when the router receives a Done message for
      the address from the link.  To be valid, the Done message MUST
      come from a link-local IPv6 Source Address, be at least 24 octets
      long, and have a correct MLD checksum.  This event is significant
      only in the "Listerners Present" state and when the router is a
      Querier.

   - "multicast-address-specific query received" occurs when a router
      receives a Multicast-Address-Specific Query for the address from
      the link.  To be valid, the Query message MUST come from a link-
      local IPv6 Source Address, be at least 24 octets long, and have a
      correct MLD checksum.  This event is significant only in the
      "Listeners Present" state and when the router is a Non-Querier.

   - "timer expired" occurs when the timer set for a multicast address
      expires.  This event is significant only in the "Listeners
      Present" or "Checking Listeners" state.

   - "retransmit timer expired" occurs when the timer set to retransmit
      a Multicast-Address-Specific Query expires.  This event is
      significant only in the "Checking Listeners" state.

   There are seven possible actions that may be taken in response to the
   above events:

   - "start timer" for the address on the link - also resets the timer
      to its initial value [Multicast Listener Interval] if the timer is
      currently running.

   - "start timer*" for the address on the link - this alternate action
      sets the timer to the minimum of its current value and either
      [Last Listener Query Interval] * [Last Listener Query Count] if
      this router is a Querier, or the Maximum Response Delay in the
      Query message * [Last Listener Query Count] if this router is a
      non-Querier.





Deering, et al.             Standards Track                    [Page 13]

RFC 2710         Multicast Listener Discovery for IPv6      October 1999


   - "start retransmit timer" for the address on the link [Last Listener
      Query Interval].

   - "clear retransmit timer" for the address on the link.

   - "send multicast-address-specific query" for the address on the
      link.  The Multicast-Address-Specific Query is sent to the address
      being queried, and has a Maximum Response Delay of [Last Listener
      Query Interval].

   - "notify routing +" internally notify the multicast routing protocol
      that there are listeners to this address on this link.

   - "notify routing -" internally notify the multicast routing protocol
      that there are no longer any listeners to this address on this
      link.

   The following state diagrams apply per group per link.  There are two
   diagrams; one for routers in Querier state and one for routers in
   Non-Querier state.  The transition between Querier and Non-Querier
   state on a link is handled specially.  All groups on that link in "No
   Listeners Present" or "Listeners Present" states switch state
   transition diagrams when the Querier/Non-Querier state transition
   occurs.  However, any groups in "Checking Listeners" state continue
   with the same state transition diagram until the "Checking Listeners"
   state is exited.  E.g. a router that starts as a Querier, receives a
   Done message for a group and then receives a Query from a router with
   a lower address (causing a transition to the Non-Querier state)
   continues to send multicast-address-specific queries for the group in
   question until it either receives a Report or its timer expires, at
   which time it starts performing the actions of a Non-Querier for this
   group.



















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RFC 2710         Multicast Listener Discovery for IPv6      October 1999


   The state transition diagram for a router in Querier state follows:

                          ________________
                         |                |
                         |                |timer expired
            timer expired|                |(notify routing -,
       (notify routing -)|  No Listeners  |clear rxmt tmr)
                 ------->|    Present     |<---------
                |        |                |          |
                |        |                |          |
                |        |________________|          |  ---------------
                |                    |               | | rexmt timer   |
                |     report received|               | |  expired      |
                |  (notify routing +,|               | | (send m-a-s   |
                |        start timer)|               | |  query,       |
      __________|______              |       ________|_|______ st rxmt |
     |                 |<------------       |                 | tmr)   |
     |                 |                    |                 |<-------
     |                 | report received    |                 |
     |                 | (start timer,      |                 |
     |                 |  clear rxmt tmr)   |                 |
     |    Listeners    |<-------------------|    Checking     |
     |     Present     | done received      |    Listeners    |
     |                 | (start timer*,     |                 |
     |                 |  start rxmt timer, |                 |
     |                 |  send m-a-s query) |                 |
 --->|                 |------------------->|                 |
|    |_________________|                    |_________________|
| report received |
| (start timer)   |
 -----------------




















Deering, et al.             Standards Track                    [Page 15]

RFC 2710         Multicast Listener Discovery for IPv6      October 1999


   The state transition diagram for a router in Non-Querier state is
   similar, but non-Queriers do not send any messages and are only
   driven by message reception.

                              ________________
                             |                |
                             |                |
                timer expired|                |timer expired
           (notify routing -)|  No Listeners  |(notify routing -)
                   --------->|    Present     |<---------
                  |          |                |          |
                  |          |                |          |
                  |          |                |          |
                  |          |________________|          |
                  |                   |                  |
                  |                   |report received   |
                  |                   |(notify routing +,|
                  |                   | start timer)     |
          ________|________           |          ________|________
         |                 |<---------          |                 |
         |                 |  report received   |                 |
         |                 |  (start timer)     |                 |
         |    Listeners    |<-------------------|     Checking    |
         |     Present     | m-a-s query rec'd  |    Listeners    |
         |                 | (start timer*)     |                 |
    ---->|                 |------------------->|                 |
   |     |_________________|                    |_________________|
   | report received |
   | (start timer)   |
    -----------------

7.  List of timers and default values

   Most of these timers are configurable.  If non-default settings are
   used, they MUST be consistent among all routers on a single link.
   Note that parentheses are used to group expressions to make the
   algebra clear.

7.1.  Robustness Variable

   The Robustness Variable allows tuning for the expected packet loss on
   a link.  If a link is expected to be lossy, the Robustness Variable
   may be increased.  MLD is robust to (Robustness Variable - 1) packet
   losses.  The Robustness Variable MUST NOT be zero, and SHOULD NOT be
   one.  Default: 2






Deering, et al.             Standards Track                    [Page 16]

RFC 2710         Multicast Listener Discovery for IPv6      October 1999


7.2.  Query Interval

   The Query Interval is the interval between General Queries sent by
   the Querier.  Default: 125 seconds.

   By varying the [Query Interval], an administrator may tune the number
   of MLD messages on the link; larger values cause MLD Queries to be
   sent less often.

7.3.  Query Response Interval

   The Maximum Response Delay inserted into the periodic General
   Queries.  Default: 10000 (10 seconds)

   By varying the [Query Response Interval], an administrator may tune
   the burstiness of MLD messages on the link; larger values make the
   traffic less bursty, as node responses are spread out over a larger
   interval.  The number of seconds represented by the [Query Response
   Interval] must be less than the [Query Interval].

7.4.  Multicast Listener Interval

   The Multicast Listener Interval is the amount of time that must pass
   before a router decides there are no more listeners for an address on
   a link.  This value MUST be ((the Robustness Variable) times (the