rfc2710.txt

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   that address, thus suppressing duplicate reports on the link.

   When a router receives a Report from a link, if the reported address
   is not already present in the router's list of multicast address
   having listeners on that link, the reported address is added to the
   list, its timer is set to [Multicast Listener Interval], and its
   appearance is made known to the router's multicast routing component.
   If a Report is received for a multicast address that is already
   present in the router's list, the timer for that address is reset to
   [Multicast Listener Interval].  If an address's timer expires, it is
   assumed that there are no longer any listeners for that address
   present on the link, so it is deleted from the list and its
   disappearance is made known to the multicast routing component.

   When a node starts listening to a multicast address on an interface,
   it should immediately transmit an unsolicited Report for that address
   on that interface, in case it is the first listener on the link.  To
   cover the possibility of the initial Report being lost or damaged, it
   is recommended that it be repeated once or twice after short delays
   [Unsolicited Report Interval].  (A simple way to accomplish this is
   to send the initial Report and then act as if a Multicast-Address-
   Specific Query was received for that address, and set a timer
   appropriately).





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RFC 2710         Multicast Listener Discovery for IPv6      October 1999


   When a node ceases to listen to a multicast address on an interface,
   it SHOULD send a single Done message to the link-scope all-routers
   multicast address (FF02::2), carrying in its Multicast Address field
   the address to which it is ceasing to listen.  If the node's most
   recent Report message was suppressed by hearing another Report
   message, it MAY send nothing, as it is highly likely that there is
   another listener for that address still present on the same link.  If
   this optimization is implemented, it MUST be able to be turned off
   but SHOULD default to on.

   When a router in Querier state receives a Done message from a link,
   if the Multicast Address identified in the message is present in the
   Querier's list of addresses having listeners on that link, the
   Querier sends [Last Listener Query Count] Multicast-Address-Specific
   Queries, one every [Last Listener Query Interval] to that multicast
   address.  These Multicast-Address-Specific Queries have their Maximum
   Response Delay set to [Last Listener Query Interval].  If no Reports
   for the address are received from the link after the response delay
   of the last query has passed, the routers on the link assume that the
   address no longer has any listeners there; the address is therefore
   deleted from the list and its disappearance is made known to the
   multicast routing component.  This process is continued to its
   resolution (i.e. until a Report is received or the last Multicast-
   Address-Specific Query is sent with no response) despite any
   transition from Querier to Non-Querier on this link.

   Routers in Non-Querier state MUST ignore Done messages.

   When a router in Non-Querier state receives a Multicast-Address-
   Specific Query, if its timer value for the identified multicast
   address is greater than [Last Listener Query Count] times the Maximum
   Response Delay specified in the message, it sets the address's timer
   to that latter value.

5.  Node State Transition Diagram

   Node behavior is more formally specified by the state transition
   diagram below.  A node may be in one of three possible states with
   respect to any single IPv6 multicast address on any single interface:

   - "Non-Listener" state, when the node is not listening to the address
      on the interface (i.e., no upper-layer protocol or application has
      requested reception of packets to that multicast address).  This
      is the initial state for all multicast addresses on all
      interfaces; it requires no storage in the node.






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RFC 2710         Multicast Listener Discovery for IPv6      October 1999


   - "Delaying Listener" state, when the node is listening to the
      address on the interface and has a report delay timer running for
      that address.

   - "Idle Listener" state, when the node is listening to the address on
      the interface and does not have a report delay timer running for
      that address.

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

   - "start listening" occurs when the node starts listening to the
      address on the interface.  It may occur only in the Non-Listener
      state.

   - "stop listening" occurs when the node stops listening to the
      address on the interface.  It may occur only in the Delaying
      Listener and Idle Listener states.

   - "query received" occurs when the node receives either a valid
      General Query message, or a valid Multicast-Address-Specific Query
      message.  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.  The Multicast Address field in the MLD
      message must contain either zero (a General Query) or a valid
      multicast address (a Multicast- Address-Specific Query).  A
      General Query applies to all multicast addresses on the interface
      from which the Query is received.  A Multicast-Address-Specific
      Query applies to a single multicast address on the interface from
      which the Query is received.  Queries are ignored for addresses in
      the Non-Listener state.

   - "report received" occurs when the node receives a valid MLD Report
      message.  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.  A Report applies only to the address
      identified in the Multicast Address field of the Report, on the
      interface from which the Report is received.  It is ignored in the
      Non-Listener or Idle Listener state.

   - "timer expired" occurs when the report delay timer for the address
      on the interface expires.  It may occur only in the Delaying
      Listener state.








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RFC 2710         Multicast Listener Discovery for IPv6      October 1999


   All other events, such as receiving invalid MLD messages or MLD
   message types other than Query or Report, are ignored in all states.

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

   - "send report" for the address on the interface.  The Report message
      is sent to the address being reported.

   - "send done" for the address on the interface.  If the flag saying
      we were the last node to report is cleared, this action MAY be
      skipped.  The Done message is sent to the link-scope all-routers
      address (FF02::2).

   - "set flag" that we were the last node to send a report for this
      address.

   - "clear flag" since we were not the last node to send a report for
      this address.

   - "start timer" for the address on the interface, using a delay value
      chosen uniformly from the interval [0, Maximum Response Delay],
      where Maximum Response Delay is specified in the Query.  If this
      is an unsolicited Report, the timer is set to a delay value chosen
      uniformly from the interval [0, [Unsolicited Report Interval] ].

   - "reset timer" for the address on the interface to a new value,
      using a delay value chosen uniformly from the interval [0, Maximum
      Response Delay], as described in "start timer".

   - "stop timer" for the address on the interface.

   In all of the following state transition diagrams, each state
   transition arc is labeled with the event that causes the transition,
   and, in parentheses, any actions taken during the transition.  Note
   that the transition is always triggered by the event; even if the
   action is conditional, the transition still occurs.














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RFC 2710         Multicast Listener Discovery for IPv6      October 1999


                             ________________
                            |                |
                            |                |
                            |                |
                            |                |
                  --------->|  Non-Listener  |<---------
                 |          |                |          |
                 |          |                |          |
                 |          |                |          |
                 |          |________________|          |
                 |                   |                  |
                 | stop listening    | start listening  | stop listening
                 | (stop timer,      |(send report,     | (send done if
                 |  send done if     | set flag,        |  flag set)
                 |  flag set)        | start timer)     |
         ________|________           |          ________|________
        |                 |<---------          |                 |
        |                 |                    |                 |
        |                 |<-------------------|                 |
        |                 |   query received   |                 |
        |     Delaying    |    (start timer)   |      Idle       |
   ---->|     Listener    |------------------->|     Listener    |
  |     |                 |   report received  |                 |
  |     |                 |    (stop timer,    |                 |
  |     |                 |     clear flag)    |                 |
  |     |_________________|------------------->|_________________|
  | query received    |        timer expired
  | (reset timer if   |        (send report,
  |  Max Resp Delay   |         set flag)
  |  < current timer) |
   -------------------

   The link-scope all-nodes address (FF02::1) is handled as a special
   case.  The node starts in Idle Listener state for that address on
   every interface, never transitions to another state, and never sends
   a Report or Done for that address.

   MLD messages are never sent for multicast addresses whose scope is 0
   (reserved) or 1 (node-local).

   MLD messages ARE sent for multicast addresses whose scope is 2
   (link-local), including Solicited-Node multicast addresses [ADDR-
   ARCH], except for the link-scope, all-nodes address (FF02::1).








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RFC 2710         Multicast Listener Discovery for IPv6      October 1999


6.  Router State Transition Diagram

   Router behavior is more formally specified by the state transition
   diagrams below.

   A router may be in one of two possible states with respect to any
   single attached link:

   - "Querier", when this router is designated to transmit MLD Queries
      on this link.

   - "Non-Querier", when there is another router designated to transmit
      MLD Queries on this link.

   The following three events can cause the router to change states:

   - "query timer expired" occurs when the timer set for query
      transmission expires.  This event is significant only when in the
      Querier state.

   - "query received from a router with a lower IP address" occurs when
      a valid MLD Query is received from a router on the same link with
      a lower IPv6 Source Address. 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.

   - "other querier present timer expired" occurs when the timer set to
      note the presence of another querier with a lower IP address on
      the link expires.  This event is significant only when in the
      Non-Querier state.

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

   - "start general query timer" for the attached link to [Query
      Interval].

   - "start other querier present timer" for the attached link to [Other
      Querier Present Interval].

   - "send general query" on the attached link.  The General Query is
      sent to the link-scope all-nodes address (FF02::1), and has a
      Maximum Response Delay of [Query Response Interval].








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