rfc2236.txt

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Fenner                      Standards Track                     [Page 6]

RFC 2236           Internet Group Management Protocol      November 1997


        If a router is not explicitly configured to use IGMPv1 and hears
        an IGMPv1 Query, it SHOULD log a warning.  These warnings MUST
        be rate-limited.

5.  Compatibility with IGMPv1 Hosts

   An IGMPv2 host may be placed on a subnet where there are hosts that
   have not yet been upgraded to IGMPv2.  The following requirement
   applies:

        The host MUST allow its Membership Report to be suppressed by
        either a Version 1 Membership Report or a Version 2 Membership
        Report.

   An IGMPv2 router may be placed on a subnet where there are hosts that
   have not yet been upgraded to IGMPv2.  The following requirements
   apply:

        If a router receives a Version 1 Membership Report, it MUST set
        a timer to note that there are version 1 hosts present which are
        members of the group for which it heard the report.  This timer
        should be the same as the [Group Membership Interval].

        If there are version 1 hosts present for a particular group, a
        router MUST ignore any Leave Group messages that it receives for
        that group.

6.  Host State Diagram

   Host behavior is more formally specified by the state transition
   diagram below.  A host may be in one of three possible states with
   respect to any single IP multicast group on any single network
   interface:

   - "Non-Member" state, when the host does not belong to the group on
     the interface.  This is the initial state for all memberships on
     all network interfaces; it requires no storage in the host.

   - "Delaying Member" state, when the host belongs to the group on the
     interface and has a report delay timer running for that membership.

   - "Idle Member" state, when the host belongs to the group on the
     interface and does not have a report delay timer running for that
     membership.







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RFC 2236           Internet Group Management Protocol      November 1997


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

   - "join group" occurs when the host decides to join the group on the
     interface.  It may occur only in the Non-Member state.

   - "leave group" occurs when the host decides to leave the group on
     the interface.  It may occur only in the Delaying Member and Idle
     Member states.

   - "query received" occurs when the host receives either a valid
     General Membership Query message, or a valid Group-Specific
     Membership Query message.  To be valid, the Query message must be
     at least 8 octets long, and have a correct IGMP checksum.  The
     group address in the IGMP header must either be zero (a General
     Query) or a valid multicast group address (a Group-Specific Query).
     A General Query applies to all memberships on the interface from
     which the Query is received.  A Group-Specific Query applies to
     membership in a single group on the interface from which the Query
     is received.  Queries are ignored for memberships in the Non-Member
     state.

   - "report received" occurs when the host receives a valid IGMP
     Membership Report message (Version 1 or Version 2).  To be valid,
     the Report message must be at least 8 octets long and have a
     correct IGMP checksum.  A Membership Report applies only to the
     membership in the group identified by the Membership Report, on the
     interface from which the Membership Report is received.  It is
     ignored for memberships in the Non-Member or Idle Member state.

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

   All other events, such as receiving invalid IGMP messages, or IGMP
   messages 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 group on the interface.  The type of report
     is determined by the state of the interface.  The Report Message is
     sent to the group being reported.








Fenner                      Standards Track                     [Page 8]

RFC 2236           Internet Group Management Protocol      November 1997


   - "send leave" for the group on the interface.  If the interface
     state says the Querier is running IGMPv1, this action SHOULD be
     skipped.  If the flag saying we were the last host to report is
     cleared, this action MAY be skipped.  The Leave Message is sent to
     the ALL-ROUTERS group (224.0.0.2).

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

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

   - "start timer" for the group on the interface, using a delay value
     chosen uniformly from the interval (0, Max Response Time], where
     Max Response time 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 group on the interface to a new value, using
     a delay value chosen uniformly from the interval (0, Max Response
     Time], as described in "start timer".

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

   In all of the following state 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 2236           Internet Group Management Protocol      November 1997


                              ________________
                             |                |
                             |                |
                             |                |
                             |                |
                   --------->|   Non-Member   |<---------
                  |          |                |          |
                  |          |                |          |
                  |          |                |          |
                  |          |________________|          |
                  |                   |                  |
                  | leave group       | join group       | leave group
                  | (stop timer,      |(send report,     | (send leave
                  |  send leave if    | set flag,        |  if flag set)
                  |  flag set)        | start timer)     |
          ________|________           |          ________|________
         |                 |<---------          |                 |
         |                 |                    |                 |
         |                 |<-------------------|                 |
         |                 |   query received   |                 |
         | Delaying Member |    (start timer)   |   Idle Member   |
    ---->|                 |------------------->|                 |
   |     |                 |   report received  |                 |
   |     |                 |    (stop timer,    |                 |
   |     |                 |     clear flag)    |                 |
   |     |_________________|------------------->|_________________|
   | query received    |        timer expired
   | (reset timer if   |        (send report,
   |  Max Resp Time    |         set flag)
   |  < current timer) |
    -------------------


   The all-systems group (address 224.0.0.1) is handled as a special
   case.  The host starts in Idle Member state for that group on every
   interface, never transitions to another state, and never sends a
   report for that group.

   In addition, a host may be in one of two possible states with respect
   to any single network interface:

   - "No IGMPv1 Router Present", when the host has not heard an IGMPv1
     style query for the [Version 1 Router Present Timeout].  This is
     the initial state.

   - "IGMPv1 Router Present", when the host has heard an IGMPv1 style
     query within the [Version 1 Router Present Timeout].




Fenner                      Standards Track                    [Page 10]

RFC 2236           Internet Group Management Protocol      November 1997


   There are two events that can cause state transitions:

   - "IGMPv1 query received", when the host receives a query with the
     Max Response Time field set to 0.

   - "timer expires", when the timer set to note the presence of an
     IGMPv1 router expires.

   And a single action that can be triggered by an event:

   - "set timer", setting the timer to its maximum value [Version 1
     Router Present Timeout] and (re)starting it.

                              ________________
                             |                |
                             |                |
                             |   No IGMPv1    |
                             |     Router     |
                             |    Present     |
                             |                |
                        ---->|                |----
                       |     |                |    |
                       |     |________________|    |
         timer expires |                           | IGMPv1 query
                       |      ________________     | received
                       |     |                |    | (set timer)
                       |     |                |    |
                       |     |                |    |
                        -----|     IGMPv1     |<---
                             |     Router     |
                             |    Present     |
                             |                |
                        ---->|                |----
                       |     |________________|    |
                       |                           |
                       | IGMPv1 query received     |
                       | (set timer)               |
                        ---------------------------













Fenner                      Standards Track                    [Page 11]

RFC 2236           Internet Group Management Protocol      November 1997


7.  Router State 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 network:

   - "Querier", when this router is designated to transmit IGMP
     Membership Queries on this network.

   - "Non-Querier", when there is another router designated to transmit
     IGMP membership Queries on this network.

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

   - "query timer expired" occurs when the timer set for query
     transmission expires.

   - "query received from a router with a lower IP address" occurs when
     an IGMP Membership Query is received from a router on the same
     network with a lower IP address.

   - "other querier present timer expired" occurs when the timer set to
     note the presence of another querier with a lower IP address on the
     network expires.

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

   - "start general query timer" for the attached network.

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

   - "send general query" on the attached network.  The General Query is
     sent to the all-systems group (224.0.0.1), and has a Max Response
     Time of [Query Response Interval].













Fenner                      Standards Track                    [Page 12]