rfc2236.txt

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RFC 2236           Internet Group Management Protocol      November 1997


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

   A router should start in the Initial state on all attached networks,
   and immediately move to Querier state.

   In addition, to keep track of which groups have members, a router may
   be in one of four possible states with respect to any single IP
   multicast group on any single attached network:

   - "No Members Present" state, when there are no hosts on the network
     which have sent reports for this multicast group.  This is the
     initial state for all groups on the router; it requires no storage
     in the router.

   - "Members Present" state, when there is a host on the network which
     has sent a Membership Report for this multicast group.






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RFC 2236           Internet Group Management Protocol      November 1997


   - "Version 1 Members Present" state, when there is an IGMPv1 host on
     the network which has sent a Version 1 Membership Report for this
     multicast group.

   - "Checking Membership" state, when the router has received a
     Leave Group message but has not yet heard a Membership Report for
     the multicast group.

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

   - "v2 report received" occurs when the router receives a Version 2
     Membership Report for the group on the interface.  To be valid, the
     Report message must be at least 8 octets long and must have a
     correct IGMP checksum.

   - "v1 report received" occurs when the router receives a Version 1
     Membership report for the group on the interface.  The same
     validity requirements apply.

   - "leave received" occurs when the router receives an IGMP Group
     Leave message for the group on the interface.  To be valid, the
     Leave message must be at least 8 octets long and must have a
     correct IGMP checksum.

   - "timer expired" occurs when the timer set for a group membership
     expires.

   - "retransmit timer expired" occurs when the timer set to retransmit
     a group-specific Membership Query expires.

   - "v1 host timer expired" occurs when the timer set to note the
     presence of version 1 hosts as group members expires.

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

   - "start timer" for the group membership on the interface - also
     resets the timer to its initial value [Group Membership Interval]
     if the timer is currently running.

   - "start timer*" for the group membership on the interface - this
     alternate action sets the timer to [Last Member Query Interval] *
     [Last Member Query Count] if this router is a Querier, or the [Max
     Response Time] in the packet * [Last Member Query Count] if this
     router is a non-Querier.





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RFC 2236           Internet Group Management Protocol      November 1997


   - "start retransmit timer" for the group membership on the interface
     [Last Member Query Interval].

   - "start v1 host timer" for the group membership on the interface,
     also resets the timer to its initial value [Group Membership
     Interval] if the timer is currently running.

   - "send group-specific query" for the group on the attached network.
     The Group-Specific Query is sent to the group being queried, and
     has a Max Response Time of [Last Member Query Interval].

   - "notify routing +" notify the routing protocol that there are
     members of this group on this connected network.

   - "notify routing -" notify the routing protocol that there are no
     longer any members of this group on this connected network.

     The state diagram for a router in Querier state follows:

































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RFC 2236           Internet Group Management Protocol      November 1997


                              ________________
 ----------------------------|                |<-----------------------
|                            |                |timer expired           |
|               timer expired|                |(notify routing -,      |
|          (notify routing -)|   No Members   |clear rxmt tmr)         |
|                    ------->|    Present     |<-------                |
|                   |        |                |       |                |
|v1 report rec'd    |        |                |       |  ------------  |
|(notify routing +, |        |________________|       | | rexmt timer| |
| start timer,      |                    |            | |  expired   | |
| start v1 host     |  v2 report received|            | | (send g-s  | |
|  timer)           |  (notify routing +,|            | |  query,    | |
|                   |        start timer)|            | |  st rxmt   | |
|         __________|______              |       _____|_|______  tmr)| |
|        |                 |<------------       |              |     | |
|        |                 |                    |              |<----- |
|        |                 | v2 report received |              |       |
|        |                 | (start timer)      |              |       |
|        | Members Present |<-------------------|    Checking  |       |
|  ----->|                 | leave received     |   Membership |       |
| |      |                 | (start timer*,     |              |       |
| |      |                 |  start rexmt timer,|              |       |
| |      |                 |  send g-s query)   |              |       |
| |  --->|                 |------------------->|              |       |
| | |    |_________________|                    |______________|       |
| | |v2 report rec'd |  |                          |                   |
| | |(start timer)   |  |v1 report rec'd           |v1 report rec'd    |
| |  ----------------   |(start timer,             |(start timer,      |
| |v1 host              | start v1 host timer)     | start v1 host     |
| |tmr    ______________V__                        | timer)            |
| |exp'd |                 |<----------------------                    |
|  ------|                 |                                           |
|        |    Version 1    |timer expired                              |
|        | Members Present |(notify routing -)                         |
 ------->|                 |-------------------------------------------
         |                 |<--------------------
 ------->|_________________| v1 report rec'd     |
| v2 report rec'd |   |   (start timer,          |
| (start timer)   |   |    start v1 host timer)  |
 -----------------     --------------------------











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RFC 2236           Internet Group Management Protocol      November 1997


   The state 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.Note that non-Queriers do not care whether a
   Membership Report message is Version 1 or Version 2.

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

8.  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.

8.1.  Robustness Variable

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





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RFC 2236           Internet Group Management Protocol      November 1997


8.2.  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 IGMP messages on the subnet; larger values cause IGMP Queries to
   be sent less often.

8.3.  Query Response Interval

   The Max Response Time inserted into the periodic General Queries.
   Default: 100 (10 seconds)

   By varying the [Query Response Interval], an administrator may tune
   the burstiness of IGMP messages on the subnet; larger values make the
   traffic less bursty, as host 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].

8.4.  Group Membership Interval

   The Group Membership Interval is the amount of time that must pass
   before a multicast router decides there are no more members of a
   group on a network.  This value MUST be ((the Robustness Variable)
   times (the Query Interval)) plus (one Query Response Interval).

8.5.  Other Querier Present Interval

   The Other Querier Present Interval is the length of time that must
   pass before a multicast router decides that there is no longer
   another multicast router which should be the querier.  This value
   MUST be ((the Robustness Variable) times (the Query Interval)) plus
   (one half of one Query Response Interval).

8.6.  Startup Query Interval

   The Startup Query Interval is the interval between General Queries
   sent by a Querier on startup.  Default: 1/4 the Query Interval.

8.7.  Startup Query Count

   The Startup Query Count is the number of Queries sent out on startup,
   separated by the Startup Query Interval.  Default: the Robustness
   Variable.






Fenner                      Standards Track                    [Page 18]