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Versions: 00 01 02 03 04 05 06 07 RFC 2236

Internet Engineering Task Force Inter-Domain Multicast Routing Working Group
INTERNET-DRAFT                                                     W. Fenner
draft-ietf-idmr-igmp-v2-05.txt                                    Xerox PARC
Obsoletes: Appendix I of RFC1112                            October 24, 1996
                                                            Expires: 4/31/97


             Internet Group Management Protocol, Version 2


Status of this Memo

This document is an Internet Draft.   Internet Drafts are working
documents of the Internet Engineering  Task Force (IETF), its  Areas,
and its  Working Groups.   Note that other  groups may also  distribute
working documents  as Internet Drafts).

Internet Drafts  are draft  documents valid  for a  maximum of  six
months.  Internet Drafts may be  updated, replaced, or  obsoleted by
other  documents at any time.   It  is not appropriate  to use Internet
Drafts as  reference material or  to cite  them  other than  as a
"working  draft" or  "work  in progress."

Please check  the I-D  abstract  listing contained  in each  Internet
Draft directory to learn the current status of this or any other
Internet Draft.

Distribution of this document is unlimited.

                                Abstract

     This draft documents IGMPv2, used by IP hosts to report their
     multicast group memberships to routers.  It replaces Appendix I of
     RFC1112.

     IGMPv2 allows group membership termination to be quickly reported
     to the routing protocol, which is important for high-bandwidth
     multicast groups and/or subnets with highly volatile group
     membership.

This document is a product of the Inter-Domain Multicast Routing working
group within the Internet Engineering Task Force.  Comments are
solicited and should be addressed to the working group's mailing list at
idmr@cs.ucl.ac.uk and/or the author(s).







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1.  Introduction

The Internet Group Management Protocol (IGMP) is used by IP hosts to
report their multicast group memberships to any immediately-neighboring
multicast routers.  This memo describes only the use of IGMP between
hosts and routers to determine group membership.  Routers that are
members of multicast groups are expected to behave as hosts as well as
routers, and may even respond to their own queries.  IGMP may also be
used between routers, but such use is not specified here.

Like ICMP, IGMP is a integral part of IP.  It is required to be
implemented by all hosts wishing to receive IP multicasts.  IGMP
messages are encapsulated in IP datagrams, with an IP protocol number of
2.  All IGMP messages described in this document are sent with IP TTL 1,
and contain the IP Router Alert option [Katz96] in their IP header.  All
IGMP messages of concern to hosts have the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Type     | Max Resp Time |           Checksum            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Group Address                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


1.1.  Type

     There are three types of IGMP messages of concern to the host-
     router interaction:

     0x11 = Membership Query
          There are two sub-types of Membership Query messages:
          - General Query, used to learn which groups have members on an
            attached network.
          - Group-Specific Query, used to learn if a particular group
            has any members on an attached network.
          These two messages are differentiated by the Group Address, as
          described in section 1.4 .  Membership Query messages are
          referred to simply as "Query" messages.

     0x16 = Version 2 Membership Report

     0x17 = Leave Group

     There is an additional type of message, for backwards-compatibility
     with IGMPv1:




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     0x12 = Version 1 Membership Report

     This document refers to Membership Reports simply as "Reports".
     When no version is specified, the statement applies equally to both
     versions.

     Unrecognized message types should be silently ignored.  New message
     types may be used by newer versions of IGMP, by multicast routing
     protocols, or other uses.

1.2.  Max Response Time

     The Max Response Time field is meaningful only in Membership Query
     messages, and specifies the maximum allowed time before sending a
     responding report in units of 1/10 second.  In all other messages,
     it is set to zero by the sender and ignored by receivers.

     Varying this setting allows IGMPv2 routers to tune the "leave
     latency" (the time between the moment the last host leaves a group
     and when the routing protocol is notified that there are no more
     members), as discussed in section 7.8.  It also allows tuning of
     the burstiness of IGMP traffic on a subnet, as discussed in section
     7.3.

1.3.  Checksum

     The checksum is the 16-bit one's complement of the one's complement
     sum of the whole IGMP message (the entire IP payload).  For
     computing the checksum, the checksum field is set to zero.  When
     receiving packets, the checksum MUST be verified before processing
     a packet.

1.4.  Group Address

     In a Membership Query message, the group address field is set to
     zero when sending a General Query, and set to the group address
     being queried when sending a Group-Specific Query.

     In a Membership Report or Leave Group message, the group address
     field holds the IP multicast group address of the group being
     reported or left.

1.5.  Other fields

     Note that IGMP messages may be longer than 8 octets, especially
     future backwards-compatible versions of IGMP.  As long as the Type
     is one that is recognized, an IGMPv2 implementation MUST ignore
     anything past the first 8 octets while processing the packet.



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     However, the IGMP checksum is always computed over the whole IP
     payload, not just over the first 8 octets.

2.  Protocol Description

Note that defaults for timer values are described later in this
document.  Timer and counter names appear in square brackets.

The term "interface" is sometimes used in this document to mean "the
primary interface on an attached network"; if a router has multiple
physical interfaces on a single network this protocol need only run on
one of them.  Hosts, on the other hand, need to perform their actions on
all interfaces that have memberships associated with them.

Multicast routers use IGMP to learn which groups have members on each of
their attached physical networks.  A multicast router keeps a list of
multicast group memberships for each attached network, and a timer for
each membership.  "Multicast group memberships" means the presence of at
least one member of a multicast group on a given attached network, not a
list of all of the members.  With respect to each of its attached
networks, a multicast router may assume one of two roles: Querier or
Non-Querier.  There is normally only one Querier per physical network.
All multicast routers start up as a Querier on each attached network.
If a multicast router hears a Query message from a router with a lower
IP address, it MUST become a Non-Querier on that network.  If a router
has not heard a Query message from another router for [Other Querier
Present Interval], it resumes the role of Querier.  Routers periodically
[Query Interval] send a General Query on each attached network for which
this router is the Querier, to solicit membership information.  On
startup, a router SHOULD send [Startup Query Count] General Queries
spaced closely together [Startup Query Interval] in order to quickly and
reliably determine membership information.  A General Query is addressed
to the all-systems multicast group (224.0.0.1), has a Group Address
field of 0, and has a Max Response Time of [Query Response Interval].

When a host receives a General Query, it sets delay timers for each
group of which it is a member on the interface from which it received
the query.  Each timer is set to a different random value, using the
highest clock granularity available on the host, selected from the range
(0, Max Response Time] with Max Response Time as specified in the Query
packet.  When a host receives a Group-Specific Query, it sets a delay
timer to a random value selected from the range (0, Max Response Time]
as above for the group being queried if it is a member on the interface
from which it received the query.  If a timer for the group is already
running, it is reset to the random value only if the requested Max
Response Time is less than the remaining value of the running timer.
When a group's timer expires, the host multicasts a Version 2 Membership
Report to the group, with IP TTL of 1.  If the host receives another



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host's Report (version 1 or 2) while it has a timer running, it stops
its timer for the specified group and does not send a Report, in order
to suppress duplicate Reports.

When a router receives a Report, it adds the group being reported to the
list of multicast group memberships on the network on which it received
the Report and sets the timer for the membership to the [Group
Membership Interval].  Repeated Reports refresh the timer.  If no
Reports are received for a particular group before this timer has
expired, the router assumes that the group has no local members and that
it need not forward remotely-originated multicasts for that group onto
the attached network.

When a host joins a multicast group, it should immediately transmit an
unsolicited Version 2 Membership Report for that group, in case it is
the first member of that group on the network.  To cover the possibility
of the initial Membership 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 Version 2 Membership Report and then act as if a
Group-Specific Query was received for that group, and set a timer
appropriately).

When a host leaves a multicast group, if it was the last host to reply
to a Query with a Membership Report for that group, it SHOULD send a
Leave Group message to the all-routers multicast group (224.0.0.2). If
it was not the last host to reply to a Query, it MAY send nothing as
there must be another member on the subnet.  This is an optimization to
reduce traffic; a host without sufficient storage to remember whether or
not it was the last host to reply MAY always send a Leave Group message
when it leaves a group.  Routers SHOULD accept a Leave Group message
addressed to the group being left, in order to accommodate
implementations of an earlier version of this standard.  Leave Group
messages are addressed to the all-routers group because other group
members have no need to know that a host has left the group, but it does
no harm to address the message to the group.

When a Querier receives a Leave Group message for a group that has group
members on the reception interface, it sends [Last Member Query Count]
Group-Specific Queries every [Last Member Query Interval] to the group
being left.  These Group-Specific Queries have their Max Response time
set to [Last Member Query Interval].  If no Reports are received after
the response time of the last query expires, the routers assume that the
group has no local members, as above.  Any Querier to non-Querier
transition is ignored during this time; the same router keeps sending
the Group-Specific Queries.

Non-Queriers MUST ignore Leave Group messages, and Queriers SHOULD



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ignore Leave Group messages for which there are no group members on the
reception interface.

When a non-Querier receives a Group-Specific Query message, if its
existing group membership timer is greater than [Last Member Query
Count] times the Max Response Time specified in the message, it sets its
group membership timer to that value.

3.  Compatibility with IGMPv1 Routers

An IGMPv2 host may be placed on a subnet where the Querier router has
not yet been upgraded to IGMPv2.  The following requirements apply:

     The IGMPv1 router will send General Queries with the Max Response
     Time set to 0.  This MUST be interpreted as a value of 100 (10
     seconds).

     The IGMPv1 router expects Version 1 Membership Reports in response
     to its Queries, and will not pay attention to Version 2 Membership
     Reports.  Therefore, a state variable MUST be kept for each
     interface, describing whether the multicast Querier on that
     interface is running IGMPv1 or IGMPv2.  This variable MUST be based
     upon whether or not an IGMPv1 query was heard in the last [Version
     1 Router Present Timeout] seconds, and MUST NOT be based upon the
     type of the last Query heard.  This state variable MUST be used to
     decide what type of Membership Reports to send for unsolicited
     Membership Reports as well as Membership Reports in response to
     Queries.

     An IGMPv2 host MAY suppress Leave Group messages on a network where
     the Querier is using IGMPv1.

An IGMPv2 router may be placed on a subnet where at least one router on
the subnet has not yet been upgraded to IGMPv2.  The following
requirements apply:

     If any IGMPv1 routers are present, the querier MUST use IGMPv1.
     This must be administratively assured; routers that desire to be
     compatible with IGMPv1 MUST have a configuration option to send
     IGMPv1 queries.  When in IGMPv1 mode, routers MUST send Periodic
     Queries with a Max Response Time of 0, and MUST ignore Leave Group
     messages.  They SHOULD also warn about receiving an IGMPv2 query,
     although such warnings MUST be rate-limited.

     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.




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4.  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.

5.  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.

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



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  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.

- "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] ].



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- "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.

                              ________________
                             |                |
                             |                |
                             |                |
                             |                |
                   --------->|   Non-Member   |<---------
                  |          |                |          |
                  |          |                |          |
                  |          |                |          |
                  |          |________________|          |
                  |                   |                  |
                  | leave group       | join group       | leave group
                  | (stop timer,      |(send report,     | (send leave if
                  |  send leave if    | set flag,        |  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



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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].

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.



























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


6.  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



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  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].

































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                                      --------------------------------
                              _______|________  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 expired
 IP address            |                           | (send general query,
 (set other querier    |      ________________     |  set gen. q. timer)
  present 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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- "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.

- "start retransmit timer" for the group membership on the interface



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  [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.




































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The state diagram for a router in Querier state follows:

                              ________________
 ----------------------------|                |<-----------------------------
|                            |                |                              |
|               timer expired|                |timer expired                 |
|          (notify routing -)|   No Members   |(notify routing -,            |
|                    ------->|    Present     |<---------  clear rxmt tmr)   |
|                   |        |                |          |                   |
|v1 report rec'd    |        |                |          |                   |
|(notify routing +, |        |________________|          |  ---------------  |
| start timer,      |                    |               | | rexmt timer   | |
| start v1 host     |  v2 report received|               | |  expired      | |
|  timer)           |  (notify routing +,|               | | (send g-s     | |
|                   |        start timer)|               | |  query,       | |
|         __________|______              |       ________|_|______ 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 timer)    |
| |tmr    ______________V__                        |                         |
| |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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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 re-
ception.  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)   |
    -----------------




















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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
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

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

7.3.  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 lest bursty, as host responses are spread out over a larger
interval.  The number of seconds represented by the [Query Response
Interval] must me less than the [Query Interval].

7.4.  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).

7.5.  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



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Robustness Variable) times (the Query Interval)) plus (one half of one
Query Response Interval).

7.6.  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.

7.7.  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.

7.8.  Last Member Query Interval

The Last Member Query Interval is the Max Response Time inserted into
Group-Specific Queries sent in response to Leave Group messages, and is
also the amount of time between Group-Specific Query messages.  Default:
10 (1 second)

This value may be tuned to modify the "leave latency" of the network.  A
reduced value results in reduced time to detect the loss of the last
member of a group.

7.9.  Last Member Query Count

The Last Member Query Count is the number of Group-Specific Queries sent
before the router assumes there are no local members.  Default: the
Robustness Variable.

7.10.  Unsolicited Report Interval

The Unsolicited Report Interval is the time between repetitions of a
host's initial report of membership in a group.  Default: 10 seconds.

7.11.  Version 1 Router Present Timeout

The Version 1 Router Present Timeout is how long a host must wait after
hearing a Version 1 Query before it may send any IGMPv2 messages.
Value: 400 seconds.

8.  Message destinations

This information is provided elsewhere in the document, but is
summarized here for convenience.





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Message Type                  Destination Group
------------                  -----------------
General Query                 ALL-SYSTEMS (224.0.0.1)
Group-Specific Query          The group being queried
Membership Report             The group being reported
Leave Message                 ALL-ROUTERS (224.0.0.2)
                              (some implementations use the group being left)


9.  Security Considerations

We consider the ramifications of a forged message of each type.

Query Message:

     A forged Query message from a machine with a lower IP address than
     the current Querier will cause Querier duties to be assigned to the
     forger.  If the forger then sends no more Query messages, other
     routers' Other Querier Present timer will time out and one will
     resume the role of Querier.  During this time, if the forger
     ignores Leave Messages, traffic might flow to groups with no
     members for up to [Group Membership Interval].

Report messages:

     A forged Report message may cause multicast routers to think there
     are members of a group on a subnet when there are not.  Forged
     Report messages from the local subnet are meaningless, since
     joining a group on a host is generally an unprivileged operation,
     so a local user may trivially gain the same result without forging
     any messages.  Forged Report messages from external sources are
     more troublesome; there are two defenses against externally forged
     Reports:
     - Ignore the Report if you cannot identify the source address of
       the packet as belonging to a subnet assigned to the interface on
       which the packet was received.  This solution means that Reports
       sent by mobile hosts without addresses on the local subnet will
       be ignored.
     - Ignore Report messages without Router Alert options [Katz96], and
       require that routers not forward Report messages.  (The
       requirement is not a requirement of generalized filtering in the
       forwarding path, since the packets already have Router Alert
       options in them).  This solution breaks backwards compatibility
       with implementations of earlier versions of this specification
       which did not require Router Alert.

     A forged Version 1 Report Message may put a router into "version 1



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     members present" state for a particular group, meaning that the
     router will ignore Leave messages.  This can cause traffic to flow
     to groups with no members for up to [Group Membership Interval].
     This can be solved by providing routers with a configuration switch
     to ignore Version 1 messages completely.  This breaks automatic
     compatibility with Version 1 hosts, so should only be used in
     situations where "fast leave" is critical.

Leave message:

     A forged Leave message will cause the Querier to send out Group-
     Specific Queries for the group in question.  This causes extra
     processing on each router and on each member of the group, but can
     not cause loss of desired traffic.

10.  Acknowledgments

IGMPv2 was designed by Rosen Sharma and Steve Deering.

11.  References

Katz96         Katz, D., "IP Router Alert Option," RFC XXXX, Cisco
               Systems, April 1996.




























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12.  Appendix I - Changes from IGMPv1

The IGMPv1 "Version" and "Type" fields are combined into a single "Type"
field.

A new IGMP Type is assigned to Version 2 Membership Report messages, so
a router may tell the difference between an IGMPv1 and IGMPv2 host
report.

A new IGMP Type is created for the IGMPv2 Leave Group message.

The Membership Query message is changed so that a previously unused
field contains a new value, the Max Response Time.

The IGMPv2 spec now specifies a querier election mechanism.  In IGMPv1,
the querier election was left up to the multicast routing protocol, and
different protocols used different mechanisms.  This could result in
more than one querier per network, so the election mechanism has been
standardized in IGMPv2.  However, this means that care must be taken
when an IGMPv2 router is trying to coexist with an IGMPv1 router that
uses a different querier election mechanism.  In particular, it means
that an IGMPv2 router must be able to act as an IGMPv1 router on a
particular network if configured to do so.  The actions required
include:

- Set the Max Response Time field to 0 in all queries.

- Ignore Leave Group messages.

The IGMPv2 spec relaxes the requirements on validity-checking for
Membership Queries and Membership Reports.  When upgrading an
implementation, be sure to remove any checks that do not belong.

The IGMPv2 spec requires the presence of the IP Router Alert option
[Katz96] in all packets described in this memo.
















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13.  Author's Address


   William C. Fenner
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA 94304
   Phone: +1 415 812 4816
   Email: fenner@parc.xerox.com










































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