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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-v2-03.txt                                    Xerox PARC
Obsoletes: Appendix I of RFC1112                                May 20, 1996
                                                           Expires: 10/31/96


             Internet Group Management Protocol, Version 2

Status of this Memo

This document is an Internet Draft.   Internet Drafts are working  docu-
ments 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
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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 Inter-
net Draft.

Distribution of this document is unlimited.

                                Abstract

     This draft documents IGMPv2, used by IP hosts to report their mul-
     ticast 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 mul-
     ticast 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 soli-
cited 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(*).  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 imple-
mented 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 mes-
sages 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
      0x16 = Version 2 Membership Report
      0x17 = Leave Group

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

      0x12 = Version 1 Membership Report

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,
_________________________
  * 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.



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     it is set to zero by the sender and ignored by receivers.

     Tuning this setting allows IGMPv2 routers to tune the "leave
     latency" of their attached subnets (the time between the moment the
     last host leaves a group and when the routing protocol is notified
     that there are no more members).  It also allows tuning of the
     amount of IGMP traffic on a subnet.

1.3.  Checksum

     The checksum is the 16-bit one's complement of the one's complement
     sum of the IGMP message.  For computing the checksum, the checksum
     field is set to zero.

1.4.  Group Address

     In a Membership Query message, the group address field is set to
     zero when sending a General Query (used to learn which groups have
     members on an attached network), and set to the group address being
     queried when sending a Group-Specific Query (used to learn if a
     particular group has any members on an attached network).

     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 should ignore
     anything past the first 8 octets.

2.  Informal Protocol Description

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

Multicast routers use IGMP to learn which groups have members on each of
their attached physical networks(*).  A multicast router keeps a list of
_________________________
  * The term "interface" is sometimes used in this  do-
cument  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.


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multicast group memberships(+) for each attached network, and a timer
for each membership.  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
becomes a Non-Querier on that network.  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) 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(*), up to the
requested Max Response Time in the Query packet.  When a host receives a
Group-Specific Query, it sets a delay timer for the group being queried
if it is a member on the interface from which it received the query.  If
a timer for a group is already running, it is reset only if the
requested Max Response Time is less than the remaining value of the run-
ning 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 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 Member-
ship 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
_________________________
  + Routers keep track of the presence of at least  one
member  of  a  multicast group on a given attached net-
work, not a list of all of the members.
  * This random value  should  use  the  highest  clock
granularity available on the host.




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of the initial Membership Report being lost or damaged, it is recom-
mended that it be repeated once or twice after short delays [Unsolicited
Report Interval].  (A simple way to accomplish this is to send the ini-
tial 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 sends 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. (+)

When a Querier receives a Leave Group message, it sets its query inter-
val for that group to a very short value [Last Member Query Interval].
The Querier sets the group membership timer to some multiple [Last
Member Query Count] of the query interval and starts sending Group-
Specific queries addressed to the group specified in the Leave Group
message.  These Group-Specific queries must have the Max Response Time
set to the query interval.  If no Reports are received before the
membership timer expires, the routers assume that the group has no local
members, as above.

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 inter-
     face, 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
_________________________
  * Note that a router should also be prepared  to  ac-
cept  a  Leave  Group  message  sent to the group being
left, as  one  widespread  IGMPv2  implementation  does
this.
  + This is an optimization to  reduce  traffic;  hosts
without  sufficient  storage to remember whether or not
they were the last host to  reply  may  always  send  a
Leave Group message when it leaves a group.




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     Present Timeout] seconds, *not* on the type of the last Query
     heard, as an IGMPv2 router may restart and send a Query before it
     realizes that it is not the Querier for the subnet.  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 optionally 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 require-
ments apply:

     If any IGMPv1 routers are present, the querier must use IGMPv1.
     Since there was no querier election specification in RFC1112, the
     rules followed by the IGMPv1 router may not agree with those in
     this document.  In this case, the IGMPv2 router must follow the
     rules established by the routing protocol for electing a querier.

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 requirements apply:

     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 Timer].

     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



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     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 inter-
     face 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 Gen-
     eral 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 mes-
sages 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:




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-    "send report" for the group on the interface.  The type of report
     is determined by the state of the interface.

-    "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 should be skipped.

-    "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 random delay
     value up to the Max Response Time in the Query.  If this is an
     unsolicited Report, the timer is set to a random delay value up to
     [Unsolicited Report Interval].

-    "reset timer" for the group on the interface to a new value, using
     a random delay value up to the requested response time in the
     Query.

-    "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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                              ________________
                             |                |
                             |                |
                             |                |
                             |                |
                   --------->|   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
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].



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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)               |
                        ---------------------------


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:



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-    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 transmis-
     sion 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 net-
     work 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 Timer].

-    "send general query" on the attached network.























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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 ini-
     tial 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 transi-
tions:

-    "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 vali-
     dity 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 pres-
     ence 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 Timer] if
     the timer is currently running.

-    "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 Timer]
     if the timer is currently running.




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-    "send group-specific query" for the group on the attached network.

-    "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)  |
 -----------------     --------------------------


_________________________
  * When entering the Checking  Membership  state,  the
timer  is  set  to [Last Member Query Interval] * [Last
Membership Query Count], not [Group Membership 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)   |
    -----------------








_________________________
  * When a non-querier router receives a Group Specific
query  in  the Members Present state, it sets its group
membership timer  to  a  multiple  [Last  Member  Query
Count]  of  the Max Response Time in the Group Specific
query.  It ignores Group Specific queries  received  in
the Checking Membership state.




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7.  List of timers and default values

Note that parentheses are used to group expressions to make the  algebra
clear.

7.1.  Query Interval

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

7.2.  Group Membership Timer

The Group Membership Timer is the amount of time that must pass before a
multicast router decides there are no more members of a group on a net-
work.  Default: (twice the Query Interval) plus (one Query Response
Interval).

7.3.  Other Querier Present Timer

The Other Querier Present Timer is the length of time that must pass
before a multicast router decides that there is no longer another multi-
cast router which should be the querier.  Default: (twice the Query
Interval) plus (one half of one Query Response Interval).

7.4.  Query Response Interval

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

7.5.  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.

7.6.  Startup Query Count

The Startup Query Count is the number of Queries sent out on startup,
separated by the Startup Query Interval.  Default: 2.

7.7.  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)





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7.8.  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: 2.

7.9.  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.

7.10.  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.  Security Considerations

Security issues are not discussed in this memo.

9.  Acknowledgments

IGMPv2 was designed by Rosen Sharma and Steve Deering.

10.  References

Katz96         Katz, D., "IP Router Alert Option," RFC XXXX, Cisco Sys-
               tems, April 1996.























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11.  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 par-
ticular 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 implemen-
tation, 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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12.  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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