Network Working Group W. Fenner
Request for Comments: 2236 Xerox PARC
Updates: 1112 November 1997
Category: Standards Track

Internet Group Management Protocol, Version 2

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1997). All Rights Reserved.

Abstract

This memo documents IGMPv2, used by IP hosts to report their
multicast group memberships to routers. It updates STD 5, 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).

1. Definitions

The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].

2. 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 [RFC2113] 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.1. 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:

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.

2.2. 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.

2.3. 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 transmitting
packets, the checksum MUST be computed and inserted into this field.
When receiving packets, the checksum MUST be verified before
processing a packet.

2.4. 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.

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

3. 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 (excluding the all-systems 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

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
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.

4. 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.
The use of IGMPv1 must be administratively configured, as there
is no reliable way of dynamically determining whether IGMPv1
routers are present on a network. Implementations MAY provide a
way for system administrators to enable the use of IGMPv1 on
their routers; in the absence of explicit configuration, the
configuration MUST default to IGMPv2. 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.

5. Compatibility with IGMPv1 Hosts

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

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

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

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

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

6. Host State Diagram

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

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

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

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

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

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

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

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

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

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

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

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

- "send report" for the group on the interface. The type of report
is determined by the state of the interface. The Report Message is
sent to the group being reported.

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

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

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

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

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

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

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

________________




---------> Non-Member <---------



________________

leave group join group leave group
(stop timer, (send report, (send leave
send leave if set flag, if flag set)
flag set) start timer)
________________ ________________
<---------

<-------------------
query received
Delaying Member (start timer) Idle Member
----> ------------------->
report received
(stop timer,
clear flag)
_________________------------------->_________________
query received timer expired
(reset timer if (send report,
Max Resp Time set flag)
< current timer)
-------------------

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

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

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

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

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)
---------------------------

7. Router State Diagram

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

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

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

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

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

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

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

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

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

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

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

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

--------------------------------
_______________ 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.

- "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
[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:

________________
---------------------------- <-----------------------
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)
----------------- --------------------------

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

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.

8.8. 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.

8.9. 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.

8.10. 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.

8.11. 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.

9. Message destinations

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

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)

Note: in older (i.e., non-standard and now obsolete) versions of
IGMPv2, hosts send Leave Messages to the group being left. A
router SHOULD accept Leave Messages addressed to the group being
left in the interests of backwards compatibility with such hosts.
In all cases, however, hosts MUST send to the ALL-ROUTERS address
to be compliant with this specification.

10. 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].

A forged Query message sent to a group with members will cause the
hosts which are members of the group to report their memberships.
This causes a small amount of extra traffic on the LAN, but causes
no protocol problems.

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 [RFC2113],
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
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].
There are two defenses against forged v1 Reports:

- To defend against externally sourced v1 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 v1 Reports sent by
mobile hosts without addresses on the local subnet will be
ignored.

- Provide 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. This solution protects against forged
version 1 reports from the local subnet as well.

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. There are two defenses against
externally forged Leave messages:

- Ignore the Leave message 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 Leave messages sent by mobile hosts without addresses on the
local subnet will be ignored.

- Ignore Leave messages without Router Alert options [RFC2113],
and require that routers not forward Leave 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.

11. Acknowledgments

IGMPv2 was designed by Rosen Sharma and Steve Deering.

12. References

RFC2119 Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

RFC2113 Katz, D., "IP Router Alert Option," RFC2113,
February 1997.

RFC1112 Deering, S., "Host Extensions for IP Multicasting",
STD 5, RFC1112, August 1989.

13. 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
[RFC2113] in all packets described in this memo.

14. Author's Address

William C. Fenner
Xerox PARC
3333 Coyote Hill Road
Palo Alto, CA 94304
Phone: +1 650 812 4816

EMail: fenner@parc.xerox.com

15. Full Copyright Statement

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