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rfc:rfc2236

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, RFC 1112.
 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 RFC 2119 [RFC 2119].

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

Fenner Standards Track [Page 1] RFC 2236 Internet Group Management Protocol November 1997

 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 [RFC 2113] 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

Fenner Standards Track [Page 2] RFC 2236 Internet Group Management Protocol November 1997

 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.

Fenner Standards Track [Page 3] RFC 2236 Internet Group Management Protocol November 1997

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

Fenner Standards Track [Page 4] RFC 2236 Internet Group Management Protocol November 1997

 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.

Fenner Standards Track [Page 5] RFC 2236 Internet Group Management Protocol November 1997

 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.

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

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

5. Compatibility with IGMPv1 Hosts

 An IGMPv2 host may be placed on a subnet where there are hosts that
 have not yet been upgraded to IGMPv2.  The following requirement
 applies:
      The host MUST allow its Membership Report to be suppressed by
      either a Version 1 Membership Report or a Version 2 Membership
      Report.
 An IGMPv2 router may be placed on a subnet where there are hosts that
 have not yet been upgraded to IGMPv2.  The following requirements
 apply:
      If a router receives a Version 1 Membership Report, it MUST set
      a timer to note that there are version 1 hosts present which are
      members of the group for which it heard the report.  This timer
      should be the same as the [Group Membership Interval].
      If there are version 1 hosts present for a particular group, a
      router MUST ignore any Leave Group messages that it receives for
      that group.

6. Host State Diagram

 Host behavior is more formally specified by the state transition
 diagram below.  A host may be in one of three possible states with
 respect to any single IP multicast group on any single network
 interface:
  1. "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.
  1. "Delaying Member" state, when the host belongs to the group on the

interface and has a report delay timer running for that membership.

  1. "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.

Fenner Standards Track [Page 7] RFC 2236 Internet Group Management Protocol November 1997

 There are five significant events that can cause IGMP state
 transitions:
  1. "join group" occurs when the host decides to join the group on the

interface. It may occur only in the Non-Member state.

  1. "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.
  1. "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.
  1. "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.
  1. "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:
  1. "send report" for the group on the interface. The type of report

is determined by the state of the interface. The Report Message is

   sent to the group being reported.

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

  1. "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).
  1. "set flag" that we were the last host to send a report for this

group.

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

this group.

  1. "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] ].
  1. "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".
  1. "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.

Fenner Standards Track [Page 9] RFC 2236 Internet Group Management Protocol November 1997

                            ________________
                           |                |
                           |                |
                           |                |
                           |                |
                 --------->|   Non-Member   |<---------
                |          |                |          |
                |          |                |          |
                |          |                |          |
                |          |________________|          |
                |                   |                  |
                | leave group       | join group       | leave group
                | (stop timer,      |(send report,     | (send leave
                |  send leave if    | set flag,        |  if flag set)
                |  flag set)        | start timer)     |
        ________|________           |          ________|________
       |                 |<---------          |                 |
       |                 |                    |                 |
       |                 |<-------------------|                 |
       |                 |   query received   |                 |
       | Delaying Member |    (start timer)   |   Idle Member   |
  ---->|                 |------------------->|                 |
 |     |                 |   report received  |                 |
 |     |                 |    (stop timer,    |                 |
 |     |                 |     clear flag)    |                 |
 |     |_________________|------------------->|_________________|
 | query received    |        timer expired
 | (reset timer if   |        (send report,
 |  Max Resp Time    |         set flag)
 |  < current timer) |
  -------------------
 The all-systems group (address 224.0.0.1) is handled as a special
 case.  The host starts in Idle Member state for that group on every
 interface, never transitions to another state, and never sends a
 report for that group.
 In addition, a host may be in one of two possible states with respect
 to any single network interface:
  1. "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.
  1. "IGMPv1 Router Present", when the host has heard an IGMPv1 style

query within the [Version 1 Router Present Timeout].

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

 There are two events that can cause state transitions:
  1. "IGMPv1 query received", when the host receives a query with the

Max Response Time field set to 0.

  1. "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:
  1. "set timer", setting the timer to its maximum value [Version 1

Router Present Timeout] and (re)starting it.

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

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

7. Router State Diagram

 Router behavior is more formally specified by the state transition
 diagrams below.
 A router may be in one of two possible states with respect to any
 single attached network:
  1. "Querier", when this router is designated to transmit IGMP

Membership Queries on this network.

  1. "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:
  1. "query timer expired" occurs when the timer set for query

transmission expires.

  1. "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.
  1. "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:
  1. "start general query timer" for the attached network.
  1. "start other querier present timer" for the attached network [Other

Querier Present Interval].

  1. "send general query" on the attached network. The General Query is

sent to the all-systems group (224.0.0.1), and has a Max Response

   Time of [Query Response Interval].

Fenner Standards Track [Page 12] RFC 2236 Internet Group Management Protocol November 1997

  1. ——————————-

_| 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. Fenner Standards Track [Page 13] RFC 2236 Internet Group Management Protocol November 1997 - "Version 1 Members Present" state, when there is an IGMPv1 host on the network which has sent a Version 1 Membership Report for this multicast group. - "Checking Membership" state, when the router has received a Leave Group message but has not yet heard a Membership Report for the multicast group. There are six significant events that can cause router state transitions: - "v2 report received" occurs when the router receives a Version 2 Membership Report for the group on the interface. To be valid, the Report message must be at least 8 octets long and must have a correct IGMP checksum. - "v1 report received" occurs when the router receives a Version 1 Membership report for the group on the interface. The same validity requirements apply. - "leave received" occurs when the router receives an IGMP Group Leave message for the group on the interface. To be valid, the Leave message must be at least 8 octets long and must have a correct IGMP checksum. - "timer expired" occurs when the timer set for a group membership expires. - "retransmit timer expired" occurs when the timer set to retransmit a group-specific Membership Query expires. - "v1 host timer expired" occurs when the timer set to note the presence of version 1 hosts as group members expires. There are six possible actions that may be taken in response to the above events: - "start timer" for the group membership on the interface - also resets the timer to its initial value [Group Membership Interval] if the timer is currently running. - "start timer*" for the group membership on the interface - this alternate action sets the timer to [Last Member Query Interval] * [Last Member Query Count] if this router is a Querier, or the [Max Response Time] in the packet * [Last Member Query Count] if this router is a non-Querier. Fenner Standards Track [Page 14] RFC 2236 Internet Group Management Protocol November 1997 - "start retransmit timer" for the group membership on the interface [Last Member Query Interval]. - "start v1 host timer" for the group membership on the interface, also resets the timer to its initial value [Group Membership Interval] if the timer is currently running. - "send group-specific query" for the group on the attached network. The Group-Specific Query is sent to the group being queried, and has a Max Response Time of [Last Member Query Interval]. - "notify routing +" notify the routing protocol that there are members of this group on this connected network. - "notify routing -" notify the routing protocol that there are no longer any members of this group on this connected network. The state diagram for a router in Querier state follows: Fenner Standards Track [Page 15] RFC 2236 Internet Group Management Protocol November 1997 —————————-| |←———————- | | |timer expired | | timer expired| |(notify routing -, | | (notify routing -)| No Members |clear rxmt tmr) | | ——→| Present |←—— | | | | | | | |v1 report rec'd | | | | ———— | |(notify routing +, | || | | rexmt timer| | | start timer, | | | | expired | | | start v1 host | v2 report received| | | (send g-s | | | timer) | (notify routing +,| | | query, | | | | start timer)| | | st rxmt | | | | | _|_| tmr)| |

←———–
←—-
v2 report received
(start timer)
Members Present ←—————— Checking
—–> leave received Membership
(start timer*,
start rexmt timer,
send g-s query)
—> ——————→
_ | | | | |v2 report rec'd | | | | | | |(start timer) | |v1 report rec'd |v1 report rec'd | | | —————- |(start timer, |(start timer, | | |v1 host | start v1 host timer) | start v1 host | | |tmr V | timer) | | |exp'd | |←——————— | | ——| | | | | Version 1 |timer expired | | | Members Present |(notify routing -) | ——→| |——————————————- | |←——————- ——→|_| v1 report rec'd | | v2 report rec'd | | (start timer, | | (start timer) | | start v1 host timer) | —————– ————————– Fenner Standards Track [Page 16] RFC 2236 Internet Group Management Protocol November 1997 The state diagram for a router in Non-Querier state is similar, but non-Queriers do not send any messages and are only driven by message reception.Note that non-Queriers do not care whether a Membership Report message is Version 1 or Version 2. | | | | timer expired| |timer expired (notify routing -)| No Members |(notify routing -) ———>| Present |←——– | | | | | | | | | | | | | || | | | | | |report received | | |(notify routing +,| | | start timer) | | | | | |←——– | | | | report received | | | | (start timer) | | | Members Present |←——————| Checking | | | g-s query rec'd | Membership | | | (start timer*) | | —→| |——————→| | | |_| |___

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

Fenner Standards Track [Page 17] RFC 2236 Internet Group Management Protocol November 1997

8.2. Query Interval

 The Query Interval is the interval between General Queries sent  by
 the Querier.  Default: 125 seconds.
 By varying the [Query Interval], an administrator may tune the number
 of IGMP messages on the subnet; larger values cause IGMP Queries to
 be sent less often.

8.3. Query Response Interval

 The Max Response Time inserted into the periodic General Queries.
 Default: 100 (10 seconds)
 By varying the [Query Response Interval], an administrator may tune
 the burstiness of IGMP messages on the subnet; larger values make the
 traffic less bursty, as host responses are spread out over a larger
 interval.  The number of seconds represented by the [Query Response
 Interval] must be less than the [Query Interval].

8.4. Group Membership Interval

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

8.5. Other Querier Present Interval

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

8.6. Startup Query Interval

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

8.7. Startup Query Count

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

Fenner Standards Track [Page 18] RFC 2236 Internet Group Management Protocol November 1997

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.

Fenner Standards Track [Page 19] RFC 2236 Internet Group Management Protocol November 1997

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:
  1. 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.
  1. Ignore Report messages without Router Alert options [RFC 2113],

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.

Fenner Standards Track [Page 20] RFC 2236 Internet Group Management Protocol November 1997

   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:
  1. 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.
  1. 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:
  1. 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.
  1. Ignore Leave messages without Router Alert options [RFC 2113],

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.

Fenner Standards Track [Page 21] RFC 2236 Internet Group Management Protocol November 1997

12. References

 RFC 2119       Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
 RFC 2113       Katz, D., "IP Router Alert Option," RFC 2113,
                February 1997.
 RFC 1112       Deering, S., "Host Extensions for IP Multicasting",
                STD 5, RFC 1112, August 1989.

Fenner Standards Track [Page 22] RFC 2236 Internet Group Management Protocol November 1997

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:
  1. Set the Max Response Time field to 0 in all queries.
  1. 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
 [RFC 2113] 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

Fenner Standards Track [Page 23] RFC 2236 Internet Group Management Protocol November 1997

15. Full Copyright Statement

 Copyright (C) The Internet Society (1997).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Fenner Standards Track [Page 24]

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