[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 3376

INTERNET-DRAFT                                Brad Cain, Nortel Networks
                                            Steve Deering, Cisco Systems
                                          Isidor Kouvelas, Cisco Systems
                                              Ajit Thyagarajan, Ericsson
Expires September 2000                                        March 2000


             Internet Group Management Protocol, Version 3
                    <draft-ietf-idmr-igmp-v3-03.txt>


STATUS OF THIS MEMO

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas,  and its working groups.  Note that other
groups may also distribute working documents as Internet- Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as work in progress.

The  list  of  current  Internet-Drafts  can  be  accessed  at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow  Directories can be accessed at
http://www.ietf.org/shadow.html.

Abstract

This document specifies Version 3 of the Internet Group Management
Protocol, IGMPv3.  IGMP is the protocol used by IPv4 systems to report
their IP multicast group memberships to neighboring multicast routers.
Version 3 of IGMP adds support for "source filtering", that is, the
ability for a system to report interest in receiving packets *only* from
specific source addresses, or from *all but* specific source addresses,
sent to a particular multicast address.  That information may be used by
multicast routing protocols to avoid delivering multicast packets from
specific sources to networks where there are no interested receivers.

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 authors.




Cain, Deering, Kouvelas, Thyagarajan                            [Page 1]


INTERNET-DRAFT                   IGMPv3                       March 2000


The capitalized 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].  Due to the lack of italics, emphasis is indicated herein by
bracketing a word or phrase in "*" characters.





                           Table of Contents


Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
The API for Requesting IP Multicast Reception  . . . . . . . . . . .   3
Multicast Reception State Maintained by Systems  . . . . . . . . . .   5
Description of the Protocol for Group Members  . . . . . . . . . . .  18
Description of the Protocol for Multicast Routers  . . . . . . . . .  23
Interoperation with Older Versions of IGMP . . . . . . . . . . . . .  33
List of Timers, Counters, and their Default Values . . . . . . . . .  37
Security Considerations  . . . . . . . . . . . . . . . . . . . . . .  40
Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . . . .  41
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  41
Appendix A.  Design Rationale  . . . . . . . . . . . . . . . . . . .  42

























Cain, Deering, Kouvelas, Thyagarajan                            [Page 2]


INTERNET-DRAFT                   IGMPv3                       March 2000


1.  INTRODUCTION

The Internet Group Management Protocol (IGMP) is used by IPv4 systems
(hosts and routers) to report their IP multicast group memberships to
any neighboring multicast routers.  Note that an IP multicast router may
itself be a member of one or more multicast groups, in which case it
performs both the "multicast router part" of the protocol (to collect
the membership information needed by its multicast routing protocol) and
the "group member part" of the protocol (to inform itself and other,
neighboring multicast routers of its memberships).

IGMP is also used for other IP multicast management functions, using
message types other than those used for group membership reporting.
This document specifies only the group membership reporting functions
and messages.

This document specifies Version 3 of IGMP.  Version 1, specified in
[RFC-1112], was the first widely-deployed version and the first version
to become an Internet Standard.  Version 2, specified in [RFC-2236],
added support for "low leave latency", that is, a reduction in the time
it takes for a multicast router to learn that there are no longer any
members of a particular group present on an attached network.  Version 3
adds support for "source filtering", that is, the ability for a system
to report interest in receiving packets *only* from specific source
addresses, or from *all but* specific source addresses, sent to a
particular multicast address.  Version 3 is designed to be interoperable
with Versions 1 and 2.


2.  THE API FOR REQUESTING IP MULTICAST RECEPTION

Within an IP system, there is (at least conceptually) an Application
Programming Interface or API used by upper-layer protocols or
application programs to ask the IP layer to enable and disable reception
of packets sent to specific IP multicast addresses.  In order to take
full advantage of the capabilities of IGMPv3, a system's IP API must
support the following operation (or any logical equivalent; for example,
see [FILTER-API]):

       IPMulticastListen ( socket, interface, multicast-address,
                           filter-mode, source-list )

where:

  "socket" is an implementation-specific parameter used to distinguish
  among different requesting entities (e.g., programs or processes)
  within the system; the socket parameter of BSD Unix system calls is a
  specific example.



Cain, Deering, Kouvelas, Thyagarajan                            [Page 3]


INTERNET-DRAFT                   IGMPv3                       March 2000


  "interface" is a local identifier of the network interface on which
  reception of the specified multicast address is to be enabled or
  disabled.  Interfaces may be physical (e.g., an Ethernet interface) or
  virtual (e.g., the endpoint of a Frame Relay virtual circuit or the
  endpoint of an IP-in-IP "tunnel").  An implementation may allow a
  special "unspecified" value to be passed as the interface parameter,
  in which case the request would apply to the "primary" or "default"
  interface of the system (perhaps established by system configuration).
  If reception of the same multicast address is desired on more than one
  interface, IPMulticastListen is invoked separately for each desired
  interface.

  "multicast-address" is the IP multicast address to which the request
  pertains.  If reception of more than one multicast address on a given
  interface is desired, IPMulticastListen is invoked separately for each
  desired multicast address.

  "filter-mode" may be either INCLUDE or EXCLUDE.  In INCLUDE mode,
  reception of packets sent to the specified multicast address is
  requested *only* from those IP source addresses listed in the source-
  list parameter.  In EXCLUDE mode, reception of packets sent to the
  given multicast address is requested from all IP source addresses
  *except* those listed in the source-list parameter.

  "source-list" is an unordered list of zero or more IP unicast
  addresses from which multicast reception is desired or not desired,
  depending on the filter mode.  An implementation MAY impose a limit on
  the size of source lists, but that limit MUST NOT be less than 64
  addresses per list.

For a given combination of socket, interface, and multicast address,
only a single filter mode and source list can be in effect at any one
time.  However, either the filter mode or the source list, or both, may
be changed by subsequent IPMulticastListen requests that specify the
same socket, interface, and multicast address.

Previous versions of IGMP did not support source filters and had a
simpler API consisting of Join and Leave operations to enable and
disable reception of a given multicast address (from *all* sources) on a
given interface.  Those Join and Leave operations are supported by the
new API as follows:

The Join operation is equivalent to

          IPMulticastListen ( socket, interface, multicast-address,
                              EXCLUDE, {} )

and the Leave operation is equivalent to:



Cain, Deering, Kouvelas, Thyagarajan                            [Page 4]


INTERNET-DRAFT                   IGMPv3                       March 2000


          IPMulticastListen ( socket, interface, multicast-address,
                              INCLUDE, {} )

where {} is an empty source list.

It is recommended that implementations continue to support the old API,
(perhaps as calls on the new API) for compatibility with pre-existing IP
multicast applications.


3.  MULTICAST RECEPTION STATE MAINTAINED BY SYSTEMS


3.1.  Socket State

For each socket on which IPMulticastListen has been invoked, the system
records the desired multicast reception state for that socket.  That
state conceptually consists of a set of records of the form:

        (interface, multicast-address, filter-mode, source-list)

The socket state evolves in response to each invocation of
IPMulticastListen on the socket, as follows:

o If the requested filter mode is INCLUDE *and* the requested source
  list is empty, then the entry corresponding to the requested interface
  and multicast address is deleted if present.  If no such entry is
  present, the request is ignored.

o If the requested filter mode is EXCLUDE *or* the requested source list
  is non-empty, then the entry corresponding to the requested interface
  and multicast address, if present, is changed to contain the requested
  filter mode and source list.  If no such entry is present, a new entry
  is created, using the parameters specified in the request.


3.2.  Interface State

In addition to the per-socket multicast reception state, a system must
also maintain or compute multicast reception state for each of its
interfaces.  That state conceptually consists of a set of records of the
form:

          (multicast-address, filter-mode, source-list)

This per-interface state is derived from the per-socket state, but may
differ from the per-socket state when different sockets have differing
filter modes and/or source lists for the same multicast address and



Cain, Deering, Kouvelas, Thyagarajan                            [Page 5]


INTERNET-DRAFT                   IGMPv3                       March 2000


interface.  For example, suppose one application or process invokes the
following operation on socket s1:

          IPMulticastListen ( s1, i, m, INCLUDE, {a, b, c} )

requesting reception on interface i of packets sent to multicast address
m, *only* if they come from source a, b, or c.  Suppose another
application or process invokes the following operation on socket s2:

          IPMulticastListen ( s2, i, m, INCLUDE, {b, c, d} )

requesting reception on the same interface i of packets sent to the same
multicast address m, *only* if they come from sources b, c, or d.  In
order to satisfy the reception requirements of both sockets, it is
necessary for interface i to receive packets sent to m from any one of
the sources a, b, c, or d.  Thus, in this example, the reception state
of interface i for multicast address m has filter mode INCLUDE and
source list {a, b, c, d}.

(After a multicast packet has been accepted from an interface by the IP
layer, its subsequent delivery to the application or process listening
on a particular socket depends on the multicast reception state of that
socket [and possibly also on other conditions, such as what transport-
layer port the socket is bound to].  So, in the above example, if a
packet arrives on interface i, destined to multicast address m, with
source address a, it may be delivered on socket s1 but not on socket
s2.)

The general rules for deriving the per-interface state from the per-
socket state are as follows:  For each distinct (interface, multicast-
address) pair that appears in any socket state, a per-interface record
is created for that multicast address on that interface.  Considering
all socket records containing the same (interface, multicast-address)
pair,

o if *any* such record has a filter mode of EXCLUDE, then the filter
  mode of the interface record is EXCLUDE, and the source list of the
  interface record is the intersection of the source lists of all socket
  records in EXCLUDE mode, minus those source addresses that appear in
  any socket record in INCLUDE mode.  For example, if the socket records
  for multicast address m on interface i are:

            from socket s1:  ( i, m, EXCLUDE, {a, b, c, d} )
            from socket s2:  ( i, m, EXCLUDE, {b, c, d, e} )
            from socket s3:  ( i, m, INCLUDE, {d, e, f} )

  then the corresponding interface record on interface i is:




Cain, Deering, Kouvelas, Thyagarajan                            [Page 6]


INTERNET-DRAFT                   IGMPv3                       March 2000


                             ( m, EXCLUDE, {b, c} )

o if *all* such records have a filter mode of INCLUDE, then the filter
  mode of the interface record is INCLUDE, and the source list of the
  interface record is the union of the source lists of all the socket
  records.  For example, if the socket records for multicast address m
  on interface i are:

            from socket s1:  ( i, m, INCLUDE, {a, b, c} )
            from socket s2:  ( i, m, INCLUDE, {b, c, d} )
            from socket s3:  ( i, m, INCLUDE, {e, f} )

  then the corresponding interface record on interface i is:

                             ( m, INCLUDE, {a, b, c, d, e, f} )


  In order to bound storage consumption, an implementation MAY impose a
  limit on the size of the source list in an interface record whose
  filter mode is INCLUDE, but that limit MUST NOT be less than 64
  addresses.  If such a limit is imposed, whenever the union of the
  source lists of all the relevant socket records would exceed that
  limit, an interface record of the following form would be created,
  instead:

                             ( m, EXCLUDE, {} )

  which enables reception of packets to multicast address m from *all*
  sources.

The above rules for deriving the interface state are (re-)evaluated
whenever an IPMulticastListen invocation modifies the socket state by
adding, deleting, or modifying a per-socket state record.  Note that a
change  of socket state does not necessarily result in a change of
interface state.


4.  MESSAGE FORMATS

IGMP messages are encapsulated in IPv4 datagrams, with an IP protocol
number of 2.  Every IGMP message described in this document is sent with
an IP Time-to-Live of 1, and carries an IP Router Alert option
[RFC-2113] in its IP header.

There are two IGMP message types of concern to the IGMPv3 protocol
described in this document:





Cain, Deering, Kouvelas, Thyagarajan                            [Page 7]


INTERNET-DRAFT                   IGMPv3                       March 2000


    Type Number (hex)   Message Name
    -----------------   ------------

          0x11          Membership Query

          0x22          Version 3 Membership Report

An implementation of IGMPv3 must also support the following three
message types, for interoperation with previous versions of IGMP (see
section 7):

          0x12          Version 1 Membership Report    [RFC-1112]

          0x16          Version 2 Membership Report    [RFC-2236]

          0x17          Version 2 Leave Group          [RFC-2236]

Unrecognized message types MUST be silently ignored.  Other message
types may be used by newer versions or extensions of IGMP, by multicast
routing protocols, or for other uses.

In this document, unless otherwise qualified, the capitalized words
"Query" and "Report" refer to IGMP Membership Queries and IGMP Version 3
Membership Reports, respectively.


4.1.  Membership Query Message

Membership Queries are sent by IP multicast routers to query the
multicast reception state of neighboring interfaces.  Queries have the
following format:




















Cain, Deering, Kouvelas, Thyagarajan                            [Page 8]


INTERNET-DRAFT                   IGMPv3                       March 2000


    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 = 0x11  | Max Resp Time |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Group Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Resv  |S| QRV |      QQI      |     Number of Sources (N)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Source Address [1]                      |
   +-                                                             -+
   |                       Source Address [2]                      |
   +-                              .                              -+
   .                               .                               .
   .                               .                               .
   +-                                                             -+
   |                       Source Address [N]                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


4.1.1.  Max Resp Time

The Max Resp Time field specifies the maximum time allowed before
sending a responding report, in units of 1/10 second.

Varying this setting allows IGMPv3 routers to tune the "leave latency"
(the time between the moment the last host leaves a group and the moment
the routing protocol is notified that there are no more members).  It
also allows tuning of the burstiness of IGMP traffic on a network.

4.1.2.  Checksum

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

4.1.3.  Group Address

The Group Address field is set to zero when sending a General Query, and
set to the IP multicast address being queried when sending a Group-
Specific Query or Group-and-Source-Specific Query (see section 4.1.9,
below).

4.1.4.  Resv (Reserved)

The Resv field is set to zero on transmission, and ignored on reception.




Cain, Deering, Kouvelas, Thyagarajan                            [Page 9]


INTERNET-DRAFT                   IGMPv3                       March 2000


4.1.5.  S Flag (Suppress Router-Side Processing)

When set to one, the S Flag indicates to any receiving multicast routers
that they are to suppress the normal processing they perform upon
hearing a Query.  It does not, however, suppress the normal "host-side"
processing of a Query that a router may be required to perform by as a
consequence of itself being a group member.

4.1.6.  QRV (Querier's Robustness Variable)

If non-zero, the QRV field contains the [Robustness Variable] value used
by the querier, i.e., the sender of the Query.  If the querier's
[Robustness Variable] exceeds 7, the maximum value of the QRV field, the
QRV is set to zero.  Receivers of a Query adopt the QRV value from the
most recently received Query as their own [Robustness Variable] value,
unless that most recently received QRV was zero, in which case the
receivers use the default [Robustness Variable] value specified in
section 8.1.

4.1.7.  QQI (Querier's Query Interval)

If non-zero, the QQI field contains the [Query Interval] value used by
the querier, i.e., the sender of the Query, in units of seconds.  If the
querier's [Query Interval] exceeds 255 seconds, the maximum value of the
QQI field, the QQI is set to zero.  Multicast routers that are not the
current querier adopt the QQI value from the most recently received
Query as their own [Query Interval] value, unless that most recently
received QQI was zero, in which case the receiving routers use the
default [Query Interval] value specified in section 8.2.

4.1.8.  Number of Sources (N)

The Number of Sources (N) field specifies how many source addresses are
present in the Query.  This number is zero in a General Query or a
Group-Specific Query, and non-zero in a Group-and-Source- Specific
Query.  This number is limited by the MTU of the network over which the
Query is transmitted.  For example, on an Ethernet with an MTU of 1500
octets, the IP header including the Router Alert option consumes 24
octets, and the IGMP fields up to including the Number of Sources (N)
field consume 12 octets, leaving 1464 octets for source addresses, which
limits the number of source addresses to 366 (1464/4).

4.1.9.  Source Address [i]

The Source Address [i] fields are a vector of n IP unicast addresses,
where n is the value in the Number of Sources (N) field.





Cain, Deering, Kouvelas, Thyagarajan                           [Page 10]


INTERNET-DRAFT                   IGMPv3                       March 2000


4.1.10.  Additional Data

If the Packet Length field in the IP header of a received Query
indicates that there are additional octets of data present, beyond the
fields described here, IGMPv3 implementations MUST include those octets
in the computation to verify the received IGMP Checksum, but MUST
otherwise ignore those additional octets.  When sending a Query, an
IGMPv3 implementation MUST NOT include additional octets beyond the
fields described here.

4.1.11.  Query Variants

There are three variants of the Query message:

1 A "General Query" is sent by a multicast router to learn the complete
  multicast reception state of the neighboring interfaces (that is, the
  interfaces attached to the network on which the Query is transmitted).
  In a General Query, both the Group Address field and the Number of
  Sources (N) field are zero.

2 A "Group-Specific Query" is sent by a multicast router to learn the
  reception state, with respect to a *single* multicast address, of the
  neighboring interfaces.  In a Group-Specific Query, the Group Address
  field contains the multicast address of interest, and the Number of
  Sources (N) field contains zero.

3 A "Group-and-Source-Specific Query" is sent by a multicast router to
  learn if any neighboring interface desires reception of packets sent
  to a specified multicast address, from any of a specified list of
  sources.  In a Group-and-Source-Specific Query, the Group Address
  field contains the multicast address of interest, and the Source
  Address [i] fields contain the source address(es) of interest.

4.1.12.  IP Destination Addresses for Queries

In IGMPv3, General Queries are sent with an IP destination address of
224.0.0.1, the all-systems multicast address.  Group-Specific and Group-
and-Source-Specific Queries are sent with an IP destination address
equal to the  multicast address of interest.  *However*, a system MUST
accept and  process any Query whose IP Destination Address field
contains *any* of the addresses (unicast or multicast) assigned to the
interface on which the Query arrives.


4.2.  Version 3 Membership Report Message

Version 3 Membership Reports are sent by IP systems to report (to
neighboring routers) the current multicast reception state, or changes



Cain, Deering, Kouvelas, Thyagarajan                           [Page 11]


INTERNET-DRAFT                   IGMPv3                       March 2000


in the multicast reception state, of their interfaces.  Reports 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 = 0x22  |    Reserved   |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Reserved            |  Number of Group Records (M)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                        Group Record [1]                       .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                        Group Record [2]                       .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   .
                                   .
                                   .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                        Group Record [M]                       .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



















Cain, Deering, Kouvelas, Thyagarajan                           [Page 12]


INTERNET-DRAFT                   IGMPv3                       March 2000


where each Group Record has the following internal format:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Record Type  |  Aux Data Len |     Number of Sources (N)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Multicast Address                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Source Address [1]                      |
   +-                                                             -+
   |                       Source Address [2]                      |
   +-                                                             -+
   .                               .                               .
   .                               .                               .
   .                               .                               .
   +-                                                             -+
   |                       Source Address [N]                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                         Auxiliary Data                        .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.2.1.  Reserved

The Reserved fields are set to zero on transmission, and ignored on
reception.

4.2.2.  Checksum

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

4.2.3.  Number of Group Records (M)

The Number of Group Records (M) field specifies how many Group Records
are present in this Report.

4.2.4.  Group Record

Each Group Record is a block of fields containing information pertaining
to the sender's membership in a single multicast group on the interface
from which the Report is sent.





Cain, Deering, Kouvelas, Thyagarajan                           [Page 13]


INTERNET-DRAFT                   IGMPv3                       March 2000


4.2.5.  Record Type

See section 4.2.12, below.

4.2.6.  Aux Data Len

The Aux Data Len field contains the length of the Auxiliary Data field
in this Group Record, in units of 32-bit words.  It may contain zero, to
indicate the absence of any auxiliary data.

4.2.7.  Number of Sources (N)

The Number of Sources (N) field specifies how many source addresses are
present in this Group Record.

4.2.8.  Multicast Address

The Multicast Address field contains the IP multicast address to which
this Group Record pertains.

4.2.9.  Source Address [i]

The Source Address [i] fields are a vector of n IP unicast addresses,
where n is the value in this record's Number of Sources (N) field.

4.2.10.  Auxiliary Data

The Auxiliary Data field, if present, contains additional information
pertaining to this Group Record.  The protocol specified in this
document, IGMPv3, does not define any auxiliary data.  Therefore,
implementations of IGMPv3 MUST NOT include any auxiliary data (i.e.,
MUST set the Aux Data Len field to zero) in any transmitted Group
Record, and MUST ignore any auxiliary data present in any received Group
Record.  The semantics and internal encoding of the Auxiliary Data field
are to be defined by any future version or extension of IGMP that uses
this field.

4.2.11.  Additional Data

If the Packet Length field in the IP header of a received Report
indicates that there are additional octets of data present, beyond the
last Group Record, IGMPv3 implementations MUST include those octets in
the computation to verify the received IGMP Checksum, but MUST otherwise
ignore those additional octets.  When sending a Report, an IGMPv3
implementation MUST NOT include additional octets beyond the last Group
Record.





Cain, Deering, Kouvelas, Thyagarajan                           [Page 14]


INTERNET-DRAFT                   IGMPv3                       March 2000


4.2.12.  Group Record Types

There are a number of different types of Group Records that may be
included in a Report message:

o A "Current-State Record" is sent by a system in response to a Query
  received on an interface.  It reports the current reception state of
  that interface, with respect to a single multicast address.  The
  Record Type of a Current-State Record may be one of the following two
  values:

    Value  Name and Meaning
    -----  ----------------

      1    MODE_IS_INCLUDE - indicates that the interface has a
           filter mode of INCLUDE for the specified multicast
           address.  The Source Address [i] fields in this Group
           Record contain the interface's source list for the
           specified multicast address, if it is non-empty.

      2    MODE_IS_EXCLUDE - indicates that the interface has a
           filter mode of EXCLUDE for the specified multicast
           address.  The Source Address [i] fields in this Group
           Record contain the interface's source list for the
           specified multicast address, if it is non-empty.

o A "Filter-Mode-Change Record" is sent by a system whenever a local
  invocation of IPMulticastListen causes a change of the filter mode
  (i.e., a change from INCLUDE to EXCLUDE, or from EXCLUDE to INCLUDE),
  of the interface-level state entry for a particular multicast address.
  The Record is included in a Report sent from the interface on which
  the change occurred.  The Record Type of a Filter-Mode-Change Record
  may be one of the following two values:

      3    CHANGE_TO_INCLUDE_MODE - indicates that the interface
           has changed to INCLUDE filter mode for the specified
           multicast address.  The Source Address [i] fields
           in this Group Record contain the interface's new
           source list for the specified multicast address,
           if it is non-empty.

      4    CHANGE_TO_EXCLUDE_MODE - indicates that the interface
           has changed to EXCLUDE filter mode for the specified
           multicast address.  The Source Address [i] fields
           in this Group Record contain the interface's new
           source list for the specified multicast address,
           if it is non-empty.




Cain, Deering, Kouvelas, Thyagarajan                           [Page 15]


INTERNET-DRAFT                   IGMPv3                       March 2000


o A "Source-List-Change Record" is sent by a system whenever a local
  invocation of IPMulticastListen causes a change of source list that is
  *not* coincident with a change of filter mode, of the interface-level
  state entry for a particular multicast address.  The Record is
  included in a Report sent from the interface on which the change
  occurred.  The Record Type of a Source-List-Change Record may be one
  of the following two values:

      5    ALLOW_NEW_SOURCES - indicates that the Source Address
           [i] fields in this Group Record contain a list of the
           additional sources that the system wishes to
           hear from, for packets sent to the specified
           multicast address.  If the change was to an INCLUDE
           source list, these are the addresses that were added
           to the list; if the change was to an EXCLUDE source
           list, these are the addresses that were deleted from
           the list.

      6    BLOCK_OLD_SOURCES - indicates that the Source Address
           [i] fields in this Group Record contain a list of the
           sources that the system no longer wishes to
           hear from, for packets sent to the specified
           multicast address.  If the change was to an INCLUDE
           source list, these are the addresses that were
           deleted from  the list; if the change was to an
           EXCLUDE source list, these are the addresses that
           were added to the list.

  If a change of source list results in both allowing new sources and
  blocking old sources, then two Group Records are sent for the same
  multicast address, one of type ALLOW_NEW_SOURCES and one of type
  BLOCK_OLD_SOURCES.


We use the term "State-Change Record" to refer to either a Filter- Mode-
Change Record or a Source-List-Change Record.

Unrecognized Record Type values MUST be silently ignored.


4.2.13.  IP Destination Addresses for Reports

Version 3 Reports are sent with an IP destination address of 224.0.0.22,
to which all IGMPv3-capable multicast routers listen.  A system that is
operating in version 1 or version 2 compatibility modes sends version 1
or version 2 Reports to the multicast group specified in the Group
Address field of the Report.  In addition, a system MUST accept and
process any version 1 or version 2 Report whose IP Destination Address



Cain, Deering, Kouvelas, Thyagarajan                           [Page 16]


INTERNET-DRAFT                   IGMPv3                       March 2000


field contains *any* of the addresses (unicast or multicast) assigned to
the interface on which the Report arrives.

4.2.14.  Notation for Group Records

In the rest of this document, we use the following notation to describe
the contents of a Group Record pertaining to a particular multicast
address:

    IS_IN ( x )  -  Type MODE_IS_INCLUDE, source addresses x
    IS_EX ( x )  -  Type MODE_IS_EXCLUDE, source addresses x
    TO_IN ( x )  -  Type CHANGE_TO_INCLUDE_MODE, source addresses x
    TO_EX ( x )  -  Type CHANGE_TO_EXCLUDE_MODE, source addresses x
    ALLOW ( x )  -  Type ALLOW_NEW_SOURCES, source addresses x
    BLOCK ( x )  -  Type BLOCK_OLD_SOURCES, source addresses x

where x is either:

o a capital letter (e.g., "A") to represent the set of source addresses,
  or

o a set expression (e.g., "A+B"), where "A+B" means the union of sets A
  and B,  "A*B" means the intersection of sets A and B, and "A-B" means
  the removal of all elements of set B from set A.


4.2.15.  Membership Report Size

If the set of Group Records required in a Report does not fit within the
size limit of a single Report message (as determined by the MTU of the
network on which it will be sent), the Group Records are sent in as many
Report messages as needed to report the entire set.

If a single Group Record contains so many source addresses that it does
not fit within the size limit of a single Report message, if its Type is
not MODE_IS_EXCLUDE, it is split into multiple Group Records, each
containing a different subset of the source addresses and each sent in a
separate Report message.  If its Type is MODE_IS_EXCLUDE, a single Group
Record is sent, containing as many source addresses as can fit, and the
remaining source addresses are not reported; though the choice of which
sources to report is arbitrary, it is preferable to report the same set
of sources in each subsequent report, rather than reporting different
sources each time.








Cain, Deering, Kouvelas, Thyagarajan                           [Page 17]


INTERNET-DRAFT                   IGMPv3                       March 2000


5.  DESCRIPTION OF THE PROTOCOL FOR GROUP MEMBERS

IGMP is an asymmetric protocol, specifying separate behaviors for group
members -- that is, hosts or routers that wish to receive multicast
packets -- and multicast routers.  This section describes the part of
IGMPv3 that applies to all group members. (Note that a multicast router
that is also a group member performs both parts of IGMPv3, receiving and
responding to its own IGMP message transmissions as well as those of its
neighbors.  The multicast router part of IGMPv3 is described in section
6.)

A system performs the protocol described in this section over all
interfaces on which multicast reception is supported, even if more than
one of those interfaces is connected to the same network.

For interoperability with multicast routers running older versions of
IGMP, systems maintain a MulticastRouterVersion variable for each
interface on which multicast reception is supported.  This section
describes the behavior of group member systems on interfaces for which
MulticastRouterVersion = 3.  The algorithm for determining
MulticastRouterVersion, and the behavior for versions other than 3, are
described in section 7.

The all-systems multicast address, 224.0.0.1, is handled as a special
case.  On all systems -- that is all hosts and routers, including
multicast routers -- reception of packets destined to the all-systems
multicast address, from all sources, is permanently enabled on all
interfaces on which multicast reception is supported.  No IGMP messages
are ever sent regarding the all-systems multicast address.

There are two types of events that trigger IGMPv3 protocol actions on an
interface:

o a change of the interface reception state, caused by a local
  invocation of IPMulticastListen.

o reception of a Query.

(Received IGMP messages of types other than Query are silently ignored,
except as required for interoperation with earlier versions of IGMP.)

The following subsections describe the actions to be taken for each of
these two cases.  In those descriptions, timer and counter names appear
in square brackets.  The default values for those timers and counters
are specified in section 8.






Cain, Deering, Kouvelas, Thyagarajan                           [Page 18]


INTERNET-DRAFT                   IGMPv3                       March 2000


5.1.  Action on Change of Interface State

An invocation of IPMulticastListen may cause the multicast reception
state of an interface to change, according to the rules in section 3.2.
Each such change affects the per-interface entry for a single multicast
address.

A change of interface state causes the system to immediately transmit a
State-Change Report from that interface.  The type and contents of the
Group Record(s) in that Report are determined by comparing the filter
mode and source list for the affected multicast address before and after
the change, according to the table below.  If no interface state existed
for that multicast address before the change (i.e., the change consisted
of creating a new per-interface record), or if no state exists after the
change (i.e., the change consisted of deleting a per-interface record),
then the "non-existent" state is considered to have a filter mode of
INCLUDE and an empty source list.

  Old State         New State         State-Change Record Sent
  ---------         ---------         ------------------------

  INCLUDE (A)       INCLUDE (B)       ALLOW (B-A), BLOCK (A-B)

  EXCLUDE (A)       EXCLUDE (B)       ALLOW (A-B), BLOCK (B-A)

  INCLUDE (A)       EXCLUDE (B)       TO_EX (B)

  EXCLUDE (A)       INCLUDE (B)       TO_IN (B)

If the computed source list for either an ALLOW or a BLOCK State-Change
Record is empty, that record is omitted from the Report message.

To cover the possibility of the State-Change Report being missed by one
or more multicast routers, it is retransmitted [Robustness Variable] - 1
more times, at intervals chosen at random from the range (0,
[Unsolicited Report Interval]).

If more changes to the same interface state entry occur before all the
retransmissions of the State-Change Report for the first change have
been completed, each such additional change triggers the immediate
transmission of a new State-Change Report.

The contents of the new transmitted report are calculated as follows.
As was done with the first report, the interface state for the affected
group before and after the latest change is compared. The report records
expressing the difference are built according to the table above.
However these records are not transmitted in a message but instead
merged with the contents of the pending report, to create the new State-



Cain, Deering, Kouvelas, Thyagarajan                           [Page 19]


INTERNET-DRAFT                   IGMPv3                       March 2000


Change report. The rules for merging the difference report resulting
from the state change and the pending report are described below.

The transmission of the merged State-Change Report terminates
retransmissions of the earlier State-Change Reports for the same
multicast address, and becomes the first of [Robustness Variable]
transmissions of State-Change Reports.

Each time a source is included in the difference report calculated
above, retransmission state for that source needs to be maintained until
[Robustness Variable] State-Change reports have been sent by the host.
This is done in order to ensure that a series of successive state
changes do not break the protocol robustness.

If the interface reception-state change that triggers the new report is
a filter-mode change, then the next [Robustness Variable] State-Change
Reports will include a Filter-Mode-Change record. This applies even if
any number of source-list changes occur in that period. The host has to
maintain retransmission state for the group until the [Robustness
Variable] State-Change reports have been sent.  When [Robustness
Variable] State-Change reports with Filter-Mode-Change records have been
transmitted after the last filter-mode change, and if source-list
changes to the interface reception have scheduled additional reports,
then the next State-Change report will include Source-List-Change
records.

Each time a State-Change Report is transmitted, the contents are
determined as follows. If the report should contain a Filter-Mode-Change
record, then if the current filter-mode of the interface is INCLUDE, a
TO_IN record is included in the report, otherwise a TO_EX record is
included. If instead the report should contain Source-List-Change
records, an ALLOW and a BLOCK record are included.  The contents of
these records are built according to the table below.

  Record   Sources included
  ------   ----------------
  TO_IN    All in the current interface state that must be forwarded
  TO_EX    All in the current interface state that must be blocked
  ALLOW    All with retransmission state that must be forwarded
  BLOCK    All with retransmission state that must be blocked

If the computed source list for either an ALLOW or a BLOCK record is
empty, that record is omitted from the State-Change report.

Note: When the first State-Change report is sent, the non-existent
pending report to merge with, can be treated as a source-change report
with empty ALLOW and BLOCK records (no sources have retransmission
state).



Cain, Deering, Kouvelas, Thyagarajan                           [Page 20]


INTERNET-DRAFT                   IGMPv3                       March 2000


5.2.  Action on Reception of a Query

When a system receives a Query, it does not respond immediately.
Instead, it delays its response by a random amount of time, bounded by
the Max Resp Time value in the received Query message.  A system may
receive a variety of Queries from different interfaces and of different
kinds (e.g., General Queries, Group-Specific Queries, and Group-and-
Source-Specific Queries), each of which may require its own delayed
response.

Before scheduling a response to a Query, the system must first consider
previously scheduled pending responses and in many cases schedule a
combined response. Therefore, the system must be able to maintain the
following state:

o A timer per interface for scheduling responses to General Queries.

o A per-group and interface timer for scheduling responses to Group-
  Specific and Group-and-Source-Specific Queries.

o A per-group list of sources to be reported in the response to a Group-
  and-Source-Specific Query.

When a new Query with the Router-Alert option arrives on an interface,
provided the system has state to report, a delay for a response is
randomly selected in the range (0, [Max-Resp-Time in Query]). The
following rules are then used to determine if a query needs to be
scheduled and the type of Query to schedule:

o If there is a pending response to a previous General Query scheduled
  sooner than the selected delay, no additional response needs to be
  scheduled.

o If the received Query is a General Query, the interface timer is used
  to schedule a response to the General Query after the selected delay.
  Any previously pending response to a General Query is canceled.

o If the received Query is a Group-Specific Query or a Group-and-Source-
  Specific Query and there is no pending response to a previous Query
  for this group, then the group timer is used to schedule a report. If
  the received Query is a Group-and-Source-Specific Query, the list of
  queried sources is recorded to be used when generating a response.

o If there already is a pending response to a previous Query scheduled
  for this group, and either the new Query is a Group-Specific Query or
  the recorded source-list associated with the group is empty, then the
  group source-list is cleared and a single response is scheduled using
  the group timer. The new response is scheduled to be sent at the



Cain, Deering, Kouvelas, Thyagarajan                           [Page 21]


INTERNET-DRAFT                   IGMPv3                       March 2000


  earliest of the remaining time for the pending report and the selected
  delay.

o If the received Query is a Group-and-Source-Specific Query and there
  is a pending response for this group with a non-empty source-list,
  then the group source list is augmented to contain the list of sources
  in the new Query and a single response is scheduled using the group
  timer. The new response is scheduled to be sent at the earliest of the
  remaining time for the pending report and the selected delay.

When the timer in a pending response record expires, the system
transmits, on the associated interface, one or more Report messages
carrying one or more Current-State Records (see section 4.2.9(1)), as
follows:

o If the expired timer is the interface timer (i.e., it is a pending
  response to a General Query), then one Current-State Record is sent
  for each multicast address for which the specified interface has
  reception state, as described in section 3.2.  The Current-Report
  Record carries the multicast address and its associated filter mode
  (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and source list.  Multiple
  Current-State Records are packed into individual Report messages, to
  the extent possible.

o If the expired timer is a group timer and the list of recorded sources
  for the that group is empty (i.e., it is a pending response to a
  Group-Specific Query), then if and only if the interface has reception
  state for that group address, a single Current-State Record is sent
  for that address.  The Current-Report Record carries the multicast
  address and its associated filter mode (MODE_IS_INCLUDE or
  MODE_IS_EXCLUDE) and source list.

o If the expired timer is a group timer and the list of recorded sources
  for that group is non-empty (i.e., it is a pending response to a
  Group-and-Source-Specific Query), then if and only if the interface
  has reception state for that group address, the contents of the
  responding Current-State Record is determined from the interface state
  and the pending response record, as specified in the following table:

                         set of sources in the
      interface state   pending response record   Current-State Record
      ---------------   -----------------------   --------------------

       INCLUDE (A)                B                   IS_IN (A*B)

       EXCLUDE (A)                B                   IS_IN (B-A)





Cain, Deering, Kouvelas, Thyagarajan                           [Page 22]


INTERNET-DRAFT                   IGMPv3                       March 2000


If the resulting Current-State Record has an empty set of source
addresses, then no response is sent.

Finally, after any required Report messages have been generated, the
source lists associated with any reported groups are cleared.


6.  DESCRIPTION OF THE PROTOCOL FOR MULTICAST ROUTERS

The purpose of IGMP is to enable each multicast router to learn, for
each of its directly attached networks, which multicast addresses are of
interest to the systems attached to those networks.  IGMP version 3 adds
the capability for a multicast router to also learn  which *sources* are
of interest to neighboring systems, for packets sent to any particular
multicast address.  The information gathered by IGMP is provided to
whichever multicast routing protocol is being used by the router, in
order to ensure that multicast packets are delivered to all networks
where there are interested receivers.

This section describes the part of IGMPv3 that is performed by multicast
routers.  Multicast routers may also themselves become members of
multicast groups, and therefore also perform the group member part of
IGMPv3, described in section 5.

A multicast router performs the protocol described in this section over
each of its directly-attached networks.  If a multicast router has more
than one interface to the same network, it only needs to operate this
protocol over one of those interfaces.  On each interface over which
this protocol is being run, the router MUST enable reception of
multicast address 224.0.0.22, from all sources (and MUST perform the
group member part of IGMPv3 for that address on that interface).

Multicast routers need to know only that *at least one* system on an
attached network is interested in packets to a particular multicast
address from a particular source; a multicast router is not required to
keep track of the interests of each individual neighboring system.

IGMPv3 is backward compatible with previous versions of the IGMP
protocol.  In order to remain backward compatible with older IGMP
systems, IGMPv3 multicast routers must also implement versions 1 and 2
of the protocol (see section 7).


6.1.  Conditions for IGMP Queries

Multicast routers send General Queries periodically to request group
membership information from an attached network.  These queries are used
to build and refresh the group membership state of systems on attached



Cain, Deering, Kouvelas, Thyagarajan                           [Page 23]


INTERNET-DRAFT                   IGMPv3                       March 2000


networks.  Systems respond to these queries by reporting their group
membership state (and their desired set of sources) with Current-State
Group Records in IGMPv3 Membership Reports.

As a member of a multicast group, a system may express interest in
receiving or not receiving traffic from particular sources.  As the
desired reception state of a system changes, it reports these changes
using Filter-Mode-Change Records or Source-List-Change Records.  These
records indicate an explicit state change in a group at a system in
either the group record's source list or its filter-mode.  When a group
membership is terminated at a system or traffic from a particular source
is no longer desired, a multicast router must query for other members of
the group or listeners of the source before deleting the group (or
source) and pruning its traffic.

To enable all systems on a network to respond to changes in group
membership, multicast routers send specific queries.  A Group- Specific
Query is sent to verify there are no systems that desire reception of
the specified group or to "rebuild" the desired reception state for a
particular group.  Group-Specific Queries are sent when a router
receives a state-change record indicating a system is leaving a group.

A Group-and-Source Specific Query is used to verify there are no systems
on a network which desire to receive a set of sources.  Group-and-Source
Specific Queries list sources for a particular group which been
requested to be no longer forwarded.  This query is sent by a multicast
router to learn if any systems desire reception of packets to the
specified group address from the specified source addresses.  Group-and-
Source Specific Queries are only sent in response to State-Change
Records and never in response to Current-State Records.  Section 4.1.8
describes each query in more detail.


6.2.  IGMP State Maintained by Multicast Routers

Multicast routers implementing IGMPv3 keep state per group per attached
network.  This group state consists of a filter-mode, a list of sources,
and various timers.  For each attached network running IGMP, a multicast
router records the desired reception state for that network.  That state
conceptually consists of a set of records of the form:

      (multicast address, group timer, filter-mode, (source records) )

Each source record is of the form:

      (source address, source timer)





Cain, Deering, Kouvelas, Thyagarajan                           [Page 24]


INTERNET-DRAFT                   IGMPv3                       March 2000


If all sources within a given group are desired, an empty source record
list is kept with filter-mode set to EXCLUDE.  This means forward all
sources for this group.  This is the IGMPv3 equivalent to a IGMPv1 or
IGMPv2 group join.


6.2.1.  Definition of Router Filter-Mode

To reduce internal state, IGMPv3 routers keep a filter-mode per group
per attached network.  This filter-mode is used to condense the total
desired reception state of a group to a minimum set such that all
systems' memberships are satisfied.  This filter-mode may change in
response to the reception of particular types of group records or when
certain timer conditions occur.  In the following sections, we use the
term "router filter-mode" to refer to the filter-mode of a particular
group within a router.  Section 6.4 describes the changes of a router
filter-mode per group record received.

Conceptually, when a group record is received, the router filter-mode
for that group is updated to represent the most number of sources
desired with the least amount of state.  As a rule, once a group record
with a filter-mode of EXCLUDE is received, the router filter-mode for
that group will be EXCLUDE.

When a router filter-mode for a group is EXCLUDE, the source record list
contains two types of sources.  The first type is the set of sources
which are not being forwarded (i.e. the set that all systems have agreed
NOT to forward).  The second set is the set which represents conflicts
in the desired reception state; this set must be forwarded.  Appendix A
describes the reasons for keeping this second set when in EXCLUDE mode.

When a router filter-mode for a group is INCLUDE, the source record list
is the list of sources desired for the group.  When a router filter-mode
for a group is INCLUDE, the source list is the total desired set of
sources for that group.  Each source in a source record must be
forwarded.

Because a reported group record with a filter-mode of EXCLUDE will cause
a router to transition its filter-mode for that group to EXCLUDE, a
mechanism for transitioning a router's filter-mode back to INCLUDE must
exist.  If all systems with a group record with filter-mode EXCLUDE mode
cease reporting, it is desirable for the router filter-mode for that
group to transition back to INCLUDE mode (which will likely cause fewer
sources to be forwarded).  This transition occurs when the group timer
expires and is explained in detail in section 6.5.






Cain, Deering, Kouvelas, Thyagarajan                           [Page 25]


INTERNET-DRAFT                   IGMPv3                       March 2000


6.2.2.  Definition of Group Timers

The group timer is only used when a group is in EXCLUDE mode and it
represents the time for the *filter-mode* of the group to expire and
switch to INCLUDE mode. We define a group timer as a decrementing timer
with a lower bound of zero kept per group per attached network.  Group
timers are updated according to the types of group records received.

A group timer expiring when a router filter-mode for the group is
EXCLUDE means there are no listeners on the attached network in EXCLUDE
mode.  At this point, a router will transition to INCLUDE filter-mode.
Section 6.5 describes the actions taken when a group timer expires while
in EXCLUDE mode.

The following table summarizes the role of the group timer.  Section 6.4
describes the details of setting the group timer per type of group
record received.

  Group
  Filter-Mode      Group Timer Value       Actions/Comments
  -----------      -----------------       ----------------

  INCLUDE          Timer >= 0              All members in INCLUDE
                                           mode.

  EXCLUDE          Timer > 0               At least one member in
                                           EXCLUDE mode.


  EXCLUDE          Timer == 0              No more listeners to
                                           group.  If all source
                                           timers have expired then
                                           delete Group Record.
                                           If there are still
                                           source record timers
                                           running, switch to
                                           INCLUDE filter-mode
                                           using those source records
                                           with running timers as the
                                           INCLUDE source record
                                           state.


6.2.3.  Definition of Source Timers

A source timer is kept per source record and is a decrementing timer
with a lower bound of zero.  Source timers are updated according to the
type and filter-mode of the group record received.  Source timers are



Cain, Deering, Kouvelas, Thyagarajan                           [Page 26]


INTERNET-DRAFT                   IGMPv3                       March 2000


always updated (for a particular group) whenever the source is present
in a received record for that group.  Section 6.4 describes the setting
of source timers per type of group records received.

A source record with a running timer with a router filter-mode for the
group of INCLUDE means that there is currently one or more systems (in
INCLUDE filter-mode) which desire to receive that source.  If a source
timer expires with a router filter-mode for the group of INCLUDE, the
router concludes that traffic from this particular source is no longer
desired on the attached network, and deletes the associated source
record.

Source timers are treated differently when a router filter-mode for a
group is EXCLUDE.  If a source record has a running timer with a router
filter-mode for the group of EXCLUDE, it means that a "conflict" has
occurred with respect to desired reception state of that source (i.e.
one system desires the source and another one doesn't).  Therefore, when
a source record has a running timer with a router filter-mode for the
group of EXCLUDE it should be forwarded.  Appendix A describes the
reasons for keeping sources that are being forwarded while in EXCLUDE
state.

If a source timer expires with a router filter-mode for the group of
EXCLUDE, the router stops forwarding traffic from this source onto the
network (and takes appropriate actions dependent on the multicast
routing protocol).

When a router filter-mode for a group is EXCLUDE, source records are
only deleted when the group timer expires.  Section 6.3 describes the
actions that should be taken dependent upon the value of a source timer.


6.3.  IGMPv3 Source-Specific Forwarding Rules

When a multicast router receives a datagram from a source destined to a
particular group, a decision has to be made whether to forward the
datagram onto an attached network or not.  This decision is first
dependent on any  multicast routing protocols present.  Assuming that
there are no other routers downstream (or that downstream routers
support source-specific pruning/grafting), the forwarding decision
depends on the presence of a group record and/or a source record.  If no
group record exists, the  datagram is not forwarded on the network.  If
a source record exists which matches the datagrams source address, the
source is forwarded  according to the router filter-mode for the group
and the value of the source timer of the source record.

To summarize, the following table describes the forwarding actions for
traffic originating from a source destined to a group.  It also



Cain, Deering, Kouvelas, Thyagarajan                           [Page 27]


INTERNET-DRAFT                   IGMPv3                       March 2000


summarizes the actions taken upon the expiration of a source timer based
on the router filter-mode of the group.

  Group
  Filter-Mode      Source Timer Value        Action
  -----------      ------------------        ------

  INCLUDE          TIMER > 0                 Forward traffic from
                                             source

  INCLUDE          TIMER == 0                Stop forwarding traffic
                                             from source and remove
                                             source record

  INCLUDE          No Source Elements        Don't forward source

  EXCLUDE          TIMER > 0                 Forward traffic from
                                             source

  EXCLUDE          TIMER == 0                Don't forward traffic
                                             from source
                                             (DO NOT remove record)

  EXCLUDE          No Source Elements        Forward traffic from
                                             source


6.4.  Action on Reception of Reports

6.4.1.  Reception of Current-State Records

When receiving Current-State Records, a router updates both its group
and source timers.  In some circumstances, the reception of a type of
group record will cause the router filter-mode for that group to change.
The table below describes the actions, with respect to state and timers
that occur to a router's state upon reception of Current-State Records.

The following notation is used to describe the updating of source
timers.  The notation ( A, B ) will be used to represent the total
number of sources for a particular group, where

 A = set of source records whose source timers > 0 (being forwarded)
 B = set of source records whose source timers = 0 (not being forwarded)

Note that there will only be two sets when a router's filter-mode for a
group is EXCLUDE.  When a router's filter-mode for a group is INCLUDE, a
single set is used to describe the set of sources being forwarded (e.g.
simply ( A ) ).



Cain, Deering, Kouvelas, Thyagarajan                           [Page 28]


INTERNET-DRAFT                   IGMPv3                       March 2000


In the following tables, abbreviations are used for several variables
(all of which are described in detail in section 8).  The variable GMI
is an abbreviation for the Group Membership Interval which is the time
in which group memberships will time out.  The variable LMQI is an
abbreviation for the Last Member Query Interval (default 1s) which is
the Maximum Response Time in Group or Group and Source Specific Queries.

Within the "Actions" section of the router state tables, we use the
notation 'A=J', which means that the set A of source records should have
their source timers set to value J.  'Delete A' means that the set A of
source records should be deleted.  'Group Timer=J' means that the Group
Timer for the group should be set to value J.

  Router State   Report Rec'd  New Router State         Actions
  ------------   ------------  ----------------         -------

  INCLUDE (A)    IS_IN (B)     INCLUDE (A+B)            (B)=GMI

  INCLUDE (A)    IS_EX (B)     EXCLUDE (A*B,B-A)        (B-A)=0
                                                        Delete (A-B)
                                                        Group Timer=GMI

  EXCLUDE (X,Y)  IS_IN (A)     EXCLUDE (X+A,Y-A)        (A)=GMI

  EXCLUDE (X,Y)  IS_EX (A)     EXCLUDE (X*A+(A-Y),Y*A)  (A-X-Y)=GMI
                                                        Delete (X-A)
                                                        Delete (Y-A)
                                                        Group Timer=GMI


6.4.2.  Reception of Filter-Mode-Change and Source-List-Change Records

When a change in the global state of a group occurs in a system, the
system sends either a Source-List-Change Record or a Filter-Mode-Change
Record for that group.  As with Current-State Records, routers must act
upon these records and possibly change their own state to reflect the
new desired membership state of the network.

Routers must query sources that are requested to be no longer forwarded
to a group.  When a router queries or receives a query for a specific
set of sources, it lowers its source timers for those source to a small
interval of Last Member Query Interval seconds.  If group records are
received in response to the queries which express interest in receiving
traffic from the queried sources, the corresponding timers are updated.

Similarly, when a router queries a specific group, it lowers its group
timer for that group to a small interval of Last Member Query Interval
seconds.  If any group records expressing EXCLUDE mode interest in the



Cain, Deering, Kouvelas, Thyagarajan                           [Page 29]


INTERNET-DRAFT                   IGMPv3                       March 2000


group are received within the interval, the group timer for the group is
updated and traffic is forwarded without any interruption.

During a query period (i.e. Last Member Query Interval seconds), a
router continues to forward traffic from the groups or sources that it
is querying.  It is not until after Last Member Query Interval seconds
without receiving a record expressing interest in the queried group or
sources that the router may prune the group or sources from the setwork.

The following table describes the changes in group state and the
action(s) taken when receiving either Filter-Mode-Change or Source-List-
Change Records.  This table also describes the queries which are sent by
the querier when a particular report is received.

We use the following notation for describing the queries which are sent.
We use the notation 'Q(G) to describe a Group-Specific Query to G.  We
use the notation 'Q(G,A)' to describe a Group-and-Source Specific Query
to G with source-list A. If source-list A is null as a result of the
action (e.g. A*B) then no query is sent as a result of the operation.

In order to maintain protocol robustness, queries sent by actions in the
table below need to be transmitted [Last Member Query Count] times
within the [Last Member Query Interval] period.

If while scheduling new queries, there are already pending queries to be
retransmitted for the same group, the new and pending queries have to be
merged. In addition, received host reports for a group with pending
queries may affect the contents of those queries. Section 6.6.3
describes the process of building and maintaining the state of pending
queries.





















Cain, Deering, Kouvelas, Thyagarajan                           [Page 30]


INTERNET-DRAFT                   IGMPv3                       March 2000


  Router State   Report Rec'd New Router State          Actions
  ------------   ------------ ----------------          -------

  INCLUDE (A)    ALLOW (B)    INCLUDE (A+B)             (B)=GMI

  INCLUDE (A)    BLOCK (B)    INCLUDE (A)               Send Q(G,A*B)

  INCLUDE (A)    ALLOW (B),   INCLUDE (A+B)             (B)=GMI
                 BLOCK (C)                              Send Q(G,A*C)

  INCLUDE (A)    TO_EX (B)    EXCLUDE (A*B,B-A)         (B-A)=0
                                                        Delete (A-B)
                                                        Send Q(G,A*B)
                                                        Group Timer=GMI

  INCLUDE (A)    TO_IN (B)    INCLUDE (A+B)             (B)=GMI
                                                        Send Q(A-B)

  EXCLUDE (X,Y)  ALLOW (A)    EXCLUDE (X+A,Y-A)         (A)=GMI

  EXCLUDE (X,Y)  BLOCK (A)    EXCLUDE (X+(A-Y),Y)       Send Q(G,A-Y)

  EXCLUDE (X,Y)  ALLOW (A),   EXCLUDE (X+A+(B-Y),Y-A)   (A)=GMI
                 BLOCK (B)                              Send Q(G,B-Y)

  EXCLUDE (X,Y)  TO_EX (A)    EXCLUDE (X*A+(A-Y),Y*A)   (A-X-Y)=LMQI
                                                        Delete (X-A)
                                                        Delete (Y-A)
                                                        Send Q(G,A-Y)
                                                        Group Timer=GMI

  EXCLUDE (X,Y)  TO_IN (A)    EXCLUDE (X+A,Y-A)         (A)=GMI
                                                        Send Q(G,X-A)
                                                        Send Q(G)


6.5.  Switching Router Filter-Modes

The group timer is used as a mechanism for transitioning the router
filter-mode from EXCLUDE to INCLUDE.

When a group timer expires with a router filter-mode of EXCLUDE, a
router assumes that there are no systems with a *filter-mode* of EXCLUDE
present on the attached network.  When a router's filter-mode for a
group is EXCLUDE and the group timer expires, the router filter-mode for
the group transitions to INCLUDE.





Cain, Deering, Kouvelas, Thyagarajan                           [Page 31]


INTERNET-DRAFT                   IGMPv3                       March 2000


A router uses source records with running source timers as its state for
the switch to a filter-mode of INCLUDE.  If there are any source records
with source timers greater than zero (i.e. being forwarded), a router
switches to filter-mode of INCLUDE using those source records.  Source
records whose timers are zero (from the previous EXCLUDE mode) are
deleted.

For example, if a router's state for a group is EXCLUDE(X,Y) and the
group timer expires for that group, the router switches to filter-mode
of INCLUDE with state INCLUDE(X).


6.6.  Action on Reception of Queries

6.6.1.  Timer Updates

When a router receives a query, it must update its timers to reflect the
correct timeout values for the group or sources being queried.  The
following table describes the timer actions for non-queriers when
receiving a Group-Specific or Group-and-Source Specific Query.  Querier
routers must also perform these actions when a query is sent.

    Query      Action
    -----      ------
    Q(G,A)     Source Timer for sources in A are lowered to LMQI
    Q(G)       Group Timer is lowered to LMQI


6.6.2.  Querier Election

IGMPv3 elects a single querier per subnet using the same querier
election mechanism as IGMPv2, namely by IP address.  When a router
receives a query with a lower IP address, it sets the Other-Querier-
Present timer to Other Querier Present Interval and ceases to send
queries on the network if it was the previously elected querier.  After
its Other-Querier Present timer expires, it should begin sending General
Queries.

If a router receives an older version query, it must use the oldest
version of IGMP on the network.  For a detailed description of
compatibility issues between IGMP versions see section 7.


6.6.3.  Building and Sending Specific Queries







Cain, Deering, Kouvelas, Thyagarajan                           [Page 32]


INTERNET-DRAFT                   IGMPv3                       March 2000


6.6.3.1.  Building and Sending Group Specific Queries

When a table action "Send Q(G)" is encountered, then the group timer
must be lowered to LMQI. The router must then immediately send a group
specific query as well as schedule [Last Member Query Count - 1] query
retransmissions to be sent within [Last Member Query Interval].

When transmitting a group specific query, if the group timer is larger
than LMQI, the "Suppress Router-Side Processing" bit is set in the query
message.


6.6.3.2.  Building and Sending Group and Source Specific Queries

When a table action "Send Q(G,X)" is encountered by a querier in the
table in section 6.4.2, the following actions must be performed for each
of the sources in X of group G, with source timer larger than LMQI:

o Set number of retransmissions for each source to [Last Member Query
  Count].

o Lower source timer to LMQI.

The router must then immediately send a group and source specific query
as well as schedule [Last Member Query Count -1] query retransmissions
to be sent within [Last Member Query Interval]. The contents of these
queries are calculated as follows.

When building a group and source specific query for a group G, two
separate query messages are sent for the group. The first one has the
"Suppress Router-Side Processing" bit set and contains all the sources
with retransmission state and timers greater than LMQI. The second has
the "Suppress Router-Side Processing" bit clear and contains all the
sources with retransmission state and timers lower or equal to LMQI. If
either of the two calculated messages does not contain any sources, then
its transmission is suppressed.

Note: If a group specific query is scheduled to be transmitted at the
same time as a group and source specific query for the same group, then
transmission of the group and source specific message with the "Suppress
Router-Side Processing" bit set may be suppressed.


7.  INTEROPERATION WITH OLDER VERSIONS OF IGMP

IGMP version 3 hosts and routers interoperate with hosts and routers
that have not yet been upgraded to IGMPv3.  This compatibility is
maintained by hosts and routers taking appropriate actions depending on



Cain, Deering, Kouvelas, Thyagarajan                           [Page 33]


INTERNET-DRAFT                   IGMPv3                       March 2000


the versions of IGMP operating on hosts and routers within a network.


7.1.  Query Version Distinctions

The IGMP version of a Membership Query message is determined as follows:

   IGMPv1 Query: length = 8 octets AND Max Response Time field is zero

   IGMPv2 Query: length = 8 octets AND Max Response Time field is
                 non-zero

   IGMPv3 Query: length >= 12 octets AND Reserved field is zero

Query messages that do not match any of the above conditions (e.g., a
Query of length 10 octets) must be silently ignored.


7.2.  Group Member Behavior

7.2.1.  In the Presence of Older Version Queriers

In order to be compatible with older version routers, IGMPv3 hosts MUST
operate in version 1 and version 2 compatibility modes.  IGMPv3 hosts
MUST keep state per local interface regarding the compatibility mode of
each attached network.  A host's compatibility mode is determined from
the Host Compatibility Mode variable which can be in one of three
states: IGMPv1, IGMPv2 or IGMPv3.   This variable is kept per interface
and is dependent on the version of General Queries heard on that
interface as well as the Older Version Querier Present timer for the
interface.

In order to switch gracefully between versions of IGMP, hosts keep both
an IGMPv1 Querier Present timer and an IGMPv2 Querier Present timer per
interface.  IGMPv1 Querier Present is set to Older Version Querier
Present Timeout seconds whenever an IGMPv1 Membership Query is received.
IGMPv2 Querier Present is set to Older Version Querier Present Timeout
seconds whenever an IGMPv2 Membership Query is received.

The Host Compatibility Mode of an interface changes whenever an older
version query (than the current compatibility mode) is heard or when
certain timer conditions occur.  When IGMPv1 Querier Present timer
expires, a host switches to Host Compatibility mode of IGMPv2 if it has
a running IGMPv2 Querier Present timer.  If it does not have a running
IGMPv2 Querier Present timer then it switches to Host Compatibility of
IGMPv3.  When IGMPv2 Querier Present timer expires, a host switches to
Host Compatibility mode of IGMPv3.




Cain, Deering, Kouvelas, Thyagarajan                           [Page 34]


INTERNET-DRAFT                   IGMPv3                       March 2000


The Host Compatibility Mode variable based on whether an older version
query was heard in the last Older Version Querier Present Timeout
seconds.  The Host Compatibility Mode is set depending on the following:

    Host Compatibility Mode           Timer State
    -----------------------           -----------

          IGMPv3 (default)            IGMPv2 Querier Present = 0
                                      and IGMPv1 Querier Present = 0

          IGMPv2                      IGMPv2 Querier Present > 0
                                      and IGMPv1 Querier Present = 0

          IGMPv1                      IGMPv1 Querier Present > 0

If a host receives a query which causes its Querier Present timers to be
updated and correspondingly its compatibility mode, it should switch
compatibility modes immediately.

When Host Compatibility Mode is IGMPv3, a host acts using the IGMPv3
protocol on that interface.  When Host Compatibility Mode is IGMPv2, a
host acts in IGMPv2 compatibility mode, using only the IGMPv2 protocol,
on that interface.  When Host Compatibility Mode is IGMPv1, a host acts
in IGMPv1 compatibility mode, using only the IGMPv1 protocol on that
interface.

An 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).

Whenever a host changes its compatability mode, it cancels all its
pending response and retransmission timers.


7.2.2.  In the Presence of Older Version Group Members

An IGMPv3 host may be placed on a network where there are hosts that
have not yet been upgraded to IGMPv3.  A host MAY allow its IGMPv3
Membership Record to be suppressed by either a Version 1 Membership
Report, or a Version 2 Membership Report.


7.3.  Multicast Router Behavior

7.3.1.  In the Presence of Older Version Queriers

IGMPv3 routers may be placed on a network where at least one router on
the network has not not yet been upgraded to IGMPv3.  The following
requirements apply:



Cain, Deering, Kouvelas, Thyagarajan                           [Page 35]


INTERNET-DRAFT                   IGMPv3                       March 2000


o If any older versions of IGMP are present on routers, the querier MUST
  use the lowest version of IGMP present on the network.  This must be
  administratively assured; routers that desire to be compatible with
  IGMPv1 and IGMPv2 MUST have a configuration option to act in IGMPv1 or
  IGMPv2 compatibility modes.  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 or
  IGMPv3 query, although such warnings MUST be rate-limited.

o If a router is not explicitly configured to use IGMPv1 and hears an
  IGMPv1 Query or IGMPv2 Query, it SHOULD log a warning.  These warnings
  MUST be rate-limited.


7.3.2.  In the Presence of Older Version Group Members

IGMPv3 routers may be placed on a network where there are hosts that
have not yet been upgraded to IGMPv3.  In order to be compatible with
older version hostd, IGMPv3 routers MUST operate in version 1 and
version 2 compatibility modes.  IGMPv3 routers keep a compatibility mode
per group record.  A group's compatibility mode is determined from the
Group Compatibility Mode variable which can be in one of three states:
IGMPv1, IGMPv2 or IGMPv3.   This variable is kept per group record and
is dependent on the version of Membership Reports heard for that group
as well as the Older Version Host Present timer for the group.

In order to switch gracefully between versions of IGMP, routers keep an
IGMPv1 Host Present timer and an IGMPv2 Host Present timer per group
record.  IGMPv1 Host Present is set to Older Version Host Present
Timeout seconds whenever an IGMPv1 Membership Report is received.
IGMPv2 Host Present is set to Older Version Host Present Timeout seconds
whenever an IGMPv2 Membership Report is received.

The Group Compatibility Mode of a group record changes whenever an older
version report (than the current compatibility mode) is heard or when
certain timer conditions occur.  When IGMPv1 Host Present timer expires,
a router switches to Group Compatibility mode of IGMPv2 if it has a
running IGMPv2 Host Present timer.  If it does not have a running IGMPv2
Host Present timer then it switches to Group Compatibility of IGMPv3.
When IGMPv2 Host Present timer expires, a router switches to Group
Compatibility mode of IGMPv3.

The Group Compatibility Mode variable based on whether an older version
report was heard in the last Older Version Host Present Timeout seconds.
The Group Compatibility Mode is set depending on the following:






Cain, Deering, Kouvelas, Thyagarajan                           [Page 36]


INTERNET-DRAFT                   IGMPv3                       March 2000


    Group Compatibility Mode          Timer State
    ------------------------          -----------

          IGMPv3 (default)            IGMPv2 Host Present = 0
                                      and IGMPv1 Host Present = 0

          IGMPv2                      IGMPv2 Host Present > 0
                                      and IGMPv1 Host Present = 0

          IGMPv1                      IGMPv1 Host Present > 0

If a host receives a report which causes its older Host Present timers
to be updated and correspondingly its compatibility mode, it should
switch compatibility modes immediately.

When Group Compatibility Mode is IGMPv3, a router acts using the IGMPv3
protocol for that group.  When Group Compatibility Mode is IGMPv2, a
router acts in IGMPv2 compatibility mode, using only the IGMPv2 protocol
for that group.  When Group Compatibility Mode is IGMPv1, a router acts
in IGMPv1 compatibility mode, using only the IGMPv1 protocol for that
group.


8.  LIST OF TIMERS, COUNTERS, AND THEIR DEFAULT VALUES

Most of these timers are configurable.  If non-default settings are
used, they MUST be consistent among all systems 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
network.  If a network 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 network; larger values cause IGMP Queries to be
sent less often.



Cain, Deering, Kouvelas, Thyagarajan                           [Page 37]


INTERNET-DRAFT                   IGMPv3                       March 2000


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 network; larger values make the
traffic lest bursty, as host responses are spread out over a larger
interval.  The number of seconds represented by the [Query Response
Interval] must 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
or a particular source 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.  It is



Cain, Deering, Kouvelas, Thyagarajan                           [Page 38]


INTERNET-DRAFT                   IGMPv3                       March 2000


also the Max Response Time inserted into Group-and-Source-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 or source.


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.  The Last Member
Query Count is also the number of Group-and-Source-Specific Queries sent
before the router assume there are no listeners to particular source.
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: 1 second.


8.11.  Older Version Querier Interval

The Older Version Querier Interval is the time-out for transitioning a
host back to IGMPv3 mode once an older version query is heard.  When an
older version query is received, hosts set their Older Version Querier
Present Timer to Older Version Querier Interval.

This value MUST be ((the Robustness Variable) times (the Query Interval
in the last Query received)) plus (one Query Response Interval).


8.12.  Older Host Present Interval

The Older Host Present Interval is the time-out for transitioning a
group back to IGMPv3 mode once an older version report is sent for that
group.  When an older version report is received, routers set their
Older Host Present Timer to Older Host Present Interval.

This value MUST be ((the Robustness Variable) times (the Query
Interval)) plus (one Query Response Interval).








Cain, Deering, Kouvelas, Thyagarajan                           [Page 39]


INTERNET-DRAFT                   IGMPv3                       March 2000


9.  SECURITY CONSIDERATIONS

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

9.1.  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].

Forged Queries may also cause hosts in exclude mode to create state,
leading to potential denial-of-service attacks.

Forged Query messages from the local network can be easily traced.
There are two measures necessary to defend against externally forged
Queries:

o Routers should not forward Queries. This is easier for a router to
  acomplish if the Query carries the Router-Alert option.

o Hosts should Ignore Queries without the Router-Alert option.


9.2.  Report messages

A forged Report message may cause multicast routers to think there are
members of a group on a network when there are not.  Forged Report
messages from the local network 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:

o Ignore the Report if you cannot identify the source address of the
  packet as belonging to a network assigned to the interface on which
  the packet was received.  This solution means that Reports sent by
  mobile hosts without addresses on the local network will be ignored.

o 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



Cain, Deering, Kouvelas, Thyagarajan                           [Page 40]


INTERNET-DRAFT                   IGMPv3                       March 2000


  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].  This can be
solved by providing routers with a configuration switch to ignore
Version 1 messages completely.  This breaks automatic compatibility with
Version 1 hosts, so should only be used in situations where "fast leave"
is critical.


10.  ACKNOWLEDGMENTS

We would like to thank Bill Fenner, Dave Thaler, Bob Quinn, Sumit Gupta,
and Michael Speer for comments and suggestions on this document.

Portions of the text of this document were copied from [RFC-1112] and
[RFC-2236].


11.  REFERENCES
[RFC-1112]   Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
             August 1989.

[RFC-2113]   Katz, D., "IP Router Alert Option," RFC 2113, April 1996.

[RFC 2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", RFC 2119, BCP14, March 1997.

[RFC-2236]   Fenner, W., "Internet Group Management Protocol, Version 2",
             RFC 2236, November 1997.

[FILTER-API] Thaler, D., B. Fenner, and B. Quinn, "Socket Interface
             Extensions for Multicast Source Filters", Work in progress,
             draft-ietf-idmr-msf-api-00.txt, February 2000.















Cain, Deering, Kouvelas, Thyagarajan                           [Page 41]


INTERNET-DRAFT                   IGMPv3                       March 2000


APPENDIX A.  DESIGN RATIONALE


A.1  The Need for State-Change Messages

IGMPv3 specifies two types of Membership Reports: Current-State and
State Change.  This section describes the rationale for the need for
both these types of Reports.

Routers need to distinguish Membership Reports that were sent in
response to Queries from those that were sent as a result of a change in
interface state.  Membership reports that are sent in response to
Membership Queries are used mainly to refresh the existing state at the
router; they typically do not cause transitions in state at the router.
Membership Reports that are sent in response to changes in interface
state require the router to take some action in response to the received
report (see Section 6.4).

The inability to distinguish between the two types of reports would
force a router to treat all Membership Reports as potential changes in
state and could result in increased processing at the router as well as
an increase in IGMP traffic on the network.


A.2  Host Suppression

In IGMPv1 and IGMPv2, a host would cancel sending a pending membership
reports if a similar report was observed from another member on the
network.  In IGMPv3, this suppression of host membership reports has
been removed.  The following points explain the reasons behind this
decision.

1 Routers may want to track per-host membership status on an interface
  This allows routers to implement fast leaves (e.g.  for layered
  multicast congestion control schemes) as well as track membership
  status for possible accounting purposes.

2 Membership Report suppression does not work well on bridged LANs.
  Many bridges and Layer2/Layer3 switches that implement IGMP snooping
  do not forward IGMP messages across LAN segments in order to prevent
  membership report suppression.  Removing membership report suppression
  eases the job of these IGMP snooping devices.

3 By eliminating membership report suppression, hosts have fewer
  messages to process; this leads to a simpler state machine
  implementation.





Cain, Deering, Kouvelas, Thyagarajan                           [Page 42]


INTERNET-DRAFT                   IGMPv3                       March 2000


4 In IGMPv3, a single membership report now bundles multiple multicast
  group records to decrease the number of packets sent.  In comparison,
  the previous versions of IGMP required that each multicast group be
  reported in a separate message.


A.3 Switching router filter modes from EXCLUDE to INCLUDE

If there exist hosts in both EXCLUDE and INCLUDE modes for a single
multicast group in a network, the router must be in EXCLUDE mode as well
(see section 6.2.1).  In EXCLUDE mode, a router forwards traffic from
all sources unless that source exists in the exclusion source list.  If
all hosts in EXCLUDE mode cease to exist, it would be desirable for the
router to switch back to INCLUDE mode seamlessly without interrupting
the flow of traffic to existing receivers.

One of the ways to accomplish this is for routers to keep track of all
sources desired by hosts that are in INCLUDE mode even though the router
itself is in EXCLUDE mode.  If the group timer now expires in EXCLUDE
mode, it implies that there are no hosts in EXCLUDE mode on the network
(otherwise a membership report from that host would have refreshed the
group timer).  The router can then switch to INCLUDE mode seamlessly
with the list of sources currently being forwarded in its source list.




























Cain, Deering, Kouvelas, Thyagarajan                           [Page 43]


INTERNET-DRAFT                   IGMPv3                       March 2000


AUTHORS' ADDRESSES

   Brad Cain
   Nortel Networks, Inc.
   600 Technology Park
   Billerica, MA 01821
   phone: +1-978-916-1316
   email: bcain@nortelnetworks.com

   Steve Deering
   Cisco Systems, Inc.
   170 Tasman Drive
   San Jose, CA 95134-1706
   phone: +1-408-527-8213
   email: deering@cisco.com

   Isidor Kouvelas
   Cisco Systems, Inc.
   170 Tasman Drive
   San Jose, CA 95134-1706
   phone: +1-408-525-0727
   email: kouvelas@cisco.com

   Ajit Thyagarajan
   Ericsson IP Infrastructure
   12120 Plum Orchard Dr.
   Silver Spring, MD 20904
   phone: +1-301-586-8200
   email: ajit@torrentnet.com






















Cain, Deering, Kouvelas, Thyagarajan                           [Page 44]


Html markup produced by rfcmarkup 1.129b, available from https://tools.ietf.org/tools/rfcmarkup/