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RFC 3376
Network Working Group B. Cain
Request for Comments: 3376 Cereva Networks
Obsoletes: 2236 S. Deering
Category: Standards Track I. Kouvelas
Cisco Systems
B. Fenner
AT&T Labs - Research
A. Thyagarajan
Ericsson
October 2002
Internet Group Management Protocol, Version 3
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
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 obsoletes RFC 2236.
Cain, et. al. Standards Track [Page 1]
RFC 3376 IGMPv3 October 2002
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The Service Interface for Requesting IP Multicast Reception . 3
3. Multicast Reception State Maintained by Systems . . . . . . . 5
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 7
5. Description of the Protocol for Group Members . . . . . . . . 19
6. Description of the Protocol for Multicast Routers . . . . . . 24
7. Interoperation with Older Versions of IGMP. . . . . . . . . . 35
8. List of Timers, Counters, and Their Default Values. . . . . . 40
9. Security Considerations . . . . . . . . . . . . . . . . . . . 43
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 47
12. Normative References. . . . . . . . . . . . . . . . . . . . . 47
13. Informative References. . . . . . . . . . . . . . . . . . . . 47
Appendix A. Design Rationale. . . . . . . . . . . . . . . . . 49
Appendix B. Summary of changes from IGMPv2. . . . . . . . . . 50
Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . 52
Full Copyright Statement. . . . . . . . . . . . . . . . . . . 53
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, as required to support
Source-Specific Multicast [SSM], 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.
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RFC 3376 IGMPv3 October 2002
Multicast Listener Discovery (MLD) is used in a similar way by IPv6
systems. MLD version 1 [MLD] implements the functionality of IGMP
version 2; MLD version 2 [MLDv2] implements the functionality of IGMP
version 3.
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.
2. The Service Interface for Requesting IP Multicast Reception
Within an IP system, there is (at least conceptually) a service
interface 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 service interface must
support the following operation:
IPMulticastListen ( socket, interface, multicast-address,
filter-mode, source-list )
where:
o "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.
o "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.
o "multicast-address" is the IP multicast address, or group, 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.
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RFC 3376 IGMPv3 October 2002
o "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.
o "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. When an operation causes the source list
size limit to be exceeded, the service interface MUST return an
error.
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. Each subsequent
request completely replaces any earlier request for the given socket,
interface and multicast address.
Previous versions of IGMP did not support source filters and had a
simpler service interface consisting of Join and Leave operations to
enable and disable reception of a given multicast address (from *all*
sources) on a given interface. The equivalent operations in the new
service interface follow:
The Join operation is equivalent to
IPMulticastListen ( socket, interface, multicast-address,
EXCLUDE, {} )
and the Leave operation is equivalent to:
IPMulticastListen ( socket, interface, multicast-address,
INCLUDE, {} )
where {} is an empty source list.
An example of an API providing the capabilities outlined in this
service interface is in [FILTER-API].
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RFC 3376 IGMPv3 October 2002
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)
At most one record per multicast-address exists for a given
interface. 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 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:
Cain, et. al. Standards Track [Page 5]
RFC 3376 IGMPv3 October 2002
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 will be delivered on
socket s1 but not on socket s2. Note that IGMP Queries and Reports
are not subject to source filtering and must always be processed by
hosts and routers.
Filtering of packets based upon a socket's multicast reception
state is a new feature of this service interface. The previous
service interface [RFC1112] described no filtering based upon
multicast join state; rather, a join on a socket simply caused the
host to join a group on the given interface, and packets destined
for that group could be delivered to all sockets whether they had
joined or not.
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} )
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RFC 3376 IGMPv3 October 2002
then the corresponding interface record on interface i is:
( m, EXCLUDE, {b, c} )
If a fourth socket is added, such as:
from socket s4: ( i, m, EXCLUDE, {} )
then the interface record becomes:
( m, EXCLUDE, {} )
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} )
An implementation MUST NOT use an EXCLUDE interface record to
represent a group when all sockets for this group are in INCLUDE
state. If system resource limits are reached when an interface
state source list is calculated, an error MUST be returned to the
application which requested the operation.
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, IP Precedence of Internetwork Control
(e.g., Type of Service 0xc0), and carries an IP Router Alert option
[RFC-2113] in its IP header. IGMP message types are registered by
the IANA [IANA-REG] as described by [RFC-3228].
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RFC 3376 IGMPv3 October 2002
There are two IGMP message types of concern to the IGMPv3 protocol
described in this document:
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:
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RFC 3376 IGMPv3 October 2002
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 Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Resv |S| QRV | QQIC | Number of Sources (N) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address [1] |
+- -+
| Source Address [2] |
+- . -+
. . .
. . .
+- -+
| Source Address [N] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1.1. Max Resp Code
The Max Resp Code field specifies the maximum time allowed before
sending a responding report. The actual time allowed, called the Max
Resp Time, is represented in units of 1/10 second and is derived from
the Max Resp Code as follows:
If Max Resp Code < 128, Max Resp Time = Max Resp Code
If Max Resp Code >= 128, Max Resp Code represents a floating-point
value as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1| exp | mant |
+-+-+-+-+-+-+-+-+
Max Resp Time = (mant | 0x10) << (exp + 3)
Small values of Max Resp Time allow 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). Larger values, especially in the exponential range,
allow tuning of the burstiness of IGMP traffic on a network.
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RFC 3376 IGMPv3 October 2002
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.
[RFC-1071]
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.
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 timer updates they
perform upon hearing a Query. It does not, however, suppress the
querier election or the normal "host-side" processing of a Query that
a router may be required to perform 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. Routers 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 or a statically configured value.
4.1.7. QQIC (Querier's Query Interval Code)
The Querier's Query Interval Code field specifies the [Query
Interval] used by the querier. The actual interval, called the
Querier's Query Interval (QQI), is represented in units of seconds
and is derived from the Querier's Query Interval Code as follows:
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RFC 3376 IGMPv3 October 2002
If QQIC < 128, QQI = QQIC
If QQIC >= 128, QQIC represents a floating-point value as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1| exp | mant |
+-+-+-+-+-+-+-+-+
QQI = (mant | 0x10) << (exp + 3)
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.
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.
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RFC 3376 IGMPv3 October 2002
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 in the multicast reception state, of their interfaces.
Reports have the following format:
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RFC 3376 IGMPv3 October 2002
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] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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RFC 3376 IGMPv3 October 2002
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.
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RFC 3376 IGMPv3 October 2002
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.
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RFC 3376 IGMPv3 October 2002
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.
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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 Source Addresses for Reports
An IGMP report is sent with a valid IP source address for the
destination subnet. The 0.0.0.0 source address may be used by a
system that has not yet acquired an IP address. Note that the
0.0.0.0 source address may simultaneously be used by multiple systems
on a LAN. Routers MUST accept a report with a source address of
0.0.0.0.
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4.2.14. 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 field contains *any* of the addresses
(unicast or multicast) assigned to the interface on which the Report
arrives.
4.2.15. 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.
4.2.16. 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 or CHANGE_TO_EXCLUDE_MODE, 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 or CHANGE_TO_EXCLUDE_MODE, a single Group
Record is sent, containing as many source addresses as can fit, and
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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.
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.)
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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.
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.
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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-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
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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).
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 derived from the Max Resp Code in the
received Query message. A system may receive a variety of Queries on
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 and interface 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]) where
Max Resp Time is derived from Max Resp Code in the received Query
message. The following rules are then used to determine if a Report
needs to be scheduled and the type of Report to schedule. The rules
are considered in order and only the first matching rule is applied.
1. If there is a pending response to a previous General Query
scheduled sooner than the selected delay, no additional response
needs to be scheduled.
2. 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.
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3. 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.
4. 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 earliest of the remaining time for the
pending report and the selected delay.
5. 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 4.2.12), as
follows:
1. 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-
State 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.
This naive algorithm may result in bursts of packets when a system
is a member of a large number of groups. Instead of using a
single interface timer, implementations are recommended to spread
transmission of such Report messages over the interval (0, [Max
Resp Time]). Note that any such implementation MUST avoid the
"ack-implosion" problem, i.e., MUST NOT send a Report immediately
on reception of a General Query.
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2. 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-State
Record carries the multicast address and its associated filter
mode (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and source list.
3. 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)
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.
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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.
(However, see Appendix A.2 point 1 for discussion.)
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 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.
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A Group-and-Source Specific Query is used to verify there are no
systems on a network which desire to receive traffic from a set of
sources. Group-and-Source Specific Queries list sources for a
particular group which have been requested to no longer be 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 4.1.11 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)
If all sources within a given group are desired, an empty source
record list is kept with filter-mode set to EXCLUDE. This means
hosts on this network want all sources for this group to be
forwarded. 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.
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Conceptually, when a group record is received, the router filter-mode
for that group is updated to cover all the requested sources using
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 which
represents conflicts in the desired reception state; this set must be
forwarded by some router on the network. The second type is the set
of sources which hosts have requested to not 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. This is the total
desired set of sources for that group. Each source in the source
record list must be forwarded by some router on the network.
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 in EXCLUDE
filter-mode cease reporting, it is desirable for the router filter-
mode for that group to transition back to INCLUDE mode. This
transition occurs when the group timer expires and is explained in
detail in section 6.5.
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.
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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 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 at least
one system desires the source. It should therefore be forwarded by a
router on the network. Appendix A describes the reasons for keeping
state for sources that have been requested to be forwarded while in
EXCLUDE state.
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If a source timer expires with a router filter-mode for the group of
EXCLUDE, the router informs the routing protocol that there is no
longer a receiver on the network interested in traffic from this
source.
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. The multicast routing
protocol in use is in charge of this decision, and should use the
IGMPv3 information to ensure that all sources/groups desired on a
subnetwork are forwarded to that subnetwork. IGMPv3 information does
not override multicast routing information; for example, if the
IGMPv3 filter-mode group for G is EXCLUDE, a router may still forward
packets for excluded sources to a transit subnet.
To summarize, the following table describes the forwarding
suggestions made by IGMP to the routing protocol for traffic
originating from a source destined to a group. It also 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 Suggest to forward traffic
from source
INCLUDE TIMER == 0 Suggest to stop forwarding
traffic from source and
remove source record. If
there are no more source
records for the group, delete
group record.
INCLUDE No Source Elements Suggest to not forward source
EXCLUDE TIMER > 0 Suggest to forward traffic
from source
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EXCLUDE TIMER == 0 Suggest to not forward
traffic from source
(DO NOT remove record)
EXCLUDE No Source Elements Suggest to 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 (Sources that at
least one host has requested to be forwarded)
B = set of source records whose source timers = 0 (Sources that IGMP
will suggest to the routing protocol not to forward)
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
requested to be forwarded (e.g., simply (A)).
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 LMQT
is an abbreviation for the Last Member Query Time, which is the total
time spent after Last Member Query Count retransmissions. LMQT
represents the "leave latency", or the difference between the
transmission of a membership change and the change in the information
given to the routing protocol.
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.
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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 (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
sources to a small interval of Last Member Query Time 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
Time seconds. If any group records expressing EXCLUDE mode interest
in the group are received within the interval, the group timer for
the group is updated and the suggestion to the routing protocol to
forward the group stands without any interruption.
During a query period (i.e., Last Member Query Time seconds), the
IGMP component in the router continues to suggest to the routing
protocol that it forwards traffic from the groups or sources that it
is querying. It is not until after Last Member Query Time seconds
without receiving a record expressing interest in the queried group
or sources that the router may prune the group or sources from the
network.
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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, once every [Last Member Query Interval].
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.
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) 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(G,A-B)
EXCLUDE (X,Y) ALLOW (A) EXCLUDE (X+A,Y-A) (A)=GMI
EXCLUDE (X,Y) BLOCK (A) EXCLUDE (X+(A-Y),Y) (A-X-Y)=Group Timer
Send Q(G,A-Y)
EXCLUDE (X,Y) TO_EX (A) EXCLUDE (A-Y,Y*A) (A-X-Y)=Group Timer
Delete (X-A)
Delete (Y-A)
Send Q(G,A-Y)
Group Timer=GMI
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RFC 3376 IGMPv3 October 2002
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.
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., requested to be
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 sends or receives a query with a clear Suppress
Router-Side Processing flag, 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 when sending or receiving
a Group-Specific or Group-and-Source Specific Query with the Suppress
Router-Side Processing flag not set.
Query Action
----- ------
Q(G,A) Source Timer for sources in A are lowered to LMQT
Q(G) Group Timer is lowered to LMQT
When a router sends or receives a query with the Suppress Router-Side
Processing flag set, it will not update its timers.
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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
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 LMQT. The router must then immediately send a
group specific query as well as schedule [Last Member Query Count -
1] query retransmissions to be sent every [Last Member Query
Interval] over [Last Member Query Time].
When transmitting a group specific query, if the group timer is
larger than LMQT, 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
LMQT:
o Set number of retransmissions for each source to [Last Member Query
Count].
o Lower source timer to LMQT.
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 every [Last Member Query Interval] over
[Last Member Query Time]. The contents of these queries are
calculated as follows.
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RFC 3376 IGMPv3 October 2002
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 LMQT. The
second has the "Suppress Router-Side Processing" bit clear and
contains all the sources with retransmission state and timers lower
or equal to LMQT. 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 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 Resp Code field is zero
IGMPv2 Query: length = 8 octets AND Max Resp Code field is
non-zero
IGMPv3 Query: length >= 12 octets
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
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RFC 3376 IGMPv3 October 2002
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
timers 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 General 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 the 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 the IGMPv2 Querier Present timer
expires, a host switches to Host Compatibility mode of IGMPv3.
The Host Compatibility Mode variable is based on whether an older
version General 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 not running
and IGMPv1 Querier Present not running
IGMPv2 IGMPv2 Querier Present running
and IGMPv1 Querier Present not running
IGMPv1 IGMPv1 Querier Present running
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.
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RFC 3376 IGMPv3 October 2002
An IGMPv1 router will send General Queries with the Max Resp Code set
to 0. This MUST be interpreted as a value of 100 (10 seconds).
An IGMPv2 router will send General Queries with the Max Resp Code set
to the desired Max Resp Time, i.e., the full range of this field is
linear and the exponential algorithm described in section 4.1.1 is
not used.
Whenever a host changes its compatibility 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 yet been upgraded to IGMPv3. The following
requirements apply:
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 Resp Code of 0
and truncated at the Group Address field (i.e., 8 bytes long), and
MUST ignore Leave Group messages. They SHOULD also warn about
receiving an IGMPv2 or IGMPv3 query, although such warnings MUST be
rate-limited. When in IGMPv2 mode, routers MUST send Periodic
Queries truncated at the Group Address field (i.e., 8 bytes long),
and SHOULD also warn about receiving an IGMPv3 query (such warnings
MUST be rate-limited). They also MUST fill in the Max Resp Time in
the Max Resp Code field, i.e., the exponential algorithm described
in section 4.1.1 is not used.
o If a router is not explicitly configured to use IGMPv1 or IGMPv2
and hears an IGMPv1 Query or IGMPv2 General Query, it SHOULD log a
warning. These warnings MUST be rate-limited.
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RFC 3376 IGMPv3 October 2002
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 hosts, 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. The IGMPv1 Host Present timer is set to Older Version
Host Present Timeout seconds whenever an IGMPv1 Membership Report is
received. The IGMPv2 Host Present timer 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 the 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 the IGMPv2 Host Present timer expires
and the IGMPv1 Host Present timer is not running, a router switches
to Group Compatibility mode of IGMPv3. Note that when a group
switches back to IGMPv3 mode, it takes some time to regain source-
specific state information. Source-specific information will be
learned during the next General Query, but sources that should be
blocked will not be blocked until [Group Membership Interval] after
that.
The Group Compatibility Mode variable is 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:
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RFC 3376 IGMPv3 October 2002
Group Compatibility Mode Timer State
------------------------ -----------
IGMPv3 (default) IGMPv2 Host Present not running
and IGMPv1 Host Present not running
IGMPv2 IGMPv2 Host Present running
and IGMPv1 Host Present not running
IGMPv1 IGMPv1 Host Present running
If a router 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 internally
translates the following IGMPv2 messages for that group to their
IGMPv3 equivalents:
IGMPv2 Message IGMPv3 Equivalent
-------------- -----------------
Report IS_EX( {} )
Leave TO_IN( {} )
IGMPv3 BLOCK messages are ignored, as are source-lists in TO_EX()
messages (i.e., any TO_EX() message is treated as TO_EX( {} )).
When Group Compatibility Mode is IGMPv1, a router internally
translates the following IGMPv1 and IGMPv2 messages for that group to
their IGMPv3 equivalents:
IGMP Message IGMPv3 Equivalent
------------ -----------------
v1 Report IS_EX( {} )
v2 Report IS_EX( {} )
In addition to ignoring IGMPv3 BLOCK messages and source-lists in
TO_EX() messages as in IGMPv2 Group Compatibility Mode, IGMPv2 Leave
messages and IGMPv3 TO_IN() messages are also ignored.
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RFC 3376 IGMPv3 October 2002
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.
8.3. Query Response Interval
The Max Response Time used to calculate the Max Resp Code 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 less bursty, as host responses are spread out over a
larger interval. The number of seconds represented by the [Query
Response Interval] must be less than the [Query Interval].
8.4. Group Membership Interval
The Group Membership Interval is the amount of time that must pass
before a multicast router decides there are no more members of a
group or a particular source on a network.
This value MUST be ((the Robustness Variable) times (the Query
Interval)) plus (one Query Response Interval).
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RFC 3376 IGMPv3 October 2002
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 used to
calculate the Max Resp Code inserted into Group-Specific Queries sent
in response to Leave Group messages. It is also the Max Response
Time used in calculating the Max Resp Code for Group-and-Source-
Specific Query messages. Default: 10 (1 second)
Note that for values of LMQI greater than 12.8 seconds, a limited set
of values can be represented, corresponding to sequential values of
Max Resp Code. When converting a configured time to a Max Resp Code
value, it is recommended to use the exact value if possible, or the
next lower value if the requested value is not exactly representable.
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 assumes there are no listeners for a
particular source. Default: the Robustness Variable.
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RFC 3376 IGMPv3 October 2002
8.10. Last Member Query Time
The Last Member Query Time is the time value represented by the Last
Member Query Interval, multiplied by the Last Member Query Count. It
is not a tunable value, but may be tuned by changing its components.
8.11. 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.
8.12. Older Version Querier Present Timeout
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).
8.13. 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).
8.14. Configuring Timers
This section is meant to provide advice to network administrators on
how to tune these settings to their network. Ambitious router
implementations might tune these settings dynamically based upon
changing characteristics of the network.
8.14.1. Robustness Variable
The Robustness Variable tunes IGMP to expected losses on a link.
IGMPv3 is robust to (Robustness Variable - 1) packet losses, e.g., if
the Robustness Variable is set to the default value of 2, IGMPv3 is
robust to a single packet loss but may operate imperfectly if more
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RFC 3376 IGMPv3 October 2002
losses occur. On lossy subnetworks, the Robustness Variable should
be increased to allow for the expected level of packet loss. However,
increasing the Robustness Variable increases the leave latency of the
subnetwork. (The leave latency is the time between when the last
member stops listening to a source or group and when the traffic
stops flowing.)
8.14.2. Query Interval
The overall level of periodic IGMP traffic is inversely proportional
to the Query Interval. A longer Query Interval results in a lower
overall level of IGMP traffic. The Query Interval MUST be equal to
or longer than the Max Response Time inserted in General Query
messages.
8.14.3. Max Response Time
The burstiness of IGMP traffic is inversely proportional to the Max
Response Time. A longer Max Response Time will spread Report
messages over a longer interval. However, a longer Max Response Time
in Group-Specific and Source-and-Group-Specific Queries extends the
leave latency. (The leave latency is the time between when the last
member stops listening to a source or group and when the traffic
stops flowing.) The expected rate of Report messages can be
calculated by dividing the expected number of Reporters by the Max
Response Time. The Max Response Time may be dynamically calculated
per Query by using the expected number of Reporters for that Query as
follows:
Query Type Expected number of Reporters
---------- ----------------------------
General Query All systems on subnetwork
Group-Specific Query All systems that had expressed interest
in the group on the subnetwork
Source-and-Group- All systems on the subnetwork that had
Specific Query expressed interest in the source and group
A router is not required to calculate these populations or tune the
Max Response Time dynamically; these are simply guidelines.
9. Security Considerations
We consider the ramifications of a forged message of each type, and
describe the usage of IPSEC AH to authenticate messages if desired.
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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].
A DoS attack on a host could be staged through forged Group-and-
Source-Specific Queries. The attacker can find out about membership
of a specific host with a general query. After that it could send a
large number of Group-and-Source-Specific queries, each with a large
source list and the Maximum Response Time set to a large value. The
host will have to store and maintain the sources specified in all of
those queries for as long as it takes to send the delayed response.
This would consume both memory and CPU cycles in order to augment the
recorded sources with the source lists included in the successive
queries.
To protect against such a DoS attack, a host stack implementation
could restrict the number of Group-and-Source-Specific Queries per
group membership within this interval, and/or record only a limited
number of sources.
Forged Query messages from the local network can be easily traced.
There are three measures necessary to defend against externally
forged Queries:
o Routers SHOULD NOT forward Queries. This is easier for a router to
accomplish if the Query carries the Router-Alert option.
o Hosts SHOULD ignore v2 or v3 Queries without the Router-Alert
option.
o Hosts SHOULD ignore v1, v2 or v3 General Queries sent to a
multicast address other than 224.0.0.1, the all-systems address.
9.2. Current-State 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:
Cain, et. al. Standards Track [Page 44]
RFC 3376 IGMPv3 October 2002
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. Report messages with a source address of 0.0.0.0 SHOULD
be accepted on any interface.
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 IGMPv1 or earlier versions of IGMPv2 which did not require
Router Alert.
A forged Version 1 Report Message may put a router into "version 1
members present" state for a particular group, meaning that the
router will ignore Leave messages. This can cause traffic to flow to
groups with no members for up to [Group Membership Interval]. 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.
A forged Version 2 Report Message may put a router into "version 2
members present" state for a particular group, meaning that the
router will ignore IGMPv3 source-specific state messages. This can
cause traffic to flow from unwanted sources for up to [Group
Membership Interval]. This can be solved by providing routers with a
configuration switch to ignore Version 2 messages completely. This
breaks automatic compatibility with Version 2 hosts, so should only
be used in situations where source include and exclude is critical.
9.3. State-Change Report Messages
A forged State-Change Report message will cause the Querier to send
out Group-Specific or Source-and-Group-Specific Queries for the group
in question. This causes extra processing on each router and on each
member of the group, but can not cause loss of desired traffic.
There are two defenses against externally forged State-Change Report
messages:
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RFC 3376 IGMPv3 October 2002
o Ignore the State-Change Report message if you cannot identify the
source address of the packet as belonging to a subnet assigned to
the interface on which the packet was received. This solution
means that State-Change Report messages sent by mobile hosts
without addresses on the local subnet will be ignored. State-
Change Report messages with a source address of 0.0.0.0 SHOULD be
accepted on any interface.
o Ignore State-Change Report messages without Router Alert options
[RFC-2113], and require that routers not forward State-Change
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.)
9.4. IPSEC Usage
In addition to these measures, IPSEC in Authentication Header mode
[AH] may be used to protect against remote attacks by ensuring that
IGMPv3 messages came from a system on the LAN (or, more specifically,
a system with the proper key). When using IPSEC, the messages sent
to 224.0.0.1 and 224.0.0.22 should be authenticated using AH. When
keying, there are two possibilities:
1. Use a symmetric signature algorithm with a single key for the LAN
(or a key for each group). This allows validation that a packet
was sent by a system with the key. This has the limitation that
any system with the key can forge a message; it is not possible to
authenticate the individual sender precisely. It also requires
disabling IPSec's Replay Protection.
2. When appropriate key management standards have been developed, use
an asymmetric signature algorithm. All systems need to know the
public key of all routers, and all routers need to know the public
key of all systems. This requires a large amount of key
management but has the advantage that senders can be authenticated
individually so e.g., a host cannot forge a message that only
routers should be allowed to send.
This solution only directly applies to Query and Leave messages in
IGMPv1 and IGMPv2, since Reports are sent to the group being reported
and it is not feasible to agree on a key for host-to-router
communication for arbitrary multicast groups.
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RFC 3376 IGMPv3 October 2002
10. IANA Considerations
All IGMP types described in this document are already assigned in
[IANA-REG].
11. Acknowledgments
We would like to thank Ran Atkinson, Luis Costa, Toerless Eckert,
Dino Farinacci, Serge Fdida, Wilbert de Graaf, Sumit Gupta, Mark
Handley, Bob Quinn, Michael Speer, Dave Thaler and Rolland Vida for
comments and suggestions on this document.
Portions of the text of this document were copied from [RFC-1112] and
[RFC-2236].
12. Normative References
[AH] Kent, S. and R. Atkinson, "IP Authentication Header",
RFC 2402, November 1998.
[IANA-REG] http://www.iana.org/assignments/igmp-type-numbers
[RFC-1112] Deering, S., "Host Extensions for IP Multicasting", STD
5, RFC 1112, August 1989.
[RFC-2113] Katz, D., "IP Router Alert Option," RFC 2113, February,
1997.
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC-2236] Fenner, W., "Internet Group Management Protocol, Version
2", RFC 2236, November 1997.
[RFC-3228] Fenner, B., "IANA Considerations for IPv4 Internet Group
Management Protocol (IGMP)", BCP 57, RFC 3228, February
2002.
13. Informative References
[RFC-1071] Braden, R., Borman, D. and C. Partridge, "Computing the
Internet checksum", RFC 1071, September 1988.
[FILTER-API] Thaler, D., B. Fenner, and B. Quinn, "Socket Interface
Extensions for Multicast Source Filters", Work in
Progress.
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RFC 3376 IGMPv3 October 2002
[SSM] Bhattacharyya, S., et. al., "An Overview of Source-
Specific Multicast (SSM)", Work in Progress.
[MLD] Deering, S., Fenner, W. and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710, October
1999.
[MLDV2] Vida, R., L. Costa, S. Fdida, S. Deering, B. Fenner, I.
Kouvelas, and B. Haberman, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", Work in Progress.
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RFC 3376 IGMPv3 October 2002
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.
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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.
Appendix B. Summary of Changes from IGMPv2
While the main additional feature of IGMPv3 is the addition of source
filtering, the following is a summary of other changes from RFC 2236.
o State is maintained as Group + List-of-Sources, not simply Group as
in IGMPv2.
o Interoperability with IGMPv1 and IGMPv2 systems is defined as
operations on the IGMPv3 state.
o The IP Service Interface has changed to allow specification of
source-lists.
o The Querier includes its Robustness Variable and Query Interval in
Query packets to allow synchronization of these variables on non-
Queriers.
o The Max Response Time in Query messages has an exponential range,
changing the maximum from 25.5 seconds to about 53 minutes, for use
on links with huge numbers of systems.
o Hosts retransmit state-change messages for increased robustness.
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RFC 3376 IGMPv3 October 2002
o Additional data sections are defined to allow later extensions.
o Report packets are sent to 224.0.0.22, to assist layer-2 switches
in "snooping".
o Report packets can contain multiple group records, to allow
reporting of full current state using fewer packets.
o Hosts no longer perform suppression, to simplify implementations
and permit explicit membership tracking.
o New Suppress Router-Side Processing (S) flag in Query messages
fixes robustness issues which were also present in IGMPv2.
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RFC 3376 IGMPv3 October 2002
Authors' Addresses
Brad Cain
Cereva Networks
Steve Deering
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA 95134-1706
Phone: +1-408-527-8213
EMail: deering@cisco.com
Bill Fenner
AT&T Labs - Research
75 Willow Rd.
Menlo Park, CA 94025
Phone: +1-650-330-7893
EMail: fenner@research.att.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
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RFC 3376 IGMPv3 October 2002
Full Copyright Statement
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Cain, et. al. Standards Track [Page 53]
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