INTERNET-DRAFT Brad Cain, Nortel Networks
Steve Deering, Cisco Systems
Ajit Thyagarajan, Ericsson
Expires June 2000 November 1999
Internet Group Management Protocol, Version 3
<draft-ietf-idmr-igmp-v3-02.txt>
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as work in progress.
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document specifies Version 3 of the Internet Group Management
Protocol, IGMPv3. IGMP is the protocol used by IPv4 systems to report
their IP multicast group memberships to neighboring multicast routers.
Version 3 of IGMP adds support for "source filtering", that is, the
ability for a system to report interest in receiving packets *only* from
specific source addresses, or from *all but* specific source addresses,
sent to a particular multicast address. That information may be used
by multicast routing protocols to avoid delivering multicast packets
from specific sources to networks where there are no interested
receivers.
This document is a product of the Inter-Domain Multicast Routing working
group within the Internet Engineering Task Force. Comments are
solicited and should be addressed to the working group's mailing list at
idmr@cs.ucl.ac.uk and/or the authors.
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The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
in this document are to be interpreted as described in [RFC 2119]. Due
to the lack of italics, emphasis is indicated herein by bracketing a word
or phrase in "*" characters.
TABLE OF CONTENTS
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The API for Requesting IP Multicast Reception. . . . . . . . . 3
3. Multicast Reception State Maintained by Systems. . . . . . . . 5
4. Message Formats. . . . . . . . . . . . . . . . . . . . . . . . 8
5. Description of the Protocol for Group Members. . . . . . . . . 18
6. Description of the Protocol for Multicast Routers. . . . . . . 22
7. Interoperation with Older Versions of IGMP . . . . . . . . . . 32
8. List of Timers, Counters, and their Default Values . . . . . . 39
9. Security Considerations. . . . . . . . . . . . . . . . . . . . 42
10. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 43
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 46
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1. INTRODUCTION
The Internet Group Management Protocol (IGMP) is used by IPv4 systems
(hosts and routers) to report their IP multicast group memberships to
any neighboring multicast routers. Note that an IP multicast router
may itself be a member of one or more multicast groups, in which case
it performs both the "multicast router part" of the protocol (to
collect the membership information needed by its multicast routing
protocol) and the "group member part" of the protocol (to inform itself
and other, neighboring multicast routers of its memberships).
IGMP is also used for other IP multicast management functions, using
message types other than those used for group membership reporting.
This document specifies only the group membership reporting functions
and messages.
This document specifies Version 3 of IGMP. Version 1, specified in
[RFC-1112], was the first widely-deployed version and the first version
to become an Internet Standard. Version 2, specified in [RFC-2236],
added support for "low leave latency", that is, a reduction in the time
it takes for a multicast router to learn that there are no longer any
members of a particular group present on an attached network. Version 3
adds support for "source filtering", that is, the ability for a system
to report interest in receiving packets *only* from specific source
addresses, or from *all but* specific source addresses, sent to a
particular multicast address. Version 3 is designed to be
interoperable with Versions 1 and 2.
2. THE API FOR REQUESTING IP MULTICAST RECEPTION
Within an IP system, there is (at least conceptually) an Application
Programming Interface or API used by upper-layer protocols or
application programs to ask the IP layer to enable and disable reception
of packets sent to specific IP multicast addresses. In order to take
full advantage of the capabilities of IGMPv3, a system's IP API must
support the following operation (or any logical equivalent):
IPMulticastListen ( socket, interface, multicast-address,
filter-mode, source-list )
where
"socket" is an implementation-specific parameter used to distinguish
among different requesting entities (e.g., programs or processes)
within the system; the socket parameter of BSD Unix system calls
is a specific example.
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"interface" is a local identifier of the network interface on which
reception of the specified multicast address is to be enabled or
disabled. Interfaces may be physical (e.g., an Ethernet interface)
or virtual (e.g., the endpoint of a Frame Relay virtual circuit or
the endpoint of an IP-in-IP "tunnel"). An implementation may allow
a special "unspecified" value to be passed as the interface
parameter, in which case the request would apply to the "primary" or
"default" interface of the system (perhaps established by system
configuration). If reception of the same multicast address is
desired on more than one interface, IPMulticastListen is invoked
separately for each desired interface.
"multicast-address" is the IP multicast address to which the request
pertains. If reception of more than one multicast address on a given
interface is desired, IPMulticastListen is invoked separately for
each desired multicast address.
"filter-mode" may be either INCLUDE or EXCLUDE. In INCLUDE mode,
reception of packets sent to the specified multicast address is
requested *only* from those IP source addresses listed in the
source-list parameter. In EXCLUDE mode, reception of packets sent
to the given multicast address is requested from all IP source
addresses *except* those listed in the source-list parameter.
"source-list" is an unordered list of zero or more IP unicast
addresses from which multicast reception is desired or not desired,
depending on the filter mode. An implementation MAY impose a limit
on the size of source lists, but that limit MUST NOT be less than
64 addresses per list.
For a given combination of socket, interface, and multicast address,
only a single filter mode and source list can be in effect at any one
time. However, either the filter mode or the source list, or both, may
be changed by subsequent IPMulticastListen requests that specify the
same socket, interface, and multicast address.
Previous versions of IGMP did not support source filters and had a
simpler API consisting of Join and Leave operations to enable and
disable reception of a given multicast address (from *all* sources) on
a given interface. Those Join and Leave operations are supported by
the new API as follows:
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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.
It is recommended that implementations continue to support the old API,
(perhaps as calls on the new API) for compatibility with pre-existing IP
multicast applications.
3. MULTICAST RECEPTION STATE MAINTAINED BY SYSTEMS
3.1 Socket State
For each socket on which IPMulticastListen has been invoked, the system
records the desired multicast reception state for that socket. That
state conceptually consists of a set of records of the form:
(interface, multicast-address, filter-mode, source-list)
The socket state evolves in response to each invocation of
IPMulticastListen on the socket, as follows:
o If the requested filter mode is INCLUDE *and* the requested source
list is empty, then the entry corresponding to the requested
interface and multicast address is deleted if present. If no such
entry is present, the request is ignored.
o If the requested filter mode is EXCLUDE *or* the requested source
list is non-empty, then the entry corresponding to the requested
interface and multicast address, if present, is changed to contain
the requested filter mode and source list. If no such entry is
present, a new entry is created, using the parameters specified in
the request.
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3.2 Interface State
In addition to the per-socket multicast reception state, a system must
also maintain or compute multicast reception state for each of its
interfaces. That state conceptually consists of a set of records of
the form:
(multicast-address, filter-mode, source-list)
This per-interface state is derived from the per-socket state, but may
differ from the per-socket state when different sockets have differing
filter modes and/or source lists for the same multicast address and
interface. For example, suppose one application or process invokes the
following operation on socket s1:
IPMulticastListen ( s1, i, m, INCLUDE, {a, b, c} )
requesting reception on interface i of packets sent to multicast
address m, *only* if they come from source a, b, or c. Suppose another
application or process invokes the following operation on socket s2:
IPMulticastListen ( s2, i, m, INCLUDE, {b, c, d} )
requesting reception on the same interface i of packets sent to the
same multicast address m, *only* if they come from sources b, c, or d.
In order to satisfy the reception requirements of both sockets, it is
necessary for interface i to receive packets sent to m from any one of
the sources a, b, c, or d. Thus, in this example, the reception state
of interface i for multicast address m has filter mode INCLUDE and
source list {a, b, c, d}.
(After a multicast packet has been accepted from an interface by the IP
layer, its subsequent delivery to the application or process listening
on a particular socket depends on the multicast reception state of that
socket [and possibly also on other conditions, such as what transport-
layer port the socket is bound to]. So, in the above example, if a
packet arrives on interface i, destined to multicast address m, with
source address a, it may be delivered on socket s1 but not on socket
s2.)
The general rules for deriving the per-interface state from the per-
socket state are as follows: For each distinct (interface, multicast-
address) pair that appears in any socket state, a per-interface record
is created for that multicast address on that interface. Considering
all socket records containing the same (interface, multicast-address)
pair,
o if *any* such record has a filter mode of EXCLUDE, then the filter
mode of the interface record is EXCLUDE, and the source list of
the interface record is the intersection of the source lists of all
socket records in EXCLUDE mode, minus those source addresses that
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appear in any socket record in INCLUDE mode. For example, if the
socket records for multicast address m on interface i are:
from socket s1: ( i, m, EXCLUDE, {a, b, c, d} )
from socket s2: ( i, m, EXCLUDE, {b, c, d, e} )
from socket s3: ( i, m, INCLUDE, {d, e, f} )
then the corresponding interface record on interface i is:
( m, EXCLUDE, {b, c} )
o if *all* such records have a filter mode of INCLUDE, then the
filter mode of the interface record is INCLUDE, and the source list
of the interface record is the union of the source lists of all the
socket records. For example, if the socket records for multicast
address m on interface i are:
from socket s1: ( i, m, INCLUDE, {a, b, c} )
from socket s2: ( i, m, INCLUDE, {b, c, d} )
from socket s3: ( i, m, INCLUDE, {e, f} )
then the corresponding interface record on interface i is:
( m, INCLUDE, {a, b, c, d, e, f} )
In order to bound storage consumption, an implementation MAY impose
a limit on the size of the source list in an interface record whose
filter mode is INCLUDE, but that limit MUST NOT be less than 64
addresses. If such a limit is imposed, whenever the union of the
source lists of all the relevant socket records would exceed that
limit, an interface record of the following form would be created,
instead:
( m, EXCLUDE, {} )
which enables reception of packets to multicast address m from *all*
sources.
The above rules for deriving the interface state are (re-)evaluated
whenever an IPMulticastListen invocation modifies the socket state by
adding, deleting, or modifying a per-socket state record. Note that a
change of socket state does not necessarily result in a change of
interface state.
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4. MESSAGE FORMATS
IGMP messages are encapsulated in IPv4 datagrams, with an IP protocol
number of 2. Every IGMP message described in this document is sent
with an IP Time-to-Live of 1, and carries an IP Router Alert option
[RFC-2113] in its IP header.
There are two IGMP message types of concern to the IGMPv3 protocol
described in this document:
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.
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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:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x11 | Max Resp Time | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Number of Sources (N) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address [1] |
+- -+
| Source Address [2] |
+- . -+
. . .
. . .
+- -+
| Source Address [N] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1.1 Max Resp Time
The Max Resp Time field specifies the maximum time allowed before
sending a responding report, in units of 1/10 second.
Varying this setting allows IGMPv3 routers to tune the "leave
latency" (the time between the moment the last host leaves a group
and the moment the routing protocol is notified that there are no
more members). It also allows tuning of the burstiness of IGMP
traffic on a network.
4.1.2 Checksum
The Checksum is the 16-bit one's complement of the one's complement
sum of the whole IGMP message (the entire IP payload). For
computing the checksum, the Checksum field is set to zero. When
receiving packets, the checksum MUST be verified before processing
a packet.
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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 4.1.8, below).
4.1.4 Reserved
The Reserved field is set to zero on transmission, and ignored on
reception.
4.1.5 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).
4.1.6 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.
4.1.7 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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4.1.8 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.
4.1.9 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.
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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:
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] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where each Group Record has the following internal format:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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.
4.2.5 Record Type
See section 4.2.12, below.
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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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4.2.12 Group Record Types
There are a number of different types of Group Records that may be
included in a Report message:
(1) 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.
(2) 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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(3) A "Source-List-Change Record" is sent by a system whenever a
local invocation of IPMulticastListen causes a change of source
list that is *not* coincident with a change of filter mode, of
the interface-level state entry for a particular multicast
address. The Record is included in a Report sent from the
interface on which the change occurred. The Record Type of a
Source-List-Change Record may be one of the following two
values:
5 ALLOW_NEW_SOURCES - indicates that the Source Address
[i] fields in this Group Record contain a list of the
additional sources that the system wishes to
hear from, for packets sent to the specified
multicast address. If the change was to an INCLUDE
source list, these are the addresses that were added
to the list; if the change was to an EXCLUDE source
list, these are the addresses that were deleted from
the list.
6 BLOCK_OLD_SOURCES - indicates that the Source Address
[i] fields in this Group Record contain a list of the
sources that the system no longer wishes to
hear from, for packets sent to the specified
multicast address. If the change was to an INCLUDE
source list, these are the addresses that were
deleted from the list; if the change was to an
EXCLUDE source list, these are the addresses that
were added to the list.
If a change of source list results in both allowing new sources
and blocking old sources, then two Group Records are sent for
the same multicast address, one of type ALLOW_NEW_SOURCES and
one of type BLOCK_OLD_SOURCES.
We use the term "State-Change Record" to refer to either a Filter-
Mode-Change Record or a Source-List-Change Record.
Unrecognized Record Type values MUST be silently ignored.
4.2.13 IP Destination Addresses for Reports
Version 3 Reports are sent with an IP destination address of
224.0.0.22, to which all IGMPv3-capable multicast routers listen.
A system that is operating in version 1 or version 2 compatibility
modes sends version 1 or version 2 Reports to the multicast group
specified in the Group Address field of the Report. In addition,
a system MUST accept and process any version 1 or version 2 Report
whose IP Destination Address field contains *any* of the addresses
(unicast or multicast) assigned to the interface on which the Report
arrives.
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4.2.14 Notation for Group Records
In the rest of this document, we use the following notation to
describe the contents of a Group Record pertaining to a particular
multicast address:
IS_IN ( x ) - Type MODE_IS_INCLUDE, source addresses x
IS_EX ( x ) - Type MODE_IS_EXCLUDE, source addresses x
TO_IN ( x ) - Type CHANGE_TO_INCLUDE_MODE, source addresses x
TO_EX ( x ) - Type CHANGE_TO_EXCLUDE_MODE, source addresses x
ALLOW ( x ) - Type ALLOW_NEW_SOURCES, source addresses x
BLOCK ( x ) - Type BLOCK_OLD_SOURCES, source addresses x
where x is either:
- a capital letter (e.g., "A") to represent the set of source
addresses, or
- a set expression (e.g., "A+B"), where "A+B" means the union
of sets A and B, "A*B" means the intersection of sets A and
B, and "A-B" means the removal of all elements of set B from
set A.
4.2.15 Membership Report Size
If the set of Group Records required in a Report does not fit within
the size limit of a single Report message (as determined by the MTU
of the network on which it will be sent), the Group Records are sent
in as many Report messages as needed to report the entire set.
If a single Group Record contains so many source addresses that it
does not fit within the size limit of a single Report message, it is
split into multiple Group Records, each containing a different subset
of the source addresses.
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5. DESCRIPTION OF THE PROTOCOL FOR GROUP MEMBERS
IGMP is an asymmetric protocol, specifying separate behaviors for group
members -- that is, hosts or routers that wish to receive multicast
packets -- and multicast routers. This section describes the part
of IGMPv3 that applies to all group members. (Note that a multicast
router that is also a group member performs both parts of IGMPv3,
receiving and responding to its own IGMP message transmissions as well
as those of its neighbors. The multicast router part of IGMPv3 is
described in section 6.)
A system performs the protocol described in this section over all
interfaces on which multicast reception is supported, even if more than
one of those interfaces is connected to the same network.
For interoperability with multicast routers running older versions of
IGMP, systems maintain a MulticastRouterVersion variable for each
interface on which multicast reception is supported. This section
describes the behavior of group member systems on interfaces for which
MulticastRouterVersion = 3. The algorithm for determining
MulticastRouterVersion, and the behavior for versions other than 3,
are described in section 7.
The all-systems multicast address, 224.0.0.1, is handled as a special
case. On all systems -- that is all hosts and routers, including
multicast routers -- reception of packets destined to the all-systems
multicast address, from all sources, is permanently enabled on all
interfaces on which multicast reception is supported. No IGMP messages
are ever sent regarding the all-systems multicast address.
There are two types of events that trigger IGMPv3 protocol actions on
an interface:
o a change of the interface reception state, caused by a local
invocation of IPMulticastListen.
o reception of a Query.
(Received IGMP messages of types other than Query are silently ignored,
except as required for interoperation with earlier versions of IGMP.)
The following subsections describe the actions to be taken for each of
these two cases. In those descriptions, timer and counter names appear
in square brackets. The default values for those timers and counters
are specified in section 8.
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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 effected 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 State-Change Report reflecting the difference between
the newest state and the state *before* the first change for which
retransmissions were not completed. The transmission of the newer
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 the newer Report.
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5.2 Action on Reception of a Query
When a system receives a Query, it does not respond immediately.
Instead, it delays its response by a random amount of time, bounded by
the Max Resp Time value in the received Query message. A system may
receive a variety of Queries from different interfaces and of different
kinds (e.g., General Queries, Group-Specific Queries, and Group-and-
Source-Specific Queries), each of which may require its own delayed
response. Therefore, the system must be able to create and temporarily
store multiple "pending response records" of the following (conceptual)
form:
(timer, interface, group, sources)
where:
"timer" is an active timer, counting down the time remaining until
a response is sent.
"interface" is the interface from which the Query was received and
to which the response will be sent.
"group" is the multicast address for which the response is pending.
For a pending response to a General Query, this will be zero.
"sources" is the set of source addresses for which the response is
pending. For a pending response to a General Query or a Group-
Specific Query, this set will be empty.
When a new Query arrives from an interface, a new pending response
record is created as follows:
o the timer is initialized to a random value chosen, using the finest
clock granularity available in the system, from the range
(0, MaxResponseTime], where MaxResponseTime is obtained from the
Max Resp Time field of the received Query.
o the interface is set to that from which the Query arrived.
o the group value is obtained from the Group Address field of the
Query (which may be zero).
o the set of sources are taken from the Source Address [i] fields of
the Query (which may be the empty set).
Then, the new pending response record is compared with the existing
pending response records in the system. If the new record is superceded
by any existing record (as defined in the next paragraph), the new
record is discarded. Otherwise, if the new record supercedes any
existing records, those existing records are discarded.
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A pending response record is said to supercede another if it pertains
to the same interface, has the same or shorter remaining timer, and
has the same or greater "coverage", where:
o a record whose group value is zero has the same or greater
coverage than any other record.
o a record whose set of sources is empty has the same or greater
coverage than any other record with the same group value.
o a record whose set of sources is non-empty has the same or greater
coverage than any other record with the same group value and a
set of sources that is the same as, or a non-empty subset of, the
first record's set.
When the timer in a pending response record expires, the system
transmits, on the interface identified in that record, one or more
Report messages carrying one or more Current-State Records (see
section 4.2.9(1)), as follows:
o If the group address in the pending response record is zero (i.e.,
it is a pending response to a General Query), then one Current-
State Record is sent for each multicast address for which the
specified interface has reception state, as described in section
3.2. The Current-Report Record carries the multicast address
and its associated filter mode (MODE_IS_INCLUDE or MODE_IS_EXCLUDE)
and source list. Multiple Current-State Records are packed into
individual Report messages, to the extent possible.
o If the group address in the pending response record is non-zero and
the set of sources in that record is empty (i.e., it is a pending
response to a Group-Specific Query), then if and only if the
interface has reception state for that group address, a single
Current-State Record is sent for that address. The Current-Report
Record carries the multicast address and its associated filter mode
(MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and source list.
o If the group address in the pending response record is non-zero and
the set of sources in that record 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:
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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 ben generated, the
pending report record with the expired timer is discarded.
6. DESCRIPTION OF THE PROTOCOL FOR MULTICAST ROUTERS
The purpose of IGMP is to enable each multicast router to learn, for
each of its directly attached networks, which multicast addresses are
of interest to the systems attached to those networks. IGMP version 3
adds the capability for a multicast router to also learn which
*sources* are of interest to neighboring systems, for packets sent to
any particular multicast address. The information gathered by IGMP is
provided to whichever multicast routing protocol is being used by the
router, in order to ensure that multicast packets are delivered to all
networks where there are interested receivers.
This section describes the part of IGMPv3 that is performed by multicast
routers. Multicast routers may also themselves become members of
multicast groups, and therefore also perform the group member part of
IGMPv3, described in section 5.
A multicast router performs the protocol described in this section
over each of its directly-attached networks. If a multicast router has
more than one interface to the same network, it only needs to operate
this protocol over one of those interfaces. On each interface over
which this protocol is being run, the router MUST enable reception of
multicast address 224.0.0.22, from all sources (and MUST perform the
group member part of IGMPv3 for that address on that interface).
Multicast routers need to know only that *at least one* system on
an attached network is interested in packets to a particular multicast
address from a particular source; the multicast router does not need
to keep track of the interests of each individual neighboring system.
A router MAY track the interests of individual systems (at the expense
of additional state). If a router chooses to keep individual interest
information, it may omit Group-Specific and Group-and-Source-Specific
Queries from its implementation. A router MUST implement IGMPv2
Group-Specific queries for backwards compatibility.
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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 sends send
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 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
system is leaving a group or when a multicast router desires to change
its filter-mode for the group (see section 6.5).
A Group-and-Source Specific Query is used to verify there are no
systems on a network which desire to receive a set of sources.
Group-and-Source Specific Queries list sources for a particular
group which been requested to be no longer forwarded. This
query is sent by a multicast router to learn if any systems desire
reception of packets to the specified group address from the specified
source addresses. Group-and-Source Specific Queries are only sent in
response to State-Change Records and never in response to Current-State
Records. Section 4.1.8 describes each query in more detail.
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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 forward all sources for this group. This is the IGMPv3 equivalent
to a IGMPv1 or IGMPv2 group join.
6.2.1 Definition of Router Filter-Mode
To reduce internal state, IGMPv3 routers keep a filter-mode per group
per attached network. This filter-mode is used to condense the total
desired reception state of a group to a minimum set such that all
systems' memberships are satisfied. This filter-mode may change in
response to the reception of particular types of group records or when
certain timer conditions occur. In the following sections, we use
the term "router filter-mode" to refer to the filter-mode of a
particular group within a router.
Conceptually, when a group record is received, the router filter-mode
for that group is updated to represent the most number of sources
desired with the least amount of state. As a rule, once a group
record with a filter-mode of EXCLUDE is received, the router
filter-mode for that group will be EXCLUDE. When a router filter-mode
for a group is INCLUDE, the source record list is the list of sources
desired for the group.
When a router filter-mode for a group is EXCLUDE, the source record
list contains two types of sources. The first type is the
set of sources which are not being forwarded (i.e. the set that all
systems have agreed NOT to forward). The second set is the set where
there are conflicts in the desired reception state; this set must
be forwarded. Appendix A describes the reasons for keeping this second
set when in EXCLUDE mode. Section 6.4 describes the changes of a
router filter-mode per group record received.
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Because a reported group record with a filter-mode of EXCLUDE will
cause a router to transition its filter-mode for that group to EXCLUDE,
a mechanism for transitioning a router's filter-mode back to INCLUDE
must exist. If all systems with a group record with filter-mode EXCLUDE
mode cease reporting, it is desirable for the router filter-mode for
that group to transition back to INCLUDE mode (which will likely cause
fewer sources to be forwarded). This transition occurs when the group
timer expires and is explained in detail in section 6.5.
6.2.2 Definition of Group Timers
Group timers represent the time for the *filter-mode* of the group
to expire. 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.
When a Current-State Record is received with a filter-mode matching the
router filter-mode for that group, the group timer is updated. Group
timers are also updated when the router filter-mode for a group is
changed and when certain State-Change Records are received.
A group timer expiring when a router filter-mode for the group is
INCLUDE means there are no listeners in INCLUDE filter-mode present
on the attached network for that group. In IGMPv3, this is the
indication that there are no longer any listeners to the group and
the group record may be deleted.
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. However, there may still be listeners with filter-mode
of INCLUDE for the group. When a group timer reaches Filter-Mode
Query Interval with the router filter-mode for the group equal to
EXCLUDE, a Group-Specific Query is sent for the group. This is to
check if there are systems in INCLUDE filter-mode on the network.
Section 6.5 details 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.
INCLUDE Timer == 0 No more listeners to
group. Delete Group
Record.
EXCLUDE Timer > 0 At least one member in
EXCLUDE mode.
EXCLUDE Timer == Filter-Mode Query group (there may
Query Interval still be systems in
INCLUDE 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 details the setting
the 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.
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Source timers are treated differently when a router filter-mode for
a group is EXCLUDE. If a source record has a running timer with a
router filter-mode for the group of EXCLUDE, it means that a "conflict"
has occurred with respect to desired reception state of that source (i.e.
one system desires the source and another one doesn't). Therefore, when a
source record has a running timer with a router filter-mode for the group
of EXCLUDE it should be forwarded. Appendix A details the reasons
for keeping sources that are being forwarded while in EXCLUDE state.
If a source timer expires with a router filter-mode for the group
of EXCLUDE, the router stops forwarding traffic from this source onto
the network (and takes appropriate actions dependent on the multicast
routing protocol).
When a router filter-mode for a group is EXCLUDE, source records are
only deleted when the group timer expires. Section 6.3 details the
actions that should be taken dependant upon the value of a source
timer.
6.3 IGMPv3 Source-Specific Forwarding Rules
When a multicast router receives a datagram from a source destined to a
particular group, a decision has to be made whether to forward the
datagram onto an attached network or not. This decision is first
dependent on any multicast routing protocols present. Assuming that
there are no other routers downstream (or that downstream routers
support source-specific pruning/grafting), the forwarding decision
depends on the presence of a group record and/or a source record. If
no group record exists, the datagram is not forwarded on the network.
If a source record exists which matches the datagrams source address,
the source is forwarded according to the router filter-mode for the
group and the value of the source timer of the source record.
To summarize, the following table details the forwarding actions
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.
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Group
Filter-Mode Source Timer Value Action
----------- ------------------ ------
INCLUDE TIMER > 0 Forward traffic from
source
INCLUDE TIMER == 0 Stop forwarding traffic
from source and remove
source record
INCLUDE No Source Elements Don't forward source
EXCLUDE TIMER > 0 Forward traffic from
source
EXCLUDE TIMER == 0 Don't forward traffic
from source
(DO NOT remove record)
EXCLUDE No Source Elements Forward traffic from
source
6.4 Action on Reception of Reports
6.4.1 Reception of Current-State Records
When receiving Current-State Records, a router updates both its group
and source timers. In some circumstances, the reception of a type of
group record will cause the router filter-mode for that group to
change. The table below describes the actions, with respect to state
and timers that occur to a router's state upon reception of
Current-State Records.
The following notation is used to describe the updating of source
timers. The notation ( A, B ) will be used to represent the total
number of sources for a particular group, where
A = set of source records whose source timers > 0 (being forwarded)
B = set of source records whose source timers = 0 (not being forwarded)
Note that there will only be two sets when a router's filter-mode
for a group is EXCLUDE. When a router's filter-mode for a group
is INCLUDE, a single set is used to describe the set of sources
being forwarded (e.g. simply ( A ) ).
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In the following tables, abbreviations are used for several variables
(all of which are described in detail in section 8). The variable GMI
is an abbreviation for the Group Membership Interval which is the time
in which group memberships will time out. The variable LMQI is an
abbreviation for the Last Member Query Interval (default 1s) which is
the Maximum Response Time in Group or Group and Source Specific Queries.
Within the "Actions" section of the router state tables, we use the
notation 'A=J', which means that the set A of source records should
have their source timers set to value J. 'Delete A' means that the set A
of source records should be deleted. 'Group Timer=J' means that the
Group Timer for the group should be set to value J.
Router State Report Rec'd New Router State Actions
------------ ------------ ---------------- -------
INCLUDE (A) IS_IN (B) INCLUDE (A+B) (B)=GMI
Group Timer=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 (B) EXCLUDE (X+(B-Y),Y*B) Group Timer=GMI
(B-Y)=GMI
Delete (Y-B)
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 a specific set of sources, it sets its
source timers for those source to a small interval of Last Member Query
Interval seconds. If group records are received which express interest
in receiving traffic from the queried sources, the corresponding timers are
updated. Similarly, when a router queries a specific group, it sets its
group timer for that group to a small interval of Last Member Query
Interval seconds. If any group records expressing interest in the group
are received within the interval, the group timer for the group is updated
and traffic is forwarded without any interruption.
During a query period (i.e. Last Member Query Interval seconds), a
router continues to forward traffic from the groups or sources that it
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is querying. It is not until after Last Member Query Interval seconds
without receiving a record expressing interest in the queried group or
sources that the router may prune the group or sources from the setwork.
The following table describes the changes in group state and the action(s)
taken when receiving either Filter-Mode-Change or Source-List-Change
Records. We use the notation 'Q(G)' to describe a Group-Specific query
to group G. We use the notation 'Q(G,A)' to describe a Group-and-Source
Specific Query to G with source-list A.
Router State Report Rec'd New Router State Actions
------------ ------------ ---------------- -------
INCLUDE (A) ALLOW (B) INCLUDE (A+B) (B)=GMI
Group Timer=GMI
INCLUDE (A) BLOCK (B) INCLUDE (A) (A*B)=LMQI
Send Q(G,A*B)
INCLUDE (A) ALLOW (B), INCLUDE (A+B) (B)=GMI
BLOCK (C) (A*C)=LMQI
Send Q(G,A*C)
Group Timer=GMI
INCLUDE (A) TO_EX (B) EXCLUDE (A*B,B-A) (A*B)=LMQI
(B-A)=0
Delete (A-B)
Send Q(G,A*B)
Group Timer=GMI
INCLUDE (A) TO_IN (B) INCLUDE (A+B) (B)=GMI
Group Timer=GMI
EXCLUDE (X,Y) ALLOW (A) EXCLUDE (X+A,Y-A) (A)=GMI
EXCLUDE (X,Y) BLOCK (A) EXCLUDE (X+(A-Y),Y) (A-Y)=LMQI
Send Q(G,A-Y)
EXCLUDE (X,Y) ALLOW (A), EXCLUDE (X+A+(B-Y),Y-A) (A)=GMI
BLOCK (B) (B-Y)=LMQI
Send Q(G,B-Y)
EXCLUDE (X,Y) TO_EX (A) EXCLUDE (X+(A-Y),Y*A) (A-Y)=LMQI
Send Q(G,A-Y)
Group Timer=GMI
Delete (Y-A)
EXCLUDE (X,Y) TO_IN (A) EXCLUDE (X+A,Y-A) (A)=GMI
(X-A)=LMQI
Send Q(G)
Group Timer=LMQI
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6.5 Switching 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 INCLUDE, a router concludes that there
are no group members present on the attached network and
deletes the group record and the associated source records.
When a router's filter-mode for a group is EXCLUDE and the group
timer expires, a query must be sent to ensure that there are no
remaining systems with filter-mode of EXCLUDE. When the group timer
reaches Filter-Mode Query Interval with a router filter-mode of
EXCLUDE for the group, a Group-Specific Query is sent for that
group. If there are systems with filter-mode of INCLUDE for the
queried group, they will report their INCLUDE state (with
Current-State Records), causing source timers for that group to be
set/reset and therefore the sources to be forwarded.
When a group timer expires with a router filter-mode of EXCLUDE, a
router assumes that there are no systems with *filter-mode of EXCLUDE*
present on the attached network. If there are any source records
with source timers greater than zero, a router switches to filter-mode
of INCLUDE using those source records with timers greater than zero (the
Group-Specific Query sent at Filter-Mode Query Interval seconds would
have caused these to be set). Source records whose timers are zero
(from the previous EXCLUDE mode) are deleted. When the router switches
its filter-mode for a group to INCLUDE, the group timer is reset to
the maximum value of the source timers of that group.
6.6 Action on Reception of Queries
IGMPv3 uses the same querier election method as previous versions. Upon
receiving an IGMPv3 General Query from another router, the querier
ceases to send General Queries and sets the OTHER_QUERIER_PRESENT timer.
Upon expiration of OTHER_QUERIER_PRESENT timer, a router becomes the
querier. Routers who are not the acting querier reset
OTHER_QUERIER_PRESENT timer upon reception of a IGMPv3 General Query.
For details of compatibility between IGMP versions see section 7.
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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 Response Time field is zero
IGMPv2 Query: length = 8 octets AND Max Response Time field is
non-zero
IGMPv3 Query: length >= 12 octets AND Reserved field is zero
Query messages that do not match any of the above conditions (e.g., a
Query of length 10 octets) must be silently ignored.
7.2 Group Member Behavior
IGMPv3 hosts can operate in version 1 and version 2 compatibility modes.
The mode in which a host operates is governed by the version of the
querier router on a network. The version of the querier can be
determined from a Membership Query.
IGMPv3 hosts keep state per local interface regarding the version of
querier on the attached network. Hosts can be in one of three states
depending on the version of querier on their attached networks. This
state is reflected by Querier Version, a state variable kept per
interface describing the version of querier on the attached network.
This state variable can have only one of three values: IGMPv3, IGMPv2,
IGMPv1. When Querier Version is IGMPv3, a host acts using the IGMPv3
protocol. When Querier Version is IGMPv2, a host acts in IGMPv2
compatibility mode, using only the IGMPv2 protocol. When Querier
Version is IGMPv1, a host acts in IGMPv1 compatibility mode, using the
IGMPv1 protocol.
If a lower version query (as compared to Querier Version) is received on
an interface, this state will change immediately to reflect the older
version querier and the host will operate in that lower version
compatibility mode. However, if a higher version query (as compared to
Querier Version) is received, it will not immediately change it's state.
This is to prevent the problem when newer version queries are sent by a
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router restarting and having not yet yielded to an older version
querier.
Each time a non-version 3 query is received, a host sets a timer: Older
Version Querier Present Timeout. The state variable, Querier Version,
reflecting the version of querier on an interface will be based on this
timer. If a host hears a newer version query (as compared to Querier
Version), it will not change its operating state until after the timer
expires.
7.2.1 In the Presence of Older Version Queriers
An IGMPv3 host may be placed on a network where the Querier router has
not yet been upgraded to IGMPv3. The following requirements apply:
An older version router expects older version Membership Reports in
response to its Queries, and will not understand Version 3
Membership Reports. Therefore, a state variable MUST be kept for
each interface, describing whether the multicast Querier on that
interface is running IGMPv1, IGMPv2, or IGMPv3. This variable
MUST be based upon whether or not the older version query was
heard in the last [Older Version Querier Present Timeout] seconds,
and MUST NOT be based upon the type of the last Query heard. This
state variable MUST be used to decide what type of Membership
Reports to send for unsolicited Membership Reports as well as
Membership Reports in response to Queries.
Version 1 Querier
An IGMPv1 router will send General Queries with the Max Response
Time set to 0. This MUST be interpreted as a value of 100 (10
seconds).
IGMPv3 hosts must send IGMPv1 Membership reports when an IGMPv1
router is present. This is IGMPv1 compatibility mode.
Version 2 Querier
IGMPv3 hosts must send IGMPv2 Membership reports when an IGMPv2
router is present. IGMPv3 hosts must use IGMPv2 Leave Group
messages when an IGMPv2 router is present. This is IGMPv2
compability mode.
The following table summarizes an IGMPv3 host response to different
types of queries:
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Querier
Version Query Type Host Response
------- ---------- -------------
IGMPv1 Membership Query IGMPv1 Membership Report
IGMPv2 Membership Query IGMPv2 Membership Report
IGMPv2 Group-Specific Query IGMPv2 Membership Report
IGMPv3 Membership Query IGMPv3 Membership Report
IGMPv3 Group-Specific Query IGMPv3 Membership Report
IGMPv3 Group-and-Source-Specific Query IGMPv3 Membership Report
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 Report to be suppressed by either a Version 1 Membership
Report, or a Version 2 Membership Report.
If a host is a member of any sources within a group reported in a V1 or
V2 membership report, then it may suppress its report by marking the
group so that it is not reported when the next IGMPv3 report is sent.
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7.2.3 Group Member Compatibility State Transition Diagram
RQ3
-----------
| |
| V
RQ1/ST ------------------- RQ2/ST
---------------| |----------------
| | V3 Mode | |
| ------->| |<------- |
| | ------------------- | |
| | | |
| |TE/DT TE/DT| |
| | | |
V | | V
------------------- ------------------
----| | | |----
RQ3| | V1 Mode |<-------------------| V2 Mode | |RQ3
--->| | RQ1/RT | |<---
------------------- ------------------
| ^ | ^ | ^
| | | | | |
------ ------ -----------
RQ1/RT RQ2 RQ2/RT
Actions
-------
ST : start Older Version Querier Present Timer
DT : delete Older Version Querier Present Timer
TE : Older Version Querier Present Timer expires
RT : reset Older Version Querier Present Timer
Events
------
RQ1 : Receive IGMPv1 Membership Query
RQ2 : Receive IGMPv2 Membership Query
RQ3 : Receive IGMPv3 Membership Query
States
------
V3 Mode : An IGMPv3 router is the present querier on an interface.
The host uses IGMPv3 Membership Reports
V2 Mode : An IGMPv2 router is the present querier on an interface.
The host uses IGMPv2 Membership Reports and IGMPv2 Leave
Group messages.
V1 Mode : An IGMPv1 router is the present querier on an interface.
The host uses IGMPv1 Membership Reports.
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7.3 Multicast Router Behavior
7.3.1 In the Presence of Older Version Queriers
IGMPv3 routers may be placed on a network where at least one router on
the network has not not yet been upgraded to IGMPv3. The following
requirements apply:
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 send IGMPv1 and IGMPv2 queries. When in IGMPv1 mode, routers
MUST send Periodic Queries with a Max Response Time of 0, and MUST
ignore Leave Group messages. They SHOULD also warn about
receiving an IGMPv2 or IGMPv3 query, although such warnings MUST
be rate-limited.
If a router is not explicitly configured to use IGMPv1 and hears
an IGMPv1 Query or IGMPv2 Query, it SHOULD log a warning. These
warnings MUST be rate-limited.
7.3.2 In the Presence of Older Version Group Members
IGMPv3 routers may be placed on a network where there are hosts that
have not yet been upgraded to IGMPv3. The following requirements apply:
IGMPv3 routers MUST keep state per group being forwarded per
interface regarding the lowest version of IGMP heard. For each
group being forwarded per interface, the state variable Oldest
Host Present is kept. Groups can be in one of three states
reflected by the state variable: Oldest Host Present.
Routers MUST act in a compatibility mode on a per group per
interface. The following table summarizes the types of messages
to be used dependent on the value of Oldest Host Present.
Oldest Host Present Messages Utilized
------------------- -----------------
IGMPv1 Version 1 Membership Queries
IGMPv2 Version 2 Membership Queries,
Version 2 Group-Specific
Membership Queries
IGMPv3 Version 3 General Membership Queries,
Version 3 Group-Specific Membership
Queries,
Version 3 Group-and-Source Specific
Membership Queries
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A router MUST keep a timer per group, Older Host Present Timeout,
if it hears an non-version 3 report for a group. This SHOULD be
set to the value of two query periods. If a router does not hear
a lower version report for the length of two query periods, it
assumes that the older version members have left and reverts to
version 3 operation for that group.
7.3.3 Router Compatibility State Transition Diagram*
RV3MR
-----------
| |
| V
RV1MR/ST ------------------- RV2MR/ST
-------------| |------------
| | V3 Mode | |
| ------->| |<------- |
| | ------------------- | |
| | | |
| |TE/DT TE/DT| |
| | | |
V | | V
----------------- -------------
----| | | |---
RV3MR| | V1 Mode |<-------------------| V2 Mode | |RV3MR
--->| | RV1MR/RT | |<--
----------------- -------------
| ^ | ^ | ^
| | | | | |
------ ------ -------
RV1MR/RT RV2MR RV2MR/RT
* with respect to a single multicast group
Actions
-------
ST : start Older Host Present Timer
DT : delete Older Host Present Timer
TE : Older Host Present Timer expires
RT : reset Older Host Present Timer
Events
------
RV1MR : Receive Version 1 Membership Report
RV2MR : Receive Version 2 Membership Report
RV3MR : Receive Version 3 Membership Report
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States
------
V3 Mode : A router operates using only IGMPv3 messages for this group.
V2 Mode : An IGMPv2 Membership Report has been heard for this group
within the last Older Host Present Timeout seconds. A
router operates using only IGMPv2 messages for this group.
V1 Mode : An IGMPv1 Membership Report has been heard for this group
within the last Older Host Present Timeout seconds. A
router operates using only IGMPv1 messages for this group.
Note: In V1 Mode and V2 Mode, the Membership Query is still a
version 3 query.
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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 inserted into the periodic General Queries.
Default: 100 (10 seconds)
By varying the [Query Response Interval], an administrator may tune the
burstiness of IGMP messages on the network; larger values make the
traffic lest bursty, as host responses are spread out over a larger
interval. The number of seconds represented by the [Query Response
Interval] must me 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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8.5 Other Querier Present Interval
The Other Querier Present Interval is the length of time that must pass
before a multicast router decides that there is no longer another
multicast router which should be the querier. This value MUST be ((the
Robustness Variable) times (the Query Interval)) plus (one half of one
Query Response Interval).
8.6 Startup Query Interval
The Startup Query Interval is the interval between General Queries sent
by a Querier on startup. Default: 1/4 the Query Interval.
8.7 Startup Query Count
The Startup Query Count is the number of Queries sent out on startup,
separated by the Startup Query Interval. Default: the Robustness
Variable.
8.8 Last Member Query Interval
The Last Member Query Interval is the Max Response Time inserted into
Group-Specific Queries sent in response to Leave Group messages. It is
also the Max Response Time inserted into Group-and-Source-Specific Query
messages. Default: 10 (1 second)
This value may be tuned to modify the "leave latency" of the network. A
reduced value results in reduced time to detect the loss of the last
member of a group or source.
8.9 Last Member Query Count
The Last Member Query Count is the number of Group-Specific Queries sent
before the router assumes there are no local members. The Last Member
Query Count is also the number of Group-and-Source-Specific Queries sent
before the router assume there are no listeners to particular source.
Default: the Robustness Variable.
8.10 Unsolicited Report Interval
The Unsolicited Report Interval is the time between repetitions of a
host's initial report of membership in a group. Default: 10 seconds.
8.11 Filter-Mode Query Interval
When a router group timer for a group with a filter-mode of EXCLUDE
reaches Filter-Mode Query Interval, a Group-Specific Query is sent
(see section 6.5). The Group-Specific Query must have its Max
Response Time set to Filter-Mode Query Interval. Default: 5 seconds.
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8.12 Older Version Querier Interval
The Older Version Querier Interval is the time-out for transitioning a
host back to IGMPv3 mode once an older version query is heard. When an
older version query is received, hosts set their Older Version Querier
Present Timer to Older Version Querier Interval.
This value MUST be ((the Robustness Variable) times (the Query Interval))
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).
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9. SECURITY CONSIDERATIONS
We consider the ramifications of a forged message of each type.
Query Message:
A forged Query message from a machine with a lower IP address than
the current Querier will cause Querier duties to be assigned to the
forger. If the forger then sends no more Query messages, other
routers' Other Querier Present timer will time out and one will
resume the role of Querier. During this time, if the forger
ignores Leave Messages, traffic might flow to groups with no
members for up to [Group Membership Interval].
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:
- 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.
- Ignore Report messages without Router Alert options [RFC-2113],
and require that routers not forward Report messages. (The
requirement is not a requirement of generalized filtering in the
forwarding path, since the packets already have Router Alert
options in them). This solution breaks backwards compatibility
with implementations of earlier versions of this specification
which did not require Router Alert.
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.
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10. ACKNOWLEDGMENTS
Some of the text of this document was copied from [RFC-1112] and
[RFC-2236].
11. REFERENCES
[RFC-1112] Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
August 1989.
[RFC-2113] Katz, D., "IP Router Alert Option," RFC 2113, April 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP14, March 1997.
[RFC-2236] Fenner, W., "Internet Group Management Protocol, Version 2",
RFC 2236, November 1997.
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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 API Rationale
The IGMPv3 API specifies only full-state requests for source
and filter-mode state. This section discusses the advantages in
using full-state API requests in comparison to an alternative design
using state-change API requests for the IGMPv3 API.
A "full-state" API operation specifies the complete reception state
of the application program, whereas a "state-change" API operation
specifies only the change in filter-mode or the incremental change
in source lists from the last reported state.
The following points summarize the differences in these APIs:
1. State-change API requests require additional filter-modes to
describe whether sources are being added or deleted or whether
the filter-mode is being changed.
2. State-change API requests require the IP layer to compute the
full state at the socket before computing the per-interface state.
3. Applications that use state-change API requests may have to
determine the IP source addresses to be added or deleted from
the full state. This leads to unnecessary computation at both
the application and the IP layers.
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4. API requests for adding and deleting sources at the same time may
have to be split across multiple requests and this could lead to
multiple messages being sent which could cause confusion at the
router (unless there is some mechanism to indicate that the
messages are somehow linked)
A.3 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.
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.4 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
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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.
AUTHORS' ADDRESSES
Brad Cain
Nortel Networks, Inc.
600 Technology Park
Billerica, MA 01821
phone: +1-978-916-1316
email: bcain@nortelnetworks.com
Steve Deering
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA 95134-1706
phone: +1-408-527-8213
email: deering@cisco.com
Ajit Thyagarajan
Ericsson IP Infrastructure
12120 Plum Orchard Dr.
Silver Spring, MD 20904
phone: +1-301-586-8200
email: ajit@torrentnet.com
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