IGMP ( Internet Group Management Protocol )

• Version 1

Version 1 was the first widely-deployed version and the first version to become an Internet Standard.


• Version 2

Version 2 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

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.

IGMPv3 supports applications that explicitly signal sources from which they want to receive traffic. With IGMPv3, receivers signal membership to a multicast host group in the following two modes:

• INCLUDE mode

In this mode, the receiver announces membership to a host group and provides a list of IP addresses (the INCLUDE list) from which it wants to receive traffic.


• EXCLUDE mode

In this mode, the receiver announces membership to a host group and provides a list of IP addresses (the EXCLUDE list) from which it does not want to receive traffic. This indicates that the host wants to receive traffic only from other sources whose IP addresses are not listed in the EXCLUDE list. To receive traffic from all sources, like in the case of the Internet Standard Multicast (ISM) service model, a host expresses EXCLUDE mode membership with an empty EXCLUDE list.


Multicast Listener Discovery (MLD) is used in a similar way by IPv6 systems. MLD version 1 implements the functionality of IGMP version 2; MLD version 2 implements the functionality of IGMP version 3.

• Membership Query Message



8			16	32 bits
Type			Max Resp Code	Checksum
Resv	S	QRV	QQIC	Number of Sources
Group address
Source Address



• Type

The message type:


0x11	Membership query. There are 2 sub-types of membership queries, general query and group-specific query.
0x16	Version 2 membership report.
0x12	Version 1 membership report. Provides compatibility with IGMPv1.
0x13	Distance Vector Multicast Routing Protocol
0x14	. PIMv1
0x15	Cisco Trace Messages.
0x16	IGMPv2 Membership Report.
0x17	Leave group
0x1E	Multicast Traceroute Response.
0x1F	Multicast Traceroute.
0x22	Version 3 membership report.
0x24	Multicast Router Advertisement.
0x25	Multicast Router Solicitation.
0x26	Multicast Router Termination.
0xF0 - 0xFF	Experimental.



• Max Resp Code

The Max Resp Code field specifies the maximum time allowed before sending a responding report. Specifies the maximum time allowed before sending a responding report in units of 1/10 second. In all other messages, it is set to 0 by the sender and ignored by the receiver.

In a Reply message, the Code field specifies the outcome of the request.


Code	Description
0	Request granted.
1	Request denied, no resources.
2	Request denied, invalid code.
3	Request denied, invalid group address.
4	Request denied, invalid access key.
5 - 255	Request pending, retry in this many seconds.



• 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.


• Group Address

In a Membership query message, The Group address is set to 0 when sending a general query. It is set to the group address being queried, when sending a group specific query or group-and-source-specific query. In a membership report of a leave group message, it holds the IP multicast group address of the group being reported or left.


• Resv (Reserved)

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


• 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.


• 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.


• QQIC (Querier's Query Interval Code)

The Querier's Query Interval Code field specifies the [Query Interval] used by the querier. 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.


• 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).


• Source Address

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.



• 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:


8	16	32 bits
Type = 0x22	Reserved	Checksum
Reserved		Number of Group Records (M)
Group Record



• Reserved

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


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


• 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.


Where each Group Record has the following internal format:


8	16	32 bits
Record Type	Aux Data Len	Number of Sources (N)
Multicast Address
Source Address
Auxiliary Data



• 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.


• Number of Sources (N)

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


• Multicast Address

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


• Source Address

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


• 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.

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


• Reception of a Query.

• Action on Change of Interface State

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

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.


• 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:

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


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


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



• 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.


• 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.


• 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.


• 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.


• Action on Reception of Reports

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.

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.

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

Example 1: QQIC



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:

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)

Example 2: Configuration Examples



The following configuration example shows how to configure a router (running IGMPv3) 

for SSM: 



          ip multicast-routing 

          ! 

          interface Ethernet3/1 

          ip address 172.21.200.203 255.255.255.0 

          ip pim sparse-dense-mode 

          ip igmp version 3 

          ! 

          interface Ethernet3/2 

          ip address 131.108.1.2 255.255.255.0 

          ip pim sparse-dense-mode 

          ip igmp version 3 

          ! 

          ip pim ssm default

Example 3: Command



The following is sample output from the show ip igmp groups command: 



Router# show ip igmp groups 

IGMP Connected Group Membership 



          Group Address     Interface         Uptime      Expires     Last Reporter 

          239.255.255.254   Ethernet3/1       1w0d        00:02:19    172.21.200.159 

          224.0.1.40        Ethernet3/1       1w0d        00:02:15    172.21.200.1 

          224.0.1.40        Ethernet3/3       1w0d        never       171.69.214.251 

          224.0.1.1         Ethernet3/1       1w0d        00:02:11    172.21.200.11 

          224.9.9.2         Ethernet3/1       1w0d        00:02:10    172.21.200.155 

          232.1.1.1         Ethernet3/1       5d21h       stopped     172.21.200.206

Example 4: Command



The following is sample output from the show ip igmp groups command with the 

group-address argument and detail keyword: 



          Router# show ip igmp groups 232.1.1.1 detail 

          Interface: Ethernet3/2 

          Group: 232.1.1.1 

          Uptime: 01:58:28 

          Group mode:     INCLUDE 

          Last reporter: 10.0.119.133 

          CSR Grp Exp: 00:02:38 

          Group source list: (C - Cisco Src Report, U - URD, R - Remote) 

            Source Address Uptime v3 Exp CSR Exp Fwd Flags 

          171.69.214.1 01:58:28 stopped 00:02:31 Yes C

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