membership reports; all IGMPv3-capable multicast devices must listen to this When a query is received, an IGMPv3 report is sent as defined in RFC multicast devices must listen to this address. RFC. defines IGMPv3. IGMPv3 . IP Multicast: IGMP Configuration Guide, Cisco IOS XE Release 3SE (Cisco. For more information about IGMPv3 group record types and membership reports, see RFC , Internet Group Management Protocol, Version.

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It is also used by connected routers to discover these group members. IP multicasting is defined as the transmission of an IP datagram to a “host group”, a set of zero or more hosts identified by a single IP destination address.

A multicast datagram is delivered to all members of its destination host group with the same “best-efforts” reliability as regular unicast IP datagrams, i. The membership of a host group is dynamic; that is, hosts may join and leave groups at any time.

Internet Group Management Protocol

There is no restriction on the location or number of members in a host group, but membership in a group may be restricted to only those hosts possessing a private access key. A host may be a member of more than lgmpv3 group at a time.

A host need not be a member of a group to send 376 to it. A host group may be permanent or transient. A permanent group has a well-known, administratively assigned IP address.

Iigmpv3 is the address, not the membership of the group, that is permanent; at any time a permanent group may have any number of members, even zero. A transient group, on the other hand, is assigned an address dynamically when the group is created, at the request 33776 a host. A transient group ceases to exist, and its address becomes eligible for reassignment, when its membership drops to zero.

The creation of transient groups and the maintenance of group membership information is the responsibility of “multicast agents”, entities that reside in internet gateways or other special-purpose hosts.

IGMP: what is the Internet Group Management Protocol? – 1&1 IONOS

There is at least one multicast agent directly attached to every IP network or subnetwork that supports IP multicasting. A host requests the creation of new groups, and joins or leaves existing groups, by exchanging messages with a neighboring agent.

Multicast agents are also responsible for internetwork delivery of multicast IP datagrams. When sending a multicast IP datagram, a host transmits it to a local network multicast address which identifies all neighboring members of the destination host group.

If the group has members on igmppv3 networks, a multicast agent becomes an additional recipient of the local multicast and relays the datagram to agents on each of those other networks, via the internet gateway system.

Finally, the agents on the other networks each transmit the datagram as a local multicast to their own neighboring members of the destination group. All of the following sections of this memo are applicable to level 2 implementations. As well as having messages corresponding to each of the operations specified above, IGMP also specifies a “deadman timer” procedure whereby hosts periodically confirm their memberships with the multicast agents. The IP module must maintain a data structure listing the IP addresses of all host groups to which the host currently belongs, ibmpv3 with each group’s loopback policy, access key, and timer variables.

This data structure is used by the IP multicast transmission service to know which outgoing datagrams to loop back, and by the reception service to know which incoming datagrams to accept.

The purpose of IGMP and the management interface operations is to maintain this data structure. Igmpg3 Internet Group Management Protocol IGMP is used between IP hosts and their immediate neighbor multicast agents to support the creation of transient groups, the addition and deletion of members of a group, and the periodic confirmation of group membership.

IGMP is an asymmetric protocol and is specified here from the point of view of a host, rather than a multicast agent. In a Create Group Request message, this field indicates if the new host group is to be public or private. In all other Request messages, this field is set to zero. For computing the checksum, the checksum field should first be ugmpv3 to 0. When the data packet is transmitted, the checksum is computed and inserted into this field. When the data packet is received, the checksum is again computed and verified against the checksum field.


If the two checksums do not match then an error has occurred. In a Confirm Group Request message, the identifier field contains zero. In all other Request messages, the identifier field contains a value to distinguish the request from other requests by the same host. In a Reply message, the identifier field contains the same value as in the corresponding Request message.

In a Create Group Request message, the group address field contains zero. In all other Request messages, the group rffc field contains a host group address. In a Create Group Reply message, the group address field contains either a newly allocated host group address if the request is granted or zero if denied.

RFC – Internet Group Management Protocol, Version 3

In all other Reply messages, the group address field contains the same host group address as in the corresponding Request message.

In a Create Group Request message, the access key field contains zero. In all other Request messages, the access key field contains the access key assigned to the host group identified in the Group Address field zero for public groups.

In a Create Group Reply message, the access key field contains either a non-zero bit number if the request for a private group is granted or zero. In all other Reply messages, the access key field contains the same access key as in the corresponding Request. IGMP is an asymmetric protocol and is specified here from the point of view of a host, rather than a multicast router. IGMP may also be used, symmetrically or asymmetrically, between multicast routers.

It is required to be implemented by all hosts conforming to level 2 of the IP multicasting specification. Multicast routers send Host Membership Query messages hereinafter called Queries to discover which host groups have members on their attached local networks.

Queries are addressed to the all-hosts group address Hosts respond to a Query by generating Host Membership Reports hereinafter called Reportsreporting each host group to which they belong on the network interface from which the Query was received.

In order to avoid an “implosion” of concurrent Reports and to reduce the total number of Reports transmitted, two techniques are used:. When a host receives a Query, rather than sending Reports immediately, it starts a report delay timer for each of its group memberships on the network interface of the incoming Query.

Each timer is set to a different, randomly-chosen value between zero and D seconds. When a timer expires, a report is generated for the corresponding host group. Thus, Reports are spread out over a D second interval instead of all occurring at once. A Report is sent with an IP destination address equal to the host group address being reported, and with an IP time-to-live of 1, so that other members of the same group on the same network can overhear the Report. If a host hears a Report for a group to which it belongs on that network, the host stops its own timer for that group and does not generate a Report for that group.

Thus, in the normal case, only one Report will be generated for each group present on the network, by the member host whose delay timer expires first. Note that the multicast routers receive all IP multicast datagrams, and therefore need not be addressed explicitly.

Further note that the routers need not know which hosts belong to a group, only that at least one host belongs to a group on a particular network. There are two exceptions to the behavior described above. First, if a report delay timer is already running for a group membership when a Query is received, that timer is not reset to a new random value, but rather allowed to continue running with its current value.

Second, a report delay timer is never set for a host’s membership in the all-hosts group If a host uses a pseudo-random number generator to compute the reporting delays, one of the host’s own individual IP address should be used as part of the seed for the generator, to reduce the chance of multiple hosts generating the same sequence of delays. Rfx host should confirm that a received Report has the same IP host group rrfc in its IP destination field and its IGMP group address field, to ensure that the host’s own Report is not cancelled by an erroneous received Report.

Multicast routers send Queries periodically to refresh their knowledge of memberships present on a particular network. If no Reports ivmpv3 received for a particular group after some number of Queries, the routers assume that that group has no local members and that they need not forward remotely-originated multicasts for that group onto the local network. Queries are normally sent infrequently no more than once a minute so as to keep the IGMP overhead on hosts and networks very low.


However, when a multicast router starts up, it may issue several closely-space Queries in order to quickly build up its knowledge of local memberships. When a ig,pv3 joins a new group, it should immediately transmit a Report for that group, rather than waiting for a Query, in case it is the first member of that group on the network. To cover the possibility of the initial Report being lost or damaged, it is recommended that it be repeated once or twice after short delays.

A simple way to accomplish this is to act as if a Query had been received for that group only, setting the group’s random report delay timer. The binding of IP host group addresses to physical hosts may be considered a generalization of igpmv3 binding of IP unicast addresses. An IP unicast address is statically bound to a single local network interface on a single IP network. An IP host group address is dynamically bound to a set of local network interfaces on a set of IP networks.

The multicast routers do not need to maintain a list of individual members of each host group. For example, a multicast router attached to an Ethernet need associate only a single Ethernet multicast address with each host group having local members, rather than a list of the members’ individual IP or Ethernet addresses. The IP multicast extensions specified in this memo are significantly different from those specified in RFC Most of the changes are due to a shift of responsibility away iympv3 the multicast routers called “multicast agents” in RFC and onto the hosts.

This new distribution of responsibility is consistent with the lightweight, soft-state gateway architecture of the Internet, and it allows the IP multicast services in the same way as the IP unicast services to be used among hosts on a single network when no router is up or present on the network.

Thus, current single-network IP broadcast applications may be migrated to the use of IP multicast before multicast routers are widely available. Host groups are identified by class D IP addresses, i. Class E IP addresses, i. In Internet standard “dotted decimal” notation, host group addresses range from This is used to address all multicast hosts on the directly connected network. There is no multicast address or any other IP address for all hosts on the total Internet.

The addresses of other well-known, permanent groups are to be published in “Assigned Numbers”. Such use is not specified here. IGMP is used between hosts and gateways on a single network to establish hosts’ membership in particular multicast groups. The gateways use this information, in igpmv3 with a multicast routing protocol, to support IP multicasting across the Internet. Without IGMP, a 33376 can still participate in multicasting local to its connected networks.

Support for local IP multicasting includes sending multicast datagrams, joining multicast groups and receiving multicast datagrams, and leaving multicast groups.

IGMP provides gateways that are capable of multicast routing with the information required to support IP multicasting across multiple networks. At this time, multicast-routing gateways are in the experimental stage and are not widely available. For hosts that are not connected efc networks with multicast-routing gateways or that do not need to receive multicast datagrams originating on other networks, IGMP serves no purpose and is therefore optional for now.

However, the rest of [RFC ] is currently recommended for the purpose of providing IP-layer access to local network multicast addressing, as a preferable alternative to local broadcast addressing. It is expected that IGMP will become recommended at some future date, when multicast-routing gateways have become more widely available.

Joining the “all-hosts” group will support strictly local uses of multicasting, e. 3376 mapping of IP Class D addresses tfc local addresses is currently specified for the following types of networks: Any network that supports broadcast but not multicast, addressing: