Internet Group Management Protocol (IGMP) is a TCP/IP network layer protocol used for informing routers of the availability of multicast groups on the network.
Multicast routing protocols are used to construct a multicast tree along which multicast data can be sent from the first-hop router to each multicast receiver avoiding users or network branches that do not need the data. In the figure above, R1 is a first-hop router, and R4, R5, and R6 last-hop routers. Multicast receivers and the routers directly connected to them (last-hop routers) constitute the third module. A multicast receiver, such as a PC that received live multimedia content is a member of a multicast group —a group of hosts who require the same content. In this figure, PC 1, PC 2, PC 3 are hosts / receivers.
IGMP is used by the last-hop routers to learn whether multicast receivers exist on the attached network segments and to maintain member relationships. IGMP is used by hosts to send Membership Report messages when they want to receive the data of multicast groups. The components in the tree modules collaborate with each other to ensure the normal working of a multicast network.
By becoming a member of a group, a multicast receiver (host) expresses int interest in receiving the data destined for the multicast address of the group. A multicast IP address can only be a destination address not a source address. Data packets destined for multicast group address can be received only by hosts that have joined the group.
To join a multicast group, a host must report its request for membership to nearby routers. These routers periodically poll the hosts in their locally attached networks to check on their membership status. When a host first joins a multicast group, it sends an IGMP message (Host Membership Query message) that is directed to a specific multicast address. The Internet Assigned Numbers Authority (IANA) has assigned that all IP multicast group addresses should fall in the range from 224.0.0.0 through 239.255.255.255. IGMP version 1 uses 224.0.0.1 as a general query address.
The routers poll the hosts regularly by sending Host Membership Query messages to determine whether any of them are still members of that group. If no hosts on the network belong to that group any longer, the router stops advertising the information to other routers so that multicast messages directed to that group are no longer forwarded to it.
IGMP messages
IGMP messages are encapsulated in IP datagrams. To indicate an IGMP packet, the IP header contains a protocol number of 2. For IGMP version 2 (defined by RFC 2236), the IP data field contains the 8-octet IGMP message shown in the figure below.
- Type—it specifies
the type of the IGMP packet (as
seen in chapter 4, RFC 3376):
- 0 x 11—specifies the type number
(hex) of a membership query packet. This is sent by a multicast
router. There are three subtypes of Query messages:
- General Query
- Group-Specific Query
- Group-and-Source-Specific Query
- 0 x 12—specifies the type number (hex) of an IGMPv1 membership report packet. This is sent by a multicast host to signal participation in a specific multicast host group.
- 0 x 16—specifies the type number (hex) of an IGMPv2 membership report packet.
- 0 x 17—specifies an IGMPv2 Leave Group packet.
- 0 x 11—specifies the type number
(hex) of a membership query packet. This is sent by a multicast
router. There are three subtypes of Query messages:
- Max resp time—this field is used in membership query messages. It specifies the maximum allowed time a host can wait before sending a corresponding report. Varying this setting allows routers to tune the leave latency. This references the time between the last host leaving a group and the time the routing protocol is notified that there are no more members.
- checksum—this field contains a 16-bit checksum.
- Class D Address—this field contains a valid multicast group address. It is used in a report packet.
IGMPv3 messages
TheIGMPv2 message format has been extended in IGMP version 3, defined in RFC 3376 (which obsoletes RFC 2236). Version 3 allows receivers to subscribe to or exclude a specific set of sources within a multicast group. To accommodate this, the 0 x 11 type membership query packet has been altered to 0 x 22. However, all IGMPv3 implementation still supports packet types 0 x 12, 0 x 16, and 0 x 17.
- Type—this remains unchanged and, for a membership query message, it has a value of 0x’11’.
- Max resp time—this field is has been changed from the IGMPv2 convention and distinguishes between
a maximum response code and a maximum response time. The time can
be determined as follows:
- If the maximum response code is less than 128, the value of maximum response code is also the maximum response time
- If the maximum response code is greater than 128, the maximum
response code represents a floating-point value denoted as follows
- Byte 0 = 1
- Bytes 1-3 = exp
- Bytes 4-7 = mant
Maximum response time = (mant OR 0x’10) << (exp + 3)
- checksum—this field contains a 16-bit checksum and is unchanged from version 2.
- Group Address—this field contains a valid multicast 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 QueryIt is used in a report packet.
- Resv—this field is reserved, and is set to zero on transmission and ignored on reception.
- S Flag—when set to 1, this field indicates that any receiving multicast routers should suppress the normal timer updates normally performed upon hearing a query.
- QRV—this field is the Querier’s Robustness Variable (QRV). The QRV was added in IGMPv3, and is used in tuning timer values for expected packet loss. The higher the value of the QRV, the more tolerant the environment is for lost packets within a domain. However, increasing the QRV also increases the latency required in detecting a problem. Routers adopt the value in this field from the most recently received query as their own robustness variable. If this value exceeds the limit of 7, it is reset to 0. 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.
- QQIC—this field is the Querier’s Query Interval Code. This value (seconds) specifies the query interval used by the originator of this query. The calculations to convert this code into the actual interval time is the same used for the maximum response code.
- Number of Sources (N)—this field indicates how many source address are contained within the message. The maximum value for this field is determined by the MTU allowed in the network.
- Source Addresses—this set of fields are a vector of NIP unicast addresses, where the value N corresponds to the Number or Sources (N) field.
When a router receives an IGMP message from a host on an attached network segment, the router learns that a multicast group resides on that segment and starts forwarding multicast traffic to the interface attached to it.
Types of IGMPv2 Messages
- General Query—a standard query message periodically sent by IGMP routers to obtain information about multicast groups. Routers use General Query messages to obtain information about all hosts on a network segment. These messages are sent to 224.0.0.1 (all multicast hosts) and the group address carried in them is 0.0.0.0. Hosts that receive a General Query message respond with a Membership Report message if they require multicast data. By default, the multicast routers send General Queries every 60 seconds.
- Group-Specific Query—when routers receive a Leave Group message from a multicast group member, they respond by sending a Group-Specific message to determine whether the group contains any members. The destination Ip address of a Group-Specific Query is the address of the group containing the host that sent the Leave Group Message. Queriers send a second Group-Specific Query after 1 second to ensure that groups are not inadvertently considered empty due to dropped or damaged packets. They only remove groups after receiving no reply to the second query.
- 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.
- Membership Report—a report message sent from hosts to
routers to join a multicast group or respond to Membership query
messages from an IGMP router.
- If a Membership Report packet is not received from a certain segment within Query intervals, the router advertises the segment as having no multicast group members, and does not send further multicast data to the segment. The maximum response time included in these General Queries indicates to hosts how much time they have to respond with a Membership Report and is set to 10 seconds by default.
- If no Membership Report message is received from a certain network segment before the Multicast Listener interval expires, the router advertises the segment as having no multicast group members and does not sent further multicast data to the segment
- Leave Group—a message sent from hosts to routers to indicate their intention to leave a specified multicast group. Leave messages are send by hosts to 24.0.0.2 to inform multicast routers on the attached network segment that the host intends to leave the multicast group.These messages include the address of the group that the host is leaving. Upon the receiving of a Leave message multicast routers sent a Group-Specific Query to the group being left as a way of determining whether any members remain in the group. If there is no response to the query, the router discontinues forwarding of multicast traffic to the group.
- Version 2 Membership Report—included for compatibility with IGMPv1.
IGMPv2 Querier
On a LAN with multiple multicast routers, it would be very inefficient for all those routers to send their own IGMP queries. Instead, the interface of one router is selected as the IGMP queries for that LAN. In IGMPv2, routers check the source address of the General Query messages send by other routers to determine their role. Any router that sees a General Query with a lower source address than its own acts as a non-querier, and in this way the router whose IP address is lowest becomes the IGMP querier.
Afterward, if routers do not receive a query message from the querier within the Other Querier Present Interval (125 seconds by default), they determine that the querier has gone down, and a new querier is selected using the same process. Once IGMP is enabled on the first-hop router R1, the router will begin sending periodically general queries to this network segment. When the hosts PC 1 an PC 3 receive these queries, the report delay timer on each host is started at a number chosen uniformly between 0 and the value of the maximum response time included in the query (10 seconds by default). This timer counts down to 0, at which points the host sends a Membership Report message.
IGMPv2 Leave Group Process (Scenario 1)
- If the host PC1 wants to leave group 224.1.1.1, it sends a Leave Group message to 224.0.0.2.
- The first-hop router R1 (theIGMPv2 Querier) responds to this message with two Group-Specific Queries (at a 1 second interval) to determine whether the network segment still contains any 224.1.1.1 members.
- If the other host PC3 has not left the group, it responds with a Membership Report. R1 continues forwarding traffic to the group.
IGMPv2 Leave Group Process (Scenario 2)
- If host PC1 wants to leave group 224.2.2.2, so it sends a Leave Group message.
- The first-hop router R1 (theIGMPv2 Querier) responds to this message with two Group-Specific Queries (at a 1 second interval). There are no 224.2.2.2 members on the segment, so R1 does not receive a response.
- Shortly thereafter, R1 decides that there are no 224.2.2.2 members remaining and stops forwarding that group multicast traffic to the segment.
Multicast Routing Table
A multicast packet is sent to a group of hosts. The multicast routers are responsible for sending the packet to all hosts that require the data. Each multicast router maintain a multicast routing table for ensuring correct data forwarding from upstream node to downstream node. A multicast routing protocol must determine upstream interface closer to data sources.
This chapter describes the purpose and scope of the document and general configurations used in this document.