Internet Group Management Protocol
Encyclopedia
The Internet Group Management Protocol (IGMP) is a communications protocol
used by hosts
and adjacent routers on IP networks to establish multicast group memberships.
IGMP is an integral part of the IP multicast
specification. It is analogous to ICMP
for unicast
connections. IGMP can be used for online streaming video and gaming
, and allows more efficient use of resources when supporting these types of applications.
IGMP is used on IPv4
networks. Multicast management on IPv6
networks is handled by Multicast Listener Discovery
(MLD) which uses ICMPv6
messaging contrary to IGMP's bare IP encapsulation.
IGMP is used between the client computer and a local multicast router. Switches
featuring IGMP snooping
derive useful information by observing these IGMP transactions. Protocol Independent Multicast
(PIM) is then used between the local and remote multicast routers, to direct multicast traffic from the multicast server to many multicast clients.
IGMP operates above the network layer
, though it does not actually act as a transport protocol.
" (RFC) documents of the Internet Engineering Task Force
(IETF). IGMPv1 is defined by RFC 1112, IGMPv2 is defined by RFC 2236 and IGMPv3 was initially defined by RFC 3376 but has since been superseded by RFC 4604 which defines both IGMPv3 and MLDv2. IGMPv2 improves over IGMPv1 by adding the ability for a host to signal desire to leave a multicast group. IGMPv3 improves over IGMPv2 mainly by adding the ability to listen to multicast originating from a set of source IP addresses only.
The FreeBSD
,IGMPv3 was added to FreeBSD in version 8.0. Linux
IGMPv3 was added in the Linux 2.5 kernel series. and Windows
operating system
s support IGMP at the host side.
For the server side implementation, the Linux case uses a daemon
such as mrouted to act as an IGMP Linux router. There are also entire routing suites (such as XORP
or Quagga
), which turn an ordinary computer into a full-fledged multicast router.
Max Resp Code : This field specifies the maximum time (in 1/10 second) allowed before sending a responding report. If the number is below 128, the value is used directly. If the value is 128 or more, it is interpreted as an exponent with mantisse.
Checksum : This is the 16-bit one's complement of the one's complement sum of the entire IGMP message.
Group Address : This is the multicast address being queried when sending a Group-Specific or Group-and-Source-Specific Query. The field is zeroed when sending a General Query.
Resv : This field is reserved. It should be zeroed when sent and ignored when received.
S (Suppress Router-side Processing) Flag : When this flag is set, it indicates to receiving routers that they are to suppress the normal timer updates.
QRV (Querier's Robustness Variable) : If this is non-zero, it contains the Robustness Variable value used by the sender of the Query. Routers should update their Robustness Variable to match the most recently received Query unless the value is zero.
QQIC (Querier's Query Interval Code) : This code is used for specify the Query Interval value (in seconds) used by the querier. If the number is below 128, the value is used directly. If the value is 128 or more, it is interpreted as an exponent with mantisse.
Number of Sources (N) : This field specifies the number of source addresses present in the Query. For General and Group-Specific Queries, this value is zero. For Group-and-Source-Specific Queries, this value is non-zero, but limited by the network's MTU.
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.
Where:
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
used by hosts
Host (network)
A network host is a computer connected to a computer network. A network host may offer information resources, services, and applications to users or other nodes on the network. A network host is a network node that is assigned a network layer host address....
and adjacent routers on IP networks to establish multicast group memberships.
IGMP is an integral part of the IP multicast
IP Multicast
IP multicast is a method of sending Internet Protocol datagrams to a group of interested receivers in a single transmission. It is often employed for streaming media applications on the Internet and private networks. The method is the IP-specific version of the general concept of multicast...
specification. It is analogous to ICMP
Internet Control Message Protocol
The Internet Control Message Protocol is one of the core protocols of the Internet Protocol Suite. It is chiefly used by the operating systems of networked computers to send error messages indicating, for example, that a requested service is not available or that a host or router could not be...
for unicast
Unicast
right|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
connections. IGMP can be used for online streaming video and gaming
Online game
An online game is a game played over some form of computer network. This almost always means the Internet or equivalent technology, but games have always used whatever technology was current: modems before the Internet, and hard wired terminals before modems...
, and allows more efficient use of resources when supporting these types of applications.
IGMP is used on IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
networks. Multicast management on IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
networks is handled by Multicast Listener Discovery
Multicast Listener Discovery
Multicast Listener Discovery is a component of the Internet Protocol Version 6 suite. MLD is used by IPv6 routers for discovering multicast listeners on a directly attached link, much like IGMP is used in IPv4. The protocol is embedded in ICMPv6 instead of using a separate protocol. MLDv1 is...
(MLD) which uses ICMPv6
ICMPv6
Internet Control Message Protocol version 6 is the implementation of the Internet Control Message Protocol for Internet Protocol version 6 defined in RFC 4443...
messaging contrary to IGMP's bare IP encapsulation.
Architecture
A network designed to deliver a multicast service using IGMP might use this basic architecture:IGMP is used between the client computer and a local multicast router. Switches
Network switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
featuring IGMP snooping
IGMP snooping
IGMP snooping is the process of listening to Internet Group Management Protocol network traffic. IGMP snooping, as implied by the name, is a feature that allows a network switch to listen in on the IGMP conversation between hosts and routers. By listening to these conversations the switch...
derive useful information by observing these IGMP transactions. Protocol Independent Multicast
Protocol Independent Multicast
Protocol-Independent Multicast is a family of multicast routing protocols for Internet Protocol networks that provide one-to-many and many-to-many distribution of data over a LAN, WAN or the Internet...
(PIM) is then used between the local and remote multicast routers, to direct multicast traffic from the multicast server to many multicast clients.
IGMP operates above the network layer
Network layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
, though it does not actually act as a transport protocol.
Standards
There are three versions of IGMP, as defined by "Request for CommentsRequest for Comments
In computer network engineering, a Request for Comments is a memorandum published by the Internet Engineering Task Force describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems.Through the Internet Society, engineers and...
" (RFC) documents of the Internet Engineering Task Force
Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
(IETF). IGMPv1 is defined by RFC 1112, IGMPv2 is defined by RFC 2236 and IGMPv3 was initially defined by RFC 3376 but has since been superseded by RFC 4604 which defines both IGMPv3 and MLDv2. IGMPv2 improves over IGMPv1 by adding the ability for a host to signal desire to leave a multicast group. IGMPv3 improves over IGMPv2 mainly by adding the ability to listen to multicast originating from a set of source IP addresses only.
Host and router implementations
The IGMP protocol is implemented on a particular host and within a router. A host requests membership to a group through its local router while a router listens for these requests and periodically sends out subscription queries.The FreeBSD
FreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
,IGMPv3 was added to FreeBSD in version 8.0. Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
IGMPv3 was added in the Linux 2.5 kernel series. and Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s support IGMP at the host side.
For the server side implementation, the Linux case uses a daemon
Daemon (computer software)
In Unix and other multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user...
such as mrouted to act as an IGMP Linux router. There are also entire routing suites (such as XORP
XORP
XORP is an open source Internet Protocol routing software suite originally designed at the International Computer Science Institute in Berkeley, California...
or Quagga
Quagga (Software)
Quagga is a network routing software suite providing implementations of Open Shortest Path First , Routing Information Protocol , Border Gateway Protocol and IS-IS for Unix-like platforms, particularly Linux, Solaris, FreeBSD and NetBSD....
), which turn an ordinary computer into a full-fledged multicast router.
Security
IGMP is vulnerable to some attacks, and firewalls commonly allow the user to disable it if not needed.IGMPv3 packet structure
IGMP messages are carried in bare IP packets with IP protocol number 2. There is no transport layer used with IGMP messaging, similar to ICMP for example.Membership Query Message
Membership Queries are sent by multicast routers to determine which multicast addresses are of interest to systems attached to its network. Routers periodically send General Queries to refresh the group membership state for all systems on its network. Group-Specific Queries are used for determining the reception state for a particular multicast address. Group-and-Source-Specific Queries allow the router to determine if any systems desire reception of messages sent to a multicast group from a source address specified in a list of unicast addresses.| bit offset | 0–3 | 4 | 5–7 | 8–15 | 16–31 | |||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | Type = 0x11 | Max Resp Code | Checksum | |||||||||||||||||||||||||||||
| 32 | Group Address | |||||||||||||||||||||||||||||||
| 64 | Resv | S | QRV | QQIC | Number of Sources (N) | |||||||||||||||||||||||||||
| 96 | Source Address [1] | |||||||||||||||||||||||||||||||
| 128 | Source Address [2] | |||||||||||||||||||||||||||||||
| . . . | ||||||||||||||||||||||||||||||||
| Source Address [N] | ||||||||||||||||||||||||||||||||
Max Resp Code : This field specifies the maximum time (in 1/10 second) allowed before sending a responding report. If the number is below 128, the value is used directly. If the value is 128 or more, it is interpreted as an exponent with mantisse.
Checksum : This is the 16-bit one's complement of the one's complement sum of the entire IGMP message.
Group Address : This is the multicast address being queried when sending a Group-Specific or Group-and-Source-Specific Query. The field is zeroed when sending a General Query.
Resv : This field is reserved. It should be zeroed when sent and ignored when received.
S (Suppress Router-side Processing) Flag : When this flag is set, it indicates to receiving routers that they are to suppress the normal timer updates.
QRV (Querier's Robustness Variable) : If this is non-zero, it contains the Robustness Variable value used by the sender of the Query. Routers should update their Robustness Variable to match the most recently received Query unless the value is zero.
QQIC (Querier's Query Interval Code) : This code is used for specify the Query Interval value (in seconds) used by the querier. If the number is below 128, the value is used directly. If the value is 128 or more, it is interpreted as an exponent with mantisse.
Number of Sources (N) : This field specifies the number of source addresses present in the Query. For General and Group-Specific Queries, this value is zero. For Group-and-Source-Specific Queries, this value is non-zero, but limited by the network's MTU.
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.
IGMPv2 packet structure
| + | Bits 0–7 | 8–15 | 16–31 | |||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | Type | Max Resp Time | Checksum | |||||||||||||||||||||||||||||
| 32 | Group Address | |||||||||||||||||||||||||||||||
Where:
- Type is Membership Query (0x11), Membership Report (IGMPv1: 0x12, IGMPv2: 0x16), Leave Group (0x17) IGMPv3 adds type Membership Report (0x22)
- Max Resp Time specifies the time limit for the corresponding report. The field has a resolution of 100 miliseconds, the value is taken directly. This field is meaningful only in Membership Query (0x11); in other messages it is set to 0 and ignored by the receiver.