Group Management

Introduction Internet Group Management Protocol (IGMP) Multicast Listener Discovery (MLD) protocol

The Need for Multicast Group Management

By definition a multicast message is sent from a source to a group of destination hosts

In IP multicasting, multicast groups have an ID called multicast group ID If a process in a host wants to receive a multicast message sent to a

particular group, it needs to somehow listens to all messages sent to that particular group

If the source and destinations of a multicast packet share a common bus (i.e. Ethernet bus) , each host only needs to know what groups have members among the processes of that host

If the source and destinations are not on the same LAN, forwarding the multicast messages to the destinations become more complicated

Hosts need to join a group by informing the multicast router on their subnetwork

Basic Host Model

Strive to make the host model simple–When sourcing data, just send the data

»Map network layer address to link layer address»Routers will figure out where receivers are and are not

–When receiving data, need to perform two actions»Tell routers what group you’re interested in (via IGMP)»Tell your LAN controller to receive for link-layer mapped address

Transmission and Delivery of Multicast Datagram

Local subnetwork transmission The source station simply addresses the IP packet to the multicast group,

the network interface card maps the Class D address to the corresponding Ethernet multicast address, and the frame is sent

Receivers that wish to capture the frame notify their IP layer that they want to receive datagrams addressed to the group

Transmission throughout the Internet Routers are required to implement a multicast routing protocol that permits

the construction of multicast delivery trees and supports multicast data packet forwarding

Each router needs to implement a group membership protocol (IGMP) that allows it to learn about the existence of group members on its directly attached subnetwork

Introduction to Group Management

Group Management:– Router accept host as members of groups or

delete them from a certain group, and maintenance of the router’s information about the group membership

Introduction to IGMP

• Internet Group Management Protocol (IGMP) is used by hosts and routers that support multicasting

• It lets all the systems on a physical network know which hosts currently belong to which multicast groups

• This information helps multicast routers know which multicast datagrams to forward onto which interfaces

• IGMP is considered to be part of IP layer

• IGMP messages are transmitted in IP datagrams

Versions of IGMP

There are 3 versions of IGMP.

•Versions 1 and 2 can only communicate group information to a multicast router.

•Version 3 adds source specific messages so that sources of a multicast group can be selectively received

• The motivation behind this additional functionality is a reduction in network traffic

Internet Group Management Protocol (IGMP)

Signaling protocol IGMPv1

– joining message– host membership query (not to a certain group address, but to the

address 224.0.0.1, using flooding)

IGMPv1 Package Format

4-bit IGMP

version (1)

4-bit IGMP

type (1-2) (unused) 16-bit checksum

32-bit group address (class D IP address)

0 3 4 7 8 15 16 31

8 bytes

An IGMP package consists of 8 bytes. The IGMP version is 1. An IGMP type of 1 is a query sent by a multicast router, and 2 is a response sent by a host

The group address is a class D IP address. In a query the group address is set to 0, and in a report it contains the group address being reported

IGMPv2

Extensions to IGMPv1:– If there are more routers, then it chooses one to be responsible

for querying the subnetwork– The routers can query the hosts according to a certain group

with a group specific query– Explicit group leave message

» The router after receiving the group leave message — by using the group specific query — does not have to wait for timing out to check if there is member of the group or not in the subnetwork.

» This decreases the latency time of the protocol

Phases of IGMP

IGMP has 2 phases:

1) When a host joins a new multicast group, it sends an IGMP message to the “all hosts” multicast address declaring its membership

• Local multicast gateways receive the message and establish necessary routing by propagating the group membership information to other multicast gateways throughout the Internet

Phases of IGMP

2) Because membership is dynamic, local multicast gateways periodically poll hosts on the local network to determine which hosts remain members of which groups

•If no host reports membership in a group after a poll, the multicast gateway assumes that no host on the network remains in that group, and stops advertising group membership to other multicast routers

IGMP Reports and Queries

1) A host sends an IGMP report when a process joins a group

2) A host does not send a report when a process leaves a group

3) A multicast router sends an IGMP query at regular intervals to see if any hosts still have processes belonging to any groups

4) A host responds to an IGMP query by sending one IGMP report for each group that still contains at least one process

IGMPv3

New property: source-filtering– The capability of the system to accept the packages addressed to

a certain multicast address from a given source address, only– however, in a certain configuration accepting the packages from

any source addresses except some source addresses– with this the source specific joining and leaving are introduced,

in such a way the hosts can (un)subscribe to/from certain sources in a group independently

– By using the source filtering the receiver can subscribe to a Source Specific Multicast (SSM) channel

Implementation of IGMPv3

• To implement source specific IGMP messages, two new types of reports were created:• an inclusion and• an exclusion group-source report

• These reports allow a host to communicate what sources in a group it wants to receive; a host utilizes an inclusion report to specify (by IP-address) what sources it wishes to receive, or an exclusion report specifying what sources it does not wish to receive

Multicast Listener Discovery (MLD)

Group management in IPv6 Listeners = that nodes, which want to receive the multicast

messages With the MLD it can be determined that whitch listener is

interested in which multicast group Two versions: MLDv1 and MLDv2 The MLDv2, similarly to IGMPv3 gives the source filtering,

Determining the required source in case of each group The MLD enable the IPv6 routers to perceive the presence of

the listeners