Chapter10 (IGMP)

8/8/2019 Chapter10 (IGMP)

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Prof. Choong Seon HONG

Chapter 10Chapter 10IGMPIGMP


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IntroductionIntroduction

Discussing the multicasting capability of the TCP/IP

protocol suite and the protocol that is involved inmulticasting, IGMP (Internet Group Management

Protocol)


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10.1 Group Management10.1 Group Management

IGMP is a group management protocol. It helps a

multicast router create and update a list of loyalmembers related to each router interface.


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Group ManagementGroup Management

In any network, there are one or more multicast routers

that distribute multicast packets to hosts or otherrouters.

The IGMP Protocol gives the multicast routers

information about the membership status of hosts

(routers) connected to the network.

Position of IGMP in the network layer


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10.2 IGMP Messages10.2 IGMP Messages

IGMPv2 Message Types


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IGMP (contd)IGMP (contd) Message Format

Type :

General and Special Query : 0x11 or 00010001Membership Report : 0x16 or 00010110

Leave Report : 0x17 or 00010111

Maximum Response Type : defining the amount of time in which aquery must be answered.

Group address :

- 0 for a general query message

- defining the group id (multicast address of the group) in the specialquery, the membership report, and leave report messages


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10.3 IGMP Operation10.3 IGMP Operation

Operation of IGMP in a Single Network

A multicast router connected to a network has a list ofmulticast addresses of the groups for which there is at

least one loyal member.


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IGMP Operations (contd)IGMP Operations (contd)

Joining a Group

A host maintains a list of processes with membership in agroup

When a process wants to join a new group, it sends its request

to the host

q The host will add the name of the process and the name of the requested

group to its list

Host only sends an IGMP report to the multicast router if this is

the first request for membership in that group.

Membership report

- In IGMP, a membership report is sent twice, one after the other.


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Monitoring Group Membership

Multicast router is responsible for monitoring all of the hostsin a LAN to see if they want to continue their membership in a

group

q the router periodically (by default, every 125s) sends a query

message. In this message, the group address field is set to 0.0.0.0

(to all systems on a LAN)

The general query message does not define a particular

group


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General Query Message

No ResponseNo Response


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Delayed Response

To prevent unnecessary traffic, IGMP uses a delayed

response strategy.


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Example 1

- Imagine there are three hosts in a network as shown in Figure. A query

message was received at time 0; the random delay time (in tenths of

seconds) for each group is shown next to the group address. Show the

sequence of report messages.


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The events occur in this sequence:

a. Time 12: The timer for 228.42.0.0 in host A expires and a

membership report is sent, which is received by the router

and every host including host B which cancels its timer for

228.42.0.0.

b. Time 30: The timer for 225.14.0.0 in host A expires and a


and every host including host C which cancels its timer for

225.14.0.0.


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c. Time 50: The timer for 251.70.0.0 in host C expires and a

membership report is sent, which is received by the routerand every host.

d. Time 70: The timer for 230.43.0.0 in host A expires and a


and every host including host A which cancels its timer for

230.43.0.0.

Note that if each host had sent a report for every group in

its list, there would have been seven reports; with thisstrategy only four reports are sent.


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10.4 Encapsulation10.4 Encapsulation


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Encapsulation (contd)Encapsulation (contd)

Destination IP Addresses

- Query : 224.0.0.1 All systems on this subnet

- Membership Report : The multicast address of the group

- Leave Report : 224.0.0.2 All routers on this subnet


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Encapsulation (contd)Encapsulation (contd)

Data Link Layer

Because the IP packet has a multicast IP address, theARP protocol cannot find the corresponding MAC

(Physical) address to forward the packet at the data link

layer.

What happens next depends on whether or not the

underlying data link layer supports physical multicastaddresses.

E l ti ( td)E l ti ( td)


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Encapsulation (contd)Encapsulation (contd) Case 1 : Physical Multicast Support

Most LANs support physical multicasting addressing

32 (25)multicast addresses at the IP level are mapped to a single

multicast address : many-to-one mappingq a host may receive packets that do not really belong to the group in

which it is involved

Physical multicast addressPhysical multicast addressfor the TCP/IP protocolfor the TCP/IP protocol


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IGMP (contd)IGMP (contd)

Case 2 : No Physical Multicast Support

Most WANs do not support physical multicast

addressing .

To send a multicast packet through these networks, a

process called tunneling is used.

In tunneling , the multicast packet is encapsulated in a

unicast packet and sent through the network, where itemerges from the other side as a multicast packet


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10.5 IGMP Package10.5 IGMP Package

Showing only the modules used in an IGMP host

involving a group table, a set of timers, and 4 softwaremodules as follows.

q a group-joining module, a group-leaving module, an input

module, and an output module

Group table


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IGMP Package (contd)IGMP Package (contd)

IGMP P k ( d)


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state : defining the state of the entry

q FREE : available for a new entry

q DELAYING : meaning that a report should be sent for this

entry when the timer matures

q IDLE : meaning that there is no timer running for the entry

Interface no.

q defining the interface through which the multicast packetis sent for this entry

Group address : multicast address which defines the

group

Reference countq meaning the number of processes still interested in this

group

IGMP P k ( td)IGMP P k ( td)


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Timers

each entry in the table in the DELAYING state has atimer to govern the sending of reports

When an expiration time matures, a signal goes to the

output module which then generates a report



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Group-joining Module

a process that wants to join a group invokes this module

module searches the group table to find an entry with

the same multicast address

q if found, the module increments the reference count to

show that one more process has joined this group

q if the multicast address is not found, the module creates a

new entry and sets the reference count to one

q Then, inform the data link layer to update its configuration

table so that this type of multicast packet can be received



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3. Request a membership report from the output module.

4. Inform the data link layer to update its configuration table.

4. Return

IGMP P k ( td)IGMP Package (contd)


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Group-Leaving Module

a process that wants to leave a group invokes thismodule

3. Request a leave report from the output



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Input Module

Random number between zero and the maximum

delay time



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IGMP Package (contd)IGMP Package (cont d)

Output Module

or a request from joining or leaving module1. If the message comes from a timer

1. if (found and state is DELAYING0

1. Create a membership report.

2. Reset the state to IDLE.

2. If message comes from the Group- joining module

1. Create a membership report3. If the message comes from group-living module

1. Create the message

4. Send the message.

5. Return

Chapter10 (IGMP)

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