1. Chapter 10 IGMP
Prof. Choong Seon HONG

2. Introduction
Discussing the multicasting capability of the TCP/IP protocol suite and the protocol that is involved in multicasting, IGMP (Internet Group Management Protocol)

3. 10.1 Group Management
IGMP is a group management protocol. It helps a multicast router create and update a list of loyal members related to each router interface.

4. Group Management
In any network, there are one or more multicast routers that distribute multicast packets to hosts or other routers.
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

5. 10.2 IGMP Messages
IGMPv2 Message Types

6. IGMP (cont’d) Type : General and Special Query : 0x11 or 00010001
Message Format
Type :
General and Special Query : 0x11 or
Membership Report : 0x16 or
Leave Report : 0x17 or
Maximum Response Type : defining the amount of time in which a query must be answered.
Group address :
- 0 for a general query message
- defining the group id (multicast address of the group) in the special query, the membership report, and leave report messages

7. 10.3 IGMP Operation
Operation of IGMP in a Single Network
A multicast router connected to a network has a list of multicast addresses of the groups for which there is at least one loyal member.

8. IGMP Operations (cont’d)
Joining a Group
A host maintains a list of processes with membership in a group
When a process wants to join a new group, it sends its request to the host
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.

9. IGMP Operations (cont’d)
Leaving a Group
When a host sees that no process is interested in a specific group, it sends a leave report.
When a router sees that none of the networks connected to its interface is interested in a specific group, it sends a leave report about that group.
No Response

10. IGMP Operations (cont’d)
Monitoring Group Membership
Multicast router is responsible for monitoring all of the hosts in a LAN to see if they want to continue their membership in a group
the router periodically (by default, every 125s) sends a query message. In this message, the group address field is set to (to all systems on a LAN)
The general query message does not define a particular group

11. IGMP Operations (cont’d)
General Query Message
No Response

12. IGMP Operations (cont’d)
Delayed Response
To prevent unnecessary traffic, IGMP uses a delayed response strategy.

13. IGMP Operations (cont’d)
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.

14. IGMP Operations (cont’d)
The events occur in this sequence:
a. Time 12: The timer for 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
b. Time 30: The timer for in host A expires and a membership report is sent, which is received by the router and every host including host C which cancels its timer for

15. IGMP Operations (cont’d)
c. Time 50: The timer for in host C expires and a membership report is sent, which is received by the router and every host.
d. Time 70: The timer for in host A expires and a membership report is sent, which is received by the router and every host including host A which cancels its timer for
Note that if each host had sent a report for every group in its list, there would have been seven reports; with this strategy only four reports are sent.

16. 10.4 Encapsulation

17. Encapsulation (cont’d)
Protocol field
The IP packet that carries an IGMP packet has a value of 2 in its protocol field.
TTL Field
The IP packet that carries an IGMP packet has a value of 1 in its TTL field.

18. Encapsulation (cont’d)
Destination IP Addresses
Query : All systems on this subnet
Membership Report : The multicast address of the group
Leave Report : All routers on this subnet

19. Encapsulation (cont’d)
Data Link Layer
Because the IP packet has a multicast IP address, the ARP 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 multicast addresses.

20. Encapsulation (cont’d)
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 mapping
a host may receive packets that do not really belong to the group in which it is involved
Physical multicast address for the TCP/IP protocol

21. IGMP (cont’d) 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 it emerges from the other side as a multicast packet

22. 10.5 IGMP Package Showing only the modules used in an IGMP host
involving a group table, a set of timers, and 4 software modules as follows.
a group-joining module, a group-leaving module, an input module, and an output module
Group table

23. IGMP Package (cont’d)

24. IGMP Package (cont’d) state : defining the state of the entry
FREE : available for a new entry
DELAYING : meaning that a report should be sent for this entry when the timer matures
IDLE : meaning that there is no timer running for the entry
Interface no.
defining the interface through which the multicast packet is sent for this entry
Group address : multicast address which defines the group
Reference count
meaning the number of processes still interested in this group

25. IGMP Package (cont’d) Timers
each entry in the table in the DELAYING state has a timer to govern the sending of reports
When an expiration time matures, a signal goes to the output module which then generates a report

26. IGMP Package (cont’d) 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
if found, the module increments the reference count to show that one more process has joined this group
if the multicast address is not found, the module creates a new entry and sets the reference count to one
Then, inform the data link layer to update its configuration table so that this type of multicast packet can be received

27. IGMP Package (cont’d)
3. Request a membership report from the output module.
4. Inform the data link layer to update its configuration table.
4. Return

28. IGMP Package (cont’d) Group-Leaving Module
a process that wants to leave a group invokes this module
3. Request a leave report from the output

29. IGMP Package (cont’d) Input Module
Random number between zero and the maximum
delay time

30. IGMP Package (cont’d) Output Module 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 report
3. If the message comes from group-living module
1. Create the message
4. Send the message.
5. Return
or a request from joining or leaving module