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CSC465 – Computer Networks
Spring 2004
Dr. J. Harrison
These slides were produced almost entirely from material by Behrouz Forouzan for the text “TCP/IP Protocol Suite (2 nd Edition)”, McGraw Hill Publisher
Chapter 10
Internet Group
Management Protocol
(IGMP)
Multicasting
- Processes may have to send the same message to a large numbers of receivers simultaneously - Example: Video-on-demand - Informing multiple stockbrokers about changes to a stock price - IGMP is necessary but not sufficient for multicasting - IGMP is a companion to the IP protocol
Position of IGMP in the network layer
CONTENTS CONTENTS
• GROUP MANAGEMENT
• IGMP MESSAGES
• IGMP OPERATION
• ENCAPSULATION
• IGMP PACKAGE
GROUP
MANAGEMENT
Multicasting and Routing
- To support multicasting there needs to be routers capable of routing multicast packets
- Routing tables must be updated using a multicasting routing protocol
- IGMP is not a multicasting routing protocol
- IGMP manages group membership
- Group membership: hosts and routers and the groups they are “interested” in (subscribe to)
Multicasting and Routing
- IGMP protocol gives the multicast routers info about the membership status of hosts (routers) connected to the network.
- A multicast router may receive thousands of multicast packets every day for different groups
- If a router has no knowledge about the membership status of the hosts it must broadcast all of the multicast packets (excess traffic)
- Better to maintain list of groups in the network for which there is at least one “loyal” member
- ICMP helps the multicast router create and update the group lists related to each interface
IGMP is a group managementIGMP is a group management
protocol. It helps a multicast routerprotocol. It helps a multicast router
create and update a list of loyalcreate and update a list of loyal
membersmembers related torelated to
each router interface.each router interface.
IGMP v MESSAGES
10.2 10.
IGMP Message Types
IGMP Message Format
Groupid (multicast address of the group) in the “special query” and “membership” and “leave” reports (0 in general query) MRT defines amt of time available to answer query (0 in reports) Checksum is calculated over the 8-byte message
Leaving a Group
- There must be a mechanism for a device to report that it no longer wishes to have membership in a group
- When a host sees that no process is interested in a specific group, it send a leave report.
- When a router determines that none of the networks connected to its interfaces is interested in a specific group, it sends a leave report about that group
Leaving a Group (con’t)
- There must be a mechanism for a device to report that it no longer wishes to have membership in a group
- When a multicasting router receives a leave report on an interface it cannot assume that all devices are disinterested in membership, just the device that sent the leave report
- The router must send a special query message that includes the groupid (multicast address)
- The router then waits for membership reports; if none arrive within the time interval the router purges the the group from its list
Leave report Monitoring Membership
- Hosts and routers use membership and leave reports to join and leave groups
- However these messages are not enough
- Consider the case where a host is taken offline before it can send its leave report
- The multicast router will never receive a leave report
- A multicast router is responsible for monitoring all of the hosts and routers on a LAN to see if they wish to continue their membership in a group
- The general query message is issued periodically
General Query Message
Note: General Query message does not define a specific group
Example 1
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. What report messages will result?
Example 1 (con’t)
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.
Example 1 (con’t)
Time 30: The timer for 225.14.0.0 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 225.14.0.
Example 1 (con’t)
Time 50: The timer for 251.70.0.0 in host C expires and a membership report is sent, which is received by the router and every host.
Example 1 (con’t)
Time 70: The timer for 230.43.0.0 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 230.43.0.0.
Example 1 (con’t)
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.
ENCAPSULATION
10.4 10.
