chapter 10 internet group management protocol network93_1... · 2002. 1. 6. · o internet group...

Chapter 10Internet Group Management Protocol

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Outlineo Group managemento IGMP messageso IGMP operationo Encapsulationo IGMP package



Introductiono Internet Group Management Protocol (IGMP)n One-to-many communication

n One of the necessary, but not sufficient, protocols that is involved in multicasting



Figure 10-1

Position of IGMP in The Network Layer

The McGraw-Hill Companies, Inc., 2000



GROUP MANAGEMENT

10.110.1




IGMPo A protocol that manages group membershipo Not a multicasting routing protocoln For multicasting, we need routing tables in

routers to route multicast packets

n This is done by the multicasting routing protocols

n However, IGMP does not address this



IGMP (Cont.)o IGMP gives the multicast routers information

about the membership status of hosts (routers) connected to the networkn Help a multicast router create and update a list of

loyal members related to each router interface



IGMP MESSAGES

10.210.2




IGMP Messageso IGMP messagesn Query

o General and special

n Membership report

n Leave report



Figure 10-2

IGMP Message Types




Figure 10-3

IGMP Message Format




IGMP Message Formato Typen Define the type of message

o 00010001: general or special queryo 00010110: membership reporto 00010111: leave report

o Maximum response timen The amount of time a query must be answeredn In tenths of a second

o Value = 100 : 10 second



IGMP Message Format (Cont.)o Checksumn Calculated over the 8-byte message

o Group addressn 0: for a general query messagen Groupid (multicast address of the group) : for the

special query, the membership report and the leave report messages



IGMP OPERATION

10.310.3




IGMP Operationo Each multicast router has a list of multicast

addressesn Each address corresponding to a group

n Each group must have at least one loyal member in one of the routing’s connecting network



IGMP Operation (Cont.)o For each groupn Only one router has the duty of distributing the

multicast packets for that group

n For example, if three routers connected to a networko Their list of groupids are mutually exclusive



Figure 10-4

IGMP Operation




IGMP Operationo Join a group

o Leave a group

o Monitor a group



Join a Groupo Host: maintain a list of process that have

membership in a groupo When a new process wants to join a new

groupn The host adds the process’s name, the requested

group’s name to its listn If this is the first entry for this group

o Send a membership report message



Join a Group (Cont.)o Router can also send membership report n If there is a new interest in a group for any of its

interface

n Note that, the membership report is sent out of all interface except the one from which the new interest comes



Figure 10-5

Membership Report




Join a Group (Cont.)o A membership report be sent twicen One after the other within a few moments

n To prevent the first one is lost or damaged



In IGMP, a membership report In IGMP, a membership report is sent twice, is sent twice,

one after the other. one after the other.




Leave a Groupo Occurrencen When a host sees that no process is interested in a

specific groupn When a router sees that none of the networks

connected to its interface is interested in a specific group

o In above both cases, send a leave report



Leave a Group (Cont.)o However, when a multicast router receives a

leave reportn It cannot immediately purge that group from its

listn Because the report comes from just one host or a

routern There may be other hosts or routers that are still

interested in that group



Leave a Group (Cont.)o Thus, the router sends a special query

message and waits for a specified response timen If no interest is received during the time

o No one is interested in that group in its networko Purge the group from its list



Figure 10-6

Leave Report




Monitor Membershipo Although we have membership report

message and leave report messagen But this is not enoughn A host may be shut down or removed

accidentallyo Thus, the multicast router must monitor all of

the hosts or routers’ group membership



Monitor Membership (Cont.)o The router periodically sends a general query

messagen By default, 255 secondsn Group address field is set to 0.0.0.0

o Does not define a particular groupn Has a maximum response time of 10 seconds

o When receives, a host or router sends a membership response if it is still interested in a group



The general query message does The general query message does not define a particular group.not define a particular group.




Figure 10-7

General Query Message




Delayed Responseo However, there may be many hosts/routers

interested in the same groupn Each would send its report messagen Flooding

o Solution: delayed responsen Only one response is sent for a group



Delayed Response (Cont.)o When a host or router receives a query

messagen It does not respond immediately and delay for a

whileo Each host or router uses a random number to

create a timern Expire between 1 to 10 seconds

o Once a waiting host or router receives a reportn It does not send the report again



Example 1Example 1

Imagine there are three hosts in a network as shown in Figure 10.8 (next slide).

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.




Figure 10-8

Example 1




SolutionSolutionThe events occur in this sequence:

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.

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 225.14.0.0.




Solution (Continued)Solution (Continued)

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.

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.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.




Query Routero Query message may create a lot of responseo To prevent unnecessary trafficn IGMP designates one router as the query router

for each networko Only the query router sends the query

messagen The other routers are passive



ENCAPSULATION

10.410.4




Figure 10-9

Encapsulation of IGMP Packet




IP Layero At the IP level: three fields are of concern

n Protocol field: 2n TTL field: one

o IGMP’s domain is LAN, no IGMP message should travel beyond the LAN

n Destination IP addresso Query: 224.0.0.1: all systems on this subnet

n All routers and all hosts will receive this packeto Membership report: the multicast address of the groupo Leave report: 224.0.0.2: all routers on this subnet

n Routers receives this packet while hosts receive and disregard it



The IP packet that carries The IP packet that carries an IGMP packet has a value of an IGMP packet has a value of

22in its protocol field.in its protocol field.




The IP packet that carries The IP packet that carries an IGMP packet has a value of an IGMP packet has a value of

11in its TTL field.in its TTL field.




Data Link Layero IGMP is encapsulated in an IP packetn IP packet will have a multicast IP address

o ARP protocol cannot find the corresponding MAC address

o Solutionn Depends on whether the underlying data link

layer support physical multicast address or not



Physical Multicast Supporto Most LANs support physical multicast

addressn Ethernet is one of them

o If the first 25 bits in an Ethernet address aren 0000000100000000010111100: physical

multicast addressn The remaining 23 bits is used to define a group



Physical Multicast Support (Cont.)o To convert an IP multicast address into an

Ethernet address, the multicast routern Extract the least significant 23 bits of a class D IP

address

n Then insert into a multicast Ethernet address

n Note, class D defines multicast addresses



Mapping Class D to Ethernet Physical Address




The Classes of IP Address



Physical Multicast Support (Cont.)o However, the group identifier of a class D IP address

is 28 (32-4) bitsn Thus, 5 (28-23) bits are not usedn 32 (2^5) multicast addresses at the IP level are mapped to

a single multicast addressn Many-to-one mapping

o Thus, a host may receive multicast packets not belong to itn Host must check the IP address since MAC address is not

completely accurate



No Physical Multicast Supporto Most WANs do not support physical multicast

addressingo To send a multicast packet through these networks

n Use the tunneling techniqueo Tunneling

n The multicast packet is encapsulated in a unicast packet and send through the network

n Then, it emerges from the other side as a multicast packet



Figure 10-11

Tunneling




IGMP PACKAGE

10.510.5




IGMP Packageo Show ony the modules used in an IGMP hosto A group tableo A set of timerso Four software modulesn A group-joining modulen A group-leaving modulen An input modulen An output module



Figure 10-12

IGMP Package




Group Tableo Group table shows whether processes in this

host belonging to this group or noto State: defines the state of the entryn FREE: no processes left in the groupn DELAYING: a report must be sent for this entry

when the timers maturesn IDLE: no timer running for this entry



Group Table (Cont.)o Interface number

n The interface through which the multicast packet is sent or received

o Group address: the multicast addresso Reference count

n The number of processes interested in the groupn When a process joins a group: + 1n When a process leave a group: -1n When zero: state = FREE



Figure 10-13

State: Free, Delaying, Idle

Reference Count: Number of processes interested

Group Table




Timerso Each entry in the DELAYING state has a

timer to govern the sending of reports

o Select a random value and if expiresn Send a signal to the output module to generate a

report



Group-Joining Moduleo Receive: a request from a process to join a groupo If found

n Increment the reference counto Else

n Create an entry with reference count set to onen Add the entry to the tablen Request a membership report from the output modulen Inform the data link layer to update its configuration table

o Then this type of multicast packet can be received



Group-Leaving Moduleo Receive: a request from a process to leave a groupo Look for the corresponding entry in the tableo If found

n Decrement the reference countn If (reference count is zero)

o If ( any timer for this entry):n Cancel the timer

o Change the state to FREEo Request a leave report from the output module

o Return



Input Moduleo Receive: an IGMP messageo Check the message typeo If (query)

n Start a timer for each entry in the table with the state IDLEn Change each IDLE state to DELAYING staten Return

o If (membership report)n Look for the corresponding entry in the tablen If ( found and state is DELAYING)

o Cancel the timer for this entryo Change the state to IDLE

o Return



Input Moduleo Hosts do not receive leave reporto Thus, the previous slide does not show this



Output Moduleo Receive: a signal from a matured timer or a request from

joining or leaving moduleo If the message comes from a timer

n If (found and state is DELAYING)o Create a membership reporto Reset the state to IDLE

o If the message comes from the group-joining modulen Create a membership report

o If the message comes from the group-leaving modulen Create a leave report

o Send the message and return


chapter 10 internet group management protocol network93_1... · 2002. 1. 6. · o internet group...

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