Lecture No. 22                                     Date :-  8 Oct. 2003                                         Scribe by :- Surya Prakash Kotha (Y3111049)

Reliable Multicast:

Suppose 3 parties are involved in WB - A, B and C
A Draws Line on WB.
Then B draws line related to A's line.
If application says that these two are casually ordered then Everybody should see updates in this sequence only.
Reordering is not desired here.

Design Goals

Design Challenges

Unicast has "fate sharing".
Meaning:  if either one of sender or receiver fails then the connection is terminated.
                   But in multicast as there  are number of senders or recievers cont decide about the connection.
 In unicast anyone (sender or receiver) can be responsible for reliability.
(In TCP, sender is responsible. It keeps on trying till it gest ack from receiver.)

Sender based approach In multi cast:
Fig. 1

So reciever based approach is fallowed in multicast.

Problem with Sequencing :

TCP uses byte number for sequencing.
However that has problem with multicast.
Solution: Application Layer Framming

Reliable Multicast Protocol

General working :

On loss detection :
  • Each node in the subset (shown inside the circle) will set timer with random values for time outs
  • One of them will eventually time outs and fires the multicast-repair-request.
  • Anybody having packet then can multicast the packet.