Reliable Multicast

1
Reliable Multicast
2
Basic Multicast

• All correct processes deliver
• Unless multicaster crashes
• Implementations
• Use reliable one-to-one comm. (B-multicast)
• Use IP-multicast

3
Reliable Multicast

• Integrity Each message delivered at most once.
• Validity Correct sender eventually delivers
  message to itself.
• Agreement Either all correct processes deliver
  message or none do.

4
Simple (Incorrect) Multicast Alg

• Process p B-multicasts m to group g
• Process q in g receives m
• Send ack to p
• Deliver m to q
• If process p does not receive ack from q, resend
  to q

5
R-multicast

• For p to multicast m to group g B-multicast(g,m)
• When q receives m from p if (m ? Received)
  add m to Received if (q ? p)
  B-multicast(g,m) Deliver m to q

6
Reliable Multicast using IP-Multicast

• Sp msgs sent by p, according to p
• Rq - msgs received by p from q

7
Team Exercise

• Compare R-multicast with reliable multicast that
  uses IP-multicast
• Which is more efficient?
• When would each be used?

8
Questions (1/2)

• What are the three requirements of reliable group
  communication?
• Give one advantage of using IP-multicast to
  implement reliable multicast.
• Give two disadvantage to using IP-multicast to
  implement reliable multicast.

9
Questions (2/2)

• How do we need to update the R-multicast
  algorithm to ensure causal ordering?
• What is the problem with using a sequencer to
  provide causal ordering?
• If we used TCP/IP for the total ordering with a
  sequencer, have we ensured
• FIFO-total ordering?
• Causal-total ordering?