Reliable Multicasting IPmulticast is unreliable Need a reliable

• Reliable Multicasting – IP-multicast is unreliable. • Need a reliable multicast layer to simplify the software design. – Like in reliable unicast (PAR), we need to add acknowledgement to ensure 100% reliability. • Sender makes sure that a packet is acknowledged by all receivers before removing it from the buffer. • Receiver makes sure that duplicated packets will not be passed to the upper layer. • Many variations in terms of how acknowledgements and retransmissions are organized.

• Reliable multicast protocols: – ACK-based – NAK-based – Ring-based – Tree-based

• ACK based protocols ACK-based protocols • Straight-forward extension of the reliable unicast protocol. • Ack-implosion problem, not scalable.

NAK-based protocols • A receiver responses only when something is wrong. • More scalable compared to the ACK-based scheme • Other mechanism must be incorporated to ensure the reliability.

Ring-based protocols • Receivers take turn to ACK. • Each data packet has only one acknowledgement packet. • Implementation is fairly complicated.

Tree-based protocols • Receivers are divided into groups. • Each group has a group leader. • Sender interacts with the group leaders. Group leaders interact with group members.

• Reliable multicast over Ethernet connected clusters: – Multicasting over Ethernet is different from multicasting in the general Internet Environment • • Broadcast Short propagation delay. Shared media network. Low error rate. • Reliable multicast over the Internet: – Very difficult because of the heterogeneous nature in the Internet. – 10 years’ research still cannot come to a conclusion • IP multicast model may need to change to support reliable multicast over the Internet!!!

Scalability of an ACK-based protocol

– Performance of the reliable multicast protocols: • Small message: ACK-based, NAK-based and Ring-based are better than tree-based.

– Performance of the reliable multicast protocols: • Large message: NAK-based is the best.

• Reference: – Ryan Lane, Scott Daniels and Xin Yuan “An Empirical Study of Reliable Multicast Protocols over Ethernet. Connected Networks. ” International Conference on Parallel Processing, 2001. Available at http: //www. cs. fsu. edu/~xyuan/cop 5570/icpp 01. ps