Transcript lect16_rmcast

• 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
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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 EthernetConnected Networks.” International Conference on
Parallel Processing, 2001. Available at
http://www.cs.fsu.edu/~xyuan/cop5570/icpp01.ps