This paper discusses scalable reliable multicast (SRM). It starts off by presenting the problems of blindly scaling unicast to multicast. Reliable delivery becomes a very difficult problem to tackle when there are multiple receivers and a layer of indirection involved. Interestingly, the authors advocate the use of Application Data Units (ADU) which is a method more closely tied to an application for requesting pieces of data.
The authors then go on to describe wb, an application that uses reliable multicast, and the design decisions that accompany it. Particular to wb are that all senders and receivers are part of the same multicast group without regard to roles, and delivery order of packets does not matter to the application. With the latter fact in mind, it seems odd that they should choose this as their example application, since part of the difficulty in multicast comes in with ordering packets correctly. To detect gaps due to losses without causing a response implosion on the sender side, receivers send periodic messages about which sequence numbers they have seen; they work together to send and respond to repair requests after first waiting some random timeout.
The paper gives the expected number of requests and timer delays for a few topologies: chain, star, and bounded-degree trees, and then shows some simulation results for these topologies. One realism missing from the simulations was the possibility of dropping repairs and repairs requests and thus requiring retransmissions of retransmissions. I hated the simulation result graphs. They weren't labeled at all, so I had to jump back and forth to the text a few times to figure out what everything was.
No comments:
Post a Comment