A Reliable Multicast Framework for Light-weight Sessions and Application Level Framing

Summary

This paper presents a general framework for building different multicast services. They argues that "multicast applications have widely different requirements for reliability." Specifically applications differ along three axis:

1. Ordering requirements. Some applications require a total ordering while others do not.
2. Number of senders. Some applications have one sender while others have many.
3. Choice of sender. In some applications multiple nodes have the data and any one can send.

Given this variation the authors strive to build a general framework that can be extended by the application.

A key aspect of their design is to make receivers responsible for reliable delivery rather than senders as done in TCP. Specifically, each receiver in the multicast group is responsible for detecting loss and requesting retransmission. Loss is detected by a gap in the sequence numbers and by periodically broadcasting the highest sequence number received from each source. When loss is detected a "repair" message is broadcast, any node that has the data can reply and all other potential senders are suppressed when the reply is seen. Similarly, if other nodes need the same data repaired they suppress sending a message if a repair is seen.

The authors implement wb a shared whiteboard system on top of their framework. In wb commands are idempotent so total ordering is not required. Also, any node can forward lost data to any other an other node and any node can be the sender of new data.

Thoughts

The idea of making the receivers responsible for recovery is fairly interesting. However, I am concerned by the amount of broadcasting that goes on. Each repair request and response is multicast to the whole group. This seems to limit the scale of the system. An alternative approach would be to have nodes send unicast repair requests to nearest neighbors.