The proposed protocol has the following salient features: • Multicast communication is reliable (all multicasts are delivered and there are no duplicates). • The sender of each multicast may select among three increasingly strong delivery ordering guarantees [HT93]: FIFO (i.e., single-source ordering); Causal (i.e., deliveries occur in an order consistent with the "happened-before" ordering of transmissions [L78,PRS97,AV97]); Total (i.e., all group members deliver multicasts in the same order and this order is consistent with causal order). • The set of senders and receivers (group) may be dynamic: a mobile host may join and leave such group at will of the application. • • Size of message headers, size of data structures at MHs, number of messages in the wired network for each multicast do not depend on the number of group members. These factors contribute to make the protocol highly scalable. To the best of our knowledge, previous reliable multicast protocols for distributed mobile systems [AB96,PRS97,AV97,YHH97,ARR97] offered at most two of the following features, supported by our proposal: (i) incomplete spatial coverage; (ii) unreliable wireless communication; (iii) dynamic membership. None such protocol was able to accommodate cell switching without any message exchange in the wired network and none supported the three delivery guarantees FIFO, Causal, Total. We have analyzed in detail the performance of the proposed protocol by simulation. We anticipate that the protocol has good performance and good scalability properties. For example, we shall see that the delivery latency with a fixed number of senders remains approximately constant up to a large number of receivers and that this latency is limited only by queuing times at "Mobile Support Stations", a factor that follows from the mismatch between the wired and wireless bandwidth and thus it is not a peculiarity of our protocol. 2.1 Overview of the protocol Our protocol is described in section 4. We present an overview here as a background for the next subsection where we discuss related work in this area. By stationary host (SH) we mean a host connected to the wired network. In our protocol, new multicasts originated by MHs have to be processed by a small subset of SHs, the coordinators, that attach proper control information to these multicasts. The details of such processing and of the related control information depend on the delivery guarantee selected by the sender. Coordinators send the multicasts to mobile support stations (MSSs), i.e., SHs connected 4 to the wired network and covering a cell each. MSSs broadcast messages from coordinators in the respective cell. MHs may miss multicasts, for any of the following reasons: (i) they roam out of coverage; or (ii) the wireless link looses the message; or (iii) they move at inopportune times, e.g., they switch cell before a multicast is broadcast in the old cell and after it has been broadcast in the new one. MHs detect missing multicasts and ask retransmissions to MSSs by means of a negative acknowledgment scheme. MSSs cache copies of past multicasts and, if a retransmission request
Another three-tier reliable multicast scheme is proposed in REF , which accommodates three increasingly strong delivery ordering guarantees: FIFO, causal, and total.
A Reliable Multicast Protocol for Distributed Mobile Systems: Design and Evaluation
IEEE Transactions on Parallel and Distributed Systems
Computer Science