Hierarchical Channels and the Relative Speed of Messages 1

FIFO communication is expensive to support. It is also quite restrictive for several applications. In this paper, we study the concept of relative speed of messages and a hierarchy of such speeds which results in a hierarchical channel . In hierarchical channels messages sent at lower levels of the hierarchy cannot be overtaken by message sent at a higher level. But lower level messages can overtake higher level messages. The exibility provided by hierarchical channels can be exploited by several distributed applications. We present examples to illustrate how FIFO ordering constraints can be relaxed for such applications using hierarchical channels , leading to increased concurrency. We present a new algorithm to implement hierarchical channels . The algorithm employs selective ooding, and is tolerant to link-failures. It is possible to calculate the statistical distribution of message propagation delays for a selective ooding implementation of hierarchical channels , as shown in the paper. So, they can be employed to implement statistical channels. If hierarchical channels are used for communication, assertions can be made about end-to-end message ordering characteristics. Thus, hierarchical channels subsume message overtaking characteristics of point-topoint links, as well as send and receive queues at intermediate nodes. Hence, hierarchical channels can support stronger assertions about message ordering, yet provide greater exibility than priority queues. Hierarchical channels can also be used to implement priority queues. Compared to Fchannels , hierarchical channels can model a greater variety of message orderings: all the way from FIFO message delivery to totally unordered message delivery.