A bang-bang principle for real-time transport protocols

Abstract

A real-time protocol is usually concerned with the transportation of a real-time data stream over a packet switched network. Among the major issues distinguishing real-time protocols from ordinary transport protocols is the problem of trading delay for loss. That is, if some loss of packets may be acceptable, and usually inevitable, the objective of the protocol is to minimize the delay of packets, subject to constraints on the acceptable loss. This is unlike usual transport protocols, which are designed to guarantee no loss at the expense of increased delays. This paper presents a model for the delay-loss tradeoffs in a family of real-time transport protocols. It is demonstrated that, under very general assumptions, an optimal real-time protocol is a bang-bang protocol: there exists a threshold queue length such that as long as the packet queue length at the sender is less than the threshold the protocol should be an ordinary positive acknowledgement with retransmissions transport protocol. However, as soon as the threshold queue length is exceeded, a newly arriving packet causes the first (or any other) packet in the queue to be discarded. Closed form expressions for the threshold buffer size are obtained and analyzed in terms of the given parameters of the system.