Designing an Internet

by David D. Clark

A typical reliable transport protocol responds to a missing packet by requesting a retransmission and delaying the delivery of any subsequent packets until the lost packet has been retransmitted. It then delivers that packet and all remaining ones in sequence. The delay while this occurs can be many times the round trip delivery time of the net, and may completely disrupt the speech reassembly algorithm. In contrast, it is very easy to cope with an occasional missing packet. The missing speech can simply be replaced by a short period of silence, which in most cases does not impair the intelligibility of the speech to the listening human. If it does, high level error correction can occur, and the listener can ask the speaker to repeat the damaged phrase. It was thus decided, fairly early in the development of the Internet architecture, that more than one transport service would be required, and the architecture must be prepared to tolerate simultaneously transports which wish to constrain reliability, delay, or bandwidth, at a minimum.