IMPROVING ROUND-TRIP TIME ESTIMATES IN RELIABLE TRANSPORT PROTOCOLS

As a reliable, end-to-end transport protocol, the Transmission Control Protocol (TCP) uses positive acknowledgements and retransmission to guarantee delivery. TCP implementations are expected to measure and adapt to changing round-trip delay so that their retransmission behavior balances user throughput and network efficiency. However, TCP suffers from a problem we call retransmission ambiguity: when an acknowledgement arrives for a datagram that has been retransmitted, there is no indication of which transmission is being acknowledged. As a result, an implementation may be unable to determine if the round-trip time it measures is for an original transmission or a retransmission of a datagram. Many existing TCP implementations do not handle this problem correctly. Furthermore, the problem of retransmission ambiguity is also a characteristic of other major transport protocols, including OSI TP4 and DECnet NSP. This paper reviews the various approaches to retransmission and presents a novel and effective approach to the retransmission ambiguity problem.