When a receiver that supports fast retransmit receives a packet with a sequence number higher than the current expected one, it proceeds as if some data was dropped. The most common such circumstance occurs because of a feature known as fast retransmit. Under some circumstances, TCP retransmits data before a particular segment's retransmission timer expires. TCP connections over high-delay links take much longer to time out than those over low-delay links, in order to avoid incorrectly timing out when a connection is merely slow rather than not present. Using this algorithm, TCP tunes itself to the usual delay of a connection. The retransmission time-out for a given segment is doubled after each retransmission of that segment. For more information on SRTT calculations, see RFC 793: Transmission Control Protocol DARPA Internet Program Protocol Specification.
This paper can be found in the ACM Digital Library at the Association for Computing Machinery.
The retransmission time-out is adjusted on the fly to match the characteristics of the connection, using Smoothed Round Trip Time (SRTT) calculations as described in Van Jacobson and Mike Karels' paper "Congestion Avoidance and Control" in Proceedings of the ACM SIGCOMM Conference on Data Communication, November 1988. If TCP does not receive an acknowledgment for the data in a given segment before the timer expires, the segment is retransmitted.
TCP starts a retransmission timer when each outbound segment is handed down to the Internet Protocol (IP) layer.
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