An Investigation into Transport Protocols and Data Transport ...

4.4. Retransmission Timeout 61
tion avoidance regime from a ‘safe’ operating at half the rate at which loss
occurred.
4.4 Retransmission Timeout
As TCP requires the receipt of acknowledgments in order to slide the TCP
window and thus maintain the flow of data between the sender and receiver,
should ack’s not be received then the TCP sender will indefinitely stall.
In order to rectify this, TCP implements a Retransmission Timeout (RTO)
which is a soft timer which is re-initialised after the receipt of an ack. It is
assumed that an RT O expiry (i.e. a period where no acks are received) is
equivalent to packet lost and therefore the missing segment should be retransmitted
to facilitate reliable delivery and progress movement of the TCP
window.
The delivery of each segment of data may experience differing latencies;
it is therefore important to have a good RTT estimator in order to determine
an appropriate RTO.
[Jac88] presented a calculation of RT O by applying a filter such that it
takes into account the variation of the average RTT, in fact this exponentially
weighted estimator devised for Tahoe is still in use today. The RTO is
calculated using a smoothed estimate of the RTT, SRTT, and a variance
estimate, RTTVAR [PA00].
SRT T ← SRT T + 1 (RT T − SRT T ) (4.6)
8
RT T V AR ← V RT T + 1 (|RT T − SRT T | − RT T V AR) (4.7)
4
RT O ← SRT T + 4 × RT T V AR (4.8)