4.2.3.1 Retransmission Timeout Calculation

A host TCP MUST implement Karn's algorithm and Jacobson's algorithm for computing the retransmission timeout ("RTO").

• Jacobson's algorithm for computing the smoothed round- trip ("RTT") time incorporates a simple measure of the variance [TCP:7].

• Karn's algorithm for selecting RTT measurements ensures that ambiguous round-trip times will not corrupt the calculation of the smoothed round-trip time [TCP:6].

This implementation also MUST include "exponential backoff" for successive RTO values for the same segment. Retransmission of SYN segments SHOULD use the same algorithm as data segments.

DISCUSSION:

There were two known problems with the RTO calculations specified in RFC-793. First, the accurate measurement of RTTs is difficult when there are retransmissions. Second, the algorithm to compute the smoothed round- trip time is inadequate [TCP:7], because it incorrectly assumed that the variance in RTT values would be small and constant. These problems were solved by Karn's and Jacobson's algorithm, respectively.

The performance increase resulting from the use of these improvements varies from noticeable to dramatic. Jacobson's algorithm for incorporating the measured RTT variance is especially important on a low-speed link, where the natural variation of packet sizes causes a large variation in RTT. One vendor found link utilization on a 9.6kb line went from 10% to 90% as a result of implementing Jacobson's variance algorithm in TCP.

The following values SHOULD be used to initialize the estimation parameters for a new connection:

1. RTT = 0 seconds.

2. RTO = 3 seconds. (The smoothed variance is to be initialized to the value that will result in this RTO).

The recommended upper and lower bounds on the RTO are known to be inadequate on large internets. The lower bound SHOULD be measured in fractions of a second (to accommodate high speed LANs) and the upper bound should be 2*MSL, i.e., 240 seconds.

DISCUSSION:

Experience has shown that these initialization values are reasonable, and that in any case the Karn and Jacobson algorithms make TCP behavior reasonably insensitive to the initial parameter choices.