Internet Core Protocols: The Definitive Guide

Excessive or Slow Retransmissions

One of the most important aspects of TCP’s reliability service is the use of acknowledgment timers (also called retransmission timers), which ensure that lost segments get recognized as such in a timely manner, resulting in the quick retransmission of that data. However, as was pointed out earlier in Section 7.1.7.4, accurately determining the correct timer values to use on any given virtual circuit is a complex process. If the value is set too low, then the timer will expire too frequently, resulting in unnecessary retransmissions. Conversely, if the value is set too high, then loss may not be detected quickly, resulting in unnecessary delays.

On some systems, you may notice that there are a high number of excessive retransmissions on new circuits (indicating that the default timer is set too low for those virtual circuits), or you may see very long gaps in between retransmissions (indicating that the default acknowledgment timer is set too high). Although these values are typically changed on a per-circuit basis as the smoothed round-trip time is calculated, the default settings can be problematic on new connections, where the smoothing has yet to begin in earnest.

For example, if the default acknowledgment timer is set to 3000 milliseconds, then the system may not detect a lost segment until three full seconds after the segment was sent. This would obviously cause problems with applications that only send one or two segments at a time (such as ...