Internet Core Protocols: The Definitive Guide

Uneven MTU multiples

Another interesting opportunity for problems occurs when the receive window size being advertised by a recipient is not an even multiple of the MTU in use on the virtual circuit. This results in problems with Silly Window Avoidance causing transmission delays. As you may recall, the Silly Window Syndrome can occur when the recipient’s TCP stack advertises a small window size, resulting in the sender transmitting a few bytes (to fill the advertised space), with the process repeating ad infinitum. In order to prevent this behavior, RFC 1122 stated that a system could only advertise a non-zero window if the receive buffers could store one fully sized segment.

However, this becomes a problem when the receive buffers on a system are not an even multiple of the MTU in use on the network, or when the application attempts to write data in blocks that are not an even multiple of the MTU. For example, if a system has a default receive window of four kilobytes but is receiving 1460-byte segments, then the sender can only transmit three fully sized segments, since the Nagle algorithm delays the fourth segment until an acknowledgment arrives for the third segment, or until a fully sized segment can be sent. However, as noted above, the recipient will not issue an acknowledgment until it has received two fully sized segments, or until a timer expires.

As such, the sender sends only three segments of data, and then stops to wait for an acknowledgment before sending the final ...