Internet Core Protocols: The Definitive Guide

Small send windows and excessively delayed acknowledgments

A variety of performance problems can also occur when the sending system is using a small send window, while the recipient is using a large receive window and is delaying acknowledgments for too long. Although this requires an odd mixture of conditions to occur, it is not so rare as to not be a problem.

For example, some server-class systems intentionally delay acknowledgments for more than every-other segment, in an attempt to reduce network utilization and overhead, with some systems delaying acknowledgments until 50% or 60% of the receive window has been filled. When this mechanism is used to exchange data with other large-scale systems on a half-duplex network, the long gaps between acknowledgments allow the network to send more data in less time. However, when this model is used on systems with small send windows (such as is common with PC-based implementations), it can then trigger serious performance problems.

Although the vast majority of “send” activity comes from server-class systems with large send windows (such as HTTP servers, or POP3 mail servers), a lot of network traffic also gets generated by PC clients that are sending mail messages with large binary attachments to an SMTP server, or who are uploading large data files to an FTP server, and so forth. Typically, these systems do not have very large send windows (most of them only have send windows that are four times the local MTU).

When these systems try ...