Internet Core Protocols: The Definitive Guide

The Nagle algorithm

The Silly Window Syndrome is indicative of a problem at the receiver’s end of the virtual circuit. Data is not being read from the receive buffers quickly, resulting in small window sizes being advertised, which in turn causes the sender to transmit small segments. The result is that lots of network traffic gets generated for very small amounts of data.

However, a sending system can also cause these kinds of problems, although for totally different reasons. Some applications (such as TELNET) are designed to send many small segments in a constant barrage, which causes high levels of network utilization for small amounts of data. Other situations in which this is a problem are applications that write data only in small chunks, such as writing 10 megabytes of data in 512-byte blocks. The number of packets that will get generated in that model are extremely wasteful of bandwidth, particularly when this same transfer could be done using larger writes.

One solution proposed to this kind of problem is the Nagle algorithm, which was originally described in RFC 896. Simply put, the Nagle algorithm suggests that segments that are smaller than the maximum size allowed (as defined by the MSS option of the recipient or the discovered MTU of the end-to-end path) should be delayed until all prior segments have been acknowledged or until a full-sized segment can be sent. This rule forces TCP stacks to merge multiple small writes into a single write, which is then sent as a single ...