Internet Core Protocols: The Definitive Guide

Network I/O Management

When an application needs to send data to another application over TCP, it writes the data to the local TCP provider, which queues the data into a send buffer. Periodically, TCP packages portions of the data into bundles (called “segments”), and passes them off to IP for delivery to the destination system, as illustrated in Figure 7.8.

Figure 7.8. An overview of TCP’s data-encapsulation process

Although this process sounds simple, it involves a lot of work, primarily due to segment-sizing issues that TCP has to deal with. For every segment that gets created, TCP has to determine the most efficient segment size to use at that particular moment, which is an extremely complex affair involving may different factors. However, this is also an extremely important service, since accurately determining the size of a segment dictates many of the performance characteristics of a virtual circuit.

For example, making the segment too small wastes network bandwidth. Every TCP segment contains at least 40 bytes of overhead for the IP and TCP headers, so if a segment only contained one byte of data, then the byte ratio of headers-to-data for that segment would be 40:1, a miserable level of throughput by anybody’s standard. Conversely, sending 400 bytes of data would change this ratio to 1:10, which is better, although still not very good. Sending four kilobytes of data would ...