Chapter 17. Network Performance, Latency,and Concurrency

In many science fiction stories, people talk with each other across astronomical distances as though they were in the same room. The technology never gets in the way. Computer networks do not work that way. They have real physical limitations. Software developers who ignore the problems of building network-enabled communication applications risk alienating their users. Let's consider some examples.

A university professor holds an online seminar with students at a number of remote locations. One student logs in using a dial-up modem over a low-quality telephone line. By default, the application attempts to send the student video and audio from each of the other participants and to have the student send video and audio to everyone else. The result is immensely frustrating to everyone. The student's connection is so saturated with video and audio data that nothing seems to work. Phone calls asking the student to turn off video do not help.

A pair of friends decides to play an online car racing game. Near the end of the game, as the two approach the finish line, each can see that the other is lurching forward and back well behind him. Each of the two players seems to cross the finish line well ahead of the other. One of them is happy to be declared the winner while the other is outraged at losing after appearing to win the race by a safe margin.

In both cases, the software is likely to be abandoned because the users' perception is that it simply does not work. And the users are right. The software was written without taking into account the limitations of computer networks. This chapter is about tailoring applications to work on the real Internet and describes strategies for adapting to different bandwidths and latency. But it cannot suggest solutions for every problem. Some games are simply unplayable unless latency is below a certain threshold and live high-resolution video conferences simply will not work at slow modem speeds.