Internet Core Protocols: The Definitive Guide

Congestion window sizing

TCP’s use of variable-length, sliding windows provides good flow control services to the receiving end of a virtual circuit. If the receiver starts having problems, it can slow down the rate at which data is being sent simply by scaling back the amount of buffer space being advertised. But if things are going well, the window can be scaled up, and traffic can flow as fast as the network will allow.

Sometimes, however, the network itself is the bottleneck. Remember that TCP segments are transmitted within IP packets, and that these packets can have their own problems outside of the virtual circuit. In particular, a forwarding device in between the two endpoints could be suffering from congestion problems, whereby it was receiving more data than it could forward, as is common with dial-up servers and application gateways.

When this occurs, the TCP segments will not arrive at their destination in a timely manner (if they make it there at all). In this scenario, the receiving system (and the virtual circuit) may be operating just fine, but problems with the underlying IP network are preventing segments from reaching their destination.

This problem is illustrated in Figure 7.11, which shows a device trying to send data to the remote endpoint, although another device on the network path is suffering from congestion problems, and has sent an ICMP Source Quench error message back to the sender, asking it to slow down the rate of data transfer.

Figure 7.11. Detecting ...