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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transmission Control Protocol (TCP)                      9  TCP Reliability and Flow Control Features and Protocol Modifications

TCP Window Management Issues 1 2 3 4 TCP Congestion Handling and Congestion Avoidance Algorithms

The Cause of Silly Window Syndrome: Inefficient Reductions of Window Size

None of what we have seen above represents a failure per se of the sliding window mechanism. It is working properly to keep the server's receive buffer filled and to manage the flow of data. The problem is that the sliding window mechanism is only concerned with managing the buffer—it doesn't take into account the inefficiency of the small segments that result when the window size is micromanaged in this way.

In essence, by sending small window size advertisements we are “winning the battles but losing the war”. Early TCP/IP researchers who discovered this phenomenon called it silly window syndrome (SWS), a play on the phrase “sliding window system” that expresses their opinion on how it behaves when it gets into this state.

The examples above show how SWS can be caused by the advertisement of small window sizes by a receiving device. It is also possible for SWS to happen if the sending device isn't careful about how it generates segments for transmission, regardless of the state of the receiver's buffers.

For example, suppose the client TCP in the example above was receiving data from the sending application in blocks of 10 bytes at a time. However, the sending TCP was so impatient to get the data to the client that it took each 10-byte block and immediately packaged it into a segment, even though the next 10-byte block was coming shortly thereafter. This would result in a needless swarm of inefficient 10-data-byte segments.

TCP Window Management Issues 1 2 3 4 TCP Congestion Handling and Congestion Avoidance Algorithms