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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

Silly Window Syndrome Avoidance Algorithms

Since SWS is caused by the basic sliding window system not paying attention to the result of decisions that create small segments, dealing with SWS is conceptually simple: change the system so that we avoid small window size advertisements, and at the same time, also avoid sending small segments. Since both the sender and recipient of data contribute to SWS, changes are made to the behavior of both to avoid SWS. These changes are collectively termed SWS avoidance algorithms.

Let's start with SWS avoidance by the receiver. As we saw in the initial example above, the receiver contributed to SWS by reducing the size of its receive window to smaller and smaller values due its being busy. This caused the right edge of the sender's send window to move by ever-smaller increments, leading to smaller and smaller segments. To avoid SWS, we simply make the rule that the receiver may not update its advertised receive window in such a way that this leaves too little usable window space on the part of the sender. In other words, we restrict the receiver from moving the right edge of the window by too small an amount. The usual minimum that the edge may be moved is either the value of the MSS parameter, or one-half the buffer size, whichever is less.

Let's see how we might use this in the example above. When the server receives the initial 360-byte segment from the client and can only process 120 bytes, it does not reduce the window size to 120. It reduces it all the way to 0, closing the window. It sends this back to the client, which will then stop and not send a small segment. Once the server has removed 60 more bytes from the buffer, it will now have 180 bytes free, half the size of the buffer. It now opens the window up to 180 bytes in size and sends the new window size to the client.

It will continue to only advertise either 0 bytes, or 180 or more, not smaller values in between. This seems to slow down the operation of TCP, but it really doesn't. Because the server is overloaded, the limiting factor in overall performance of the connection is the rate at which the server can clear the buffer. We are just exchanging many small segments for a few larger ones.

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