TCP silly window syndrome

Problem statement

Laymen explanation

This problem occurs when data are passed to the sending TCP entity in large blocks, but an interactive application on the receiving side reads 1 byte at a time. we get greatly reduced performance because we are sending a small amount of data in a segment with at least 40 bytes of header overhead.

Technical explanation

This diagram shows one example of how the phenomenon known as TCP silly window syndrome can arise. The client is trying to send data as fast as possible to the server, which is very busy and cannot clear its buffers promptly. Each time the client sends data the server reduces its receive window. The size of the messages the client sends shrinks until it is only sending very small, inefficient segments.

Let's suppose this happens. Now, eventually, the server will remove some of the data from this buffer. Let's say it removes 40 bytes by the time the first closed-window “probe” from the client arrives. The server then reopens the window to a size of 40 bytes. The client is still desperate to send data as fast as possible, so it generates a 40-byte segment. And so it goes, with likely all the remaining data passing from the client to the server in tiny segments until either the client runs out of data, or the server more quickly clears the buffer.

Now imagine the worst-case scenario. This time, it is the application process on the server that is overloaded. It is drawing data from the buffer one byte at a time. Every time it removes a byte from the server's buffer, the server's TCP opens the window with a window size of exactly 1 and puts this in the Window field in an acknowledgment to the client. The client then sends a segment with exactly one byte, refilling the buffer until the application draws off the next byte.

Solution

Modern TCP implementations incorporate a set of SWS avoidance algorithms. When receiving, devices are programmed not to advertise very small windows, waiting instead until there is enough room in the buffer for one of a reasonable size. Transmitters use Nagle’s algorithm to ensure that small segments are not generated when there are unacknowledged bytes outstanding.

Reference

http://www.tcpipguide.com/free/t_TCPSillyWindowSyndromeandChangesTotheSlidingWindow.htm