TCP Windows

From a performance point of view, one of the main limitations of the original TCP specification is the 16 bits window field in the TCP header. As this field indicates the current size of the receive window in bytes, it limits the TCP receive window at 65535 bytes. This limitation was not a severe problem when TCP was designed since at that time high-speed wide area networks offered a maximum bandwidth of 56 kbps. However, in today’s network, this limitation is not acceptable anymore. The table below provides the rough 19 maximum throughput that can be achieved by a TCP connection with a 64 KBytes window in function of the connection’s round-trip-time.

To solve this problem, a backward compatible extension that allows TCP to use larger receive windows was proposed in RFC 1323. Today, most TCP implementations support this option. The basic idea is that instead of storing snd.wnd and rcv.wndas 16 bits integers in the TCB, they should be stored as 32 bits integers. As the TCP segment header only contains 16 bits to place the window field, it is impossible to copy the value of snd.wnd in each sent TCP segment. Instead the header contains snd.wnd >> S where S is the scaling factor ( 0 ≤ S ≤ 14) negotiated during connection establishment. The client adds its proposed scaling factor as a TCP option in the SYN segment. If the server supports RFC 1323, it places in the SYN+ACK segment the scaling factor that it uses when advertising its own receive window. The local and remote scaling factors are included in the TCB. If the server does not support RFC 1323, it ignores the received option and no scaling is applied.

By using the window scaling extensions defined in RFC 1323, TCP implementations can use a receive buffer of up to 1 GByte. With such a receive buffer, the maximum throughput that can be achieved by a single TCP connection becomes:

These throughputs are acceptable in today’s networks. However, there are already servers having 10 Gbps interfaces. Early TCP implementations had fixed receiving and sending buffers 20. Today’s high performance implementations are able to automatically adjust the size of the sending and receiving buffer to better support high bandwidth flows [SMM1998].

Source: Olivier Bonaventure, https://s3.amazonaws.com/saylordotorg-resources/wwwresources/site/wp-content/uploads/2012/02/Computer-Networking-Principles-Bonaventure-1-30-31-OTC1.pdf
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