9.8 The Window Scale Factor

The window declared by the destination has 2 B allocated in the TCP header. The destination can therefore declare a window that is between 0 and 65535 B. These windows are too small for gigabyte networks. One solution is to use the window scale factor optional item in the TCP segment header. This option can only be used in segments that initialize a connection (segments with the SYN flag).

Using the window scale factor item, both sides of the connection agree to scale the window by a factor of 0 to 14. Let us call this factor n. The agreed factor can be different in each direction.

The window scale factor is used in an interesting way. If the source proposes a window of size k big and proposes to scale it by n, then the destination understands that the window proposed by the source is k x 2ⁿ (that is, it increases the width of the window by n bits).

The largest declared window possible is 65535x2¹⁴ =1073725440=1G-16384. Thus with this window, the maximum amount...