Editing Transmission Control Protocol (section)

===Window scaling===
{{Main|TCP window scale option}}
For more efficient use of high-bandwidth networks, a larger TCP window size may be used. A 16-bit TCP window size field controls the flow of data and its value is limited to 65,535 bytes. Since the size field cannot be expanded beyond this limit, a scaling factor is used. The [[TCP window scale option]], as defined in {{harvtxt|RFC 1323}}, is an option used to increase the maximum window size to 1 gigabyte. Scaling up to these larger window sizes is necessary for [[TCP tuning]].

The window scale option is used only during the TCP 3-way handshake. The window scale value represents the number of bits to left-shift the 16-bit window size field when interpreting it. The window scale value can be set from 0 (no shift) to 14 for each direction independently. Both sides must send the option in their SYN segments to enable window scaling in either direction.

Some routers and packet firewalls rewrite the window scaling factor during a transmission. This causes sending and receiving sides to assume different TCP window sizes. The result is non-stable traffic that may be very slow. The problem is visible on some sites behind a defective router.<ref>{{cite web |url=https://lwn.net/Articles/92727/ |title=TCP window scaling and broken routers |website=LWN.net |access-date=2016-07-21 |archive-date=2020-03-31 |archive-url=https://web.archive.org/web/20200331213612/https://lwn.net/Articles/92727/ |url-status=live }}</ref>