Connected: An Internet Encyclopedia
2.1 Window Scale Option

Up: Connected: An Internet Encyclopedia
Up: Requests For Comments
Up: RFC 1072
Up: 2. TCP WINDOW SCALE OPTION
Prev: 2. TCP WINDOW SCALE OPTION
Next: 2.2 Using the Window Scale Option

2.1 Window Scale Option

2.1 Window Scale Option

This three-byte option may be sent in a SYN segment by a TCP (1) to indicate that it is prepared to do both send and receive window scaling, and (2) to communicate a scale factor to be applied to its receive window. The scale factor is encoded logarithmically, as a power of 2 (presumably to be implemented by binary shifts).

Note: the window in the SYN segment itself is never scaled.

      TCP Window Scale Option:

      Kind: 3

             +---------+---------+---------+
             | Kind=3  |Length=3 |shift.cnt|
             +---------+---------+---------+

Here shift.cnt is the number of bits by which the receiver right- shifts the true receive-window value, to scale it into a 16-bit value to be sent in TCP header (this scaling is explained below). The value shift.cnt may be zero (offering to scale, while applying a scale factor of 1 to the receive window). This option is an offer, not a promise; both sides must send Window Scale options in their SYN segments to enable window scaling in either direction.


Next: 2.2 Using the Window Scale Option

Connected: An Internet Encyclopedia
2.1 Window Scale Option