The three-byte Window Scale option may be sent in a SYN segment by a TCP. It has two purposes: (1) indicate that the TCP is prepared to do both send and receive window scaling, and (2) communicate a scale factor to be applied to its receive window. Thus, a TCP that is prepared to scale windows should send the option, even if its own scale factor is 1. The scale factor is limited to a power of two and encoded logarithmically, so it may be implemented by binary shift operations.
TCP Window Scale Option (WSopt):
Kind: 3 Length: 3 bytes
+---------+---------+---------+
| Kind=3 |Length=3 |shift.cnt|
+---------+---------+---------+
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. If window scaling is enabled, then the TCP that sent this option will right-shift its true receive-window values by 'shift.cnt' bits for transmission in SEG.WND. The value 'shift.cnt' may be zero (offering to scale, while applying a scale factor of 1 to the receive window).
This option may be sent in an initial <SYN> segment (i.e., a segment with the SYN bit on and the ACK bit off). It may also be sent in a <SYN,ACK> segment, but only if a Window Scale op- tion was received in the initial <SYN> segment. A Window Scale option in a segment without a SYN bit should be ignored.
The Window field in a SYN (i.e., a <SYN> or <SYN,ACK>) segment itself is never scaled.