RFC 1323 Using the Window Scale Option

snd_wscale : 4,    /* Window scaling received from sender    */
rcv_wscale : 4;    /* Window scaling to send to receiver    */

/* Maximal number of window scale according to RFC1323 */
#define TCP_MAX_WSCALE        14U

 Using the Window Scale Option

      A model implementation of window scaling is as follows, using the
      notation of RFC-793 [Postel81]:

      *    All windows are treated as 32-bit quantities for storage in
           the connection control block and for local calculations.
           This includes the send-window (SND.WND) and the receive-
           window (RCV.WND) values, as well as the congestion window.

      *    The connection state is augmented by two window shift counts,
           Snd.Wind.Scale and Rcv.Wind.Scale, to be applied to the
           incoming and outgoing window fields, respectively.

      *    If a TCP receives a <SYN> segment containing a Window Scale
           option, it sends its own Window Scale option in the <SYN,ACK>
           segment.

      *    The Window Scale option is sent with shift.cnt = R, where R
           is the value that the TCP would like to use for its receive
           window.

      *    Upon receiving a SYN segment with a Window Scale option
           containing shift.cnt = S, a TCP sets Snd.Wind.Scale to S and
           sets Rcv.Wind.Scale to R; otherwise, it sets both
           Snd.Wind.Scale and Rcv.Wind.Scale to zero.

      *    The window field (SEG.WND) in the header of every incoming
           segment, with the exception of SYN segments, is left-shifted
           by Snd.Wind.Scale bits before updating SND.WND:
              SND.WND = SEG.WND << Snd.Wind.Scale
           (assuming the other conditions of RFC793 are met, and using
           the "C" notation "<<" for left-shift).

      *    The window field (SEG.WND) of every outgoing segment, with
           the exception of SYN segments, is right-shifted by
           Rcv.Wind.Scale bits:
              SEG.WND = RCV.WND >> Rcv.Wind.Scale.

      TCP determines if a data segment is "old" or "new" by testing
      whether its sequence number is within 2**31 bytes of the left edge
      of the window, and if it is not, discarding the data as "old".  To
      insure that new data is never mistakenly considered old and vice-
      versa, the left edge of the sender's window has to be at most
      2**31 away from the right edge of the receiver's window.
      Similarly with the sender's right edge and receiver's left edge.
      Since the right and left edges of either the sender's or
      receiver's window differ by the window size, and since the sender
      and receiver windows can be out of phase by at most the window
      size, the above constraints imply that 2 * the max window size
      must be less than 2**31, or

           max window < 2**30

      Since the max window is 2**S (where S is the scaling shift count)
      times at most 2**16 - 1 (the maximum unscaled window), the maximum
      window is guaranteed to be < 2*30 if S <= 14.  Thus, the shift
      count must be limited to 14 (which allows windows of 2**30 = 1
      Gbyte).  If a Window Scale option is received with a shift.cnt
      value exceeding 14, the TCP should log the error but use 14
      instead of the specified value.

      The scale factor applies only to the Window field as transmitted
      in the TCP header; each TCP using extended windows will maintain
      the window values locally as 32-bit numbers.  For example, the
      "congestion window" computed by Slow Start and Congestion
      Avoidance is not affected by the scale factor, so window scaling
      will not introduce quantization into the congestion window.