title: MSS and tcp window size
body: <p>during communication between A and B ,
  When A is sending data to B , the segment size is going to depend on B's window size.
  Initially , after B successfully receives data, it doubles window size and this continues.
  Now B knows its MSS. So while sending the segment size to A, it can make sure that it does not exceed MSS value. 
  Then why B has to inform MSS to A at the first place ?</p>