code: func EncodeMSSOption(mss uint32, b []byte) int {
  	// mssOptionSize is the number of bytes in a valid MSS option.
  	const mssOptionSize = 4
  
  	if len(b) < mssOptionSize {
  		return 0
  	}
  	b[0], b[1], b[2], b[3] = TCPOptionMSS, mssOptionSize, byte(mss>>8), byte(mss)
  	return mssOptionSize
  }
docs: // EncodeMSSOption encodes the MSS TCP option with the provided MSS values in
  // the supplied buffer. If the provided buffer is not large enough then it just
  // returns without encoding anything. It returns the number of bytes written to
  // the provided buffer.