code: func (s *sender) sendSegment(data buffer.VectorisedView, flags byte, seq seqnum.Value) *tcpip.Error {
  	s.lastSendTime = time.Now()
  	if seq == s.rttMeasureSeqNum {
  		s.rttMeasureTime = s.lastSendTime
  	}
  
  	rcvNxt, rcvWnd := s.ep.rcv.getSendParams()
  
  	// Remember the max sent ack.
  	s.maxSentAck = rcvNxt
  
  	return s.ep.sendRaw(data, flags, seq, rcvNxt, rcvWnd)
  }
docs: // sendSegment sends a new segment containing the given payload, flags and
  // sequence number.