code: func (s *segment) logicalLen() seqnum.Size {
  	l := seqnum.Size(s.data.Size())
  	if s.flagIsSet(header.TCPFlagSyn) {
  		l++
  	}
  	if s.flagIsSet(header.TCPFlagFin) {
  		l++
  	}
  	return l
  }
docs: // logicalLen is the segment length in the sequence number space. It's defined
  // as the data length plus one for each of the SYN and FIN bits set.