code: func logicalLen(tcp header.TCP) seqnum.Size {
  	l := dataLen(tcp)
  	flags := tcp.Flags()
  	if flags&header.TCPFlagSyn != 0 {
  		l++
  	}
  	if flags&header.TCPFlagFin != 0 {
  		l++
  	}
  	return l
  }
docs: // logicalLen calculates the logical length of the TCP segment.