code: func (l *listenContext) createCookie(id stack.TransportEndpointID, seq seqnum.Value, data uint32) seqnum.Value {
  	ts := timeStamp()
  	v := l.cookieHash(id, 0, 0) + uint32(seq) + (ts << tsOffset)
  	v += (l.cookieHash(id, ts, 1) + data) & hashMask
  	return seqnum.Value(v)
  }
docs: // createCookie creates a SYN cookie for the given id and incoming sequence
  // number.