tcp: mss honoring; accept syn with ECE/CWR flags; add RSTQueue type (#41)

* tcp: mss honoring; accept syn with ECE/CWR flags; add RSTQueue type

* tcp: move option logic to own file

* remove prints in pcap

* add MSS send threshold inspired by linux/freebsd/lwip thresh

tcp/rst.go

@@ -0,0 +1,63 @@
+package tcp
+
+import "github.com/soypat/lneto/internal"
+
+// RSTQueue is a small fixed-size queue of pending stateless RST responses.
+// It is not safe for concurrent use; callers must synchronize access.
+type RSTQueue struct {
+	buf [4]rstEntry
+	len uint8
+}
+
+type rstEntry struct {
+	remoteAddr [4]byte
+	remotePort uint16
+	localPort  uint16
+	seq        Value
+	ack        Value
+	flags      Flags
+}
+
+// Queue enqueues a RST response. Silently drops if srcaddr is not IPv4 or queue is full.
+func (q *RSTQueue) Queue(srcaddr []byte, remotePort, localPort uint16, seq, ack Value, flags Flags) {
+	if len(srcaddr) == 4 && q.len < uint8(len(q.buf)) {
+		entry := &q.buf[q.len]
+		copy(entry.remoteAddr[:], srcaddr)
+		entry.remotePort = remotePort
+		entry.localPort = localPort
+		entry.seq = seq
+		entry.ack = ack
+		entry.flags = flags
+		q.len++
+	}
+}
+
+// Pending returns the number of queued RST entries.
+func (q *RSTQueue) Pending() int { return int(q.len) }
+
+// Drain writes one pending RST to the carrier buffer and returns the TCP frame length written.
+// Returns (0, nil) if the queue is empty or offsetToIP < 0.
+func (q *RSTQueue) Drain(carrierData []byte, offsetToIP, offsetToFrame int) (int, error) {
+	if q.len == 0 || offsetToIP < 0 {
+		return 0, nil
+	}
+	q.len--
+	entry := &q.buf[q.len]
+	tfrm, err := NewFrame(carrierData[offsetToFrame:])
+	if err != nil {
+		return 0, nil
+	}
+	tfrm.SetSourcePort(entry.localPort)
+	tfrm.SetDestinationPort(entry.remotePort)
+	tfrm.SetSegment(Segment{
+		SEQ:   entry.seq,
+		ACK:   entry.ack,
+		Flags: entry.flags,
+	}, 5)
+	tfrm.SetUrgentPtr(0)
+	err = internal.SetIPAddrs(carrierData[offsetToIP:offsetToFrame], 0, nil, entry.remoteAddr[:])
+	if err != nil {
+		return 0, nil
+	}
+	return sizeHeaderTCP, nil
+}