Basic implementation of TCP over XDP for Knot server

including connection handling for kxdpgun in TCP mode.

The implementation is not perfect!! But the interface with other Knot parts is stable.

src/libknot/xdp/protocols.h

@@ -49,8 +49,10 @@ enum {
 	PROT_TCP_OPT_ENDOP = 0,
 	PROT_TCP_OPT_NOOP  = 1,
 	PROT_TCP_OPT_MSS   = 2,
+	PROT_TCP_OPT_WSC   = 3, // window scale
 
 	PROT_TCP_OPT_LEN_MSS = 4,
+	PROT_TCP_OPT_LEN_WSC = 3,
 };
 
 inline static void *prot_read_tcp(void *data, knot_xdp_msg_t *msg, uint16_t *src_port, uint16_t *dst_port)
@@ -185,7 +187,7 @@ inline static size_t prot_write_hdrs_len(const knot_xdp_msg_t *msg)
 	}
 
 	if (msg->flags & KNOT_XDP_MSG_TCP) {
-		res += sizeof(struct tcphdr) - sizeof(struct udphdr);
+		res += sizeof(struct tcphdr) - sizeof(struct udphdr) + 4; // 4 == PROT_TCP_OPT_LEN_WSC + align
 
 		if (msg->flags & KNOT_XDP_MSG_MSS) {
 			res += PROT_TCP_OPT_LEN_MSS;
@@ -268,7 +270,7 @@ inline static void prot_write_tcp(void *data, const knot_xdp_msg_t *msg, void *d
 	tcp->dest    = dst_port;
 	tcp->seq     = htobe32(msg->seqno);
 	tcp->ack_seq = htobe32(msg->ackno);
-	tcp->window  = htobe16(0x8000); // TODO proper window size handling in TCP streams
+	tcp->window  = htobe16(0xffff); // Practically infinite window (see also WSC option below)
 	tcp->check   = 0; // Temporarily initialize before checksum calculation.
 
 	tcp->syn = ((msg->flags & KNOT_XDP_MSG_SYN) ? 1 : 0);
@@ -277,6 +279,11 @@ inline static void prot_write_tcp(void *data, const knot_xdp_msg_t *msg, void *d
 	tcp->rst = ((msg->flags & KNOT_XDP_MSG_RST) ? 1 : 0);
 
 	uint8_t *hdr_end = data + sizeof(*tcp);
+	hdr_end[0] = PROT_TCP_OPT_WSC;
+	hdr_end[1] = PROT_TCP_OPT_LEN_WSC;
+	hdr_end[2] = 14; // Maximum possible.
+	hdr_end += PROT_TCP_OPT_LEN_WSC;
+	*hdr_end++ = PROT_TCP_OPT_NOOP;
 	if (msg->flags & KNOT_XDP_MSG_MSS) {
 		uint16_t mss = htobe16(1460); // TODO: set proper MSS value
 		hdr_end[0] = PROT_TCP_OPT_MSS;