From 5ded633782709710c42648c71154d6062e83b7a6 Mon Sep 17 00:00:00 2001
From: Guus Sliepen <guus@meshlink.io>
Date: Mon, 3 Jul 2017 23:16:01 +0200
Subject: [PATCH] Add UDP semantics.

Basically, never retransmit, and whatever we get we immediately pass up to
the application. The only thing we do handle the TCP way is the FIN packet.

This also introduces the use of the aux entry in the header. We know that
older versions of UTCP ignore this field, and also ignore any extra data
in SYN packets, so we can add anything here in a backwards-compatible way.
---
 utcp.c | 66 +++++++++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 51 insertions(+), 15 deletions(-)

diff --git a/utcp.c b/utcp.c
index 8143e0e..c650ab7 100644
--- a/utcp.c
+++ b/utcp.c
@@ -127,6 +127,10 @@ static bool fin_wanted(struct utcp_connection *c, uint32_t seq) {
 	}
 }
 
+static bool is_reliable(struct utcp_connection *c) {
+	return c->flags & UTCP_RELIABLE;
+}
+
 static inline void list_connections(struct utcp *utcp) {
 	debug("%p has %d connections:\n", utcp, utcp->nconnections);
 	for(int i = 0; i < utcp->nconnections; i++)
@@ -389,20 +393,27 @@ struct utcp_connection *utcp_connect_ex(struct utcp *utcp, uint16_t dst, utcp_re
 	c->recv = recv;
 	c->priv = priv;
 
-	struct hdr hdr;
-
-	hdr.src = c->src;
-	hdr.dst = c->dst;
-	hdr.seq = c->snd.iss;
-	hdr.ack = 0;
-	hdr.wnd = c->rcv.wnd;
-	hdr.ctl = SYN;
-	hdr.aux = 0;
+	struct {
+		struct hdr hdr;
+		uint8_t init[4];
+	} pkt;
+
+	pkt.hdr.src = c->src;
+	pkt.hdr.dst = c->dst;
+	pkt.hdr.seq = c->snd.iss;
+	pkt.hdr.ack = 0;
+	pkt.hdr.wnd = c->rcv.wnd;
+	pkt.hdr.ctl = SYN;
+	pkt.hdr.aux = 0x0101;
+	pkt.init[0] = 1;
+	pkt.init[1] = 0;
+	pkt.init[2] = 0;
+	pkt.init[3] = flags & 0x7;
 
 	set_state(c, SYN_SENT);
 
-	print_packet(utcp, "send", &hdr, sizeof hdr);
-	utcp->send(utcp, &hdr, sizeof hdr);
+	print_packet(utcp, "send", &pkt, sizeof pkt);
+	utcp->send(utcp, &pkt, sizeof pkt);
 
 	gettimeofday(&c->conn_timeout, NULL);
 	c->conn_timeout.tv_sec += utcp->timeout;
@@ -514,7 +525,7 @@ ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
 		return -1;
 	}
 
-	// Add data to send buffer
+	// Exit early if we have nothing to send.
 
 	if(!len)
 		return 0;
@@ -524,6 +535,8 @@ ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
 		return -1;
 	}
 
+	// Add data to send buffer.
+
 	len = buffer_put(&c->sndbuf, data, len);
 	if(len <= 0) {
 		errno = EWOULDBLOCK;
@@ -532,7 +545,11 @@ ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
 
 	c->snd.last += len;
 	ack(c, false);
-	if(!timerisset(&c->rtrx_timeout))
+	if(!is_reliable(c)) {
+		c->snd.una = c->snd.nxt = c->snd.last;
+		buffer_get(&c->sndbuf, NULL, c->sndbuf.used);
+	}
+	if(is_reliable(c) && !timerisset(&c->rtrx_timeout))
 		start_retransmit_timer(c);
 	return len;
 }
@@ -750,6 +767,12 @@ static void handle_in_order(struct utcp_connection *c, const void *data, size_t
 
 
 static void handle_incoming_data(struct utcp_connection *c, uint32_t seq, const void *data, size_t len) {
+	if(!is_reliable(c)) {
+		c->recv(c, data, len);
+		c->rcv.nxt = seq + len;
+		return;
+	}
+
 	uint32_t offset = seqdiff(seq, c->rcv.nxt);
 	if(offset + len > c->rcvbuf.maxsize)
 		abort();
@@ -826,6 +849,19 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 				goto reset;
 			}
 
+			// Parse auxilliary information
+			if(hdr.aux) {
+				if(hdr.aux != 0x0101 || len < 4 || ((uint8_t *)data)[0] != 1) {
+					len = 1;
+					goto reset;
+				}
+				c->flags = ((uint8_t *)data)[3] & 0x7;
+				data += 4;
+				len -= 4;
+			} else {
+				c->flags = UTCP_TCP;
+			}
+
 			// Return SYN+ACK, go to SYN_RECEIVED state
 			c->snd.wnd = hdr.wnd;
 			c->rcv.irs = hdr.seq;
@@ -1052,7 +1088,7 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 			break;
 		}
 	} else {
-		if(!len) {
+		if(!len && is_reliable(c)) {
 			c->dupack++;
 			if(c->dupack == 3) {
 				debug("Triplicate ACK\n");
@@ -1072,7 +1108,7 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 		timerclear(&c->conn_timeout); // It will be set anew in utcp_timeout() if c->snd.una != c->snd.nxt.
 		if(c->snd.una == c->snd.last)
 			stop_retransmit_timer(c);
-		else
+		else if(is_reliable(c))
 			start_retransmit_timer(c);
 	}
 
-- 
2.39.2