X-Git-Url: http://git.meshlink.io/?p=utcp;a=blobdiff_plain;f=utcp.c;h=d09b9e7d526e80c4ce39590c19c718904fe32590;hp=75553d72567be387667b7ced7871cac32d0d6a46;hb=f1035e971bb894203bdfba6cafbaf0bb30f197eb;hpb=5d25d40b5baf4bd60ccc1dc6db65b3e5b094e2cd

diff --git a/utcp.c b/utcp.c
index 75553d7..d09b9e7 100644
--- a/utcp.c
+++ b/utcp.c
@@ -1,6 +1,6 @@
 /*
     utcp.c -- Userspace TCP
-    Copyright (C) 2014 Guus Sliepen <guus@tinc-vpn.org>
+    Copyright (C) 2014-2017 Guus Sliepen <guus@tinc-vpn.org>
 
     This program is free software; you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -45,14 +45,19 @@
 #endif
 
 #ifndef timersub
-#define timersub(a, b, r) do {\
-	(r)->tv_sec = (a)->tv_sec - (b)->tv_sec;\
-	(r)->tv_usec = (a)->tv_usec - (b)->tv_usec;\
-	if((r)->tv_usec < 0)\
-		(r)->tv_sec--, (r)->tv_usec += 1000000;\
-} while (0)
+#define timersub(a, b, r)\
+	do {\
+		(r)->tv_sec = (a)->tv_sec - (b)->tv_sec;\
+		(r)->tv_usec = (a)->tv_usec - (b)->tv_usec;\
+		if((r)->tv_usec < 0)\
+			(r)->tv_sec--, (r)->tv_usec += USEC_PER_SEC;\
+	} while (0)
 #endif
 
+static inline size_t max(size_t a, size_t b) {
+	return a > b ? a : b;
+}
+
 #ifdef UTCP_DEBUG
 #include <stdarg.h>
 
@@ -65,92 +70,248 @@ static void debug(const char *format, ...) {
 
 static void print_packet(struct utcp *utcp, const char *dir, const void *pkt, size_t len) {
 	struct hdr hdr;
-	if(len < sizeof hdr) {
-		debug("%p %s: short packet (%zu bytes)\n", utcp, dir, len);
+
+	if(len < sizeof(hdr)) {
+		debug("%p %s: short packet (%lu bytes)\n", utcp, dir, (unsigned long)len);
 		return;
 	}
 
-	memcpy(&hdr, pkt, sizeof hdr);
-	fprintf (stderr, "%p %s: len=%zu, src=%u dst=%u seq=%u ack=%u wnd=%u ctl=", utcp, dir, len, hdr.src, hdr.dst, hdr.seq, hdr.ack, hdr.wnd);
-	if(hdr.ctl & SYN)
+	memcpy(&hdr, pkt, sizeof(hdr));
+	debug("%p %s: len=%lu, src=%u dst=%u seq=%u ack=%u wnd=%u aux=%x ctl=", utcp, dir, (unsigned long)len, hdr.src, hdr.dst, hdr.seq, hdr.ack, hdr.wnd, hdr.aux);
+
+	if(hdr.ctl & SYN) {
 		debug("SYN");
-	if(hdr.ctl & RST)
+	}
+
+	if(hdr.ctl & RST) {
 		debug("RST");
-	if(hdr.ctl & FIN)
+	}
+
+	if(hdr.ctl & FIN) {
 		debug("FIN");
-	if(hdr.ctl & ACK)
+	}
+
+	if(hdr.ctl & ACK) {
 		debug("ACK");
+	}
 
-	if(len > sizeof hdr) {
-		debug(" data=");
-		for(int i = sizeof hdr; i < len; i++) {
-			const char *data = pkt;
-			debug("%c", data[i] >= 32 ? data[i] : '.');
+	if(len > sizeof(hdr)) {
+		uint32_t datalen = len - sizeof(hdr);
+		const uint8_t *data = (uint8_t *)pkt + sizeof(hdr);
+		char str[datalen * 2 + 1];
+		char *p = str;
+
+		for(uint32_t i = 0; i < datalen; i++) {
+			*p++ = "0123456789ABCDEF"[data[i] >> 4];
+			*p++ = "0123456789ABCDEF"[data[i] & 15];
 		}
+
+		*p = 0;
+
+		debug(" data=%s", str);
 	}
 
 	debug("\n");
 }
 #else
-#define debug(...)
-#define print_packet(...)
+#define debug(...) do {} while(0)
+#define print_packet(...) do {} while(0)
 #endif
 
 static void set_state(struct utcp_connection *c, enum state state) {
 	c->state = state;
-	if(state == ESTABLISHED)
+
+	if(state == ESTABLISHED) {
 		timerclear(&c->conn_timeout);
+	}
+
 	debug("%p new state: %s\n", c->utcp, strstate[state]);
 }
 
-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++)
-		debug("  %u -> %u state %s\n", utcp->connections[i]->src, utcp->connections[i]->dst, strstate[utcp->connections[i]->state]);
+static bool fin_wanted(struct utcp_connection *c, uint32_t seq) {
+	if(seq != c->snd.last) {
+		return false;
+	}
+
+	switch(c->state) {
+	case FIN_WAIT_1:
+	case CLOSING:
+	case LAST_ACK:
+		return true;
+
+	default:
+		return false;
+	}
+}
+
+static bool is_reliable(struct utcp_connection *c) {
+	return c->flags & UTCP_RELIABLE;
 }
 
 static int32_t seqdiff(uint32_t a, uint32_t b) {
 	return a - b;
 }
 
+// Buffer functions
+// TODO: convert to ringbuffers to avoid memmove() operations.
+
+// Store data into the buffer
+static ssize_t buffer_put_at(struct buffer *buf, size_t offset, const void *data, size_t len) {
+	debug("buffer_put_at %lu %lu %lu\n", (unsigned long)buf->used, (unsigned long)offset, (unsigned long)len);
+
+	size_t required = offset + len;
+
+	if(required > buf->maxsize) {
+		if(offset >= buf->maxsize) {
+			return 0;
+		}
+
+		len = buf->maxsize - offset;
+		required = buf->maxsize;
+	}
+
+	if(required > buf->size) {
+		size_t newsize = buf->size;
+
+		if(!newsize) {
+			newsize = required;
+		} else {
+			do {
+				newsize *= 2;
+			} while(newsize < required);
+		}
+
+		if(newsize > buf->maxsize) {
+			newsize = buf->maxsize;
+		}
+
+		char *newdata = realloc(buf->data, newsize);
+
+		if(!newdata) {
+			return -1;
+		}
+
+		buf->data = newdata;
+		buf->size = newsize;
+	}
+
+	memcpy(buf->data + offset, data, len);
+
+	if(required > buf->used) {
+		buf->used = required;
+	}
+
+	return len;
+}
+
+static ssize_t buffer_put(struct buffer *buf, const void *data, size_t len) {
+	return buffer_put_at(buf, buf->used, data, len);
+}
+
+// Get data from the buffer. data can be NULL.
+static ssize_t buffer_get(struct buffer *buf, void *data, size_t len) {
+	if(len > buf->used) {
+		len = buf->used;
+	}
+
+	if(data) {
+		memcpy(data, buf->data, len);
+	}
+
+	if(len < buf->used) {
+		memmove(buf->data, buf->data + len, buf->used - len);
+	}
+
+	buf->used -= len;
+	return len;
+}
+
+// Copy data from the buffer without removing it.
+static ssize_t buffer_copy(struct buffer *buf, void *data, size_t offset, size_t len) {
+	if(offset >= buf->used) {
+		return 0;
+	}
+
+	if(offset + len > buf->used) {
+		len = buf->used - offset;
+	}
+
+	memcpy(data, buf->data + offset, len);
+	return len;
+}
+
+static bool buffer_init(struct buffer *buf, uint32_t len, uint32_t maxlen) {
+	memset(buf, 0, sizeof(*buf));
+
+	if(len) {
+		buf->data = malloc(len);
+
+		if(!buf->data) {
+			return false;
+		}
+	}
+
+	buf->size = len;
+	buf->maxsize = maxlen;
+	return true;
+}
+
+static void buffer_exit(struct buffer *buf) {
+	free(buf->data);
+	memset(buf, 0, sizeof(*buf));
+}
+
+static uint32_t buffer_free(const struct buffer *buf) {
+	return buf->maxsize - buf->used;
+}
+
 // Connections are stored in a sorted list.
 // This gives O(log(N)) lookup time, O(N log(N)) insertion time and O(N) deletion time.
 
 static int compare(const void *va, const void *vb) {
+	assert(va && vb);
+
 	const struct utcp_connection *a = *(struct utcp_connection **)va;
 	const struct utcp_connection *b = *(struct utcp_connection **)vb;
 
+	assert(a && b);
 	assert(a->src && b->src);
 
 	int c = (int)a->src - (int)b->src;
-	if(c)
+
+	if(c) {
 		return c;
+	}
+
 	c = (int)a->dst - (int)b->dst;
 	return c;
 }
 
 static struct utcp_connection *find_connection(const struct utcp *utcp, uint16_t src, uint16_t dst) {
-	if(!utcp->nconnections)
+	if(!utcp->nconnections) {
 		return NULL;
+	}
+
 	struct utcp_connection key = {
 		.src = src,
 		.dst = dst,
 	}, *keyp = &key;
-	struct utcp_connection **match = bsearch(&keyp, utcp->connections, utcp->nconnections, sizeof *utcp->connections, compare);
+	struct utcp_connection **match = bsearch(&keyp, utcp->connections, utcp->nconnections, sizeof(*utcp->connections), compare);
 	return match ? *match : NULL;
 }
 
 static void free_connection(struct utcp_connection *c) {
 	struct utcp *utcp = c->utcp;
-	struct utcp_connection **cp = bsearch(&c, utcp->connections, utcp->nconnections, sizeof *utcp->connections, compare);
+	struct utcp_connection **cp = bsearch(&c, utcp->connections, utcp->nconnections, sizeof(*utcp->connections), compare);
 
 	assert(cp);
 
 	int i = cp - utcp->connections;
-	memmove(cp + i, cp + i + 1, (utcp->nconnections - i - 1) * sizeof *cp);
+	memmove(cp, cp + 1, (utcp->nconnections - i - 1) * sizeof(*cp));
 	utcp->nconnections--;
 
-	free(c->sndbuf);
+	buffer_exit(&c->rcvbuf);
+	buffer_exit(&c->sndbuf);
 	free(c);
 }
 
@@ -167,32 +328,45 @@ static struct utcp_connection *allocate_connection(struct utcp *utcp, uint16_t s
 			errno = ENOMEM;
 			return NULL;
 		}
+
 		src = rand() | 0x8000;
-		while(find_connection(utcp, src, dst))
+
+		while(find_connection(utcp, src, dst)) {
 			src++;
+		}
 	}
 
 	// Allocate memory for the new connection
 
 	if(utcp->nconnections >= utcp->nallocated) {
-		if(!utcp->nallocated)
+		if(!utcp->nallocated) {
 			utcp->nallocated = 4;
-		else
+		} else {
 			utcp->nallocated *= 2;
-		struct utcp_connection **new_array = realloc(utcp->connections, utcp->nallocated * sizeof *utcp->connections);
-		if(!new_array)
+		}
+
+		struct utcp_connection **new_array = realloc(utcp->connections, utcp->nallocated * sizeof(*utcp->connections));
+
+		if(!new_array) {
 			return NULL;
+		}
+
 		utcp->connections = new_array;
 	}
 
-	struct utcp_connection *c = calloc(1, sizeof *c);
-	if(!c)
+	struct utcp_connection *c = calloc(1, sizeof(*c));
+
+	if(!c) {
+		return NULL;
+	}
+
+	if(!buffer_init(&c->sndbuf, DEFAULT_SNDBUFSIZE, DEFAULT_MAXSNDBUFSIZE)) {
+		free(c);
 		return NULL;
+	}
 
-	c->sndbufsize = DEFAULT_SNDBUFSIZE;
-	c->maxsndbufsize = DEFAULT_MAXSNDBUFSIZE;
-	c->sndbuf = malloc(c->sndbufsize);
-	if(!c->sndbuf) {
+	if(!buffer_init(&c->rcvbuf, DEFAULT_RCVBUFSIZE, DEFAULT_MAXRCVBUFSIZE)) {
+		buffer_exit(&c->sndbuf);
 		free(c);
 		return NULL;
 	}
@@ -201,7 +375,11 @@ static struct utcp_connection *allocate_connection(struct utcp *utcp, uint16_t s
 
 	c->src = src;
 	c->dst = dst;
+#ifdef UTCP_DEBUG
+	c->snd.iss = 0;
+#else
 	c->snd.iss = rand();
+#endif
 	c->snd.una = c->snd.iss;
 	c->snd.nxt = c->snd.iss + 1;
 	c->rcv.wnd = utcp->mtu;
@@ -212,40 +390,109 @@ static struct utcp_connection *allocate_connection(struct utcp *utcp, uint16_t s
 	// Add it to the sorted list of connections
 
 	utcp->connections[utcp->nconnections++] = c;
-	qsort(utcp->connections, utcp->nconnections, sizeof *utcp->connections, compare);
+	qsort(utcp->connections, utcp->nconnections, sizeof(*utcp->connections), compare);
 
 	return c;
 }
 
-struct utcp_connection *utcp_connect(struct utcp *utcp, uint16_t dst, utcp_recv_t recv, void *priv) {
+static inline uint32_t absdiff(uint32_t a, uint32_t b) {
+	if(a > b) {
+		return a - b;
+	} else {
+		return b - a;
+	}
+}
+
+// Update RTT variables. See RFC 6298.
+static void update_rtt(struct utcp_connection *c, uint32_t rtt) {
+	if(!rtt) {
+		debug("invalid rtt\n");
+		return;
+	}
+
+	struct utcp *utcp = c->utcp;
+
+	if(!utcp->srtt) {
+		utcp->srtt = rtt;
+		utcp->rttvar = rtt / 2;
+		utcp->rto = rtt + max(2 * rtt, CLOCK_GRANULARITY);
+	} else {
+		utcp->rttvar = (utcp->rttvar * 3 + absdiff(utcp->srtt, rtt)) / 4;
+		utcp->srtt = (utcp->srtt * 7 + rtt) / 8;
+		utcp->rto = utcp->srtt + max(utcp->rttvar, CLOCK_GRANULARITY);
+	}
+
+	if(utcp->rto > MAX_RTO) {
+		utcp->rto = MAX_RTO;
+	}
+
+	debug("rtt %u srtt %u rttvar %u rto %u\n", rtt, utcp->srtt, utcp->rttvar, utcp->rto);
+}
+
+static void start_retransmit_timer(struct utcp_connection *c) {
+	gettimeofday(&c->rtrx_timeout, NULL);
+	c->rtrx_timeout.tv_usec += c->utcp->rto;
+
+	while(c->rtrx_timeout.tv_usec >= 1000000) {
+		c->rtrx_timeout.tv_usec -= 1000000;
+		c->rtrx_timeout.tv_sec++;
+	}
+
+	debug("timeout set to %lu.%06lu (%u)\n", c->rtrx_timeout.tv_sec, c->rtrx_timeout.tv_usec, c->utcp->rto);
+}
+
+static void stop_retransmit_timer(struct utcp_connection *c) {
+	timerclear(&c->rtrx_timeout);
+	debug("timeout cleared\n");
+}
+
+struct utcp_connection *utcp_connect_ex(struct utcp *utcp, uint16_t dst, utcp_recv_t recv, void *priv, uint32_t flags) {
 	struct utcp_connection *c = allocate_connection(utcp, 0, dst);
-	if(!c)
+
+	if(!c) {
 		return NULL;
+	}
+
+	assert((flags & ~0xf) == 0);
 
+	c->flags = flags;
 	c->recv = recv;
 	c->priv = priv;
 
-	struct hdr hdr;
+	struct {
+		struct hdr hdr;
+		uint8_t init[4];
+	} pkt;
 
-	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;
+	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;
 
+	start_retransmit_timer(c);
+
 	return c;
 }
 
+struct utcp_connection *utcp_connect(struct utcp *utcp, uint16_t dst, utcp_recv_t recv, void *priv) {
+	return utcp_connect_ex(utcp, dst, recv, priv, UTCP_TCP);
+}
+
 void utcp_accept(struct utcp_connection *c, utcp_recv_t recv, void *priv) {
 	if(c->reapable || c->state != SYN_RECEIVED) {
 		debug("Error: accept() called on invalid connection %p in state %s\n", c, strstate[c->state]);
@@ -261,56 +508,66 @@ void utcp_accept(struct utcp_connection *c, utcp_recv_t recv, void *priv) {
 static void ack(struct utcp_connection *c, bool sendatleastone) {
 	int32_t left = seqdiff(c->snd.last, c->snd.nxt);
 	int32_t cwndleft = c->snd.cwnd - seqdiff(c->snd.nxt, c->snd.una);
-	char *data = c->sndbuf + seqdiff(c->snd.nxt, c->snd.una);
+	debug("cwndleft = %d\n", cwndleft);
 
 	assert(left >= 0);
 
-	if(cwndleft <= 0)
+	if(cwndleft <= 0) {
 		cwndleft = 0;
+	}
 
-	if(cwndleft < left)
+	if(cwndleft < left) {
 		left = cwndleft;
+	}
 
-	if(!left && !sendatleastone)
+	if(!left && !sendatleastone) {
 		return;
+	}
 
 	struct {
 		struct hdr hdr;
-		char data[c->utcp->mtu];
-	} pkt;
+		uint8_t data[];
+	} *pkt;
 
-	pkt.hdr.src = c->src;
-	pkt.hdr.dst = c->dst;
-	pkt.hdr.ack = c->rcv.nxt;
-	pkt.hdr.wnd = c->snd.wnd;
-	pkt.hdr.ctl = ACK;
-	pkt.hdr.aux = 0;
+	pkt = malloc(sizeof(pkt->hdr) + c->utcp->mtu);
+
+	if(!pkt) {
+		return;
+	}
+
+	pkt->hdr.src = c->src;
+	pkt->hdr.dst = c->dst;
+	pkt->hdr.ack = c->rcv.nxt;
+	pkt->hdr.wnd = c->snd.wnd;
+	pkt->hdr.ctl = ACK;
+	pkt->hdr.aux = 0;
 
 	do {
 		uint32_t seglen = left > c->utcp->mtu ? c->utcp->mtu : left;
-		pkt.hdr.seq = c->snd.nxt;
+		pkt->hdr.seq = c->snd.nxt;
 
-		memcpy(pkt.data, data, seglen);
+		buffer_copy(&c->sndbuf, pkt->data, seqdiff(c->snd.nxt, c->snd.una), seglen);
 
 		c->snd.nxt += seglen;
-		data += seglen;
 		left -= seglen;
 
-		if(c->state != ESTABLISHED && !left && seglen) {
-			switch(c->state) {
-			case FIN_WAIT_1:
-			case CLOSING:
-				seglen--;
-				pkt.hdr.ctl |= FIN;
-				break;
-			default:
-				break;
-			}
+		if(seglen && fin_wanted(c, c->snd.nxt)) {
+			seglen--;
+			pkt->hdr.ctl |= FIN;
 		}
 
-		print_packet(c->utcp, "send", &pkt, sizeof pkt.hdr + seglen);
-		c->utcp->send(c->utcp, &pkt, sizeof pkt.hdr + seglen);
+		if(!c->rtt_start.tv_sec) {
+			// Start RTT measurement
+			gettimeofday(&c->rtt_start, NULL);
+			c->rtt_seq = pkt->hdr.seq + seglen;
+			debug("Starting RTT measurement, expecting ack %u\n", c->rtt_seq);
+		}
+
+		print_packet(c->utcp, "send", pkt, sizeof(pkt->hdr) + seglen);
+		c->utcp->send(c->utcp, pkt, sizeof(pkt->hdr) + seglen);
 	} while(left);
+
+	free(pkt);
 }
 
 ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
@@ -323,14 +580,16 @@ ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
 	switch(c->state) {
 	case CLOSED:
 	case LISTEN:
-	case SYN_SENT:
-	case SYN_RECEIVED:
 		debug("Error: send() called on unconnected connection %p\n", c);
 		errno = ENOTCONN;
 		return -1;
+
+	case SYN_SENT:
+	case SYN_RECEIVED:
 	case ESTABLISHED:
 	case CLOSE_WAIT:
 		break;
+
 	case FIN_WAIT_1:
 	case FIN_WAIT_2:
 	case CLOSING:
@@ -341,55 +600,45 @@ 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)
+	if(!len) {
 		return 0;
+	}
 
 	if(!data) {
 		errno = EFAULT;
 		return -1;
 	}
 
-	uint32_t bufused = seqdiff(c->snd.nxt, c->snd.una);
-
-	/* Check our send buffer.
-	 * - If it's big enough, just put the data in there.
-	 * - If not, decide whether to enlarge if possible.
-	 * - Cap len so it doesn't overflow our buffer.
-	 */
-
-	if(len > c->sndbufsize - bufused && c->sndbufsize < c->maxsndbufsize) {
-		uint32_t newbufsize;
-		if(c->sndbufsize > c->maxsndbufsize / 2)
-			newbufsize = c->maxsndbufsize;
-		else
-			newbufsize = c->sndbufsize * 2;
-		if(bufused + len > newbufsize) {
-			if(bufused + len > c->maxsndbufsize)
-				newbufsize = c->maxsndbufsize;
-			else
-				newbufsize = bufused + len;
-		}
-		char *newbuf = realloc(c->sndbuf, newbufsize);
-		if(newbuf) {
-			c->sndbuf = newbuf;
-			c->sndbufsize = newbufsize;
-		}
-	}
+	// Add data to send buffer.
 
-	if(len > c->sndbufsize - bufused)
-		len = c->sndbufsize - bufused;
+	len = buffer_put(&c->sndbuf, data, len);
 
-	if(!len) {
-		errno == EWOULDBLOCK;
+	if(len <= 0) {
+		errno = EWOULDBLOCK;
 		return 0;
 	}
 
-	memcpy(c->sndbuf + bufused, data, len);
 	c->snd.last += len;
 
+	// Don't send anything yet if the connection has not fully established yet
+
+	if(c->state == SYN_SENT || c->state == SYN_RECEIVED) {
+		return len;
+	}
+
 	ack(c, false);
+
+	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;
 }
 
@@ -399,14 +648,267 @@ static void swap_ports(struct hdr *hdr) {
 	hdr->dst = tmp;
 }
 
+static void retransmit(struct utcp_connection *c) {
+	if(c->state == CLOSED || c->snd.last == c->snd.una) {
+		debug("Retransmit() called but nothing to retransmit!\n");
+		stop_retransmit_timer(c);
+		return;
+	}
+
+	struct utcp *utcp = c->utcp;
+
+	struct {
+		struct hdr hdr;
+		uint8_t data[];
+	} *pkt;
+
+	pkt = malloc(sizeof(pkt->hdr) + c->utcp->mtu);
+
+	if(!pkt) {
+		return;
+	}
+
+	pkt->hdr.src = c->src;
+	pkt->hdr.dst = c->dst;
+	pkt->hdr.wnd = c->rcv.wnd;
+	pkt->hdr.aux = 0;
+
+	switch(c->state) {
+	case SYN_SENT:
+		// Send our SYN again
+		pkt->hdr.seq = c->snd.iss;
+		pkt->hdr.ack = 0;
+		pkt->hdr.ctl = SYN;
+		pkt->hdr.aux = 0x0101;
+		pkt->data[0] = 1;
+		pkt->data[1] = 0;
+		pkt->data[2] = 0;
+		pkt->data[3] = c->flags & 0x7;
+		print_packet(c->utcp, "rtrx", pkt, sizeof(pkt->hdr) + 4);
+		utcp->send(utcp, pkt, sizeof(pkt->hdr) + 4);
+		break;
+
+	case SYN_RECEIVED:
+		// Send SYNACK again
+		pkt->hdr.seq = c->snd.nxt;
+		pkt->hdr.ack = c->rcv.nxt;
+		pkt->hdr.ctl = SYN | ACK;
+		print_packet(c->utcp, "rtrx", pkt, sizeof(pkt->hdr));
+		utcp->send(utcp, pkt, sizeof(pkt->hdr));
+		break;
+
+	case ESTABLISHED:
+	case FIN_WAIT_1:
+	case CLOSE_WAIT:
+	case CLOSING:
+	case LAST_ACK:
+		// Send unacked data again.
+		pkt->hdr.seq = c->snd.una;
+		pkt->hdr.ack = c->rcv.nxt;
+		pkt->hdr.ctl = ACK;
+		uint32_t len = seqdiff(c->snd.last, c->snd.una);
+
+		if(len > utcp->mtu) {
+			len = utcp->mtu;
+		}
+
+		if(fin_wanted(c, c->snd.una + len)) {
+			len--;
+			pkt->hdr.ctl |= FIN;
+		}
+
+		c->snd.nxt = c->snd.una + len;
+		c->snd.cwnd = utcp->mtu; // reduce cwnd on retransmit
+		buffer_copy(&c->sndbuf, pkt->data, 0, len);
+		print_packet(c->utcp, "rtrx", pkt, sizeof(pkt->hdr) + len);
+		utcp->send(utcp, pkt, sizeof(pkt->hdr) + len);
+		break;
+
+	case CLOSED:
+	case LISTEN:
+	case TIME_WAIT:
+	case FIN_WAIT_2:
+		// We shouldn't need to retransmit anything in this state.
+#ifdef UTCP_DEBUG
+		abort();
+#endif
+		stop_retransmit_timer(c);
+		goto cleanup;
+	}
+
+	start_retransmit_timer(c);
+	utcp->rto *= 2;
+
+	if(utcp->rto > MAX_RTO) {
+		utcp->rto = MAX_RTO;
+	}
+
+	c->rtt_start.tv_sec = 0; // invalidate RTT timer
+
+cleanup:
+	free(pkt);
+}
+
+/* Update receive buffer and SACK entries after consuming data.
+ *
+ * Situation:
+ *
+ * |.....0000..1111111111.....22222......3333|
+ * |---------------^
+ *
+ * 0..3 represent the SACK entries. The ^ indicates up to which point we want
+ * to remove data from the receive buffer. The idea is to substract "len"
+ * from the offset of all the SACK entries, and then remove/cut down entries
+ * that are shifted to before the start of the receive buffer.
+ *
+ * There are three cases:
+ * - the SACK entry is after ^, in that case just change the offset.
+ * - the SACK entry starts before and ends after ^, so we have to
+ *   change both its offset and size.
+ * - the SACK entry is completely before ^, in that case delete it.
+ */
+static void sack_consume(struct utcp_connection *c, size_t len) {
+	debug("sack_consume %lu\n", (unsigned long)len);
+
+	if(len > c->rcvbuf.used) {
+		debug("All SACK entries consumed");
+		c->sacks[0].len = 0;
+		return;
+	}
+
+	buffer_get(&c->rcvbuf, NULL, len);
+
+	for(int i = 0; i < NSACKS && c->sacks[i].len;) {
+		if(len < c->sacks[i].offset) {
+			c->sacks[i].offset -= len;
+			i++;
+		} else if(len < c->sacks[i].offset + c->sacks[i].len) {
+			c->sacks[i].len -= len - c->sacks[i].offset;
+			c->sacks[i].offset = 0;
+			i++;
+		} else {
+			if(i < NSACKS - 1) {
+				memmove(&c->sacks[i], &c->sacks[i + 1], (NSACKS - 1 - i) * sizeof(c->sacks)[i]);
+				c->sacks[NSACKS - 1].len = 0;
+			} else {
+				c->sacks[i].len = 0;
+				break;
+			}
+		}
+	}
+
+	for(int i = 0; i < NSACKS && c->sacks[i].len; i++) {
+		debug("SACK[%d] offset %u len %u\n", i, c->sacks[i].offset, c->sacks[i].len);
+	}
+}
+
+static void handle_out_of_order(struct utcp_connection *c, uint32_t offset, const void *data, size_t len) {
+	debug("out of order packet, offset %u\n", offset);
+	// Packet loss or reordering occured. Store the data in the buffer.
+	ssize_t rxd = buffer_put_at(&c->rcvbuf, offset, data, len);
+
+	if(rxd < 0 || (size_t)rxd < len) {
+		abort();
+	}
+
+	// Make note of where we put it.
+	for(int i = 0; i < NSACKS; i++) {
+		if(!c->sacks[i].len) { // nothing to merge, add new entry
+			debug("New SACK entry %d\n", i);
+			c->sacks[i].offset = offset;
+			c->sacks[i].len = rxd;
+			break;
+		} else if(offset < c->sacks[i].offset) {
+			if(offset + rxd < c->sacks[i].offset) { // insert before
+				if(!c->sacks[NSACKS - 1].len) { // only if room left
+					debug("Insert SACK entry at %d\n", i);
+					memmove(&c->sacks[i + 1], &c->sacks[i], (NSACKS - i - 1) * sizeof(c->sacks)[i]);
+					c->sacks[i].offset = offset;
+					c->sacks[i].len = rxd;
+				} else {
+					debug("SACK entries full, dropping packet\n");
+				}
+
+				break;
+			} else { // merge
+				debug("Merge with start of SACK entry at %d\n", i);
+				c->sacks[i].offset = offset;
+				break;
+			}
+		} else if(offset <= c->sacks[i].offset + c->sacks[i].len) {
+			if(offset + rxd > c->sacks[i].offset + c->sacks[i].len) { // merge
+				debug("Merge with end of SACK entry at %d\n", i);
+				c->sacks[i].len = offset + rxd - c->sacks[i].offset;
+				// TODO: handle potential merge with next entry
+			}
+
+			break;
+		}
+	}
+
+	for(int i = 0; i < NSACKS && c->sacks[i].len; i++) {
+		debug("SACK[%d] offset %u len %u\n", i, c->sacks[i].offset, c->sacks[i].len);
+	}
+}
+
+static void handle_in_order(struct utcp_connection *c, const void *data, size_t len) {
+	// Check if we can process out-of-order data now.
+	if(c->sacks[0].len && len >= c->sacks[0].offset) { // TODO: handle overlap with second SACK
+		debug("incoming packet len %lu connected with SACK at %u\n", (unsigned long)len, c->sacks[0].offset);
+		buffer_put_at(&c->rcvbuf, 0, data, len); // TODO: handle return value
+		len = max(len, c->sacks[0].offset + c->sacks[0].len);
+		data = c->rcvbuf.data;
+	}
+
+	if(c->recv) {
+		ssize_t rxd = c->recv(c, data, len);
+
+		if(rxd < 0 || (size_t)rxd != len) {
+			// TODO: handle the application not accepting all data.
+			abort();
+		}
+	}
+
+	if(c->rcvbuf.used) {
+		sack_consume(c, len);
+	}
+
+	c->rcv.nxt += len;
+}
+
+
+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();
+	}
+
+	if(offset) {
+		handle_out_of_order(c, offset, data, len);
+	} else {
+		handle_in_order(c, data, len);
+	}
+}
+
+
 ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
+	const uint8_t *ptr = data;
+
 	if(!utcp) {
 		errno = EFAULT;
 		return -1;
 	}
 
-	if(!len)
+	if(!len) {
 		return 0;
+	}
 
 	if(!data) {
 		errno = EFAULT;
@@ -418,16 +920,17 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	// Drop packets smaller than the header
 
 	struct hdr hdr;
-	if(len < sizeof hdr) {
+
+	if(len < sizeof(hdr)) {
 		errno = EBADMSG;
 		return -1;
 	}
 
 	// Make a copy from the potentially unaligned data to a struct hdr
 
-	memcpy(&hdr, data, sizeof hdr);
-	data += sizeof hdr;
-	len -= sizeof hdr;
+	memcpy(&hdr, ptr, sizeof(hdr));
+	ptr += sizeof(hdr);
+	len -= sizeof(hdr);
 
 	// Drop packets with an unknown CTL flag
 
@@ -436,6 +939,53 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 		return -1;
 	}
 
+	// Check for auxiliary headers
+
+	const uint8_t *init = NULL;
+
+	uint16_t aux = hdr.aux;
+
+	while(aux) {
+		size_t auxlen = 4 * (aux >> 8) & 0xf;
+		uint8_t auxtype = aux & 0xff;
+
+		if(len < auxlen) {
+			errno = EBADMSG;
+			return -1;
+		}
+
+		switch(auxtype) {
+		case AUX_INIT:
+			if(!(hdr.ctl & SYN) || auxlen != 4) {
+				errno = EBADMSG;
+				return -1;
+			}
+
+			init = ptr;
+			break;
+
+		default:
+			errno = EBADMSG;
+			return -1;
+		}
+
+		len -= auxlen;
+		ptr += auxlen;
+
+		if(!(aux & 0x800)) {
+			break;
+		}
+
+		if(len < 2) {
+			errno = EBADMSG;
+			return -1;
+		}
+
+		memcpy(&aux, ptr, 2);
+		len -= 2;
+		ptr += 2;
+	}
+
 	// Try to match the packet to an existing connection
 
 	struct utcp_connection *c = find_connection(utcp, hdr.dst, hdr.src);
@@ -445,8 +995,9 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	if(!c) {
 		// Ignore RST packets
 
-		if(hdr.ctl & RST)
+		if(hdr.ctl & RST) {
 			return 0;
+		}
 
 		// Is it a SYN packet and are we LISTENing?
 
@@ -459,24 +1010,55 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 
 			// Try to allocate memory, otherwise send a RST back
 			c = allocate_connection(utcp, hdr.dst, hdr.src);
+
 			if(!c) {
 				len = 1;
 				goto reset;
 			}
 
+			// Parse auxilliary information
+			if(init) {
+				if(init[0] < 1) {
+					len = 1;
+					goto reset;
+				}
+
+				c->flags = init[3] & 0x7;
+			} else {
+				c->flags = UTCP_TCP;
+			}
+
 			// Return SYN+ACK, go to SYN_RECEIVED state
 			c->snd.wnd = hdr.wnd;
 			c->rcv.irs = hdr.seq;
 			c->rcv.nxt = c->rcv.irs + 1;
 			set_state(c, SYN_RECEIVED);
 
-			hdr.dst = c->dst;
-			hdr.src = c->src;
-			hdr.ack = c->rcv.irs + 1;
-			hdr.seq = c->snd.iss;
-			hdr.ctl = SYN | ACK;
-			print_packet(c->utcp, "send", &hdr, sizeof hdr);
-			utcp->send(utcp, &hdr, sizeof hdr);
+			struct {
+				struct hdr hdr;
+				uint8_t data[4];
+			} pkt;
+
+			pkt.hdr.src = c->src;
+			pkt.hdr.dst = c->dst;
+			pkt.hdr.ack = c->rcv.irs + 1;
+			pkt.hdr.seq = c->snd.iss;
+			pkt.hdr.wnd = c->rcv.wnd;
+			pkt.hdr.ctl = SYN | ACK;
+
+			if(init) {
+				pkt.hdr.aux = 0x0101;
+				pkt.data[0] = 1;
+				pkt.data[1] = 0;
+				pkt.data[2] = 0;
+				pkt.data[3] = c->flags & 0x7;
+				print_packet(c->utcp, "send", &pkt, sizeof(hdr) + 4);
+				utcp->send(utcp, &pkt, sizeof(hdr) + 4);
+			} else {
+				pkt.hdr.aux = 0;
+				print_packet(c->utcp, "send", &pkt, sizeof(hdr));
+				utcp->send(utcp, &pkt, sizeof(hdr));
+			}
 		} else {
 			// No, we don't want your packets, send a RST back
 			len = 1;
@@ -491,11 +1073,15 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	// In case this is for a CLOSED connection, ignore the packet.
 	// TODO: make it so incoming packets can never match a CLOSED connection.
 
-	if(c->state == CLOSED)
+	if(c->state == CLOSED) {
+		debug("Got packet for closed connection\n");
 		return 0;
+	}
 
 	// It is for an existing connection.
 
+	uint32_t prevrcvnxt = c->rcv.nxt;
+
 	// 1. Drop invalid packets.
 
 	// 1a. Drop packets that should not happen in our current state.
@@ -511,57 +1097,65 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	case LAST_ACK:
 	case TIME_WAIT:
 		break;
+
 	default:
+#ifdef UTCP_DEBUG
 		abort();
+#endif
+		break;
 	}
 
 	// 1b. Drop packets with a sequence number not in our receive window.
 
 	bool acceptable;
 
-	if(c->state == SYN_SENT)
+	if(c->state == SYN_SENT) {
 		acceptable = true;
+	} else if(len == 0) {
+		acceptable = seqdiff(hdr.seq, c->rcv.nxt) >= 0;
+	} else {
+		int32_t rcv_offset = seqdiff(hdr.seq, c->rcv.nxt);
 
-	// TODO: handle packets overlapping c->rcv.nxt.
-#if 0
-	// Only use this when accepting out-of-order packets.
-	else if(len == 0)
-		if(c->rcv.wnd == 0)
-			acceptable = hdr.seq == c->rcv.nxt;
-		else
-			acceptable = (seqdiff(hdr.seq, c->rcv.nxt) >= 0 && seqdiff(hdr.seq, c->rcv.nxt + c->rcv.wnd) < 0);
-	else
-		if(c->rcv.wnd == 0)
-			// We don't accept data when the receive window is zero.
-			acceptable = false;
-		else
-			// Both start and end of packet must be within the receive window
-			acceptable = (seqdiff(hdr.seq, c->rcv.nxt) >= 0 && seqdiff(hdr.seq, c->rcv.nxt + c->rcv.wnd) < 0)
-				|| (seqdiff(hdr.seq + len + 1, c->rcv.nxt) >= 0 && seqdiff(hdr.seq + len - 1, c->rcv.nxt + c->rcv.wnd) < 0);
-#else
-	if(c->state != SYN_SENT)
-		acceptable = hdr.seq == c->rcv.nxt;
-#endif
+		// cut already accepted front overlapping
+		if(rcv_offset < 0) {
+			acceptable = len > (size_t) - rcv_offset;
+
+			if(acceptable) {
+				ptr -= rcv_offset;
+				len += rcv_offset;
+				hdr.seq -= rcv_offset;
+			}
+		} else {
+			acceptable = seqdiff(hdr.seq, c->rcv.nxt) >= 0 && seqdiff(hdr.seq, c->rcv.nxt) + len <= c->rcvbuf.maxsize;
+		}
+	}
 
 	if(!acceptable) {
-		debug("Packet not acceptable, %u  <= %u + %zu < %u\n", c->rcv.nxt, hdr.seq, len, c->rcv.nxt + c->rcv.wnd);
+		debug("Packet not acceptable, %u <= %u + %lu < %u\n", c->rcv.nxt, hdr.seq, (unsigned long)len, c->rcv.nxt + c->rcvbuf.maxsize);
+
 		// Ignore unacceptable RST packets.
-		if(hdr.ctl & RST)
+		if(hdr.ctl & RST) {
 			return 0;
-		// Otherwise, send an ACK back in the hope things improve.
-		goto ack;
+		}
+
+		// Otherwise, continue processing.
+		len = 0;
 	}
 
 	c->snd.wnd = hdr.wnd; // TODO: move below
 
 	// 1c. Drop packets with an invalid ACK.
-	// ackno should not roll back, and it should also not be bigger than snd.nxt.
+	// ackno should not roll back, and it should also not be bigger than what we ever could have sent
+	// (= snd.una + c->sndbuf.used).
+
+	if(hdr.ctl & ACK && (seqdiff(hdr.ack, c->snd.last) > 0 || seqdiff(hdr.ack, c->snd.una) < 0)) {
+		debug("Packet ack seqno out of range, %u <= %u < %u\n", c->snd.una, hdr.ack, c->snd.una + c->sndbuf.used);
 
-	if(hdr.ctl & ACK && (seqdiff(hdr.ack, c->snd.nxt) > 0 || seqdiff(hdr.ack, c->snd.una) < 0)) {
-		debug("Packet ack seqno out of range, %u %u %u\n", hdr.ack, c->snd.una, c->snd.nxt);
 		// Ignore unacceptable RST packets.
-		if(hdr.ctl & RST)
+		if(hdr.ctl & RST) {
 			return 0;
+		}
+
 		goto reset;
 	}
 
@@ -570,67 +1164,111 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	if(hdr.ctl & RST) {
 		switch(c->state) {
 		case SYN_SENT:
-			if(!(hdr.ctl & ACK))
+			if(!(hdr.ctl & ACK)) {
 				return 0;
+			}
+
 			// The peer has refused our connection.
 			set_state(c, CLOSED);
 			errno = ECONNREFUSED;
-			if(c->recv)
+
+			if(c->recv) {
 				c->recv(c, NULL, 0);
+			}
+
 			return 0;
+
 		case SYN_RECEIVED:
-			if(hdr.ctl & ACK)
+			if(hdr.ctl & ACK) {
 				return 0;
+			}
+
 			// We haven't told the application about this connection yet. Silently delete.
 			free_connection(c);
 			return 0;
+
 		case ESTABLISHED:
 		case FIN_WAIT_1:
 		case FIN_WAIT_2:
 		case CLOSE_WAIT:
-			if(hdr.ctl & ACK)
+			if(hdr.ctl & ACK) {
 				return 0;
+			}
+
 			// The peer has aborted our connection.
 			set_state(c, CLOSED);
 			errno = ECONNRESET;
-			if(c->recv)
+
+			if(c->recv) {
 				c->recv(c, NULL, 0);
+			}
+
 			return 0;
+
 		case CLOSING:
 		case LAST_ACK:
 		case TIME_WAIT:
-			if(hdr.ctl & ACK)
+			if(hdr.ctl & ACK) {
 				return 0;
+			}
+
 			// As far as the application is concerned, the connection has already been closed.
 			// If it has called utcp_close() already, we can immediately free this connection.
 			if(c->reapable) {
 				free_connection(c);
 				return 0;
 			}
+
 			// Otherwise, immediately move to the CLOSED state.
 			set_state(c, CLOSED);
 			return 0;
+
 		default:
+#ifdef UTCP_DEBUG
 			abort();
+#endif
+			break;
 		}
 	}
 
+	uint32_t advanced;
+
+	if(!(hdr.ctl & ACK)) {
+		advanced = 0;
+		goto skip_ack;
+	}
+
 	// 3. Advance snd.una
 
-	uint32_t advanced = seqdiff(hdr.ack, c->snd.una);
-	uint32_t prevrcvnxt = c->rcv.nxt;
+	advanced = seqdiff(hdr.ack, c->snd.una);
+	prevrcvnxt = c->rcv.nxt;
 
 	if(advanced) {
+		// RTT measurement
+		if(c->rtt_start.tv_sec) {
+			if(c->rtt_seq == hdr.ack) {
+				struct timeval now, diff;
+				gettimeofday(&now, NULL);
+				timersub(&now, &c->rtt_start, &diff);
+				update_rtt(c, diff.tv_sec * 1000000 + diff.tv_usec);
+				c->rtt_start.tv_sec = 0;
+			} else if(c->rtt_seq < hdr.ack) {
+				debug("Cancelling RTT measurement: %u < %u\n", c->rtt_seq, hdr.ack);
+				c->rtt_start.tv_sec = 0;
+			}
+		}
+
 		int32_t data_acked = advanced;
 
 		switch(c->state) {
-			case SYN_SENT:
-			case SYN_RECEIVED:
-				data_acked--;
-				break;
-			// TODO: handle FIN as well.
-			default:
-				break;
+		case SYN_SENT:
+		case SYN_RECEIVED:
+			data_acked--;
+			break;
+
+		// TODO: handle FIN as well.
+		default:
+			break;
 		}
 
 		assert(data_acked >= 0);
@@ -638,42 +1276,57 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 		int32_t bufused = seqdiff(c->snd.last, c->snd.una);
 		assert(data_acked <= bufused);
 
-		// Make room in the send buffer.
-		// TODO: try to avoid memmoving too much. Circular buffer?
-		uint32_t left = bufused - data_acked;
-		if(data_acked && left)
-			memmove(c->sndbuf, c->sndbuf + data_acked, left);
+		if(data_acked) {
+			buffer_get(&c->sndbuf, NULL, data_acked);
+		}
+
+		// Also advance snd.nxt if possible
+		if(seqdiff(c->snd.nxt, hdr.ack) < 0) {
+			c->snd.nxt = hdr.ack;
+		}
 
 		c->snd.una = hdr.ack;
 
 		c->dupack = 0;
 		c->snd.cwnd += utcp->mtu;
-		if(c->snd.cwnd > c->maxsndbufsize)
-			c->snd.cwnd = c->maxsndbufsize;
+
+		if(c->snd.cwnd > c->sndbuf.maxsize) {
+			c->snd.cwnd = c->sndbuf.maxsize;
+		}
 
 		// Check if we have sent a FIN that is now ACKed.
 		switch(c->state) {
 		case FIN_WAIT_1:
-			if(c->snd.una == c->snd.last)
+			if(c->snd.una == c->snd.last) {
 				set_state(c, FIN_WAIT_2);
+			}
+
 			break;
+
 		case CLOSING:
 			if(c->snd.una == c->snd.last) {
 				gettimeofday(&c->conn_timeout, NULL);
 				c->conn_timeout.tv_sec += 60;
 				set_state(c, TIME_WAIT);
 			}
+
 			break;
+
 		default:
 			break;
 		}
 	} else {
-		if(!len) {
+		if(!len && is_reliable(c)) {
 			c->dupack++;
-			if(c->dupack >= 3) {
+
+			if(c->dupack == 3) {
 				debug("Triplicate ACK\n");
 				//TODO: Resend one packet and go to fast recovery mode. See RFC 6582.
-				//abort();
+				//We do a very simple variant here; reset the nxt pointer to the last acknowledged packet from the peer.
+				//Reset the congestion window so we wait for ACKs.
+				c->snd.nxt = c->snd.una;
+				c->snd.cwnd = utcp->mtu;
+				start_retransmit_timer(c);
 			}
 		}
 	}
@@ -682,23 +1335,39 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 
 	if(advanced) {
 		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.nxt)
-			timerclear(&c->rtrx_timeout);
+
+		if(c->snd.una == c->snd.last) {
+			stop_retransmit_timer(c);
+		} else if(is_reliable(c)) {
+			start_retransmit_timer(c);
+		}
 	}
 
+skip_ack:
 	// 5. Process SYN stuff
 
 	if(hdr.ctl & SYN) {
 		switch(c->state) {
 		case SYN_SENT:
+
 			// This is a SYNACK. It should always have ACKed the SYN.
-			if(!advanced)
+			if(!advanced) {
 				goto reset;
+			}
+
 			c->rcv.irs = hdr.seq;
 			c->rcv.nxt = hdr.seq;
-			set_state(c, ESTABLISHED);
+
+			if(c->shut_wr) {
+				c->snd.last++;
+				set_state(c, FIN_WAIT_1);
+			} else {
+				set_state(c, ESTABLISHED);
+			}
+
 			// TODO: notify application of this somehow.
 			break;
+
 		case SYN_RECEIVED:
 		case ESTABLISHED:
 		case FIN_WAIT_1:
@@ -708,9 +1377,13 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 		case LAST_ACK:
 		case TIME_WAIT:
 			// Ehm, no. We should never receive a second SYN.
-			goto reset;
+			return 0;
+
 		default:
+#ifdef UTCP_DEBUG
 			abort();
+#endif
+			return 0;
 		}
 
 		// SYN counts as one sequence number
@@ -721,12 +1394,14 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 
 	if(c->state == SYN_RECEIVED) {
 		// This is the ACK after the SYNACK. It should always have ACKed the SYNACK.
-		if(!advanced)
+		if(!advanced) {
 			goto reset;
+		}
 
 		// Are we still LISTENing?
-		if(utcp->accept)
+		if(utcp->accept) {
 			utcp->accept(c, c->src);
+		}
 
 		if(c->state != ESTABLISHED) {
 			set_state(c, CLOSED);
@@ -740,68 +1415,71 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 		case SYN_SENT:
 		case SYN_RECEIVED:
 			// This should never happen.
+#ifdef UTCP_DEBUG
 			abort();
+#endif
+			return 0;
+
 		case ESTABLISHED:
 		case FIN_WAIT_1:
 		case FIN_WAIT_2:
 			break;
+
 		case CLOSE_WAIT:
 		case CLOSING:
 		case LAST_ACK:
 		case TIME_WAIT:
 			// Ehm no, We should never receive more data after a FIN.
 			goto reset;
+
 		default:
+#ifdef UTCP_DEBUG
 			abort();
+#endif
+			return 0;
 		}
 
-		ssize_t rxd;
-
-		if(c->recv) {
-			rxd = c->recv(c, data, len);
-			if(rxd != len) {
-				// TODO: once we have a receive buffer, handle the application not accepting all data.
-				fprintf(stderr, "c->recv(%p, %p, %zu) returned %zd\n", c, data, len, rxd);
-				abort();
-			}
-			if(rxd < 0)
-				rxd = 0;
-			else if(rxd > len)
-				rxd = len; // Bad application, bad!
-		} else {
-			rxd = len;
-		}
-
-		c->rcv.nxt += len;
+		handle_incoming_data(c, hdr.seq, ptr, len);
 	}
 
 	// 7. Process FIN stuff
 
-	if(hdr.ctl & FIN) {
+	if((hdr.ctl & FIN) && hdr.seq + len == c->rcv.nxt) {
 		switch(c->state) {
 		case SYN_SENT:
 		case SYN_RECEIVED:
 			// This should never happen.
+#ifdef UTCP_DEBUG
 			abort();
+#endif
+			break;
+
 		case ESTABLISHED:
 			set_state(c, CLOSE_WAIT);
 			break;
+
 		case FIN_WAIT_1:
 			set_state(c, CLOSING);
 			break;
+
 		case FIN_WAIT_2:
 			gettimeofday(&c->conn_timeout, NULL);
 			c->conn_timeout.tv_sec += 60;
 			set_state(c, TIME_WAIT);
 			break;
+
 		case CLOSE_WAIT:
 		case CLOSING:
 		case LAST_ACK:
 		case TIME_WAIT:
 			// Ehm, no. We should never receive a second FIN.
 			goto reset;
+
 		default:
+#ifdef UTCP_DEBUG
 			abort();
+#endif
+			break;
 		}
 
 		// FIN counts as one sequence number
@@ -821,13 +1499,14 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	// - or we got an ack, so we should maybe send a bit more data
 	//   -> sendatleastone = false
 
-ack:
-	ack(c, prevrcvnxt != c->rcv.nxt);
+	ack(c, len || prevrcvnxt != c->rcv.nxt);
 	return 0;
 
 reset:
 	swap_ports(&hdr);
 	hdr.wnd = 0;
+	hdr.aux = 0;
+
 	if(hdr.ctl & ACK) {
 		hdr.seq = hdr.ack;
 		hdr.ctl = RST;
@@ -836,14 +1515,16 @@ reset:
 		hdr.seq = 0;
 		hdr.ctl = RST | ACK;
 	}
-	print_packet(utcp, "send", &hdr, sizeof hdr);
-	utcp->send(utcp, &hdr, sizeof hdr);
+
+	print_packet(utcp, "send", &hdr, sizeof(hdr));
+	utcp->send(utcp, &hdr, sizeof(hdr));
 	return 0;
 
 }
 
 int utcp_shutdown(struct utcp_connection *c, int dir) {
-	debug("%p shutdown %d\n", c ? c->utcp : NULL, dir);
+	debug("%p shutdown %d at %u\n", c ? c->utcp : NULL, dir, c ? c->snd.last : 0);
+
 	if(!c) {
 		errno = EFAULT;
 		return -1;
@@ -855,23 +1536,47 @@ int utcp_shutdown(struct utcp_connection *c, int dir) {
 		return -1;
 	}
 
-	// TODO: handle dir
+	if(!(dir == UTCP_SHUT_RD || dir == UTCP_SHUT_WR || dir == UTCP_SHUT_RDWR)) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	// TCP does not have a provision for stopping incoming packets.
+	// The best we can do is to just ignore them.
+	if(dir == UTCP_SHUT_RD || dir == UTCP_SHUT_RDWR) {
+		c->recv = NULL;
+	}
+
+	// The rest of the code deals with shutting down writes.
+	if(dir == UTCP_SHUT_RD) {
+		return 0;
+	}
+
+	// Only process shutting down writes once.
+	if(c->shut_wr) {
+		return 0;
+	}
+
+	c->shut_wr = true;
 
 	switch(c->state) {
 	case CLOSED:
-		return 0;
 	case LISTEN:
+		errno = ENOTCONN;
+		return -1;
+
 	case SYN_SENT:
-		set_state(c, CLOSED);
 		return 0;
 
 	case SYN_RECEIVED:
 	case ESTABLISHED:
 		set_state(c, FIN_WAIT_1);
 		break;
+
 	case FIN_WAIT_1:
 	case FIN_WAIT_2:
 		return 0;
+
 	case CLOSE_WAIT:
 		set_state(c, CLOSING);
 		break;
@@ -885,40 +1590,40 @@ int utcp_shutdown(struct utcp_connection *c, int dir) {
 	c->snd.last++;
 
 	ack(c, false);
-	return 0;
-}
 
-int utcp_close(struct utcp_connection *c) {
-	if(utcp_shutdown(c, SHUT_RDWR))
-		return -1;
-	c->reapable = true;
+	if(!timerisset(&c->rtrx_timeout)) {
+		start_retransmit_timer(c);
+	}
+
 	return 0;
 }
 
-int utcp_abort(struct utcp_connection *c) {
+static bool reset_connection(struct utcp_connection *c) {
 	if(!c) {
 		errno = EFAULT;
-		return -1;
+		return false;
 	}
 
 	if(c->reapable) {
 		debug("Error: abort() called on closed connection %p\n", c);
 		errno = EBADF;
-		return -1;
+		return false;
 	}
 
-	c->reapable = true;
+	c->recv = NULL;
+	c->poll = NULL;
 
 	switch(c->state) {
 	case CLOSED:
-		return 0;
+		return true;
+
 	case LISTEN:
 	case SYN_SENT:
 	case CLOSING:
 	case LAST_ACK:
 	case TIME_WAIT:
 		set_state(c, CLOSED);
-		return 0;
+		return true;
 
 	case SYN_RECEIVED:
 	case ESTABLISHED:
@@ -940,70 +1645,54 @@ int utcp_abort(struct utcp_connection *c) {
 	hdr.wnd = 0;
 	hdr.ctl = RST;
 
-	print_packet(c->utcp, "send", &hdr, sizeof hdr);
-	c->utcp->send(c->utcp, &hdr, sizeof hdr);
-	return 0;
+	print_packet(c->utcp, "send", &hdr, sizeof(hdr));
+	c->utcp->send(c->utcp, &hdr, sizeof(hdr));
+	return true;
 }
 
-static void retransmit(struct utcp_connection *c) {
-	if(c->state == CLOSED || c->snd.nxt == c->snd.una)
+// Closes all the opened connections
+void utcp_abort_all_connections(struct utcp *utcp) {
+	if(!utcp) {
+		errno = EINVAL;
 		return;
+	}
 
-	struct utcp *utcp = c->utcp;
+	for(int i = 0; i < utcp->nconnections; i++) {
+		struct utcp_connection *c = utcp->connections[i];
 
-	struct {
-		struct hdr hdr;
-		char data[c->utcp->mtu];
-	} pkt;
+		if(c->reapable || c->state == CLOSED) {
+			continue;
+		}
 
-	pkt.hdr.src = c->src;
-	pkt.hdr.dst = c->dst;
+		reset_connection(c);
 
-	switch(c->state) {
-		case LISTEN:
-			// TODO: this should not happen
-			break;
+		if(c->recv) {
+			errno = 0;
+			c->recv(c, NULL, 0);
+		}
+	}
 
-		case SYN_SENT:
-			pkt.hdr.seq = c->snd.iss;
-			pkt.hdr.ack = 0;
-			pkt.hdr.wnd = c->rcv.wnd;
-			pkt.hdr.ctl = SYN;
-			print_packet(c->utcp, "rtrx", &pkt, sizeof pkt.hdr);
-			utcp->send(utcp, &pkt, sizeof pkt.hdr);
-			break;
+	return;
+}
 
-		case SYN_RECEIVED:
-			pkt.hdr.seq = c->snd.nxt;
-			pkt.hdr.ack = c->rcv.nxt;
-			pkt.hdr.ctl = SYN | ACK;
-			print_packet(c->utcp, "rtrx", &pkt, sizeof pkt.hdr);
-			utcp->send(utcp, &pkt, sizeof pkt.hdr);
-			break;
+int utcp_close(struct utcp_connection *c) {
+	if(utcp_shutdown(c, SHUT_RDWR) && errno != ENOTCONN) {
+		return -1;
+	}
 
-		case ESTABLISHED:
-		case FIN_WAIT_1:
-			pkt.hdr.seq = c->snd.una;
-			pkt.hdr.ack = c->rcv.nxt;
-			pkt.hdr.ctl = ACK;
-			uint32_t len = seqdiff(c->snd.nxt, c->snd.una);
-			if(c->state == FIN_WAIT_1)
-				len--;
-			if(len > utcp->mtu)
-				len = utcp->mtu;
-			else {
-				if(c->state == FIN_WAIT_1)
-					pkt.hdr.ctl |= FIN;
-			}
-			memcpy(pkt.data, c->sndbuf, len);
-			print_packet(c->utcp, "rtrx", &pkt, sizeof pkt.hdr + len);
-			utcp->send(utcp, &pkt, sizeof pkt.hdr + len);
-			break;
+	c->recv = NULL;
+	c->poll = NULL;
+	c->reapable = true;
+	return 0;
+}
 
-		default:
-			// TODO: implement
-			abort();
+int utcp_abort(struct utcp_connection *c) {
+	if(!reset_connection(c)) {
+		return -1;
 	}
+
+	c->reapable = true;
+	return 0;
 }
 
 /* Handle timeouts.
@@ -1012,149 +1701,272 @@ static void retransmit(struct utcp_connection *c) {
  * The return value is the time to the next timeout in milliseconds,
  * or maybe a negative value if the timeout is infinite.
  */
-int utcp_timeout(struct utcp *utcp) {
+struct timeval utcp_timeout(struct utcp *utcp) {
 	struct timeval now;
 	gettimeofday(&now, NULL);
 	struct timeval next = {now.tv_sec + 3600, now.tv_usec};
 
 	for(int i = 0; i < utcp->nconnections; i++) {
 		struct utcp_connection *c = utcp->connections[i];
-		if(!c)
+
+		if(!c) {
 			continue;
+		}
 
+		// delete connections that have been utcp_close()d.
 		if(c->state == CLOSED) {
 			if(c->reapable) {
 				debug("Reaping %p\n", c);
 				free_connection(c);
 				i--;
 			}
+
 			continue;
 		}
 
 		if(timerisset(&c->conn_timeout) && timercmp(&c->conn_timeout, &now, <)) {
 			errno = ETIMEDOUT;
 			c->state = CLOSED;
-			if(c->recv)
+
+			if(c->recv) {
 				c->recv(c, NULL, 0);
+			}
+
 			continue;
 		}
 
 		if(timerisset(&c->rtrx_timeout) && timercmp(&c->rtrx_timeout, &now, <)) {
+			debug("retransmit()\n");
 			retransmit(c);
 		}
 
-		if(c->poll && c->sndbufsize < c->maxsndbufsize / 2)
-			c->poll(c, c->maxsndbufsize - c->sndbufsize);
+		if(c->poll) {
+			if((c->state == ESTABLISHED || c->state == CLOSE_WAIT)) {
+				uint32_t len =  buffer_free(&c->sndbuf);
 
-		if(timerisset(&c->conn_timeout) && timercmp(&c->conn_timeout, &next, <))
-			next = c->conn_timeout;
+				if(len) {
+					c->poll(c, len);
+				}
+			} else if(c->state == CLOSED) {
+				c->poll(c, 0);
+			}
+		}
 
-		if(c->snd.nxt != c->snd.una) {
-			c->rtrx_timeout = now;
-			c->rtrx_timeout.tv_sec++;
-		} else {
-			timerclear(&c->rtrx_timeout);
+		if(timerisset(&c->conn_timeout) && timercmp(&c->conn_timeout, &next, <)) {
+			next = c->conn_timeout;
 		}
 
-		if(timerisset(&c->rtrx_timeout) && timercmp(&c->rtrx_timeout, &next, <))
+		if(timerisset(&c->rtrx_timeout) && timercmp(&c->rtrx_timeout, &next, <)) {
 			next = c->rtrx_timeout;
+		}
 	}
 
 	struct timeval diff;
+
 	timersub(&next, &now, &diff);
-	if(diff.tv_sec < 0)
-		return 0;
-	return diff.tv_sec * 1000 + diff.tv_usec / 1000;
+
+	return diff;
 }
 
-struct utcp *utcp_init(utcp_accept_t accept, utcp_pre_accept_t pre_accept, utcp_send_t send, void *priv) {
-	struct utcp *utcp = calloc(1, sizeof *utcp);
-	if(!utcp)
-		return NULL;
+bool utcp_is_active(struct utcp *utcp) {
+	if(!utcp) {
+		return false;
+	}
 
+	for(int i = 0; i < utcp->nconnections; i++)
+		if(utcp->connections[i]->state != CLOSED && utcp->connections[i]->state != TIME_WAIT) {
+			return true;
+		}
+
+	return false;
+}
+
+struct utcp *utcp_init(utcp_accept_t accept, utcp_pre_accept_t pre_accept, utcp_send_t send, void *priv) {
 	if(!send) {
 		errno = EFAULT;
 		return NULL;
 	}
 
+	struct utcp *utcp = calloc(1, sizeof(*utcp));
+
+	if(!utcp) {
+		return NULL;
+	}
+
 	utcp->accept = accept;
 	utcp->pre_accept = pre_accept;
 	utcp->send = send;
 	utcp->priv = priv;
-	utcp->mtu = 1000;
-	utcp->timeout = 60;
+	utcp->mtu = DEFAULT_MTU;
+	utcp->timeout = DEFAULT_USER_TIMEOUT; // sec
+	utcp->rto = START_RTO; // usec
 
 	return utcp;
 }
 
 void utcp_exit(struct utcp *utcp) {
-	if(!utcp)
+	if(!utcp) {
 		return;
+	}
+
 	for(int i = 0; i < utcp->nconnections; i++) {
-		if(!utcp->connections[i]->reapable)
-			debug("Warning, freeing unclosed connection %p\n", utcp->connections[i]);
-		free(utcp->connections[i]->sndbuf);
-		free(utcp->connections[i]);
+		struct utcp_connection *c = utcp->connections[i];
+
+		if(!c->reapable)
+			if(c->recv) {
+				c->recv(c, NULL, 0);
+			}
+
+		buffer_exit(&c->rcvbuf);
+		buffer_exit(&c->sndbuf);
+		free(c);
 	}
+
 	free(utcp->connections);
 	free(utcp);
 }
 
 uint16_t utcp_get_mtu(struct utcp *utcp) {
-	return utcp->mtu;
+	return utcp ? utcp->mtu : 0;
 }
 
 void utcp_set_mtu(struct utcp *utcp, uint16_t mtu) {
 	// TODO: handle overhead of the header
-	utcp->mtu = mtu;
+	if(utcp) {
+		utcp->mtu = mtu;
+	}
+}
+
+void utcp_reset_timers(struct utcp *utcp) {
+	if(!utcp) {
+		return;
+	}
+
+	struct timeval now, then;
+
+	gettimeofday(&now, NULL);
+
+	then = now;
+
+	then.tv_sec += utcp->timeout;
+
+	for(int i = 0; i < utcp->nconnections; i++) {
+		utcp->connections[i]->rtrx_timeout = now;
+		utcp->connections[i]->conn_timeout = then;
+		utcp->connections[i]->rtt_start.tv_sec = 0;
+	}
+
+	if(utcp->rto > START_RTO) {
+		utcp->rto = START_RTO;
+	}
 }
 
 int utcp_get_user_timeout(struct utcp *u) {
-	return u->timeout;
+	return u ? u->timeout : 0;
 }
 
 void utcp_set_user_timeout(struct utcp *u, int timeout) {
-	u->timeout = timeout;
+	if(u) {
+		u->timeout = timeout;
+	}
 }
 
 size_t utcp_get_sndbuf(struct utcp_connection *c) {
-	return c->maxsndbufsize;
+	return c ? c->sndbuf.maxsize : 0;
 }
 
 size_t utcp_get_sndbuf_free(struct utcp_connection *c) {
-	return c->maxsndbufsize - c->sndbufsize;
+	if(!c) {
+		return 0;
+	}
+
+	switch(c->state) {
+	case SYN_SENT:
+	case SYN_RECEIVED:
+	case ESTABLISHED:
+	case CLOSE_WAIT:
+		return buffer_free(&c->sndbuf);
+
+	default:
+		return 0;
+	}
 }
 
 void utcp_set_sndbuf(struct utcp_connection *c, size_t size) {
-	c->maxsndbufsize = size;
-	if(c->maxsndbufsize != size)
-		c->maxsndbufsize = -1;
+	if(!c) {
+		return;
+	}
+
+	c->sndbuf.maxsize = size;
+
+	if(c->sndbuf.maxsize != size) {
+		c->sndbuf.maxsize = -1;
+	}
+}
+
+size_t utcp_get_rcvbuf(struct utcp_connection *c) {
+	return c ? c->rcvbuf.maxsize : 0;
+}
+
+size_t utcp_get_rcvbuf_free(struct utcp_connection *c) {
+	if(c && (c->state == ESTABLISHED || c->state == CLOSE_WAIT)) {
+		return buffer_free(&c->rcvbuf);
+	} else {
+		return 0;
+	}
+}
+
+void utcp_set_rcvbuf(struct utcp_connection *c, size_t size) {
+	if(!c) {
+		return;
+	}
+
+	c->rcvbuf.maxsize = size;
+
+	if(c->rcvbuf.maxsize != size) {
+		c->rcvbuf.maxsize = -1;
+	}
 }
 
 bool utcp_get_nodelay(struct utcp_connection *c) {
-	return c->nodelay;
+	return c ? c->nodelay : false;
 }
 
 void utcp_set_nodelay(struct utcp_connection *c, bool nodelay) {
-	c->nodelay = nodelay;
+	if(c) {
+		c->nodelay = nodelay;
+	}
 }
 
 bool utcp_get_keepalive(struct utcp_connection *c) {
-	return c->keepalive;
+	return c ? c->keepalive : false;
 }
 
 void utcp_set_keepalive(struct utcp_connection *c, bool keepalive) {
-	c->keepalive = keepalive;
+	if(c) {
+		c->keepalive = keepalive;
+	}
 }
 
 size_t utcp_get_outq(struct utcp_connection *c) {
-	return seqdiff(c->snd.nxt, c->snd.una);
+	return c ? seqdiff(c->snd.nxt, c->snd.una) : 0;
 }
 
 void utcp_set_recv_cb(struct utcp_connection *c, utcp_recv_t recv) {
-	c->recv = recv;
+	if(c) {
+		c->recv = recv;
+	}
 }
 
 void utcp_set_poll_cb(struct utcp_connection *c, utcp_poll_t poll) {
-	c->poll = poll;
+	if(c) {
+		c->poll = poll;
+	}
+}
+
+void utcp_set_accept_cb(struct utcp *utcp, utcp_accept_t accept, utcp_pre_accept_t pre_accept) {
+	if(utcp) {
+		utcp->accept = accept;
+		utcp->pre_accept = pre_accept;
+	}
 }