X-Git-Url: http://git.meshlink.io/?p=utcp;a=blobdiff_plain;f=utcp.c;h=04d2b194c20f193aa44f2d4006b9afc1bd356b29;hp=2ed32175dc0f98c9b2bc045f7a62fb43caeb5089;hb=HEAD;hpb=7baf55a28cf48163c007ee2ff56678371d78c990

diff --git a/utcp.c b/utcp.c
index 2ed3217..04d2b19 100644
--- a/utcp.c
+++ b/utcp.c
@@ -27,8 +27,7 @@
 #include <stdbool.h>
 #include <string.h>
 #include <unistd.h>
-#include <sys/time.h>
-#include <sys/socket.h>
+#include <time.h>
 
 #include "utcp_priv.h"
 
@@ -44,16 +43,50 @@
 #undef poll
 #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 += USEC_PER_SEC;\
-	} while (0)
+#ifndef UTCP_CLOCK
+#if defined(CLOCK_MONOTONIC_RAW) && defined(__x86_64__)
+#define UTCP_CLOCK CLOCK_MONOTONIC_RAW
+#else
+#define UTCP_CLOCK CLOCK_MONOTONIC
+#endif
 #endif
 
+static void timespec_sub(const struct timespec *a, const struct timespec *b, struct timespec *r) {
+	r->tv_sec = a->tv_sec - b->tv_sec;
+	r->tv_nsec = a->tv_nsec - b->tv_nsec;
+
+	if(r->tv_nsec < 0) {
+		r->tv_sec--, r->tv_nsec += NSEC_PER_SEC;
+	}
+}
+
+static int32_t timespec_diff_usec(const struct timespec *a, const struct timespec *b) {
+	return (a->tv_sec - b->tv_sec) * 1000000 + (a->tv_nsec - b->tv_nsec) / 1000;
+}
+
+static bool timespec_lt(const struct timespec *a, const struct timespec *b) {
+	if(a->tv_sec == b->tv_sec) {
+		return a->tv_nsec < b->tv_nsec;
+	} else {
+		return a->tv_sec < b->tv_sec;
+	}
+}
+
+static void timespec_clear(struct timespec *a) {
+	a->tv_sec = 0;
+	a->tv_nsec = 0;
+}
+
+static bool timespec_isset(const struct timespec *a) {
+	return a->tv_sec;
+}
+
+static long CLOCK_GRANULARITY; // usec
+
+static inline size_t min(size_t a, size_t b) {
+	return a < b ? a : b;
+}
+
 static inline size_t max(size_t a, size_t b) {
 	return a > b ? a : b;
 }
@@ -61,71 +94,85 @@ static inline size_t max(size_t a, size_t b) {
 #ifdef UTCP_DEBUG
 #include <stdarg.h>
 
-static void debug(const char *format, ...) {
+#ifndef UTCP_DEBUG_DATALEN
+#define UTCP_DEBUG_DATALEN 20
+#endif
+
+static void debug(struct utcp_connection *c, const char *format, ...) {
+	struct timespec tv;
+	char buf[1024];
+	int len;
+
+	clock_gettime(CLOCK_REALTIME, &tv);
+	len = snprintf(buf, sizeof(buf), "%ld.%06lu %u:%u ", (long)tv.tv_sec, tv.tv_nsec / 1000, c ? c->src : 0, c ? c->dst : 0);
 	va_list ap;
 	va_start(ap, format);
-	vfprintf(stderr, format, ap);
+	len += vsnprintf(buf + len, sizeof(buf) - len, format, ap);
 	va_end(ap);
+
+	if(len > 0 && (size_t)len < sizeof(buf)) {
+		fwrite(buf, len, 1, stderr);
+	}
 }
 
-static void print_packet(struct utcp *utcp, const char *dir, const void *pkt, size_t len) {
+static void print_packet(struct utcp_connection *c, const char *dir, const void *pkt, size_t len) {
 	struct hdr hdr;
 
 	if(len < sizeof(hdr)) {
-		debug("%p %s: short packet (%lu bytes)\n", utcp, dir, (unsigned long)len);
+		debug(c, "%s: short packet (%lu bytes)\n", dir, (unsigned long)len);
 		return;
 	}
 
 	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) {
-		debug("RST");
-	}
+	uint32_t datalen;
 
-	if(hdr.ctl & FIN) {
-		debug("FIN");
+	if(len > sizeof(hdr)) {
+		datalen = min(len - sizeof(hdr), UTCP_DEBUG_DATALEN);
+	} else {
+		datalen = 0;
 	}
 
-	if(hdr.ctl & ACK) {
-		debug("ACK");
-	}
 
-	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;
+	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];
-		}
+	for(uint32_t i = 0; i < datalen; i++) {
+		*p++ = "0123456789ABCDEF"[data[i] >> 4];
+		*p++ = "0123456789ABCDEF"[data[i] & 15];
+	}
 
-		*p = 0;
+	*p = 0;
 
-		debug(" data=%s", str);
-	}
+	debug(c, "%s: len %lu src %u dst %u seq %u ack %u wnd %u aux %x ctl %s%s%s%s%s data %s\n",
+	      dir, (unsigned long)len, hdr.src, hdr.dst, hdr.seq, hdr.ack, hdr.wnd, hdr.aux,
+	      hdr.ctl & SYN ? "SYN" : "",
+	      hdr.ctl & RST ? "RST" : "",
+	      hdr.ctl & FIN ? "FIN" : "",
+	      hdr.ctl & ACK ? "ACK" : "",
+	      hdr.ctl & MF ? "MF" : "",
+	      str
+	     );
+}
 
-	debug("\n");
+static void debug_cwnd(struct utcp_connection *c) {
+	debug(c, "snd.cwnd %u snd.ssthresh %u\n", c->snd.cwnd, ~c->snd.ssthresh ? c->snd.ssthresh : 0);
 }
 #else
 #define debug(...) do {} while(0)
 #define print_packet(...) do {} while(0)
+#define debug_cwnd(...) do {} while(0)
 #endif
 
 static void set_state(struct utcp_connection *c, enum state state) {
 	c->state = state;
 
 	if(state == ESTABLISHED) {
-		timerclear(&c->conn_timeout);
+		timespec_clear(&c->conn_timeout);
 	}
 
-	debug("%p new state: %s\n", c->utcp, strstate[state]);
+	debug(c, "state %s\n", strstate[state]);
 }
 
 static bool fin_wanted(struct utcp_connection *c, uint32_t seq) {
@@ -153,12 +200,40 @@ static int32_t seqdiff(uint32_t a, uint32_t b) {
 }
 
 // Buffer functions
-// TODO: convert to ringbuffers to avoid memmove() operations.
+static bool buffer_wraps(struct buffer *buf) {
+	return buf->size - buf->offset < buf->used;
+}
+
+static bool buffer_resize(struct buffer *buf, uint32_t newsize) {
+	char *newdata = realloc(buf->data, newsize);
+
+	if(!newdata) {
+		return false;
+	}
+
+	buf->data = newdata;
+
+	if(buffer_wraps(buf)) {
+		// Shift the right part of the buffer until it hits the end of the new buffer.
+		// Old situation:
+		// [345......012]
+		// New situation:
+		// [345.........|........012]
+		uint32_t tailsize = buf->size - buf->offset;
+		uint32_t newoffset = newsize - tailsize;
+		memmove(buf->data + newoffset, buf->data + buf->offset, tailsize);
+		buf->offset = newoffset;
+	}
+
+	buf->size = newsize;
+	return true;
+}
 
 // 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);
+	debug(NULL, "buffer_put_at %lu %lu %lu\n", (unsigned long)buf->used, (unsigned long)offset, (unsigned long)len);
 
+	// Ensure we don't store more than maxsize bytes in total
 	size_t required = offset + len;
 
 	if(required > buf->maxsize) {
@@ -170,32 +245,41 @@ static ssize_t buffer_put_at(struct buffer *buf, size_t offset, const void *data
 		required = buf->maxsize;
 	}
 
+	// Check if we need to resize the buffer
 	if(required > buf->size) {
 		size_t newsize = buf->size;
 
 		if(!newsize) {
-			newsize = required;
-		} else {
-			do {
-				newsize *= 2;
-			} while(newsize < required);
+			newsize = 4096;
 		}
 
+		do {
+			newsize *= 2;
+		} while(newsize < required);
+
 		if(newsize > buf->maxsize) {
 			newsize = buf->maxsize;
 		}
 
-		char *newdata = realloc(buf->data, newsize);
-
-		if(!newdata) {
+		if(!buffer_resize(buf, newsize)) {
 			return -1;
 		}
+	}
 
-		buf->data = newdata;
-		buf->size = newsize;
+	uint32_t realoffset = buf->offset + offset;
+
+	if(buf->size - buf->offset <= offset) {
+		// The offset wrapped
+		realoffset -= buf->size;
 	}
 
-	memcpy(buf->data + offset, data, len);
+	if(buf->size - realoffset < len) {
+		// The new chunk of data must be wrapped
+		memcpy(buf->data + realoffset, data, buf->size - realoffset);
+		memcpy(buf->data, (char *)data + buf->size - realoffset, len - (buf->size - realoffset));
+	} else {
+		memcpy(buf->data + realoffset, data, len);
+	}
 
 	if(required > buf->used) {
 		buf->used = required;
@@ -208,52 +292,116 @@ 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;
+// Copy data from the buffer without removing it.
+static ssize_t buffer_copy(struct buffer *buf, void *data, size_t offset, size_t len) {
+	// Ensure we don't copy more than is actually stored in the buffer
+	if(offset >= buf->used) {
+		return 0;
+	}
+
+	if(buf->used - offset < len) {
+		len = buf->used - offset;
 	}
 
-	if(data) {
-		memcpy(data, buf->data, len);
+	uint32_t realoffset = buf->offset + offset;
+
+	if(buf->size - buf->offset <= offset) {
+		// The offset wrapped
+		realoffset -= buf->size;
 	}
 
-	if(len < buf->used) {
-		memmove(buf->data, buf->data + len, buf->used - len);
+	if(buf->size - realoffset < len) {
+		// The data is wrapped
+		memcpy(data, buf->data + realoffset, buf->size - realoffset);
+		memcpy((char *)data + buf->size - realoffset, buf->data, len - (buf->size - realoffset));
+	} else {
+		memcpy(data, buf->data + realoffset, 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) {
+static ssize_t buffer_call(struct utcp_connection *c, struct buffer *buf, size_t offset, size_t len) {
+	if(!c->recv) {
+		return len;
+	}
+
+	// Ensure we don't copy more than is actually stored in the buffer
 	if(offset >= buf->used) {
 		return 0;
 	}
 
-	if(offset + len > buf->used) {
+	if(buf->used - offset < len) {
 		len = buf->used - offset;
 	}
 
-	memcpy(data, buf->data + offset, len);
-	return len;
-}
+	uint32_t realoffset = buf->offset + offset;
 
-static bool buffer_init(struct buffer *buf, uint32_t len, uint32_t maxlen) {
-	memset(buf, 0, sizeof(*buf));
+	if(buf->size - buf->offset <= offset) {
+		// The offset wrapped
+		realoffset -= buf->size;
+	}
 
-	if(len) {
-		buf->data = malloc(len);
+	if(buf->size - realoffset < len) {
+		// The data is wrapped
+		ssize_t rx1 = c->recv(c, buf->data + realoffset, buf->size - realoffset);
 
-		if(!buf->data) {
-			return false;
+		if(rx1 < buf->size - realoffset) {
+			return rx1;
 		}
+
+		// The channel might have been closed by the previous callback
+		if(!c->recv) {
+			return len;
+		}
+
+		ssize_t rx2 = c->recv(c, buf->data, len - (buf->size - realoffset));
+
+		if(rx2 < 0) {
+			return rx2;
+		} else {
+			return rx1 + rx2;
+		}
+	} else {
+		return c->recv(c, buf->data + realoffset, len);
 	}
+}
 
-	buf->size = len;
-	buf->maxsize = maxlen;
-	return true;
+// Discard data from the buffer.
+static ssize_t buffer_discard(struct buffer *buf, size_t len) {
+	if(buf->used < len) {
+		len = buf->used;
+	}
+
+	if(buf->size - buf->offset <= len) {
+		buf->offset -= buf->size;
+	}
+
+	if(buf->used == len) {
+		buf->offset = 0;
+	} else {
+		buf->offset += len;
+	}
+
+	buf->used -= len;
+
+	return len;
+}
+
+static void buffer_clear(struct buffer *buf) {
+	buf->used = 0;
+	buf->offset = 0;
+}
+
+static bool buffer_set_size(struct buffer *buf, uint32_t minsize, uint32_t maxsize) {
+	if(maxsize < minsize) {
+		maxsize = minsize;
+	}
+
+	buf->maxsize = maxsize;
+
+	return buf->size >= minsize || buffer_resize(buf, minsize);
 }
 
 static void buffer_exit(struct buffer *buf) {
@@ -262,7 +410,7 @@ static void buffer_exit(struct buffer *buf) {
 }
 
 static uint32_t buffer_free(const struct buffer *buf) {
-	return buf->maxsize - buf->used;
+	return buf->maxsize > buf->used ? buf->maxsize - buf->used : 0;
 }
 
 // Connections are stored in a sorted list.
@@ -360,12 +508,12 @@ static struct utcp_connection *allocate_connection(struct utcp *utcp, uint16_t s
 		return NULL;
 	}
 
-	if(!buffer_init(&c->sndbuf, DEFAULT_SNDBUFSIZE, DEFAULT_MAXSNDBUFSIZE)) {
+	if(!buffer_set_size(&c->sndbuf, DEFAULT_SNDBUFSIZE, DEFAULT_MAXSNDBUFSIZE)) {
 		free(c);
 		return NULL;
 	}
 
-	if(!buffer_init(&c->rcvbuf, DEFAULT_RCVBUFSIZE, DEFAULT_MAXRCVBUFSIZE)) {
+	if(!buffer_set_size(&c->rcvbuf, DEFAULT_RCVBUFSIZE, DEFAULT_MAXRCVBUFSIZE)) {
 		buffer_exit(&c->sndbuf);
 		free(c);
 		return NULL;
@@ -382,9 +530,13 @@ static struct utcp_connection *allocate_connection(struct utcp *utcp, uint16_t s
 #endif
 	c->snd.una = c->snd.iss;
 	c->snd.nxt = c->snd.iss + 1;
-	c->rcv.wnd = utcp->mtu;
 	c->snd.last = c->snd.nxt;
-	c->snd.cwnd = utcp->mtu;
+	c->snd.cwnd = (utcp->mss > 2190 ? 2 : utcp->mss > 1095 ? 3 : 4) * utcp->mss;
+	c->snd.ssthresh = ~0;
+	debug_cwnd(c);
+	c->srtt = 0;
+	c->rttvar = 0;
+	c->rto = START_RTO;
 	c->utcp = utcp;
 
 	// Add it to the sorted list of connections
@@ -406,44 +558,50 @@ static inline uint32_t absdiff(uint32_t a, uint32_t b) {
 // Update RTT variables. See RFC 6298.
 static void update_rtt(struct utcp_connection *c, uint32_t rtt) {
 	if(!rtt) {
-		debug("invalid rtt\n");
+		debug(c, "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);
+	if(!c->srtt) {
+		c->srtt = rtt;
+		c->rttvar = rtt / 2;
 	} 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);
+		c->rttvar = (c->rttvar * 3 + absdiff(c->srtt, rtt)) / 4;
+		c->srtt = (c->srtt * 7 + rtt) / 8;
 	}
 
-	if(utcp->rto > MAX_RTO) {
-		utcp->rto = MAX_RTO;
+	c->rto = c->srtt + max(4 * c->rttvar, CLOCK_GRANULARITY);
+
+	if(c->rto > MAX_RTO) {
+		c->rto = MAX_RTO;
 	}
 
-	debug("rtt %u srtt %u rttvar %u rto %u\n", rtt, utcp->srtt, utcp->rttvar, utcp->rto);
+	debug(c, "rtt %u srtt %u rttvar %u rto %u\n", rtt, c->srtt, c->rttvar, c->rto);
 }
 
 static void start_retransmit_timer(struct utcp_connection *c) {
-	gettimeofday(&c->rtrx_timeout, NULL);
-	c->rtrx_timeout.tv_usec += c->utcp->rto;
+	clock_gettime(UTCP_CLOCK, &c->rtrx_timeout);
+
+	uint32_t rto = c->rto;
 
-	while(c->rtrx_timeout.tv_usec >= 1000000) {
-		c->rtrx_timeout.tv_usec -= 1000000;
+	while(rto > USEC_PER_SEC) {
 		c->rtrx_timeout.tv_sec++;
+		rto -= USEC_PER_SEC;
 	}
 
-	debug("timeout set to %lu.%06lu (%u)\n", c->rtrx_timeout.tv_sec, c->rtrx_timeout.tv_usec, c->utcp->rto);
+	c->rtrx_timeout.tv_nsec += rto * 1000;
+
+	if(c->rtrx_timeout.tv_nsec >= NSEC_PER_SEC) {
+		c->rtrx_timeout.tv_nsec -= NSEC_PER_SEC;
+		c->rtrx_timeout.tv_sec++;
+	}
+
+	debug(c, "rtrx_timeout %ld.%06lu\n", c->rtrx_timeout.tv_sec, c->rtrx_timeout.tv_nsec);
 }
 
 static void stop_retransmit_timer(struct utcp_connection *c) {
-	timerclear(&c->rtrx_timeout);
-	debug("timeout cleared\n");
+	timespec_clear(&c->rtrx_timeout);
+	debug(c, "rtrx_timeout cleared\n");
 }
 
 struct utcp_connection *utcp_connect_ex(struct utcp *utcp, uint16_t dst, utcp_recv_t recv, void *priv, uint32_t flags) {
@@ -468,7 +626,7 @@ struct utcp_connection *utcp_connect_ex(struct utcp *utcp, uint16_t dst, utcp_re
 	pkt.hdr.dst = c->dst;
 	pkt.hdr.seq = c->snd.iss;
 	pkt.hdr.ack = 0;
-	pkt.hdr.wnd = c->rcv.wnd;
+	pkt.hdr.wnd = c->rcvbuf.maxsize;
 	pkt.hdr.ctl = SYN;
 	pkt.hdr.aux = 0x0101;
 	pkt.init[0] = 1;
@@ -478,10 +636,10 @@ struct utcp_connection *utcp_connect_ex(struct utcp *utcp, uint16_t dst, utcp_re
 
 	set_state(c, SYN_SENT);
 
-	print_packet(utcp, "send", &pkt, sizeof(pkt));
+	print_packet(c, "send", &pkt, sizeof(pkt));
 	utcp->send(utcp, &pkt, sizeof(pkt));
 
-	gettimeofday(&c->conn_timeout, NULL);
+	clock_gettime(UTCP_CLOCK, &c->conn_timeout);
 	c->conn_timeout.tv_sec += utcp->timeout;
 
 	start_retransmit_timer(c);
@@ -495,11 +653,11 @@ struct utcp_connection *utcp_connect(struct utcp *utcp, uint16_t dst, utcp_recv_
 
 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]);
+		debug(c, "accept() called on invalid connection in state %s\n", c, strstate[c->state]);
 		return;
 	}
 
-	debug("%p accepted, %p %p\n", c, recv, priv);
+	debug(c, "accepted %p %p\n", c, recv, priv);
 	c->recv = recv;
 	c->priv = priv;
 	set_state(c, ESTABLISHED);
@@ -507,19 +665,22 @@ 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);
-	debug("cwndleft = %d\n", cwndleft);
+	int32_t cwndleft = is_reliable(c) ? min(c->snd.cwnd, c->snd.wnd) - seqdiff(c->snd.nxt, c->snd.una) : MAX_UNRELIABLE_SIZE;
 
 	assert(left >= 0);
 
 	if(cwndleft <= 0) {
-		cwndleft = 0;
-	}
-
-	if(cwndleft < left) {
+		left = 0;
+	} else if(cwndleft < left) {
 		left = cwndleft;
+
+		if(!sendatleastone || cwndleft > c->utcp->mss) {
+			left -= left % c->utcp->mss;
+		}
 	}
 
+	debug(c, "cwndleft %d left %d\n", cwndleft, left);
+
 	if(!left && !sendatleastone) {
 		return;
 	}
@@ -527,23 +688,17 @@ static void ack(struct utcp_connection *c, bool sendatleastone) {
 	struct {
 		struct hdr hdr;
 		uint8_t data[];
-	} *pkt;
-
-	pkt = malloc(sizeof(pkt->hdr) + c->utcp->mtu);
-
-	if(!pkt) {
-		return;
-	}
+	} *pkt = c->utcp->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.wnd = is_reliable(c) ? c->rcvbuf.maxsize : 0;
 	pkt->hdr.ctl = ACK;
 	pkt->hdr.aux = 0;
 
 	do {
-		uint32_t seglen = left > c->utcp->mtu ? c->utcp->mtu : left;
+		uint32_t seglen = left > c->utcp->mss ? c->utcp->mss : left;
 		pkt->hdr.seq = c->snd.nxt;
 
 		buffer_copy(&c->sndbuf, pkt->data, seqdiff(c->snd.nxt, c->snd.una), seglen);
@@ -551,6 +706,14 @@ static void ack(struct utcp_connection *c, bool sendatleastone) {
 		c->snd.nxt += seglen;
 		left -= seglen;
 
+		if(!is_reliable(c)) {
+			if(left) {
+				pkt->hdr.ctl |= MF;
+			} else {
+				pkt->hdr.ctl &= ~MF;
+			}
+		}
+
 		if(seglen && fin_wanted(c, c->snd.nxt)) {
 			seglen--;
 			pkt->hdr.ctl |= FIN;
@@ -558,21 +721,23 @@ static void ack(struct utcp_connection *c, bool sendatleastone) {
 
 		if(!c->rtt_start.tv_sec) {
 			// Start RTT measurement
-			gettimeofday(&c->rtt_start, NULL);
+			clock_gettime(UTCP_CLOCK, &c->rtt_start);
 			c->rtt_seq = pkt->hdr.seq + seglen;
-			debug("Starting RTT measurement, expecting ack %u\n", c->rtt_seq);
+			debug(c, "starting RTT measurement, expecting ack %u\n", c->rtt_seq);
 		}
 
-		print_packet(c->utcp, "send", pkt, sizeof(pkt->hdr) + seglen);
+		print_packet(c, "send", pkt, sizeof(pkt->hdr) + seglen);
 		c->utcp->send(c->utcp, pkt, sizeof(pkt->hdr) + seglen);
-	} while(left);
 
-	free(pkt);
+		if(left && !is_reliable(c)) {
+			pkt->hdr.wnd += seglen;
+		}
+	} while(left);
 }
 
 ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
 	if(c->reapable) {
-		debug("Error: send() called on closed connection %p\n", c);
+		debug(c, "send() called on closed connection\n");
 		errno = EBADF;
 		return -1;
 	}
@@ -580,7 +745,7 @@ ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
 	switch(c->state) {
 	case CLOSED:
 	case LISTEN:
-		debug("Error: send() called on unconnected connection %p\n", c);
+		debug(c, "send() called on unconnected connection\n");
 		errno = ENOTCONN;
 		return -1;
 
@@ -595,7 +760,7 @@ ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
 	case CLOSING:
 	case LAST_ACK:
 	case TIME_WAIT:
-		debug("Error: send() called on closing connection %p\n", c);
+		debug(c, "send() called on closed connection\n");
 		errno = EPIPE;
 		return -1;
 	}
@@ -627,8 +792,15 @@ ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
 
 	// Add data to send buffer.
 
-	if(is_reliable(c) || (c->state != SYN_SENT && c->state != SYN_RECEIVED)) {
+	if(is_reliable(c)) {
 		len = buffer_put(&c->sndbuf, data, len);
+	} else if(c->state != SYN_SENT && c->state != SYN_RECEIVED) {
+		if(len > MAX_UNRELIABLE_SIZE || buffer_put(&c->sndbuf, data, len) != (ssize_t)len) {
+			errno = EMSGSIZE;
+			return -1;
+		}
+	} else {
+		return 0;
 	}
 
 	if(len <= 0) {
@@ -652,15 +824,15 @@ ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
 
 	if(!is_reliable(c)) {
 		c->snd.una = c->snd.nxt = c->snd.last;
-		buffer_get(&c->sndbuf, NULL, c->sndbuf.used);
+		buffer_discard(&c->sndbuf, c->sndbuf.used);
 	}
 
-	if(is_reliable(c) && !timerisset(&c->rtrx_timeout)) {
+	if(is_reliable(c) && !timespec_isset(&c->rtrx_timeout)) {
 		start_retransmit_timer(c);
 	}
 
-	if(is_reliable(c) && !timerisset(&c->conn_timeout)) {
-		gettimeofday(&c->conn_timeout, NULL);
+	if(is_reliable(c) && !timespec_isset(&c->conn_timeout)) {
+		clock_gettime(UTCP_CLOCK, &c->conn_timeout);
 		c->conn_timeout.tv_sec += c->utcp->timeout;
 	}
 
@@ -673,10 +845,9 @@ static void swap_ports(struct hdr *hdr) {
 	hdr->dst = tmp;
 }
 
-static void retransmit(struct utcp_connection *c) {
+static void fast_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);
+		debug(c, "fast_retransmit() called but nothing to retransmit!\n");
 		return;
 	}
 
@@ -685,17 +856,61 @@ static void retransmit(struct utcp_connection *c) {
 	struct {
 		struct hdr hdr;
 		uint8_t data[];
-	} *pkt;
+	} *pkt = c->utcp->pkt;
 
-	pkt = malloc(sizeof(pkt->hdr) + c->utcp->mtu);
+	pkt->hdr.src = c->src;
+	pkt->hdr.dst = c->dst;
+	pkt->hdr.wnd = c->rcvbuf.maxsize;
+	pkt->hdr.aux = 0;
+
+	switch(c->state) {
+	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 = min(seqdiff(c->snd.last, c->snd.una), utcp->mss);
 
-	if(!pkt) {
+		if(fin_wanted(c, c->snd.una + len)) {
+			len--;
+			pkt->hdr.ctl |= FIN;
+		}
+
+		buffer_copy(&c->sndbuf, pkt->data, 0, len);
+		print_packet(c, "rtrx", pkt, sizeof(pkt->hdr) + len);
+		utcp->send(utcp, pkt, sizeof(pkt->hdr) + len);
+		break;
+
+	default:
+		break;
+	}
+}
+
+static void retransmit(struct utcp_connection *c) {
+	if(c->state == CLOSED || c->snd.last == c->snd.una) {
+		debug(c, "retransmit() called but nothing to retransmit!\n");
+		stop_retransmit_timer(c);
 		return;
 	}
 
+	struct utcp *utcp = c->utcp;
+
+	if(utcp->retransmit) {
+		utcp->retransmit(c);
+	}
+
+	struct {
+		struct hdr hdr;
+		uint8_t data[];
+	} *pkt = c->utcp->pkt;
+
 	pkt->hdr.src = c->src;
 	pkt->hdr.dst = c->dst;
-	pkt->hdr.wnd = c->rcv.wnd;
+	pkt->hdr.wnd = c->rcvbuf.maxsize;
 	pkt->hdr.aux = 0;
 
 	switch(c->state) {
@@ -709,7 +924,7 @@ static void retransmit(struct utcp_connection *c) {
 		pkt->data[1] = 0;
 		pkt->data[2] = 0;
 		pkt->data[3] = c->flags & 0x7;
-		print_packet(c->utcp, "rtrx", pkt, sizeof(pkt->hdr) + 4);
+		print_packet(c, "rtrx", pkt, sizeof(pkt->hdr) + 4);
 		utcp->send(utcp, pkt, sizeof(pkt->hdr) + 4);
 		break;
 
@@ -718,7 +933,7 @@ static void retransmit(struct utcp_connection *c) {
 		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));
+		print_packet(c, "rtrx", pkt, sizeof(pkt->hdr));
 		utcp->send(utcp, pkt, sizeof(pkt->hdr));
 		break;
 
@@ -731,22 +946,24 @@ static void retransmit(struct utcp_connection *c) {
 		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;
-		}
+		uint32_t len = min(seqdiff(c->snd.last, c->snd.una), utcp->mss);
 
 		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
+		// RFC 5681 slow start after timeout
+		uint32_t flightsize = seqdiff(c->snd.nxt, c->snd.una);
+		c->snd.ssthresh = max(flightsize / 2, utcp->mss * 2); // eq. 4
+		c->snd.cwnd = utcp->mss;
+		debug_cwnd(c);
+
 		buffer_copy(&c->sndbuf, pkt->data, 0, len);
-		print_packet(c->utcp, "rtrx", pkt, sizeof(pkt->hdr) + len);
+		print_packet(c, "rtrx", pkt, sizeof(pkt->hdr) + len);
 		utcp->send(utcp, pkt, sizeof(pkt->hdr) + len);
+
+		c->snd.nxt = c->snd.una + len;
 		break;
 
 	case CLOSED:
@@ -762,16 +979,17 @@ static void retransmit(struct utcp_connection *c) {
 	}
 
 	start_retransmit_timer(c);
-	utcp->rto *= 2;
+	c->rto *= 2;
 
-	if(utcp->rto > MAX_RTO) {
-		utcp->rto = MAX_RTO;
+	if(c->rto > MAX_RTO) {
+		c->rto = MAX_RTO;
 	}
 
 	c->rtt_start.tv_sec = 0; // invalidate RTT timer
+	c->dupack = 0; // cancel any ongoing fast recovery
 
 cleanup:
-	free(pkt);
+	return;
 }
 
 /* Update receive buffer and SACK entries after consuming data.
@@ -793,15 +1011,15 @@ cleanup:
  * - 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);
+	debug(c, "sack_consume %lu\n", (unsigned long)len);
 
 	if(len > c->rcvbuf.used) {
-		debug("All SACK entries consumed");
+		debug(c, "all SACK entries consumed\n");
 		c->sacks[0].len = 0;
 		return;
 	}
 
-	buffer_get(&c->rcvbuf, NULL, len);
+	buffer_discard(&c->rcvbuf, len);
 
 	for(int i = 0; i < NSACKS && c->sacks[i].len;) {
 		if(len < c->sacks[i].offset) {
@@ -823,46 +1041,52 @@ static void sack_consume(struct utcp_connection *c, size_t len) {
 	}
 
 	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);
+		debug(c, "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);
+	debug(c, "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();
+	if(rxd <= 0) {
+		debug(c, "packet outside receive buffer, dropping\n");
+		return;
+	}
+
+	if((size_t)rxd < len) {
+		debug(c, "packet partially outside receive buffer\n");
+		len = rxd;
 	}
 
 	// 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);
+			debug(c, "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);
+					debug(c, "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");
+					debug(c, "SACK entries full, dropping packet\n");
 				}
 
 				break;
 			} else { // merge
-				debug("Merge with start of SACK entry at %d\n", i);
+				debug(c, "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);
+				debug(c, "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
 			}
@@ -872,28 +1096,38 @@ static void handle_out_of_order(struct utcp_connection *c, uint32_t offset, cons
 	}
 
 	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);
+		debug(c, "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) {
+		if(rxd != (ssize_t)len) {
 			// TODO: handle the application not accepting all data.
 			abort();
 		}
 	}
 
+	// Check if we can process out-of-order data now.
+	if(c->sacks[0].len && len >= c->sacks[0].offset) {
+		debug(c, "incoming packet len %lu connected with SACK at %u\n", (unsigned long)len, c->sacks[0].offset);
+
+		if(len < c->sacks[0].offset + c->sacks[0].len) {
+			size_t offset = len;
+			len = c->sacks[0].offset + c->sacks[0].len;
+			size_t remainder = len - offset;
+
+			ssize_t rxd = buffer_call(c, &c->rcvbuf, offset, remainder);
+
+			if(rxd != (ssize_t)remainder) {
+				// TODO: handle the application not accepting all data.
+				abort();
+			}
+		}
+	}
+
 	if(c->rcvbuf.used) {
 		sack_consume(c, len);
 	}
@@ -901,20 +1135,54 @@ static void handle_in_order(struct utcp_connection *c, const void *data, size_t
 	c->rcv.nxt += len;
 }
 
+static void handle_unreliable(struct utcp_connection *c, const struct hdr *hdr, const void *data, size_t len) {
+	// Fast path for unfragmented packets
+	if(!hdr->wnd && !(hdr->ctl & MF)) {
+		if(c->recv) {
+			c->recv(c, data, 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;
+		c->rcv.nxt = hdr->seq + len;
 		return;
 	}
 
-	uint32_t offset = seqdiff(seq, c->rcv.nxt);
+	// Ensure reassembled packet are not larger than 64 kiB
+	if(hdr->wnd >= MAX_UNRELIABLE_SIZE || hdr->wnd + len > MAX_UNRELIABLE_SIZE) {
+		return;
+	}
 
-	if(offset + len > c->rcvbuf.maxsize) {
-		abort();
+	// Don't accept out of order fragments
+	if(hdr->wnd && hdr->seq != c->rcv.nxt) {
+		return;
 	}
 
+	// Reset the receive buffer for the first fragment
+	if(!hdr->wnd) {
+		buffer_clear(&c->rcvbuf);
+	}
+
+	ssize_t rxd = buffer_put_at(&c->rcvbuf, hdr->wnd, data, len);
+
+	if(rxd != (ssize_t)len) {
+		return;
+	}
+
+	// Send the packet if it's the final fragment
+	if(!(hdr->ctl & MF)) {
+		buffer_call(c, &c->rcvbuf, 0, hdr->wnd + len);
+	}
+
+	c->rcv.nxt = hdr->seq + len;
+}
+
+static void handle_incoming_data(struct utcp_connection *c, const struct hdr *hdr, const void *data, size_t len) {
+	if(!is_reliable(c)) {
+		handle_unreliable(c, hdr, data, len);
+		return;
+	}
+
+	uint32_t offset = seqdiff(hdr->seq, c->rcv.nxt);
+
 	if(offset) {
 		handle_out_of_order(c, offset, data, len);
 	} else {
@@ -940,13 +1208,12 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 		return -1;
 	}
 
-	print_packet(utcp, "recv", data, len);
-
 	// Drop packets smaller than the header
 
 	struct hdr hdr;
 
 	if(len < sizeof(hdr)) {
+		print_packet(NULL, "recv", data, len);
 		errno = EBADMSG;
 		return -1;
 	}
@@ -954,12 +1221,21 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	// Make a copy from the potentially unaligned data to a struct hdr
 
 	memcpy(&hdr, ptr, sizeof(hdr));
+
+	// Try to match the packet to an existing connection
+
+	struct utcp_connection *c = find_connection(utcp, hdr.dst, hdr.src);
+	print_packet(c, "recv", data, len);
+
+	// Process the header
+
 	ptr += sizeof(hdr);
 	len -= sizeof(hdr);
 
 	// Drop packets with an unknown CTL flag
 
-	if(hdr.ctl & ~(SYN | ACK | RST | FIN)) {
+	if(hdr.ctl & ~(SYN | ACK | RST | FIN | MF)) {
+		print_packet(NULL, "recv", data, len);
 		errno = EBADMSG;
 		return -1;
 	}
@@ -1013,10 +1289,6 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 
 	bool has_data = len || (hdr.ctl & (SYN | FIN));
 
-	// Try to match the packet to an existing connection
-
-	struct utcp_connection *c = find_connection(utcp, hdr.dst, hdr.src);
-
 	// Is it for a new connection?
 
 	if(!c) {
@@ -1055,6 +1327,7 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 				c->flags = UTCP_TCP;
 			}
 
+synack:
 			// Return SYN+ACK, go to SYN_RECEIVED state
 			c->snd.wnd = hdr.wnd;
 			c->rcv.irs = hdr.seq;
@@ -1070,7 +1343,7 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 			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.wnd = c->rcvbuf.maxsize;
 			pkt.hdr.ctl = SYN | ACK;
 
 			if(init) {
@@ -1079,13 +1352,15 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 				pkt.data[1] = 0;
 				pkt.data[2] = 0;
 				pkt.data[3] = c->flags & 0x7;
-				print_packet(c->utcp, "send", &pkt, sizeof(hdr) + 4);
+				print_packet(c, "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));
+				print_packet(c, "send", &pkt, sizeof(hdr));
 				utcp->send(utcp, &pkt, sizeof(hdr));
 			}
+
+			start_retransmit_timer(c);
 		} else {
 			// No, we don't want your packets, send a RST back
 			len = 1;
@@ -1095,13 +1370,13 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 		return 0;
 	}
 
-	debug("%p state %s\n", c->utcp, strstate[c->state]);
+	debug(c, "state %s\n", strstate[c->state]);
 
 	// 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) {
-		debug("Got packet for closed connection\n");
+		debug(c, "got packet for closed connection\n");
 		return 0;
 	}
 
@@ -1157,7 +1432,7 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 		}
 
 		if(!acceptable) {
-			debug("Packet not acceptable, %u <= %u + %lu < %u\n", c->rcv.nxt, hdr.seq, (unsigned long)len, c->rcv.nxt + c->rcvbuf.maxsize);
+			debug(c, "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) {
@@ -1167,6 +1442,15 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 			// Otherwise, continue processing.
 			len = 0;
 		}
+	} else {
+#if UTCP_DEBUG
+		int32_t rcv_offset = seqdiff(hdr.seq, c->rcv.nxt);
+
+		if(rcv_offset) {
+			debug(c, "packet out of order, offset %u bytes", rcv_offset);
+		}
+
+#endif
 	}
 
 	c->snd.wnd = hdr.wnd; // TODO: move below
@@ -1182,7 +1466,7 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	}
 
 	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);
+		debug(c, "packet ack seqno out of range, %u <= %u < %u\n", c->snd.una, hdr.ack, c->snd.una + c->sndbuf.used);
 
 		// Ignore unacceptable RST packets.
 		if(hdr.ctl & RST) {
@@ -1287,13 +1571,13 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 		// 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);
+				struct timespec now;
+				clock_gettime(UTCP_CLOCK, &now);
+				int32_t diff = timespec_diff_usec(&now, &c->rtt_start);
+				update_rtt(c, diff);
 				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);
+				debug(c, "cancelling RTT measurement: %u < %u\n", c->rtt_seq, hdr.ack);
 				c->rtt_start.tv_sec = 0;
 			}
 		}
@@ -1313,11 +1597,17 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 
 		assert(data_acked >= 0);
 
+#ifndef NDEBUG
 		int32_t bufused = seqdiff(c->snd.last, c->snd.una);
 		assert(data_acked <= bufused);
+#endif
 
 		if(data_acked) {
-			buffer_get(&c->sndbuf, NULL, data_acked);
+			buffer_discard(&c->sndbuf, data_acked);
+
+			if(is_reliable(c)) {
+				c->do_poll = true;
+			}
 		}
 
 		// Also advance snd.nxt if possible
@@ -1327,13 +1617,28 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 
 		c->snd.una = hdr.ack;
 
-		c->dupack = 0;
-		c->snd.cwnd += utcp->mtu;
+		if(c->dupack) {
+			if(c->dupack >= 3) {
+				debug(c, "fast recovery ended\n");
+				c->snd.cwnd = c->snd.ssthresh;
+			}
+
+			c->dupack = 0;
+		}
+
+		// Increase the congestion window according to RFC 5681
+		if(c->snd.cwnd < c->snd.ssthresh) {
+			c->snd.cwnd += min(advanced, utcp->mss); // eq. 2
+		} else {
+			c->snd.cwnd += max(1, (utcp->mss * utcp->mss) / c->snd.cwnd); // eq. 3
+		}
 
 		if(c->snd.cwnd > c->sndbuf.maxsize) {
 			c->snd.cwnd = c->sndbuf.maxsize;
 		}
 
+		debug_cwnd(c);
+
 		// Check if we have sent a FIN that is now ACKed.
 		switch(c->state) {
 		case FIN_WAIT_1:
@@ -1345,8 +1650,8 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 
 		case CLOSING:
 			if(c->snd.una == c->snd.last) {
-				gettimeofday(&c->conn_timeout, NULL);
-				c->conn_timeout.tv_sec += 60;
+				clock_gettime(UTCP_CLOCK, &c->conn_timeout);
+				c->conn_timeout.tv_sec += utcp->timeout;
 				set_state(c, TIME_WAIT);
 			}
 
@@ -1356,18 +1661,38 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 			break;
 		}
 	} else {
-		if(!len && is_reliable(c)) {
+		if(!len && is_reliable(c) && c->snd.una != c->snd.last) {
 			c->dupack++;
+			debug(c, "duplicate ACK %d\n", c->dupack);
 
 			if(c->dupack == 3) {
-				debug("Triplicate ACK\n");
-				//TODO: Resend one packet and go to fast recovery mode. See RFC 6582.
-				//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);
+				// RFC 5681 fast recovery
+				debug(c, "fast recovery started\n", c->dupack);
+				uint32_t flightsize = seqdiff(c->snd.nxt, c->snd.una);
+				c->snd.ssthresh = max(flightsize / 2, utcp->mss * 2); // eq. 4
+				c->snd.cwnd = min(c->snd.ssthresh + 3 * utcp->mss, c->sndbuf.maxsize);
+
+				if(c->snd.cwnd > c->sndbuf.maxsize) {
+					c->snd.cwnd = c->sndbuf.maxsize;
+				}
+
+				debug_cwnd(c);
+
+				fast_retransmit(c);
+			} else if(c->dupack > 3) {
+				c->snd.cwnd += utcp->mss;
+
+				if(c->snd.cwnd > c->sndbuf.maxsize) {
+					c->snd.cwnd = c->sndbuf.maxsize;
+				}
+
+				debug_cwnd(c);
 			}
+
+			// We got an ACK which indicates the other side did get one of our packets.
+			// Reset the retransmission timer to avoid going to slow start,
+			// but don't touch the connection timeout.
+			start_retransmit_timer(c);
 		}
 	}
 
@@ -1376,10 +1701,10 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	if(advanced) {
 		if(c->snd.una == c->snd.last) {
 			stop_retransmit_timer(c);
-			timerclear(&c->conn_timeout);
+			timespec_clear(&c->conn_timeout);
 		} else if(is_reliable(c)) {
 			start_retransmit_timer(c);
-			gettimeofday(&c->conn_timeout, NULL);
+			clock_gettime(UTCP_CLOCK, &c->conn_timeout);
 			c->conn_timeout.tv_sec += utcp->timeout;
 		}
 	}
@@ -1397,7 +1722,7 @@ skip_ack:
 			}
 
 			c->rcv.irs = hdr.seq;
-			c->rcv.nxt = hdr.seq;
+			c->rcv.nxt = hdr.seq + 1;
 
 			if(c->shut_wr) {
 				c->snd.last++;
@@ -1406,10 +1731,12 @@ skip_ack:
 				set_state(c, ESTABLISHED);
 			}
 
-			// TODO: notify application of this somehow.
 			break;
 
 		case SYN_RECEIVED:
+			// This is a retransmit of a SYN, send back the SYNACK.
+			goto synack;
+
 		case ESTABLISHED:
 		case FIN_WAIT_1:
 		case FIN_WAIT_2:
@@ -1417,8 +1744,8 @@ skip_ack:
 		case CLOSING:
 		case LAST_ACK:
 		case TIME_WAIT:
-			// Ehm, no. We should never receive a second SYN.
-			return 0;
+			// This could be a retransmission. Ignore the SYN flag, but send an ACK back.
+			break;
 
 		default:
 #ifdef UTCP_DEBUG
@@ -1426,9 +1753,6 @@ skip_ack:
 #endif
 			return 0;
 		}
-
-		// SYN counts as one sequence number
-		c->rcv.nxt++;
 	}
 
 	// 6. Process new data
@@ -1480,7 +1804,7 @@ skip_ack:
 			return 0;
 		}
 
-		handle_incoming_data(c, hdr.seq, ptr, len);
+		handle_incoming_data(c, &hdr, ptr, len);
 	}
 
 	// 7. Process FIN stuff
@@ -1504,8 +1828,8 @@ skip_ack:
 			break;
 
 		case FIN_WAIT_2:
-			gettimeofday(&c->conn_timeout, NULL);
-			c->conn_timeout.tv_sec += 60;
+			clock_gettime(UTCP_CLOCK, &c->conn_timeout);
+			c->conn_timeout.tv_sec += utcp->timeout;
 			set_state(c, TIME_WAIT);
 			break;
 
@@ -1560,14 +1884,14 @@ reset:
 		hdr.ctl = RST | ACK;
 	}
 
-	print_packet(utcp, "send", &hdr, sizeof(hdr));
+	print_packet(c, "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 at %u\n", c ? c->utcp : NULL, dir, c ? c->snd.last : 0);
+	debug(c, "shutdown %d at %u\n", dir, c ? c->snd.last : 0);
 
 	if(!c) {
 		errno = EFAULT;
@@ -1575,7 +1899,7 @@ int utcp_shutdown(struct utcp_connection *c, int dir) {
 	}
 
 	if(c->reapable) {
-		debug("Error: shutdown() called on closed connection %p\n", c);
+		debug(c, "shutdown() called on closed connection\n");
 		errno = EBADF;
 		return -1;
 	}
@@ -1635,7 +1959,7 @@ int utcp_shutdown(struct utcp_connection *c, int dir) {
 
 	ack(c, false);
 
-	if(!timerisset(&c->rtrx_timeout)) {
+	if(!timespec_isset(&c->rtrx_timeout)) {
 		start_retransmit_timer(c);
 	}
 
@@ -1649,7 +1973,7 @@ static bool reset_connection(struct utcp_connection *c) {
 	}
 
 	if(c->reapable) {
-		debug("Error: abort() called on closed connection %p\n", c);
+		debug(c, "abort() called on closed connection\n");
 		errno = EBADF;
 		return false;
 	}
@@ -1689,7 +2013,7 @@ static bool reset_connection(struct utcp_connection *c) {
 	hdr.wnd = 0;
 	hdr.ctl = RST;
 
-	print_packet(c->utcp, "send", &hdr, sizeof(hdr));
+	print_packet(c, "send", &hdr, sizeof(hdr));
 	c->utcp->send(c->utcp, &hdr, sizeof(hdr));
 	return true;
 }
@@ -1753,10 +2077,10 @@ int utcp_abort(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.
  */
-struct timeval utcp_timeout(struct utcp *utcp) {
-	struct timeval now;
-	gettimeofday(&now, NULL);
-	struct timeval next = {now.tv_sec + 3600, now.tv_usec};
+struct timespec utcp_timeout(struct utcp *utcp) {
+	struct timespec now;
+	clock_gettime(UTCP_CLOCK, &now);
+	struct timespec next = {now.tv_sec + 3600, now.tv_nsec};
 
 	for(int i = 0; i < utcp->nconnections; i++) {
 		struct utcp_connection *c = utcp->connections[i];
@@ -1768,7 +2092,7 @@ struct timeval utcp_timeout(struct utcp *utcp) {
 		// delete connections that have been utcp_close()d.
 		if(c->state == CLOSED) {
 			if(c->reapable) {
-				debug("Reaping %p\n", c);
+				debug(c, "reaping\n");
 				free_connection(c);
 				i--;
 			}
@@ -1776,7 +2100,7 @@ struct timeval utcp_timeout(struct utcp *utcp) {
 			continue;
 		}
 
-		if(timerisset(&c->conn_timeout) && timercmp(&c->conn_timeout, &now, <)) {
+		if(timespec_isset(&c->conn_timeout) && timespec_lt(&c->conn_timeout, &now)) {
 			errno = ETIMEDOUT;
 			c->state = CLOSED;
 
@@ -1791,14 +2115,15 @@ struct timeval utcp_timeout(struct utcp *utcp) {
 			continue;
 		}
 
-		if(timerisset(&c->rtrx_timeout) && timercmp(&c->rtrx_timeout, &now, <)) {
-			debug("retransmit()\n");
+		if(timespec_isset(&c->rtrx_timeout) && timespec_lt(&c->rtrx_timeout, &now)) {
+			debug(c, "retransmitting after timeout\n");
 			retransmit(c);
 		}
 
 		if(c->poll) {
-			if((c->state == ESTABLISHED || c->state == CLOSE_WAIT)) {
-				uint32_t len =  buffer_free(&c->sndbuf);
+			if((c->state == ESTABLISHED || c->state == CLOSE_WAIT) && c->do_poll) {
+				c->do_poll = false;
+				uint32_t len = buffer_free(&c->sndbuf);
 
 				if(len) {
 					c->poll(c, len);
@@ -1808,18 +2133,18 @@ struct timeval utcp_timeout(struct utcp *utcp) {
 			}
 		}
 
-		if(timerisset(&c->conn_timeout) && timercmp(&c->conn_timeout, &next, <)) {
+		if(timespec_isset(&c->conn_timeout) && timespec_lt(&c->conn_timeout, &next)) {
 			next = c->conn_timeout;
 		}
 
-		if(timerisset(&c->rtrx_timeout) && timercmp(&c->rtrx_timeout, &next, <)) {
+		if(timespec_isset(&c->rtrx_timeout) && timespec_lt(&c->rtrx_timeout, &next)) {
 			next = c->rtrx_timeout;
 		}
 	}
 
-	struct timeval diff;
+	struct timespec diff;
 
-	timersub(&next, &now, &diff);
+	timespec_sub(&next, &now, &diff);
 
 	return diff;
 }
@@ -1849,13 +2174,24 @@ struct utcp *utcp_init(utcp_accept_t accept, utcp_pre_accept_t pre_accept, utcp_
 		return NULL;
 	}
 
+	utcp_set_mtu(utcp, DEFAULT_MTU);
+
+	if(!utcp->pkt) {
+		free(utcp);
+		return NULL;
+	}
+
+	if(!CLOCK_GRANULARITY) {
+		struct timespec res;
+		clock_getres(UTCP_CLOCK, &res);
+		CLOCK_GRANULARITY = res.tv_sec * USEC_PER_SEC + res.tv_nsec / 1000;
+	}
+
 	utcp->accept = accept;
 	utcp->pre_accept = pre_accept;
 	utcp->send = send;
 	utcp->priv = priv;
-	utcp->mtu = DEFAULT_MTU;
 	utcp->timeout = DEFAULT_USER_TIMEOUT; // sec
-	utcp->rto = START_RTO; // usec
 
 	return utcp;
 }
@@ -1884,6 +2220,7 @@ void utcp_exit(struct utcp *utcp) {
 	}
 
 	free(utcp->connections);
+	free(utcp->pkt);
 	free(utcp);
 }
 
@@ -1891,11 +2228,31 @@ uint16_t utcp_get_mtu(struct utcp *utcp) {
 	return utcp ? utcp->mtu : 0;
 }
 
+uint16_t utcp_get_mss(struct utcp *utcp) {
+	return utcp ? utcp->mss : 0;
+}
+
 void utcp_set_mtu(struct utcp *utcp, uint16_t mtu) {
-	// TODO: handle overhead of the header
-	if(utcp) {
-		utcp->mtu = mtu;
+	if(!utcp) {
+		return;
+	}
+
+	if(mtu <= sizeof(struct hdr)) {
+		return;
 	}
+
+	if(mtu > utcp->mtu) {
+		char *new = realloc(utcp->pkt, mtu + sizeof(struct hdr));
+
+		if(!new) {
+			return;
+		}
+
+		utcp->pkt = new;
+	}
+
+	utcp->mtu = mtu;
+	utcp->mss = mtu - sizeof(struct hdr);
 }
 
 void utcp_reset_timers(struct utcp *utcp) {
@@ -1903,9 +2260,9 @@ void utcp_reset_timers(struct utcp *utcp) {
 		return;
 	}
 
-	struct timeval now, then;
+	struct timespec now, then;
 
-	gettimeofday(&now, NULL);
+	clock_gettime(UTCP_CLOCK, &now);
 
 	then = now;
 
@@ -1918,13 +2275,19 @@ void utcp_reset_timers(struct utcp *utcp) {
 			continue;
 		}
 
-		c->rtrx_timeout = now;
-		c->conn_timeout = then;
+		if(timespec_isset(&c->rtrx_timeout)) {
+			c->rtrx_timeout = now;
+		}
+
+		if(timespec_isset(&c->conn_timeout)) {
+			c->conn_timeout = then;
+		}
+
 		c->rtt_start.tv_sec = 0;
-	}
 
-	if(utcp->rto > START_RTO) {
-		utcp->rto = START_RTO;
+		if(c->rto > START_RTO) {
+			c->rto = START_RTO;
+		}
 	}
 }
 
@@ -1969,6 +2332,8 @@ void utcp_set_sndbuf(struct utcp_connection *c, size_t size) {
 	if(c->sndbuf.maxsize != size) {
 		c->sndbuf.maxsize = -1;
 	}
+
+	c->do_poll = is_reliable(c) && buffer_free(&c->sndbuf);
 }
 
 size_t utcp_get_rcvbuf(struct utcp_connection *c) {
@@ -2036,6 +2401,7 @@ void utcp_set_recv_cb(struct utcp_connection *c, utcp_recv_t recv) {
 void utcp_set_poll_cb(struct utcp_connection *c, utcp_poll_t poll) {
 	if(c) {
 		c->poll = poll;
+		c->do_poll = is_reliable(c) && buffer_free(&c->sndbuf);
 	}
 }
 
@@ -2057,36 +2423,49 @@ void utcp_expect_data(struct utcp_connection *c, bool expect) {
 
 	if(expect) {
 		// If we expect data, start the connection timer.
-		if(!timerisset(&c->conn_timeout)) {
-			gettimeofday(&c->conn_timeout, NULL);
+		if(!timespec_isset(&c->conn_timeout)) {
+			clock_gettime(UTCP_CLOCK, &c->conn_timeout);
 			c->conn_timeout.tv_sec += c->utcp->timeout;
 		}
 	} else {
 		// If we want to cancel expecting data, only clear the timer when there is no unACKed data.
 		if(c->snd.una == c->snd.last) {
-			timerclear(&c->conn_timeout);
+			timespec_clear(&c->conn_timeout);
 		}
 	}
 }
 
 void utcp_offline(struct utcp *utcp, bool offline) {
+	struct timespec now;
+	clock_gettime(UTCP_CLOCK, &now);
+
 	for(int i = 0; i < utcp->nconnections; i++) {
 		struct utcp_connection *c = utcp->connections[i];
 
-		if(!c->reapable) {
-			utcp_expect_data(c, offline);
-
-			// If we are online again, reset the retransmission timers, but keep the connection timeout as it is,
-			// to prevent peers toggling online/offline state frequently from keeping connections alive
-			// if there is no progress in sending actual data.
-			if(!offline) {
-				gettimeofday(&utcp->connections[i]->rtrx_timeout, NULL);
-				utcp->connections[i]->rtt_start.tv_sec = 0;
+		if(c->reapable) {
+			continue;
+		}
+
+		utcp_expect_data(c, offline);
+
+		if(!offline) {
+			if(timespec_isset(&c->rtrx_timeout)) {
+				c->rtrx_timeout = now;
+			}
+
+			utcp->connections[i]->rtt_start.tv_sec = 0;
+
+			if(c->rto > START_RTO) {
+				c->rto = START_RTO;
 			}
 		}
 	}
+}
 
-	if(!offline && utcp->rto > START_RTO) {
-		utcp->rto = START_RTO;
-	}
+void utcp_set_retransmit_cb(struct utcp *utcp, utcp_retransmit_t retransmit) {
+	utcp->retransmit = retransmit;
+}
+
+void utcp_set_clock_granularity(long granularity) {
+	CLOCK_GRANULARITY = granularity;
 }