X-Git-Url: http://git.meshlink.io/?p=utcp;a=blobdiff_plain;f=utcp.c;h=c026ce7b9de58b2bdefb71ece87bac739e2c04e4;hp=1fae32cd2eaf44430f93311a0787034c58e2861d;hb=1c136554d0e3c3fb89106c349e7613869d63ca2c;hpb=d5888c10a181d6247e407f480524d36be6319b3e

diff --git a/utcp.c b/utcp.c
index 1fae32c..c026ce7 100644
--- a/utcp.c
+++ b/utcp.c
@@ -19,6 +19,7 @@
 
 #define _GNU_SOURCE
 
+#include <assert.h>
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -31,6 +32,31 @@
 
 #include "utcp_priv.h"
 
+#ifndef EBADMSG
+#define EBADMSG         104
+#endif
+
+#ifndef SHUT_RDWR
+#define SHUT_RDWR 2
+#endif
+
+#ifdef poll
+#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 += 1000000;\
+} while (0)
+#endif
+
+#ifndef max
+#define max(a, b) ((a) > (b) ? (a) : (b))
+#endif
+
 #ifdef UTCP_DEBUG
 #include <stdarg.h>
 
@@ -49,7 +75,7 @@ static void print_packet(struct utcp *utcp, const char *dir, const void *pkt, si
 	}
 
 	memcpy(&hdr, pkt, sizeof hdr);
-	fprintf (stderr, "%p %s: src=%u dst=%u seq=%u ack=%u wnd=%u ctl=", utcp, dir, hdr.src, hdr.dst, hdr.seq, hdr.ack, hdr.wnd);
+	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)
 		debug("SYN");
 	if(hdr.ctl & RST)
@@ -81,6 +107,19 @@ static void set_state(struct utcp_connection *c, enum state state) {
 	debug("%p new state: %s\n", c->utcp, strstate[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 inline void list_connections(struct utcp *utcp) {
 	debug("%p has %d connections:\n", utcp, utcp->nconnections);
 	for(int i = 0; i < utcp->nconnections; i++)
@@ -91,14 +130,108 @@ 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) {
+	if(buf->maxsize <= buf->used)
+		return 0;
+
+	debug("buffer_put_at %zu %zu %zu\n", buf->used, offset, len);
+
+	size_t required = offset + len;
+	if(required > buf->maxsize) {
+		if(offset >= buf->maxsize)
+			return 0;
+		abort();
+		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 < buf->used + len);
+		}
+		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;
-	if(!a->src || !b->src)
-		abort();
+
+	assert(a && b);
+	assert(a->src && b->src);
+
 	int c = (int)a->src - (int)b->src;
 	if(c)
 		return c;
@@ -120,14 +253,14 @@ static struct utcp_connection *find_connection(const struct utcp *utcp, uint16_t
 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);
-	if(!cp)
-		abort();
+
+	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->sndbuf);
 	free(c);
 }
 
@@ -166,10 +299,12 @@ static struct utcp_connection *allocate_connection(struct utcp *utcp, uint16_t s
 	if(!c)
 		return NULL;
 
-	c->sndbufsize = DEFAULT_SNDBUFSIZE;
-	c->maxsndbufsize = DEFAULT_MAXSNDBUFSIZE;
-	c->sndbuf = malloc(c->sndbufsize);
-	if(!c->sndbuf) {
+	if(!buffer_init(&c->sndbuf, DEFAULT_SNDBUFSIZE, DEFAULT_MAXSNDBUFSIZE)) {
+		free(c);
+		return NULL;
+	}
+
+	if(!buffer_init(&c->rcvbuf, DEFAULT_RCVBUFSIZE, DEFAULT_MAXRCVBUFSIZE)) {
 		free(c);
 		return NULL;
 	}
@@ -178,7 +313,11 @@ 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;
@@ -200,6 +339,7 @@ struct utcp_connection *utcp_connect(struct utcp *utcp, uint16_t dst, utcp_recv_
 		return NULL;
 
 	c->recv = recv;
+	c->priv = priv;
 
 	struct hdr hdr;
 
@@ -237,11 +377,9 @@ 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);
 
-	fprintf(stderr, "ack, left=%d, cwndleft=%d, sendatleastone=%d\n", left, cwndleft, sendatleastone);
-	if(left < 0)
-		abort();
+	assert(left >= 0);
 
 	if(cwndleft <= 0)
 		cwndleft = 0;
@@ -254,41 +392,39 @@ static void ack(struct utcp_connection *c, bool sendatleastone) {
 
 	struct {
 		struct hdr hdr;
-		char data[c->utcp->mtu];
-	} pkt;
+		char 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.ack = c->rcv.nxt;
-	pkt.hdr.wnd = c->snd.wnd;
-	pkt.hdr.ctl = ACK;
-	pkt.hdr.aux = 0;
+	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);
+		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) {
@@ -329,44 +465,13 @@ ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
 		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;
-		}
-	}
-
-	if(len > c->sndbufsize - bufused)
-		len = c->sndbufsize - bufused;
-
-	if(!len) {
-		errno == EWOULDBLOCK;
+	len = buffer_put(&c->sndbuf, data, len);
+	if(len <= 0) {
+		errno = EWOULDBLOCK;
 		return 0;
 	}
 
-	memcpy(c->sndbuf + bufused, data, len);
 	c->snd.last += len;
-
 	ack(c, false);
 	return len;
 }
@@ -377,7 +482,192 @@ static void swap_ports(struct hdr *hdr) {
 	hdr->dst = tmp;
 }
 
-int utcp_recv(struct utcp *utcp, const void *data, size_t len) {
+static void retransmit(struct utcp_connection *c) {
+	if(c->state == CLOSED || c->snd.nxt == c->snd.una)
+		return;
+
+	struct utcp *utcp = c->utcp;
+
+	struct {
+		struct hdr hdr;
+		char data[];
+	} *pkt;
+
+	pkt = malloc(sizeof pkt->hdr + c->utcp->mtu);
+	if(!pkt)
+		return;
+
+	pkt->hdr.src = c->src;
+	pkt->hdr.dst = c->dst;
+
+	switch(c->state) {
+		case SYN_SENT:
+			// Send our SYN again
+			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;
+
+		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
+			timerclear(&c->rtrx_timeout);
+			break;
+	}
+
+	free(pkt);
+}
+
+// Update receive buffer and SACK entries after consuming data.
+static void sack_consume(struct utcp_connection *c, size_t len) {
+	debug("sack_consume %zu\n", len);
+	if(len > c->rcvbuf.used)
+		abort();
+
+	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].offset = 0;
+			c->sacks[i].len -= len - c->sacks[i].offset;
+			i++;
+		} else {
+			if(i < NSACKS - 1) {
+				memmove(&c->sacks[i], &c->sacks[i + 1], (NSACKS - 1 - i) * sizeof c->sacks[i]);
+				c->sacks[i + 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 < 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;
+				}
+				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 %zu connected with SACK at %u\n", 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 != 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) {
+	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) {
 	if(!utcp) {
 		errno = EFAULT;
 		return -1;
@@ -474,6 +764,8 @@ int utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 
 	// 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.
@@ -490,7 +782,10 @@ int utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	case TIME_WAIT:
 		break;
 	default:
+#ifdef UTCP_DEBUG
 		abort();
+#endif
+		break;
 	}
 
 	// 1b. Drop packets with a sequence number not in our receive window.
@@ -501,42 +796,38 @@ int utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 		acceptable = true;
 
 	// TODO: handle packets overlapping c->rcv.nxt.
-#if 0
+#if 1
 	// 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);
+		acceptable = seqdiff(hdr.seq, c->rcv.nxt) >= 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);
+		acceptable = seqdiff(hdr.seq, c->rcv.nxt) >= 0 && seqdiff(hdr.seq, c->rcv.nxt) + len <= c->rcvbuf.maxsize;
 #else
 	if(c->state != SYN_SENT)
 		acceptable = hdr.seq == c->rcv.nxt;
 #endif
 
 	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 + %zu < %u\n", c->rcv.nxt, hdr.seq, len, c->rcv.nxt + c->rcvbuf.maxsize);
 		// Ignore unacceptable RST packets.
 		if(hdr.ctl & RST)
 			return 0;
 		// Otherwise, send an ACK back in the hope things improve.
-		goto ack;
+		ack(c, true);
+		return 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.
-
-	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);
+	// 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.una + c->sndbuf.used) > 0 &&
+			  seqdiff(hdr.ack, c->snd.nxt) > 0) // TODO: simplify this if
+			 || 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);
 		// Ignore unacceptable RST packets.
 		if(hdr.ctl & RST)
 			return 0;
@@ -589,27 +880,49 @@ int utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 			set_state(c, CLOSED);
 			return 0;
 		default:
+#ifdef UTCP_DEBUG
 			abort();
+#endif
+			break;
 		}
 	}
 
 	// 3. Advance snd.una
 
 	uint32_t advanced = seqdiff(hdr.ack, c->snd.una);
-	c->snd.una = hdr.ack;
+	prevrcvnxt = c->rcv.nxt;
 
 	if(advanced) {
-		debug("%p advanced %u\n", utcp, advanced);
-		// Make room in the send buffer.
-		// TODO: try to avoid memmoving too much. Circular buffer?
-		uint32_t left = seqdiff(c->snd.nxt, hdr.ack);
-		if(left)
-			memmove(c->sndbuf, c->sndbuf + advanced, left);
+		int32_t data_acked = advanced;
+
+		switch(c->state) {
+			case SYN_SENT:
+			case SYN_RECEIVED:
+				data_acked--;
+				break;
+			// TODO: handle FIN as well.
+			default:
+				break;
+		}
+
+		assert(data_acked >= 0);
+
+		int32_t bufused = seqdiff(c->snd.last, c->snd.una);
+		assert(data_acked <= bufused);
+
+		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;
-		debug("%p increasing cwnd to %u\n", utcp, c->snd.cwnd);
+		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) {
@@ -630,9 +943,13 @@ int utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	} else {
 		if(!len) {
 			c->dupack++;
-			if(c->dupack >= 3) {
+			if(c->dupack == 3) {
 				debug("Triplicate ACK\n");
-				abort();
+				//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;
 			}
 		}
 	}
@@ -640,7 +957,7 @@ int utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	// 4. Update timers
 
 	if(advanced) {
-		timerclear(&c->conn_timeout); // It should be set anew in utcp_timeout() if c->snd.una != c->snd.nxt.
+		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);
 	}
@@ -669,7 +986,10 @@ int utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 			// Ehm, no. We should never receive a second SYN.
 			goto reset;
 		default:
+#ifdef UTCP_DEBUG
 			abort();
+#endif
+			return 0;
 		}
 
 		// SYN counts as one sequence number
@@ -699,7 +1019,10 @@ int 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:
@@ -711,32 +1034,26 @@ int utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 			// Ehm no, We should never receive more data after a FIN.
 			goto reset;
 		default:
+#ifdef UTCP_DEBUG
 			abort();
+#endif
+			return 0;
 		}
 
-		int rxd;
-
-		if(c->recv) {
-			rxd = c->recv(c, data, len);
-			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, data, 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;
@@ -755,7 +1072,10 @@ int utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 			// Ehm, no. We should never receive a second FIN.
 			goto reset;
 		default:
+#ifdef UTCP_DEBUG
 			abort();
+#endif
+			break;
 		}
 
 		// FIN counts as one sequence number
@@ -769,14 +1089,14 @@ int utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 		}
 	}
 
-	if(!len && !advanced)
-		return 0;
-
-	if(!len && !(hdr.ctl & SYN) && !(hdr.ctl & FIN))
-		return 0;
+	// Now we send something back if:
+	// - we advanced rcv.nxt (ie, we got some data that needs to be ACKed)
+	//   -> sendatleastone = true
+	// - or we got an ack, so we should maybe send a bit more data
+	//   -> sendatleastone = false
 
 ack:
-	ack(c, true);
+	ack(c, prevrcvnxt != c->rcv.nxt);
 	return 0;
 
 reset:
@@ -797,7 +1117,7 @@ reset:
 }
 
 int utcp_shutdown(struct utcp_connection *c, int dir) {
-	debug("%p shutdown %d\n", c->utcp, dir);
+	debug("%p shutdown %d at %u\n", c ? c->utcp : NULL, dir, c ? c->snd.last : 0);
 	if(!c) {
 		errno = EFAULT;
 		return -1;
@@ -809,12 +1129,26 @@ 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;
 
 	switch(c->state) {
 	case CLOSED:
-		return 0;
 	case LISTEN:
+		errno = ENOTCONN;
+		return -1;
+
 	case SYN_SENT:
 		set_state(c, CLOSED);
 		return 0;
@@ -845,6 +1179,8 @@ int utcp_shutdown(struct utcp_connection *c, int dir) {
 int utcp_close(struct utcp_connection *c) {
 	if(utcp_shutdown(c, SHUT_RDWR))
 		return -1;
+	c->recv = NULL;
+	c->poll = NULL;
 	c->reapable = true;
 	return 0;
 }
@@ -861,6 +1197,8 @@ int utcp_abort(struct utcp_connection *c) {
 		return -1;
 	}
 
+	c->recv = NULL;
+	c->poll = NULL;
 	c->reapable = true;
 
 	switch(c->state) {
@@ -899,74 +1237,13 @@ int utcp_abort(struct utcp_connection *c) {
 	return 0;
 }
 
-static void retransmit(struct utcp_connection *c) {
-	if(c->state == CLOSED || c->snd.nxt == c->snd.una)
-		return;
-
-	struct utcp *utcp = c->utcp;
-
-	struct {
-		struct hdr hdr;
-		char data[c->utcp->mtu];
-	} pkt;
-
-	pkt.hdr.src = c->src;
-	pkt.hdr.dst = c->dst;
-
-	switch(c->state) {
-		case LISTEN:
-			// TODO: this should not happen
-			break;
-
-		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;
-
-		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;
-
-		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;
-
-		default:
-			// TODO: implement
-			abort();
-	}
-}
-
 /* Handle timeouts.
  * One call to this function will loop through all connections,
  * checking if something needs to be resent or not.
  * 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};
@@ -997,6 +1274,9 @@ int utcp_timeout(struct utcp *utcp) {
 			retransmit(c);
 		}
 
+		if(c->poll && buffer_free(&c->sndbuf) && (c->state == ESTABLISHED || c->state == CLOSE_WAIT))
+			c->poll(c, buffer_free(&c->sndbuf));
+
 		if(timerisset(&c->conn_timeout) && timercmp(&c->conn_timeout, &next, <))
 			next = c->conn_timeout;
 
@@ -1013,27 +1293,36 @@ int utcp_timeout(struct utcp *utcp) {
 
 	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);
+bool utcp_is_active(struct utcp *utcp) {
 	if(!utcp)
-		return NULL;
+		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; // s
 
 	return utcp;
 }
@@ -1044,7 +1333,7 @@ void utcp_exit(struct utcp *utcp) {
 	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);
+		buffer_exit(&utcp->connections[i]->sndbuf);
 		free(utcp->connections[i]);
 	}
 	free(utcp->connections);
@@ -1052,48 +1341,97 @@ void utcp_exit(struct utcp *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;
 }
 
 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) {
+	if(c && (c->state == ESTABLISHED || c->state == CLOSE_WAIT))
+		return buffer_free(&c->sndbuf);
+	else
+		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) {
+	if(c)
+		c->recv = recv;
+}
+
+void utcp_set_poll_cb(struct utcp_connection *c, utcp_poll_t 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;
+	}
 }