X-Git-Url: http://git.meshlink.io/?p=utcp;a=blobdiff_plain;f=utcp.c;h=1b498a2897d0c4063b568040c24804c6235f636d;hp=186b63e90bba16798f7fe4d897c40e4c554cfd19;hb=0c29e6bb0a943f9d06067628f894495dcc01c5ba;hpb=a4e2223575a4b0659bcbffeb73e636d5ba5847bc

diff --git a/utcp.c b/utcp.c
index 186b63e..1b498a2 100644
--- a/utcp.c
+++ b/utcp.c
@@ -53,6 +53,10 @@
 } while (0)
 #endif
 
+#ifndef max
+#define max(a, b) ((a) > (b) ? (a) : (b))
+#endif
+
 #ifdef UTCP_DEBUG
 #include <stdarg.h>
 
@@ -103,6 +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++)
@@ -117,16 +134,30 @@ static int32_t seqdiff(uint32_t a, uint32_t b) {
 // TODO: convert to ringbuffers to avoid memmove() operations.
 
 // Store data into the buffer
-static ssize_t buffer_put(struct buffer *buf, const void *data, size_t len) {
+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;
-	if(len > buf->maxsize - buf->used)
-		len = buf->maxsize - buf->used;
-	if(len > buf->size - buf->used) {
+
+	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;
-		do {
-			newsize *= 2;
-		} while(newsize < buf->used + len);
+		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);
@@ -135,11 +166,17 @@ static ssize_t buffer_put(struct buffer *buf, const void *data, size_t len) {
 		buf->data = newdata;
 		buf->size = newsize;
 	}
-	memcpy(buf->data + buf->used, data, len);
-	buf->used += len;
+
+	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)
@@ -164,9 +201,11 @@ static ssize_t buffer_copy(struct buffer *buf, void *data, size_t offset, size_t
 
 static bool buffer_init(struct buffer *buf, uint32_t len, uint32_t maxlen) {
 	memset(buf, 0, sizeof *buf);
-	buf->data = malloc(len);
-	if(!len)
-		return false;
+	if(len) {
+		buf->data = malloc(len);
+		if(!buf->data)
+			return false;
+	}
 	buf->size = len;
 	buf->maxsize = maxlen;
 	return true;
@@ -265,11 +304,20 @@ static struct utcp_connection *allocate_connection(struct utcp *utcp, uint16_t s
 		return NULL;
 	}
 
+	if(!buffer_init(&c->rcvbuf, DEFAULT_RCVBUFSIZE, DEFAULT_MAXRCVBUFSIZE)) {
+		free(c);
+		return NULL;
+	}
+
 	// Fill in the details
 
 	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;
@@ -285,6 +333,46 @@ static struct utcp_connection *allocate_connection(struct utcp *utcp, uint16_t s
 	return c;
 }
 
+// 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 + abs(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(struct utcp *utcp, uint16_t dst, utcp_recv_t recv, void *priv) {
 	struct utcp_connection *c = allocate_connection(utcp, 0, dst);
 	if(!c)
@@ -329,6 +417,7 @@ 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);
 
 	assert(left >= 0);
 
@@ -347,7 +436,7 @@ static void ack(struct utcp_connection *c, bool sendatleastone) {
 	} *pkt;
 
 	pkt = malloc(sizeof pkt->hdr + c->utcp->mtu);
-	if(!pkt->data)
+	if(!pkt)
 		return;
 
 	pkt->hdr.src = c->src;
@@ -366,16 +455,16 @@ static void ack(struct utcp_connection *c, bool sendatleastone) {
 		c->snd.nxt += 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;
+		}
+
+		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);
@@ -431,6 +520,8 @@ ssize_t utcp_send(struct utcp_connection *c, const void *data, size_t len) {
 
 	c->snd.last += len;
 	ack(c, false);
+	if(!timerisset(&c->rtrx_timeout))
+		start_retransmit_timer(c);
 	return len;
 }
 
@@ -459,11 +550,8 @@ static void retransmit(struct utcp_connection *c) {
 	pkt->hdr.dst = c->dst;
 
 	switch(c->state) {
-		case LISTEN:
-			// TODO: this should not happen
-			break;
-
 		case SYN_SENT:
+			// Send our SYN again
 			pkt->hdr.seq = c->snd.iss;
 			pkt->hdr.ack = 0;
 			pkt->hdr.wnd = c->rcv.wnd;
@@ -473,6 +561,7 @@ static void retransmit(struct utcp_connection *c) {
 			break;
 
 		case SYN_RECEIVED:
+			// Send SYNACK again
 			pkt->hdr.seq = c->snd.nxt;
 			pkt->hdr.ack = c->rcv.nxt;
 			pkt->hdr.ctl = SYN | ACK;
@@ -482,31 +571,157 @@ static void retransmit(struct utcp_connection *c) {
 
 		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.nxt, c->snd.una);
-			if(c->state == FIN_WAIT_1)
-				len--;
+			uint32_t len = seqdiff(c->snd.last, c->snd.una);
 			if(len > utcp->mtu)
 				len = utcp->mtu;
-			else {
-				if(c->state == FIN_WAIT_1)
-					pkt->hdr.ctl |= FIN;
+			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;
 
-		default:
-			// TODO: implement
+		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.
+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) {
@@ -623,7 +838,10 @@ ssize_t 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.
@@ -634,28 +852,19 @@ ssize_t 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;
@@ -667,10 +876,11 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	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.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);
+	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);
 		// Ignore unacceptable RST packets.
 		if(hdr.ctl & RST)
 			return 0;
@@ -723,7 +933,10 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 			set_state(c, CLOSED);
 			return 0;
 		default:
+#ifdef UTCP_DEBUG
 			abort();
+#endif
+			break;
 		}
 	}
 
@@ -733,6 +946,20 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 	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) {
@@ -753,6 +980,10 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 		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;
@@ -783,9 +1014,9 @@ ssize_t utcp_recv(struct utcp *utcp, const void *data, size_t len) {
 				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.
-				//This will cause us to start retransmitting, but at the same speed as the incoming ACKs arrive,
-				//thus preventing a drop in speed.
+				//Reset the congestion window so we wait for ACKs.
 				c->snd.nxt = c->snd.una;
+				c->snd.cwnd = utcp->mtu;
 			}
 		}
 	}
@@ -794,8 +1025,10 @@ 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
+			start_retransmit_timer(c);
 	}
 
 	// 5. Process SYN stuff
@@ -822,7 +1055,10 @@ ssize_t 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
@@ -852,7 +1088,10 @@ 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:
@@ -864,36 +1103,26 @@ ssize_t 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;
 		}
 
-		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.
-				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, 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;
@@ -912,7 +1141,10 @@ ssize_t 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
@@ -954,7 +1186,7 @@ reset:
 }
 
 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;
@@ -966,12 +1198,26 @@ 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;
@@ -1103,13 +1349,6 @@ struct timeval utcp_timeout(struct utcp *utcp) {
 		if(timerisset(&c->conn_timeout) && timercmp(&c->conn_timeout, &next, <))
 			next = c->conn_timeout;
 
-		if(c->snd.nxt != c->snd.una) {
-			c->rtrx_timeout = now;
-			c->rtrx_timeout.tv_sec++;
-		} else {
-			timerclear(&c->rtrx_timeout);
-		}
-
 		if(timerisset(&c->rtrx_timeout) && timercmp(&c->rtrx_timeout, &next, <))
 			next = c->rtrx_timeout;
 	}
@@ -1119,22 +1358,34 @@ struct timeval utcp_timeout(struct utcp *utcp) {
 	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
+	utcp->rto = START_RTO; // us
 
 	return utcp;
 }
@@ -1153,63 +1404,97 @@ 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->sndbuf.maxsize;
+	return c ? c->sndbuf.maxsize : 0;
 }
 
 size_t utcp_get_sndbuf_free(struct utcp_connection *c) {
-	if(c->state == ESTABLISHED || c->state == CLOSE_WAIT)
+	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) {
+	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;
+	}
 }