Use total free memory as advertised window. This seems to be sufficient

to avoid drops even on slow NICs.

src/include/gpxe/tcp.h

@@ -211,41 +211,30 @@ struct tcp_mss_option {
 #define MIN_PKB_LEN	MAX_HDR_LEN + 100 /* To account for padding by LL */
 
 /**
- * Advertised TCP window size
+ * Maxmimum advertised TCP window size
  *
- * 
- * Our TCP window is actually limited by the amount of space available
- * for RX packets in the NIC's RX ring; we tend to populate the rings
- * with far fewer descriptors than a typical driver.  This would
- * result in a desperately small window size, which kills WAN download
- * performance; the maximum bandwidth on any link is limited to
- *
- *    max_bandwidth = ( tcp_window / round_trip_time )
- *
- * With a 4kB window, which probably accurately reflects our amount of
- * buffer space, and a WAN RTT of say 200ms, this gives a maximum
- * achievable bandwidth of 20kB/s, which is not acceptable.
- *
- * We therefore aim to process packets as fast as they arrive, and
- * advertise an "infinite" window.  If we don't process packets as
- * fast as they arrive, then we will drop packets and have to incur
- * the retransmission penalty.
+ * We estimate the TCP window size as the amount of free memory we
+ * have.  This is not strictly accurate (since it ignores any space
+ * already allocated as RX buffers), but it will do for now.
  *
  * Since we don't store out-of-order received packets, the
  * retransmission penalty is that the whole window contents must be
- * resent.
+ * resent.  This suggests keeping the window size small, but bear in
+ * mind that the maximum bandwidth on any link is limited to
+ *
+ *    max_bandwidth = ( tcp_window / round_trip_time )
  *
- * We choose to compromise on a window size of 64kB (which is the
- * maximum that can be represented without using TCP options).  This
- * gives a maximum bandwidth of 320kB/s at 200ms RTT, which is
- * probably faster than the actual link bandwidth.  It also limits
- * retransmissions to 64kB, which is reasonable.
+ * With a 48kB window, which probably accurately reflects our amount
+ * of free memory, and a WAN RTT of say 200ms, this gives a maximum
+ * bandwidth of 240kB/s.  This is sufficiently close to realistic that
+ * we will need to be careful that our advertised window doesn't end
+ * up limiting WAN download speeds.
  *
  * Finally, since the window goes into a 16-bit field and we cannot
  * actually use 65536, we use a window size of (65536-4) to ensure
  * that payloads remain dword-aligned.
  */
-#define TCP_WINDOW_SIZE	( 65536 - 4 )
+#define TCP_MAX_WINDOW_SIZE	( 65536 - 4 )
 
 /**
  * Advertised TCP MSS

src/net/netdevice.c

@@ -398,8 +398,13 @@ static void net_step ( struct process *process ) {
 		/* Poll for new packets */
 		netdev_poll ( netdev, -1U );
 
-		/* Process received packets */
-		while ( ( pkb = netdev_rx_dequeue ( netdev ) ) ) {
+		/* Process at most one received packet.  Give priority
+		 * to getting packets out of the NIC over processing
+		 * the received packets, because we advertise a window
+		 * that assumes that we can receive packets from the
+		 * NIC faster than they arrive.
+		 */
+		if ( ( pkb = netdev_rx_dequeue ( netdev ) ) ) {
 			DBGC ( netdev, "NETDEV %p processing %p\n",
 			       netdev, pkb );
 			netdev->ll_protocol->rx ( pkb, netdev );

src/net/tcp.c

@@ -6,6 +6,7 @@
 #include <timer.h>
 #include <vsprintf.h>
 #include <gpxe/pkbuff.h>
+#include <gpxe/malloc.h>
 #include <gpxe/retry.h>
 #include <gpxe/tcpip.h>
 #include <gpxe/tcp.h>
@@ -265,6 +266,7 @@ static int tcp_senddata_conn ( struct tcp_connection *conn, int force_send ) {
 	unsigned int flags;
 	size_t len;
 	size_t seq_len;
+	size_t window;
 	int rc;
 
 	/* Allocate space to the TX buffer */
@@ -322,6 +324,12 @@ static int tcp_senddata_conn ( struct tcp_connection *conn, int force_send ) {
 	if ( seq_len )
 		start_timer ( &conn->timer );
 
+	/* Estimate window size */
+	window = freemem;
+	if ( window > TCP_MAX_WINDOW_SIZE )
+		window = TCP_MAX_WINDOW_SIZE;
+	window &= ~0x03; /* Keep everything dword-aligned */
+
 	/* Fill up the TCP header */
 	payload = pkb->data;
 	if ( flags & TCP_SYN ) {
@@ -338,7 +346,7 @@ static int tcp_senddata_conn ( struct tcp_connection *conn, int force_send ) {
 	tcphdr->ack = htonl ( conn->rcv_ack );
 	tcphdr->hlen = ( ( payload - pkb->data ) << 2 );
 	tcphdr->flags = flags;
-	tcphdr->win = htons ( TCP_WINDOW_SIZE );
+	tcphdr->win = htons ( window );
 	tcphdr->csum = tcpip_chksum ( pkb->data, pkb_len ( pkb ) );
 
 	/* Dump header */
@@ -492,7 +500,7 @@ static int tcp_send_reset ( struct tcp_connection *conn,
 	tcphdr->ack = in_tcphdr->seq;
 	tcphdr->hlen = ( ( sizeof ( *tcphdr ) / 4 ) << 4 );
 	tcphdr->flags = ( TCP_RST | TCP_ACK );
-	tcphdr->win = htons ( TCP_WINDOW_SIZE );
+	tcphdr->win = htons ( TCP_MAX_WINDOW_SIZE );
 	tcphdr->csum = tcpip_chksum ( pkb->data, pkb_len ( pkb ) );
 
 	/* Dump header */