Advertise a larger MSS to improve TCP performance.

src/include/gpxe/tcp.h

@@ -27,6 +27,18 @@ struct tcp_header {
 	uint16_t urg;		/* Urgent pointer */
 };
 
+/**
+ * TCP MSS option
+ */
+struct tcp_mss_option {
+	uint8_t kind;
+	uint8_t length;
+	uint16_t mss;
+};
+
+/** Code for the TCP MSS option */
+#define TCP_OPTION_MSS 2
+
 /*
  * TCP flags
  */
@@ -212,7 +224,17 @@ struct tcp_header {
  * guess an arbitrary number that is empirically as large as possible
  * while avoiding retransmissions due to dropped packets.
  */
-#define TCP_WINDOW_SIZE	2048
+#define TCP_WINDOW_SIZE	4096
+
+/**
+ * Advertised TCP MSS
+ *
+ * We currently hardcode this to a reasonable value and hope that the
+ * sender uses path MTU discovery.  The alternative is breaking the
+ * abstraction layer so that we can find out the MTU from the IP layer
+ * (which would have to find out from the net device layer).
+ */
+#define TCP_MSS 1460
 
 /** TCP maximum segment lifetime
  *

src/net/tcp.c

@@ -229,6 +229,8 @@ static int tcp_senddata_conn ( struct tcp_connection *conn, int force_send ) {
 	struct tcp_application *app = conn->app;
 	struct pk_buff *pkb;
 	struct tcp_header *tcphdr;
+	struct tcp_mss_option *mssopt;
+	void *payload;
 	unsigned int flags;
 	size_t len;
 	size_t seq_len;
@@ -289,13 +291,20 @@ static int tcp_senddata_conn ( struct tcp_connection *conn, int force_send ) {
 		start_timer ( &conn->timer );
 
 	/* Fill up the TCP header */
+	payload = pkb->data;
+	if ( flags & TCP_SYN ) {
+		mssopt = pkb_push ( pkb, sizeof ( *mssopt ) );
+		mssopt->kind = TCP_OPTION_MSS;
+		mssopt->length = sizeof ( *mssopt );
+		mssopt->mss = htons ( TCP_MSS );
+	}
 	tcphdr = pkb_push ( pkb, sizeof ( *tcphdr ) );
 	memset ( tcphdr, 0, sizeof ( *tcphdr ) );
 	tcphdr->src = conn->local_port;
 	tcphdr->dest = conn->peer.st_port;
 	tcphdr->seq = htonl ( conn->snd_seq );
 	tcphdr->ack = htonl ( conn->rcv_ack );
-	tcphdr->hlen = ( ( sizeof ( *tcphdr ) / 4 ) << 4 );
+	tcphdr->hlen = ( ( payload - pkb->data ) << 2 );
 	tcphdr->flags = flags;
 	tcphdr->win = htons ( TCP_WINDOW_SIZE );
 	tcphdr->csum = tcpip_chksum ( pkb->data, pkb_len ( pkb ) );