Limit xmit window to one MTU. (Path MTU discovery not yet

implemented; should be done at some point.)

src/include/gpxe/tcp.h

@@ -237,6 +237,14 @@ struct tcp_mss_option {
  */
 #define TCP_MAX_WINDOW_SIZE	( 65536 - 4 )
 
+/**
+ * Path MTU
+ *
+ * We really ought to implement Path MTU discovery.  Until we do,
+ * anything with a path MTU greater than this may fail.
+ */
+#define TCP_PATH_MTU 1460
+
 /**
  * Advertised TCP MSS
  *