[tcp] Update maximum window size to 2MB

The current maximum window size of 256kB was calculated based on rough
link bandwidth and RTT measurements taken in 2012, and is too small to
avoid filling the TCP window on some modern links.

Update the list of typical link bandwidth and RTT figures to reflect
the modern world, and increase the maximum window size accordingly.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/tcp.h

@@ -339,21 +339,27 @@ struct tcp_options {
  *    a) Gigabit LAN: expected bandwidth 125MB/s, typical RTT 0.5ms,
  *       minimum required window 64kB
  *
- *    b) Home Internet connection: expected bandwidth 10MB/s, typical
- *       RTT 25ms, minimum required window 256kB
+ *    b) 10-Gigabit LAN: expected bandwidth 1250MB/s, typical RTT
+ *       0.5ms, minimum required window 640kB
  *
- *    c) WAN: expected bandwidth 2MB/s, typical RTT 100ms, minimum
- *       required window 200kB.
+ *    c) Home Internet connection: expected bandwidth 50MB/s, typical
+ *       RTT 25ms, minimum required window 1280kB
+ *
+ *    d) International WAN: expected bandwidth 50MB/s, typical RTT
+ *       25ms, minimum required window 1280kB
+ *
+ *    e) Intercontinental WAN: expected bandwidth 5MB/s, typical RTT
+ *       250ms, minimum required window 1280kB.
  *
  * The maximum possible value for the TCP window size is 1GB (using
  * the maximum window scale of 2**14).  However, it is advisable to
  * keep the window size as small as possible (without limiting
  * bandwidth), since in the event of a lost packet the window size
  * represents the maximum amount that will need to be retransmitted.
  *
- * We therefore choose a maximum window size of 256kB.
+ * We therefore choose a (rounded up) maximum window size of 2048kB.
  */
-#define TCP_MAX_WINDOW_SIZE	( 256 * 1024 )
+#define TCP_MAX_WINDOW_SIZE	( 2048 * 1024 )
 
 /**
  * Path MTU