Reduce IPv4 MTU to 1280.

Seems to fix #773.
2016-07-12 01:06:01 -04:00
parent af8ddd090e
commit ece4280401
1 changed files with 6 additions and 4 deletions
@@ -120,11 +120,13 @@ namespace Network {
     * IPv4 MTU. Don't use full Ethernet-derived MTU,
     * mobile networks have high tunneling overhead.
     *
-     * About 95% of IPv4 TCP MSS I see are >= 1360.
-     * An IP MTU is 20 bytes larger.
-     * We let smaller MTUs fragment.
+     * As of July 2016, VPN traffic over Amtrak Acela wifi seems to be
+     * dropped if tunnelled packets are 1320 bytes or larger.  Use a
+     * 1280-byte IPv4 MTU for now.
+     *
+     * We may have to implement ICMP-less PMTUD (RFC 4821) eventually.
     */
-    static const int DEFAULT_IPV4_MTU = 1380;
+    static const int DEFAULT_IPV4_MTU = 1280;
    /* IPv6 MTU. Use the guaranteed minimum to avoid fragmentation. */
    static const int DEFAULT_IPV6_MTU = 1280;