From ece4280401d67526bc9e932dbc9db5f4cb10b3d2 Mon Sep 17 00:00:00 2001 From: John Hood Date: Tue, 12 Jul 2016 01:06:01 -0400 Subject: [PATCH] Reduce IPv4 MTU to 1280. Seems to fix #773. --- src/network/network.h | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/src/network/network.h b/src/network/network.h index 5d14b43..5267a45 100644 --- a/src/network/network.h +++ b/src/network/network.h @@ -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;