google path mtu?

Mikael Abrahamsson swmike at
Fri Jan 23 07:18:55 CET 2015

On Fri, 23 Jan 2015, Mikael Abrahamsson wrote:

> Anyone know how I can check in OSX for what destinations it has received 
> PTB packets and what the PMTU it think it has for these destionations?

Found it:

$ netstat -f inet6 -narlW
Destination                             Gateway                         Flags        Refs      Use    Mtu    Netif Expire
default                                 fe80::9272:40ff:fe07:cc74%en0   UGc            14        0   1500      en0

$ netstat -f inet6 -narlW | grep -v 1500 | grep en0
2620:109:c007:102::5be1:f881            fe80::9272:40ff:fe07:cc74%en0   UGHWIi          2      237   1280      en0
2a00:1450:4005:801::1017                fe80::9272:40ff:fe07:cc74%en0   UGHWIi          1      454   1280      en0
2a00:1450:400f:804::2000                fe80::9272:40ff:fe07:cc74%en0   UGHWIi          2       73   1280      en0
2a00:1450:400f:804::200e                fe80::9272:40ff:fe07:cc74%en0   UGHWIi          2     1476   1280      en0
2a00:1450:400f:805::2003                fe80::9272:40ff:fe07:cc74%en0   UGHWIi          1       20   1280      en0
2a00:1450:4010:c07::bd                  fe80::9272:40ff:fe07:cc74%en0   UGHWIi          1      820   1280      en0

So I guess the problem this time was some Google servers sending me 
PTB=1280 and then Chrome not taking this into account when sending UDP 
packets when using QUIC, resulting in fragmented IPv6 packets (which works 
very badly in real life), and then not handling this situation by doing 
fall-back to something else.

Mikael Abrahamsson    email: swmike at

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