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Topics - vsauer

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Questions & Answers / No working MTU can be found! Was: MTU issue?
« on: June 22, 2017, 04:22:30 AM »
Hi folks,

I have a strange problem with one of my IPv6 uplinks.
The uplink is a VDSL100 of Deutsche Telekom with PPPoE and MTU of 1492 terminated on a linux-based PC (Brand "PC Engines" model APU2 with Ubuntu 14.04).

Here's the uplink interface:

ppp0      Link encap:Point-to-Point Protocol
          inet addr:84.119.111.111  P-t-P:62.155.246.13  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1492  Metric:1

Basically, I set the MTU of the he-ipv6 interface as well as in the tunnelbroker web-interface to 1472.
The tunnel goes to Frankfurt (216.66.80.30).

he-ipv6   Link encap:IPv6-in-IPv4
          inet6 addr: fe80::5495:b377/64 Scope:Link
          inet6 addr: 2001:470:1111:aaaa::2/64 Scope:Global
          UP POINTOPOINT RUNNING NOARP  MTU:1472  Metric:1


root@bh:~$ ip tun sh
he-ipv6: ipv6/ip  remote 216.66.80.30  local any  ttl 255  6rd-prefix 2002::/16


In iptables and ip6tables I enabled mss-clamping for ppp0 or rather he-ipv6.

Ping works but TCP-Connections get interrupted after a few packets which looks like an MTU issue.
Tracepath claims the MTU of 1472 as okay:

root@bh:~$ tracepath6 xxx
 1?: [LOCALHOST]                        0.139ms pmtu 1472
 1: xxx-2.tunnel.tserv6.fra1.ipv6.he.net              12.648ms
 1:  xxx-2.tunnel.tserv6.fra1.ipv6.he.net              13.633ms
 2:  ve399.core1.fra1.he.net                              16.649ms
[....]
 7:  xxx2.kettenbach-it.de                                  12.957ms reached
     Resume: pmtu 1472 hops 7 back 7


Here's what ping with different packet sizes says:


root@bh:~$ ping6 -s 1472 -M do ipv6.google.com
PING ipv6.google.com(fra16s18-in-x0e.1e100.net) 1472 data bytes
ping: local error: Message too long, mtu=1472

root@bh:~$ ping6 -s 1425 -M do ipv6.google.com
PING ipv6.google.com(fra16s18-in-x0e.1e100.net) 1425 data bytes
ping: local error: Message too long, mtu=1472

root@bh:~$ ping6 -s 1424 -M do ipv6.google.com
PING ipv6.google.com(fra16s18-in-x0e.1e100.net) 1424 data bytes
72 bytes from fra16s18-in-x0e.1e100.net: icmp_seq=1 ttl=58 (truncated)


Obviously, packets larger than 1424 do not go through and packets smaller or equal 1424 get truncated.

If I lower the MTU to the minimun of 1280, the pings look like this:


PING ipv6.google.com(fra16s18-in-x0e.1e100.net) 1280 data bytes
ping: local error: Message too long, mtu=1280

PING ipv6.google.com(fra16s18-in-x0e.1e100.net) 1233 data bytes
ping: local error: Message too long, mtu=1280

PING ipv6.google.com(fra16s18-in-x0e.1e100.net) 1232 data bytes
72 bytes from fra16s18-in-x0e.1e100.net: icmp_seq=1 ttl=58 (truncated)



Wha do packets always get truncated???

I don't know what's going on here and I don't know could be the next (debugging) steps.
Does anybody have a hint for me?

Volker


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