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IPv6 Path MTU problems

Posts: 29
Thanks: 15
Registered: ‎15-10-2011

IPv6 Path MTU problems

Berkeley's "Netalyzr" is giving me a warning about IPv6 Path MTU:
Your system can send fragmented traffic, but can not receive fragmented traffic over IPv6.
The path between your network and our system supports an MTU of at least 1488 bytes. The path between our system and your network has an MTU of 1492 bytes.
The network failed to properly generate an ICMP6 "too big" message. The path MTU bottleneck that fails to properly report the ICMP "too big" is between 2a02:16c8:1:2::11 and *. The path between our system and your network does not appear to handle fragmented IPv6 traffic properly.

My local ppp end point is on the machine where I ran Netalyzr and has an MTU or 1492 bytes to Plusnet's system.  My local firewall isn't showing any dropped packets.  I don't have shell access to a box on the net with IPv6 connectivity at the moment, making debugging the problem a bit tricky.
Looking at the traceroute which Netalyzr gives (where they traceroute back to me) I can't tell if the problem is between Plusnet and Plusnet, or between Plusnet and Me.
2607:f740:b::2 (0 ms)
2001:550:2:8b::4:1 (1 ms)
2001:550::154:26:19:54 (15 ms)
2001:550:4::3c (14 ms) (13 ms) (15 ms) (20 ms) (19 ms) (84 ms) (191 ms) (84 ms)
2a02:16c8:1:2::11 (82 ms)
Tracerouting the other way looks like this:
root@zebedee:~# traceroute6 2607:f740:b::2
traceroute to 2607:f740:b::2 (2607:f740:b::2), 30 hops max, 80 byte packets
1  2a02:16c8:0:1::1c (2a02:16c8:0:1::1c)  127.830 ms  128.487 ms  129.364 ms
2  2a02:16c8:1:2::13 (2a02:16c8:1:2::13)  8.618 ms  8.853 ms  9.266 ms
3  2a02:16c8::b (2a02:16c8::b)  8.790 ms  8.748 ms  8.959 ms
4 (2001:7f8:17::1b1b:1)  15.310 ms  15.522 ms  15.735 ms
5 (2001:470:0:2cd::1)  18.201 ms  18.654 ms  18.851 ms
6 (2001:7f8:4::b62:1)  10.190 ms  9.058 ms  9.255 ms
7 (2001:728:0:2000::116)  8.804 ms  8.762 ms  8.830 ms
8 (2001:728:0:2000::16)  85.290 ms  15.434 ms  16.115 ms
9 (2001:418:0:2000::1c6)  17.472 ms  16.800 ms  18.022 ms
10 (2001:418:0:2000::1b1)  122.998 ms  109.478 ms  108.943 ms
11 (2001:418:0:2000::72)  114.887 ms  108.451 ms  110.471 ms
12  2607:f740:b::2 (2607:f740:b::2)  97.740 ms  92.345 ms  92.791 ms

Anyone else have the same problem?
... needs a web browser with a java plugin, or alternatively, download and run their commandline client.
Community Gaffer
Community Gaffer
Posts: 16,814
Thanks: 4,843
Fixes: 314
Registered: ‎04-04-2007

Re: IPv6 Path MTU problems

Hi Tim,
I get the same MTU fragmentation warning, however my traceroute completes:

IPv6 Traceroute (?): OK –
It takes 16 network hops for IPv6 traffic to pass from our IPv6 server to your system, as shown below. For each hop, the time it takes to traverse it is shown in parentheses.
    2607:f740:b::2 (0 ms)
    2001:550:2:8b::4:1 (1 ms)
    2001:550::154:26:19:54 (14 ms)
    2001:550:4::3c (14 ms) (13 ms) (15 ms) (14 ms) (19 ms) (96 ms) (84 ms) (90 ms)
    2a02:16c8:1:2::7 (93 ms)
    2a02:16c8:1:2::17 (91 ms)
    2a02:16c8:[REDACTED] (102 ms)

It's having no visible effect on my connection from what I can tell though. I did originally have a problem with MTU fragmentation when I first configured my router but I overcame that as per the thread here.

Bob Pullen
Plusnet Product Team
If I've been helpful then please give thanks ⤵

Posts: 6,824
Thanks: 1
Registered: ‎27-10-2012

Re: IPv6 Path MTU problems

Can you install tracepath on your server to see where on a traceroute the PMTU is lower?
I've found that routers often have many MTU settings by default (some of which are lower than those set or displayed in the web gui).
Posts: 1,990
Thanks: 5
Registered: ‎11-12-2013

Re: IPv6 Path MTU problems

Andy the MTU jumbo frame workaround I think isnt working 100%, I had to change my router back to 1492 MTU as it was preventing me downloading windows 8.1 updates.
Just a warning for you.