topic IPv6 Path MTU problems in IPv6 Trial

IPv6 Path MTU problems

TimSmall — Thu, 12 Jun 2014 16:11:19 GMT

Hello,
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)
te-11-1-0.edge2.Washington4.Level3.net (13 ms)
vl-80.edge2.Washington1.Level3.net (15 ms)
vl-4061.car1.NewYork2.Level3.net (20 ms)
vl-4041.edge1.NewYork1.Level3.net (19 ms)
vl-4086.edge3.London1.Level3.net (84 ms)
vl-51.car5.London1.Level3.net (191 ms)
PLUSNET-TEC.car5.London1.Level3.net (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 lonap.he.net (2001:7f8:17::1b1b:1) 15.310 ms 15.522 ms 15.735 ms
5 10ge3-1.core1.lon2.he.net (2001:470:0:2cd::1) 18.201 ms 18.654 ms 18.851 ms
6 ge-0.linx.londen03.uk.bb.gin.ntt.net (2001:7f8:4::b62:1) 10.190 ms 9.058 ms 9.255 ms
7 ae-0.r23.londen03.uk.bb.gin.ntt.net (2001:728:0:2000::116) 8.804 ms 8.762 ms 8.830 ms
8 ae-3.r22.amstnl02.nl.bb.gin.ntt.net (2001:728:0:2000::16) 85.290 ms 15.434 ms 16.115 ms
9 ae-0.r23.amstnl02.nl.bb.gin.ntt.net (2001:418:0:2000::1c6) 17.472 ms 16.800 ms 18.022 ms
10 ae-7.r21.asbnva02.us.bb.gin.ntt.net (2001:418:0:2000::1b1) 122.998 ms 109.478 ms 108.943 ms
11 ae-2.r05.asbnva02.us.bb.gin.ntt.net (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?
http://netalyzr.icsi.berkeley.edu/
... needs a web browser with a java plugin, or alternatively, download and run their commandline client.
Cheers,
Tim.

Re: IPv6 Path MTU problems

bobpullen — Fri, 13 Jun 2014 11:08:17 GMT

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)
    te-11-1-0.edge2.Washington4.Level3.net (13 ms)
    vl-70.edge2.Washington1.Level3.net (15 ms)
    vl-4068.edge2.Washington12.Level3.net (14 ms)
    vl-4041.edge1.NewYork1.Level3.net (19 ms)
    vl-4086.edge3.London1.Level3.net (96 ms)
    vl-51.car5.London1.Level3.net (84 ms)
    PLUSNET-TEC.car5.London1.Level3.net (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.

Re: IPv6 Path MTU problems

AndyH — Mon, 16 Jun 2014 08:28:45 GMT

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).

Re: IPv6 Path MTU problems

chrcoluk — Wed, 10 Dec 2014 19:03:05 GMT

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.