IPv6 fragmentation weirdness

[Kevin Oberman]

> Date: Fri, 15 May 2009 00:09:02 +0200 (CEST)
> From: sthaug at nethelp.no
> 
> > First, why is the kernel fragmenting this at all as it fits in the
> > interface MTU?
> 
> Good question, I definitely disagree with this behavior and would say
> that it breaks POLA. But it's documented (see the ping6 -m option).
> 
> > Can anyone fetch anything from ftp.funet.fi via IPv6? I suspect it is
> > something in the path that is blocking my traffic, so others may not see
> > this, but I think the root issues is the kernel fragmenting packets way
> > below MTU size.
> 
> I just picked up a copy of the 7.2 bootonly ISO image using IPv6. Slow
> but usable. My path (from Oslo, Norway) is:
> 
> sthaug at lab1% traceroute6 ftp.funet.fi
> traceroute6 to ftp.funet.fi (2001:708:10:9::20:1) from 2001:8c0:8b00:1::2, 64 hops max, 12 byte packets
>  1  ge-0-0-9-515.br1.fn3.no.catchbone.net  0.254 ms  4.917 ms  0.203 ms
>  2  c10G-ge-5-1-0.cr2.osls.no.catchbone.net  0.485 ms  0.408 ms  0.399 ms
>  3  c10G-xe-4-1-0.br1.osls.no.catchbone.net  0.364 ms  0.351 ms  0.361 ms
>  4  2001:2000:3083:6::1  9.006 ms  8.848 ms  8.966 ms
>  5  s-ipv6-b1-link.ipv6.telia.net  19.481 ms  19.590 ms  19.412 ms
>  6  2001:2000:3080:d::2  110.907 ms  109.056 ms  119.495 ms
>  7  helsinki0-rtr.funet.fi  116.305 ms  123.534 ms  119.472 ms
>  8  csc0-x0000-helsinki0.ipv6.funet.fi  118.873 ms  117.439 ms  116.054 ms
>  9  ftp.funet.fi  115.777 ms  116.087 ms  117.735 ms
> 
> Note that the IPv6 transit from Telia is tunnelled, and the RTT is awful
> compared to IPv4 (IPv4 RTT to ftp.funet.fi from the same box is around
> 17 ms).
> 
> Steinar Haug, Nethelp consulting, sthaug at nethelp.no
> 

Thanks, Steinar.

I just re-read the man page and I had misunderstood what it was
saying. That still leave me baffled as to what is happening.

My path is, as would be expected, very different.
traceroute6 to ftp.funet.fi (2001:708:10:9::20:1) from 2001:400:0:40::200:101, 64 hops max, 12 byte packets
 1  esnet.rt1.ams.nl.geant2.net (2001:798:29:10aa::9)  83.998 ms  115.099 ms  85.969 ms
 2  so-7-0-0.rt2.cop.dk.geant2.net (2001:798:cc:1501:2201::1)  101.692 ms  96.955 ms  96.868 ms
 3  nordunet-gw.rt2.cop.dk.geant2.net (2001:798:15:10aa::2)  179.931 ms  205.407 ms  195.268 ms
 4  dk-uni.nordu.net (2001:948:0:f055::2)  210.468 ms se-fre.nordu.net (2001:948:0:f03f::1)  187.479 ms dk-uni.nordu.net (2001:948:0:f055::2)  190.578 ms
 5  se-tug.nordu.net (2001:948:0:f049::2)  188.170 ms  213.538 ms se-tug.nordu.net (2001:948:0:f056::1)  183.273 ms
 6  helsinki0-rtr.funet.fi (2001:948:0:f035::2)  188.114 ms  189.214 ms  192.192 ms
 7  csc0-x0000-helsinki0.ipv6.funet.fi (2001:708:0:f000::1:2)  186.166 ms  190.181 ms  186.669 ms
 8  ftp.funet.fi (2001:708:10:9::20:1)  186.251 ms  198.591 ms  205.987 ms

This is exactly the same as my IPv4 path. Something along this path is
silently refusing to pass a packet at the start of the transfer and that
screams MTU.

If I ping from a Juniper router, I can get 1482 byte packets through, so
I suspect that there is a tunnel somewhere. But FreeBSD boxes die at the
lower limit.

Does the kernel fragmentation only affect ICMP or are TCP packet also
fragmented at 1280 bytes?
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751