On Thu, 14 May 2009, Kevin Oberman wrote:
Hi,
Date: Fri, 15 May 2009 00:09:02 +0200 (CEST)
From: sth...@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:
sth...@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, sth...@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?
WRT to TCP you may also want to check the hostcache:
sysctl net.inet.tcp.hostcache.list
--
Bjoern A. Zeeb The greatest risk is not taking one.
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