[Netalyzr] IPv6 fragmentation problem reported - but why?

Nicholas Weaver nweaver at ICSI.Berkeley.EDU
Wed Sep 18 07:52:27 PDT 2013

Thanks.  This suggests a bug in our V6 server, such as the MTU discovery server having crashed.

I will attempt to debug this in the next couple of hours.

On Sep 18, 2013, at 9:01 AM, Stefan Winter <stefan.winter at restena.lu> wrote:

> Hello,
> I've just run netalyzr checks on our network. We are an ISP, and have a
> pretty good grasp of all technology involved, and are rather proud of
> our well-implemented and double-checked IPv6 connectivity.
> That's why I was very baffled to see an IPv6 fragmentation problem being
> reported. It's test 44 of this client-side transcript:
> http://n1.netalyzr.icsi.berkeley.edu/transcript/id=43ca253f-2716-4ddc7253-7ffa-40c8-87ef/side=client/sort
> It shows that IPv6 fragments can be sent and received between my host
> and Netalyzr just fine. The tool then goes on to try and find the MTU on
> the IPv6 link and its binary search goes down to MTU = 0. ?!?
> Given that we're able to send and receive packets at all, the 0 looks
> rather impossible to me.
> The log is not verbose enough for me to figure out *what* exactly this
> test is doing though, and if there's anything we might need to fix.
> I did, of course, check the obvious candidate: yes, ICMPv6
> packet-too-big is allowed bidirectionally on both of the firewalls this
> test traverses. After those comes the backbone, which is out of my reach.
> If someone cares enough to take a look at this, I'd very happy.
> Greetings,
> Stefan Winter
> -- 
> Stefan WINTER
> Ingenieur de Recherche
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Nicholas Weaver                  it is a tale, told by an idiot,
nweaver at icsi.berkeley.edu                full of sound and fury,
510-666-2903                                 .signifying nothing
PGP: http://www1.icsi.berkeley.edu/~nweaver/data/nweaver_pub.asc

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