[Netalyzr] IPv6 fragmentation problem reported - but why?

Stefan Winter stefan.winter at restena.lu
Wed Sep 18 06:01:15 PDT 2013


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:


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.


Stefan Winter

Ingenieur de Recherche
Fondation RESTENA - Réseau Téléinformatique de l'Education Nationale et
de la Recherche
6, rue Richard Coudenhove-Kalergi
L-1359 Luxembourg

Tel: +352 424409 1
Fax: +352 422473

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recipient's key is known to me

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