The issue here is that customers (the ones that browse the broken web sites), 
don’t know about MTU, ICMP, etc.

So I guess is in your side as the “provider” of the content, who is the 
interested party in making sure it works for “all” your possible customers.

Up to now, every time I’ve seen this problem was just related to ICMPv6 being 
filtered, as many folks do in IPv4 …

By the way, interesting article, I didn’t read it before:


-----Mensaje original-----
De: <> en nombre de 
Paul Stewart <>
Responder a: <>
Fecha: viernes, 14 de octubre de 2016, 13:52
Para: Mikael Abrahamsson <>
Asunto: Re: contact with One & One ?

    At $$$job we run quite a bit of dual stack towards customers as an ISP 
(mainly PPPoE) - our own public website fails the PTB test and quite honestly 
we’ve never fixed it.  it works for lots of customer/visitors but breaks for 
others (and they fail back to IPv4) - we thought it was only external tunnel 
visitors but have found out otherwise… never fully understood what was going on 
and I keep meaning to look at it .. 
    NGINX front ends load balanced via anycast … pretty standard Ubuntu 
16.04LTS setup on the server side.  From what I’ve read it seems to be an ECMP 
related problem like what CloudFlare published a blog about … 
    > On Oct 14, 2016, at 7:45 AM, Mikael Abrahamsson <> wrote:
    > On Fri, 14 Oct 2016, JORDI PALET MARTINEZ wrote:
    >> I think is time to retire happy-eye-balls, it is the only way the people 
will react to those issues!
    > Happy eyeballs doesn't solve PMTU blackhole.
    > So this is actually customer breakage occuring, but I imagine lots of 
ISPs are actually doing MSS re-write and/or announcing lower than 1500 MTU on 
the customer LAN, so even if a customer has PPPoE with 1492 MTU, they still 
won't see this problem.
    > I have seen swedish authorities websites with same 
"won't-respond-to-PTB", no answer there either to fault reports.
    > -- 
    > Mikael Abrahamsson    email:

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