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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