Re: What do you consider acceptable packet / session modification for a network operator?

[Saku Ytti via NANOG <nanog () lists nanog org>]

On Fri, 26 Dec 2025 at 16:17, Marco Moock via NANOG
<nanog () lists nanog org> wrote:

> > No, if you don't want the headache of having to deal with every goofy
> > little situation where PMTUD doesn't work and you _know_ you have a
> > link with an MTU under 1500 (common with ISPs using PPPOE to the
> > customer premise equipment) then you clamp the TCP MSS. You don't like
> > it. But you do it anyway because tech support hours are expensive and
> > that results in fewer of them.
>
> I've never seen that yet at the ISPs I use.

Maybe I am misreading, I'm reading that physical MTU is 1500B out of
which PPPoE headers eat. So the 1500B user packet wont fit.

You're saying you've never seen an ISP adjust TCP MSS here? I must
have misread, because I've never seen an ISP not adjust here.

Funnily enough, there is absolutely no need. My ISP bought gear which
can do >1500B, they control both ends of the link, there is a PPP
option to negotiate MTU. So my ISP could have just simply configured
physical MTU above 1500B, even potentially only when it is their own
CPE specifically asking >1500B. And never have to clamp. Yet, they
clamp, because it is so ingrained in the industry, people are not even
asking why we are doing this.

-- 
  ++ytti
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