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