On Fri, 26 Dec 2025 at 16:17, Marco Moock via NANOG <[email protected]> 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 _______________________________________________ NANOG mailing list https://lists.nanog.org/archives/list/[email protected]/message/66QZ3LQ7J3ABEVFLVAGIJL7QI5VGPP6N/
