On Fri, Dec 26, 2025 at 8:07 AM Marco Moock via NANOG <[email protected]> wrote: > Am 26.12.2025 um 17:47:14 Uhr schrieb Saku Ytti: > > 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. > > If that fixes the problem, PMTU discovery (mandatory for IPv6 and IPv4 > with DF bit) is broken and that means UDP, IPsec, GRE etc. all fail.
Correct: PMTUD on the Internet is broken. ISPs work around this by engineering a clean 1500 byte path everywhere they can and clamping the MSS the few places that they can't. There's a reason we haven't moved up to 9kb ethernet frames on the server and eyeball LANs. This is that reason. > The ISPs I used emit ICMP packet too big messages. Everybody emits them. Too many don't make it to the destination. Regards, Bill Herrin -- For hire. https://bill.herrin.us/resume/ _______________________________________________ NANOG mailing list https://lists.nanog.org/archives/list/[email protected]/message/IOTN4VSUD4THDSMNWCBRSVOO6RJBAJOC/
