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

[William Herrin via NANOG <nanog () lists nanog org>]

On Fri, Dec 26, 2025 at 8:07 AM Marco Moock via NANOG
<nanog () lists nanog org> 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/
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