I was not aware that much (if any?) IPv4 code cared about PMTUD - I thought that only gained provenance in IPv6 Outside of setting DF I guess? But I've had no issues with it, but I only accept/transit 1500 MTU so perhaps I don't see an issue where one might exist -----Original Message----- From: Saku Ytti <saku@ytti.fi> Sent: Monday, September 7, 2026 3:42 AM To: Gary Sparkes <gary@kisaracorporation.com> Cc: North American Network Operators Group <nanog@lists.nanog.org>; Vasilenko Eduard <vasilenko.eduard@huawei.com> Subject: Re: Charging for IPs On Mon, 7 Sept 2026 at 10:38, Gary Sparkes <gary@kisaracorporation.com> wrote:
For direct peering, RFC1918 is valid and usable, so that could be an option providers give - I have a few sessions where I'm using RFC1918 addresses that the provider allocated to me for that first hop session.
RFC1918 breaks PTMUD pretty reliably. You can in many platforms decide to originate ICMP messages from e.g. loopback instead of the RFC1918 interface. But in that case, why bother with the RFC1918, far-side may route it internally and create attack vector to protect against that you have to consider all kinds of attacks towards that RFC1918. With RFC8950 you don't have to, you only have to worry about the IPv6. If you don't have IPv6 and you can originate ICMP from loop, maybe IPv4 LL address is less risky than RFC1918? -- ++ytti