On Tue, Oct 21, 2025 at 8:49 AM Jen Linkova <[email protected]> wrote:
> On Wed, Oct 22, 2025 at 2:00 AM Bill Fenner <[email protected]> wrote: > >> Per > https://datatracker.ietf.org/doc/statement-iesg-statement-on-clarifying-the-use-of-bcp-14-key-words/, > explanations about when the "SHOULD" can be bypassed (I note that there is > a `whenever` but this does not seem to cover all cases). > > >> And I would assume that basic MTU & fragmentation consideration should > be added. > > > > > > Good point regarding the MTU; I lean towards prohibiting insertion. > > I assume this excludes RFC7915 cases, right? RFC7915 doesn't talk about ways in which we might change the message and how to avoid violating the MTU, so I think that has to be addressed in this document. So this is either SHOULD NOT or MUST NOT insert the bits if it causes the ICMP message to exceed the MTU? Or, SHOULD or MUST fragment the ICMP message if insertion of the bits causes the message to exceed the MTU? Should the rules be the same for IPv4 and IPv6, or different? > So we are talking about > more text to Section 4 to discuss insertion by intermediate nodes > which are not translating between protocols? > I think whatever we add has to be applicable to anyone who inserts an extension to a message in progress. > In a previous age I would have said SHOULD NOT insert if it results in > the new packet exceeding the MTU, and let it be obvious that if you want to > do it you have to deal with the consequences of fragmentation; now that > you've pointed out the IESG statement I'm more inclined to say MUST NOT > because I don't want to get into the details. > > It is only possible to append that object to a very limited list of > error messages. > I do not think we want an ICMP error message to get fragmented, so > clearly intermediate nodes MUST NOT insert the structure if the > resulting packet exceeds min. MTU for the given protocol family. > (Practically it means it's impossible to do insertion for IPv4...) > Well, it can't really be IPv4 minimum MTU, since RFC4884 says you have to include 128 bytes of the original packet, which already blows past the minimum MTU, but maybe 576. So, is the text The extension MUST NOT be inserted if it causes an ICMPv4 message to grow beyond the <words that mean 576> or an ICMPv6 message to grow beyond <words that mean 1280>. appropriate? I guess <words that mean 1280> is "IPv6 minimum MTU"; what words mean 576? "IPv4 minimum reassembly buffer"? Should this be SHOULD NOT with the escape clause that if you know via some out-of-band mechanism that the path from you to the receiver of the ICMP message has a sufficient MTU that you can send it anyway? Bill
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