Alan Lo created a detailed proposal for the contents of such a file, which basically contained definitions for standard packet header formats like Ethernet, IPv4, IPv6, TCP, UDP, and a dozen or two others.

Alan sent me the document he wrote privately, and gave me permission to publish it under the Apache 2 license, which I have done here for now, in case others take up this work besides me:

• https://github.com/jafingerhut/p4-guide/tree/master/standard-header-defns

It was discussed in (I believe) the P4 language design work group meeting during 2021.

I do recall that one of the last things discussed on this topic was to postpone it until we had implemented P4 namespaces (#3), to prevent people from having conflicts in the type names between the proposed new file, and existing header definitions they might already be using. Unfortunately the stalling of the P4 namespaces work also ended up stalling the introduction of a 'stdheaders.p4' file.

I would propose that we create a 'stdheaders.p4' file, even in the absence of an implementation of P4 namespaces, and simply tell people "use it if you like". They can always either (a) avoid using stdheaders.p4, or (b) #include it, but create their own custom definitions if they wish, as long as their custom definitions use different type names.