This conversation is missing some fundamental points – really the most important points – which are the minimum sizes guaranteed to work everywhere. For IPv6, the minimum MTU/MRU are 1280/1500. For IPv4, they are only 68/576 but since the IPv4 network supports fragmentation we can nominally designate the IPv4 minimum MTU as 576 also if we clear the DF bit. It means that, without probing or having some divine knowledge of paths that have not been previously visited, the ONLY sizes guaranteed to work are 1280 for IPv6 and 576 for IPv4.
Now take the case of Multinet where a path may traverse multiple concatenated IP networks of arbitrary IP protocol versions - remember “Catenet”? Since there may be no advanced knowledge of network IP protocol versions, the most we can absolutely and for sure count on across the entire path is 576. What this gives us is not the *maximum packet size*; instead, it determines the *minimum cell size*. We know that a 576 cell will traverse all paths, so we never send a non-final cell smaller than this (which might trigger a tiny fragment alarm). But, we can certainly send packets larger than the path MTU - *much* larger in many cases. And for paths that support them, we can also send Brian’s jumbograms. Speaking of jumbos, on a different list I made a point about how this all harkens back to RFC1149 and RFC6214. But nowadays, we can imagine substituting a memory stick for the slips of paper with whitestuff and blackstuff and those avian carriers will transport jumbos just fine. Not quite unlimited MTU, but close enough. Fred
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