On 16/02/2017 10:12, Joe Touch wrote: > Brian (et al.), > > > On 2/10/2017 11:45 AM, Brian E Carpenter wrote: >>> practice the >>> Internet breaks the mechanism. However it breaks it is a way that seems >>> disruptive to some user traffic. The document is really guidance >>> one how hosts might use ICMP for optimization, and arguable need >>> not be a standard at all. >> I think that's a mischaracterisation of the mechanism (and the draft). >> PMTUD is not an optimisation. Without it, you get black holes > PMTUD is an optimization to avoid fragmentation. > > Without it, you use fragmentation (which has overheads and other > consequences, notably for IPv4). In IPv6, you don't even know you *need* fragmentation without PMTUD, since only the source is allowed to fragment. (That was one of the many failure modes for 6to4, which is why the only safe approach was to use 1280 always.) > However, it is only *with* PMTUD (and ICMP blocking) that you get black > holes. True for v4. Brian