RFC 4821 PMTUD is that "negotiation" that is "lacking". It is there. It is deployed. It actually works. No more relying on someone sending the ICMP packets through in order for PMTUD to work!
For some value of "works". There are way too many places filtering ICMP for PMTUD to work consistently. PMTUD is *not* the solution, unfortunately.
I guess you missed the part about 4821 PMTUD does not rely on ICMP.
Modern PMTUD does not rely on ICMP and works even where it is filtered.
As long as the implementations are few and far between: https://www.psc.edu/~mathis/MTU/ http://www.ietf.org/mail-archive/web/rrg/current/msg05816.html the traditional ICMP-based PMTUD is what most of use face today. Steinar Haug, Nethelp consulting, sthaug@nethelp.no