Re: [pmtud] Re: [dccp] PMTU issues

[Eddie Kohler <kohler@icir.org>]

Hi Fred,

* PLPMTUD is useful.
* Designing PMTUD so that it works in the absence of ICMP feedback seems
  necessary.

BUT

* Suitable ICMP feedback hints might significantly improve the performance
  of a transport protocol.
* We can program our transports to react to ICMP as a hint -- i.e., not
  trust it, but use it to optimize performance.
* So ICMP should not be "needed", but it might, and probably would, be quite
  helpful in some cases.
* For instance, not all packetization layers have as easy a time as TCP
  with packet size changes.  The smooth ramp-up suggested in PLPMTUD may
  require intervention from the application for example.  For good
  performance, these applications may apply PMTUD in unexpected ways --
  they might start large, for example.  ICMP feedback would really help
  them.
* ICMP is not a significant cause of Internet congestion and need not ever
  become one (mark it less-than-best-effort).

I still think your overloading of ECN capable as "PLPMTUD capable, don't
send ICMP" is not necessary, a bad idea, and will not fly.

Eddie

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