[e2e] MTU - IP layer

Loki Jorgenson ljorgenson at apparentnetworks.com
Thu Apr 21 17:46:36 PDT 2005

Joe wrote:

> MTU is a link payload issue. Path MTU is the min of the MTUs on the
> path; it is path MTU that is defined E2E.

  And that doesn't seem like a problem?  I guess if RFC 1191 was
reliably implemented and Layer 2 fed back to the end-to-end....

> Short of that, the only other way is POSITIVE feedback - try larger
> packets and see what gets through. If it does, report back and use
> size. That's already under consideration in the IETF "pmtud" working

>From the early drafts I looked at they were proposing, as you suggest, a
"probing for packet loss by size defines pMTU" mechanism  - is that
still the case then?  That doesn't sound like positive feedback per se.
The idea of ICMP DF Set probing (a la RFC1191) at least seemed like
positive feedback, if only best-effort....


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