Re: [dccp] PMTU issues

[Eddie Kohler <kohler@icir.org>]

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> It's also possible for DCCP to get a better initial estimate of the PMTU,
> and apparently there are other problems with the current DCCP PMTUD
> mechanism (using ICMP "can't fragment" messages).  

Yes -- The current draft tries to make clear (but fails to) that a new,
PLPMTUD-style MTU discovery mechanism is acceptable. There are API problems
with such a scheme, but we absolutely allow it.

If you can accept an extra RTT or two at connection initiation time, I
think you can do pretty well:

    Request -->
            <-- Response
    then, all in 1 RTT:
    Padded Sync(512 bytes) -->     /* actual #s would need to fit CC
    Padded Sync(1280 bytes) -->	       mechanism */
    Padded Sync(4000 bytes) -->
            <-- SyncReply(1)	   /* SyncReplies sent only for those
	    <-- SyncReply(2)	      packets that got through */
	    <-- SyncReply(3)
            
> So, my question for the DCCP people, is it worth changing DCCP PMTUD?

It's certainly worth describing it better, and perhaps mentioning a set of
acceptable procedures.  Have you any suggestions for useful procedures
here?

Eddie