Subject: [Tsvwg] Diffserv codepoint seems strange for Re-ECN <draft-briscoe-tsvwg-re-ecn-tcp-00>

From: Bob Briscoe <[email-masked]>
Date: Wed, 09 Nov 2005 03:20:12 +0000

Sally,

Your assumption/question about confining the re-ECN semantics for the 
ECT(0) & ECT(1) codepoints solely to certain DSCPs threw me - I've realised 
why - it doesn't make much sense (at least to me).

The network /middleboxes currently don't read the nonce codepoints, only 
the receiver. Diffserv codepoints are for reading by the network. So I 
assume your ECN-alternate-semantics draft only applied to marking 
semantics, not to the (current) nonce ECT codepoint semantics which are e2e.

So, rather than a Diffserv codepoint, in the re-ECN draft I used the TCP 
handshake to negotiate re-ECN capability between sender and receiver. I 
give a compatibility mode where the re-ECN sender can work with a nonce 
receiver, or basic ECN for that matter. It's optional, so if an implementer 
doesn't do the compatibility mode a re-ECN sender falls back to basic ECN.

But the re-ECN draft does suggest that the network/middleboxes read and 
have semantics for these two codepoints. In the incremental deployment 
section I explain how this would work with nonce senders and receivers. The 
basic idea is that a nonce sender appears to be declaring 50% path 
congestion to the network. So policers should be configured permissively on 
first deployment. Then, they can be gradually tightened up to encourage 
upgrade to re-ECN.

Of course the word 'upgrade' assumes re-ECN is 'better' than the nonce.

This is all in the re-ECN draft - pls read the details there which I think 
you'll find is less strange than using DSCPs. Perhaps strange is a little 
too strong. But certainly confining re-ECN to certain DSCPs is 
unnecessarily over-constraining.

(The essence of this backwards compatibility was also in the re-feedback 
SIGCOMM paper - don't know where the Diffserv idea came from).

Cheers

Bob

____________________________________________________________________________
Bob Briscoe, <[email-masked]>      Networks Research Centre, BT Research
B54/130 Adastral Park,Martlesham Heath,Ipswich,IP5 3RE,UK.   +44 1473 645196  

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