[tcpm] Review pt1/2: draft-bensley-tcpm-dctcp-00
Bob Briscoe <bob.briscoe@bt.com> Wed, 26 February 2014 15:11 UTC
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Date: Wed, 26 Feb 2014 15:10:50 +0000
To: sbens@microsoft.com, lars@netapp.com, dthaler@microsoft.com
From: Bob Briscoe <bob.briscoe@bt.com>
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Subject: [tcpm] Review pt1/2: draft-bensley-tcpm-dctcp-00
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Stephen, Lars, Dave, 1) Applicability and Incremental Deployment =========================================== Although it's called Data Centre TCP, you haven't said what it's deployment applicability is. Under "4. Deployment Issues" you mention a "heterogeneous datacenter". What does that mean? I believe DCTCP does not co-exist well with legacy TCP flows (it starves itself without special measures in the buffers). See <http://www.ietf.org/proceedings/88/slides/slides-88-tsvwg-20.pdf> For instance, what would you say to these two areas of potential applicability: a) My company operates a private network connecting the data centres of hundreds of companies. But it's infeasible to get everyone to change to DCTCP all at once. The above link to our slides at the last IETF is a proposal to deploy DCTCP incrementally, which could work on such a large private community of interest network and potentially on the public Internet as well: b) We are considering using DCTCP within corporate LANs (at least where app-layer proxying means there are no (or very few) flows within the LAN that also cross the wide area). 2) "Intended Status: Standards Track" ===================================== * Without having stated otherwise, Standards Track implies this could be for the public Internet. * If solely for homogeneous data centres (to document an existing implementation for the record), then Informational would be appropriate * If you are prepared to allow the spec to evolve as it progresses, then I would suggest Experimental would be appropriate. 3) Capability Negotiation of the new TCP wire protocol. ======================================================= " 4. Deployment Issues ... DCTCP requires changes on both the sender and the receiver, so in a heterogeneous datacenter, all the endpoints should support DCTCP and should be configured to use it. " The ECN echoing behaviour of the receiver (S.2.2) alters the TCP wire protocol without any negotiation. In "a heterogeneous data centre" DCTCP and non-DCTCP implementations will not realise they are not built to interwork, and their different uses of ECE feedback will horribly confuse each other. Slide 14 of the presentation linked earlier suggests a roadmap of standards actions necessary to deploy DCTCP in heterogenous data centres. Would you be happy to break DCTCP down into these parts? component. . . . . . . . . . . . . IETF wg document #1 redefine meaning of ECN CE . . . . tsvwg Expt update to RFC3168 #2 specify ECN behaviour in AQM algos aqm CoDel, PIE, (RED++?) #3 specify change to TCP feedback . . tcpm draft-ietf-accurate-ecn-reqs #4 specify change to TCP sender algo tcpm Expt update to RFC5861 If so, I suggest your draft could focus on #4 (specify change to TCP sender algo). In place of your S.2.2, you would need to cite some of the following for #3 (specify change to TCP feedback): * draft-ietf-tcpm-accecn-reqs-05 identifies capability negotiation as a problem. It is close to tcpm WGLC. The tcpm WG wanted a requirements doc before we could specify a protocol solution. * draft-kuehlewind-accurate-ecn proposes a solution to capability negotiation (the one we originally specified and implemented for ConEx) * draft-kuehlewind-accurate-ecn-option proposes alternative capability negotiation based on TCP options. 4) Flaw in receiver feedback algo. ================================== The feedback algo in S.2.2 is badly flawed because of the possibility of losing pure ACKs. See <http://tools.ietf.org/html/draft-ietf-tcpm-accecn-reqs-05#appendix-A>. This is why we've suggested alternative(s). I will send more examples offlist to show it is flawed in many more ways than just the one example. 5) Sender Algo ============== I have a suggestion for an algo that naturally auto-configures to RTT. It would replace the algo in "2.3. Processing Congestion Indications on the Sender". Mohammad Alizadeh worked on similar but different ideas in his SIGMETRICS paper about DCTCP. However, I'll send that in pt2/2 - got to rush now. Bob ________________________________________________________________ Bob Briscoe, BT
- [tcpm] Review pt1/2: draft-bensley-tcpm-dctcp-00 Bob Briscoe