Re: [multipathtcp] draft-boucadair-mptcp-plain-mode-07

Hi Joe:

Comments below prefaced by DA>

-----Original Message-----
From: multipathtcp [mailto:multipathtcp-bounces@ietf.org] On Behalf Of Joe Touch
Sent: Friday, June 03, 2016 10:21 AM
To: Markus.Brunner3@swisscom.com
Cc: multipathtcp@ietf.org; sarikaya@ieee.org
Subject: Re: [multipathtcp] draft-boucadair-mptcp-plain-mode-07

On 6/3/2016 12:19 AM, Markus.Brunner3@swisscom.com wrote:
> Hi Joe,
>
> If you mean with dynamic routing a packet by packet routing decision, that could work however, how do you incorporate the path characteristic feedback into that decision. 
That's what multipath routing has been doing for years.

AFAIK, the mechanisms in MPTCP are an instance inspired by existing multipath routing.

> Here the MPTCP mechanisms just come in handy and do the job quite well, but as you say we pay with some complexity. That complexity we feel,will have anyway with whatever solution we are going.

Perhaps, but MPTCP is the wrong solution for an IP tunnel for many reasons:

- it will layer congestion control on top of congestion control for all transports in the payload IP that already support it (which they should)
DA> Would that not be true of any tunnel mechanism that used feedback of the tunnel leg as well as the e2e application?

- it fails to preserve payload IP messages inside a single TCP segment, which increases loss due to a lack of fate-sharing and increases latency and jitter
DA> I apologize for not getting your point here.

- by recovering from loss, it increases latency and jitter
DA> AFAIK, any loss does that irrespective of the delivery mechanism. In the hybrid access case you are correcting loss with a smaller RTT are you not?

- it can use only one load monitoring mechanism (the one built-in)
DA> How many did you want?

- it can use only one traffic distribution mechanism (the one built-in)
DA> If you are saying it is "implementation constrained", well go figure :-)  

That's a lot of baggage vs. just using tunnels and existing dynamic multipath routing -- and the latter doesn't require a new spec.

DA> Can you point me to the feedback mechanisms embodied in making dynamic multipath work?  And if you are striping flows across multiple paths (absolute requirement in my application), any indication of how successful this is in minimizing the residence time of packets in the reordering function?

DA> If all we are talking about is a slightly smarter ECMP , IMO it does not cut it.

Dave

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