Re: [multipathtcp] A question related to MPTCP control overhead

Sayee Kompalli Chakravartula <sayee.kompalli.chakravartula@huawei.com> Sun, 09 April 2017 14:25 UTC

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From: Sayee Kompalli Chakravartula <sayee.kompalli.chakravartula@huawei.com>
To: "multipathtcp@ietf.org" <multipathtcp@ietf.org>
Thread-Topic: [multipathtcp] A question related to MPTCP control overhead
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Date: Sun, 9 Apr 2017 14:25:27 +0000
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Subject: Re: [multipathtcp] A question related to MPTCP control overhead
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Dear Olivier,
Thanks for the response. While preparing reply to your observation, I came across your 2013 Sigcomm paper "Are TCP Extensions Middlebox-proof?", and I believe that I should read that paper before I reply you.

I thought of a rough sketch of solution without regard to what middleboxes can do, which essentially assumes a state-machine at both ends. Before I embark on designing a solution, if at all feasible, I wanted to know how others think of such an issue. Hope to reply you soon.

Sayee


-----Original Message-----
From: Olivier Bonaventure [mailto:Olivier.Bonaventure@uclouvain.be] 
Sent: Friday, April 07, 2017 9:41 PM
To: Sayee Kompalli Chakravartula; multipathtcp@ietf.org
Subject: Re: [multipathtcp] A question related to MPTCP control overhead

Hello,
>
> This is the first time I'm posting a question to the group!! I work 
> for Huawei Technologies, India at Bangalore. For quite some time I've 
> been reading material on MPTCP, and of late, I have been thinking of a 
> potential issue related to MPTCP control overhead. I'm wondering 
> whether the issue I articulated in the following is worth considering, 
> and if so, I would love to hear opinion from the members of the group. 
> I thank you all in advance!

Thanks for your email. Bascially, you would like to have a Multipath TCP connection that starts without DSS on this initial subflow and then enables the DSS when a second subflow has been established.

This would reduce the overhead when MPTCP connections are composed of a single subflow. However, in the presence of middleboxes, it looks very difficult to design a solution that would preserve connectivity under all circumstances.

Assume that MPTCP starts without any DSS option and only uses them after the establishment of the second subflow. If the bytestream has been affected by a middlebox (e.g. a NAT with an FTP ALG) that adds or remove bytes in the payload, when MPTCP will switch to using the DSS? they will be desynchronised with the actual sequence numbers that the receiver observers. There are other corner cases where DSS options are removed from packets with data but not from SYNs that could also result in various problems.

The idea looks nice, but there are unfortunately many devils hidden in the details


Olivier