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Re: [multipathtcp] Number of DSS to store

"Hampel, K Georg (K Georg)" <georg.hampel@alcatel-lucent.com> Mon, 20 August 2012 14:46 UTC

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From: "Hampel, K Georg (K Georg)" <georg.hampel@alcatel-lucent.com>
To: Christoph Paasch <christoph.paasch@uclouvain.be>, Anumita Biswas <anumita_biswas@apple.com>
Thread-Topic: [multipathtcp] Number of DSS to store
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Date: Mon, 20 Aug 2012 14:45:41 +0000
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Cc: "multipathtcp@ietf.org" <multipathtcp@ietf.org>, Mahesh M <Mahesh.M@citrix.com>, Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Subject: Re: [multipathtcp] Number of DSS to store
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Quite honestly, I think we are getting lost in details. There is very little implementation and real-world experience with MPTCP. The simplest possible implementation sends ONE DSS per packet and does NO buffering of payload-w/o-mapping or mappings-w/o-payload and it may in fact provide proper operation in the vast majority of use cases. Exceptions should be based on real-world trials rather than speculation.

I agree with Christoph that on sender side, DSS should have priority over SACK blocks.

Georg


-----Original Message-----
From: Christoph Paasch [mailto:christoph.paasch@uclouvain.be] 
Sent: Wednesday, August 01, 2012 8:45 AM
To: Anumita Biswas
Cc: Alan Ford (alanford); Hampel, K Georg (K Georg); multipathtcp@ietf.org; Mahesh M; Ilpo Järvinen
Subject: Re: [multipathtcp] Number of DSS to store

Hi Anumita & Alan,

On Tuesday 31 July 2012 21:44:35 Anumita Biswas wrote:
> A DSS option may not be sent with corresponding data if there isn't  enough
> tcp option space available such as due to the presence of SACK blocks.

Then the number of SACK-blocks could get reduced for the packet that needs to 
hold the DSS-option, or the SACK-blocks can be on a separate subflow-ack.

SACK-blocks don't necessarily need to be sent reliably. However, the DSS-
option has to be sent reliably. That's why it is good to couple it together 
with the data as delivery is thus guaranteed.


Alan, in your slides yesterday you said:
"A mapping for subflow seqno x MUST not be 
sent before the mappings for subflow seqno 
0 .. x-1 have been sent."

This will still force receivers to be able to store an uncertain number of 
DSS-mappings.

I believe we should say:
"A mapping, covering the subflow seqno's x to y MUST be sent on one of these 
segments."

That way the receiver only has to store one single DSS-mapping option per 
subflow.


Cheers,
Christoph

-- 
IP Networking Lab --- http://inl.info.ucl.ac.be
MultiPath TCP in the Linux Kernel --- http://mptcp.info.ucl.ac.be
Université Catholique de Louvain
--

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