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Re: [tcpm] Discussion -- Pause/Resume mechanism on TCP ?

Alexander Zimmermann <alexander.zimmermann@nets.rwth-aachen.de> Wed, 21 April 2010 12:01 UTC

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From: Alexander Zimmermann <alexander.zimmermann@nets.rwth-aachen.de>
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Date: Wed, 21 Apr 2010 14:01:05 +0200
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To: Lars Eggert <lars.eggert@nokia.com>
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Subject: Re: [tcpm] Discussion -- Pause/Resume mechanism on TCP ?
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Hi,

Am 21.04.2010 um 12:00 schrieb Lars Eggert:

> Hi,
> 
> ...
> 
> Even proposals that are agnostic to link technology have issues, e.g., Freeze TCP, which I cited since it sounded similar to your proposal. For example, there is an implicit assumption that path characteristics (available bandwidth, RTT, etc.) do not change during the freeze period - when they do, the thawed connection can significantly overshoot along the new path, or it can underperform (and will take a long time to probe for extra capacity if it is in congestion avoidance mode). And obviously, there is an assumption that you somehow know when to freeze and thaw. 
> 

Exactly!!! This IMO the reason why TCP freeze would not work (in general).

Copied from RFC 2861:

3. Description
"When a TCP sender has sufficient data available to fill the available network capacity for that flow, cwnd and ssthresh get set to appropriate values for the network conditions. When a TCP sender stops sending, the flow stops sampling the network conditions, and so the value of the congestion window may become inaccurate."

BTW: Read the introduction of this RFC. It gave me an excellent insight why TCP freeze is a bad idea...

Alex

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