netdev - Re: [PATCH] tcp-illinois: incorrect beta usage

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Date:	Wed, 28 Nov 2007 16:43:31 -0800
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Lachlan Andrew <lachlan.andrew@...il.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	shaoliu@...nceton.edu, Douglas Leith <doug.leith@...m.ie>,
	Robert Shorten <robert.shorten@...m.ie>, netdev@...r.kernel.org
Subject: Re: [PATCH] tcp-illinois: incorrect beta usage

Lachlan Andrew wrote:
> Thanks Stephen.
>
> A related problem (largely due to the published algorithm itself) is
> that Illinois is very aggressive when it over-estimates the maximum
> RTT.
>
> At high load (say 200Mbps and 200ms RTT), a backlog of packets builds
> up just after a loss, causing the RTT estimate to become large.  This
> makes Illinois think that *all* losses are due to corruption not
> congestion, and so only back off by 1/8 instead of 1/2.
>
> I can't think how to fix this except by better RTT estimation, or
> changes to Illinois itself.  Currently, I ignore RTT measurements when
>    sacked_out != 0    and have a heuristic "RTT aging" mechanism, but
> that's pretty ugly.
>
> Cheers,
> Lachlan
>
>   
Ageing the RTT estimates needs to be done anyway.
Maybe something can be reused from H-TCP. The two are closely related.

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