[RFC] TCP illinois max rtt aging

[Stephen Hemminger <shemminger@linux-foundation.org>]

On Wed, 28 Nov 2007 21:26:12 -0800
"Shao Liu" <shaoliu@Princeton.EDU> wrote:

> Hi Stephen and Lachlan,
> 
> Thanks for pointing out and fixing this bug.
> 
> For the max RTT problem, I have considered it also and I have some idea on
> improve it. I also have some other places to improve. I will summarize all
> my new ideas and send you an update. For me to change it, could you please
> give me a link to download to latest source codes for the whole congestion
> control module in Linux implementation, including the general module for all
> algorithms, and the implementation for specific algorithms like TCP-Illinois
> and H-TCP? 
> 
> Thanks for the help!
> -Shao
> 
> 
> 
> -----Original Message-----
> From: Stephen Hemminger [mailto:shemminger@linux-foundation.org] 
> Sent: Wednesday, November 28, 2007 4:44 PM
> To: Lachlan Andrew
> Cc: David S. Miller; Herbert Xu; shaoliu@Princeton.EDU; Douglas Leith;
> Robert Shorten; netdev@vger.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.
>

The following adds gradual aging of max RTT.

--- a/net/ipv4/tcp_illinois.c	2007-11-29 08:58:35.000000000 -0800
+++ b/net/ipv4/tcp_illinois.c	2007-11-29 09:37:33.000000000 -0800
@@ -63,7 +63,10 @@ static void rtt_reset(struct sock *sk)
 	ca->cnt_rtt = 0;
 	ca->sum_rtt = 0;
 
-	/* TODO: age max_rtt? */
+	/* add slowly fading memory for maxRTT to accommodate routing changes */
+	if (ca->max_rtt > ca->base_rtt)
+		ca->max_rtt = ca->base_rtt
+			+ (((ca->max_rtt - ca->base_rtt) * 31) >> 5);
 }
 
 static void tcp_illinois_init(struct sock *sk)