[c-nsp] TCP behavior under strict CAR rate-limiting

[Phil Bedard]

If the normal burst value is too low you may always be exceeding the  
normal burst limit.  You can do a show int rate-limit to look at  
current burst values.

Phil

On Jun 19, 2008, at 12:00 PM, Christopher Hunt wrote:

> Sorry,
> The rate-limit statement that results in 0.2mbps throughput is:
> rate-limit input 10000000 5000 5000 conform-action transmit exceed- 
> action drop
>
> -------- Original Message --------
> Subject: 	TCP behavior under strict CAR rate-limiting
> Date: 	Thu, 19 Jun 2008 08:11:36 -0700
> From: 	Christopher Hunt <chunt at reachone.com>
> Organization: 	ReachONE Internet Inc.
> To: 	cisco-nsp at puck.nether.net
>
>
>
> Please excuse me if this is off-topic.  I'm needing some expert TCP  
> analysis and not sure where to turn.  I'm experimenting with CAR  
> rate-limiting using the Cisco IOS.  I am using Iperf to test TCP  
> throughput, PRTG to poll the Cisco interface traffic levels each  
> second and Wireshark to capture the packets.  This is a lab, so  
> there are no public IPs in use.    Using the rate-limit statement  
> "rate-limit input 10000000 1875000 3750000 conform-action transmit  
> exceed-action drop" I see expected behavior, that is an average of  
> 10mbps with bursts up to ~18mbps.
> Using the rate-limit statement "rate-limit input 10000000 1875000  
> 3750000 conform-action transmit exceed-action drop" I see an average  
> of 0.02mbps (200 bits/second) with no bursting at all.
> I expected that the cause would be excessive drops by the Cisco  
> interface causing the TCPs on either end to negotiate a low  
> tcp_receive_window, but the packets don't bear that out.  The window  
> sizes are staying high. The cisco interface doesn't show many drops  
> nor does it ever seem to receive the 10mbps rate, even for a  
> second.  I expect that even retransmits, duplicate ACKS etc. would  
> increment packet counters on the Cisco.Any ideas what else could  
> cause low throughput besides a low tcp_receive_window?
> -- 
> Christopher Hunt
>
>
>
> -- 
> Christopher Hunt
> ReachONE Internet, Inc.
> (888)820-7559
>
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