[c-nsp] High latency/jitter at times on a NPE-G1?

[Min Qiu]

No need for a complicate lab.  In this case, all we need is 
to ping through the box to a well defined target (let say a 
PC with known good respond time) and compare the result(round 
trip time and rtt jitter) of pinging straight to the target:

  test 1:  ping ---> target
  test 2:  ping ---> NPE-GE --> target 

The statistical difference of test 1 and 2 has to be introduced 
by NPE-GE.

Min

> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net 
> [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Mark Rogaski
> Sent: Thursday, June 01, 2006 10:36 AM
> To: cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] High latency/jitter at times on a NPE-G1?
> 
> 
> An entity claiming to be Rodney Dunn (rodunn at cisco.com) wrote:
> : If you want to prove it put a sniffer/analyzer on both sides
> : of the box and time sync the traces.
> : 
> : That is just about the ONLY way you can 100% prove the box
> : is introducing latency/jitter in the forwarding stream.
> 
> Actually, jitter can be measured with a single analyzer by 
> measuring the
> variation of the timestamp deltas.  Latency is trickier to 
> measure, but as
> long as you have bidirectional traffic you can measure the 
> the net round
> trip delay encountered.  Just pick a packet A travelling to 
> the destination
> and a packet B returning from the destination.
> 
> Let A_1 and B_1 be the timestamps for the respective packets 
> on analyzer #1
> and A_2 and B_2 be the timestamps for the respective packets 
> on analyzer
> #2.
> 
> Net RTD = abs(abs(A_1 - B_1) - abs(A_2 - B_2))
> 
> This doesn't tell you how much latency is being introduced, 
> but it does
> tell you if the box is introducing latency.  The nice part is 
> you don't
> need to sync clocks on the analyzers.
> 
> 
> : 
> : I'd be very amazed if it's the 72xx because it's software forwarding
> : and once the packet goes in the switching vector it can't be
> : held up other than if there is congestion for the most part.
> : 
> 
> Actually, if the device is doing the forwarding under load 
> I'd be surprised
> if it wasn't introducing latency.
> 
> Mark
> 
> -- 
> []                    |  A computer lets you make more 
> mistakes faster than
> [] Mark Rogaski       |  any invention in human history - 
> with the possible
> [] wendigo at pobox.com  |  exceptions of handguns and tequila.
> [] mrogaski at cpan.org  |  -- Mitch Ratliffe, Technology Review 
> April, 1992
> []                    |
>