Subject: Latency and load

From: David Newman <[email-masked]>
Date: Thu, 19 Aug 1999 16:46:06 -0400

BMWGer's,

I'd like to start a discussion regarding the offered load to be used
when benchmarking latency of routers and switches.

I'm asking for two reasons:

1. Numerous devices from numerous vendors exhibit relatively high
latency when offered loads are at line rate, and substantially lower
latency with lower offered loads.

I believe this occurs because of congestion internal to the DUT/SUT. In
other words, congestion can still occur even when the offered load
itself should not produce congestion. In my experience, reducing the
offered load (to somewhere between 1 percent and 99 percent--it varies
by device) will clear the internal congestion and produce far lower
latency measurements.

2. I haven't been able to locate clear documentation on what offered
load should be used when making latency measurements. Section 26.2 of
RFC 2544 instructs testers to "first determine throughput" and then
"send a stream of frames ... at the determined throughput rate." 

This is problematic, since there isn't necessarily a strong correlation
between throughput and latency. I have encountered devices with
line-rate throughput that nonetheless exhibit less-than-optimal latency
when the offered load is 100 percent. Although RFC 1242 makes clear that
throughput doesn't necessarily mean line rate, there's still a problem:
I know of at least one device where throughput is 99 percent of the
theoretical maximum, yet latency doesn't settle down until the offered
rate is 96 percent or less. 

Maybe this issue should be clarified in the switching methodology. 

I would be very curious to hear what offered loads other BMWGer's use in
measuring latency.

dn

---

From: "mike winslow" <[email-masked]>
Date: Fri, 20 Aug 1999 01:00:47 -0400

It's quite possible that your test equipment  may be applying a timestamp to
the message prior to transmission.

If this is the case, then the measured latency will reflect the time in
queue awaiting transmission at the test equipment.

You can either calibrate your tests by looping the test equipment back on
itself (if possible) in order to evaluate the latency due just to the
tester, or better yet, apply a dual port monitor at the ingress and egress
points to make a truer measurement.

Mike Winslow
Director of Products, North America
Radcom Equipment Inc
1-800-RADCOM4 ext. 304
(pager) 1-800-609-9283
(car)  201-220-3218
(text page) [email-masked]
www.radcom-inc.com

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]]On Behalf Of David Newman
Sent: Thursday, August 19, 1999 4:46 PM
To: [email-masked]
Subject: Latency and load

BMWGer's,

dn