Subject: [bmwg] Congestion Definition in dsmterm03

From: "Perser, Jerry" <[email-masked]>
Date: Tue, 3 Sep 2002 14:46:55 -0700

Kevin,

Below is new definition (and discussion) for congestion.

The plan is to put it in the 04 version along with a change to "delay
variation."  I am current changing "delay variation" to "jitter" because way
we are using the term complies with the use of "jitter" in RFC1889 and
RFC2598 more that "delay variation" in draft-ietf-ippm-ipdv-10.txt (which
IESG has approved as a Proposed Standard).

3.1.3 Congestion

   Definition:
A condition in which one or more egress interfaces are offered more packets
than are forwarded.

   Discussion:
This condition is a superset of the overload definition [2]. The overload
definition assumes the congestion is introduced strictly by the tester on
ingress of a DUT/SUT. That may or may not be the case here.

Another difference between congestion and overload occurs when the SUT
comprises multiple elements, in that congestion may occur at multiple
points. Consider an SUT comprising multiple edge devices exchanging traffic
with a single core device. Depending on traffic patterns, the edge devices
may induce congestion on multiple egress interfaces on the core device. In
contrast, overload [1] deals only with overload on ingress.

Throughput [1] defines the lower boundary of congestion.  Throughput is the
maximum offered rate with no congestion.  At offered rates above throughput,
the DUT/SUT is considered congested.

Ingress observations alone are not sufficient to cover all cases in which
congestion may occur. A device with an infinite amount of memory could
buffer an infinite amount of packets, and eventually forward all of them.
However, these packets may or may not be forwarded during the test duration.
Even though ingress interfaces accept all packets without loss, this
hypothetical device may still be congested.

The definition presented here explicitly defines congestion as an event
observable on egress interfaces. Regardless of internal architecture, any
device that cannot forward packets on one or more egress interfaces is
congested.

___________________________________________________________ 
Jerry Perser                         Spirent Communications
Director of Test Methodology         SmartBits Division 
Phone: 818.676.2320                  26750 Agoura Road 
Lab:   818.676.2337                  Calabasas, CA, 91302 
Cell:  818.292.6457                  Corp:  818.676.2300

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