Subject: [bmwg] dsmterm-05 - Forwarding Congestion

From: "Perser, Jerry" <[email-masked]>
Date: Mon, 17 Feb 2003 11:19:20 -0800

Here are the changes from "Congestion" to "Forwarding Congestion".  Besides
the term, we have added two new paragraphs.  One defines what delay is not
used in determining Forwarding Congestion.  The other states that Forwarding
Congestion is different than Congestion Collapse and Incipient Congestion.

Jerry.

===================================================================

3.1.3 Forwarding 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 [Ma98]. 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 Forwarding Congestion and overload occurs when
the SUT comprises multiple elements, in that Forwarding 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 [Br91] deals only with overload on
ingress.

Packet Loss, not increased Delay, is the metric to indicate the condition of
Forwarding Congestion.  Packet Loss is a deterministic indicator of
Forwarding Congestion.  While increased delay may be an indicator of
Forwarding Congestion, it is a non-deterministic indicator of Forwarding
Congestion.  External observation of increased delay to indicate congestion
is in effect external observation of Incipient Congestion. [Ra99] states
that it is impractical to build a black-box test to externally observe
Incipient Congestion in a router.  For the purpose of "black-box" testing a
DUT/SUT, Packet Loss as the indicator of Forwarding Congestion is used.

Throughput [Br91] defines the lower boundary of Forwarding Congestion.
Throughput is the maximum offered rate with no Forwarding Congestion.  At
offered rates above throughput, the DUT/SUT is considered congested.

Ingress observations alone are not sufficient to cover all cases in which
Forwarding 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.

Forwarding Congestion, indicated by occurrence of packet loss, is one type
of congestion for a DUT/SUT.  Congestion Collapse [Na84] is defined as the
state in which buffers are full and all arriving packets must be dropped
across the network.  Incipient Congestion [Ra99] is defined as congestion
that produces increased delay without packet loss.

The definition presented here explicitly defines Forwarding 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
under Forwarding Congestion.

   Measurement units:
     none

   See Also:
Gateway Congestion Control Survey [Ma91]
___________________________________________________________ 
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

_______________________________________________
bmwg mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/bmwg