Subject: RE: [bmwg] dsmterm-05 - Forwarding Congestion

From: Michele Bustos <[email-masked]>
Date: Tue, 4 Mar 2003 16:54:27 -0800

Dear Scott,

Thank you for your clear explanation. :)

Unfortunately, I did not gain that same level of understanding from reading
the doc.

"Throughput is the maximum offered rate with no Forwarding Congestion."

It appears from your explanation that the point of the throughput reference
is to provide an example, however the doc reads as though it's a
redefinition of throughput in relation to the more general definition of
'congestion', as suggested in an earlier thread by Kevin, defined in
rfc1254.

http://www1.ietf.org/mail-archive/working-groups/bmwg/current/msg00283.html

kind regards,

/m

-----Original Message-----
From: Scott Poretsky [mailto:[email-masked]]
Sent: Tuesday, March 04, 2003 3:46 PM
To: Michele Bustos; Jerry Perser (E-mail)
Cc: [email-masked]
Subject: RE: [bmwg] dsmterm-05 - Forwarding Congestion

At 01:40 PM 3/4/03 -0800, Michele Bustos wrote:

In section 3.1.3 Forwarding Congestion:

"At offered rates above throughput, the DUT/SUT is considered congested."

Can you explain this?

In my understanding of the classic definition of throughput and congestion,
the DUT/SUT could be congested long before any packets are dropped. 

This is called "Incipient Congestion" as defined in RFC 2481.  This is the
case in which queues are building, but not full.  Incipient Congestion is
externally observable as increased delay.  A full queue results in packet
loss.  We have defined the term Forwarding Congestion to describe the
condition in which queues are full and packet loss is externally observable.
RFC 2481 states that black-box measurement of Incipient Congestion is not
practical.  Increased delay is non-deterministic indicator of congestion,
but packet loss is a deterministic indicator of congestion.  For the
practical purposes of DiffServ measurement, we have specified Forwarding
Congestion as the indicator for congestion.  For reference, here is the
discussion from the draft....

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.

thx,
/m
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---

From: Kevin Dubray <[email-masked]>
Date: Tue, 04 Mar 2003 20:19:31 -0500

A comment on the thread:

The nature of my question on the topic was to a) reconcile input
offered at the Atlanta BMWG meeting, b) understand why "lossful"
in the term's handle isn't desirable. (Acutally the minutes make
reference to the notion of "loss-indicated congestion" - my mistake).

An alternative viewpoint offered to me by the principals of the
draft reflects 1) forwarding congestion is really only offered
in the context of this benchmarking draft (and its
subsequent methodology) and 2) use of "lossful" with the notion
of "forwarding congestion" is editorially redundant given the
discussion of "forwarding congestion" in the I-D.

I assure you no worms were harmed in the opening of this can. :-)

-Kevin

---

From: Kevin Dubray <[email-masked]>
Date: Tue, 04 Mar 2003 20:19:31 -0500

A comment on the thread:

-Kevin