Subject: FW: [PCN] Feedback on draft-briscoe-tsvwg-cl-phb-02.txt

From: "Geib, Ruediger" <[email-masked]>
Date: Thu, 21 Sep 2006 09:27:14 +0200

Hi Michael,

while it is obvious that packets should start to get congestion marked after the admission rate has been passed, it is still not clear to me, what happens if the average traffic passes the admission rate for a longer duration. I'd prefer if the following was possible:

- settable admission rate, e.g. 50% of the link 
  bandwidth (marking starts after it's passed).

- settable maximum pcn rate, e.g. 80% of the 
  link bandwidth (dropping starts after it's passed).

If PCN works like that, single link failures would result in busy tones for new call attempts, but quality degradation for admitted calls could be avoided by planning (if desired). If I got 1) of your comments right, you propose to let bursts pass the admission rate, but still drop traffic after the admission rate is passed for times longer than burst intervals?

Regards,

Rüdiger

-----Original Message-----
From: Michael Menth [mailto:[email-masked]]
Sent: Thursday, September 21, 2006 8:23 AM
To: [email-masked]
Subject: [PCN] Feedback on draft-briscoe-tsvwg-cl-phb-02.txt

Hi all,

I quickly browsed through the draft on Pre-Congestion Notification marking
http://www.ietf.org/internet-drafts/draft-briscoe-tsvwg-cl-phb-02.txt
and want to give some comments.

1) I don't understand why Section 2 proposes a virtual queue with a
a probability-based marking when the queue size is within two thresholds.
What is the benefit of such an approach compared with the one of a queue
with a single threshold from which on packets are labelled if they exceed
this limit? Is this maybe a relict from an RED-based ECN-labelling which
might be useful in the presence of TCP traffic? From my perspective, the
task of admission marking is just to mark those packets that exceed the
configured admission rate and a virtual queue is used only to account for
the burstiness of the traffic and to allow some tolerance regarding the
rate. This is similar to the "cell delay variation tolerance (CDVT)" and
the "burst tolerance" in ATM.

2) I was puzzled by the fact that the Admission Marker uses a virtual
queue and that the preemption marker uses a token bucket. I'd prefer
if a solution is given in a similar manner, e.g., two virtual queues
that differ with respect to the following issues:
- Admission marking considers all realtime packets whereas preemption
marking considers only non-preemption-marked packets.
- The admission marker wants to find out whether the configured admission
rate is exceeded and if so, it wants to label all packets. Therefore,
the queue has infinite "buffer size" (probably set to a reasonable
value to recognize quickly if the rate has reduced) and all packets
beyond a certain threshold are labelled.
- In contrast, the preemption marker wants to mark packets that exceed
the configured preemption rate. Therefore, - if a virtual queue is
also used in this case - only those packets exceeding the threshold
are preemption-marked and these packets do not increase the queue
size.
- As the admission marker cannot be based on a token bucket, I consider
the virtual queue as the more appropriate unifying mechanism for both
markers.

Note that this would only describe the behavior of the markers, the
implementation can be done differently.

Best wishes,

    Michael

-- 
Dr. Michael Menth, Assistant Professor
University of Wuerzburg, Institute of Computer Science
Am Hubland, D-97074 Wuerzburg, Germany, room B206
phone: (+49)-931/888-6644, fax: (+49)-931/888-6632
mailto:[email-masked]
http://www3.informatik.uni-wuerzburg.de/research/ngn

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---

From: Michael Menth <[email-masked]>
Date: Thu, 21 Sep 2006 11:21:43 +0200

Hi Ruediger,

>   link bandwidth (dropping starts after it's passed).
Flow preemption starts - not packet dropping - just to avoid confusion! 
This is a virtual queue for the only purpose of metering traffic. These 
mechanisms do not operate on the physical buffer contents.

> If PCN works like that, single link failures would result in busy tones for new call attempts, but quality degradation for admitted calls could be avoided by planning (if desired). 
That's perfectly what's intended in the draft - at least I understood it 
this way - and I find it pretty good.

> If I got 1) of your comments right, you propose to let bursts pass the admission rate, but still drop traffic after the admission rate is passed for times longer than burst intervals?
>   
The marker tolerates short time burst above the 
configured-admission-rate and the size of the burst is configurable. If 
bursts are larger, packets are admission marked - not dropped! Again, 
this is not my proposal, that's what essentially Section 2.2 in the 
draft with  "min-marking-threshold=max-marking-threshold". I just 
wondered what the RED-like queue management should be good for and I 
don't like the different approaches to explain the two different markers 
(virtual queue, token bucket). However, I perfectly agree with their 
behavior.

Best wishes,

    Michael