Subject: RE: [PCN] Feedback to draft-briscoe-tsvwg-cl-architecture-03.txt

From: <[email-masked]>
Date: Fri, 13 Oct 2006 21:22:27 +0100

Whilst updating the draft I'm looking at this idea of yours

-----Original Message-----
From: Michael Menth [mailto:[email-masked]] 
Sent: 20 September 2006 07:44
To: Anna Charny (acharny)
Cc: [email-masked]
Subject: Re: [PCN] Feedback to
draft-briscoe-tsvwg-cl-architecture-03.txt

[snip]
>
> 7) Section 5.5
>
> Another potential solution: add the declared rate to the SAR at the
> ingress gateway as long as new flows send hardly any traffic. This
> information must be obtained from the corresponding policers which are
> also under the control of the ingress gateway. Or add it for an
initial
> period of e.g. 10 s?
>
>        [Anna]Just to understand what you mean: this is a proposal to
> solve 	 which aspect of the problem?
>   
This proposal tries to cope with flash crowds, i.e. with many almost 
simultaneous admission requests. The problem is that the AC can respect 
the impact of  newly admitted flows only after some time when their 
typical sent rate influences the admission marking in the routers. I 
think that there should be a mechanism to find out the minimum rate 
between two border routers that is still admissible. The sum of the 
overall declared rates of new flows should not consume more than this 
rate. A flow is new as long as its typical rate has not yet hat an 
impact on the packet marking, and to keep things simple, for an initial 
time of maybe 10 s.

[phil] your idea sounds interesting, but not quite sure how it works -
how do you find out the minimum rate between two border routers that is
still admissible?
This doesn't seem easy to me. You can't just subtract the current
ingress rate from some static maximum capacity for the aggregate between
the two border routers. Because this capacity depends on what all the
other aggregates are doing that share some of the same links as this
aggregate. 

I think you suggested in another email [sorry if I got this wrong, it's
getting late here..] sending a msg from ingress to egress that quizzed
each router en route about how much capacity it has left (below the
configured-admission-rate [aka admissible-link-rate]). I'm not sure I
like this - we're trying to have a solution that makes minimal impact on
existing routers, processing this sort of msg would be a significant
change?

Thanks
phil

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---

From: Michael Menth <[email-masked]>
Date: Mon, 16 Oct 2006 09:02:57 +0200

Dear Phil,

yes, I proposed "quizzing" the routers along the way for their unused CL 
capacity. When only little capacity is left, the ingress slows down its 
admission rate to, e.g. (1 Mbit/s)/10 s. Note that this does not require 
the routers to keep any states.

But if quizzing still too complex, "slow-admission marking" by 
intermediate PCN nodes could be used:

1) no marking: there is lot's of capacity on all links and new flows can 
be admitted as fast as possible
2) slow-admission marking: only little admissible capacity is left and 
the link runs the risk being overloaded by a flash crowd when a large 
rate of new flows is admitted within a short interval, therefore, 
additional flows should be admitted only slowly.
3) stop-admission marking: the admissible link capacity is consumed and 
no more flows over this link should be admitted
4) excess-traffic marking: the link carries too much traffic and flows 
should be preempted

Example:
a) slow-admission threshold: 10 Mbit/s
b) admissible link capacity: 20 Mbit/s
c) tolerable link capacity 40 Mbit/s
d) physical link bandwidth: 50 Mbit/s

a) If the CL traffic rate on a link exceeds the slow-admission 
threshold, all packets are slow-admission marked; the egress signals the 
information back to the respective ingress routers which slow down their 
admission rate for the specific egress to, e.g., 1 (Mbit/s)/10 s. As a 
consequence, flash crowds cannot swamp a medium loaded system.
b)  If the CL traffic rate on a link exceeds the admissible link 
capacity, all packets are stop-admission marked; the egress signals the 
information back to the respective ingress routers and the they stop 
admitting further calls to the specific egress.
c) If the CL traffic rate on a link exceeds the tolerable link capacity, 
all excess packets are excess-traffic marked; the egress signals the 
information back to the respective ingress routers and the they preempt 
the necessary number of calls to the specific egress.
d) If the CL traffic rate on a link exceeds the physical link capacity, 
packet loss and heavy delay occurs.

As 1), 3), and 4) already exist, two bits are needed for encoding the 
marking anyway, therefore, the implementation of 2) wouldn't need 
another bit. As slow-admission marking is technically the same as 
stop-admission marking, this solution should be compliant with the 
existing philosophy.

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