[PCN] Comments about CL and SM edge behavior

[Michael Menth <[e-mail]>]

Hi all,

as I cannot participate in this IETF meeting, I would like to stimulate 
some discussion regarding the issues I find important. The PCN WG has 
currently two edge behaviors: SM and CL.
http://www3.informatik.uni-wuerzburg.de/staff/menth/Publications/papers/draft-charny-pcn-single-marking-edge-behaviour-00.txt
http://tools.ietf.org/html/draft-taylor-pcn-cl-edge-behaviour-00

I would like to propose the following changes to both of them:
1) In CL and SM, the PCN egress node currently signals rates of marked 
and unmarked traffic periodically to  the PCN ingress node which takes 
the AC decision based on this marking information.
Proposal: Calculate new AC state (block/admit) locally at the PCN egress 
node and signal only changes to PCN ingress node.
Reason: a) This reduces unnecessary signalling. b) It allows simple 
integration of other AC methods. Is that needed? Yes! CLE-based AC 
cannot block empty ingress-egress aggregates (IEAs) and there will be 
many of them. Probing for IEAs helps and is simple: explicit probes are 
periodically sent from ingress to egress (not per flow!). The AC state 
is set to block when a marked probe packet is received  and set back to 
admit after some time. More on these options see Section VI.B of
http://www3.informatik.uni-wuerzburg.de/~menth/Publications/papers/Menth08-PCN-Overview.pdf

2) I think the current flow termination mechanisms of CL and SM have the 
following two shortcomings:
* The PCN egress node measures rates of marked and umarked traffic, 
calculates the edge-to-edge supportable rate (ESR), and sends it to the 
PCN ingress node. The PCN ingress node measures the ingress rate (IR) 
and calculates the termination rate TR=IR-ESR. This requires that the 
values for IR and ESR must timely correspond each other to avoid 
potential overtermination. You may call that a measurement bias. See 
Section IV.C of 
http://www3.informatik.uni-wuerzburg.de/~menth/Publications/papers/Menth08-Sub-9.pdf
* The current flow termination mechanism requires preferential dropping 
of marked packets to avoid overtermination. When a PCN domain uses both 
upgraded PCN nodes and legacy nodes that are not aware of this packet 
dropping strategy, then overtermination occurs if legacy nodes drop 
unmarked packets in case of congestion. This case is not in charter but 
still relevant.

Proposal 2.a) solves only the first problem:
So far, the FT entity chooses a set of flows such that their overall 
rate equals the termination rate, and terminates these flows. Proposed 
change: the FT entity chooses a set of flows such that their overall 
rate equals the edge-to-edge supportable rate and terminates the 
remaining flows. This makes the correlation of correct measurement 
values to calculate the termination rate obsolete.
More on these options see Section VII.A.1 and VII.B of:
http://www3.informatik.uni-wuerzburg.de/~menth/Publications/papers/Menth08-PCN-Overview.pdf

Proposal 2.b) solves both problem, but works only for CL:
Measure only marked traffic and take it as direct estimate for the 
termination rate.
More on these options see Section VII.B.1.a of:
http://www3.informatik.uni-wuerzburg.de/~menth/Publications/papers/Menth08-PCN-Overview.pdf
Advantage: the above problems are solved as this mechanism is not prone 
to overtermination due to measurement bias or dropping of special packets.
Disadvantage: the proposed method requires several termination steps in 
case of very large overload when marked packets are lost.

3) Currently, PCN ingress nodes terminate flows when necessary.
Proposal: let the PCN egress nodes terminate the flows.
Reason: a) This makes termination signalling obsolete. b) The PCN egress 
node can track for which flows marked packets were recently received 
(marked flows). When only marked flows are terminated, CL's FT mechanism 
takes correct termination decisions and SM's FT mechanism takes better 
termination decisions in case of multipath routing. To support multipath 
routing, the alternative is to signal the information about marked flows 
from the PCN egress to the ingress. This principle can be applied to any 
termination mechanism. More on these options see Section VII.A of:
http://www3.informatik.uni-wuerzburg.de/~menth/Publications/papers/Menth08-PCN-Overview.pdf

I would be happy to see discussion about these architectural 
modifications that do not change the heart of SM and CL but obviously 
improve them.

Regards,

    Michael

-- 
Dr. Michael Menth, Assistant Professor
University of Wuerzburg, Institute of Computer Science
Am Hubland, D-97074 Wuerzburg, Germany, room B206
phone: [phone], fax: [phone]
mailto:[e-mail]
http://www3.informatik.uni-wuerzburg.de/research/ngn