Subject: [PCN] Summary

From: Michael Menth <[email-masked]>
Date: Fri, 22 Sep 2006 15:42:15 +0200

Summary

A "controlled load (CL) region" consists of ingress and egress gateways 
(routers) and of interior routers. An admission control (AC) function 
explicitly admits flows that want to use the CL service of the CL region 
and that enter and leave the region at its ingress and egress routers. 
The ingress and egress gateways are also called border routers and the 
overall CL traffic between two border routers is called a 
"border-to-border (b2b) CL aggregate". The flows need traffic 
descriptors that are enforced by a policing function at the ingress 
routers and they may also be used for the admission decision.

Each link in such a CL region is associated with a configurable 
"admissible CL link rate" and a configurable "supportable CL link rate" 
whereby the supportable CL link rate is usually significantly larger 
than the admissible CL link rate. When a link carries more traffic than 
its admissible link CL rate, no additional flows should be admitted to 
the network. Although flows are only admitted as long as the admissible 
CL link rate is not exceeded, the admitted traffic on a link can exceed 
the supportable CL link rate, e.g., due to changed flow behavior or due 
to redirected traffic after failures. In such a case, counter actions 
may be taken to reduce the traffic on the link, e.g., by preempting 
already admitted CL flows in order to restore the QoS of the CL service.

The AC function for a new flow entering the network at ingress router R0 
and leaving it at egress router R1 relies on feedback from the egress 
router R1 about the existing b2b CL aggregate between R0 and R1. This 
feedback is generated as follows. All routers meter the rate of the CL 
traffic on their outgoing links and mark the packets with "admission 
saturation" if the admissible CL link rate is exceeded. Egress router R1 
controls the admission saturation marks for each of its b2b CL
aggregates separately, and if the packets from R0 are admission 
saturation marked, no further flows from R0 to R1 should be admitted. 
This must be signaled from the egress R1 to the ingress R0. As the 
inter-arrival time of CL packets on a link varies, the rate of the CL 
traffic on the link is also subject to fluctuations. As a consequence, 
the admissible CL link rate may be sometimes exceeded. To get more 
stable information about the admission saturation for a b2b aggregate, 
the egress routers measure the fraction of the traffic that is 
associated with an admission saturation mark, and provide a "b2b 
admission saturation estimate" based on a moving average. If this value 
is large (close to 1), no further flows should be admitted.

A potential preemption function relies on the feedback from the egress 
routers about the b2b CL aggregates. This feedback is generated as 
follows. All routers meter the rate of the CL traffic on their outgoing 
links and mark only those packets with
"excess traffic" that exceed the respective supportable CL link rate. 
The egress routers measure for each b2b CL aggregate the rate of all 
received packets ("b2b received rate") and the rate of received packets 
with an excess traffic mark ("b2b excess rate"). If the b2b excess rate 
of an b2b CL aggregate is larger than zero, the rate of the b2b CL 
aggregate should be reduced by its b2b excess rate under the assumption 
that no packet loss occurred; otherwise, the b2b sent rate is required
from the corresponding ingress router to calculate the required traffic 
rate reduction: (sent rate - (received rate - excess rate)).

Thus, the following mapping of the terminology is used (here : Section 
1.2 of the draft):
http://www.ietf.org/internet-drafts/draft-briscoe-tsvwg-cl-architecture-03.txt 

admissible link rate: configured-admission-rate
supportable link rate: configured-preemption-rate
admission saturation mark: admission mark
excess traffic mark: preemption mark
b2b CL aggregate: CL-region-aggregate
b2b admission saturation estimate: congestion-level-estimate
b2b sent rate: ingress-aggregate-rate
b2b received rate: no equivalent
b2b excess traffic rate: no equivalent
An equivalent for sustainable-aggregate-rate is intentionally avoided
as this is only what it means if packets of the respective aggregate are
preemption marked.

I like the new notation better than the old one for the following reasons:
- The attributes "b2b" and "link" show the scope of the measured 
quantities which is of extreme importance for the understanding of how 
the measurement-based AC works.
- "admissible" and "supportable" express a threshold rather than an 
action (admission, preemption).
- The mark "excess traffic" just indicates that the admissible link rate 
is exceeded, but it does not imply any action like preemption.
- The mark "admission saturation" indicates that no further flows should 
be admitted whereas the mark "admission" can lead to confusion.
- This motivates also the "b2b admission saturation estimate".

I do not propose this nomenclature as the best one and that it should be 
copied, but I'd like to point out that the terminology can be made more 
precise.

Kind regards,

    Michael

-- 
Dr. Michael Menth, Assistant Professor
University of Wuerzburg, Institute of Computer Science
e CL link rate, e.g., due to changed flow behavior or due 
to redirected traffic after failures. In such a case, counter actions 
may be taken to reduce the traffic on the link, e.g., by preempting 
already admitted CL flows in order to restore the QoS of the CL service.

The AC function for a new flow entering the network at ingress router R0 
and leaving it at egress router R1 relies on feedback from the egress 
router R1 about the existing b2b CL aggregate between R0 and R1. This 
feedback is generated as follows. All routers meter the rate of the CL 
traffic on their outgoing links and mark the packets with "admission 
saturation" if the admissible CL link rate is exceeded. Egress router R1 
controls the admission saturation marks for each of its b2b CL
aggregates separately, and if the packets from R0 are admission 
saturation marked, no further flows from R0 to R1 should be admitted. 
This must be signaled from the egress R1 to the ingress R0. As the 
inter-arrival time of CL packets on a link varies, the rate of the CL 
traffic on the link is also subject to fluctuations. As a consequence, 
the admissible CL link rate may be sometimes exceeded. To get more 
stable information about the admission saturation for a b2b aggregate, 
the egress routers measure the fraction of the traffic that is 
associated with an admission saturation mark, and provide a "b2b 
admission saturation estimate" based on a moving average. If this value 
is large (close to 1), no further flows should be admitted.

A potential preemption function relies on the feedback from the egress 
routers about the b2b CL aggregates. This feedback is generated as 
follows. All routers meter the rate of the CL traffic on their outgoing 
links and mark only those packets with
"excess traffic" that exceed the respective supportable CL link rate. 
The egress routers measure for each b2b CL aggregate the rate of all 
received packets ("b2b received rate") and the rate of received packets 
with an excess traffic mark ("b2b excess rate"). If the b2b excess rate 
of an b2b CL aggregate is larger than zero, the rate of the b2b CL 
aggregate should be reduced by its b2b excess rate under the assumption 
that no packet loss occurred; otherwise, the b2b sent rate is required
from the corresponding ingress router to calculate the required traffic 
rate reduction: (sent rate - (received rate - excess rate)).

Thus, the following mapping of the terminology is used (here : Section 
1.2 of the draft):
http://www.ietf.org/internet-drafts/draft-briscoe-tsvwg-cl-architecture-03.txt 

admissible link rate: configured-admission-rate
supportable link rate: configured-preemption-rate
admission saturation mark: admission mark
excess traffic mark: preemption mark
b2b CL aggregate: CL-region-aggregate
b2b admission saturation estimate: congestion-level-estimate
b2b sent rate: ingress-aggregate-rate
b2b received rate: no equivalent
b2b excess traffic rate: no equivalent
An equivalent for sustainable-aggregate-rate is intentionally avoided
as this is only what it means if packets of the respective aggregate are
preemption marked.

I like the new notation better than the old one for the following reasons:
- The attributes "b2b" and "link" show the scope of the measured 
quantities which is of extreme importance for the understanding of how 
the measurement-based AC works.
- "admissible" and "supportable" express a threshold rather than an 
action (admission, preemption).
- The mark "excess traffic" just indicates that the admissible link rate 
is exceeded, but it does not imply any action like preemption.
- The mark "admission saturation" indicates that no further flows should 
be admitted whereas the mark "admission" can lead to confusion.
- This motivates also the "b2b admission saturation estimate".

I do not propose this nomenclature as the best one and that it should be 
copied, but I'd like to point out that the terminology can be made more 
precise.

Kind regards,

    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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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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