Subject: [PCN] PCN architecture, use of phrases like "exceeds rate"

From: <[email-masked]>
Date: Wed, 8 Aug 2007 18:37:13 +0100

I wrote a new / replacement para in the intro - to try to overcome bob's
rant - which complained about using phrases like "do marking when the
current rate exceeds a reference rate" [copied at end] - whilst still
giving the innocent reader (if such a person exists) a sporting chance
of understanding what it's all about:

<<Two rates, a PCN-lower-rate and a PCN-upper-rate, can be associated
with each link of the PCN-domain. Each rate is used by an algorithm
(specified in another document) that determines how and when a number of
PCN-packets are marked, and how the markings are encoded in packet
headers. PCN-egress-nodes make measurements of the packet markings and
send information as necessary to the nodes that make the decision about
which PCN-flows to accept/reject or terminate, based on this
information. Another document will describe decision-making algorithms.
Overall the aim is to enable PCN-nodes to give an "early warning" of
potential congestion before there is any significant build-up of
PCN-packets in the queue; the admission control mechanism limits the
PCN-traffic on each link to *roughly* its PCN-lower-rate and the flow
termination mechanism limits the PCN-traffic on each link to *roughly*
its PCN-upper-rate.>>

I know the last bit mentions rate - hopefully it's ok by relating it to
the overall system performance, rather than how each link does marking.
I'm not sure whether "*roughly*" is really necessary or whether you
think i cheated. 

I also added to the definitions for example:

<<Threshold-marking: a PCN-marking algorithm such that all PCN-traffic
is marked if the PCN-traffic exceeds a particular rate (either the
PCN-lower-rate or PCN-upper-rate). NOTE: The definition reflects the
overall intent of the algorithm rather than its instantaneous behaviour,
since the rate measured at a particular moment depends on the algorithm,
its implementation and the traffic's variance as well as its rate.>>

** bob's rant

> * Marking is when "current" rate is "above" a reference rate?

> "   o  Admission-marking: the marking of PCN-packets by a PCN-node to

>        indicate that the PCN-traffic on a link is above the
configured-

>        admissible-rate.

>     o  Termination-marking: the marking of PCN-packets by a PCN-node
to

>        indicate that the PCN-traffic on a link is above the
configured-

>        termination-rate.

> "

> These marking definitions and other examples hilighted in magenta in
the

> above PDF prejudge marking mechanisms as rate measurements.

> We don't just have marking when the _current_rate is _above_ the
reference

> rate. We can have marking when the _current_ rate is _below_, or when
the

> _past_ rate was _above_.

> The configured rates are parameters of a (to be defined) mechanism.
These

> mechanisms will not have to mark packets when the rate is above these

> configured rates. That depends on the mechanism.

> This sounds picky, but it's actually a big rant I have about all this

> loose

> discussion of rate (yes, the issue of what rate means is mentioned in
the

> draft, but we need to be careful about such sloppy usage). IMHO, it's

> important to scale our conception of time down to inter-arrival times,
to

> be precise. Then the rate is packet size/interarrival time. Therefore
seen

> at a small enough timescale, rate might be varying rapidly above and
below

> the configured rate.

> A marking mechanism could be implemented by a virtual queue rather
than a

> rate measurement. When the offered instantaneous rate has been below
the

> configured-admission-rate for many packets (perhaps hundreds), marking

> will

> and should still occur,... if the virtual queue is still above a
marking

> threshold even if it is emptying rather than filling. This can be a

> deliberate design choice that says we still want admission marking
when

> there has been recent overload to allow time for the control loop to
work.

> It ensures a greater safety margin is automatically created by the
marking

> mechanism when recent traffic had higher variance. This was the
marking

> algorithm that was most studied and simulated in CL-PHB draft.

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