Subject: [PCN] Marking Converter for Excess-Marked Traffic

From: Michael Menth <[email-masked]>
Date: Mon, 07 Jul 2008 23:29:33 +0200

Hi,

we recently submitted a mechanism that converts PCN markings obtained 
based on an admissible rate (PCN lower rate) to equivalent PCN markings 
based on a supportable rate (PCN upper rate). This mechanism might be 
useful to implement marked flow termination with single marking, i.e. 
when only excess marked packets based on the admissible rate are 
available but excess marked packets based on the supportable rate are 
required. The mechanism is described in
http://www.ietf.org/internet-drafts/draft-menth-pcn-marking-converter-00.txt

Early simulation results show that the mechanism work somehow, but it 
has its limitations especially when many small ingress-egress-aggregates 
share a single link which is a realistic case. A summary of these 
results is given in Section 8 of
http://www.ietf.org/internet-drafts/draft-menth-pcn-performance-03.txt

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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---

From: <[email-masked]>
Date: Wed, 16 Jul 2008 17:36:58 +0100

Michael

I found this an interesting idea. Although I couldn't follow the draft
100% (could do with more explanation what the pseudocode is trying to
do! But it seems right to me).

If I get it right, this draft is about edge behaviour (PCN-egress-node)
for termination. It's assuming that an operator has only a single
codepoint and wants to achieve both PCN adm ctrl & termination, so is
using 'single marking' (ie using the Excess traffic meter function of
http://tools.ietf.org/id/draft-eardley-pcn-marking-behaviour-01.txt).
Anna has already described how edges can do admission control. And how
edges can do flow termination, using the approach of the egress
measuring the unmarked rate & the ingress the rate being sent (towards
this egress) & terminating the difference (Method A). 

You say that your idea enables an egress to terminate without rate
measurements, and so is a variant of the "terminate marked flows" idea.

As a WG we have the objective to produce Info RFCs describing edge
behaviour. Of course we have lots of possibilities & could try and
document several of them, but it would seem most realistic to me to try
& do one edge behaviour for admission and one for termination - at least
to start with; tackle more after one is done. 
I think it would be best to choose something that works for 'single
marking', including in the case where there's lots of dropping. At the
moment, to me the most promising choice for termination is therefore
Method A. however, your draft opens up another, interesting,
possibility.

Quick qus
- in your simulations, when you terminate a flow does it instantly cause
a reduction in bit rate on the bottleneck link, or do you simulate a
RTT?
- if there is a RTT, I'd have expected that need to do something similar
to EMFT (to avoid over-terminating, only terminate a flow with some
random probability)
- can you give an intuitive explanation of why there's over-termination
if the load on the bottleneck link comes from many
ingress-egress-aggregates? I didn't follow your explanation.
- when you say "measured rate termination [also] suffers from this", do
you mean Method A, or do you mean where the egress measures the rate of
marked traffic & terminates that. (the two should be equivalent, if
there's no dropping in the domain)
- have you studied the impact of dropping on this? (I assume it'll be
similar to EMFT, where it just slows down the rate at which the
PCN-domain goes from overload to being OK))

thanks
phil

{ -----Original Message-----
{ From: [email-masked] [mailto:[email-masked]] On Behalf Of
{ Michael Menth
{ Sent: 07 July 2008 22:30
{ To: pcn
{ Subject: [PCN] Marking Converter for Excess-Marked Traffic
{ 
{ Hi,
{ 
{ we recently submitted a mechanism that converts PCN markings obtained
{ based on an admissible rate (PCN lower rate) to equivalent PCN
markings
{ based on a supportable rate (PCN upper rate). This mechanism might be
{ useful to implement marked flow termination with single marking, i.e.
{ when only excess marked packets based on the admissible rate are
{ available but excess marked packets based on the supportable rate are
{ required. The mechanism is described in
{ http://www.ietf.org/internet-drafts/draft-menth-pcn-marking-converter-
{ 00.txt
{ 
{ Early simulation results show that the mechanism work somehow, but it
{ has its limitations especially when many small
ingress-egress-aggregates
{ share a single link which is a realistic case. A summary of these
{ results is given in Section 8 of
{ http://www.ietf.org/internet-drafts/draft-menth-pcn-performance-03.txt
{ 
{ 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
{ 
{ _______________________________________________
{ PCN mailing list
{ [email-masked]
{ https://www.ietf.org/mailman/listinfo/pcn
_______________________________________________
PCN mailing list
[email-masked]
https://www.ietf.org/mailman/listinfo/pcn

---

From: Michael Menth <[email-masked]>
Date: Wed, 16 Jul 2008 21:27:16 +0200

Hi Phil,

> http://tools.ietf.org/id/draft-eardley-pcn-marking-behaviour-01.txt).
The limitation "only a single DSCP is available for PCN marking" does 
not mean that only one marking scheme can be applied. The encoding 
scheme in 
http://www.ietf.org/internet-drafts/draft-menth-pcn-psdm-encoding-00.txt 
uses only one DSCP for PCN marking and supports concurrent excess and 
exhaustive marking such that probe-based admission control can be based 
on exhaustive marking  (with admissible rate as reference rate) and flow 
termination can be based on excess marking (with supportable rate as 
reference rate).

> a reduction in bit rate on the bottleneck link, or do you simulate a
We assume that a flow stops sending packets D_T time after the egress 
node decides that it should be terminated. We set D_T=200 ms, so it's in 
the order of an RTT.

> to EMFT (to avoid over-terminating, only terminate a flow with some
Yes. EMFT is used for termination.

> ingress-egress-aggregates? I didn't follow your explanation.
Assume  that there is no supportable rate (SR) pre-congestion, but (AR) 
pre-congestion. Then a certain amount of traffic is excess-marked. If 
the fraction of marked traffic is larger for a first aggregate and 
smaller for a second aggregate, the first aggregate will see marked  
packets even after conversion  and hence flows will be terminated.  This 
is why overtermination occurs. This happens more easily if the number of 
aggregates on a link is large because then packet markings within a 
aggregates become more random, i.e. they are not spaced in more or less 
regular intervals.

> there's no dropping in the domain)
I mean method A.

> PCN-domain goes from overload to being OK))
No, we have not looked at it.

Some additional comments. From our simulations we have seen that marked 
flow termination based on converted markings has limitations such that I 
don't see it as a significant improvement compared to Single Marking. It 
might work better with multipath routing, but also not 100% correctly.

Regards,

    Michael

---

From: <[email-masked]>
Date: Thu, 17 Jul 2008 14:08:45 +0100

{ 
{ > - can you give an intuitive explanation of why there's
over-termination
{ > if the load on the bottleneck link comes from many
{ > ingress-egress-aggregates? I didn't follow your explanation.
{ >
{ Assume  that there is no supportable rate (SR) pre-congestion, but
(AR)
{ pre-congestion. Then a certain amount of traffic is excess-marked. If
{ the fraction of marked traffic is larger for a first aggregate and
{ smaller for a second aggregate, the first aggregate will see marked
{ packets even after conversion  and hence flows will be terminated.
This
{ is why overtermination occurs. This happens more easily if the number
of
{ aggregates on a link is large because then packet markings within a
{ aggregates become more random, i.e. they are not spaced in more or
less
{ regular intervals.
{ 
does the over-termination get solved if you slow down the termination a
bit? (ie EMFT has lower probability of terminating a (Converted) marked
flow)

thanks
[phil

---

From: Michael Menth <[email-masked]>
Date: Thu, 17 Jul 2008 17:47:59 +0200

Hi Phil,

good question. I believe not because the reason for overtermination is 
different. But we take it as a hint for further studies. The findings of 
our quick simulations tell us it's a nice idea that has its drawbacks 
under challenging conditions. It might be useful under some 
circumstances, but we need to find out what they are.

Regards,

    Michael