Re: [PCN] Concensus questions from Thursday's PCN meeting

Hi Ruediger

I will try to explain this more clearly.
Here are the assumptions:
* In the PCN domain we assume that ECMP routing is possible!
* ingress-eggress-aggregate can contain flows that are passing 
through one path or more paths (when ECMP routing is used).

* routers are currently dropping packets randomly. Thus marked and unmarked
packets will be 
  dropped randomly

* When excess rate measurements are used, the triggering of admission
control and flow 
  termination are done at the egress by using the CLE. One example of this
trigger is:
  CLE > 1%. Note that CLE = marked/(unmarked + marked). This means that if
this trigger is not activated while a severe
  congestion occurs in the PCN domain, then the operation of the PCN domain
will completely collapse.

My statement is:
If the routers do not preferentially drop marked packets then the PCN domain
operation, even in corner case and misconfiguration 
situations, is more robust and more stable than in the situation that the
router is preferentially dropping marked packets.

The below example show such a corner case!

I will describe two situations:

Situation 1:
Consider that an ingress-egress-aggregate due to ECMP routing it 
includes flows that are passing from at least two paths.
Assume that path1 supports a maximum bandwidth capacity of C. Now consider 
that the maximum bandwdith capacity of path2 is 15*C.
Consider also that both paths are fully utilized.

Consider that preferentially marked packets are dropped and that path2
fails.
Now assume that all (maximum) traffic passing through path2 will be rerouted
through path1. 
If a bottleneck router located in path1, say Rbott, is misconfigured (i.e.,
from the point of 
view of having wrongly too high configured-admissible-rate value), or even
if it is well configured,
then it can be possible that CLE measured at the egress cannot reach 1%.
Note that 
CLE = marked packets/ (marked packets + unmarked packets).
This is because Rbott will just allow an excess rate to pass through that is

equal to: C - configured-admissible-rate. The rest of the marked packets, so
the 
rest of the excess rate, will be dropped by Rbott, since this router is 
preferentially dropping marked packets.
Furthermore, note that due to the ECMP routing flows that are belonging to 
the same ingress-egress-aggregate and that are passing through another path,

than path1, which is not congested, will forward traffic towards the egress
node
that will be unmarked.
This will mean that the CLE > 1% will not be triggered, this will mean that 
the flow termination will not be triggered and that the operation of the PCN
domain 
will collapse completely.

Situation 2:
Now consider that the router just operates as currently, i.e., no
preferential drop, 
randomly dropping marked and unmarked packets.
Consider also that the two paths described in Situation 1 above are used, 
ECMP routing is used, and that they are fully utilized.
Assume also that path2 fails and that the path2 traffic is rerouted on
path1.
Now the CLE value will have a higher probability of reaching the triggering
value of 1%.

This is because the routers in path1 will drop randomly marked and unmarked
packets.
It is more certain that in this case the CLE will reach 1% due to the
following reason.
In this example it is assumed that the maximum bandwidth capacity supported
by path2 is 15*C. 
This means that after rerouting the traffic from path2 into path1, the ratio
between marked 
packets and unmarked packets that are passing thorugh path1 can be equal or
higher than 16.
Note that the routers in path1 will mark the excess rate above C, thus
16 * C (i.e., the rerouted traffic from path2) as marked.
The above observation holds also for the situation that the routers are
preferentially 
dropping unmarked packets.

Thus when routers are preferentially dropping marked packets the robustness
of the 
PCN domain operation is decreasing and in some cases it severely decreases, 
which could cause the complete collapse of the PCN domain operation.

Best regards,
Georgios

> -----Original Message-----
> From: Geib, Ruediger [mailto:Ruediger.Geib@t-systems.com] 
> Sent: maandag 17 maart 2008 9:42
> To: karagian@cs.utwente.nl
> Cc: pcn@ietf.org
> Subject: RE: [PCN] Concensus questions from Thursday's PCN meeting
> 
> Hi Georgios,
> 
> PCN is designed for deployment in traffic engineered 
> networks. Please decribe, how to engineer a high performance 
> carrier backbone and then review the assumptions your 
> discussion is based on.
> 
> Regards,
> 
> Rudiger
> 
> | -----Original Message-----
> | From: Georgios Karagiannis [mailto:karagian@cs.utwente.nl]
> | Sent: Monday, March 17, 2008 9:34 AM
> | To: Geib, Rüdiger
> | Cc: pcn@ietf.org
> | Subject: RE: [PCN] Concensus questions from Thursday's PCN meeting
> | 
> | Hi Ruediger
> | 
> | What do you mean?
> | Do you mean that you do not want to discuss corner cases 
> (ECMP related
> | cases) that could
> | collapse the PCN domain operation?
> | What I am saying is that if the routers do not preferentially drop 
> | marked packets then the PCN domain operation is more robust 
> and more 
> | stable than in the situation that the router is preferentially 
> | dropping marked packets.
> | 
> | Are you saying that this statement is not right?
> | 
> | 
> | Best regards,
> | Georgios
> |  
>

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