Subject: [PCN] PCN architecture, revised sub-section on probing

From: <[email-masked]>
Date: Wed, 8 Aug 2007 23:15:27 +0100

This is to flag up that I wrote a revised sub-section (in Section 5,
Detailed functional architecture) about probing. to reflect (hopefully!)
some other discussion we had on the list in july. 

5.5.  Probing functions

   Probing functions are optional, and can be used for admission

   control.  A PCN-ingress-node generates and sends probe packets in

   order to test the pre-congestion level.  Probing is useful or even

   essential under the following conditions:

   o  when an ingress-egress-aggregate carries no traffic (or too little

      traffic for the PCN-egress-node to accurately make the

      "measurements of PCN-traffic" that are required for an admission

      decision).  It may be that the traffic levels on other ingress-

      egress-aggregates are so high that a new flow shouldn't be

      admitted on the 'empty' ingress-egress-aggregate.  Probing is

      useful to check this.

   o  in the presence of multipath routing (ECMP) between the PCN-

      boundary-nodes, when some paths are pre-congested there may be

      other paths which aren't pre-congested.  Probing is useful to

      determine whether the new flow would follow a path that isn't pre-

      congested and hence can be admitted.

   Probe packets may be simple data addressed to the PCN-egress-node and

   require no protocol standardisation, although there will be best

   practice for their number, size and rate.  There are two

   possibilities for how probing is triggered:

   o  the PCN-egress-node requests (signals) the PCN-ingress-node to

      generate probe traffic

   o  if the PCN-ingress-node knows which PCN-egress-node is associated

      with the destination address in the admission request, then the

      PCN-ingress-node could know it has no reservation with that PCN-

      egress-node and unilaterally start probing.

   The probing functions are:

   o  Make decision that probing is needed

   o  (if required) Communicate the request that probing is needed - the

      PCN-egress-node signals to the PCN-ingress-node that probe traffic

      is needed

   o  Generate probe traffic - the PCN-ingress-node generates the probe

      traffic.  The appropriate number (or rate) of probe packets will

      depend on the PCN-marking algorithm; for example an excess-rate-

      marking algorithm generates fewer PCN-marks than a threshold-

      marking algorithm.

   o  Forward probe packets - as far as PCN-interior-nodes are

      concerned, probe packets must be handled the same as (ordinary

      data) PCN-packets.

   o  Consume probe packets - the PCN-egress-node consumes probe packets

      to ensure that they don't travel beyond the PCN-domain.

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---

From: Michael Menth <[email-masked]>
Date: Thu, 09 Aug 2007 13:10:35 +0200

Hi Phil,

the text misses the fact that probing is more difficult in the presence 
of ECMP. In this case, probe packets must have the same header as future 
data packets to be routed on the same path in order to give answer 
whether precongestion is on that path or not. This is an essential 
aspect of probing and requires protocol standardization.

Regards,

    Michael

[email-masked] wrote:
> Detailed functional architecture) about probing. to reflect 
> (hopefully!) some other discussion we had on the list in july.
>
>  
>
> 5.5.  Probing functions
>
>  
>
> ------------------------------------------------------------------------
>

-- 
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

---

From: "Jozef Babiarz" <[email-masked]>
Date: Fri, 10 Aug 2007 13:27:37 -0400

Phil and all, my comments below are denoted by [Joe]. 

Regards, Joe 
email:[email-masked] 
Telephone:613-763-6098 

________________________________

From: [email-masked] [mailto:[email-masked]] 
Sent: August 8, 2007 6:15 PM
To: [email-masked]
Subject: [PCN] PCN architecture, revised sub-section on probing

5.5.  Probing functions

[Joe] We need to decide if probing is optional or a mandatory function.
I think that we agree that it's needed if there is ECMP or some other
form of multipath routing. We also need probing when there is no traffic
present between ingress - egress for an aggregate or when there is only
one flow per ingress -egress. Probing is also very useful to verify that
media path is present between ingress and egress before new flow is
admitted. This is useful to avid the condition of a person answering the
phone and having no audio path available. When there are no tunnels
between ingress-egress nodes, probing can be used to discover egress
node and relay edge nodes address (more on this later).  Finally, we are
not sure if probing during admission of new flows would address issues
of over admission or provide some mitigation against DoS attacks. We
should have more information on this in few weeks. On the other hand,
probing is not need if we know that there is no ECMP or multipath
routing in the network and all ingress - egress aggregates will have at
least one flow present before pre-congestion occurs. So maybe in this
section we should word it such that it explains when probing is needed
and what problems it solves. [end]

      useful to check this.

[Joe] Agree that it's needed when there is no traffic between
ingress-egress-aggregate. I also believe that probing should be very
light weight, meaning that probing consume very little bandwidth in the
network. [end]

[Joe] Probe packets need to have the same source/destination IP address,
source/destination port number, protocol ID and DSCP value as media
packets otherwise the network may route probe packets along a different
path then media. [end]

      generate probe traffic

[Joe] I'm assuming this is to address the case when there is no traffic
flowing in a pre setup ingress-egress aggregate? Comment, it will not be
needed if we probe during admission of new flows. [end]

[Joe] Do not understand how the PCN-ingress-node knows which
PCN-egress-node the new flow that is going to be admitted will be routed
through? Can you explain what you are thinking about here? [end]

      is needed

[Joe] I think the PCN-ingress-node needs to trigger probing. For
admission control, it has a lot more information then the egress node.
The PCN-ingress-node has or needs to have source/destination IP address,
source/destination port numbers, protocol ID and DSCP value of the flow
that is to be admitted. Using this information the PCN-ingress-node
sends probe packets marked with IP header information as the new flow
will once admitted to discover which PCN-egress-node the new flow will
go through. The PCN-egress-node needs to intercept the probe packets and
send back to probe originating PCN-ingress-node the pre-congestion
information of the path between ingress-egress. For bi-directional flows
this needs to be done in both directions before admission decision is
made. [end]

      marking algorithm.

[Joe] I do not think and from simulation I've seen to date that
excess-rate-marking approach for admission control will work with
probing. Excess-rate-marking requires a lot of packets to be received
before any meaningful results can be obtained. [end]

      data) PCN-packets.

[Joe] Yes, the PCN-egress-node needs to consume probe packets. [end]

[Joe] Final note on probing and admission control. I've been exchanging
emails with Michael Menth on this subject and we are planning to write a
short draft that goes into more details in this area. Planning to send
it out to the list in September time frame. [end]

---

From: <[email-masked]>
Date: Wed, 15 Aug 2007 08:34:38 +0100

Yep, needs adding - at least for the second reason for probing (ECMP)

phil

> -----Original Message-----
> From: Michael Menth [mailto:[email-masked]]
> Sent: 09 August 2007 12:11
> To: Eardley,PL,Philip,CXR9 R
> Cc: PCN IETF list
> Subject: Re: [PCN] PCN architecture, revised sub-section on probing
> 
> Hi Phil,
>

---

From: <[email-masked]>
Date: Wed, 15 Aug 2007 09:06:35 +0100

In-line

phil

-----Original Message-----
From: Jozef Babiarz [mailto:[email-masked]] 
Sent: 10 August 2007 18:28
To: Eardley,PL,Philip,CXR9 R; [email-masked]
Subject: RE: [PCN] PCN architecture, revised sub-section on probing

Regards, Joe 
email:[email-masked] 
Telephone:613-763-6098 

5.5.  Probing functions

[Joe] We need to decide if probing is optional or a mandatory function.
I think that we agree that it's needed if there is ECMP or some other
form of multipath routing. 

[phil] talking specifically about the multipath case for probing. yes I
agree it's needed in principle. But it isn't clear to me whether you'd
choose to use it in practice - it would mean you'd always probe for
every call request, which could be a significant overhead & delay etc.
perhaps it's better to take the slight risk that you over-admit; or even
tunnel all data between ingress & egress (effectively turn off ecmp). To
me it looks like an operational choice that isn't clear-cut. 

We also need probing when there is no traffic present between ingress -
egress for an aggregate or when there is only one flow per ingress
-egress. 

[phil] yes

Probing is also very useful to verify that media path is present between
ingress and egress before new flow is admitted. This is useful to avid
the condition of a person answering the phone and having no audio path
available. 

[phil] not sure I get this. at least if there's traffic present on
ingress-egress-aggregate or probe gets through then this is ok.

When there are no tunnels between ingress-egress nodes, probing can be
used to discover egress node and relay edge nodes address (more on this
later).  

[phil] ok, I can imagine a probing mechanism might allow you to get this
info. Seems too specific to mention in this section?

Finally, we are not sure if probing during admission of new flows would
address issues of over admission or provide some mitigation against DoS
attacks. We should have more information on this in few weeks. 

[phil] sounds good, look forward to hearing more!

On the other hand,  probing is not need if we know that there is no ECMP
or multipath routing in the network and all ingress - egress aggregates
will have at least one flow present before pre-congestion occurs. So
maybe in this section we should word it such that it explains when
probing is needed and what problems it solves. 

[phil] I tried to word it like this!

[end]

      useful to check this.

[phil] yes (at least for the ecmp case)

      generate probe traffic

[phil]. Yes. This can be clarified. 

[phil] for example if it's seen the destination address before. Someone
mentioned this case, bob I think. 

      is needed

[phil] I think we're talking at cross-purposes here, I agree with what
you've written. I was thinking of the case of no traffic in
ingress-egress-aggregate where it's the egress that realises this &
tells the ingress to start probing. Bi-directional sessions are
mentioned in Section 6 (not a probing specific issue).

      marking algorithm.

[phil] seems important to me to try and quantify this - is this
something the simuations can get a handle on? should be added to list of
comparison factors when we're choosing a marking algorithm.

      data) PCN-packets.

[phil] excellent!

---

From: Michael Menth <[email-masked]>
Date: Wed, 15 Aug 2007 11:04:30 +0200

Hi Phil,

> [Joe] I do not think and from simulation I’ve seen to date that 
> [phil] seems important to me to try and quantify this – is this 
> something the simuations can get a handle on? should be added to list 
> of comparison factors when we’re choosing a marking algorithm.
>
This does not even require simulations. Assume that "excess-rate" 
marking is used for admission marking and admission is stopped if more 
than 3% of the traffic is marked. Then, at least 33 packets are needed 
for probing to admit a call. To obtain statistical reliability of 90%, 
several hundred probes are required.

Regards,

Michael

---

From: "Jozef Babiarz" <[email-masked]>
Date: Wed, 15 Aug 2007 17:10:03 -0400

See two comments in line marked with [Joe1]

Regards, Joe 
email:[email-masked] 
Telephone:613-763-6098 

From: [email-masked] [mailto:[email-masked]] 
Sent: August 15, 2007 4:07 AM
To: Babiarz, Jozef (CAR:0S03); [email-masked]
Subject: RE: [PCN] PCN architecture, revised sub-section on probing

In-line

phil

-----Original Message-----
From: Jozef Babiarz [mailto:[email-masked]] 
Sent: 10 August 2007 18:28
To: Eardley,PL,Philip,CXR9 R; [email-masked]
Subject: RE: [PCN] PCN architecture, revised sub-section on probing

Regards, Joe 
email:[email-masked] 
Telephone:613-763-6098 

5.5.  Probing functions

[Joe1] I see your point. Maybe we need to state under what conditions
probing should be used and let the operator decide. 

[phil] yes

[Joe1] Yes. This is useful when there is no traffic in the aggregate or
if the solution does not relay on ingress-egress aggregate and has
higher importance when we get into multi-domain configurations. 

[end]

      useful to check this.

      generate probe traffic

      is needed

      marking algorithm.

      data) PCN-packets.

[phil] excellent!