Subject: [PCN] Single DSCP for Admission and Flow Termination Marking

From: "Jozef Babiarz" <[email-masked]>
Date: Sun, 30 Mar 2008 15:10:05 -0400

I was thinking about the encoding and tunneling problem for PCN and I
think there may be a method by which a single DSCP and the ECN field can
be used to signal both admission control and flow termination using ECN
bits. Would like others to think about this and point out any issues
that I may have missed.

The proposed approach can use the EF-admit DSCP for PCN.

My assumptions:
The EF-admit DSCP is used for end-to-end admission control as proposed
by Fred Baker and the PCN wg also reuses this DSCP for PCN based
admission control and flow termination function for edge-to-edge. Should
note that I believe this can also be used for end-to-end PCN based
admission and termination but this is currently out of scope so I will
not discuss.

Proposed encoding:
A packet that is market with EF-admit DSCP means that it is admission
control with that ECN bits be used in the following way:
ECN = 00, means that packet is not-PCN-capable and that admission
control is done some other way.
Interior nodes remark ECN = "01" packets to "11" if traffic rate is
above PCN-lower-rate.
Interior nodes remark ECN = "10" packets to "11" if traffic rate is
above PCN-upper-rate. 

How it works:
For PCN based admission control this approach requires that "probe"
packet be sent at flow setup to determine if a new flow can be admitted.
Example of a probe packet is a RSVP Path message sent from
ingress-edge-node to egress-edge-node. The RSVP Path packet is market
with EF-admit DSCP and has its ECN bits set to "01". Interior nodes
meter EF-admit packets against the PCN-lower-rate and the result of the
meter is passed to the marker where the marker remarks "01" packet to
"11" if PCN-lower-rate was exceeded. The metering and marking behavior
is the so called "threshold marking" as defined in CL or 3sm drafts for
admission control.

For PCN based flow termination, once a flow is admitted,
ingress-edge-node marks admitted PCN traffic with ECN bits to "10".
Interior nodes meter EF-admit packets against PCN-upper-rate and the
results of the meter is passed to the marker where the maker remarks
"10" packets to "11" based on "excess-rate marking" with or without
marking frequency reduction. 

The egress-edge-node knows if the "11" market packet is a probe or it
belongs to an admitted flow therefore it can interpret what the marking
means. 

This sounds simple, so what have I missed? 
This approach works with tunneling, as ECN CE state is copied from outer
to inner IP header on de-encapsulation.
I believe that this marking also satisfies guidelines in RFC4774.
Are there any conditions where this marking approach will not work?

One final note:
If PCN wg decides to reuse the EF-admit DSCP for PCN than we need to ask
Fred Baker to add a statement in to his draft that the ECN bits in the
IP header are used for other function and that RFC3168 does not apply to
EF-admit DSCP as per guidance in RFC4774.

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

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---

From: Lars Eggert <[email-masked]>
Date: Mon, 31 Mar 2008 10:15:46 -0700

Hi, Joe,

> packet be sent at flow setup to determine if a new flow can be  

one quick comment: do I understand this correctly that this encoding  
would require probing, i.e., make it a mandatory part of the  
architecture?

Lars

---

From: Michael Menth <[email-masked]>
Date: Tue, 01 Apr 2008 00:40:57 +0200

Hi Joe,

if I understand correctly, your idea is the following:

Apply marking based on admissible rate (AR) only to probe packets (01) 
and possibly switch to 11 in case of AR-pre-congestion
Apply marking based on supportable rate (SR) only to data packets (10) 
and possibly switch to 11 in case of SR-pre-congestion
Probe and data packets can be differentiated (somehow) by the egress 
node, therefore, the egress knows whether 11 means AM or TM.

Lars pointed out that this approach implicitly assumes that AR-marking 
is applied only to probe packets and I share his view. Per-flow probing 
is one option for this architecture, which could be implicitly done with 
RSVP-PATH messages or explicite probes. Another option are regular probe 
messages per IEA to keep its admission/block state up to date. This 
should not be a problem when regular transmission of CLE values from 
egress to the ingress nodes is also considered feasible (cf. CL, SM). 
It's basically a ping every e.g. 100ms between communicating 
ingress-egress node pairs.

What we lose: picky-backing pre-congestion notification concerning 
admissible rates onto data packets
What we gain: possibility of using more accurate admission control based 
on threshold marking (applicable to smaller IEAs) combined with flow 
termination, ability to cope with ECMP (assumption: per-flow probing), 
and independence of admissible and supportable rates (SR=u*AR not 
needed) per link (simpler/more flexible configuration of PCN rate 
thresholds) while using only a single DSCP.

Regards,

Michael

Jozef Babiarz wrote:
> think there may be a method by which a single DSCP and the ECN field 
> can be used to signal both admission control and flow termination 
> using ECN bits. Would like others to think about this and point out 
> any issues that I may have missed.
>
> admission control and flow termination function for edge-to-edge. 
> Should note that I believe this can also be used for end-to-end PCN 
> based admission and termination but this is currently out of scope so 
> I will not discuss.
>
> Proposed encoding:
>
> Interior nodes remark ECN = “01” packets to “11” if traffic rate is 
> above PCN-lower-rate.
>
> Interior nodes remark ECN = “10” packets to “11” if traffic rate is 
> above PCN-upper-rate.
>
> How it works:
>
> For PCN based admission control this approach requires that “probe” 
> packet be sent at flow setup to determine if a new flow can be 
> admitted. Example of a probe packet is a RSVP Path message sent from 
> with EF-admit DSCP and has its ECN bits set to “01”. Interior nodes 
> meter EF-admit packets against the PCN-lower-rate and the result of 
> the meter is passed to the marker where the marker remarks “01” packet 
> to “11” if PCN-lower-rate was exceeded. The metering and marking 
> behavior is the so called “threshold marking” as defined in CL or 3sm 
> drafts for admission control.
>
> ingress-edge-node marks admitted PCN traffic with ECN bits to “10”. 
> “10” packets to “11” based on “excess-rate marking” with or without 
> The egress-edge-node knows if the “11” market packet is a probe or it 
> belongs to an admitted flow therefore it can interpret what the 
> marking means.
>
> This approach works with tunneling, as ECN CE state is copied from 
> outer to inner IP header on de-encapsulation.
>
> If PCN wg decides to reuse the EF-admit DSCP for PCN than we need to 
> ask Fred Baker to add a statement in to his draft that the ECN bits in 
> the IP header are used for other function and that RFC3168 does not 
> apply to EF-admit DSCP as per guidance in RFC4774.
>
> ///Regards, Joe/
>
> email:[email-masked]
>
> Telephone:613-763-6098
>
> ------------------------------------------------------------------------
>

-- 
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: Mon, 31 Mar 2008 19:33:02 -0400

This encoding requires for the ingress and egress-edge-nodes to be able
to differentiate which packet that is "11" marking represents
pre-congestion marking for admission control and which represent for
flow termination. So for example if the ingress-edge-node sends
RSVP-PATH message the egress-edge-node would know that this packet is a
probe and "11" marking means that traffic level along the path is above
the PCN-low-rate, therefore would send response to ingress not to admit
the new flow. So yes this encoding approach would require some form of
probing or exchange of information between ingress and egress nodes
before a new flow is admitted. Without "probing" the ingress-edge-node
nodes does not know what path or egress-edge-node the new flow once
admitted will be routed. 

Does probing need to be mandatory? I believe that it should be highly
recommended as it addresses citrine problems in PCN architecture.  

Regards, Joe
email:[email-masked]
Telephone:613-763-6098
-----Original Message-----
From: Lars Eggert [mailto:[email-masked]] On Behalf Of Lars
Eggert
Sent: March 31, 2008 1:16 PM
To: Babiarz, Jozef (CAR:0S03)
Cc: [email-masked]
Subject: Re: [PCN] Single DSCP for Admission and Flow Termination
Marking

Hi, Joe,

On 2008-3-30, at 12:10, ext Jozef Babiarz wrote:
> admitted.

Lars