Subject: Re: CLP bit to QoS mapping

From: Mustapha Aissaoui <[email-masked]>
Date: Mon, 21 Jun 2004 14:49:51 -0400

Hi Fabio,
the mapping of the CLP bit to EXP is discussed in Section 11 of
draft-ietf-pwe3-atm-encap-05.txt. The authors did not specify rules
for the mapping nor values of the EXP field. This is usually a user
configurable mapping on a PE.

There are two cases to consider when using cell concatenation in any
of the two cell modes (1:1 and N:1). If the ATM VC is of a
CLP-transparent conformance definition (i.e., CBR, VBR.1), then the
value of the CLP bit does not matter. A single value of EXP should
reflect the QoS treatment required from the MPLS network. If it is a
CLP significant conformance definition, i.e., VBR.2 and VBR.3, then
a rule should be used in the mapping. An example of a rule is the
one used for mapping the CLP bit into the control word in the AAL5
modes. But this should be user-configurable.

Regards,
Mustapha. 
--------
Fabio Poggi wrote:
> 
> Hi all,
> I've read draft-martini-l2circuit-encap-mpls-07 and I've one question
> regarding cap. 4.2.4 "CLP bit to Quality of Service mapping":
> How this mapping is resolved in case of 'ATM Cells mode' encapsulation ?
> For example, in the diagram illustrating 'ATM Cells mode' encapsulation
> with two different ATM cells provided in the document. Conceptually the two
> ATM cells could have different CLP values. How these, probably different,
> values are used to set the 'unique' QoS field in the MPLS label stack ?
> 
> Is it possible to assume the same rule defined in 'ATM AAL5 CPCS-SDU Mode'
> for L (CLP) bit ?
> 
> Fabio
> 
> _______________________________________________
> pwe3 mailing list
> [email-masked]
> https://www1.ietf.org/mailman/listinfo/pwe3

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From: Luca Martini <[email-masked]>
Date: Mon, 21 Jun 2004 15:23:13 -0600

>values are used to set the 'unique' QoS field in the MPLS label stack ?
( if I understand correctly you are talking about cell concatenation )
Then each cell has to be encapsulated with it's own MPLS label stack.

>Is it possible to assume the same rule defined in 'ATM AAL5 CPCS-SDU Mode'
Currently, the preferred method is to have the ATM cell with the 
different CLP setting have it's own mpls label stack.
Luca

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From: Mustapha Aissaoui <[email-masked]>
Date: Tue, 22 Jun 2004 12:34:16 -0400

Luca,
see one comment below.

Mustapha.
-----------
> different CLP setting have it's own mpls label stack.

MA> Are you suggesting concatenation be aborted when a cell of a
different CLP value than the "currently concatenated cells" arrives?
I think this could be too disruptive and may not achieve the
objective of bandwidth efficiency. In my view, it is much easier to
use a rule similar to the one described for the AAL5 modes. Note
that such a rule would only apply to CLP-significant service
categories. For a CLP-transparent service category, there is no need
to differentiate between CLP0 and CLP1 cells.

---

From: Luca Martini <[email-masked]>
Date: Tue, 22 Jun 2004 18:27:03 -0600

>to differentiate between CLP0 and CLP1 cells. 
Well , the method described for AAL5 modes will cause ATM analyzers to 
detect errors. In AAL5 more this is not much of a problem as it ends up 
being a packet anyway. But in cell mode, an ATM analyzer will see the 
wrong data dropped by the mpls network. Not much, but still some people 
are very picky about it.
I am not saying that using the described AAL5 method for CPL to MPLS EXP 
bit marking is wrong, as it will probably be fine for 99% of the 
applications. But there is no reason why the concatenation timer cannot 
be triggered , and the packet sent if a cell with different CLP setting 
arrives ...

Luca

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From: Mustapha Aissaoui <[email-masked]>
Date: Wed, 23 Jun 2004 12:54:10 -0400

Luca,
see one clarification below.

Mustapha.
------------
> be triggered , and the packet sent if a cell with different CLP setting

MA> This case is different from the AAL5 behaviour. In
concatenation, The individual CLP setting of each cell is actually
maintained in the encapsulation. It is copied into the CLP field in
the ATM specific header in both the N:1 and 1:1 cell modes. The
remote PE will recover each indidividual cell CLP setting. The
compression of the CLP values is only performed for marking the EXP
field in the shim header.

---

From: Luca Martini <[email-masked]>
Date: Wed, 23 Jun 2004 15:42:31 -0600

Yes, so if you have congestion in the PSN, traffic marked with CPL=0 
will be dropped causing a SLA issue.

Luca