Subject: Comments on draft-ietf-pwe3-atm-encap-00

From: [email-masked]
Date: Thu, 14 Nov 2002 17:19:19 +0000

Thanks to everyone who participated in pulling together the combined 
working group draft for ATM, draft-ietf-pwe3-atm-encap-00.txt.

I have a few comments and questions regarding the draft.

Section 4.1. The control word: 
In the draft, the 1:1, N:1, PDU and SDU modes have different control 
words. The control word shown in the figure in Sec 4.1 is in fact only 
applicable to the N:1 and the SDU modes, and not the 1:1 and PDU modes. 
The document is currently confusing and I think it would be clearer if the 
discussion of the detailed structure is moved to the N:1 and SDU sections, 
to be replaced by a top level view of the control word, together with the 
existing text on the functionality of the control word. The detailed 
structure of the control word for each mode is then discussed along with 
that mode. I can draft some text changes if the working group and editor 
agree, since this requires shifting some text around.

Section 5: ATM
In the introduction to the 1:1 cell mode, the encapsulations are basically 
the same, but with the addition of a VCI for VPCs. This Please could I 
propose the following replacement text for paragraph 2:
"The One-to-one mode specifies an encapsulation method that maps one ATM 
VCC or one ATM VPC to one Pseudo-Wire. For VCCs, the VPI/VCI is not 
included. For VPCs, the VPI is not included. Cells from one VCC or one VPC 
may be concatenated."

Section 6 and Section 7.1: 
We need to have a discussion about which cell modes are mandatory and 
which are optional. As the draft stands, only the N:1 cell mode is 
mandatory. However, as has already been discussed at length, there are 
issues with both fault diagnosis and QoS support in this mode because for 
N>1, you cannot identify individual connections once they are multiplexed 
on the pseudo-wire. It therefore makes sense to also make the 1:1 cell 
mode mandatory which does not suffer to this extent from these problems 
and allows operators to maintain many of the operational and provisioning 
procedures that they use today. Furthermore, it is apparent that the 1:1 
and N:1 cell modes have different applicabilities. Section 6.1 explicitly 
states the applicability of the 1:1 cell mode (as does Section 7.4 in the 
case of N:1). I would like to propose that which modes are mandatory for 
which applications is guided by the applicability statements. 

Section 6.6 ATM VCC Services: 
This section seems to be redundant.

Section 7.1 N:1 cell mode:
Do you intend that this mode is only able to carry ATM connections from 
the same ATM port on one pseudo-wire? If not, then somewhere it needs to 
be stated that the VCI/VPI values for the cells on different ATM 
connections on the same pseudo-wire must be unique so that the VCCs/VPCs 
can be demultiplexed properly at the egress PE. This means that VCI/VPI 
values must be translated to unique values if ATM connections from more 
than one port are to be carried on the pseudo-wire. 
If this is what you intend in this mode, then please consider adding a 
note below figure 8 as follows: 
"When multiple VCCs or VPCs are transported in one pseudo-wire VPI/VCI 
values MUST be unique. When the multiple VCCs or VPCs, are from different 
a physical transmission path it may be necessary to assign unique VPI/VCI 
values to the ATM connections. If they are from the same physical 
transmission path, the VPI/VCI values are unique."

Section 7.1 Paragraph 6, Sec7.5 Paragraph 1:
Please could you explain why you mention a SAR function here? A SAR in ATM 
is generally understood to be the function that segments an AAL SDU/PDU 
into ATM cell payloads, or reassembles ATM cell payloads into a complete 
SDU/PDU. Neither the 1:1 or N:1 cell mode have a SAR function, so 
mentioning it here is confusing. Please could I suggest that the text 
would be clearer if this statement is removed.

Section 7.4, Bullet iii:
Again, SAR is mentioned here, but cell concatenation is not a SAR process 
in the ATM sense as we are not segmenting or reassembling anything in the 
ATM layer. Please could the words "Like any SAR scheme," be removed. 

Section 7.5: Review of cell header information:
By "review", do you mean "processing" of the ATM cell header? If so, you 
always remove the HEC in the cell header in the ATM-PSN direction, and 
regenerate a HEC in the PSN-ATM direction. Therefore, you always process 
the ATM cell header and do not carry it transparently. Also, if you have 
to translate the VCI/VPI as described above, then this will also entail 
processing. I think it is really the ATM cell payload that is 
transparently carried, but not all of the header.
Also, in the last sentence of paragraph 1, and in paragraph 2, it is 
stated that service requirements can be derived from the ATM cell header. 
This is not completely true. It is true that you can look at the cell loss 
priority bit and map that to a drop precedence in the PSN, for example. 
However, if you are to map ATM QoS to a PSN CoS, you also need much more 
context information about the ATM connection (service category, 
conformance definition, QoS objectives, traffic parameters etc) that are 
only available through signalling or provisioning when you 
establish/modify the ATM connection.

Best regards,

Matthew