Subject: RE: RE: [802.1] 802.1ad Customer FCS Retention

From: Keith Knightson <[email-masked]>
Date: Mon, 24 Nov 2003 10:35:09 -0500

At 3:55 PM -0800 23/11/03, Mick Seaman wrote:
>Not withstanding the claim that an Ethernet frame has some called
>characteristic information (CI) that one cannot change and that FCS is part
>of it, no such notion (CI) exist in the MAC Service or Internal Sublayer
>Service or bridge specifications (or I believe in ISO 7498 including its
>addenda and interpretations) and Keith is entirely correct in saying that
>the MAC Service (as delivered to an LLC Entity and specified in ISO 15802-1
>,formerly ISO 10039) does not have FCS as a parameter.
>
>Moreover the MAC Service is provided at an abstract interface in a system
>and there is no requirement to associate an FCS with the parameters of the
>service until they have been encoded and transmitted in a frame by media
>access method specific functions. The coverage of an FCS is not from service
>user to service user, it is across media.
>
>Mick
>
Mick,

Exaclty, so.

The FCS is not part of the Service Data Unit (i.e the client layer, 
for those who insist) delivered to the MAC Sublayer  (i.e. the server 
layer) for transmission.   If the transmission is a PW one simply 
uses the framing of that instead of the MAC Sublayer, IP or whatever 
it is that is "acting" as the transmission layer for the next link. 
The PE is clearly not a repeater. Layers are terminated. Relay 
functions are included.

IMHO, it is thus the SDU, (i.e. the frame and not the transmission 
framing information) that is to be encapsulation by the PW. 
Otherwise, there is no architectural criteria on which to base the 
distinction between any of the layers in the layered stack, and one 
could argue that a lot of  other other stuff should also be 
encapsulated along with the frame.  The notion of the separation 
between service user and provider layers is key in this discussion.

Anyway this is all these concepts are extremely well documented in 
the IEEE docs, so I do not see why other different and dubious 
approaches should be taken.

Regards

Keith

---

From: mark seery <[email-masked]>
Date: Mon, 24 Nov 2003 08:19:34 -0800

Keith Knightson wrote:

> <snip> The PE is clearly not a repeater. Layers are terminated. Relay 
> functions are included.
>
Keith, I think there is a disconnect being caused by the fact that a PW 
is envisioned in multiple applications. In a wire service, depending on 
how the carrier defines the service to the subscriber, there are 
potential implications. VPLS does as you say involve some type of relay 
function.

On the more general issue, Neil is arguing for something based on the 
principle of layer encapsulation (similar to object encapsulation in 
software) and which is also reflected in the minimum intervention 
principle stated in the PW architecture; the theory being that a tunnel 
is more extensible if you do not change the payload. This is a general 
principle, and there may be occassions where it does not apply when 
dealing with the real world. What 802 defines as a SDU and what is an 
SDU is to a PW may legitimately be two different things depending on the 
service definition; which is not to say your points are not good.

If the practicality is that vendors will use the same mac chips for both 
wire and VPLS services, and if those mac chips act in a way that does 
not support retaining the FCS, then the discussion becomes moot, except 
to highlight an unmet desire on the behalf of some operators to have an 
end to end indication of whether they corrupted the subsriber's data 
anywhere in the network. It could be argued that if all links have a 
32-bit CRC and all equipment does not introduce errors above a similarly 
prescribed rate, then that is achieved. The only question that would 
remain is whether the operator knows where corruption is occuring, when 
it does.

mark