Subject: RE: pwe3-ethernet-encap - FCS (requirements)

From: "Christopher N. \"Nick\" DelRegno" <[email-masked]>
Date: Wed, 24 Sep 2003 07:50:58 -0500

Yes, with clarification:

1)  Port-mode packets at egress must be identical to packets at ingress.
VLAN/DLCI mode packets can be changed at egress (by design) so the identical
requirement doesn't apply.

2)  Yes.

3)  Inspected is less of an issue.  Changed is a greater issue for "private
line" purists.  

4)  Yes, either in the egress PE or via L2 packet drops at next L2 hop.

5)  Not necessarily.  The point of the FCS retention is that the original
packet can be checked for error somewhere after the PW transit.  That could
exist in the egress PE or elsewhere.

6)  I think FCS retention may have value for non port-mode PWs as well, as a
means to ensure against transit faults.  However, in order to use this, one
would have to check the FCS prior to rewriting the VLAN/DLCI upon egress,
adding to the complexity of the implementation.  

Nick

-----Original Message-----
From: Stewart Bryant [mailto:[email-masked]] 
Sent: Wednesday, September 24, 2003 2:20 AM
To: Christopher N. "Nick" DelRegno
Cc: [email-masked]
Subject: pwe3-ethernet-encap - FCS (requirements)

Please can we park the proposed solutions for a while
and agree on the requirements.

As I understand the requirements they are:

1) Packet at egress must be identical to packet at ingress.

2) Frames delivered across the PW with the same probability
of undetected error as provided by the CRC of the originating
packet.

3) Customer traffic must not be inspected.

4) The network operator to be able to detect errors in PW traffic
induced by their network at same time, or before the customer
detects.

5) Mechanism to detect the problem must be confined to the PE.

Non-requirements

6) Requirements 1..5 do not apply to non-port mode services.

My questions:

a) Have I stated the above requirements correctly?

b) Are there any additional requirements?

c) Are there any additional non-requirements?

d) Why item 6?

Stewart

---

From: Eric Rosen <[email-masked]>
Date: Thu, 25 Sep 2003 10:37:00 -0400

> 1) Packet at egress must be identical to packet at ingress. 

If this is  the requirement, then Forward Error  Correction is mandated, and
it must be able to correct an arbitrary number of bit errors. 

I  think  a  more  accurate  statement  of the  requirement  would  be  "the
probability of undetected bit errors in the packet must be below a specified
threshold". 

> 3) Customer traffic must not be inspected. 

doesn't really appear to make sense. 

>    induced by their network at same time, or before the customer

This should probably  be rephrased as "the likelihood  that an error induced
in the SP network will be detected  by the customer but not by the SP should
be below a specified threshold". 

> 6) Requirements 1..5 do not apply to non-port mode services. 

It's hard to see why the requirements would be different in this case.  What
people are really  saying, I think, is not that only  the port-mode case has
these requirements, but that only in the port-mode case can a cost-effective
solution be provided.  The counterpoints are that: 

- if it isn't needed in the non-port-mode case, it can't really be needed in
  the port-mode case;  (to argue that it is needed, one  would have to argue
  that the expectations of port mode customers are really different than the
  expectations of other customers);

- even in  the port-mode case the  solution doesn't really seem  to be cost-
  effective, as  it buys a very  small increase in  coverage, adds overhead,
  adds complexity, and won't always  be available; one could of course argue
  that the benefit in customer perception is enough to put it over the top.