[PWE3] FCS retention in PWE3

[Stewart Bryant <stbryant@cisco.com>]

Plan of Record
--------------

The plan of record in PWE3 with respect to FCS retention is that we
would change the requirements draft to optionally permit the retention
of FCS (currently is says must remove FCS), leave the base drafts
as they are (they say remove FCS), and test the consensus of the WG
for a new draft that calls up the optional retention of FCS for some
encapsulations.

Consensus
---------

Danny and I have reviewed the discussion on the list and agree that
there is consensus for the new draft, and that we should proceed with
the plan of record.

We hence ask Andy to resubmit (draft-malis-pwe3-fcs-retention-00.txt)
as a WG draft.

Applicability statement
----------------------

Looking at the discussion so far, I think that we need to take great
care in our drafting of the applicability statement in the FCS
retention draft.

To quote section 7 of our requirements draft:

"An emulated service SHOULD be as similar to the native service as
possible, but NOT REQUIRED to be identical. The applicability
statement of a PWE3 service MUST report limitations of the emulated
service."

This means that our WG goal is to provide the necessary and
sufficient degree of emulation, not perfection. In the specific
case of Ethernet FCS retention, I would take this to mean that
we must provide the necessary degree of faithfulness required
to support the assumptions made in IEEE 802.1.

I have seen three cases made for FCS retention in the mails to
the list.

1) IEEE 802.1 assumptions
2) OAM
3) Data integrity

These need to be addressed in FCS retention draft applicability
statement.


IEEE 802.1 assumptions
----------------------

This is best summarized by the notes from Norman Finn:

"PW is just another medium to a bridge.  However, *all* media
known to bridges, at least as far as IEEE 802.1 are concerned,
are 802 media, and all 802 media have 32-bit CRCs.  If you connect
a medium that doesn't have at least a 32-bit CRC to a bridge,
then the resultant bridged LAN will suffer a significant
degradation in its ability to transport frames without error,
because bridges depend on reliable media.  And then the routers
on top of the bridges don't work so well."

and

"If I'm in total control, and the PW is running over known
Ethernet links, then I can get away without the CRC in the PW.
I have no problem whatsoever with the current definition of PWs
without CRC.

"But, if I don't have tight control over the physical media over
which the PW runs, then I really need a CRC on the PW for the
bridge to be able to trust the "wire" and thus provide the same
level of data integrity that bridges offer over physical
Ethernets.  So, I claim that bridges need a CRC option on PWs."

That is to say PWE3 provides a service to 802.1 and needs by one
method or another to provide the necessary and sufficient
degree of wire emulation such that their design assumptions hold.


OAM
---

The original reason that FCS retention raised was because of an
OAM issue. A malfunctioning bridge caused a network error, and
it was thought that FCS retention would be useful in generating
an alarm and diagnosing the fault. PWE3 is also developing an
OAM method based on VCCV. We need to understand those
circumstances where FCS retention is the best OAM technique,
and those where VCCV is the better approach.


Data integrity
--------------

Some folks want FCS retention as a means of assuring the data
integrity of the PW service. The degree of enhancement in data
integrity when using FCS retention, compared to the base drafts
is something that we need to treat carefully, because CRC32
offers a relatively weak degree of data assurance (4 or 5 bit
corruption). The IETF standard method of ensuring data
integrity is IPSec, and IPSec is called up in the security
section of the PWE3 architecture for just this purpose. 802.1
are planning their own method of enhanced data assurance which
we would pass this transparently over the PW. Within the PWE3
context the IETF method of providing a similar degree of
data assurance would be to run IPSec between the PEs. We
therefore need to describe the improvement in data integrity
achieved through FCS retention and compare this to the
improvements achieved through the use of IPSec.

Regards

Stewart




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