Re: [PWE3] RE: [802.1] 802.1ad Customer FCS Retention

[Norman Finn <nfinn@cisco.com>]

Scott,

Over what media can routers run?  Wet string?  There are media in use
that have, in effect, only a 16-bit checksum.

If the endpoints of the PW have knowledge of the underlying media on which
the PW runs, and if those media have satisfactory data integrity protection,
and if the methods of encapsulation and decapsulation have a suitable
mathematical and/or implementation basis to suppose that they won't introduce
significant errors, then you're done.

On the other hand, one of the purposes of layering is precisely to insulate
the upper layers from knowledge of the lower.  If the ends of the PW do not
know that the data is as safe as an Ethernet frame, then the Ether-over-
whatever (i.e. PW) application needs its own checksum over its own data.  This
checksum certainly covers every bit of the Ethernet frame payload.  If the
checksum chosen is a polynomial CRC, presumably the IEEE 802 one, then it has
the property of being extendable to cover some PW header information without
compromise, just as it is used to cover added 802.1Q tags.  I'm not expert
enough on the various PW techniques to tell you for sure whether that's needed
or not, but I doubt that it's necessary.

-- Norm

Scott Kotrla wrote:
> Norman,
>
> What media are bridges going to be running PW over that does not have a
> CRC?  Are you talking about a CRC of the PW payload or a CRC of the PW
> packet including both payload and header?    
> Thanks,
>
> Scott
>
> -----Original Message-----
> From: owner-stds-802-1@majordomo.ieee.org
> [mailto:owner-stds-802-1@majordomo.ieee.org] On Behalf Of mark seery
> Sent: Tuesday, November 18, 2003 10:16 AM
> To: Norman Finn
> Cc: stds-802-1@ieee.org; pwe3@ietf.org
> Subject: Re: [PWE3] RE: [802.1] 802.1ad Customer FCS Retention
>
>
> Hi Norman,
>
> This is a good formulation of the problem. 
> thanks,
> mark
>
> ---- Norman Finn <nfinn@cisco.com> wrote:
>
> > Mark,
> >
> > You say below,
> >
> > > but a PW will be running over a physical media itself already,
> possibly
>
> > > even SONET.............
> >
> > Exactly!  Here, the trail gets a bit murky.  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.
> >
> > -- Norm


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