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

[neil.2.harrison@bt.com]

Norm....As per our discussion today face-face.  I don't really see what the problem is here.  The client layer network should not care about the server layer network (and vice versa)....we need this layer independence for lots of reasons.  An ethernet traffic unit has a FCS.  This should simply be transported transparently in the payload area of whatever the server layer is....and the server layer should use its own (OAM) 'integrity mechanisms' irrespective of the client.  Problems arise when people start doing things like client layer compression and thus forming inter-layer dependencies.

Further, as we agreed today and as I posted on an earlier mail to this thread, the ethernet FCS is a CRC that can be linerarly updated for adaptation function changes (eg VLAN ID change) on each link-connection (as provided by some server layer trail technology) whilst preserving its end-end intergrity check of the rest of the ethernet traffic unit between its trail termination points.

regards, Neil

> -----Original Message-----
> From: Norman Finn [mailto:nfinn@cisco.com]
> Sent: 18 November 2003 08:56
> To: mark seery
> Cc: stds-802-1@ieee.org; pwe3@ietf.org
> Subject: Re: [PWE3] RE: [802.1] 802.1ad Customer FCS Retention
> 
> 
> 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
> 
> mark seery wrote:
> > 
> > Norman Finn wrote:
> > 
> >> <snip>
> >> In other words, routers pass about $0.93 of the data 
> integrity buck to 
> >> bridges,
> >> and bridges pass about $0.75 of that to the data links.  
> The buck has 
> >> to stop
> >> somewhere, and the data link is where it stops.  The PW 
> has to take 
> >> the same
> >> hit as the physical media.
> >>
> >> -- Norm
> > 
> > 
> > but a PW will be running over a physical media itself 
> already, possibly 
> > even SONET.............
> > 
> > mark
> > 
> > 
> > 
> > 
> 
> 
> 
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