Subject: Re: Ethernet over MPLS

From: Eric Rosen <[email-masked]>
Date: Thu, 27 Mar 2003 13:18:47 -0500

I wish we could discuss this without resorting to ad hominems.  

I myself have tried at least three times to get tagged mode removed from the
specifications, on the grounds that (as has finally been generally noticed),
it doesn't really make much sense when looked at from a network architecture
point of view.

However, there  is a real  practical problem solved  by tagged mode,  and we
need to look at it and decide what to do about it. 

The underlying problem  is that people tend to design  hardware in ways that
are  locally optimized.   For  instance, when  designing ethernet  switching
hardware, one might  decide that all tag manipulation is  done on the output
card, with the  input card passing the tags transparently.   (Or it might be
the other  way around, with  all the tag  processing done only on  the input
card and the  output card passing them transparently.)  This  is fine if all
ethernet frames  passing through the box  receive both the  input and output
ethernet processing.  However, if you try to connect two of these boxes with
a pseudowire,  you immediately run into  a problem: at the  ingress box, the
frames never  received the output  ethernet processing.  The  consequence is
that if a received frame has a provider tag, the hardware cannot support raw
mode. 

Now  our choice  is  whether  or not  to  provide a  standard  which can  be
supported by hardware of this sort. 

It's  pretty  clear  that a  lot  of  the  deployed hardware  that  provides
pseudowires is like  this, and that service providers  will continue to want
interoperability with the deployed hardware. 

It would be  possible to describe tagged mode only  in an Informational doc,
but I  think this  would end up  being more  confusing to the  industry than
having tagged  mode appear  in the  standard, with suitable  text as  to its
purpose and its applicability.  (As has been pointing out, the applicability
statement for tagged mode would be quite a challenge to write!)

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From: Muneyoshi Suzuki <[email-masked]>
Date: Fri, 28 Mar 2003 09:42:39 +0900

> However, if you try to connect two of these boxes with

Could you clarify the reason why the hw cannot support raw mode if a
frame has a "provider tag"?

Muneyoshi Suzuki
Nippon Telegraph and Telephone Corp.