Subject: Re: Is IP Layer 2 Interworking useful?

From: Joe Touch <[email-masked]>
Date: Fri, 26 Apr 2002 07:17:40 -0700

Eric Rosen wrote:
> Yushun> My question  is, when you have  different L2 protocols  at the ends,
> Yushun> isn't it  time to try L3  solutions? I just don't  see going through
> Yushun> all this troubles  and inventing new protocols justify  what you can
> Yushun> simply do with L3 (i.e., IP). 
> 
> Perhaps there are some service providers who think that the trouble involved
> in providing the  necessary ARP mediation is possibly  less than the trouble
> involved in providing an L3VPN service. 
> 
>>From the customer perspective, if the  customer wants an L2VPN, I doubt that
> the SP will be willing to say "look,  since we can't give you the same L2 at
> both ends, you need to use an  L3VPN service; after all, you don't expect us
> to go to any extra trouble for you, do you?"

Then why not run IP over ethernet over IP at that point?

In order to have an L2VPN you must have the same L2 at the endpoints, 
otherwise the semantics of the L2 are necessarily undefined. If that's 
what the customer wants, the put the L2-A to L2-B translation at the 
edge of the customer, and run IP in your VPN. It's more reliable, and 
the solution has already been implemented.

Joe

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From: Yu-Shun Wang <[email-masked]>
Date: Fri, 26 Apr 2002 11:46:39 -0700

Hi,

Please see comments & questions inline.

> involved in providing an L3VPN service. 

Ok, I agree that's debatable. But one purpose of IP is to have a global
naming/addressing space, and there's actually a working routing architecture
established for IP. Don't get me wrong, I am not saying it's perfect, but it
works. By introducing the ARP mediation, the effect is to broker locally
scoped (whether intentionally so or just a artifect of deployment) address
space across the Internet, and you still need to perform packet encap-decap
*just like what IP & ARP already do*.

Why go through the process of distributing more L2/MAC address globally
when one (IP) is already in place?

> to go to any extra trouble for you, do you?"

Since you brought up the "provider-client" dialog, I'll play along. :-)
The provider should say: "Listen, we know what you want, and we can do it.
But there's another solution that is already in place and working (hint: IP).
We can just build the VPN on top of that (and it's call L3VPN.)"
If the marketing people can not do that, then I'd say the provider(s) have
a bigger problem and it's not technical.

The clients can ask for anything, but that doesn't mean as an engineer,
we need to comply all their requests or there isn't room to negotiate.
(Or do we?)

yushun.
____________________________________________________________________________
Yu-Shun Wang <[email-masked]>               Information Sciences Institute
                                            University of Southern California