Subject: RE: I-D ACTION:draft-nanji-l2tp-eth-00.txt

From: Theimer Thomas <[email-masked]>
Date: Fri, 7 Jul 2000 16:27:06 +0200

Suhail Nanji writes:
> The end user is not necessarily running PPP.  The modem is
> designed to be inexpensive and is also not running PPP.  This is a very
> common model for both DSL and Cable.  
> 
	[Theimer Thomas]  
	Two questions for clarification:

	1. What is the link layer between modem and access concentrator ? If the answer is ATM or FR, then why is bridging over ATM/FR not sufficient ?

	2. If the modem is so inexpensive that it doesn't even run PPP, would such a modem be capable of running L2TP ?

	Regards,

	Thomas

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From: Suhail Nanji <[email-masked]>
Date: Fri, 7 Jul 2000 11:38:26 -0700 (PDT)

> 	1. What is the link layer between modem and access concentrator ? If the answer is ATM or FR, then why is bridging over ATM/FR not sufficient ?

Bridging is sufficient to get it to the first access concentrator but from
there some protocol is needed to have the Ethernet frame tunneled to
another access concentrator which would be the point of termination.

> 	2. If the modem is so inexpensive that it doesn't even run PPP,
would such a modem be capable of running L2TP ?

The modem would not be running L2TP.  The modem would simply be bridging
traffic.  L2TP is between the first access concentrator and another access
concentrator.

---

From: James Carlson <[email-masked]>
Date: Fri, 7 Jul 2000 20:42:02 -0400 (EDT)

> another access concentrator which would be the point of termination.

OK, so if I understand this correctly, you want something like this:

	PC<--Ethernet-->modem<--DSL-->conc1<--ATM-->conc2

where "conc1" is the first concentrator and "conc2" is the second
concentrator.  Is this correct?

If so, then why not run regular IP over Ethernet between the PC and
conc1, using any convenient encapsulation over the DSL link
(Cisco-style HDLC comes to mind; ATM would also do the trick but would
be needlessly more expensive), and then have conc1 behave as a
learning bridge (called a "switch" by marketing droids these days)?
The link to conc2 can then be either ATM LANE (yecch, but solves the
problem simply), a static VC per MAC (simpler, but like LANE doesn't
scale nicely), or a routed IP over ATM link (a little more complex,
but scales nicely).

Why is IP-over-PPPoE-over-foo-over-L2TP-over-bar necessary?  It only
complicates the software in the PC and breaks (at least) RFC 1661.

-- 
James Carlson                                  <[email-masked]>
"PPP Design and Debugging" --- http://people.ne.mediaone.net/carlson/ppp

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From: Suhail Nanji <[email-masked]>
Date: Mon, 10 Jul 2000 10:01:33 -0700 (PDT)

> concentrator.  Is this correct?

No, my picture would look something like this:

	PC<--Ethernet-->modem<--DSL-->DSLAM<--ATM-->conc1<--IP-->conc2

The access concentrators are connected via an IP network.  This is why
tunneling is needed to transport the Ethernet frame.