Subject: Do L2 control protocols run end-to-end, or locally?

From: David Clear <[email-masked]>
Date: Tue, 25 Jun 2002 12:06:58 -0700

This is a pretty basic question.  Please bear with me.

Do L2 control protocols run end-to-end (i.e. across the PW VC)?

I'm specifically looking at L2TPv3 (but no doubt the same applies
to MPLS).  Take the following scenario:

   +------+        +------+        +------+       +------+
   |      |   X1   |      |  PSN   |      |  X2   |      |
   |  R1  |--------|  R2  |--....--|  R3  |-------|  R4  |
   |      |        |      |        |      |       |      |
   +------+        +------+        +------+       +------+
                               Y
           <=====================================>
                     Emulated L2 Service

Here, R2 ad R3 perform L2 tunneling so as to provide R1 and R4 an L2 VC.

+ If the VC is carrying, say, a Frame Relay subinterface, does
  LMI run independently across X1 and X2 (providing local "Line Protocol Up"
  status, or is it tunneled across Y, to provide R1 and R4 end-to-end
status?

+ Does it matter if a different DLCI is used at X1 and X2?

+ Does the same hold true for HDLC keepalives and PPP NCP, IPCP, Ethernet
  BPDUs, etc?

I appreciate any help.

Regards,
David.

---

From: "W. Mark Townsley" <[email-masked]>
Date: Tue, 25 Jun 2002 22:50:46 +0200

> to MPLS).  Take the following scenario:

Please see draft-ietf-l2tpext-pwe3-fr-00.txt (soon to be -01)

> + Does it matter if a different DLCI is used at X1 and X2?

In L2TP, the entire FR framing, including the DLCIs, are carried untouched in
the data packets. If DLCI translation is to be done, it is up to the
implementation at R2 and R3.  

> + Does the same hold true for HDLC keepalives and PPP NCP, IPCP, Ethernet

Most of the time, such control packets are carried inband. 

Look for draft-ietf-l2tpext-pwe3-hdlc-00.txt and
draft-townsley-pwe3-l2tpv3-00.txt which should appear very soon for more
information.

> 
> I appreciate any help.
> 
> Regards,
> David.
> 
> _______________________________________________
> pwe3 mailing list
> [email-masked]
> https://www1.ietf.org/mailman/listinfo/pwe3

---

From: "Claude Kawa" <[email-masked]>
Date: Tue, 25 Jun 2002 17:02:02 -0400

It will be better if we use the terminology and reference model of PWE3
requirements and framework drafts.

In the case of frame relay, LMI runs between a PE and a local CE as in the
following "diagram". where we have two local LMI (between CE1/PE2 and
CE2/PE2) running independently of each others.

                               CE1----PE1....PSN.....PE2----CE2

What's the protocol used between the PEs to convey LMI active bit?  This is
something that has not been discussed in details on this list; there are a
number of possibilities.                           

To answer the question about DLCI it is recommended to read first:
http://www.ietf.org/internet-drafts/draft-martini-frame-encap-mpls-01.txt

where everything is answered about DLCI unless we missed something.

Claude Kawa

-----Original Message-----
From: David Clear [mailto:[email-masked]]
Sent: Tuesday, June 25, 2002 3:07 PM
To: [email-masked]'
Subject: [PWE3] Do L2 control protocols run end-to-end, or locally?

I appreciate any help.

Regards,
David.

---

From: Giles Heron <[email-masked]>
Date: 26 Jun 2002 18:25:50 +0000

and to follow up on PPP, HDLC and Ethernet.

In these cases (from the MPLS/draft-martini side of the house at least -
I can't speak for L2TPv3) any control frames are carried in-band.  The
drafts treat these frames as being user data.

Giles

-- 
=================================================================
Giles Heron    Principal Network Architect    PacketExchange Ltd.
ph: +44 7880 506185              "if you build it they will yawn"
=================================================================

---

From: "W. Mark Townsley" <[email-masked]>
Date: Wed, 26 Jun 2002 21:17:30 +0200

> drafts treat these frames as being user data.

Same in L2TPv3, unless you turn on some of the "dial-oriented" features of PPP.
If you need support for some things like ACCM, which may break when you carry
PPP inband as user data (as is pointed out in the martini specs as well), you
might need these. But, in its most basic form, L2TPv3 passes PPP and HDLC data
in much the same fashion as the draft-martini side of the house.

L2TPv3 does add the suggestion to set a Priority bit in the L2TPv3 header for in
band control packets if the sender has the ability to classify them. The
receiver MAY use this as an indication that the packet should be given a higher
priority, particularly during times of congestion. This can be particularly
important for links which carry a very sensitive echo mechansim for dead link
detection (such as a PPP LCP 'keepalive').

- Mark

---

From: Shahram Davari <[email-masked]>
Date: Wed, 26 Jun 2002 13:34:27 -0700

Hi David,

Here are my views.

-Shahram

> to MPLS).  Take the following scenario:

It depends on the mode of operation. In frame switching modes (in which
R2 and R3 do frame switching and change DLCI) LMI can be run independently
across X1 and X2. In transparent port mode LMI is run between R1 and R4
and should be tunneled through PSN. The reason is that in port mode R1
should think that it is directly connected to R4.

> + Does it matter if a different DLCI is used at X1 and X2?

if you mean no switching is done at R2 and R3, yes it does
matter as I explained above. However, if you mean by coincidence 
they are the same, then no it does not matter.