Subject: RE: Confusion about terminology in draft-ietf-mpls-atm-02.txt

From: "Ballou, Ken" <[email-masked]>
Date: Mon, 10 Jan 2000 13:34:55 -0500

OK, I think I understand.  (Thanks also to Jeremy Lawrence for his
clarification.)

Might I suggest that the definition of "ATM-LSR" needs to be more
restrictive?  As I read the definition, it does not preclude reassembling
the incoming cells into a complete frame and then deciding how to forward
the frame (scenario 2 Eric describes).  Perhaps changing the wording to
"forwards cells between these interfaces without first reassembling the
frame using only labels carried ..." would make the definition clearer?  I
believe this is the distinction between scenarios 1 and 2.

I think this also makes clear that an edge LSR is not a member of an ATM-LSR
domain, since it is spelled out that just having an LC-ATM interface does
not automatically imply being an ATM-LSR.

					- Ken

-----Original Message-----
From: Eric Rosen [mailto:[email-masked]]
Sent: Monday, January 10, 2000 12:43 PM
To: Ballou, Ken
Cc: [email-masked]
Subject: Re: Confusion about terminology in draft-ietf-mpls-atm-02.txt 

Let me try to clarify.

Ken> Question 1:

Ken> In section 3, the following definition is given for an ATM-LSR:

     An ATM-LSR is  a LSR with a number of  LC-ATM interfaces which forwards
     cells  between these  interfaces using  labels  carried in  the VCI  or
     VPI/VCI field.

Ken> Since the  definition of  "LC-ATM interface" specifies  explicitly that
Ken> the "top label is inferred either from the contents of the VCI field or
Ken> the combined contents  of the VPI and VCI fields,"  would it be correct
Ken> to state that  the clause "which forwards cells  ..." in the definition
Ken> of an  ATM-LSR is redundant?  That  is, simply containing  at least one
Ken> LC-ATM interface  makes an LSR  be an ATM-LSR?   (I'm not trying  to be
Ken> uselessly pedantic here.  This is one cause of my confusion later.) 

An  LSR which receives  cells on  an LC-ATM  interface might  do one  of two
things:

1. For  each cell  it receives,  it forwards  that cell  out  another LC-ATM
   interface, replacing the VPI/VCI field. 

2. It reassembles  a set  of cells into  a full  frame, then decides  how to
   forward that frame,  perhaps based (wholly or partially)  on the incoming
   VPI/VCI value which the cells carried. 

If  it does  1, it  is  functioning as  an ATM-LSR.   If  it does  2, it  is
functioning  as a  frame-based  LSR.  Note  that  2 is  compatible with  the
outgoing interface  being an  LC-ATM interface as  well.  In this  case, the
frame gets reassembled when it arrives, and then gets segmented before it is
transimtted. 

Ken> Question 2:

Ken> A frame-based LSR  is defined as "a LSR  which forwards complete frames
Ken> between  its  interfaces."   My  interpretation of  "forwards  complete
Ken> frames" is

The intended interpretation is that the entire frame is present before it is
transmitted.  Whether the frame is  interspersed with others on the outgoing
interface doesn't matter.

Ken> Does  the definition of  frame-based LSR  require that  complete frames
Ken> appear on  both the  ingress and the  egress interface?  (That's  how I
Ken> interpret it.)  Or  does it mean that there is a  complete frame on _at
Ken> least one_ interface? 

Neither.   A  host  which is  connected  to  two  ATM  switches could  be  a
frame-based LSR, if each of the ATM switches is functioning as an ATM-LSR.

Ken> I could word this question another way.  Here's a router with three
Ken> interfaces:

Ken> gigabit     +--------+  ATM OC-12
Ken> ethernet    |        +-------------
Ken> ------------+        |
Ken>             |        +-------------
Ken>             +--------+  ATM OC-12

Ken> Assume that the  two ATM OC-12 interfaces are  LC-ATM interfaces.  Does
Ken> this router then meet the definition of "frame-based LSR"? 

If this  router is not  an ATM switch,  but an ATM  endsystem, then it  is a
frame-based LSR.   This is probably the  case you are thinking  of.  In this
case, the router can certainly be an Edge LSR.

If this router is an ATM switch with a gigabit ethernet interface (perhaps a
somewhat unusual configuration), then it  might well be that it functions as
a  frame-based  LSR  when  forwarding  between  the  ethernet  and  the  ATM
interfaces,  but  as  an  ATM-LSR   when  forwarding  between  the  two  ATM
interfaces. 

Ken> Question 3:

Ken> The Edge Set of an ATM-LSR domain is defined as "the set of frame-based
Ken> LSRs  which  are   connected  to  members  of  the   domain  by  LC-ATM
Ken> interfaces."  Since at ATM-LSR domain  is defined as "a set of ATM-LSRs
Ken> which  are mutually  interconnected by  LC-ATM interfaces,"  I conclude
Ken> that the "Edge  LSRs," which are the members of the  Edge Set, are also
Ken> members of the ATM-LSR domain. 

Since an  LSR can have  an LC-ATM interface  without being an  ATM-LSR, your
conclusion is a non-sequitur.

---

From: Grenville Armitage <[email-masked]>
Date: Mon, 10 Jan 2000 10:45:04 -0800

"Ballou, Ken" wrote:
	[..]
> believe this is the distinction between scenarios 1 and 2.

I dont see the need for any more wordage. The ATM-LSR definition talks
about forwarding cells, and Eric's scenario 2 refers to forwarding
frames. Remember, the definition is based on what the LSR does
internally, not the format in which the blobs of MPLS data arrive
and leave over external interfaces.

cheers,
gja
________________________________________________________________________
Grenville Armitage                            http://mh005.infi.net/~gja
Bell Labs Research Silicon Valley