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

From: "Ballou, Ken" <[email-masked]>
Date: Fri, 7 Jan 2000 14:43:48 -0500

I have been struggling mightily with the terminology and definitions in
"MPLS using LDP and ATM VC Switching."  First, I notice the draft is dated
April, 1999, and expired October, 1999.  What is the current status of this
document?  (I'll call that "Question 0." :-)

Question 1:

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.

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

Question 2:

A frame-based LSR is defined as "a LSR which forwards complete frames
between its interfaces."  My interpretation of "forwards complete frames" is
that once the first bit of the layer 3 PDU (along with any layer 2
encapsulation) appears on the physical link, there are no intervening bits
until the entire frame is sent.  (So, for example, ethernet forwards
complete frames, but ATM-LSRs do not [since cells for other VPI/VCIs can be
freely intermixed with cells for the current frame].)

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

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

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

Assume that the two ATM OC-12 interfaces are LC-ATM interfaces.  Does this
router then meet the definition of "frame-based LSR"?  I assume the LC-ATM
interfaces do not forward complete frames, since cells can be interleaved.
But if the definition of "frame-based LSR" requires finding a pair of
interfaces which forward complete interfaces, this can't be a frame-based
LSR.  (I'm hung up on this because I'm trying to understand whether this
device can be an "Edge LSR.")

Question 3:

The Edge Set of an ATM-LSR domain is defined as "the set of frame-based LSRs
which are connected to members of the domain by LC-ATM interfaces."  Since
at ATM-LSR domain is defined as "a set of ATM-LSRs which are mutually
interconnected by LC-ATM interfaces," I conclude that the "Edge LSRs," which
are the members of the Edge Set, are also members of the ATM-LSR domain.
However, in the last sentence of the first paragraph of section 8, I read:
"these 'Edge LSRs' are not themselves ATM-LSRs".  This appears to contradict
the definitions of section 3.

Thanks in advance for any enlightenment.

					- Ken

---

From: Eric Gray <[email-masked]>
Date: Fri, 07 Jan 2000 15:39:31 -0500

> document?  (I'll call that "Question 0." :-)

It's in the process of becoming an RFC.  Whether it is
currently in the RFC editor's hands I don't know.  Once
it gets to this state, there is no need to 'refresh' it
in order to keep it from expiring.

> This is one cause of my confusion later.)

In the english language, redundancy is often a good thing.

> Or does it mean that there is a complete frame on _at least one_ interface?

In forwarding a complete frame from an input interface to
an output interface, the frame would appear first on one,
and then on the other - wouldn't it?

> ethernet    |        +-------------

Ah.

My feeling is that an LSR is necessarily a frame based LSR
if it receives whole frames (as in this case) since they
will be translated into AAL5 PDUs before being pushed down
to the ATM layer and broken into cells.

It also follows that the LSR is frame based in the reverse
direction because the ATM cells would be re-assembled into
an AAL-5 PDU in order to be transmitted as whole frames via
an ethernet interface.

> the definitions of section 3.

Well, not so much so if you assume that the definition of 
frame based is actually as I've explained it above.  There
is the fuzziness that surrounds an LSR that has multiple
LC-ATM interfaces and at least one interface that is not 
an ATM interface.  In this case, forwarding between LC-ATM
interfaces will not be frame-based while forwarding between
any interface and a non-ATM interface will be.

I would think of this as a hybrid LSR.

---

From: Jeremy Lawrence <[email-masked]>
Date: Sat, 08 Jan 2000 17:39:58 +1100

Some comments...

>"Ballou, Ken" wrote:
[...]
>In the english language, redundancy is often a good thing.

It's not redundant. A frame-based LSR can have one or more LC-ATM
interfaces. An ATM-LSR, in addition, forwards cells. This often
implies that it has limited ability do deal with frame-based
operations like pushing and popping labels, and LDP has some
features to deal with this (e.g. an assumption that ATM-LSRs
cannot do penultimate hop popping).

> > Or does it mean that there is a complete frame on _at least one_ interface?

No, a frame-based LSR can have one or more LC-ATM interfaces.

> > router then meet the definition of "frame-based LSR"? 

Assuming it could deal with whole frames and frame-based operations
like pushing and popping several labels, then it would be a frame-based
LSR.

> > are the members of the Edge Set, are also members of the ATM-LSR domain.

Edge LSRs cannot be members of the ATM-LSR domain as they are not
ATM-LSRs.

Hope this helps,

Jeremy

---

From: Eric Rosen <[email-masked]>
Date: Mon, 10 Jan 2000 12:43:18 -0500

Let me try to clarify.

Ken> Question 1:

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

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.