Subject: VC "unmerging" - MPLS using ATM VC switching related

From: alper altinordu ee stnt <[email-masked]>
Date: Wed, 20 Oct 1999 15:35:22 -0400 (EDT)

Hi All,

I have a question regarding to MPLS using ATM VC switching. I would like
to know how AAL5 frames are separated once they are VC merged. It seems
that there's no way to separate the frames without processing the upper
level (i.e. IP) header.

Let me specify my question more precisely:
 
(Payload, VPI/VCI)  <- Let this represent an ATM cell
 
[Payload, VPI/VCI)  <- and this be an AAL5 EoP cell
 
SW-1 is a VC Merge capable switch
It merges the VCs a/b and c/d to the VC x/y
 
A is the destination of the cells of the VC a/b
C is the destination of the cells of the VC c/d
 
[5,a/b)_(2,a/b)_(1,a/b)  _____                         _____ 
                       \_|_ _|_         [4,x/y)(3,x/y)_|_ _|_____ Dest A
                        _|_/ | [5,x/y)(2,x,y)(1,x/y)   |  \|
___[4,c/d)___(3,c/d)___/ |S-1|                         |S-2|----- Dest C
                         |___|                         |___|
 
Switch 1 (S-1) receives packets P-1 consisting of cells with payloads 1,2
and 5 and P-2 with payloads 3 and 4. S-1 reassembles (or just buffers
until the EoP cells are received) the packets and VC merges them on the
VC x/y and transmits P-2 first followed by P-1 to S-2.
 
Now the question is how does S-2 know it should send the first incoming
packet (P2) to "Dest C" output port and the second incoming one (P1) to
the "Dest A" output port? Is there a way to know this without reassembling
the packets and processing the IP (or other upper level) header?
 
Thank you
 
Alper Altinordu

---

From: "Peter Ashwood-Smith" <[email-masked]>
Date: Thu, 21 Oct 1999 08:01:20 -0400

> that there's no way to separate the frames without processing the upper

  That is correct. Once the packets arrive at the end of the label switched
path and the last header is popped they must be routed.

  Cheers,

  Peter Ashwood-Smith

---

From: Robert Raszuk <[email-masked]>
Date: Thu, 21 Oct 1999 05:16:39 -0700

Actually I think this is not always the case. Remember that the multiple
label headers can be prepended into the ALL5 frame before SAR action on
ingress. The value of the top most lable value is mapped to VPI/VCI
space in the cells. So when the SAR on the other end reasembles the
cells into ALL5 frames either VC merged or not we still have all the
original lable information. We can pop the top, switch based on the top
label or the bottom one if the depth of the stack is = 2. 

So the bottom line is that this can already tell us how to forward the
packet out of the egress LSR without any need for any IP lookup. (The
aggregate lable would be an exception of course).

Robert

> Peter Ashwood-Smith wrote:
> 

-- 
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