RE: [PWE3] re: Proposed way to move forward for ATM/MPLS in pwe3

[neil.2.harrison@bt.com]

John,

Of all the ATMoverMPLS encapsulation variations I have seen to date yours
(ie the one you recently submitted to ATMF) is by far the clearest/best IMO.
However, it is not sufficient just to carry the client layer (in this case
ATM) fault management information transparently in a failure-free case, we
must have correct defect handling behaviour under dynamic failure
conditions...and this area has still to be addressed correctly in all the
drafts (though I do recognise that you at least are aware of this).

regards, Neil

> -----Original Message-----
> From: Rutemiller, John [mailto:John.Rutemiller@marconi.com]
> Sent: 28 May 2002 16:54
> To: pwe3@ietf.org
> Subject: RE: [PWE3] re: Proposed way to move forward for ATM/MPLS in
> pwe3
> 
> 
> Khalid,
> 
> The way to move forward is to simplify. Not to expand.
> 
> 
> I've been looking at PDU mode and trying to figure out if there
> is an easier way to provide the same functionality. What I've
> come up with (thanks to input from others) follows at the end of
> this message.
> 
> There are some important characteristics to note with what I
> propose compared to PDU mode:
> 
> 1) It is a variation of Cell Mode
> 2) Works for all AAL types (not AAL5 specific)
>    - When a frame is sent can be triggered by max_delay (any AAL)
>      or by max_MTU (for AAL5 if desired), or by the PTI bit value
> 3) The arrival of an ATM cell causes the transmission of at
>    most one frame
>    - Admin/OAM cells are sent as the last cell of a frame
>    - PDU mode will send two frames in response to the arrival
>      of an admin/OAM cell
> 4) Transmission is at least as efficient as PDU mode
> 5) Like PDU mode, is not transparent to user cell EFCI and CLP
> 
> 
> As a way to move forward, I would like to replace PDU mode with
> what I propose. It is simpler than PDU mode, provides equivalent
> functionality and is not AAL specific.
> 
> John
> 
> 
> -----------------------------------------------------------------
> 
> 
> ==> Procedures at the Ingress Side
> 
> The procedures for processing arriving cells preserves the 
> value of the ATM
> user-to-user bit located in PTI bit position 3 and all PTI bits for
> administrative cells. Processing is based on the following rules:
> 
> 1)	All cells, except the last cell of a frame, MUST be a 
> user cell with
> the PTI bit value PTI=0x0.
> 
> 2)	The last cell in a frame may be either a user cell with any PTI
> value (PTI=0xx), or an administrative cell (PTI=1xx).
> 
> 3)	Administrative cells MUST be transmitted only as the 
> last cell in a
> frame.
> 
> 4)	User cells with PTI=0x1 MUST be transmitted only as the 
> last cell in
> a frame.
> 
> 
> 
> 
> 
> The pseudo code for processing an arriving cell is as follows:
> 
> If starting a new frame
> 		Initialize the A-PTI/C and U-PTI/C fields to zero
> 
> Append arriving cell payload to frame buffer
> 
> If PTI=0xx then
> 	U-PTI = arriving-PTI
> 		U-CLP = U-CLP | arriving-CLP
> Else If PTI=1xx then
> 	A-PTI = arriving-PTI
> 		A-CLP = arriving-CLP
> Endif
> 
> If PTI != 0x0 or (FrameSize => MaxFrameSsize)
> 		Send Frame
> 	Endif
> 
> The value of the user cell EFCI is always the value of the 
> last arriving
> cell.
> 
> The value of the user cell UU bit is always set to zero when 
> the PTI=0x0. If
> the PTI=0x1, the UU bit is set to one and the frame will be 
> sent. Only the
> last cell in a frame can set the PTI=0x1 value.
> 
> The value of the admin cell PTI bits are always preserved 
> separately and the
> frame is always sent immediately after appending the admin 
> cell payload.
> 
> 
> ==> Procedures at the Egress Side
> 
> The procedures for processing arriving frames generates ATM 
> cells from using
> the PTI/CLP values carried in the first byte of the frame.
> 
> 
> 
> The pseudo code for processing an arriving frame is as follows:
> 
> While (not last cell payload)
> 		PTI = U-PTI & 010
> 		CLP = U-CLP
> 		Prepend VPI/VCI
> 		Send cell
> 	End
> 
> 	If A-PTI/CLP != 0000 then
> 		PTI = U-PTI
> 		CLP = U-CLP
> 		Prepend VPI/VCI
> 		Send cell
> 	Else
> 		PTI = A-PTI
> 		CLP = A-CLP
> 		Prepend VPI/VCI
> 		Send cell
> 	Endif
> 
> 
> 
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