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RE: [RPRWG] Cut through definition?

If this scheme is adopted then I don't see how this would work with cut through mode and GFP.
Hence it is no longer L1 agnostics. Should we not support cut-through then?
Is this a bad thing for RPR? I think I will let the someone else answer the question!
 
I would like to make it clear that we, Nortel, are like everybody else, that we will support
GFP on our equipment when the 3rd party silicon vendors sell GFP compliant chips.
 
What is the original goal for preemption? Are you providing a solution that is more
complicated than it's worth. What about test equipment complexity.
 
Lastly, there are those who believe that RPR is more than just for packet based network.
 
 
Harry
-----Original Message-----
From: William Dai [mailto:wdai@xxxxxxxxxxxx]
Sent: Thursday, April 12, 2001 3:17 PM
To: stds-802-17@xxxxxxxx
Subject: Re: [RPRWG] Cut through definition?

Although I'm not a big preemptive transfer fan, but I think this topic deserves detailed
discussion before we rush into any conclusion. What changes me is the discussion of
Jumbe Frame support on RPR, not long ago it was 2KB, now it is 9KB, what about the
ultimate 64KB in the future ?
 
By saying that, I'm proposing neither ATM cell like structure nor slotted ring structure,
and since RPR MAC is L1 agnostic, physical signalling trick cannot be used either.
 
Let me give one example of preemptive transfer definition here and let's discuss what
is so complicated (simple) about it.
 
    1.    There are 3 MAC classes of traffic (H, M, L,).
    2.    Preemption is allowed only for "Transit" H traffic to preempt "Transmit" M or L traffic.
    3.    Preempted segment is not allowed to be prempted again.
    4.    Preempted "Transmit" traffic will be scheduled to tranfer right after "Transit" H traffic,
           independent of classes.
    5.    Each Packet transfer will be inserted an "IDLE/Escape" word for every 256 or 512
           (for the sake of alignment/padding concern) byte as the preemptive inserion point.
    6.    Jumbo frame is not supported for H class.
 
By the way, SONET clock distribution is not needed. After all, RPR is a packet based network.
 
 
Best Regards
 
William Dai
         
 
 
----- Original Message -----
From: Harry Peng
Sent: Thursday, April 12, 2001 7:23 AM
Subject: RE: [RPRWG] Cut through definition?

Exactly my point.
 
"we should keep it simple and not Segment packets. " i.e. Do not preempt.
 
Regards,
 
Harry
 
-----Original Message-----
From: Necdet Uzun [mailto:nuzun@xxxxxxxxxxxxxxxx]
Sent: Wednesday, April 11, 2001 6:22 PM
To: Peng, Harry [SKY:1E11:EXCH]
Cc: Sushil Pandhi; Leon Bruckman; 'davidvja@xxxxxxxxxxx'; stds-802-17@xxxxxxxx
Subject: Re: [RPRWG] Cut through definition?

I am not clear how the proposed preemption method works.

Does a high priority transit packet preempt a low priority add packet?
Can a high priority add packet also preempt a low priority transit packet?
What happens if a previously preempted add packet contends with a same priority packet that was also preempted in an upstream node?
What happens if a previously preempted add packet contents with a same priority previously preempted transit packet that follows a high priority preempting transit packet with a clock cycle gap in between due to clock mismatch?
Do we require a SONET clock to be distributed on the ring?
Is RPR MAC layer one agnostic?

Thanks.

Necdet

Harry Peng wrote:

 

Complexity what complexity:

In the tandem path, if a high priority packet can preempt a low priority packet at
arbitrary boundary then the preempted logic will have to deal with a tandem packet that
is already pre-empted.
This means the fastest pre-emption response time is on internal word size and the pre-empted packet
will have to pad to word boundaries to make live easier.
Furthermore the tandem receiver will have to respond to within one clock cycle as it is the
atomic size. What is the word size for 10G 64 bits 128 bits? What about for 40G or higher.

Unless, you are will to have cells. Then why not use ATM.

I agree that we should keep it simple and not Segment packets.

Regards,

Harry
 

-----Original Message-----
From: Sushil Pandhi [mailto:Sushil.Pandhi@xxxxxxxxxxxxxxx]
Sent: Wednesday, April 11, 2001 10:33 AM
To: Leon Bruckman
Cc: 'davidvja@xxxxxxxxxxx'; stds-802-17@xxxxxxxx
Subject: Re: [RPRWG] Cut through definition?
 

I agree with Leon  that 'pre-emption' will increase the complexity.  ATM solves
this by segmenting the message into
smaller size cells and reassembling cells, and using this  approach adds a lot
of complexity.

If we do not have preemption, and assuming 1522 byte frame just starts
transmission before synchronouus traffic
can be sent, 1522 byte frame at OC-3 rate will take about 82.6 micro-seconds.
If we assume that in the ring, at 32
nodes the same situation arises then  we have about 2.6 msec delay because not
doing preemption.  So I doubt  preemption
will give us much advantage.

-Sushil

Leon Bruckman wrote:

> DVJ
> My personal view is that preempting lower traffic in the middle of a packet
> adds complexity that is not really needed. At 1G, the transmission time for
> a 1500 bytes packet is 12 usec, so the worst case for a 256 ring will be 3.1
> msec of added delay because of low packets being transmitted and not
> preeempted. Furthermore, the probability of the worst case is very small. We
> did some simulations with the following assumptions:
> - There is always a low priority packet being transmitted by the node
> - High priority packet may arrive at any time during the low priority packet
> transmission (equal probability)
> Some of the results were presented during the January interim (by Gal Mor).
>
> For a 128 nodes ring operating at 1G the preeemption gain will still be in
> the msec range with very high probability, and this can easily be absorbed
> by the jitter buffers at the receiver.
> Leon
>
> -----Original Message-----
> From: David V. James [mailto:davidvja@xxxxxxxxxxx]
> Sent: Friday, April 06, 2001 4:27 AM
> To: Carey Kloss; Devendra Tripathi
> Cc: stds-802-17@xxxxxxxx
> Subject: RE: [RPRWG] Cut through definition?
>
> All,
>
> Relative to the discussion of cut through, et. al.
> My perception is that a cutthrough node has two
> insertion buffers, for classA (provisioned synchronous)
> and classB (provisioned asynchronous).
>
> The preferred transmit order is as follows:
>   a) classA insertion buffer (always)
>   b) classA transmit traffic (subject to provisioned rate)
>   c) asynchronous traffic.
> The classA insertion buffer only needs to be the size of
> the maximum packets sent by this node, plus (perhaps) some
> extra symbols to deal with hardware decoding latencies.
>
> The classB insertion buffer is to deal with the accumulation
> asynchronous packets that occurs when (worst case) full asynchronous
> is coming in/out and rate-limited synchronous is being transmitted.
> The size of the classB buffer is on the order of several upsteam-link
> delays times rateOfSynchronous/rateOfLink ratio.
>
> Order of the asynchronous traffic (c) depends on the classB
> buffer-filled status, prenegotiated vs. consumed rates, and
> the size of the asynchronous backlog in the client.
>
> The asynchronous transmit buffer is a bit schitzophrenic on its
> behavior. It should be in the client (not the MAC) because that
> allows packets to be reordered/inserted/deleted until the just
> before transmission time. However, the amount of traffic in the
> asynchronous transmit queue may influence the MAC queue-selection
> and throttle-signal assertion properties.
>
> I personally favor allowing cut-through synchronous traffic to
> preempt asynchronous, even in the middle of a packet. That's
> yields the lowest possible jitter, but at some encoding complexity
> costs.
>
> DVJ