RE: [RPRWG] Cut through definition?
What about Ring control packets ? Are they given same priority as transit
packet or
there is one more priority level ?
Regards,
Devendra Tripathi
VidyaWeb, Inc
90 Great Oaks Blvd #206
San Jose, Ca 95119
Tel: (408)226-6800,
Direct: (408)363-2375
Fax: (408)226-6862
> -----Original Message-----
> From: owner-stds-802-17@xxxxxxxx [mailto:owner-stds-802-17@xxxxxxxx]On
> Behalf Of Carey Kloss
> Sent: Wednesday, March 28, 2001 6:07 PM
> To: stds-802-17@xxxxxxxx
> Subject: [RPRWG] Cut through definition?
>
>
>
> I would like to revisit the cut-through vs. store and forward, if nobody
> objects?
>
> The last discussion ended with a wish to get a more concrete definition
> of cut-through. Towards that end, I would like to put out my own
> understanding, and generate feedback on what's specifically different in
> current schemes:
>
> >From what I understand, cut-through exists as Sanjay has explained it:
> 1. Transit (pass-thru) traffic always has priority over transmit
> (add-on) traffic, regardless of class.
> 2. There is a small (1-2 MTU) transit buffer to hold incoming transit
> traffic when sending transmit traffic.
> 3. All prioritization happens at a higher layer, when deciding what to
> transmit.
>
> I was also wondering if there is any agreement on cut-through congestion
> control mechanisms? Looking through the presentations on the RPR
> website, I've seen a number of schemes, and this is my understanding
> from the slides. Please correct me if I've misunderstood:
>
> 1. The simplest, local fairness, which I'm not sure that anyone is
> implementing: When HOL timer times out for high-pri traffic, send a
> congestion packet upstream. This will stall the upstream neighbor from
> sending low-pri traffic (after some delay).
>
> 2. Fujitsu: Keep a cache of the most active source nodes. If a node has
> an HOL timer time out, it sends a unicast "pause" message to throttle
> the most active source for a time. After another timeout, it will send
> more "pause" messages to other sources. This can be extended to cover
> multiple priorities, although I didn't see it explicitly stated in the
> slides.
>
> 3. Nortel, iPT-CAP: When an HOL timer expires, the node calculates the
> number of sources sending through the congested link, and apportions the
> link fairly (if the link is 150M, and there are 3 sources, it decides
> that each souce can use 50M). To do this, it sets its B/W cap to 50M,
> and then sends a message upstream to tell other nodes to start sending
> at only 50M. Once the affected link becomes uncongested, new messages
> are sent upstream, advising that more B/W is now available. This will
> converge to a fair B/W allocation.
>
> 4. Dynarc: Token passing and credits. No detailed description. What is
> the "goodput"?
>
> 5. Lantern: Per-SLA weighted fairness, with remaining bandwidth
> apportioned fairly to SLAs. There wasn't a good explanation of
> congestion handling, though. If the per-SLA rate limits are strictly
> enforced to stop congestion, and traffic is bursty, what happens to the
> "goodput"?
>
> Thanks a lot,
> --Carey Kloss
>
>
> Sanjay Agrawal wrote:
>
>
> Please see comments inline.
>
> -Sanjay
>
> -----Original Message-----
> From: William Dai [mailto:wdai@xxxxxxxxxxxx]
> Sent: Thursday, March 22, 2001 2:37 PM
> To: Sanjay Agrawal; 'Devendra Tripathi'; Ajay Sahai; Ray Zeisz
> Cc: stds-802-17@xxxxxxxx
> Subject: Re: [RPRWG] MAC Question
>
> My understanding of the cut-through definition in Sanjay's
> example is
> 1. Pass-through packet is allowed to transmit before it is
> completely received.
>
> [Sanjay Agarwal]
> Not necessarily. You have same result if you forward packet
> after you completely receive it or you start
> transmitting before you receive. In the formar case you have one
> packet delay, in latter you don't. 1500byte at 10
> gig gives you 1.2 microseconds.
>
> 2. There is only one transit buffer (regardless of
> class).
> [Sanjay Agarwal]
> Yes that is what proposed cut through schemes have. If you
> have multiple classes of service and you allow
> priority than you have to arbitrate between add and pass classes
> of traffic at that moment it becomes store and
> forward.
>
> 3. Scheduling Algorithm always give pass-through traffic
> (regardlesss of class)
> preference over add-on traffic.
> [Sanjay Agarwal]
>
> Yes that is what proposed cut through schemes have. If you
> don't give pass higher priority than you don't have
> cut through scheme.
> which somewhat contradicts with his first statement. Thus the
> interesting results.
> No. it doesn't.
>
> The debate should be based on a solid definition of cut-through
> transmission, otherwise
> there will be no convergence at the end of the discussion.
>
> I agree.
>
> I fully agree Sanjay's first statement, but want to add that
> each class should have its
> own transit buffer, (personally I prefer having 3 classes
> supported as RPR MAC services).
> Whether each transit buffer should reside in MAC layer or
> systemlayer is up to further
> discussion. Under this context, Circuit Emulation (or some may
> prefer to call it
> Synchronous) class will benefit from the cut-through transit.
> [Sanjay Agarwal]
> I don't agree in the present cut through proposals case.
> Unless you want to define cut though differently.
> Ideally it could further benefit from preemptive
> transmission (yet another definition to be solidly defined).
>
> William Dai
>
>
>