----- Original Message -----
Sent: Friday, May 04, 2001 11:43
AM
Subject: RE: [RPRWG] More comments on
preemption
Hi William,
If we use 2 cos bits than we have 4 classes. Might as well
introduce best effort class that provides not guarantees. Best effort
traffic will fill spaces in the unused link bandwidth.
We could have 3 classes of service out of which only 4 are
defined and rest are there for future extentions.
Also, wouldn't reassembly at the destination RPR mac would
be big task. You have no idea how many outstanding preempted mac fragments
there are. Multiple nodes could be sending preempted packets from two
directions of the ring to a destination node. I think the of job
fragmentation and reassembly in the RPR mac chip will be
difficult.
-Sanjay K. Agrawal
Luminous Networks
-----Original Message-----
From:
William Dai [mailto:wdai@xxxxxxxxxxxx]
Sent: Thursday, May 03, 2001 8:28 PM
To: stds-802-17@xxxxxxxx
Subject: Re: [RPRWG]
More comments on preemption
Kanaiya,
There has been a lot of discussion regarding preemption in
the
"Cut through Definition ?" email thread a while
ago.
Let me just recapture what I proposed in that thread for
your
reference (with minor corrections and
additions). It may contradict
with the proposed GFP
packet encapsulation requirement, but hope
it is
enough to correct your misconception about preemption.
1. There are 3 MAC level classes of traffic (H, M, L,). H
and M traffic
insertion is
subjuct to self policing according to their respectively
provisioned rate, while L traffic insertion is
subject to the "Fairness"
ring
access algorithm only.
2. Preemption is allowed only
for H traffic to preempt M or L traffic,
3. Each M
and L packet transfer will be inserted an "IDLE/Escape"
word for every 256 byte (for the sake of
alignment/padding concern)
as the
preemptive insertion point.
4. Preemptive insertion
is allowed only at the preemptive insertion
point of onging M or L traffic.
5. Preempted "Leftover" traffic will be scheduled to transfer
right
after the H traffic is
transferred, regardless of classes, and it could
be subject to further preemption when new H
traffic arrives.
6. M and L traffic are allowed to
do store&forward (packet-wise)
transit on the ring (to reduce the complexity of
reassembly task at
the final
receiver), while H traffic is allowed to do both cut-through
and store&forward transit on the
ring.
7. Jumbo frame is not supported for H
class.
All conditions need to apply at the same time.
Regards
William Dai
----- Original Message -----
From:
"Kanaiya Vasani" <kanaiya@xxxxxxxxxxxxxx>
To:
<stds-802-17@xxxxxxxx>
Sent: Thursday, May 03,
2001 11:07 AM
Subject: RE: [RPRWG] More comments on
preemption
>
> William,
>
> Maybe you can further elaborate on
this. What happens to the pre-empted
>
packet(frame)? How do we deal with the portion of the packet(frame)that
is
> already transmitted?
>
> Thanks,
>
> - Kanaiya
>
> -----Original Message-----
> From: William Dai [mailto:wdai@xxxxxxxxxxxx]
> Sent: Tuesday, May 01, 2001 2:34 PM
> To: stds-802-17@xxxxxxxx
> Subject:
Re: [RPRWG] More comments on preemption
>
>
>
> Preemption does NOT drop packets.
>
> William Dai (minority
member)
>
>
> ----- Original Message -----
> From: "Kanaiya Vasani" <kanaiya@xxxxxxxxxxxxxx>
> To: <stds-802-17@xxxxxxxx>
> Sent: Tuesday, May 01, 2001 1:40 PM
>
Subject: RE: [RPRWG] More comments on preemption
>
>
>
>
> > There has been some good discussion
around the subject of preemption.
> Looks
> > like a majority of the active members on the
reflector would prefer to
> leave
> > it out.
> >
> > I too believe that there shouldn't be any
preemption within the MAC for
> the
> > following reasons:
>
>
> > 1. The RPR MAC shall be defined with
a set of transmission performance
> >
specifications - worst case packet delay, packet jitter tolerance,
packet
> > loss, etc. -
similar to other transmission and transport technologies.
In
> > this context, the MAC packet
loss shall be zero under normal conditions.
>
> Pre-emption results in dropping of frames, and therefore should not be
a
> > function of the MAC.
> >
> > 2. Packet loss is also an
important component of a service level
>
agreement.
> > Service providers obviously
want packet loss to be as close to zero as
> >
possible, and the MAC must do its part to help the overall system
achieve
> > this objective.
Dropping packets or causing CRC errors to support
> > pre-emption is not desirable.
>
>
> > Regards,
>
>
> > - Kanaiya
> >
> > -----Original
Message-----
> > From: Leon Bruckman [mailto:leonb@xxxxxxxxxxxxx]
> > Sent: Sunday, April 22, 2001 10:15 AM
> > To: 'William Dai'; stds-802-17@xxxxxxxx
> > Subject: RE: [RPRWG] More comments on
preemption
> >
>
>
> >
> >
William,
> > You are right that the additional
delay variation added by each
additional
> > node becomes lower as the number of nodes already
taken into
consideration
> > increases. But the maximum delay variation will not
decrease as the
number
>
> of nodes increases.
> > So the simulation
shows the limit to the delay variation, under the
noted
> > assumptions.
> > You already corrected your second observation, so I
understand it is OK.
> > Leon
> >
> > -----Original
Message-----
> > From: William Dai [mailto:wdai@xxxxxxxxxxxx]
> > Sent: Thursday, April 19, 2001 9:18 PM
> > To: stds-802-17@xxxxxxxx
> > Subject: Re: [RPRWG] More comments on preemption
> >
> >
> >
> > Leon,
> >
> > Simulation may
or may not catch the worst case situation. There are 128
> > nodes in your simulation model, the sheer number of nodes
which makes
> > it look like the "toughest"
you can get. While I believe it is good to
> >
evaluate
> > the delay, but it make the jitter
evaluation more difficult. Why ?
because
> > the
> > the
probability of getting minimum delay (packet pass through 127 nodes
> > without being blocked by Jumbo frame insertion)
and the probability of
> > getting maximum
delay (packet pass through 127 nodes and being blocked
> > by Jumbo frame insertion at every node) diminish quickly as
the number
of
> >
nodes increases.
> >
> > Secondly, assume we're comparing a 100Mbps traffic flow
going through
> > 1G ring vs. 10G ring with
the same number of nodes and same traffic
> >
generation models, AND on the other end of the anti-jitter buffer,
traffic
> > will be extracted
out at 100Mbps for the same flow. In theory, the size
> > of the anti-jitter buffer and the delay caused by the
anti-buffer should
> be
> > the SAME. It should not be a surprise because 10G ring is
only 10 times
> > wider than 1G ring, not 10
times faster for the 100Mbps traffic flow.
>
>
> > I'm not a simulation believer
(although I used to be in that field), but
I
> > do
> > respect
those people who is doing that. It is just a tool used by
PEOPLE.
> >
> >
> > Best regards
> >
> > William
Dai
> >
>
>