----- 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
> >
>
>