Subject: RE: [Diffserv] Intuitive EF ?

From: Shahram Davari <[email-masked]>
Date: Wed, 26 Jul 2000 10:46:53 -0700

Grenville,

I think you misunderstood my comment. What I am trying to say is that
example 1.2.2 wants to show an output stream which is intuitively EF
conformant (as you and I both agree) but does not fulfill the conformance
definitions of RFC-2598 as mentioned in my email. I never said there is a
problem with this behavior. I just pointed out that according to RFC-2598
this flow is not EF conformant, while it should be.

Regards,
-Shahram

>-----Original Message-----
>From: Grenville Armitage [mailto:[email-masked]]
>Sent: Wednesday, July 26, 2000 1:37 PM
>To: Shahram Davari
>Cc: DiffServ
>Subject: Re: [Diffserv] Intuitive EF ?
>
>
>Shahram,
>
>Shahram Davari wrote:
>	[..]
>> 1.2.1 works fine, but 1.2.2 DOES NOT. For example consider 
>the time interval
>> of 4T to 6T which is 3T that starts with an EF bit 
>(consistent with you
>> interpretation). This time interval (3T) is greater than 2T 
>(which is the
>> time interval to send an MTU size packet at configured rate 
>C/2). Now during
>> this time interval only one EF packet has been sent (the one 
>at time 4T). So
>> the average rate during this interval is MTU/3T = C/3 which 
>is less than C/2
>> which was the configured rate. Do you have any other intuition
>> interpretations for this example?
>
>I have no problem with 1.2.2. The source stream shows EF packets
>every 3T, the output stream shows EF packets every 3T, and there
>doesn't appear to be any jitter added.
>
>My intuition (which seems to be born out by most router designs) is
>that packets can't go out if they haven't yet arrived. So this
>example where, by design, the source EF stream is running at less
>than 'configured rate' is simply wonderful - underutilized EF class,
>great, no problem, we can sleep easy.
>
>cheers,
>gja
>_______________________________________________________________
>_________
>Grenville Armitage                    
>http://members.home.net/garmitage/
>Bell Labs Research Silicon Valley
>

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---

From: Grenville Armitage <[email-masked]>
Date: Wed, 26 Jul 2000 11:15:29 -0700

Since your observed conformance failure is of the form "the EF packets
are coming out slower than the configured rate", and I firmly believe
most router designers know packets can't come out that haven't
already gone in, I take this scenario as intuitively part of
the existing RFC2598 definition, and therefore conformant.

There's a saying that you only need QoS architectures when there's
contention. Perhaps we need more explicit text in RFC2598 reflecting
that truth. It would certainly appear to remove the supposed conformance
failure in 1.2.2.

cheers,
gja

> this flow is not EF conformant, while it should be.
	[..]
________________________________________________________________________
Grenville Armitage                    http://members.home.net/garmitage/
Bell Labs Research Silicon Valley

---

From: Dimitrios Stiliadis <[email-masked]>
Date: Wed, 26 Jul 2000 15:14:00 -0400 (EDT)

Grenville,

> the existing RFC2598 definition, and therefore conformant.

Your observation is correct. However, there are boundary
conditions. Assume a router with a lot of latency. 
The EF definition as is, requires exact jitter behavior
for very small time scales.(delays between any two packets).

During the latency of the router, a lot of packets might
arrive and not reach the outgoing link. As a result,
altough the router looks backlogged from the outside,
it doesn't realy send packets with the EF rate yet.
If the flow continues for an arbitrary amount of time,,
you will see the EF rate pretty soon. However, you can
always create some bursty traffic scenarios that will
make a compliant implementation look wrong from the 
outside world. 

In the end of the day, compliance or not can not be 
an intuitive matter ? Can it ?

Dimitri

---

From: Grenville Armitage <[email-masked]>
Date: Wed, 26 Jul 2000 14:59:37 -0700

Dimitrios Stiliadis wrote:
	[..]
> it doesn't realy send packets with the EF rate yet.

That's okay so far - the outside world just characterizes
this as part of the path's latency.

> you will see the EF rate pretty soon.

(On the egress?)  Agreed.

> However, you can

I'm not thinking clearly here - can't quite see the
failure mode you're referring to.

(Mind you, I'm also assuming that we're not talking
about theoretical routers with arbitrarily large
latencies - market forces would remove such routers
from consideration anyway :)

> an intuitive matter ? Can it ?

Agreed in theory. Though in practice many IETF documents carry
assumed world-views (call them "intuitive assumptions") around
inside. Interoperability trials basically test how well each
implementor shared (either accidentally or through coordination)
a spec's world-views.

cheers,
gja