Subject: [Diffserv] clarifications on new terms (draft-ietf-diffserv-new-terms-04)

From: "Sergio Andreozzi" <[email-masked]>
Date: Thu, 3 May 2001 18:39:37 +0300

Hi everybody,

could you tell me if the following sentences are correct? (If no, please
comment why):

1. PHB groups (a single PHB is a special case of a PHB group) MUST operate
entirely independently between each other

1. _instances_ of a _type_ PHB group MUST operate entirely independently
between each other

2. to operate entirely independently means each _istance_ of PHB receives
individual buffer space and bandwith, and mechanisms that implement it don't
use any information coming from mechanisms that implement a different
_istance_ of PHB

3. Class selector PHB requirements specify how PHB associated to codepoints
xxx000 should behave for backwards compatibility;
   we can assert these PHBs operate indipendently, being single PHBs

Thanks

Sergio Andreozzi

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From: Brian E Carpenter <[email-masked]>
Date: Thu, 03 May 2001 12:10:29 -0500

Sergio,

Some PHBs explicitly allow for over-booking of capacity and for the
dropping of packets due to instantaneous overload. AF is an example.
If you are over-booking the total capacity, it is clearly not mathematically
possible for the behavior aggregates to be independent, since excess traffic
in one aggregate will compete with the excess in another.

Even in the case of EF, where no over-booking is allowed, if two EF aggregates
are sharing a link, they will influence each other's jitter.

  Brian

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From: "Sergio Andreozzi" <[email-masked]>
Date: Thu, 3 May 2001 23:54:19 +0300

>are sharing a link, they will influence each other's jitter.
Yes, I understand that two EF aggregates entering the same node from two
different links, and so from two different ingress interfaces, but exiting
from the same egress interface, they experience the same EF PHB and can
influence the each other's jitter.

My doubt is on a PHB group abstraction level. I would like to better
understand the concept of independence among PHB groups.

In the draft-ietf-diffserv-new-terms-04 to better understand AF PHB the
concept of _type_ and _instance_ of a PHB have been indirectly introduced.

The idea comes from high-level programming languages. When you need several
instances of the same data structure, you describe only once the structure
(defining the type), then you create several instances and you can assign
different data values to each instance.

The same for the AF PHB. You want to define the structure of an Assured
Forwarding PHB group (composed by three PHB with three different level of
precedence).
After that you create four "AF PHB instances" of this "AF PHB group type"
and you assign codepoints to each group. Each AF PHB group implementation
can have different parameters for the mechanisms (e.g. for RED queues). Each
packet in one of these instances MUST be forwarded independently from
packets in another instance.

My question is: what does "MUST be forwarded independently" mean?

>It seems to me that EF might be able to do overbooking also since not all
EF
>traffic will be in active state
>even in the "busy (peak) hour". Just think of the voice traffic, during
busy
>hour a 4-to-one or 8-to-one concentration ratio (overbooking) is frequently
>used". Would you comment on this?
>Cheng C. Chen

I still don't have strong experience on services configuration. Is this a
problem of policing at the network boundary? If you augment the max number
of voice connections that can be enstablished, maybe you increase the
upstream traffic of an interior node. The EF PHB will behave as implemented.
If an unlimited preemption is allowed, all the voice connections could find
enough resources.
But this doesn't mean to over-book the EF PHB.

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From: Nabil Seddigh <[email-masked]>
Date: Thu, 03 May 2001 17:20:34 -0400

> My question is: what does "MUST be forwarded independently" mean?

My recollection is that such a line was put in the AF RFC 
to reflect the fact that there is no REQUIRED priority 
amongst the AF classes. Each class may be independently 
allocated a portion of the link bandwidth.

---
Best,
Nabil Seddigh
[email-masked]

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From: Brian E Carpenter <[email-masked]>
Date: Thu, 03 May 2001 17:00:18 -0500

Exactly. You have 4 independent automata, each following its own rules. 
The fact that the 4 automata happen to be built the same way is incidental.

However, we must not forget that in the real world they share at least one
common resource, the output line.

    Brian

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From: demir <[email-masked]>
Date: Thu, 3 May 2001 16:45:38 -0700 (PDT)

In addition to that:
i) a DS node MUST NOT aggregate two or more AF classes together, otherwise
there is an obvious dependency that is not possible in AF PHB group
ii) Drop precedence of an AF PHB group has nothing to do with others
i.e. There is no relationship between drop precedence of AF class 1 packet
having DSCP of 001010 (Low Drop Prec) and AF class 2 packet having DSCP of
010010 (again Low Drop Prec) or AF class 1 packet having DSCP of 001010
(Low Drop Prec) and AF class 2 packet having DSCP of 010110 (High Drop
Prec)
ii) preempting the configured forwarding resources among the classes of AF
PHB group is not allowed for each PHB group
(I am not sure about "excess" resources, but I assume it is allowed to
preempt)

However, there could be a relation where RFC2597 states as:
  "This memo does not specify that any particular relationship hold
   between AF PHB groups and other implemented PHB groups; it requires
   only that whatever relationship is chosen be documented.
   Implementations MAY allow either or both of these relationships to be
   configurable.  It is expected that this level of configuration
   flexibility will prove valuable to many network administrators."

I am not sure how this affects the "independence" criteria.

Alper K. Demir

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From: "Sergio Andreozzi" <[email-masked]>
Date: Fri, 4 May 2001 12:35:53 +0300

one example of dependence among PHBs of a PHB group (e.g. AF1 PHB) is to use
a RIO-C algorithm to calculate the average queue lenght for packets. This
algorithm considers all packets belonging to AF1. Therefore there is an
explicit dipendence among AF11, AF12 and AF13. Dropping algorithm for yellow
packets use information coming from red and green packets marked as AF1.

Conversely, for instance, AF2 PHB group doesn't use information coming from
other PHB groups. So I can assert it forwards packets indipendently.

"Packets in one AF class MUST be forwarded independently from packets in
another AF class"  [RFC2597:3]

Could you explain what "MUST be forwarded independently" mean? And how this

Sergio Andreozzi

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From: Kathleen Nichols <[email-masked]>
Date: Fri, 04 May 2001 10:09:25 -0700

Sergio Andreozzi wrote:
...
> 
> Could you explain what "MUST be forwarded independently" mean? 

...
At the time this was being bandied about that phrase was considered
to be a close equivalent of "must have separate queues" but a less
restrictive way of saying so (that is, didn't preclude a range of
implementations that might become popular).

	Kathie