Subject: RE: [PWE3] [Fwd: Proposed text for SAToP congestion section]

From: "Gray, Eric" <[email-masked]>
Date: Tue, 3 Jan 2006 14:14:00 -0500

Stewart,
 
    Agreed.  The problem then is to define "the right thing to do" and
"circumstances"
as policy knobs that can be controlled by the service operator.
 
    The "right thing to do" might be defined in terms of prioritizing
traffic types while 
the "circumstances" might be defined in terms of specific congestion
thresholds.  I
believe the point is that we should not try to define things in this level
of detail in an
IETF specification.
 
--
Eric

  _____  

From: [email-masked] [mailto:[email-masked]] On Behalf Of
Stewart Bryant
Sent: Tuesday, January 03, 2006 2:05 PM
To: [email-masked]
Cc: Yaakov Stein; pwe3; Scott W Brim
Subject: Re: [PWE3] [Fwd: Proposed text for SAToP congestion section]

Eric W Gray wrote:

Yaakov,

    Think of the difference between "congestion" and "acute resource
shortage" in perhaps more familiar 
terms of breathing difficulty.  Congestion is in fact a frequent, if not
normal, condition.  Acute resource 
shortage is pathological.

Perhaps we should use the term severe congestion. No one is thinking 
that we should shut down the TDM service because of a few dropped packets.

    We should not attempt to dictate how services are treated under what is
likely to be a common  mode.

    As for discontinuing a CBR service because it is consuming more than a
"fair share", perhaps you did
not understand the AD's comments, or perhaps those comments are wrong.  It
is not particulary meaningful
to define a CBR service if it does not continue to provide CBR delivery
under conditions of competition for
(possibly scarce) resources.

Only up to a point. There will be some point at which CBR traffic load has 
to be shed to allow higher priority TDM traffic to continue, or to allow the

network infrastruture itself to continue.

Perhaps the right analogy is with electricity supply. Cutting supply is very

serious and not done lightly, it is only done after more capacity has been
added, 
then the voltage has been reduced, and then even then the low priority users
are 
shut down first.

TDM shutdown should only be undertaken when it really is "the right
thing to do under the circumstances"

- Stewart

--
Eric

Yaakov Stein wrote:

8.  Congestion Control

As explained in [PWE3-ARCH], the PSN carrying the PW may be subject to

congestion, with congestion characteristics depending on PSN type, 

network architecture, configuration, and loading. During congestion 

the PSN may exhibit packet loss and PDV that will impact

s/will/might/ ... since it is the goal that it *not* impact the PW.

[YJS] OK - reasonable

the timing and data integrity of the SAToP PW.  During intervals of 

accute congestion SAToP will not be able to maintain service.

If the PW is engineered with sufficient resources, then congestion alone

will not be a problem, since other traffic could be dropped as needed.

Failures might be a problem but not congestion itself.

Perhaps you could say "during intervals of acute resource shortage".

[YJS] I don't see the major difference between congestion and "acute

resource shortage".

In any case, the title of the section is "congestion control",

and we were specifically asked to provide a solution for what

SAToP does under "congestion".

In

addition, since SAToP PWs carry constant bit rate (CBR) services 

across the PSN, they cannot behave in a TCP-friendly manner prescribed

by [RFC2914].  In the presence of services that reduce transmission 

rate, SAToP PWs will thus consume more than their fair share (although

being CBR, the percentage of total bandwidth they consume remains 

constant)

I don't understand combining "fair share" and CBR.  When you engineer a

PW for CBR, it removes it from consideration of "fairness" of

distribution of resources.  I would delete this paragraph ...

[YJS] Unfortunately, this is precisely the paragraph we were asked

to write in Allison's DISCUSS.

and SHOULD be halted.

... except for this.  During intervals of acute resource shortage, when

the PW service requirements cannot be met, the service should be halted.

[YJS] you mean SHOULD :)

Whenever possible, SAToP PWs should be run over traffic-engineered 

PSNs providing bandwidth allocation and admission control mechanisms.

IntServ-enabled domains providing the Guaranteed Service (GS) or 

DiffServ-enabled domains using EF (expedited

forwarding) are examples of traffic-engineered PSNs. Such PSNs will 

minimize loss and delay while providing some degree of isolation of 

the SAToP PW's effects from neighboring streams.

I would move this paragraph up to the top.  Considerations of all of the

other issues depend on this.

[YJS] Actually I tried this in one of the first attempts.

I believe that we should start with what the architecture draft

tells us, and then differentiate between the best effort

and the TE cases. Only then does it make sense to say

that TE SHOULD be used IF AVAILABLE.

The PEs SHOULD monitor for congestion (by using explicit congestion 

notification, VCCV, or by measuring packet loss) in order to ensure 

that the SAToP service may be maintained. Note that SAToP packet loss 

rate may always be measured unintrusively since the expected packet 

arrival rate is known and fixed.  When significant congestion is 

detected the SAToP service SHOULD be terminated.

Again, congestion itself is not the problem.  "Resource shortage" as a

general term?

[YJS] as above

The rest is okay.

[YJS]  :)

Thanks ... Scott

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---

From: Stewart Bryant <[email-masked]>
Date: Tue, 03 Jan 2006 20:10:21 +0000

Gray, Eric wrote:

> believe the point is that we should not try to define things in this 
> level of detail in an
> IETF specification.

Eric

Do you want to provide some alternative text that addresses ALL of the 
issues
raised in Alison's discuss?

Without addressing the issues we cannot move forward.

Stewart

> --
> Eric
>
>     ------------------------------------------------------------------------
>     From: [email-masked] [mailto:[email-masked]] On
>     Behalf Of Stewart Bryant
>>         Think of the difference between "congestion" and "acute
>>     resource shortage" in perhaps more familiar
>>     terms of breathing difficulty.  Congestion is in fact a frequent,
>>     if not normal, condition.  Acute resource
>>         We should not attempt to dictate how services are treated
>>     under what is likely to be a common  mode.
>>
>>         As for discontinuing a CBR service because it is consuming
>>     more than a "fair share", perhaps you did
>>     not understand the AD's comments, or perhaps those comments are
>>     wrong.  It is not particulary meaningful
>>     to define a CBR service if it does not continue to provide CBR
>>     delivery under conditions of competition for
>     serious and not done lightly, it is only done after more capacity
>     has been added,
>     then the voltage has been reduced, and then even then the low
>     priority users are
>     shut down first.
>
>>------------------------------------------------------------------------
>>
>------------------------------------------------------------------------
>