Subject: Fw: MPLS and CAC

From: "Erickson Trejo-Reyes" <[email-masked]>
Date: Mon, 28 Aug 2000 21:42:43 +0100

Thank you both David and Curtis for your kind answers. Refering the following statement in the possible configuration:

> While CAC is fully supported it can essentially be ignored by setting a very high overbooking factor... 

> With the high overbooking, these tunnels come up anyway.  If there are preferred services, they get preferred treatment so the overbooking doesn't matter to them.  Best effort services temporarily get squeezed... therefore oversubscribing is much more preferrable to having CAC kick in and losing 100% of the traffic until things settle down.

By overbooking you mean that, for example, if a voice source can be treated as a VBR source with 64 KB peak bit rate and activity factor of 0.4, then the reservation should be made to a higher value than the computed effective bandwidth (and obviously higher than the average bit rate of 0.4 times 64KB)?

I had thought that, if an MPLS node is due to use only certain percentage of its total capacity to serve, for example, guaranteed services, it would not be too harmful to make reservations (up to the available percentage) only based on the sustained rate, with the only possible consequence of temporary squeezing the throughput of best-effort services. Comments?

Again, thanks in advanced,

Erickson

---

From: "C. Chen" <[email-masked]>
Date: Mon, 28 Aug 2000 15:51:38 -0500

Mr.  Erickson,

It seems to me that overbooking factor determination is not simply PCR *
activity factor (e.g., .4). I was wondering wether there is a IETF draft
defining overbooking factor that you know of ? If not, I would like to
suggest that we work together with people who might be interested in
participating to make a draft contribution on this suject.

Best Regards,

Cheng C. Chen

Erickson Trejo-Reyes wrote:

>  Thank you both David and Curtis for your kind answers. Refering the
> following statement in the possible configuration: > While CAC is
> fully supported it can essentially be ignored by setting a very high
> overbooking factor... > With the high overbooking, these tunnels come
> up anyway.  If there are preferred services, they get preferred
> treatment so the overbooking doesn't matter to them.  Best effort
> services temporarily get squeezed... therefore oversubscribing is much
> more preferrable to having CAC kick in and losing 100% of the traffic
> until things settle down. By overbooking you mean that, for example,
> if a voice source can be treated as a VBR source with 64 KB peak bit
> rate and activity factor of 0.4, then the reservation should be made
> to a higher value than the computed effective bandwidth (and obviously
> higher than the average bit rate of 0.4 times 64KB)? I had thought
> that, if an MPLS node is due to use only certain percentage of its
> total capacity to serve, for example, guaranteed services, it would
> not be too harmful to make reservations (up to the available
> percentage) only based on the sustained rate, with the only possible
> consequence of temporary squeezing the throughput of best-effort
> services. Comments? Again, thanks in advanced,  Erickson

---

From: David Charlap <[email-masked]>
Date: Mon, 28 Aug 2000 19:43:48 -0400

Erickson Trejo-Reyes wrote:
> 
> By overbooking you mean that, for example, if a voice source can be
> treated as a VBR source with 64 KB peak bit rate and activity factor
> of 0.4, then the reservation should be made to a higher value than the
> computed effective bandwidth (and obviously higher than the average
> bit rate of 0.4 times 64KB)?

The idea behind overbooking is simple.  You accept requests for
resources that you don't have.

Under certain circumstances, this is OK.  For instance:

- You know that the overbooked reservations are temporary and will go
  away (eg. the overload is due to a failure somewhre and will be moved
  to another switch when the rest of the network realizes this and
  reroutes the tunnels.)

- You know that most of these tunnels aren't going to be running at peak
  capacity.  This is dangerous for an ISP to assume - especially when
  legal contracts demand a certain service level and you don't know the
  nature of the data, but it can be acceptible in a situation where you
  do know what kind of data will be flowing through the tunnels.

- You know that the tunnesl aren't used much during certain times of the
  day.  For instance, many corporate links (especially those that are
  not behind a publicly-accessible server) almost go idle at night, when
  most employees have gone home.  Again, you have to know your customers
  here.

> I had thought that, if an MPLS node is due to use only certain
> percentage of its total capacity to serve, for example, guaranteed
> services, it would not be too harmful to make reservations (up to the
> available percentage) only based on the sustained rate, with the only
> possible consequence of temporary squeezing the throughput of
> best-effort services. Comments?

Squeezing best-effor services due to a lot of reservations is not
overbooking.  That's just the nature of best-effort traffic.

Overbooking is where (for instance) a 10G switch grants an aggregate of
20G among all of its tunnels.  If the tunnels don't actually use more
than 10G total, you're OK - if not, then you cause their guarantees to
be violated.

Whether or not this is acceptible will depend on why you're overbooking
(is it something transitory that will quickly go away, or is it a
deliberate management decision), and what kind of service your customers
are actually paying for.

-- David

---

From: Aimin Sang <[email-masked]>
Date: Tue, 29 Aug 2000 10:24:29 -0500 (CDT)

Dear David,

Thanks for your good insights regarding the overbooking factor. I have

>   to another switch when the rest of the network realizes this and

Could you please give us a more detailed example (when "this is OK") to
set overbooking factor? Say: The overbooked reservation is temporary
because of the re-routing, but who set the overbooking factor and how? 
It is set by the network operator from time to time, or the CAC algorithm
automatically?

Further, I guess you mean that the CAC's decision of overbooking can be
balanced by traffic engineering. If that is right, are these two control 
at the same time-scale?

Given the GCAC in PNNI 1.0, should the overbooking factor be a coeffient
of AvCR (e.g. 120% * AvCR), or of the actual allocated bandwidth (or
effective bandwidth) to each flow? By functionality, they should bring the
same effect.

Considering the highly non-stationarity of real network traffic, the
overbooking factor can hardly works well (with high utilization and low
loss) all the time. So does the overbooking factor reflect the real
"over- reservation of the bandwidth resources", or it is just a tuning
knob of the specific CAC algorithm?

>   do know what kind of data will be flowing through the tunnels.
This is a very good point.

> deliberate management decision), and what kind of service your customers

The last paragraph gave me very good insights, but I still do know how to
overbook in case of something transitory. Maybe you mean the traffic burst
which comes and goes very quickly, and the overbooking factor is just
fixed during that period.

If a local CAC gives the network operator an interface to tune the
overbooking factor, I think the corresponding tuning guideline should
follows tightly. Could you please comment on this, or give some example
about the tuning guideline?

Many thanks,

Sincerely,

Aimin
****************************************************************
                Aimin Sang, Ph.D Candidate
                Telecommunication Networks
                ENS 106, ECE Department
                Univ. of Texas at Austin

                Mailing/Home Address:
                1515 Rio Grand Dr., Apt. 723, Plano, TX 75075

                Fax:   (214)-291-2280
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"You can never be too rich, too thin, or have too much bandwidth"
*****************************************************************

---

From: David Charlap <[email-masked]>
Date: Tue, 29 Aug 2000 12:55:10 -0400

Aimin Sang wrote:
> 
> Could you please give us a more detailed example (when "this is OK")
> to set overbooking factor? Say: The overbooked reservation is
> temporary because of the re-routing, but who set the overbooking
> factor and how? It is set by the network operator from time to time,
> or the CAC algorithm automatically?

I would hope that every implementation would require the operator to set
this factor.

> Further, I guess you mean that the CAC's decision of overbooking can
> be balanced by traffic engineering. If that is right, are these two
> control at the same time-scale?

The idea here is not that you want the switch to remain overbooked, but
that temporary overbooking is better than lost connections.

If a link fails, a fast-reroute algorithm may shunt the LSP onto another
router.  Later on, when the routing tables stabilize, the ingress router
(perhaps in response to an operator command) may choose to reroute that
LSP again, onto a less-heavily trafficked switch.

If a lot of LSPs get shunted onto a single fallback router, this router
may not have enough resources to maintain all of their original QoS
levels.  At this point, the failover switch has one of two choices.  It
can refuse to accept those LSPs that it can't satisfy, or it can
overbook them.

If it refuses to accept the LSPs, then those connections will go down
until the ingress router can re-signal with a new path.  The LSPs that
did get rerouted will continue to operate at their former QoS levels.

If it overbooks, then none of the connections will go down, but the QoS
on all of them will suffer until the ingress node can reroute them
elsewhere.

There are advantages to either scenario.  For some networks, it is
better that all connections remain up, at reduced quality.  For some
networks, it is better that some connections go down, so that the rest
remain at their normal quality levels.

As for how to actually implement overbooking, I'll leave that part of
the answer to someone with more experience in this area.

-- David

---

From: "C. Chen" <[email-masked]>
Date: Tue, 29 Aug 2000 12:59:57 -0500

I agree with Mr. Charlap with few additional comments which will be marked
with ">>"

Best Regards,

Cheng C. Chen

>> It seems to me that for LSP set up using LDP is almost like ATM PVC and
hence overbooking is almost a necessity and has to be implemented in
conjunction with CAC and routing algorithm due to:
1. Inaccuracy of traffic descriptor supplied by customeror operator of
initial LSP setup
2. Traffic non-coincidence effect due to different time zone
3. Dynamic traffic changes
4. Not all traffic are in active busrst state simultaneously (This used to
call concentration ratio in voice switch) even during busy hour.

>
> >> OVerbooking can also be effective in increasing network utilization
> efficiency even during normal operations. In our experiences with our
> customer  network overbooking is aiming mostly on normal operation.

---

From: "Steven Wright" <[email-masked]>
Date: Tue, 29 Aug 2000 14:08:25 -0400

When you talk of overbooking & CAC in an MPLS network are you talking about
the admission of flows to an LSP ( e.g. the packet classification) or the
admission/establishment of LSPs to an interface/link. These seem to be two
separate CAC decisions ...
regards
Steven Wright

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]On Behalf Of David
> Charlap
> Sent: Tuesday, August 29, 2000 12:55 PM
> To: [email-masked]
> Subject: Re: Fw: MPLS and CAC
>
>
> Aimin Sang wrote:
> >

---

From: David Charlap <[email-masked]>
Date: Tue, 29 Aug 2000 14:15:45 -0400

Steven Wright wrote:
> 
> When you talk of overbooking & CAC in an MPLS network are you talking
> about the admission of flows to an LSP ( e.g. the packet
> classification) or the admission/establishment of LSPs to an
> interface/link. These seem to be two separate CAC decisions ...

I've been talking about overbooking resources used by transit routers
when making the reservations needed for LSP setup.

-- David

---

From: Aimin Sang <[email-masked]>
Date: Tue, 29 Aug 2000 13:50:02 -0500 (CDT)

Thanks for the answer. I think it maybe better to have the re-routing
for link failure separated from the flow's QoS violation for overbooked
CAC. Of course if link failure is defined as a seriously violated QoS,
then they have to co-work together.

I am expecting someone to give me more insights
about overbooking implementation in CAC, especially for controlled load
service, or CBR/VBR in ATM.

Thanks.

Sincerely,

Aimin
		
                Mailing/Home Address: 
		
                Fax:   (214)-291-2280

---

From: Curtis Villamizar <[email-masked]>
Date: Tue, 29 Aug 2000 21:48:57 -0400

In message <[email-masked]>, "Erickson Trejo-Re
yes" writes:
> 
> By overbooking you mean that, for example, if a voice source can be =
> treated as a VBR source with 64 KB peak bit rate and activity factor of =
> 0.4, then the reservation should be made to a higher value than the =
> computed effective bandwidth (and obviously higher than the average bit =
> rate of 0.4 times 64KB)?

I meant that the allowable sum of reservations could greatly exceed
the desireable sum of reservations.  The ingress would have to be
smart enough to lay out trafffic such that load was well distributed
and was far below the values where CAC would limit the sum of
reservations.

> I had thought that, if an MPLS node is due to use only certain =
> percentage of its total capacity to serve, for example, guaranteed =
> services, it would not be too harmful to make reservations (up to the =
> available percentage) only based on the sustained rate, with the only =
> possible consequence of temporary squeezing the throughput of =

Instead of limiting the guaranteed services to 30% by setting the
reservable bandwidth very low, the ingress can be configured to avoid
any links which are approaching or have over 30% guaranteed service.
Right now the only way to know that it is guaranteed service and now
BE is to give then different holding priorities but that is changing.

Curtis

---

From: "Bora Akyol" <[email-masked]>
Date: Tue, 29 Aug 2000 22:12:57 -0700

Of course overbooking is a poor man's substitute for calculating the
equivalent capacity of the traffic that is passing through the tunnel.

Nevertheless, overbooking is commonly used and people like it (for many
reasons).

Bora

---

From: Curtis Villamizar <[email-masked]>
Date: Wed, 30 Aug 2000 09:54:35 -0400

> equivalent capacity of the traffic that is passing through the tunnel.

wrt equivalent capacity:

How do you propose calculating the equivalent capacity of best effort
Internet traffic?  What happens to your calculation when a peer
network (ie: some other AS not under your control) loses a link and
prefers a different inter-provider interconnect?  What happens to your
equivalent capacity predictions when a hot web site or other event
appears (sports events, major news events, etc)?

There is a great deal of unpredictability in Internet traffic
patterns.

wrt overbooking:

In addition to the MPLS usage that is analogous to equivalent capacity
in ATM, overbooking is there to handle the transient traffic layout
when a fault occurs.  The layout is then gradually optimized.

Curtis