Francois Le Faucheur, Editor
Cisco Systems, Inc.
IETF Internet Draft
Expires: April, 2002
Document: draft-ietf-tewg-diff-te-russian-00.txt October, 2002
Russian Dolls Bandwidth Constraints Model for
Diff-Serv-aware MPLS Traffic Engineering
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Abstract
This document provides a detailed description of a candidate
Bandwidth Constraints model for Diff-Serv-aware MPLS Traffic
Engineering, which is referred to as the Russian Dolls model.
Summary for Sub-IP related Internet Drafts
RELATED DOCUMENTS:
draft-ietf-tewg-diff-te-reqts-06.txt
draft-ietf-tewg-diff-te-proto-02.txt
WHERE DOES IT FIT IN THE PICTURE OF THE SUB-IP WORK
This ID is a Working Group document of the TE Working Group.
WHY IS IT TARGETED AT THIS WG(s)
TEWG is responsible for specifying protocol extensions for support of
Diff-Serv-aware MPLS Traffic Engineering.
Le Faucheur, et. al 1
Russian Dolls model for DS-TE October 2002
JUSTIFICATION
The TEWG charter states that "This will entail verification and
review of the Diffserv requirements in the WG Framework document and
initial specification of how these requirements can be met through
use and potentially expansion of existing protocols."
In line with this, the TEWG is in the process of selecting bandwidth
constraints model for Diff-Serv-aware MPLS Traffic Engineering. This
document describes one bandwidth constraints model under
consideration.
1. Introduction
[DSTE-REQ] presents the Service Providers requirements for support of
Diff-Serv-aware MPLS Traffic Engineering (DS-TE). This includes the
fundamental requirement to be able to enforce different bandwidth
constraints for different classes of traffic.
[DSTE-REQ] also defines the concept of Bandwidth Constraint Models
for DS-TE and states that "The DS-TE technical solution must specify
one default bandwidth constraint model which must be supported by any
DS-TE implementation. However, additional bandwidth constraint models
may also be specified."
This document provides a more detailed description of one particular
Bandwidth Constraint model introduced in [DSTE-REQ] which is called
the Russian Dolls model. The Russian Dolls model is one of the
candidate models under consideration by the TEWG for selection of the
default bandwidth constraints model and/or of optional additional
models.
[DSTE-PROTO] specifies the IGP and RSVP-TE signaling extensions for
support of DS-TE. These extensions support the Russian Dolls model.
Considerations on the Russian Dolls model are provided in [RUSSIAN-
CONS] and [BCMODEL].
2. Contributing Authors
This document was the collective work of several. The text and
content of this document was contributed by the editor and the co-
authors listed below. (The contact information for the editor appears
in Section 11, and is not repeated below.)
Jim Boyle Kireeti Kompella
Protocol Driven Networks, Inc. Juniper Networks, Inc.
1381 Kildaire Farm Road #288 1194 N. Mathilda Ave.
Cary, NC 27511, USA Sunnyvale, CA 94099
Phone: (919) 852-5160 Email: kireeti@juniper.net
Email: jboyle@pdnets.com
Le Faucheur et. al 2
Russian Dolls model for DS-TE October 2002
William Townsend Thomas D. Nadeau
Tenor Networks Cisco Systems, Inc.
100 Nagog Park 250 Apollo Drive
Acton, MA 01720 Chelmsford, MA 01824
Phone: +1-978-264-4900 Phone: +1-978-244-3051
Email: Email: tnadeau@cisco.com
btownsend@tenornetworks.com
Darek Skalecki
Nortel Networks
3500 Carling Ave,
Nepean K2H 8E9
Phone: +1-613-765-2252
Email: dareks@nortelnetworks.com
3. Definitions
For readability a number of definitions from [DSTE-REQ] are repeated
here:
Class-Type (CT): the set of Traffic Trunks crossing a link that is
governed by a specific set of Bandwidth Constraints. CT is used for
the purposes of link bandwidth allocation, constraint based routing
and admission control. A given Traffic Trunk belongs to the same CT
on all links.
TE-Class: A pair of:
i. a Class-Type
ii. a preemption priority allowed for that Class-Type. This
means that an LSP transporting a Traffic Trunk from
that Class-Type can use that preemption priority as the
set-up priority, as the holding priority or both.
4. Russian Dolls Model Definition
The "Russian Dolls" model is defined in the following manner
(assuming for now that the optional per-CT Local Overbooking
Multipliers defined in [DSTE-PROTO] are not used - i.e. LOM[c]=1 ,
0<=c<=7 ):
o Maximum Number of Bandwidth Constraints (MaxBC)= Maximum
Number of Class-Types (MaxCT) = 8
o All LSPs supporting Traffic Trunks from CTc (with
b<=c<=7) use no more than BCb i.e.:
- All LSPs from CT7 use no more than BC7
- All LSPs from CT6 and CT7 use no more than BC6
- All LSPs from CT5, CT6 and CT7 use no more than BC5
- etc.
- All LSPs from CT0, CT1,... CT7 use no more than BC0
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Russian Dolls model for DS-TE October 2002
Purely for illustration purposes, the diagram below represents the
Russian Doll Bandwidth Constraints model in a pictorial manner when 3
Class-Types are active:
I------------------------------------------------------I
I-------------------------------I I
I--------------I I I
I CT2 I CT2+CT1 I CT2+CT1+CT0 I
I--------------I I I
I-------------------------------I I
I------------------------------------------------------I
I-----BC2------>
I----------------------BC1------>
I---------------------------------------------BC0------>
While simpler or, conversely, more flexible/sophisticated Bandwidth
Constraints models can be defined, the Russian Dolls model is an
attractive trade-off for the following reasons:
- Network administrators generally find it superior to the
basic Maximum Allocation model consisting of a single
independent BC per CT (which, in typical deployment scenarios
may result in either capacity wastage, low priority Traffic
Trunk starvation and/or degradation of QoS objectives).
- network administrators generally find it sufficient for the
real life deployments currently anticipated (e.g. it
addresses all the scenarios described in [DSTE-REQ] as
illustrated in Appendix A below).
- it remains simple and only requires limited protocol
extensions, while more sophisticated Bandwidth Constraints
model may require more complex extensions.
More details on the properties of the Russian Dolls model can be
found in [RUSSIAN-CONS] and [BCMODEL].
As a simple example usage of the "Russian Doll" Bandwidth Constraints
Model, a network administrator using one CT for Voice (CT1) and one
CT for data (CT0) might configure on a given link:
- BC0 = 2.5 Gb/s (i.e. Voice + Data is limited to 2.5 Gb/s)
- BC1= 1.5 Gb/s (i.e. Voice is limited to 1.5 Gb/s).
5. Example Formulas for Computing "Unreserved TE-Class [i]" with
Russian Dolls Model
As specified in [DSTE-PROTO], formulas for computing "Unreserved TE-
Class [i]" MUST reflect all of the Bandwidth Constraints relevant to
the CT associated with TE-Class[i], and thus, depend on the Bandwidth
Constraints Model.
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Russian Dolls model for DS-TE October 2002
Keeping in mind, as explained in [DSTE-PROTO], that details of
admission control algorithms as well as formulas for computing
"Unreserved TE-Class [i]" are outside the scope of the IETF work, we
provide in this section, for illustration purposes, an example of how
values for the unreserved bandwidth for TE-Class[i] might be computed
with the Russian Dolls model, assuming:
- the basic admission control algorithm which simply deducts
the exact bandwidth of any established LSP from all of the
Bandwidth Constraints relevant to the CT associated with that
LSP.
- the optional per-CT Local Overbooking Multipliers are not
used (.i.e. LOM[c]=1, 0<= c <=7).
We assume that:
TE-Class [i] <--> < CTc , preemption p>
in the configured TE-Class mapping.
Let us define "Reserved(CTb,q)" as the sum of the bandwidth reserved
by all established LSPs which belong to CTb and have a holding
priority of q. Note that if q and CTb do not form one of the 8
possible configured TE-Classes, then there can not be any established
LSP which belong to CTb and have a holding priority of q, so in that
case Reserved(CTb,q)=0.
For readability, formulas are first shown assuming only 4 CTs are
active. The formulas are then extended to cover the cases where more
CTs are used.
If CTc = CT0, then "Unreserved TE-Class [i]" =
[ BC0 - SUM ( Reserved(CTb,q) ) ] for q <= p and 0 <= b <= 3
If CTc = CT1, then "Unreserved TE-Class [i]" =
MIN [
[ BC1 - SUM ( Reserved(CTb,q) ) ] for q <= p and 1 <= b <= 3,
[ BC0 - SUM ( Reserved(CTb,q) ) ] for q <= p and 0 <= b <= 3
]
If CTc = CT2, then "Unreserved TE-Class [i]" =
MIN [
[ BC2 - SUM ( Reserved(CTb,q) ) ] for q <= p and 2 <= b <= 3,
[ BC1 - SUM ( Reserved(CTb,q) ) ] for q <= p and 1 <= b <= 3,
[ BC0 - SUM ( Reserved(CTb,q) ) ] for q <= p and 0 <= b <= 3
]
If CTc = CT3, then "Unreserved TE-Class [i]" =
MIN [
[ BC3 - SUM ( Reserved(CTb,q) ) ] for q <= p and 3 <= b <= 3,
[ BC2 - SUM ( Reserved(CTb,q) ) ] for q <= p and 2 <= b <= 3,
[ BC1 - SUM ( Reserved(CTb,q) ) ] for q <= p and 1 <= b <= 3,
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Russian Dolls model for DS-TE October 2002
[ BC0 - SUM ( Reserved(CTb,q) ) ] for q <= p and 0 <= b <= 3
]
The formula can be generalized to 8 active CTs and expressed in a
more compact way in the following:
"Unreserved TE-Class [i]" =
MIN [
[ BCc - SUM ( Reserved(CTb,q) ) ] for q <= p and c <= b <= 7,
. . .
[ BC0 - SUM ( Reserved(CTb,q) ) ] for q <= p and 0 <= b <= 7,
]
where:
TE-Class [i] <--> < CTc , preemption p>
in the configured TE-Class mapping.
6. Support of Optional Local Overbooking Method
We remind the reader that, as discussed in [DSTE-PROTO], the
"LSP/link size overbooking" method (which does not use the Local
Overbooking Multipliers - LOMs-) is expected to be sufficient in many
DS-TE environments. It is expected that the optional Local
Overbooking method (and LOMs) would only be used in specific
environments, in particular where different overbooking ratios need
to be enforced on different links of the DS-TE domain and cross-
effect of overbooking across CTs needs to be accounted for very
accurately.
This section discusses the impact of the optional local overbooking
method on the Russian Dolls model and associated rules and formula.
This is only applicable in the cases where the optional local
overbooking method is indeed supported by the DS-TE LSRs and actually
deployed.
6.1. Russian Dolls Model Definition With Local Overbooking
Let us define "Reserved(CTc)" as the sum of the bandwidth reserved by
all established LSPs which belong to CTc.
Let us define "Normalised(CTc)" as "Reserved(CTc)/LOM(c)".
As specified in [DSTE-PROTO], when the optional Local Overbooking
method is supported, the bandwidth constraints MUST be applied to
"Normalised(CTc)" rather than to "Reserved(CTc)". Thus, when the
optional Local Overbooking method is supported, the "Russian Doll"
model definition MUST be extended in the following manner:
- Maximum Number of Bandwidth Constraints (MaxBC)= Maximum
Number of Class-Types (MaxCT) = 8
- SUM [ Normalised(CTc), for b<=c<=7 ] <= BCb i.e.:
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Russian Dolls model for DS-TE October 2002
o [ Normalised(CT7) ] <= BC7
o [ Normalised(CT6) + Normalised(CT7) ] <= BC6
o [ Normalised(CT5) + Normalised(CT6) +
Normalised(CT7) ] <= BC5
o etc.
o [ Normalised(CT0) + Normalised(CT1) +
...
Normalised(CT6) + Normalised(CT7) ] <= BC0
Purely for illustration purposes, the diagram below represents the
Russian Doll Bandwidth Constraints model in a pictorial manner when 3
Class-Types are active and the local overbooking method is used:
I--------------------------------------------------------------I
I-----------------------------------------I Normalised(CT2) I
I--------------------I Normalised(CT2) I + I
I Normalised(CT2) I + I Normalised(CT1) I
I--------------------I Normalised(CT1) I + I
I-----------------------------------------I Normalised(CT0) I
I--------------------------------------------------------------I
I--------BC2--------->
I-------------------------BC1------------->
I-----------------------------------------------BC0------------>
6.2. Example Formulas for Computing "Unreserved TE-Class [i]" With
Local Overbooking
Again, keeping in mind that details of admission control algorithms
as well as formulas for computing "Unreserved TE-Class [i]" are
outside the scope of the IETF work, we provide in this section, for
illustration purposes, an example of how values for the unreserved
bandwidth for TE-Class[i] might be computed with the Russian Dolls
model, assuming:
- the basic admission control algorithm which simply deducts
the exact bandwidth of any established LSP from all of the
Bandwidth Constraints relevant to the CT associated with that
LSP.
- the optional per-CT Local Overbooking Multipliers are used.
When the optional local overbooking method is supported, the example
generalized formula of section 5 becomes:
"Unreserved TE-Class [i]" =
LOM(c) x MIN [
[ BCc - SUM ( Normalised(CTb,q) ) ] for q <= p and c <= b <= 7,
. . .
[ BC0 - SUM ( Normalised(CTb,q) ) ] for q <= p and 0 <= b <= 7,
]
where:
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Russian Dolls model for DS-TE October 2002
- TE-Class [i] <--> < CTc , preemption p>
in the configured TE-Class mapping.
- Normalised(CTb,q) = Reserved(CTb/q)/LOM(b)
6.3. Example Usage of LOM
To illustrate usage of the local overbooking method with the Russian
Dolls model, let's consider a DS-TE deployment where two CTs (CT0 for
data and CT1 for voice) and a single preemption priority are used.
The TE-Class mapping is the following:
TE-Class <--> CT, preemption
==============================
0 CT0, 0
1 CT1, 0
rest unused
Let's assume that on a given link, BCs and LOMs are configured in the
following way:
BC0 = 200
BC1 = 100
LOM(0) = 4 (i.e. = 400%)
LOM(1) = 2 (i.e. = 200%)
Let's further assume that the DS-TE LSR uses the example formulas
presented above for computing unreserved bandwidth values.
If there is no established LSP on the considered link, the LSR will
advertise for that link in IGP :
Unreserved TE-Class [0] = 4 x (200 - 0/4 - 0/2 )= 800
Unreserved TE-Class [1] = 2 x (100- 0/2) = 200
Note again that these values advertised for Unreserved Bandwidth are
larger than BC1 and BC0.
If there is only a single established LSP, with CT=CT0 and BW=100,
the LSR will advertise:
Unreserved TE-Class [0] = 4 x (200 - 100/4 - 0/2 )=700
Unreserved TE-Class [1] = 2 x (100- 0/2) = 200
If there is only a single established LSP, with CT=CT1 and BW=100,
the LSR will advertise:
Unreserved TE-Class [0] = 4 x (200 - 0/4 - 100/2 )= 600
Unreserved TE-Class [1] = 2 x (100- 100/2) = 100
Note that the impact of an LSP on the unreserved bandwidth of a CT
does not depend only on the LOM for that CT: it also depends on the
LOM for the CT of the LSP. This can be seen in our example. A BW=100
tunnel affects Unreserved
CT0 twice as much if it is a CT1 tunnel, than if it is a CT0 tunnel.
It reduces Unreserved CTO by 200 (800->600) rather than by 100
(800->700). This is because LOM(1) is half as big as LOM(0). This
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Russian Dolls model for DS-TE October 2002
illustrates why the local overbooking method allows very fine
accounting of cross-effect of overbooking across CTs, as compared
with the LSP/link size overbooking method.
If there are two established LSPs, one with CT=CT1 and BW=100 and one
with CT=CT0 and BW=100, the LSR will advertise:
Unreserved TE-Class [0] = 4 x (200 - 100/4 - 100/2) = 500
Unreserved TE-Class [1] = 2 x (100 - 100/2) = 100
If there are two LSPs established, one with CT=CT1 and BW=100, and
one with CT=CT0 and BW=480, the LSR will advertise:
Unreserved TE-Class [0] = 4 x (200 - 480/4 - 100/2) = 120
Unreserved TE-Class [1] = 2 x MIN [ (200 - 480/4 - 100/2),
(100 - 100/2) ]
= 2 x MIN [ 30, 50 ]
= 60
7. Security Considerations
No new security considerations are raised by this document; they are
the same as in the DS-TE requirements document [DSTE-REQ].
8. Acknowledgments
We thank Martin Tatham for his earlier contribution in this work.
9. Normative References
[DSTE-REQ] Le Faucheur et al, Requirements for support of Diff-Serv-
aware MPLS Traffic Engineering, draft-ietf-tewg-diff-te-reqts-06.txt,
September 2002.
[DSTE-PROTO] Le Faucheur et al, Protocol extensions for support of
Diff-Serv-aware MPLS Traffic Engineering, draft-ietf-tewg-diff-te-
proto-02.txt, October 2002.
10. Informative References
[RUSSIAN-CONS] Le Faucheur, "Considerations on Bandwidth Constraints
Model for DS-TE", draft-lefaucheur-tewg-russian-dolls-00.txt, June
2002.
[BCMODEL] Lai, "Bandwidth Constraints Models for DS-TE",
draft-wlai-tewg-bcmodel-00.txt, June 2002.
[OSPF-TE] Katz, Yeung, Traffic Engineering Extensions to OSPF, draft-
katz-yeung-ospf-traffic-08.txt, September 2002.
Le Faucheur et. al 9
Russian Dolls model for DS-TE October 2002
[ISIS-TE] Smit, Li, IS-IS extensions for Traffic Engineering, draft-
ietf-isis-traffic-04.txt, August 2001.
[RSVP-TE] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[DIFF-MPLS] Le Faucheur et al, "MPLS Support of Diff-Serv", RFC3270,
May 2002.
11. Editor's Address:
Francois Le Faucheur
Cisco Systems, Inc.
Village d'Entreprise Green Side - Batiment T3
400, Avenue de Roumanille
06410 Biot-Sophia Antipolis
France
Phone: +33 4 97 23 26 19
Email: flefauch@cisco.com
Appendix A - Addressing [DSTE-REQ] Scenarios
This Appendix provides examples of how the Russian Dolls Model
Bandwidth Constraints model can be used to support each of the
scenarios described in [DSTE-REQ].
1. Scenario 1: Limiting Amount of Voice
By configuring on every link:
- Bandwidth Constraint 1 (for CT1=Voice) = "certain percentage"
of link capacity
- BC0 (for CT1=Voice + CT0= Data) = link capacity
By configuring:
- every CT1/Voice TE-LSP with preemption =0
- every CT0/Data TE-LSP with preemption =1
DS-TE with the Russian Dolls model will address all the requirements:
- amount of Voice traffic limited to desired percentage on
every link
- data traffic capable of using all remaining link capacity
- voice traffic capable of preempting other traffic
2. Scenario 2: Maintain Relative Proportion of Traffic Classes
By configuring on every link:
- BC2 (for CT2) = e.g. 45%
- BC1 (for CT1+CT2) = e.g. 80%
- BC0 (for CT0+CT1+CT2) = e.g.100%
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Russian Dolls model for DS-TE October 2002
DS-TE with the Russian Dolls model will ensure that the amount of
traffic of each Class Type established on a link is within acceptable
levels as compared to the resources allocated to the corresponding
Diff-Serv PHBs regardless of which order the LSPs are routed in,
regardless of which preemption priorities are used by which LSPs and
regardless of failure situations. Optional automatic adjustment of
Diff-Sev scheduling configuration could be used for maintaining very
strict relationship between amount of established traffic of each
Class Type and corresponding Diff-Serv resources.
3. Scenario 3: Guaranteed Bandwidth Services
By configuring on every link:
- BC1 (for CT1) = "given" percentage of link bandwidth
(appropriate to achieve the Guaranteed Bandwidth service's
QoS objectives)
- BC0 (for CT0+CT1) = 100% of link bandwidth
DS-TE with the Russian Dolls model will ensure that the amount of
Guaranteed Bandwidth Trafic established on every link remains below
the given percentage so that it will always meet its QoS objectives.
At the same time it will allow traffic engineering of the rest of the
traffic such that links can be filled up.
Le Faucheur et. al 11
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