Francois Le Faucheur
Cisco Systems, Inc.
Waisum Lai
AT&T Labs
IETF Internet Draft
Expires: December, 2003
Document: draft-ietf-tewg-diff-te-mam-00.txt June, 2003
Maximum Allocation Bandwidth Constraints Model for
Diff-Serv-aware MPLS Traffic Engineering
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Abstract
This document provides specification for one Bandwidth Constraints
model for Diff-Serv-aware MPLS Traffic Engineering, which is referred
to as the Maximum Allocation Model.
Summary for Sub-IP related Internet Drafts
RELATED DOCUMENTS:
draft-ietf-tewg-diff-te-reqts-07.txt
draft-ietf-tewg-diff-te-proto-04.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.
Le Faucheur 1
Maximum Allocation Model for DS-TE June 2003
WHY IS IT TARGETED AT THIS WG(s)
TEWG is responsible for specifying protocol extensions for support of
Diff-Serv-aware MPLS Traffic Engineering.
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 specifying bandwidth constraints model
for Diff-Serv-aware MPLS Traffic Engineering. This document specifies
one particular bandwidth constraints model.
Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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
at least one bandwidth constraint model and MAY specify multiple
Bandwidth Constraints models."
This document provides a detailed description of one particular
Bandwidth Constraint model for DS-TE which is introduced in [DSTE-
REQ] and called the Maximum Allocation Model (MAM).
[DSTE-PROTO] specifies the IGP and RSVP-TE signaling extensions for
support of DS-TE. These extensions support MAM.
2. 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.
Le Faucheur et. al 2
Maximum Allocation Model for DS-TE June 2003
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.
"Reserved (CTc)": For a given Class-Type CTc ( 0 <= c <= MaxCT-1),
let us define "Reserved(CTc)" as the sum of the bandwidth reserved by
all established LSPs which belong to CTc.
We also introduce the following definition:
Reserved(CTb,q) : 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.
3. Maximum Allocation Model Definition
MAM is defined in the following manner:
o Maximum Number of Bandwidth Constraints (MaxBC)=
Maximum Number of Class-Types (MaxCT) = 8
o for each value of c in the range 0 <= c <= (MaxCT - 1):
Reserved (CTc) <= BCc <= Max-Reservable-Bandwidth,
o SUM (Reserved(CTc)) <= Max-Reservable-Bandwidth
where the SUM is across all values of c in the range
0 <= c <= (MaxCT - 1)
A DS-TE LSR implementing MAM MUST support enforcement of bandwidth
constraints in compliance with this definition.
To increase the degree of bandwidth sharing among the different CTs,
the sum of bandwidth constraints may exceed the Maximum Reservable
Bandwidth, so that the following relationship may hold true:
o SUM (BCc) > Max-Reservable-Bandwidth,
where the SUM is across all values of c in the range
0 <= c <= (MaxCT - 1)
The sum of bandwidth constraints may also be equal to (or below) the
Maximum Reservable Bandwidth. In that case, the Maximum Reservable
Bandwidth does not actually constrain CT bandwidth reservations (in
other words, the 3rd bullet item of the MAM definition above will
never effectively come into play). This is because the 2nd bullet
item of the MAM definition above implies that:
SUM (reserved(CTc)) <= SUM (BCc)
and we assume here that
Le Faucheur et. al 3
Maximum Allocation Model for DS-TE June 2003
SUM (BCc) <= Maximum Reservable Bandwidth
therefore, it will always be true that:
SUM (Reserved(CTc)) <= Max-Reservable-Bandwidth.
Both preemption within a Class-Type and across Class-Types is
allowed.
Where 8 Class-Types are active, the MAM bandwidth constraints can
also be expressed in the following way:
- All LSPs from CT7 use no more than BC7
- All LSPs from CT6 use no more than BC6
- All LSPs from CT5 use no more than BC5
- etc.
- All LSPs from CT0 use no more than BC0
- All LSPs from all CTs collectively use no more than the
Maximum Reservable Bandwidth
Purely for illustration purposes, the diagram below represents MAM in
a pictorial manner when 3 Class-Types are active:
I----------------------------I
<---BC0---> I
I---------I I
I I I
I CT0 I I
I I I
I---------I I
I I
I I
<-------BC1-------> I
I-----------------I I
I I I
I CT1 I I
I I I
I-----------------I I
I I
I I
<-----BC2-----> I
I-------------I I
I I I
I CT2 I I
I I I
I-------------I I
I I
I CT0+CT1+CT2 I
I I
I----------------------------I
<--Max Reservable Bandwidth-->
Le Faucheur et. al 4
Maximum Allocation Model for DS-TE June 2003
(Note that, in this illustration, the sum BC0 + BC1 + BC2 exceeds the
Max Reservable Bandwidth.)
While more flexible/sophisticated Bandwidth Constraints models can be
defined (and are indeed defined - see [DSTE-RDM]), the Maximum
Allocation Model is attractive in some DS-TE environments for the
following reasons:
- Network administrators generally find MAM simple and
intuitive
- MAM matches simple bandwidth control policies that Network
Administrators may want to enforce such as setting individual
bandwidth constraint for a given type of traffic (aka. Class-
Type) and simultaneously limit the aggregate of reserved
bandwidth across all types of traffic.
- MAM can be used in a way which ensures isolation across
Class-Types, whether preemption is used or not.
- MAM can simultaneously achieve isolation, bandwidth
efficiency and protection against QoS degradation of the
premium CT.
- MAM only requires limited protocol extensions such as the
ones defined in [DSTE-PROTO].
MAM may not be attractive in some DS-TE environments because:
- MAM cannot simultaneously achieve isolation, bandwidth
efficiency and protection against QoS degradation of CTs
other than the Premium CT.
Additional considerations on the properties of MAM and its comparison
with RDM can be found in [BC-CONS] and [BC-MODEL].
As a very simple example usage of the MAM Model, a network
administrator using one CT for Voice (CT1) and one CT for Data (CT0)
might configure on a given 2.5 Gb/s link:
- BC0 = 2 Gb/s (i.e. Data is limited to 2 Gb/s)
- BC1 = 1 Gb/s (i.e. Voice is limited to 1 Gb/s)
- Maximum Reservable Bandwidth = 2.5 Gb/s (i.e. aggregate Data
+ Voice is limited to 2.5 Gb/s)
4. Example Formulas for Computing "Unreserved TE-Class [i]" with
Maximum Allocation 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. Thus, a DS-TE LSR implementing MAM MUST reflect
the MAM bandwidth constraints defined in section 3 above when
computing "Unreserved TE-Class [i]".
Le Faucheur et. al 5
Maximum Allocation Model for DS-TE June 2003
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 MAM, 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.
Then:
"Unreserved TE-Class [i]" =
MIN [
[ BCc - SUM ( Reserved(CTc,q) ) ] for q <= p ,
[ Max-Res-Bw - SUM (Reserved(CTb,q)) ] for q <= p and 0 <= b <= 7,
]
where:
TE-Class [i] <--> < CTc , preemption p>
in the configured TE-Class mapping.
5. Security Considerations
Security considerations related to the use of DS-TE are discussed in
[DSTE-PROTO]. Those apply independently of the Bandwidth Constraints
model, including MAM specified in this document.
6. Acknowledgments
A lot of the material in this document has been derived from ongoing
discussions within the TEWG work. This involved many people including
Jerry Ash and Dimitry Haskin.
7. Normative References
[DSTE-REQ] Le Faucheur et al, Requirements for support of Diff-Serv-
aware MPLS Traffic Engineering, draft-ietf-tewg-diff-te-reqts-07.txt,
February 2003.
[DSTE-PROTO] Le Faucheur et al, Protocol extensions for support of
Diff-Serv-aware MPLS Traffic Engineering, draft-ietf-tewg-diff-te-
proto-04.txt, June 2003
[RFC2119] S. Bradner, Key words for use in RFCs to Indicate
Requirement Levels, RFC2119, March 1997.
Le Faucheur et. al 6
Maximum Allocation Model for DS-TE June 2003
8. Informative References
[BC-CONS] Le Faucheur, "Considerations on Bandwidth Constraints Model
for DS-TE", draft-lefaucheur-tewg-russian-dolls-00.txt, June 2002.
[BC-MODEL] Lai, "Bandwidth Constraints Models for DS-TE",
draft-wlai-tewg-bcmodel-00.txt, June 2002.
[DSTE-RDM] Le Faucheur et al., "Russian Dolls Bandwidth Constraints
Model for Diff-Serv-aware MPLS Traffic Engineering",
draft-ietf-tewg-diff-te-russian-03.txt, June 2003
[OSPF-TE] Katz et al., Traffic Engineering Extensions to OSPF,
draft-katz-yeung-ospf-traffic-09.txt, October 2002.
[ISIS-TE] Smit et al., IS-IS extensions for Traffic Engineering,
draft-ietf-isis-traffic-04.txt, December 2002.
[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.
[DSTE-MAR] Ash, G., "Max Allocation with Reservation Bandwidth
Constraint Model for MPLS/DiffServ TE & Performance Comparisons",
Work In Progress.
9. Authors' 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
Wai Sum Lai
AT&T Labs
200 Laurel Avenue
Middletown, New Jersey 07748, USA
Phone: (732) 420-3712
Email: wlai@att.com
Le Faucheur et. al 7
Maximum Allocation Model for DS-TE June 2003
Appendix A - Addressing [DSTE-REQ] Scenarios
This Appendix provides examples of how the Maximum Allocation
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
- Bandwidth Constraint 0 (for CT0=Data) = link capacity (or a
constraint specific to data traffic)
- Max Reservable Bandwidth = link capacity
By configuring:
- every CT1/Voice TE-LSP with preemption =0
- every CT0/Data TE-LSP with preemption =1
DS-TE with the Maximum Allocation 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 (or
up to its own specific constraint)
- 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% of link capacity
- BC1 (for CT1) = e.g. 35% of link capacity
- BC0 (for CT0) = e.g.100% of link capacity
- Max Reservable Bandwidth = link capacity
DS-TE with the Maximum Allocation 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.
By also configuring:
- every CT2/Voice TE-LSP with preemption =0
- every CT1/Premium Data TE-LSP with preemption =1
- every CT0/Best-Effort TE-LSP with preemption =2
DS-TE with the Maximum Allocation Model will also ensure that:
- CT2 Voice LSPs always have first preemption priority in order
to use the CT2 capacity
- CT1 Premium Data LSPs always have second preemption priority
in order to use the CT1 capacity
Le Faucheur et. al 8
Maximum Allocation Model for DS-TE June 2003
- Best-Effort can use up to link capacity whatever is left by
CT2 and CT1.
Optional automatic adjustment of Diff-Serv 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 QoS objectives of the Guaranteed
Bandwidth service)
- BC0 (for CT0=Data) = link capacity (or a constraint specific
to data traffic)
- Max Reservable Bandwidth = link capacity
DS-TE with the Maximum Allocation Model will ensure that the amount
of Guaranteed Bandwidth Traffic 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 (or limited to
the specific constraint for such traffic).
Le Faucheur et. al 9
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