RFC4125: Maximum Allocation Bandwidth Constraints Model for Diffserv-aware MPLS Traffic Engineering
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Related keywords:
(DS-TE)
(maximum allocation model)
Network Working Group F. Le Faucheur
Request for Comments: 4125 Cisco Systems, Inc.
Category: Experimental W. Lai
AT&T Labs
June 2005
Maximum Allocation Bandwidth Constraints Model for
Diffserv-aware MPLS Traffic Engineering
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document provides specifications for one Bandwidth Constraints
Model for Diffserv-aware MPLS Traffic Engineering, which is referred
to as the Maximum Allocation Model.
Table of Contents
1. Introduction ....................................................2
1.1. Specification of Requirements ..............................2
2. Definitions .....................................................2
3. Maximum Allocation Model Definition .............................3
4. Example Formulas for Computing "Unreserved TE-Class [i]" with
Maximum Allocation Model.........................................6
5. Security Considerations .........................................7
6. IANA Considerations .............................................7
7. Acknowledgements ................................................7
Appendix A: Addressing [DSTE-REQ] Scenarios.........................8
Normative References...............................................10
Informative References.............................................10
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1. Introduction
[DSTE-REQ] presents the Service Providers requirements for support of
Diffserv-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 Constraints Model
for DS-TE and states that "The DS-TE technical solution MUST specify
at least one Bandwidth Constraints Model and MAY specify multiple
Bandwidth Constraints Models."
This document provides a detailed description of one particular
Bandwidth Constraints 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.
1.1. 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].
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.
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.
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A number of recovery mechanisms, under investigation or specification
in the IETF, take advantage of the concept of bandwidth sharing
across particular sets of LSPs. "Shared Mesh Restoration" in
[GMPLS-RECOV] and "Facility-based Computation Model" in [MPLS-BACKUP]
are example mechanisms that increase bandwidth efficiency by sharing
bandwidth across backup LSPs protecting against independent failures.
To ensure that the notion of "Reserved (CTc)" introduced in
[DSTE-REQ] is compatible with such a concept of bandwidth sharing
across multiple LSPs, the wording of the "Reserved (CTc)" definition
provided in [DSTE-REQ] is generalized into the following:
Reserved (CTc): For a given Class-Type CTc ( 0 <= c <= MaxCT ), let
us define "Reserved(CTc)" as the total amount of the
bandwidth reserved by all the established LSPs which
belong to CTc.
With this generalization, the Maximum Allocation Model definition
provided in this document is compatible with Shared Mesh Restoration
defined in [GMPLS-RECOV], so that DS-TE and Shared Mesh Protection
can operate simultaneously. This assumes that Shared Mesh
Restoration operates independently within each DS-TE Class-Type and
does not operate across Class-Types (for example, backup LSPs
protecting Primary LSPs of CTx also need to belong to CTx; Excess
Traffic LSPs sharing bandwidth with Backup LSPs of CTx also need to
belong to CTx).
We also introduce the following definition:
Reserved(CTb,q): Let us define "Reserved(CTb,q)" as the total amount
of the bandwidth reserved by all the established
LSPs that 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 cannot
be any established LSPs that belongs to CTb and has
a holding priority of q; therefore, in this 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,
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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
SUM (BCc) <= Maximum Reservable Bandwidth.
Therefore, it will always be true that:
SUM (Reserved(CTc)) <= Max-Reservable-Bandwidth.
Both preemption within a CT and across CTs is allowed.
Where 8 CTs 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
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- 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 CTs 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-->
(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
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- MAM matches simple bandwidth control policies that Network
Administrators may want to enforce, such as setting an
individual Bandwidth Constraint for a given type of traffic
(a.k.a. 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 of usage of MAM, 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 when computing
"Unreserved TE-Class [i]".
As explained in [DSTE-PROTO], the details of admission control
algorithms, as well as formulas for computing "Unreserved TE-Class
[i]", are outside the scope of the IETF work. Keeping that in mind,
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we provide in this section an example, for illustration purposes, of
how values for the unreserved bandwidth for TE-Class[i] might be
computed with MAM. In the example, we assume the use of 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. IANA Considerations
[DSTE-PROTO] defines a new name space for "Bandwidth Constraints
Model Id". The guidelines for allocation of values in that name
space are detailed in section 13.1 of [DSTE-PROTO]. In accordance
with these guidelines, IANA has assigned a Bandwidth Constraints
Model Id for MAM from the range 0-239 (which is to be managed as per
the "Specification Required" policy defined in [IANA-CONS]).
Bandwidth Constraints Model Id 1 was allocated by IANA to MAM.
7. Acknowledgements
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.
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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].
A.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
A.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
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DS-TE with the Maximum Allocation Model will ensure that the amount
of traffic of each CT established on a link is within acceptable
levels as compared to the resources allocated to the corresponding
Diffserv Per Hop Behaviors (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
- Best-Effort can use up to link capacity of what is left by CT2
and CT1.
Optional automatic adjustment of Diffserv scheduling configuration
could be used for maintaining very strict relationships between the
amounts of established traffic of each CT and corresponding Diffserv
resources.
A.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
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the rest of the traffic such that links can be filled up (or limited
to the specific constraint for such traffic).
Normative References
[DSTE-REQ] Le Faucheur, F. and W. Lai, "Requirements for Support
of Differentiated Services-aware MPLS Traffic
Engineering", RFC 3564, July 2003.
[DSTE-PROTO] Le Faucheur, F., Ed., "Protocol Extensions for Support
of Diffserv-aware MPLS Traffic Engineering", RFC 4124,
June 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[IANA-CONS] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.
Informative References
[BC-CONS] Le Faucheur, F., "Considerations on Bandwidth
Constraints Model for DS-TE", Work in Progress, June
2002.
[BC-MODEL] Lai, W., "Bandwidth Constraints Models for
Differentiated Services (Diffserv)-aware MPLS Traffic
Engineering: Performance Evaluation", RFC 4128, June
2005.
[DSTE-RDM] Le Faucheur, F., Ed., "Russian Dolls Bandwidth
Constraints Model for Diffserv-aware MPLS Traffic
Engineering", RFC 4127, June 2005.
[GMPLS-RECOV] Lang, et al., "Generalized MPLS Recovery Functional
Specification", Work in Progress.
[MPLS-BACKUP] Vasseur, et al., "MPLS Traffic Engineering Fast
reroute: Bypass Tunnel Path Computation for Bandwidth
Protection", Work in Progress.
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Authors' Addresses
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
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