One document matched: draft-lefaucheur-diff-te-mam-00.txt
Francois Le Faucheur
Cisco Systems, Inc.
IETF Internet Draft
Expires: April, 2002
Document: draft-lefaucheur-diff-te-mam-00.txt February, 2003
Maximum Allocation Bandwidth Constraints Model for
Diff-Serv-aware MPLS Traffic Engineering
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are
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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-03.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.
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Maximum Allocation Model for DS-TE February 2003
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.
TE-Class: A pair of:
i. a Class-Type
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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 ) ,let
us define "Reserved(CTc)" as the sum of the bandwidth reserved by all
established LSPs which belong to CTc.
The following definition from [DSTE-PROTO] is also repeated here:
Normalised(CTc) : let us define "Normalised(CTc)" as
"Reserved(CTc)/LOM(c)", where LOM (c) is the Local Overbooking
Multiplier for CTc defined in [DSTE-PROTO].
We also introduce the following definitions:
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.
Normalised(CTc,q) let us define "ormalised(CTc,q)" as
"Reserved(CTc/q) / LOM(c)", where LOM (c) is the Local Overbooking
Multiplier for CTc defined in [DSTE-PROTO].
3. Maximum Allocation Model Definition
MAM 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 for each value of b in the range 0 <= b <= (MaxCT - 1):
Reserved (CTb) <= BCb,
A DS-TE LSR implementing MAM MUST support enforcement of bandwidth
constraints in compliance with this definition.
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
Purely for illustration purposes, the diagram below represents MAM in
a pictorial manner when 3 Class-Types are active:
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Maximum Allocation Model for DS-TE February 2003
I--------------II-------------II-------------I
I CT2 II CT1 II CT0 I
I--------------II-------------II-------------I
<-----BC2------><-----BC1-----><-----BC0----->
While more flexible/sophisticated Bandwidth Constraints models can be
defined (and are indeed defined - see [RDM]) , the Maximum Allocation
Model is attractive in some DS-TE environments for the following
reasons:
- Network administrators generally find MAM extremely simple
and intuitive
- MAM matches simple bandwidth control policies that Network
Administrators may want to enforce (i.e. set aside a fixed
chunk of bandwidth for a given type of traffic (aka. Class-
Type).
- MAM can be used in a way which ensures very strict isolation
across Class-Types, so that each Class-Type is guaranteed its
share of bandwidth no matter the level of contention by other
classes, 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 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 = 1.5 Gb/s (i.e. Data is limited to 1.5 Gb/s)
- BC1 = 1 Gb/s (i.e. Voice is limited to 1 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]".
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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.
- the optional per-CT Local Overbooking Multipliers are not
used (.i.e. LOM[c]=1, 0<= c <=7).
Then:
"Unreserved TE-Class [i]" =
[ BCc - SUM ( Reserved(CTc,q) ) ] for q <= p
where:
TE-Class [i] <--> < CTc , preemption p>
in the configured TE-Class mapping.
5. 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 MAM 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.
5.1. Maximum Allocation Model Definition With Local Overbooking
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 MAM definition is
extended in the following manner:
- Maximum Number of Bandwidth Constraints (MaxBC)= Maximum
Number of Class-Types (MaxCT) = 8
- for each value of b in the range 0 <= b <= (MaxCT - 1):
Normalised(CTb) <= BCb,
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A DS-TE LSR implementing MAM and implementing the optional Local
Overbooking method MUST support enforcement of bandwidth constraints
in compliance with this extended definition.
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-------------------II-------------------II--------------------I
I Normalised(CT2) II Normalised(CT1) II Normalised(CT0) I
I-------------------II-------------------II--------------------I
<--------BC2--------><--------BC1--------><--------BC0--------->
5.2. Example Formulas for Computing "Unreserved TE-Class [i]" With
Local Overbooking
A DS-TE LSR implementing MAM and implementing the optional Local
Overbooking method MUST reflect the MAM bandwidth constraints defined
in section 5.1 above when computing "Unreserved TE-Class [i]".
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 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.
- the optional per-CT Local Overbooking Multipliers are used.
When the optional local overbooking method is supported, the example
generalized formula of section 4 becomes:
"Unreserved TE-Class [i]" =
LOM(c) x [ BCc - SUM ( Normalised(CTc,q) ) ] for q <= p ,
Or, equivalently:
"Unreserved TE-Class [i]" =
[ LOM(c) x BCc ] - SUM ( Reserved (CTc,q) ) for q <= p ,
where:
TE-Class [i] <--> < CTc , preemption p>
in the configured TE-Class mapping.
5.3. Example Usage of LOM
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Maximum Allocation Model for DS-TE February 2003
To illustrate usage of the local overbooking method with MAM, 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)= 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)= 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) = 800
Unreserved TE-Class [1] = 2 x (100- 100/2) = 100
6. 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.
7. 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, Waisum Lai and Dimitry Haskin.
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Maximum Allocation Model for DS-TE February 2003
8. 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-03.txt, February 2003
[RFC2119] S. Bradner, Key words for use in RFCs to Indicate
Requirement Levels, RFC2119, March 1997.
9. 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.
[RDM] Le Faucheur et al., "Russian Dolls Bandwidth Constraints Model
for Diff-Serv-aware MPLS Traffic Engineering",
draft-ietf-tewg-diff-te-russian-01.txt, February 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.
10. Author'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
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