Francois Le Faucheur,
                                                     Cisco Systems, Inc.

IETF Internet Draft
Expires: December, 2002
Document: draft-lefaucheur-tewg-russian-dolls-00.txt         June, 2002

             Considerations on Bandwidth Constraints Models
                               for DS-TE

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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  This document provides input for the selection of a default Bandwidth
  Constraints Model for Diff-Serv-aware MPLS Traffic Engineering (DS-

  It discusses a number of considerations on Bandwidth Constraints
  Models and how the Maximum Allocation Model and the Russian Dolls
  Model address these considerations.

  While this document may not exhaustively cover all possible
  considerations for selection of a Bandwidth Constraints model, we
  feel it covers the most important considerations for practical DS-TE

  We conclude that the Russian Dolls Bandwidth Constraint Model is a
  good default Bandwidth Constraint Model for DS-TE.

Le Faucheur                                                          1

                Considerations on BC Models for DS-TE       June 2002

1.      Introduction

  [DSTE-REQ] presents the Service Providers requirements for support of
  Diff-Serv-aware MPLS Traffic Engineering (DS-TE). [DSTE-REQ] states
  that a default Bandwidth Constraints Model must be specified as part
  of the DS-TE solution. The purpose of such a default model is to
  ensure that there is at least one common Bandwidth Constraints model
  implementation across various vendors equipment in order to allow for
  easier deployment of DS-TE.

  Note that additional Bandwidth Constraints models may also be
  specified and supported by DS-TE implementations.

2.      Terminology

  Section 3.3 of [DSTE-REQ] describes two examples of Bandwidth
  Constraints Models.

  The first example uses a separate, independent Bandwidth Constraint
  (BC) for each Class-type (CT). We refer to this model as the Maximum
  Allocation Model or MAM.

  With MAM, the Bandwidth Constraints are defined in the following
       All LSPs supporting Traffic Trunks from CTc use no more than BCc

  For example, when 3 CTs are used with MAM:
        - sum (CT0) <= BC0
        - sum (CT1) <= BC1
        - sum (CT2) <= BC2

  For illustration purposes, on a link of 100 unit of bandwidth where
  three CTs are used, the network administrator might then configure
  BC0=30, BC1= 50, BC2=20 such that:
       - All LSPs supporting Traffic Trunks from CT0 use no more than
          30 (e.g. Voice <= 30)
       - All LSPs supporting Traffic Trunks from CT1 use no more than
          50 (e.g. Premium Data <= 50)
       - All LSPs supporting Traffic Trunks from CT2 use no more than
          20 (e.g. Best Effort <= 20)

  The second example is the Russian Dolls Model. We refer to it as the
  RDM. More details can also be found on the RDM in section 9 and
  Appendix C of [DSTE-PROTO].

  With RDM, the Bandwidth Constraints are defined in the following
        BCb is the constraint that bounds the total bandwidth used by
        all LSPs supporting Traffic Trunks from all class types CTn,
        where b <= n < M, M being the number of class-types used in the

 Le Faucheur                                                         2

                Considerations on BC Models for DS-TE       June 2002

  For example, when 3 CTs are used with RDM (M=3):
        - sum (CT0+CT1+CT2) <= BC0
        - sum (CT1+CT2) <= BC1
        - sum (CT2) <= BC2

  For illustration purposes, on a link of 100 units of bandwidth where
  three CTs are used, the network administrator might then configure
  BC0=100, BC1= 80, BC2=60 such that
       - All LSPs supporting Traffic Trunks from CT2 use no more than
          60 (e.g. Voice <= 60)
       - All LSPs supporting Traffic Trunks from CT1 or CT2 use no
          more than 80 (e.g. Voice + Premium Data <= 80)
       - All LSPs supporting Traffic Trunks from CT0 or CT1 or CT2 use
          no more than 100 (e.g. Voice + Premium Data + Best Effort <=

3.      Considerations

3.1.    Canonical DS-TE Deployment

  For easier discussion of the considerations below, we consider an
  example DS-TE deployment which we refer to as the canonical DS-TE

  The canonical DS-TE deployment is characterized by:
        - 3 Class-Types :
             o CT2 for real-time PHB Scheduling Class(es) (PSCs; see
             o CT1 for low-loss PSCs
             o CT0 for other PSCs (e.g. Best Effort)
        - CT2 needs high preemption priority(ies) to ensure placement
          of such traffic as close as possible to its shortest path.
        - CT1 needs medium preemption priority(ies) to ensure CT1
          traffic is as close as possible to its shortest path, but
          without forcing some CT2 traffic further away from its own
          shortest path.
        - CT0 only needs low preemption priority(ies) as its QoS
          objectives can be accommodated, if necessary, by paths
          relatively further away from their shortest path as long as
          they satisfy the bandwidth/resources constraints.

  Note that although we always refer to the canonical DS-TE deployment
  in the discussion below, the discussion also applies to other
  deployment scenarios.

3.2.    Canonical Set of DS-TE Objectives

  The considerations below stem from a set of typical practical
  objectives sought in DS-TE deployment scenarios. We refer to those as
  the canonical set of DS-TE objectives. This set includes:

 Le Faucheur                                                         3

                Considerations on BC Models for DS-TE       June 2002

        - Diff-Serv QoS enforcement. We wish to use the DS-TE Bandwidth
          Constraints to ensure the respective QoS performance targets
          of the various Diff-Serv Behavior Aggregates are always met
          regardless of the actual demand of LSPs across all CTs.

        - Avoiding bandwidth wastage. If bandwidth is not used to
          establish LSPs of a given CT, this bandwidth should be
          available for use by other CTs, as long as it does not
          compromise the previous objective. This is also referred to
          as achieving efficient bandwidth sharing across CTs. Note
          that we are talking about use of bandwidth in the control
          plane for Constraint Based Routing and not about use of
          bandwidth in the data plane. Diff-Serv PHB implementations
          are responsible for achieving efficient bandwidth sharing in
          the data plane, e.g. through use of work-conserving
          scheduling algorithms).

        - Avoiding starvation of Best-Effort traffic (considering that
          when preemption is used, Best-Effort LSPs are typically
          granted low preemption priority).

3.3.    Avoiding Wastage and QoS Degradation simultaneously

  MAM can not simultaneously protect against both bandwidth wastage and
  QoS degradation.  With MAM:

        - EITHER, one configures Bandwidth Constraints so that that SUM
          (BCi) <= link bandwidth.
  Then, significant bandwidth wastage can occur because whenever a CT
  is not using its bandwidth, this bandwidth cannot be used by other
  Consider the example where the link has 100 units of bandwidth and
  BC0=35, BC1=35 and BC2=30. If,  Sum (bandwidth of all LSPs of CT2)=
  10, then LSPs of CT0 are still limited to 35 and LSPs of CT1 to 35,
  so that 20 units of bandwidth will go wasted.

        - OR, one configures Bandwidth Constraints so that SUM (BCi)
  exceeds link bandwidth.
  Then, constraint-based routing and admission control can not protect
  against aggregate congestion and associated QoS degradation.
  Consider the example where a link has 100 units of bandwidth and
  BC0=70, BC1=70 and BC2=50. Then, the router performing admission
  control for that link will accept establishment of LSPs whose sum
  across all classes can reach 190, far exceeding the link capacity.
  Note that this could result in QoS degradation not just on CT0 LSPs
  but also on CT1 LSPs (for example if there are 60 units of CT1 LSP
  established and 50 units of CT2 LSPs, which totals to 110% of link
  capacity, it is clear that CT1 will experience QoS degradation.
  (Depending on the scheduler used, CT2 might also experience

 Le Faucheur                                                         4

                Considerations on BC Models for DS-TE       June 2002

  RDM, by contrast, can simultaneously protect well against bandwidth
  wastage (i.e. achieve efficient bandwidth sharing across CTs) and
  protect against QoS degradation. This is because the recursiveness of
  Bandwidth Constraints always allows a CT to make use of what is left
  over by the previous CT. For example, whatever is unused by CT2 can
  be reused by CT1, whatever is unused by CT1 and CT2 will be reused by
  CT0 since CT0 is only constrained collectively with CT1 and CT2 by
  BC0. Yet RDM can avoid QoS degradation by separately constraining the
  amount of real-time traffic as well as the amount of real-time plus
  low-loss traffic (see also section 3.5 below). We note that RDM does
  not completely remove the possibility of conceivable bandwidth
  wastage. However, we also observe that:
        - it considerably reduces this risk, as compared to MAM, by
          effectively constraining CTs collectively and thus always
          giving the opportunity to reuse the unused bandwidth to all
          CTs with smaller numerical indexes. So even if it does not
          give such opportunity to all other CTs (which would be the
          ideal  if bandwidth wastage were the only concern) it does
          provide many opportunities for bandwidth reuse.
        - the remaining conceivable bandwidth wastage scenarios in RDM
          may be more or less inevitable anyway to meet QoS objectives
          of Diff-Serv classes. Therefore these scenarios do not really
          represent bandwidth wastage due to the BC model itself. For
          example, a conceivable bandwidth wastage scenario with RDM is
          when there is little CT0 demand, and a heavy CT1 and CT2
          demand. Bandwidth may be considered wasted since some CT1/CT2
          demand will get rejected even if link is not fully used
          (since CT2+CT1 is limited by BC1 below link capacity). But
          limiting CT1 and CT2 to less than link capacity collectively,
          even when there is no CT0 traffic, is probably necessary
          anyway to ensure the QoS objectives of low-delay and low-loss
          traffic are met. So such a bandwidth wastage can be seen as
          largely inevitable since accepting more CT1/CT2 traffic would
          eventually result in QoS degradation. In fact, such
          "bandwidth wastage" may be seen as desirable in order to
          avoid QoS degradation.

3.4.    Avoiding Starvation of Best-Effort Traffic

  In typical DS-TE deployment scenario, MAM does not effectively
  protect low priority traffic (ie CT0) against starvation.

  With MAM, in order to avoid the bandwidth wastage issue pointed out
  above, BC1 and BC2 need to be configured as high as possible. Yet to
  avoid QoS degradation of CT1 and CT2, BC1 and BC2 need to be
  configured so that BC1+BC2 is below link capacity. So, it is expected
  that in practise, BC1 and BC2 would be commonly configured so that
  BC1+BC2 is just slightly below link capacity - say to "capacity minus
  small-delta". Since, when preemption is used, CT0 traffic is
  typically granted low preemption priorities (to maximize QoS
  performance of CT1 and CT2 traffic), whenever CT1 and CT2 traffic are
  grabbing all their allowed resources, CT0 traffic will be starved out

 Le Faucheur                                                         5

                Considerations on BC Models for DS-TE       June 2002

  and left with only "small-delta" units of bandwidth. Increasing the
  value of "small-delta" to alleviate CT0 starvation could be done, but
  this would be at the cost of increasing bandwidth wastage.

  With RDM, low priority traffic (i.e. CT0) may be well protected
  against starvation, regardless of the preemption priority it uses, by
  setting BC1 (which constrains CT1 + CT2 traffic) to less than link
  capacity, thus ensuring that the required capacity is left for CT0.

3.5.    Diff-Serv QoS Enforcement Objective

  DS-TE allows enforcement of different Bandwidth Constraints. These
  multiple Bandwidth Constraints can be useful to pursue multiple
  different goals.

  One such goal is the "Diff-Serv QoS enforcement objective" identified
  above in the set of canonical DS-TE objectives. This objective is to
  ensure that the respective QoS performance targets for the Diff-Serv
  PSC(s) belonging to each CT are met. In other words, the Bandwidth
  Constraints are used to control the distribution of traffic across
  the various CTs so that the traffic load submitted to the various
  PHBs/PSCs activated on the links is compatible with their respective
  targeted QoS performance. In that context, DS-TE can be seen as a
  form of aggregate admission control for Diff-Serv.

  Other goals relate more to how resources can be apportioned across
  different classes in order to address some Service Provider policy.
  We refer to such goals as "Policy goals". An example of a policy goal
  would be the desire to limit the amount of traffic that Best Effort
  traffic may be able to use on a given link to a certain level (even
  if going beyond that level would not result in degradation of the QoS

  We feel that, while addressing policy goals is highly desirable,
  addressing the Diff-Serv QoS enforcement objective well is of
  paramount importance, as this is the most fundamental thrust behind
  DS-TE (ie being Diff-Serv-aware as opposed to being aware of generic
  policy classes).

  We feel that RDM is a much more natural match to the Diff-Serv QoS
  enforcement objective than MAM because:
        - when there is no CT1 and CT2 traffic, there is typically no
          need to limit CT0 traffic below "link capacity" in order to
          meet CT0 traffic QoS objectives.
             o RDM will not unnecessary limit CT0 traffic when there is
               no CT1 and CT2 traffic since the only constraint
               applying to CT0 is that CT0+CT1+CT2 <= BC0.
             o To avoid unnecessarily limiting CT0 when there is no CT1
               and CT2 traffic, MAM would have to have its BC0
               configured to "link capacity". This means that
               BC0+BC1+BC2 would significantly exceed link capacity

 Le Faucheur                                                         6

                Considerations on BC Models for DS-TE       June 2002

               resulting in significant QoS degradation whenever there
               is CT1 and/or CT2 traffic.
        - When there is some CT1 and CT2 traffic, it is typically
          useful to effectively limit CT0 to whatever is left over by
          CT1 and CT2 from the link capacity, in order to maintain CT0
          traffic QoS objectives (since the CT1/CT2 PHBs will typically
          be configured so that they are granted the
          bandwidth/resources they require for CT1 and CT2 traffic).
             o RDM naturally limits CT0 to whatever is left by CT1 and
               CT2 since CT0 is effectively limited by BC0-CT1-CT2.
             o To limit CT0 to whatever is left over by CT1 and CT2 in
               all cases, MAM would have to configure BC0 to a value
               smaller than ("link capacity"-BC1-BC2) which would be
               very small if not equal to zero, unless significant
               bandwidth wastage is tolerated.
        - Similarly, intuition (as well as some experimental
          observations) suggests that it is efficient to collectively
          limit CT1 and CT2. This enables CT1 to effectively make full
          use of whatever bandwidth hasn't been used by CT2 to avoid
          bandwidth wastage, while protecting CT1 traffic from QoS
          degradation by constraining CT1 more tightly as the amount of
          CT2 traffic increases. In simple terms, if there is less
          real-time (CT2) traffic established on the link, the link can
          take up more low-loss (CT1) traffic without QoS degradation
          of the low loss traffic (assuming appropriate configuration
          of the PHBs on the link or assuming dynamic adjustment of the
             o RDM naturally limits CT1 and CT2 collectively via BC1.
             o MAM cannot naturally limit CT1 depending on the amount
               of CT2 traffic.

4.      Conclusions

  Considering that:

        - other Bandwidth Constraints Models can be defined in addition
          to the default model to address less typical/more complex
          deployment scenarios
        - the Russian Dolls Model matches very well the canonical DS-TE
          objectives in the canonical DS-TE deployment scenario (as
          well as many other practical deployment scenarios)

  we recommend selecting the Russian Dolls Model as the default model
  for DS-TE.

5.      Security Considerations

  No new security considerations are raised by this document. Those are
  the same as the ones mentioned in [DSTE-REQ].

 Le Faucheur                                                         7

                Considerations on BC Models for DS-TE       June 2002

6.      Acknowledgments

  We thank Bruce Davie for his review and recommendations.


  [DSTE-REQ] Le Faucheur et al, Requirements for support of Diff-Serv-
  aware MPLS Traffic Engineering, draft-ietf-tewg-diff-te-reqts-05.txt,
  June 2002.

   [DSTE-PROTO] Le Faucheur et al, Protocol extensions for support of
  Diff-Serv-aware MPLS Traffic Engineering, draft-ietf-tewg-diff-te-
  proto-01.txt, June 2002.

  [DIFF-NEW] Grossman, " New Terminology and Clarifications for
  Diffserv ", RFC3260, April 2002.

Author's Address:

  Francois Le Faucheur
  Cisco Systems, Inc.
  Village d'Entreprise Green Side - Batiment T3
  400, Avenue de Roumanille
  06410 Biot-Sophia Antipolis
  Phone: +33 4 97 23 26 19

 Le Faucheur                                                         8