[Search] [txt|pdfized|bibtex] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01                                                         
Internet Draft                                                    J. Wroclawski
draft-ietf-issll-ds-map-01.txt                                          MIT LCS
Expires August, 2001                                                  A. Charny
                                                                  Cisco Systems
                                                                 February, 2001

        Integrated Service Mappings for Differentiated Services Networks

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 working
    documents of the Internet Engineering Task Force (IETF), its Areas, and
    its Working Groups.  Note that other groups may also distribute working
    documents as Internet Drafts.

    Internet Drafts are draft documents valid for a maximum of six months.
    Internet Drafts may be updated, replaced, or obsoleted by other
    documents at any time.  It is not appropriate to use Internet Drafts as
    reference material or to cite them other than as a "working draft" or
    "work in progress."

    The list of current Internet-Drafts can be accessed at

    The list of Internet-Draft Shadow Directories can be accessed at

    To learn the current status of any Internet-Draft, please check the
    "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
    Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe),
    ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

    This document is a product of the ISSLL working group of the Internet
    Engineering Task Force.  Please address comments to the group's mailing
    list at issll@mercury.lcs.mit.edu, with a copy to the authors.
    Copyright (C) The Internet Society (2001).  All Rights Reserved.


    This document describes mappings of IETF Integrated Services onto IETF
    differentiated services networks.  These mappings allow appropriately
    engineered and configured differentiated service network clouds to play
    the role of "network elements" in the Integrated Services framework, and
    thus to be used as components of an overall end-to-end Integrated
    Services QoS solution.

1. Introduction

    The IETF Integrated Services framework [INTSERV] defines mechanisms and
    interfaces for providing network Quality of Service control useful for
    applications that require more predictable network service than is

Wroclawski and Charny            Expires: August, 2001               [page  1 ]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

    available with the traditional best-effort IP delivery model.  Provision
    of end-to-end QoS control in the Intserv model is based on the
    concatenation of "network elements" along the data transmission path.
    When all of the concatenated network elements implement one of the
    defined Intserv "services" [G,CL], the resulting data transmission path
    will deliver a known, controlled QoS defined by the particular Intserv
    service in use.

    The IETF Differentiated Services framework [DIFFSERV] defines a number
    of mechanisms for differentiating different traffic streams within a
    network and providing different levels of delivery service to those
    different streams.  These mechanisms include differentiated per-hop
    queuing and forewarding behaviors, as well as behaviors such as traffic
    classification, metering, policing and shaping that are intended to be
    used at the edge or boundary of a diffserv cloud.  Crucially, the
    Differentiated Services framework manages traffic forwarding behavior
    within a diffserv cloud at the aggregate level, rather than the
    per-application-flow level.

    The availability of Differentiated Services per-hop and cloud-edge
    behaviors, together with additional mechanisms to statically or
    dynamically limit the absolute level of traffic within a traffic class,
    allows an IETF Differentiated Services network cloud to act as a network
    element within the Integrated Services framework.  In other words, an
    appropriately designed, configured and managed Diffserv network cloud
    can act as one component of an overall end-to-end QoS controlled data
    path using the Integrated Services framework, and therefore support the
    delivery of Intserv QoS services.

    This document is one of a set that together describe the usage of
    Differentiated Services networks in this manner.  This document
    describes methods for implementing Intserv using Diffserv network
    behaviors and mechanisms.  Companion documents [RSVPAGGR, DCLASS] define
    extensions to the RSVP signaling protocol [RSVP] that are useful in this
    environment.It is recommended that readers be familiar with the overall
    framework in which these mappings and protocols are expected to be used;
    this framework is described fully in [ISDSFRAME].

    Within this document, Section 2 describes the overall approach and
    discusses issues that are independent of the class of Intserv service
    being implemented.  Section 3 discusses implementation of the Controlled
    Load service.  Section 4 discusses implementation of a mathematically
    correct Guaranteed service, and presents information about the
    performance and limitations of this implementation.  Section 5 discusses
    implementation of close approximations to the Guaranteed service that
    may be acceptable in some circumstances and may allow more efficient use
    of network resources.  Section 6 briefly describes the relationship of
    the mechanisms described here to the Intserv Null Service [NULL].

2. Basics
2.1. Components

    Figure 1 shows the basic use of a Diffserv network cloud as an Intserv

Wroclawski and Charny            Expires: August, 2001               [page  2 ]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

    network element. In this figure, Intserv functions within the
    non-Diffserv regions take place at the level of individual
    switches, routers, subnets, and similar ojects. In contrast, the
    entire Diffserv region acts as a _single_ Intserv network element;
    using components of the Diffserv architecture to implement the
    behaviors expected of an object in the Intserv environment.

             ________         ______________         ________
            /        \       /              \       /        \
           /          \     /                \     /          \
    |---| |        |---|   |---|          |---|   |---|        | |---|
    |Tx |-|--O--O--|ER1|---|BR1|          |BR2|---|ER2|--O--O--|-|Rx |
    |---| |        |-- |   |---|          |---|   |---|        | |---|
           \          /     \                /     \          /
            \________/       \______________/       \________/

        Non-Diffserv region   Diffserv region     Non-Diffserv region

                 Figure 1: Sample Network Configuration                          Figure 1 <ascii art TBA>

    The figure shows that required Intserv network element functions are
    mapped to the Diffserv cloud as follows:

    - Traffic scheduling.  The Intserv traffic scheduling function is
      supported by appropriately selected, configured, and provisioned PHB's
      within the Diffserv network.  These PHB's, when concatenated along the
      path of traffic flow, must provide a scheduling result that adequately
      approximates the result defined by the Intserv service.

      In general, the PHB concatenation will only be able to approximate the
      defined Intserv service over a limited range of operating conditions
      (level of traffic, allocated resources, and the like).  In that case,
      other elements of the network, such as shapers and policers, must
      ensure that the traffic conditions seen by the PHB's stay within this

    - Traffic classification.  The Intserv framework requires that each
      network element (re)classify arriving traffic into flows for further
      processing.  This requirement is based on the architectural assumption
      that network elements should be independent, and not depend on other
      network elements for correct operation.

        NOTE: the Intserv framework does not specify the granularity of a
        flow.  Intserv is often associated with per-application or
        per-session end-to-end flows, but in fact any collection of packets
        that can be described by an appropriate classifier can be treated as
        an Intserv traffic flow.

      When Intserv is mapped to Diffserv, packets must be classified into
      flows, policed, shaped, and marked with the appropriate DSCP before
      they enter the interior of the diffserv cloud.  Strictly speaking, the
      independence requirement stated above implies that the ingress
      boundary router of each diffserv cloud must implement a MF classifier
      to perform the classification function.  However, in keeping with the

Wroclawski and Charny            Expires: August, 2001               [page  3 ]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

      diffserv model, it is permissible to push the flow classification
      function further towards the edge of the network if appropriate
      agreements are in place.  For example, flows may be classified and
      marked by the upstream edge router if the Diffserv network is prepared
      to trust this router.

    - Policing and shaping.  In terms of location in the network, these
      functions are similar to traffic classification.  A strict
      interpretation of the Intserv framework would require that the ingress
      boundary router of the diffserv cloud perform these functions.  In
      practice, they may be pushed to an upstream edge router if appropriate
      agreements are in place.

      Note that moving the shaping function upstream of the diffserv ingress
      boundary router may result in poorer overall QoS performance.  This is
      because if shaping is performed at the boundary router, a single
      shaper can be applied to all of the traffic in the service class,
      whereas if the shaping is performed upstream separate shapers will be
      applied to the traffic from each upstream node.  As discussed further
      in Section 4, the single shaper may be preferable in some

    - Admission control.  The quantitative Intserv services (Guaranteed and
      Controlled Load) require that some form of admission control limit the
      amount of arriving traffic relative to the available resources.  Two
      issues are of interest; the method used by the diffserv cloud to
      determine whether sufficient resources are available, and the method
      used by the overall network to query the diffserv cloud about this

      Within the cloud, the admission control *mechanism* is closely related
      to resource allocation.  If some form of static resource allocation
      (provisioning) is used, the admission control function can be
      performed by any network component that is aware of this allocation,
      such as a properly configured boundary router.  If resource allocation
      within the network cloud is dynamic (a dynamic "bandwidth broker" or
      signaling protocol) then this protocol can also perform the admission
      control function, by refusing to admit new traffic when it determines
      that it cannot allocate new resources to match.

      The admission control *mechanism* used is independent of the admission
      control *algorithm* used to determine whether sufficient resources are
      available to admit a new traffic flow.  The algorithm used may range
      from simple peak-rate allocation to a complex statistical
      measurement-based approach.  The choice of algorithm is dependent on
      the Intserv service to be supported.  Admission control algorithms
      appropriate for each service are <not yet> discussed in the service
      specific sections below.

      The admission control mechanism used within the diffserv cloud is also
      independent of the mechanism used by the outside world to request
      service from the cloud.  As an example, end-to-end RSVP might be used
      together with any form of interior admission control mechanism -
      static provisioning, a central bandwidth broker, or aggregate RSVP
      internal signalling.

Wroclawski and Charny            Expires: August, 2001               [page  4 ]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

2.2. Per-Cloud versus Per-Path Control

    The key to providing absolute, quantitative QoS services within a
    diffserv network is to ensure that at each hop in the network the
    resources allocated to the PHB's used for these services are sufficient
    to handle the arriving traffic.  As described above, this can be done
    through a spectrum of mechanisms ranging from static provisioning to
    dynamic per-hop signaling within the cloud.  Two situations are

    - With per-cloud provisioning, sufficient resources are made available
      in the network so that traffic arriving at an ingress point can flow
      to *any* egress point without violating the PHB resource allocation
      requirements.  In this case, admission control and traffic management
      decisions need not be based on destination information.

    - With per-path provisioning, resources are made available in the
      network to ensure that the PHB resource allocation requirements will
      not be violated if traffic arriving at an ingress point flows to one
      (in the unicast case) specific egress point.  This requires that
      admission control and resource allocation mechanisms take into account
      the egress point of traffic entering the network, but results in more
      efficient resource utilization.

Two points are important to note:

    - Both approaches are valuable, but all functions must adopt the same
      approach.  Particularly, if resource allocation is per-path, traffic
      shaping and policing, and hence classification must be destination
      aware as well.

    - The per-cloud vs per-path decision is independent of decisions about
      static vs.  dynamic provisioning.  It is often assumed that dynamic
      provisioning is necessarily per-path, while static provisioning is
      more likely to be per-cloud.  In reality, all four options may be
      useful in differing circumstances.

3. Implementation of the Controlled Load Service

3.1. Summary of CL Requirements

    The essence of the Controlled Load service is that traffic using it
    experiences the performance expected of an unloaded network.  The CL
    specification [CL] refines this definition.

    - Controlled Load traffic is described by a token bucket Tspec.  When
      traffic is conformant to the Tspec, network elements will forward it
      with queuing delay not greater than that caused by the traffic's own
      burstiness - that is, the result of the source emitting a burst of
      size B into a logical network with capacity R. Further, in doing this
      no packets will be discarded due to queue overflow.  Statistically
      rare deviations from this ideal behavior are permitted.  A measure of
      the "quality" of a CL service is how rare these deviations are.

Wroclawski and Charny            Expires: August, 2001               [page  5 ]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

        NOTE: the actual behavior requirements stated in the CL spec are
        slightly more detailed than what is presented here.

    - Network elements must not assume that that arrival of nonconformant
      traffic for a specific controlled-load flow will be unusual, or
      indicative of error.  In certain circumstances large numbers of
      packets will fail the conformance test *as a matter of normal
      operation*.  Some aspects of the behavior of a CL network element in
      the presence of nonconformant traffic are specified.

      (These circumstances include elements carrying traffic from adaptive
      applications that use the CL service to provide a floor on performance
      but constantly try to do better, elements acting as the "split points"
      of a multicast distribution tree or carrying multi-source aggregate
      flows, such as those generated by RSVP's wildcard or shared-explicit
      reservation styles supporting a shared reservation).

      In the presence of nonconformant packets arriving for one or more
      controlled-load flows, each network element must ensure locally that
      the following requirements are met:

      1) The network element MUST continue to provide the contracted
      quality of service to those controlled-load flows not experiencing
      excess traffic.

      2) The network element SHOULD prevent excess controlled-load
      traffic from unfairly impacting the handling of arriving best-
      effort traffic.

      3) Consistent with points 1 and 2, the network element MUST attempt
      to forward the excess traffic on a best-effort basis if sufficient
      resources are available.

    These points lead to two observations about a well implemented CL service.

    - CL traffic can be sorted into "delay classes" based on burstiness.
      Highly bursty flows, having a large ratio of Tspec parameters B/R,
      should expect to experience more queuing delay than their
      low-burstiness counterparts.  Thus, a good CL implementation will sort
      the offered CL traffic into sub-classes that are expecting roughly
      equivalent delay, and queue these subclasses independently to achieve
      this result.

    - The CL specification leaves open the precise treatment of
      nonconformant traffic, giving only the minimum requirements listed

        NOTE: The phrase "best effort basis" in the portion of the CL spec
        quoted above has sometimes been taken to mean "the traffic must be
        placed in the best effort traffic class and treated identically to
        BE traffic".  This interpretation is incorrect.  It is easy to see
        this at one level, because if nonconformant CL traffic from
        non-adaptive applications is simply lumped in with adaptive
        best-effort traffic it will tend to unfairly impact that traffic, in

Wroclawski and Charny            Expires: August, 2001               [page  6 ]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

        contravention of point 2).  However, the intent of the specification
        is more general.  An appropriate reading is "nonconformant CL
        traffic should be transmitted, when possible, in the way that is
        most advantageous to users and applications, subject to the
        requirements on non-interference with other traffic".  This allows
        the CL service to be used both to provide a specific QoS for
        non-adaptive applications and as to provide a "floor" or minimum QoS
        for adaptive applications.

3.2. Implementation of CL using the AF Per-Hop Behavior

    The CL service can be supported most effectively using an appropriately
    designed and configured Assured Forwarding PHB implementation [AF] as
    the data forwarding element.  This approach SHOULD be used whenever

    The basics of the AF-based approach are as follows:

    - Sort the offered CL traffic into delay classes based on the B/R
      ratio of the Tspec.  The packets of each delay class will be forwarded
      using a separate instance of the AF PHB.

    - For each delay class, construct an aggregate Tspec for the admitted
      traffic according to the rule for summing Tspecs given in [CL].  This
      Tspec will be used to police the traffic for conformance at the
      ingress to the diffserv cloud.

    - For each delay class, police arriving packets against the token
      bucket Tspec derived above.  Mark conforming packets with a DSCP
      indicating the selected AF instance, and highest priority forwarding
      within that instance.  Mark nonconformant packets with a DSCP
      indicating the selected AF instance, and lowest priority forwarding
      within that instance.

    - At each node within the diffserv network, configure each AF instance
      appropriately by:

     a) setting the actual queue size (or alternatively the dropping
        parameters for high priority packets) to limit queuing delay to the
        delay class's target.  (In other words, packets that have been
        delayed beyond the class target should be dropped).

     b) setting the dropping parameters for low priority packets to drop
        such packets as soon as any significant non-transient queuing of
        these packets is detected.

     c) setting the service rate of the AF instance to a bandwidth
        sufficient to meet the delay and loss behavior requirements of the
        CL spec when only high-priority packets are present.

    - Implement an admission control algorithm that ensures that at each
      hop in the network the level of conformant traffic offered to each AF
      instance is equal to or less than that provisioned for in step 4c
      above (or alternatively dynamically allocates more bandwidth to the
      relevant AF instance when required).

Wroclawski and Charny            Expires: August, 2001               [page  7 ]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

    In addition to these basic actions, two subtleties with the use of AF
    must be observed.

    First the relationship between different AF instances, and between AF
    and other PHBs, must be more tightly constrained than is required by the
    the base AF specification.

    - Bandwidth should be allocated between AF and BE (and any other
      relevant PHB's) in such a way that AF cannot simply steal all
      best-effort bandwidth on demand.  A simple WFQ or CBQ scheduler can
      meet this requirement.

    - The bandwidth allocation relationship between different AF instances
      must be known.  Two likely relationships are

      o Bandwidth is allocated to each AF instance independently, as with
        a WFQ scheduler.

      o Bandwidth is allocated across the AF instances used for CL service
        on a priority basis, with the AF instance supporting the lowest
        delay class of CL having the highest priority.

    Either of these approaches may be used.  However the choice of approach
    affects the admission control decision, and must be taken into account.
    In the first case, admission control decisions may be made for each CL
    delay class independently.  In the second case, admission control
    decisions for high priority classes will affect lower priority classes,
    which must be taken into account.

    The second subtlety is that the implementation of AF must service the AF
    classes in a timely manner, by ensuring that the bandwidth allocated to
    an AF instance is made available at a time-scale substantially shorter
    than the delay target of the class.  This requirement is slightly
    stronger than that stated in the AF specification.  In practice, any
    implementation using a common queuing algorithm is likely to be able to
    meet this requirement unless other PHB's, such as EF, are served at
    higher priority.  When that is true, the traffic seen by the higher
    priority PHB will also require limiting and shaping in order to ensure
    that the CL AF instances receive bandwidth on a timely basis.

    The overall result of this procedure is an implementation of the CL
    service with the following characteristics:

    - Conformant CL traffic is carried according to the CL requirements.

    - Resources are used efficiently by aggregating traffic with similar
      requirements, but supporting multiple delay classes for traffic with
      widely differing requirements.

    - Non-CL traffic is carried whenever resources permit, and is not
      reordered with respect to the CL flow's conformant traffic.

    - Nonconformant CL traffic is not able to disrupt traffic of other
      classes, particular BE.

Wroclawski and Charny            Expires: August, 2001               [page  8 ]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

3.2.1 CL/AF Admission Control Approaches


3.3. Implementation of CL using the EF Per-Hop Behavior

    It is also possible to implement an approximation of the Controlled Load
    service using the Diffserv Expedited Forwarding [EF] PHB as the traffic
    scheduling element.  This approach is not preferred, because of two
    significant limitations.  Therefore, this approach SHOULD NOT be used
    unless the AF-based approach is not available.

    - Because there is only one EF scheduling class per node, it is
      impossible to sort the Controlled Load traffic into queuing delay
      classes, as described above for the AF implementation.  Instead, all
      CL traffic must be handled as one scheduling class, and sufficient
      resources must be allocated to the class to cause *all* CL traffic to
      meet the queuing delay expectations of the most demanding flows.

    - Because the EF PHB requires a hard limit on the amount of traffic
      passing through it, a CL service implemented using EF cannot handle
      nonconformant (over-Tspec) traffic gracefully, as can be done with AF.
      Instead, nonconformant traffic must either be discarded at the ingress
      of the Diffserv cloud or remarked into a different behavior aggregate,
      and thus potentially reordered in transit.  Either of these behaviors
      is less desirable than the one obtained from the AF-based
      implementation above.

    Notwithstanding these limitations, it may be useful to implement a CL
    approximation based on the EF PHB when the Diffserv network does not
    support the AF PHB, or when the implementation of the AF PHB cannot
    assure the forwarding of traffic in a sufficiently timely manner.  In
    this case:

    - All CL traffic is marked with a DSCP corresponding to the EF PHB.

    - A single aggregate Tspec for all CL traffic is computed for each
      network ingress.

    - Arriving CL traffic is policed against this Tspec, and nonconformant
      traffic is either discarded or remarked as BE, at the preference of
      the network operator.

    - At each hop within the network the EF PHB must receive a bandwidth
      allocation sufficient to meet the requirements given in the EF
      specification when the arriving CL traffic is at the Tspec level for
      that point within the network.

    - The topology of the network must be designed so that the
      instantaneous queuing delay caused by fan-in to a node will exceed the
      CL requirements rarely or never.  In practice, this will be a concern
      only with very high fan-in topologies.

Wroclawski and Charny            Expires: August, 2001               [page  9 ]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

4. Implementation of the Guaranteed Service

    The Guaranteed service [G] offers a strict mathematical assurance of
    both throughput and queuing delay, assuming only that the network is
    functioning correctly.  A key concept of the Guaranteed service is that
    "error terms", referred to as C and D in the specification, are provided
    by the network element to the customer, allowing the customer to
    calculate the bandwidth it must request from the network in order to
    achieve a particular queuing delay target.  Thus, the two important
    tasks in implementing a Guaranteed service network element are providing
    the traffic scheduling, policing, and shaping functions needed to
    support a hard bound on performance, and characterizing the network
    element's error terms so that the customer of the service can accurately
    characterize the network path and deduce what level of resources must be

    Our strategy for implementing these capabilities within a diffserv cloud
    revolves around the use of the EF PHB for Guaranteed traffic, together
    with the shaping and policing functions necessary to obtain a
    performance bound in this context.  The basic traffic policing and
    shaping requirements for Guaranteed service are discussed more fully in
    the service specification.

    Delay through a Diffserv cloud can be roughly classified into
    propagation and serialization delay, shaping/reshaping delays at the
    boundary, and queuing delay inside the cloud.  In order to determine the
    error terms C_dc and D_dc for the Diffserv cloud needed for end-to-end
    determination of end-to-end delay, each of these delay components need
    to be evaluated.  The difficulty in characterizing C_dc and D_dc is that
    unlike the Intserv model, where the C and D terms are a local property
    of the router, in the case of Diffserv cloud these terms depend not only
    on the topology of the cloud, but also on the internal traffic
    characteristics of potentially _all_ EF traffic in the cloud.

    Hence, the existence of upper bounds on delay through the cloud implies
    centralized knowledge about the topology of the cloud and traffic
    characterization.  In turn, dependence of the delay bounds on traffic
    characterization at any ingress point to the cloud implies the existence
    of a policy that defines traffic characterization rules, as well as
    implementation mechanisms at _all_ ingress points in the network that
    enforce that policy.

    These considerations imply that determination of the bound on the delay
    through the Diffserv cloud should be performed off-line, perhaps as part
    of a traffic management algorithm, based on the knowledge of the
    topology, traffic patterns, shaping policies, and other relevant
    parametersof the cloud.  These parameters are discussed in the following
    sections with respect of each delay component.

    Once the delay bounds and determined, the corresponding error terms C_dc
    and D_dc are configured into the appropriate intserv-capable edge
    routers, as discussed below.

Wroclawski and Charny            Expires: August, 2001               [page  10]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

4.1 Propagation and Serialization Delay.

    These delay components can be bounded by modeling the Diffserv cloud as
    a sequence of at most h links, each of which of at most length C. The
    parameters (h, C) determine the so-called "diameter" of the cloud.  The
    knowledge of this diameter can then be used to obtain upper bounds on
    the propagation and serialization delay through the cloud.

4.2 Shaping delay.

    The Diffserv EF PHB assumes that traffic entering the Diffserv region is
    conditioned at the Diffserv cloud boundary.  In the framework of Figure
    1, shaping is expected to take place at the ingress edge router ER1, and
    optionally at the boundary router BR1.  Granularity of such shaping is
    implementation dependent, and can range from microflow shaping to
    aggregate shaping.  The granularity of aggregation can be "all EF
    traffic between a particular ingress-egress pair", which is frequently
    referred to as "pipe model", or "all EF traffic originating at a given
    ingress to all possible destinations", which is frequently referred to
    as "hose model".

    In addition to ingress shaping, the Diffserv model allows re-shaping
    traffic at the egress point.  As for the case of ingress shaping, the
    egress shaping can be implemented either at BR2 or ER2.

    The effect of different choices of the location and granularity of
    shaping on the delay guarantees that can be provided by a Diffserv cloud
    will be discussed in section ??.  In this section we consider the effect
    of this choices on the C and D terms advertised by the Interv-capable
    routers ER1 and ER2.  Note that the Intserv capable router downstream
    from the Diffserv cloud (ER2 in the reference network of Figure 1) is
    responsible for exporting the C and D terms of the Diffserv cloud.

4.2.1. Shaping at the Edge Routers

    If shaping is performed at the ingress edge router ER1, and reshaping,
    if any, is performed at ER2, but there is no shaping implemented inside
    the Diffserv cloud, the shaping/reshaping delay is part of the total
    delay advertised by the edge routers ER1 and ER2, and hence the
    corresponding C and D terms are exported by the Intserv-capable edge
    routers.  These will be denoted as C_is, D_is, C_es, D_es respectively,
    where the indices _is and _es denote "ingress shaper" and "egress
    shaper".  The values of these parameters are implementation dependent.

    Since the Diffserv cloud itself does not perform any shaping in this
    case, its C_dc should be set to zero.  The determination of the value of
    D_dc and factors affecting it are discussed in section 4.4 below.

4.2.2  Shaping at the boundary routers

    In the case where shaping is performed by the boundary routers, shaping
    and reshaping delay become part of the delay of the Diffserv cloud and
    hence have to be accounted for in the C_dc and D_dc error terms.  Note

Wroclawski and Charny            Expires: August, 2001               [page  11]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

    that depending on the shaping implementation, the rate-dependent error
    term may not necessarily be zero, and hence ingress shaping may add a
    non-zero component to the C_dc value of the Diffserv cloud.

    Since the ingress shaping delay depends on the shaping implementation
    and shaping granularity at the border router, and since different border
    routers may implement different shaping algorithms, it seems natural to
    dedicate the responsibility to export the error terms for ingress
    shaping delay to the ingress edge router(s) attached to the border

    It is important to note that in the case of aggregate shaping, the
    shaping delay may be a function of the combined burst and combined rate
    of all microflows comprising the shaped aggregate (note that the
    aggregate may consist of microflows arriving from different ingress

    To enable an existence of a meaningful upper bound on the shaping delay
    the shapers at the edge routers must be configured in such a way as to
    ensure the existence of the bound on the shaping delay at the boundary
    router.  This may be accomplished by emposing a policy such as "token
    bucket parameters of all flows requiring G support entering the diffserv
    cloud from any edge router should satisfy the condition (r>=r_min,
    b<=b_max).  Such conditions would enable token bucket characterization
    of the aggregate stream, which in combination with the properties of the
    shaping implementation would enable the computation of an upper bound
    for a particular microflow.

    If the egress boundary router implements reshaping on an aggregate
    basis, just as in the case if ingress shaping, the egress reshaping
    delay of a microflow depends on the combined rate and burstiness of the
    aggregate which is being reshaped.  Aggregate burstiness depends, among
    other things, on the parameters of ingress shapers and on the delay
    bound of the diffserv cloud incurred by all microflows after the last
    shaping point.

    The C and D terms corresponding to the egress boundary shaping must be
    configured at the egress edge router, which is responsible for exporting
    the egress shaping component of the C and D terms of the Diffserv cloud.

    In addition, just as in section 4.2.1, the egress edge router is
    responsible for exporting the D_ds component of the delay inside the
    diffserv cloud which is not due to the shaping or reshaping delays.

4.2.3. Shaping inside the Diffserv cloud

    While the Diffserv model does not prevent shaping inside the cloud as
    well as at the boundaries, this draft will concentrate on the most
    common case when all internal interfaces of any node in the diffserv
    cloud implement work-conserving aggregate class-based scheduling only.

Wroclawski and Charny            Expires: August, 2001               [page  12]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

4.3  Queuing delay

    Queuing delay experienced by a given packet is caused by two reasons:
    contention with other packets in the scheduler and the interruption of
    service experienced by the scheduler as a whole.  A typical example of
    the latter is the delay in a single processor system when the processor
    schedules some tasks other than packet scheduler.  If a bound on this
    latter portion of the delay is known for all routers inside the diffserv
    cloud, then the contribution of this delay component can be bounded by
    multiplying this bound by the max hop count h.

    The component of the queuing delay due to contention with other packets
    in the link scheduler will be discussed in detail in section 4.4.  For
    the sake of brevity, in the rest of this draft the term queuing delay
    will be used to refer to just the portion of the queuing delay due to
    contention with other packets in the scheduler.

4.4. Queueing delay bounds in the Diffserv Cloud

    The main difficulty in obtaining hard delay bounds for an arbitrary
    topology cloud arises from the assumption of aggregate scheduling inside
    the cloud.  When a packet of some flow f traverses a sequence of
    aggregate queues, its worst case delay may depend on the traffic of
    other flows which do not even share a single queue with the flow.
    Moreover, the delay of a packet p of flow f at time t may be affected by
    flows whose last packets have exited the network long before the first
    packet of flow f entered the network [CHARNY].

    The ability to provide hard delay bounds in a Diffserv cloud with
    aggregate scheduling must rely on cooperation of all devices in the
    cloud, as well as strict constraints on the traffic entering the cloud.

    It has been demonstrated that the knowledge of the following parameters
    global to the cloud is essential for the ability to provide strict
    queuing delay guarantees across the Diffserv cloud [CHARNY],[LEBOUDEC]:

    - limited number of hops of any flow across the cloud (denoted h)

    - low (bounded) ratio of the load of EF traffic to the service rate of
      the EF queue on any link in the cloud (denoted u)

    - minimum rate of the shaped aggregate (denoted r_min)

    - maximum token bucket depth of an edge-to-edge aggregate (denoted

    - minimum service rate of the EF queue (denoted S)

    - maximum deviation of the amount of service of the EF queue from the
      ideal fluid service at rate S (denoted E)

Wroclawski and Charny            Expires: August, 2001               [page  13]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

    Currently, the only known delay bound that holds for an arbitrary
    topology and arbitrary route distribution is given in [LeBouldec] by

                 D = (E/S + ub_max/r_min)x h/(1-u(h-1))

    which holds for any utilization u<1/(h-1).  This bound holds for the
    case when the capacity of any single link is substantially smaller than
    the total capacity of all interfaces of any router.  (This bound may be
    slightly improved if the capacity of a single link is not negligible
    compared to the total router capacity [LeBouldec]).  Unfortunately, this
    bound explodes when u=1/(h-1).

    Some knowledge on either the topology or the routes in the cloud may
    yield to an improved bound.  For example, for a class of network
    topologies which includes a multistage network it can be shown [CHARNY]
    that the bound is given by

                D = (E/S + ub_max/r_min)x((1+u)^h-1)/u

    While this bound holds for any utilization, due to the exponential term
    the delay grows very fast with the increase in utilization u.

    Unfortunately, at the moment no bound is known for a general topology
    with utilization greater than 1/(h-1).  It can be shown [CHARNY], that
    for utilization values greater than 1/(h-1), for any value of delay D
    one can always construct a network such that the delay in that network
    is greater than D. This implies that either no bound exists at all, or
    if a bound does exist, it must depend on some additional characteristics
    of the network other than just h and u.

    The practical implication of these results is that, barring new results
    on delay bounds, the amount of traffic requiring end-to-end Guaranteed
    service across the diffserv cloud should be rather small.  Furthermore,
    it also implies that if substantial amount of other EF traffic is
    present in the network, in order to ensure strict delay bounds for GS
    traffic, buffering and scheduling mechanisms must exist that ensure
    separation of the GS traffic using EF PHB from other traffic using EF

4.5.  Relationship to Bandwidth Allocation Techniques and Traffic
      Conditioning Models

4.5.1.  Availability of sufficient bandwidth

    As discussed in Section 4.4, in order to provide a strict delay bound
    across the Diffserv cloud the ratio of the EF load to the service rate
    of the EF queue has to be deterministically bounded on all links in the
    network.  This can be either ensured by signaled admission control (such
    as using RSVP aggregation techniques [RSVPAGGR] or by a static
    provisioning mechanism.  It should be noted that if provisioning is
    used, then to ensure deterministic load/service rate ratio on all link
    the network should be strongly overprovisioned to account for possible
    inaccuracy of traffic matrix estimates.

Wroclawski and Charny            Expires: August, 2001               [page  14]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

    In either case deterministic availability of sufficient bandwidth on all
    links is a necessary condition for the ability to provide deterministic
    delay guarantees.

4.5.2.  Effect of Shaping Granularity on Delay Bounds

    A related, although different issue for the ability to provide delay
    deterministic delay guarantees is the granularity of the ingress
    shaping.  The implications of different choices on the resulting delay
    bounds are discussed in the following subsections.  Per-microflow shaping

    The known worst case delay bound is linear in the ratio b_max/r_min.  In
    the case of microflow shaping, the minimal rate of the microflow can be
    quite small, resulting in a large delay bound.  There is a substantial
    advantage therefore in aggregating many small microflows into an
    aggregate and shaping the aggregate as a whole.  While in principle
    there is a range of choices for aggregation, this document will consider
    only two: edge-to-edge aggregation and edge-to-everywhere aggregation. Shaping of edge-to-edge aggregates

    This type of shaping is natural for explicit bandwidth reservation
    techiques.  In this case r_min and b_max relate to the rate and token
    bucket depth of the border-to-border aggregates.  Since the delay bound
    is linear in b_max/r_min, aggregating as many microflows sharing the
    same border-to-border pair as possible results in the increase of r_min,
    and hence in the decrease of the delay bound.  The location of the
    shaper at the border router is therefore beneficial for reducing the
    edge-to-edge delay bound.  Shaping of edge-to-everywhere aggregates.

    This type of shaping is frequently assumed in conjunction with bandwidth
    provisioning.  The effect of this choice on delay bounds depends on
    exactly how provisioning is done.  One possibility for provisioning the
    network is to estimate edge-to-edge demand matrix for EF traffic and
    ensure that there is sufficient capacity to accommodate this demand,
    assuming that the traffic matrix is accurate enough.  Another option is
    to make no assumption on the edge-to-edge EF traffic distribution, but
    rather admit a certain amount of EF traffic at each ingress edge,
    regardless of the destination edge, and provision the network in such a
    way that even if _all_ traffic from _all_ sources happens to pass
    through a single bottleneck link, the capacity of that link is
    sufficient to ensure the appropriate load to service rate ratio for the
    EF traffic.

    Depending on which of the two choices for provisioning is chosen,
    shaping of the edge-to-everywhere aggregate has the opposite effect on
    the delay bound.

    In the case of "edge-to-edge provisioning", the bandwidth of any link
    may be sufficient to accommodate the _actual_ load of EF traffic while

Wroclawski and Charny            Expires: August, 2001               [page  15]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

    remaining within the target utilization bound.  Hence, it is the minimal
    rate and the maximum burst size of the _actual_ edge-to-edge aggregates
    sharing any link that effect the delay bound.  However, aggregate
    edge-to-all shaping may result in individual substreams of the shaped
    aggregate being shaped to a much higher rate than the expected rate of
    that substream.  When the edge-to-everywhere aggregate splits inside the
    network into different substreams going to different destinations, each
    of those substreams may have in the worst case substantially larger
    burstiness than the token bucket depth of the aggregate
    edge-to-everywhere stream.  This results in substantial increase of the
    worst case delay over the edge-to-edge shaping model.  Moreover, in this
    case the properties of ingress shapers do not provide sufficient
    information to bound the worst case delay, since it is the burstiness of
    the _substreams_ inside the shaped aggregates that is needed, but is

    In contrast, if the "worst case" provisioning is assumed, the network is
    provisioned in such a way that each link can accommodate all the traffic
    even if all edge-to-everywhere aggregates end up sharing this link.  In
    this case the r_min and b_max of the edge-to-everywhere aggregate should
    be used without modification in the formula for the delay bound.
    Intuitively, in this case the actual traffic distribution can only be
    better than the worst case, in which all the aggregate traffic at a
    given ingress is destined to the same "worst case egress".

    Note that the "worst case" provisioning model targeting a particular
    utilization bound results in substantially more overprovisioning than
    the the "point-to-point" provisioning using an estimated traffic matrix,
    or explicit point-to point bandwidth allocation using signaled admission

4.6 Concatenation of Diffserv Clouds

    In the case where one or more Diffserv clouds are concatenated via an
    Intserv-capable node, the total delay is simply a concatenation of
    delays computed for each individual intserv-diffserv-insterv segment
    along the path.  However, obtaining end-to-end delay bound for a
    concatenation of Diffserv clouds via nodes implementing aggregate
    scheduling is a more complicated problem which requires further

5.  Implementation of Resource Efficient Close Approximations to the
     Guaranteed Service


6. Relationship to the Null Service

    The Intserv "Null Service" [NULL] differs from other defined services by
    not expressing any quantitative network performance requirements.  Use
    of the Null Service where an Intserv service class is required allows an
    application or host requesting QoS control service to express policy
    related information to the network without making a specific

Wroclawski and Charny            Expires: August, 2001               [page  16]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

    quantitative QoS request.  The assumption is that the network policy
    management and control elements will use this information to select an
    appropriate QoS for the requesting entity, and take whatever action is
    required to provide this QoS.

    One possibility is that the network policy mechanisms will determine
    that a quantitative end-to-end QoS is appropriate for this entity, and
    that this QoS can be provided using Intserv mechanisms.  In this case,
    the Null service selector can be replaced, at the first hop router or
    elsewhere along the path, with a different Intserv service class and
    related parameter information.  Once this occurs, the situation with
    respect to the use of Diffserv networks to provide the desired QoS is
    identical to that described above for these other services.

    A second alternative is that the network policy mechanisms determine
    that the requesting entity should receive a relative, rather than
    absolute (quantitative) level of service.  In this case, the packets are
    marked with the appropriate DSCP, but the admission control actions
    described above are not necessary.

7. Security Considerations

    <None yet>

8. References

    [AF] Heinanen, J., Baker, F., Weiss, W., Wroclawski, J., "Assured
    Forwarding PHB Group", RFC 2597, June 1999.

    [CHARNY] Anna Charny, "Delay Bounds in a Network with Aggregate
    Scehduling", work in progress,

    [CL] Wroclawski, J., "Specification of the Controlled-Load Network
    Element Service", RFC 2211, September 1997

    [DCLASS] Bernet, Y., "Format of the RSVP DCLASS Object", RFC 2996,
    November 2000

    [DIFFSERV] Blake, S., Black, D., Carlson, M., Davies, E., Wang Z.,
    Weiss, W., "An Architecture for Differentiated Service", RFC 2475,
    December 1998.

    [EF] Jacobson, V., Nichols, K., Poduri, K., "An Expedited Forwarding
    PHB", RFC 2598, June 1999.

    [G] Schenker, S., Partridge, C., Guerin, R., "Specification of
    Guaranteed Quality of Service", RFC 2212 September 1997

    [GENCHAR] Shenker, S., Wroclawski, J., "General Characterization
    Parameters for Integrated Service Network Elements", RFC 2215, September

Wroclawski and Charny            Expires: August, 2001               [page  17]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

    [INTSERV] Clark, D. et al.  "Integrated Services in the Internet
    Architecture: an Overview" RFC1633, June 1994

    [ISDSFRAME] Bernet, Ford, Yavatkar, Baker, Zhang, Speer, Braden,
    Davie, Wroclawski, Felstaine, "A Framework for Integrated
    Services Operation over Diffserv Networks", RFC 2998, November 2000

    [LEBOUDEC] Jean-Yves LeBoudec, "A Proven Delay Bound in a Network with
    Aggregate Scheduling", work in progress,

    [NULL] Bernet, Y., Smith, A., Davie, B., "Specification of the Null
    Service Type", RFC 2297, November 2000

    [RSVP] Braden, R., L. Zhang, S. Berson, S. Herzog, S. Jamin, "Resource
    Reservation Protocol (RSVP) - Version 1 Functional Specification", RFC
    2205, September 1997

    [RSVPAGGR] Baker, F., Iturralde, C., Le Faucheur, F., Davie, B.,
    "Aggregation of RSVP for IPv4 and IPv6 Reservations", Internet Draft

    [RSVPINTSERV] Wroclawski, J., "The use of RSVP with IETF Integrated
    Services", RFC 2210, September 1997.

9. Authors' addresses

    John Wroclawski
    MIT Laboratory for Computer Science
    545 Technology Sq., Cambridge, MA  02139, USA
    EMail: jtw@lcs.mit.edu

    Anna Charny
    Cisco Systems
    300 Apollo Drive, Chelmsford, MA 01824, USA
    Email: acharny@cisco.com

10. Full Copyright

    Copyright (C) The Internet Society 2001.  All Rights Reserved.

    This document and translations of it may be copied and furnished
    to others, and derivative works that comment on or otherwise
    explain it or assist in its implementation may be prepared, copied,
    published and distributed, in whole or in part, without
    restriction of any kind, provided that the above copyright notice
    and this paragraph are included on all such copies and derivative
    works.  However, this document itself may not be modified in any
    way, such as by removing the copyright notice or references to the
    Internet Society or other Internet organizations, except as needed
    for the purpose of developing Internet standards in which case the
    procedures for copyrights defined in the Internet Standards

Wroclawski and Charny            Expires: August, 2001               [page  18]

INTERNET DRAFT          draft-ietf-issll-ds-map-01.txt           February, 2001

    process must be followed, or as required to translate it into
    languages other than English.

    The limited permissions granted above are perpetual and will not
    be revoked by the Internet Society or its successors or assigns.

    This document and the information contained herein is provided on

Wroclawski and Charny            Expires: August, 2001               [page  19]