rfc3086.txt

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Network Working Group                                         K. Nichols
Request for Comments: 3086                                 Packet Design
Category: Informational                                     B. Carpenter
                                                                     IBM
                                                              April 2001


       Definition of Differentiated Services Per Domain Behaviors
                   and Rules for their Specification

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

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

Abstract

   The differentiated services framework enables quality-of-service
   provisioning within a network domain by applying rules at the edges
   to create traffic aggregates and coupling each of these with a
   specific forwarding path treatment in the domain through use of a
   codepoint in the IP header.  The diffserv WG has defined the general
   architecture for differentiated services and has focused on the
   forwarding path behavior required in routers, known as "per-hop
   forwarding behaviors" (or PHBs).  The WG has also discussed
   functionality required at diffserv (DS) domain edges to select
   (classifiers) and condition (e.g., policing and shaping) traffic
   according to the rules.  Short-term changes in the QoS goals for a DS
   domain are implemented by changing only the configuration of these
   edge behaviors without necessarily reconfiguring the behavior of
   interior network nodes.

   The next step is to formulate examples of how forwarding path
   components (PHBs, classifiers, and traffic conditioners) can be used
   to compose traffic aggregates whose packets experience specific
   forwarding characteristics as they transit a differentiated services
   domain.  The WG has decided to use the term per-domain behavior, or
   PDB, to describe the behavior experienced by a particular set of
   packets as they cross a DS domain.  A PDB is characterized by
   specific metrics that quantify the treatment a set of packets with a
   particular DSCP (or set of DSCPs) will receive as it crosses a DS
   domain.  A PDB specifies a forwarding path treatment for a traffic
   aggregate and, due to the role that particular choices of edge and



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   PHB configuration play in its resulting attributes, it is where the
   forwarding path and the control plane interact.  The measurable
   parameters of a PDB should be suitable for use in Service Level
   Specifications at the network edge.

   This document defines and discusses Per-Domain Behaviors in detail
   and lays out the format and required content for contributions to the
   Diffserv WG on PDBs and the procedure that will be applied for
   individual PDB specifications to advance as WG products.  This format
   is specified to expedite working group review of PDB submissions.

Table of Contents

    1. Introduction ................................................ 2
    2. Definitions ................................................. 4
    3. The Value of Defining Edge-to-Edge Behavior ................. 5
    4. Understanding PDBs .......................................... 7
    5. Format for Specification of Diffserv Per-Domain Behaviors ...13
    6. On PDB Attributes ...........................................16
    7. A Reference Per-Domain Behavior .............................19
    8. Guidelines for Advancing PDB Specifications .................21
    9. Security Considerations .....................................22
   10. Acknowledgements ............................................22
       References ..................................................22
       Authors' Addresses ..........................................23
       Full Copyright Statement ....................................24

1 Introduction

   Differentiated Services allows an approach to IP Quality of Service
   that is modular, incrementally deployable, and scalable while
   introducing minimal per-node complexity [RFC2475].  From the end
   user's point of view, QoS should be supported end-to-end between any
   pair of hosts.  However, this goal is not immediately attainable.  It
   will require interdomain QoS support, and many untaken steps remain
   on the road to achieving this.  One essential step, the evolution of
   the business models for interdomain QoS, will necessarily develop
   outside of the IETF.  A goal of the diffserv WG is to provide the
   firm technical foundation that allows these business models to
   develop.  The first major step will be to support edge-to-edge or
   intradomain QoS between the ingress and egress of a single network,
   i.e., a DS Domain in the terminology of RFC 2474.  The intention is
   that this edge-to-edge QoS should be composable, in a purely
   technical sense, to a quantifiable QoS across a DS Region composed of
   multiple DS domains.






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   The Diffserv WG has finished the first phase of standardizing the
   behaviors required in the forwarding path of all network nodes, the
   per-hop forwarding behaviors or PHBs.  The PHBs defined in RFCs 2474,
   2597 and 2598 give a rich toolbox for differential packet handling by
   individual boxes.  The general architectural model for diffserv has
   been documented in RFC 2475.  An informal router model [MODEL]
   describes a model of traffic conditioning and other forwarding
   behaviors.  However, technical issues remain in moving "beyond the
   box" to intradomain QoS models.

   The ultimate goal of creating scalable end-to-end QoS in the Internet
   requires that we can identify and quantify behavior for a group of
   packets that is preserved when they are aggregated with other packets
   as they traverse the Internet.  The step of specifying forwarding
   path attributes on a per-domain basis for a set of packets
   distinguished only by the mark in the DS field of individual packets
   is critical in the evolution of Diffserv QoS and should provide the
   technical input that will aid in the construction of business models.
   This document defines and specifies the term "Per-Domain Behavior" or
   PDB to describe QoS attributes across a DS domain.

   Diffserv classification and traffic conditioning are applied to
   packets arriving at the boundary of a DS domain to impose
   restrictions on the composition of the resultant traffic aggregates,
   as distinguished by the DSCP marking , inside the domain.  The
   classifiers and traffic conditioners are set to reflect the policy
   and traffic goals for that domain and may be specified in a TCA
   (Traffic Conditioning Agreement).  Once packets have crossed the DS
   boundary, adherence to diffserv principles makes it possible to group
   packets solely according to the behavior they receive at each hop (as
   selected by the DSCP).  This approach has well-known scaling
   advantages, both in the forwarding path and in the control plane.
   Less well recognized is that these scaling properties only result if
   the per-hop behavior definition gives rise to a particular type of
   invariance under aggregation.  Since the per-hop behavior must be
   equivalent for every node in the domain, while the set of packets
   marked for that PHB may be different at every node, PHBs should be
   defined such that their characteristics do not depend on the traffic
   volume of the associated BA on a router's ingress link nor on a
   particular path through the DS domain taken by the packets.
   Specifically, different streams of traffic that belong to the same
   traffic aggregate merge and split as they traverse the network.  If
   the properties of a PDB using a particular PHB hold regardless of how
   the temporal characteristics of the marked traffic aggregate change
   as it traverses the domain, then that PDB scales.  (Clearly this
   assumes that numerical parameters such as bandwidth allocated to the
   particular PDB may be different at different points in the network,
   and may be adjusted dynamically as traffic volume varies.)  If there



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   are limits to where the properties hold, that translates to a limit
   on the size or topology of a DS domain that can use that PDB.
   Although useful single-link DS domains might exist, PDBs that are
   invariant with network size or that have simple relationships with
   network size and whose properties can be recovered by reapplying
   rules (that is, forming another diffserv boundary or edge to re-
   enforce the rules for the traffic aggregate) are needed for building
   scalable end-to-end quality of service.

   There is a clear distinction between the definition of a Per-Domain
   Behavior in a DS domain and a service that might be specified in a
   Service Level Agreement.  The PDB definition is a technical building
   block that permits the coupling of classifiers, traffic conditioners,
   specific PHBs, and particular configurations with a resulting set of
   specific observable attributes which may be characterized in a
   variety of ways.  These definitions are intended to be useful tools
   in configuring DS domains, but the PDB (or PDBs) used by a provider
   is not expected to be visible to customers any more than the specific
   PHBs employed in the provider's network would be.  Network providers
   are expected to select their own measures to make customer-visible in
   contracts and these may be stated quite differently from the
   technical attributes specified in a PDB definition, though the
   configuration of a PDB might be taken from a Service Level
   Specification (SLS).  Similarly, specific PDBs are intended as tools
   for ISPs to construct differentiated services offerings; each may
   choose different sets of tools, or even develop their own, in order
   to achieve particular externally observable metrics.  Nevertheless,
   the measurable parameters of a PDB are expected to be among the
   parameters cited directly or indirectly in the Service Level
   Specification component of a corresponding SLA.

   This document defines Differentiated Services Per-Domain Behaviors
   and specifies the format that must be used for submissions of
   particular PDBs to the Diffserv WG.

2 Definitions

   The following definitions are stated in RFCs 2474 and 2475 and are
   repeated here for easy reference:

   " Behavior Aggregate: a collection of packets with the same codepoint
     crossing a link in a particular direction.

   " Differentiated Services Domain: a contiguous portion of the
     Internet over which a consistent set of differentiated services
     policies are administered in a coordinated fashion.  A
     differentiated services domain can represent different




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     administrative domains or autonomous systems, different trust
     regions, different network technologies (e.g., cell/frame), hosts
     and routers, etc.  Also DS domain.

   " Differentiated Services Boundary: the edge of a DS domain, where
     classifiers and traffic conditioners are likely to be deployed.  A
     differentiated services boundary can be further sub-divided into
     ingress and egress nodes, where the ingress/egress nodes are the
     downstream/upstream nodes of a boundary link in a given traffic
     direction.  A differentiated services boundary typically is found
     at the ingress to the first-hop differentiated services-compliant
     router (or network node) that a host's packets traverse, or at the
     egress of the last-hop differentiated services-compliant router or
     network node that packets traverse before arriving at a host.  This
     is sometimes referred to as the boundary at a leaf router.  A
     differentiated services boundary may be co-located with a host,
     subject to local policy.  Also DS boundary.

   To these we add:

   " Traffic Aggregate: a collection of packets with a codepoint that
     maps to the same PHB, usually in a DS domain or some subset of a DS
     domain.  A traffic aggregate marked for the foo PHB is referred to
     as the "foo traffic aggregate" or "foo aggregate" interchangeably.
     This generalizes the concept of Behavior Aggregate from a link to a
     network.

   " Per-Domain Behavior: the expected treatment that an identifiable or
     target group of packets will receive from "edge-to-edge" of a DS
     domain.  (Also PDB.)  A particular PHB (or, if applicable, list of
     PHBs) and traffic conditioning requirements are associated with
     each PDB.

   " A Service Level Specification (SLS) is a set of parameters and
     their values which together define the service offered to a traffic
     stream by a DS domain.  It is expected to include specific values
     or bounds for PDB parameters.

3 The Value of Defining Edge-to-Edge Behavior