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Network Working Group                                          Y. Bernet
Request for Comments: 3290                                     Microsoft
Category: Informational                                         S. Blake
                                                                Ericsson
                                                             D. Grossman
                                                                Motorola
                                                                A. Smith
                                                        Harbour Networks
                                                                May 2002


           An Informal Management Model for Diffserv Routers

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 (2002).  All Rights Reserved.

Abstract

   This document proposes an informal management model of Differentiated
   Services (Diffserv) routers for use in their management and
   configuration.  This model defines functional datapath elements
   (e.g., classifiers, meters, actions, marking, absolute dropping,
   counting, multiplexing), algorithmic droppers, queues and schedulers.
   It describes possible configuration parameters for these elements and
   how they might be interconnected to realize the range of traffic
   conditioning and per-hop behavior (PHB) functionalities described in
   the Diffserv Architecture.

Table of Contents

   1 Introduction .................................................    3
   2 Glossary .....................................................    4
   3 Conceptual Model .............................................    7
   3.1 Components of a Diffserv Router ............................    7
   3.1.1 Datapath .................................................    7
   3.1.2 Configuration and Management Interface ...................    9
   3.1.3 Optional QoS Agent Module ................................   10
   3.2 Diffserv Functions at Ingress and Egress ...................   10
   3.3 Shaping and Policing .......................................   12
   3.4 Hierarchical View of the Model .............................   12
   4 Classifiers ..................................................   13



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   4.1 Definition .................................................   13
   4.1.1 Filters ..................................................   15
   4.1.2 Overlapping Filters ......................................   15
   4.2 Examples ...................................................   16
   4.2.1 Behavior Aggregate (BA) Classifier .......................   16
   4.2.2 Multi-Field (MF) Classifier ..............................   17
   4.2.3 Free-form Classifier .....................................   17
   4.2.4 Other Possible Classifiers ...............................   18
   5 Meters .......................................................   19
   5.1 Examples ...................................................   20
   5.1.1 Average Rate Meter .......................................   20
   5.1.2 Exponential Weighted Moving Average (EWMA) Meter .........   21
   5.1.3 Two-Parameter Token Bucket Meter .........................   21
   5.1.4 Multi-Stage Token Bucket Meter ...........................   22
   5.1.5 Null Meter ...............................................   23
   6 Action Elements ..............................................   23
   6.1 DSCP Marker ................................................   24
   6.2 Absolute Dropper ...........................................   24
   6.3 Multiplexor ................................................   25
   6.4 Counter ....................................................   25
   6.5 Null Action ................................................   25
   7 Queuing Elements .............................................   25
   7.1 Queuing Model ..............................................   26
   7.1.1 FIFO Queue ...............................................   27
   7.1.2 Scheduler ................................................   28
   7.1.3 Algorithmic Dropper ......................................   30
   7.2 Sharing load among traffic streams using queuing ...........   33
   7.2.1 Load Sharing .............................................   34
   7.2.2 Traffic Priority .........................................   35
   8 Traffic Conditioning Blocks (TCBs) ...........................   35
   8.1 TCB ........................................................   36
   8.1.1 Building blocks for Queuing ..............................   37
   8.2 An Example TCB .............................................   37
   8.3 An Example TCB to Support Multiple Customers ...............   42
   8.4 TCBs Supporting Microflow-based Services ...................   44
   8.5 Cascaded TCBs ..............................................   47
   9 Security Considerations ......................................   47
   10 Acknowledgments .............................................   47
   11 References ..................................................   47
   Appendix A. Discussion of Token Buckets and Leaky Buckets ......   50
   Authors' Addresses .............................................   55
   Full Copyright Statement........................................   56









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1.  Introduction

   Differentiated Services (Diffserv) [DSARCH] is a set of technologies
   which allow network service providers to offer services with
   different kinds of network quality-of-service (QoS) objectives to
   different customers and their traffic streams.  This document uses
   terminology defined in [DSARCH] and [NEWTERMS] (some of these
   definitions are included here in Section 2 for completeness).

   The premise of Diffserv networks is that routers within the core of
   the network handle packets in different traffic streams by forwarding
   them using different per-hop behaviors (PHBs).  The PHB to be applied
   is indicated by a Diffserv codepoint (DSCP) in the IP header of each
   packet [DSFIELD].  The DSCP markings are applied either by a trusted
   upstream node, e.g., a customer, or by the edge routers on entry to
   the Diffserv network.

   The advantage of such a scheme is that many traffic streams can be
   aggregated to one of a small number of behavior aggregates (BA),
   which are each forwarded using the same PHB at the router, thereby
   simplifying the processing and associated storage.  In addition,
   there is no signaling other than what is carried in the DSCP of each
   packet, and no other related processing that is required in the core
   of the Diffserv network since QoS is invoked on a packet-by-packet
   basis.

   The Diffserv architecture enables a variety of possible services
   which could be deployed in a network.  These services are reflected
   to customers at the edges of the Diffserv network in the form of a
   Service Level Specification (SLS - see [NEWTERMS]).  Whilst further
   discussion of such services is outside the scope of this document
   (see [PDBDEF]), the ability to provide these services depends on the
   availability of cohesive management and configuration tools that can
   be used to provision and monitor a set of Diffserv routers in a
   coordinated manner.  To facilitate the development of such
   configuration and management tools it is helpful to define a
   conceptual model of a Diffserv router that abstracts away
   implementation details of particular Diffserv routers from the
   parameters of interest for configuration and management.  The purpose
   of this document is to define such a model.

   The basic forwarding functionality of a Diffserv router is defined in
   other specifications; e.g., [DSARCH, DSFIELD, AF-PHB, EF-PHB].

   This document is not intended in any way to constrain or to dictate
   the implementation alternatives of Diffserv routers.  It is expected
   that router implementers will demonstrate a great deal of variability
   in their implementations.  To the extent that implementers are able



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   to model their implementations using the abstractions described in
   this document, configuration and management tools will more readily
   be able to configure and manage networks incorporating Diffserv
   routers of assorted origins.

   This model is intended to be abstract and capable of representing the
   configuration parameters important to Diffserv functionality for a
   variety of specific router implementations.  It is not intended as a
   guide to system implementation nor as a formal modeling description.
   This model serves as the rationale for the design of an SNMP MIB
   [DSMIB] and for other configuration interfaces (e.g., other policy-
   management protocols) and, possibly, more detailed formal models
   (e.g., [QOSDEVMOD]): these should all be consistent with this model.

   o  Section 3 starts by describing the basic high-level blocks of a
      Diffserv router.  It explains the concepts used in the model,
      including the hierarchical management model for these blocks which
      uses low-level functional datapath elements such as Classifiers,
      Actions, Queues.

   o  Section 4 describes Classifier elements.

   o  Section 5 discusses Meter elements.

   o  Section 6 discusses Action elements.

   o  Section 7 discusses the basic queuing elements of Algorithmic
      Droppers, Queues, and Schedulers and their functional behaviors
      (e.g., traffic shaping).

   o  Section 8 shows how the low-level elements can be combined to
      build modules called Traffic Conditioning Blocks (TCBs) which are
      useful for management purposes.

   o  Section 9 discusses security concerns.

   o  Appendix A contains a brief discussion of the token bucket and
      leaky bucket algorithms used in this model and some of the
      practical effects of the use of token buckets within the Diffserv
      architecture.

2.  Glossary

   This document uses terminology which is defined in [DSARCH].  There
   is also current work-in-progress on this terminology in the IETF and
   some of the definitions provided here are taken from that work.  Some





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   of the terms from these other references are defined again here in
   order to provide additional detail, along with some new terms
   specific to this document.

   Absolute      A functional datapath element which simply discards all
   Dropper       packets arriving at its input.

   Algorithmic   A functional datapath element which selectively
   Dropper       discards packets that arrive at its input, based on a
                 discarding algorithm.  It has one data input and one
                 output.

   Classifier    A functional datapath element which consists of filters
                 that select matching and non-matching packets.  Based
                 on this selection, packets are forwarded along the
                 appropriate datapath within the router.  A classifier,
                 therefore, splits a single incoming traffic stream into
                 multiple outgoing streams.

   Counter       A functional datapath element which updates a packet
                 counter and also an octet counter for every
                 packet that passes through it.

   Datapath      A conceptual path taken by packets with particular
                 characteristics through a Diffserv router.  Decisions
                 as to the path taken by a packet are made by functional
                 datapath elements such as Classifiers and Meters.

   Filter        A set of wildcard, prefix, masked, range and/or exact
                 match conditions on the content of a packet's
                 headers or other data, and/or on implicit or derived
                 attributes associated with the packet.  A filter is
                 said to match only if each condition is satisfied.

   Functional    A basic building block of the conceptual router.
   Datapath      Typical elements are Classifiers, Meters, Actions,
   Element       Algorithmic Droppers, Queues and Schedulers.

   Multiplexer   A multiplexor.
   (Mux)

   Multiplexor   A functional datapath element that merges multiple
   (Mux)         traffic streams (datapaths) into a single traffic
                 stream (datapath).







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   Non-work-     A property of a scheduling algorithm such that it
   conserving    services packets no sooner than a scheduled departure
                 time, even if this means leaving packets queued
                 while the output (e.g., a network link or connection
                 to the next element) is idle.

   Policing      The process of comparing the arrival of data packets
                 against a temporal profile and forwarding, delaying
                 or dropping them so as to make the output stream
                 conformant to the profile.

   Queuing       A combination of functional datapath elements
   Block         that modulates the transmission of packets belonging
                 to a traffic streams and determines their
                 ordering, possibly storing them temporarily or
                 discarding them.

   Scheduling    An algorithm which determines which queue of a set
   algorithm     of queues to service next.  This may be based on the
                 relative priority of the queues, on a weighted fair
                 bandwidth sharing policy or some other policy. Such
                 an algorithm may be either work-conserving or non-
                 work-conserving.

   Service-Level A set of parameters and their values which together