rfc2990.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                         G. Huston
Request for Comments: 2990                                      Telstra
Category: Informational                                   November 2000


                Next Steps for the IP QoS Architecture

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 (2000).  All Rights Reserved.

Abstract

   While there has been significant progress in the definition of
   Quality of Service (QoS) architectures for internet networks, there
   are a number of aspects of QoS that appear to need further
   elaboration as they relate to translating a set of tools into a
   coherent platform for end-to-end service delivery.  This document
   highlights the outstanding architectural issues relating to the
   deployment and use of QoS mechanisms within internet networks, noting
   those areas where further standards work may assist with the
   deployment of QoS internets.

   This document is the outcome of a collaborative exercise on the part
   of the Internet Architecture Board.

Table of Contents

    1. Introduction ...........................................   2
    2. State and Stateless QoS ................................   4
    3. Next Steps for QoS Architectures .......................   6
       3.1 QoS-Enabled Applications ...........................   7
       3.2 The Service Environment ............................   9
       3.3 QoS Discovery ......................................  10
       3.4 QoS Routing and Resource Management ................  10
       3.5 TCP and QoS ........................................  11
       3.6 Per-Flow States and Per-Packet classifiers .........  13
       3.7 The Service Set ....................................  14
       3.8 Measuring Service Delivery .........................  14
       3.9 QoS Accounting .....................................  15
       3.10 QoS Deployment Diversity ..........................  16
       3.11 QoS Inter-Domain signaling ........................  17



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       3.12 QoS Deployment Logistics ..........................  17
    4. The objective of the QoS architecture ..................  18
    5. Towards an end-to-end QoS architecture .................  19
    6. Conclusions ............................................  21
    7. Security Considerations ................................  21
    8. References .............................................  22
    9. Acknowledgments ........................................  23
   10. Author's Address .......................................  23
   11. Full Copyright Statement ...............................  24

1. Introduction

   The default service offering associated with the Internet is
   characterized as a best-effort variable service response.  Within
   this service profile the network makes no attempt to actively
   differentiate its service response between the traffic streams
   generated by concurrent users of the network.  As the load generated
   by the active traffic flows within the network varies, the network's
   best effort service response will also vary.

   The objective of various Internet Quality of Service (QoS) efforts is
   to augment this base service with a number of selectable service
   responses.  These service responses may be distinguished from the
   best-effort service by some form of superior service level, or they
   may be distinguished by providing a predictable service response
   which is unaffected by external conditions such as the number of
   concurrent traffic flows, or their generated traffic load.

   Any network service response is an outcome of the resources available
   to service a load, and the level of the load itself.  To offer such
   distinguished services there is not only a requirement to provide a
   differentiated service response within the network, there is also a
   requirement to control the service-qualified load admitted into the
   network, so that the resources allocated by the network to support a
   particular service response are capable of providing that response
   for the imposed load.  This combination of admission control agents
   and service management elements can be summarized as "rules plus
   behaviors". To use the terminology of the Differentiated Service
   architecture [4], this admission control function is undertaken by a
   traffic conditioner (an entity which performs traffic conditioning
   functions and which may contain meters, markers, droppers, and
   shapers), where the actions of the conditioner are governed by
   explicit or implicit admission control agents.

   As a general observation of QoS architectures, the service load
   control aspect of QoS is perhaps the most troubling component of the
   architecture.  While there are a wide array of well understood
   service response mechanisms that are available to IP networks,



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   matching a set of such mechanisms within a controlled environment to
   respond to a set of service loads to achieve a completely consistent
   service response remains an area of weakness within existing IP QoS
   architectures.  The control elements span a number of generic
   requirements, including end-to-end application signaling, end-to-
   network service signaling and resource management signaling to allow
   policy-based control of network resources.  This control may also
   span a particular scope, and use 'edge to edge' signaling, intended
   to support particular service responses within a defined network
   scope.

   One way of implementing this control of imposed load to match the
   level of available resources is through an application-driven process
   of service level negotiation (also known as application signaled
   QoS).  Here, the application first signals its service requirements
   to the network, and the network responds to this request.  The
   application will proceed if the network has indicated that it is able
   to carry the additional load at the requested service level.  If the
   network indicates that it cannot accommodate the service requirements
   the application may proceed in any case, on the basis that the
   network will service the application's data on a best effort basis.
   This negotiation between the application and the network can take the
   form of explicit negotiation and commitment, where there is a single
   negotiation phase, followed by a commitment to the service level on
   the part of the network.  This application-signaled approach can be
   used within the Integrated Services architecture, where the
   application frames its service request within the resource
   reservation protocol (RSVP), and then passes this request into the
   network.  The network can either respond positively in terms of its
   agreement to commit to this service profile, or it can reject the
   request.  If the network commits to the request with a resource
   reservation, the application can then pass traffic into the network
   with the expectation that as long as the traffic remains within the
   traffic load profile that was originally associated with the request,
   the network will meet the requested service levels.  There is no
   requirement for the application to periodically reconfirm the service
   reservation itself, as the interaction between RSVP and the network
   constantly refreshes the reservation while it remains active.  The
   reservation remains in force until the application explicitly
   requests termination of the reservation, or the network signals to
   the application that it is unable to continue with a service
   commitment to the reservation [3].  There are variations to this
   model, including an aggregation model where a proxy agent can fold a
   number of application-signaled reservations into a common aggregate
   reservation along a common sub-path, and a matching deaggregator can
   reestablish the collection of individual resource reservations upon
   leaving the aggregate region [5].  The essential feature of this
   Integrated Services model is the "all or nothing" nature of the



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   model.  Either the network commits to the reservation, in which case
   the requestor does not have to subsequently monitor the network's
   level of response to the service, or the network indicates that it
   cannot meet the resource reservation.

   An alternative approach to load control is to decouple the network
   load control function from the application.  This is the basis of the
   Differentiated Services architecture.  Here, a network implements a
   load control function as part of the function of admission of traffic
   into the network, admitting no more traffic within each service
   category as there are assumed to be resources in the network to
   deliver the intended service response.  Necessarily there is some
   element of imprecision in this function given that traffic may take
   an arbitrary path through the network.  In terms of the interaction
   between the network and the application, this takes the form of a
   service request without prior negotiation, where the application
   requests a particular service response by simply marking each packet
   with a code to indicate the desired service.  Architecturally, this
   approach decouples the end systems and the network, allowing a
   network to implement an active admission function in order to
   moderate the workload that is placed upon the network's resources
   without specific reference to individual resource requests from end
   systems.  While this decoupling of control allows a network's
   operator greater ability to manage its resources and a greater
   ability to ensure the integrity of its services, there is a greater
   potential level of imprecision in attempting to match applications'
   service requirements to the network's service capabilities.

2. State and Stateless QoS

   These two approaches to load control can be characterized as state-
   based and stateless approaches respectively.

   The architecture of the Integrated Services model equates the
   cumulative sum of honored service requests to the current reserved
   resource levels of the network.  In order for a resource reservation
   to be honored by the network, the network must maintain some form of
   remembered state to describe the resources that have been reserved,
   and the network path over which the reserved service will operate.
   This is to ensure integrity of the reservation.  In addition, each
   active network element within the network path must maintain a local
   state that allows incoming IP packets to be correctly classified into
   a reservation class.  This classification allows the packet to be
   placed into a packet flow context that is associated with an
   appropriate service response consistent with the original end-to-end
   service reservation.  This local state also extends to the function





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   of metering packets for conformance on a flow-by-flow basis, and the
   additional overheads associated with maintenance of the state of each
   of these meters.

   In the second approach, that of a Differentiated Services model, the
   packet is marked with a code to trigger the appropriate service
   response from the network elements that handles the packet, so that
   there is no strict requirement to install a per-reservation state on
   these network elements.  Also, the end application or the service
   requestor is not required to provide the network with advance notice
   relating to the destination of the traffic, nor any indication of the
   intended traffic profile or the associated service profile.  In the
   absence of such information any form of per-application or per-path
   resource reservation is not feasible.  In this model there is no
   maintained per-flow state within the network.

   The state-based Integrated Services architectural model admits the
   potential to support greater level of accuracy, and a finer level of
   granularity on the part of the network to respond to service
   requests.  Each individual application's service request can be used
   to generate a reservation state within the network that is intended
   to prevent the resources associated with the reservation to be
   reassigned or otherwise preempted to service other reservations or to
   service best effort traffic loads.  The state-based model is intended
   to be exclusionary, where other traffic is displaced in order to meet
   the reservation's service targets.

   As noted in RFC2208 [2], there are several areas of concern about the
   deployment of this form of service architecture.  With regard to
   concerns of per-flow service scalability, the resource requirements
   (computational processing and memory consumption) for running per-
   flow resource reservations on routers increase in direct proportion
   to the number of separate reservations that need to be accommodated.
   By the same token, router forwarding performance may be impacted
   adversely by the packet-classification and scheduling mechanisms
   intended to provide differentiated services for these resource-
   reserved flows.  This service architecture also poses some challenges
   to the queuing mechanisms, where there is the requirement to allocate
   absolute levels of egress bandwidth to individual flows, while still
   supporting an unmanaged low priority best effort traffic class.