rfc2816.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                                         A. Ghanwani
Request for Comments: 2816                                Nortel Networks
Category: Informational                                           W. Pace
                                                                      IBM
                                                            V. Srinivasan
                                                    CoSine Communications
                                                                 A. Smith
                                                         Extreme Networks
                                                                M. Seaman
                                                                  Telseon
                                                                 May 2000


                  A Framework for Integrated Services
           Over Shared and Switched IEEE 802 LAN Technologies

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

   This memo describes a framework for supporting IETF Integrated
   Services on shared and switched LAN infrastructure.  It includes
   background material on the capabilities of IEEE 802 like networks
   with regard to parameters that affect Integrated Services such as
   access latency, delay variation and queuing support in LAN switches.
   It discusses aspects of IETF's Integrated Services model that cannot
   easily be accommodated in different LAN environments.  It outlines a
   functional model for supporting the Resource Reservation Protocol
   (RSVP) in such LAN environments.  Details of extensions to RSVP for
   use over LANs are described in an accompanying memo [14].  Mappings
   of the various Integrated Services onto IEEE 802 LANs are described
   in another memo [13].











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Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Document Outline . . . . . . . . . . . . . . . . . . . . .  4
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Frame Forwarding in IEEE 802 Networks  . . . . . . . . . .  5
       4.1. General IEEE 802 Service Model  . . . . . . . . . . .  5
       4.2. Ethernet/IEEE 802.3 . . . . . . . . . . . . . . . . .  7
       4.3. Token Ring/IEEE 802.5 . . . . . . . . . . . . . . . .  8
       4.4. Fiber Distributed Data Interface  . . . . . . . . . . 10
       4.5. Demand Priority/IEEE 802.12 . . . . . . . . . . . . . 10
   5.  Requirements and Goals . . . . . . . . . . . . . . . . . . 11
       5.1. Requirements  . . . . . . . . . . . . . . . . . . . . 11
       5.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . 13
       5.3. Non-goals . . . . . . . . . . . . . . . . . . . . . . 14
       5.4. Assumptions . . . . . . . . . . . . . . . . . . . . . 14
   6.  Basic Architecture . . . . . . . . . . . . . . . . . . . . 15
       6.1. Components  . . . . . . . . . . . . . . . . . . . . . 15
             6.1.1. Requester Module  . . . . . . . . . . . . . . 15
             6.1.2. Bandwidth Allocator . . . . . . . . . . . . . 16
             6.1.3. Communication Protocols . . . . . . . . . . . 16
       6.2. Centralized vs.  Distributed Implementations  . . . . 17
   7.  Model of the Bandwidth Manager in a Network  . . . . . . . 18
       7.1. End Station Model . . . . . . . . . . . . . . . . . . 19
             7.1.1. Layer 3 Client Model  . . . . . . . . . . . . 19
             7.1.2. Requests to Layer 2 ISSLL . . . . . . . . . . 19
             7.1.3. At the Layer 3 Sender . . . . . . . . . . . . 20
             7.1.4. At the Layer 3 Receiver . . . . . . . . . . . 21
       7.2. Switch Model  . . . . . . . . . . . . . . . . . . . . 22
             7.2.1. Centralized Bandwidth Allocator . . . . . . . 22
             7.2.2. Distributed Bandwidth Allocator . . . . . . . 23
       7.3. Admission Control . . . . . . . . . . . . . . . . . . 25
       7.4. QoS Signaling . . . . . . . . . . . . . . . . . . . . 26
             7.4.1. Client Service Definitions  . . . . . . . . . 26
             7.4.2. Switch Service Definitions  . . . . . . . . . 27
   8.  Implementation Issues  . . . . . . . . . . . . . . . . . . 28
       8.1. Switch Characteristics  . . . . . . . . . . . . . . . 29
       8.2. Queuing . . . . . . . . . . . . . . . . . . . . . . . 30
       8.3. Mapping of Services to Link Level Priority  . . . . . 31
       8.4. Re-mapping of Non-conforming Aggregated Flows . . . . 31
       8.5. Override of Incoming User Priority  . . . . . . . . . 32
       8.6. Different Reservation Styles  . . . . . . . . . . . . 32
       8.7. Receiver Heterogeneity  . . . . . . . . . . . . . . . 33
   9.  Network Topology Scenarios   . . . . . . . . . . . . . . . 35
       9.1. Full Duplex Switched Networks . . . . . . . . . . . . 36
       9.2. Shared Media Ethernet Networks  . . . . . . . . . . . 37
       9.3. Half Duplex Switched Ethernet Networks  . . . . . . . 38
       9.4. Half Duplex Switched and Shared Token Ring Networks . 39



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       9.5. Half Duplex and Shared Demand Priority Networks . . . 40
   10. Justification  . . . . . . . . . . . . . . . . . . . . . . 42
   11. Summary  . . . . . . . . . . . . . . . . . . . . . . . . . 43
   References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
   Security Considerations  . . . . . . . . . . . . . . . . . . . 45
   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 45
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 46
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . 47

1. Introduction

   The Internet has traditionally provided support for best effort
   traffic only.  However, with the recent advances in link layer
   technology, and with numerous emerging real time applications such as
   video conferencing and Internet telephony, there has been much
   interest for developing mechanisms which enable real time services
   over the Internet.  A framework for meeting these new requirements
   was set out in RFC 1633 [8] and this has driven the specification of
   various classes of network service by the Integrated Services working
   group of the IETF, such as Controlled Load and Guaranteed Service
   [6,7].  Each of these service classes is designed to provide certain
   Quality of Service (QoS) to traffic conforming to a specified set of
   parameters.  Applications are expected to choose one of these classes
   according to their QoS requirements.  One mechanism for end stations
   to utilize such services in an IP network is provided by a QoS
   signaling protocol, the Resource Reservation Protocol (RSVP) [5]
   developed by the RSVP working group of the IETF. The IEEE under its
   Project 802 has defined standards for many different local area
   network technologies.  These all typically offer the same MAC layer
   datagram service [1] to higher layer protocols such as IP although
   they often provide different dynamic behavior characteristics -- it
   is these that are important when considering their ability to support
   real time services.  Later in this memo we describe some of the
   relevant characteristics of the different MAC layer LAN technologies.
   In addition, IEEE 802 has defined standards for bridging multiple LAN
   segments together using devices known as "MAC Bridges" or "Switches"
   [2].  Recent work has also defined traffic classes, multicast
   filtering, and virtual LAN capabilities for these devices [3,4].
   Such LAN technologies often constitute the last hop(s) between users
   and the Internet as well as being a primary building block for entire
   campus networks.  It is therefore necessary to provide standardized
   mechanisms for using these technologies to support end-to-end real
   time services.  In order to do this, there must be some mechanism for
   resource management at the data link layer.  Resource management in
   this context encompasses the functions of admission control,
   scheduling, traffic policing, etc.  The ISSLL (Integrated Services





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   over Specific Link Layers) working group in the IETF was chartered
   with the purpose of exploring and standardizing such mechanisms for
   various link layer technologies.

2. Document Outline

   This document is concerned with specifying a framework for providing
   Integrated Services over shared and switched LAN technologies such as
   Ethernet/IEEE 802.3, Token Ring/IEEE 802.5, FDDI, etc.  We begin in
   Section 4 with a discussion of the capabilities of various IEEE 802
   MAC layer technologies.  Section 5 lists the requirements and goals
   for a mechanism capable of providing Integrated Services in a LAN.
   The resource management functions outlined in Section 5 are provided
   by an entity referred to as a Bandwidth Manager (BM). The
   architectural model of the BM is described in Section 6 and its
   various components are discussed in Section 7.  Some implementation
   issues with respect to link layer support for Integrated Services are
   examined in Section 8.  Section 9 discusses a taxonomy of topologies
   for the LAN technologies under consideration with an emphasis on the
   capabilities of each which can be leveraged for enabling Integrated
   Services.  This framework makes no assumptions about the topology at
   the link layer.  The framework is intended to be as exhaustive as
   possible; this means that it is possible that all the functions
   discussed may not be supportable by a particular topology or
   technology, but this should not preclude the usage of this model for
   it.

3. Definitions

   The following is a list of terms used in this and other ISSLL
   documents.

   -  Link Layer or Layer 2 or L2:  Data link layer technologies such as
      Ethernet/IEEE 802.3 and Token Ring/IEEE 802.5 are referred to as
      Layer 2 or L2.

   -  Link Layer Domain or Layer 2 Domain or L2 Domain:  Refers to a set
      of nodes and links interconnected without passing through a L3
      forwarding function.  One or more IP subnets can be overlaid on a
      L2 domain.

   -  Layer 2 or L2 Devices:  Devices that only implement Layer 2
      functionality as Layer 2 or L2 devices.  These include IEEE 802.1D
      [2] bridges or switches.

   -  Internetwork Layer or Layer 3 or L3:  Refers to Layer 3 of the ISO
      OSI model.  This memo is primarily concerned with networks that
      use the Internet Protocol (IP) at this layer.



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   -  Layer 3 Device or L3 Device or End Station:  These include hosts
      and routers that use L3 and higher layer protocols or application
      programs that need to make resource reservations.

   -  Segment:  A physical L2 segment that is shared by one or more
      senders.  Examples of segments include:  (a) a shared Ethernet or
      Token Ring wire resolving contention for media access using CSMA
      or token passing; (b) a half duplex link between two stations or
      switches; (c) one direction of a switched full duplex link.

   -  Managed Segment:  A managed segment is a segment with a DSBM
      (designated subnet bandwidth manager, see [14]) present and
      responsible for exercising admission control over requests for
      resource reservation.  A managed segment includes those
      interconnected parts of a shared LAN that are not separated by
      DSBMs.

   -  Traffic Class:  Refers to an aggregation of data flows which are
      given similar service within a switched network.

   -  Subnet:  Used in this memo to indicate a group of L3 devices
      sharing a common L3 network address prefix along with the set of
      segments making up the L2 domain in which they are located.

   -  Bridge/Switch:  A Layer 2 forwarding device as defined by IEEE
      802.1D [2].  The terms bridge and switch are used synonymously in
      this memo.

4. Frame Forwarding in IEEE 802 Networks

4.1. General IEEE 802 Service Model

   The user_priority is a value associated with the transmission and
   reception of all frames in the IEEE 802 service model.  It is
   supplied by the sender that is using the MAC service and is provided
   along with the data to a receiver using the MAC service.  It may or
   may not be actually carried over the network.  Token Ring/IEEE 802.5
   carries this value encoded in its FC octet while basic Ethernet/IEEE
   802.3 does not carry it.  IEEE 802.12 may or may not carry it
   depending on the frame format in use.  When the frame format in use
   is IEEE 802.5, the user_priority is carried explicitly.  When IEEE
   802.3 frame format is used, only the two levels of priority
   (high/low) that are used to determine access priority can be
   recovered.  This is based on the value of priority encoded in the
   start delimiter of the IEEE 802.3 frame.






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