rfc1812.txt

<VC++网络游戏建摸与实现>源代码

   11. REFERENCES .........................................  133
   APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ......  145



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RFC 1812         Requirements for IP Version 4 Routers         June 1995


   APPENDIX B. GLOSSARY ...................................  146
   APPENDIX C. FUTURE DIRECTIONS ..........................  152
   APPENDIX D. Multicast Routing Protocols ................  154
   D.1 Introduction .......................................  154
   D.2 Distance  Vector  Multicast  Routing  Protocol  -
       DVMRP ..............................................  154
   D.3 Multicast Extensions to OSPF - MOSPF ...............  154
   D.4 Protocol Independent Multicast - PIM ...............  155
   APPENDIX E Additional Next-Hop  Selection  Algorithms
        ...................................................  155
   E.1. Some Historical Perspective .......................  155
   E.2. Additional Pruning Rules ..........................  157
   E.3 Some Route Lookup Algorithms .......................  159
   E.3.1 The Revised Classic Algorithm ....................  159
   E.3.2 The Variant Router Requirements Algorithm ........  160
   E.3.3 The OSPF Algorithm ...............................  160
   E.3.4 The Integrated IS-IS Algorithm ...................  162
   Security Considerations ................................  163
   APPENDIX F: HISTORICAL ROUTING PROTOCOLS ...............  164
   F.1 EXTERIOR GATEWAY PROTOCOL - EGP ....................  164
   F.1.1 Introduction .....................................  164
   F.1.2 Protocol Walk-through ............................  165
   F.2 ROUTING INFORMATION PROTOCOL - RIP .................  167
   F.2.1 Introduction .....................................  167
   F.2.2 Protocol Walk-Through ............................  167
   F.2.3 Specific Issues ..................................  172
   F.3 GATEWAY TO GATEWAY PROTOCOL - GGP ..................  173
   Acknowledgments ........................................  173
   Editor's Address .......................................  175

1. INTRODUCTION

  This memo replaces for RFC 1716, "Requirements for Internet Gateways"
  ([INTRO:1]).

  This memo defines and discusses requirements for devices that perform
  the network layer forwarding function of the Internet protocol suite.
  The Internet community usually refers to such devices as IP routers or
  simply routers; The OSI community refers to such devices as
  intermediate systems.  Many older Internet documents refer to these
  devices as gateways, a name which more recently has largely passed out
  of favor to avoid confusion with application gateways.

  An IP router can be distinguished from other sorts of packet switching
  devices in that a router examines the IP protocol header as part of
  the switching process.  It generally removes the Link Layer header a
  message was received with, modifies the IP header, and replaces the
  Link Layer header for retransmission.



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  The authors of this memo recognize, as should its readers, that many
  routers support more than one protocol.  Support for multiple protocol
  suites will be required in increasingly large parts of the Internet in
  the future.  This memo, however, does not attempt to specify Internet
  requirements for protocol suites other than TCP/IP.

  This document enumerates standard protocols that a router connected to
  the Internet must use, and it incorporates by reference the RFCs and
  other documents describing the current specifications for these
  protocols.  It corrects errors in the referenced documents and adds
  additional discussion and guidance for an implementor.

  For each protocol, this memo also contains an explicit set of
  requirements, recommendations, and options.  The reader must
  understand that the list of requirements in this memo is incomplete by
  itself.  The complete set of requirements for an Internet protocol
  router is primarily defined in the standard protocol specification
  documents, with the corrections, amendments, and supplements contained
  in this memo.

  This memo should be read in conjunction with the Requirements for
  Internet Hosts RFCs ([INTRO:2] and [INTRO:3]).  Internet hosts and
  routers must both be capable of originating IP datagrams and receiving
  IP datagrams destined for them.  The major distinction between
  Internet hosts and routers is that routers implement forwarding
  algorithms, while Internet hosts do not require forwarding
  capabilities.  Any Internet host acting as a router must adhere to the
  requirements contained in this memo.

  The goal of open system interconnection dictates that routers must
  function correctly as Internet hosts when necessary.  To achieve this,
  this memo provides guidelines for such instances.  For simplification
  and ease of document updates, this memo tries to avoid overlapping
  discussions of host requirements with [INTRO:2] and [INTRO:3] and
  incorporates the relevant requirements of those documents by
  reference.  In some cases the requirements stated in [INTRO:2] and
  [INTRO:3] are superseded by this document.

  A good-faith implementation of the protocols produced after careful
  reading of the RFCs should differ from the requirements of this memo
  in only minor ways.  Producing such an implementation often requires
  some interaction with the Internet technical community, and must
  follow good communications software engineering practices.  In many
  cases, the requirements in this document are already stated or implied
  in the standard protocol documents, so that their inclusion here is,
  in a sense, redundant.  They were included because some past
  implementation has made the wrong choice, causing problems of
  interoperability, performance, and/or robustness.



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  This memo includes discussion and explanation of many of the
  requirements and recommendations.  A simple list of requirements would
  be dangerous, because:

  o Some required features are more important than others, and some
     features are optional.

  o Some features are critical in some applications of routers but
     irrelevant in others.

  o There may be valid reasons why particular vendor products that are
     designed for restricted contexts might choose to use different
     specifications.

  However, the specifications of this memo must be followed to meet the
  general goal of arbitrary router interoperation across the diversity
  and complexity of the Internet.  Although most current implementations
  fail to meet these requirements in various ways, some minor and some
  major, this specification is the ideal towards which we need to move.

  These requirements are based on the current level of Internet
  architecture.  This memo will be updated as required to provide
  additional clarifications or to include additional information in
  those areas in which specifications are still evolving.

1.1 Reading this Document

1.1.1 Organization

  This memo emulates the layered organization used by [INTRO:2] and
  [INTRO:3].  Thus, Chapter 2 describes the layers found in the Internet
  architecture.  Chapter 3 covers the Link Layer.  Chapters 4 and 5 are
  concerned with the Internet Layer protocols and forwarding algorithms.
  Chapter 6 covers the Transport Layer.  Upper layer protocols are
  divided among Chapters 7, 8, and 9.  Chapter 7 discusses the protocols
  which routers use to exchange routing information with each other.
  Chapter 8 discusses network management.  Chapter 9 discusses other
  upper layer protocols.  The final chapter covers operations and
  maintenance features.  This organization was chosen for simplicity,
  clarity, and consistency with the Host Requirements RFCs.  Appendices
  to this memo include a bibliography, a glossary, and some conjectures
  about future directions of router standards.

  In describing the requirements, we assume that an implementation
  strictly mirrors the layering of the protocols.  However, strict
  layering is an imperfect model, both for the protocol suite and for
  recommended implementation approaches.  Protocols in different layers
  interact in complex and sometimes subtle ways, and particular



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  functions often involve multiple layers.  There are many design
  choices in an implementation, many of which involve creative breaking
  of strict layering.  Every implementor is urged to read [INTRO:4] and
  [INTRO:5].

  Each major section of this memo is organized into the following
  subsections:

  (1) Introduction

  (2) Protocol Walk-Through - considers the protocol specification
       documents section-by-section, correcting errors, stating
       requirements that may be ambiguous or ill-defined, and providing
       further clarification or explanation.

  (3) Specific Issues - discusses protocol design and implementation
       issues that were not included in the walk-through.

  Under many of the individual topics in this memo, there is
  parenthetical material labeled DISCUSSION or IMPLEMENTATION.  This
  material is intended to give a justification, clarification or
  explanation to the preceding requirements text.  The implementation
  material contains suggested approaches that an implementor may want to
  consider.  The DISCUSSION and IMPLEMENTATION sections are not part of
  the standard.

1.1.2 Requirements

  In this memo, the words that are used to define the significance of
  each particular requirement are capitalized.  These words are:

  o MUST
     This word means that the item is an absolute requirement of the
     specification.  Violation of such a requirement is a fundamental
     error; there is no case where it is justified.

  o MUST IMPLEMENT
     This phrase means that this specification requires that the item be
     implemented, but does not require that it be enabled by default.

  o MUST NOT
     This phrase means that the item is an absolute prohibition of the
     specification.

  o SHOULD
     This word means that there may exist valid reasons in particular
     circumstances to ignore this item, but the full implications should
     be understood and the case carefully weighed before choosing a



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     different course.

  o SHOULD IMPLEMENT
     This phrase is similar in meaning to SHOULD, but is used when we
     recommend that a particular feature be provided but does not
     necessarily recommend that it be enabled by default.

  o SHOULD NOT
     This phrase means that there may exist valid reasons in particular
     circumstances when the described behavior is acceptable or even
     useful.  Even so, the full implications should be understood and
     the case carefully weighed before implementing any behavior
     described with this label.

  o MAY
     This word means that this item is truly optional.  One vendor may
     choose to include the item because a particular marketplace
     requires it or because it enhances the product, for example;
     another vendor may omit the same item.

1.1.3 Compliance

  Some requirements are applicable to all routers.  Other requirements
  are applicable only to those which implement particular features or
  protocols.  In the following paragraphs, relevant refers to the union
  of the requirements applicable to all routers and the set of
  requirements applicable to a particular router because of the set of
  features and protocols it has implemented.

  Note that not all Relevant requirements are stated directly in this
  memo.  Various parts of this memo incorporate by reference sections of
  the Host Requirements specification, [INTRO:2] and [INTRO:3].  For
  purposes of determining compliance with this memo, it does not matter
  whether a Relevant requirement is stated directly in this memo or
  merely incorporated by reference from one of those documents.

  An implementation is said to be conditionally compliant if it
  satisfies all the Relevant MUST, MUST IMPLEMENT, and MUST NOT
  requirements.  An implementation is said to be unconditionally
  compliant if it is conditionally compliant and also satisfies all the
  Relevant SHOULD, SHOULD IMPLEMENT, and SHOULD NOT requirements.  An
  implementation is not compliant if it is not conditionally compliant