rfc1716.txt

RFC 的详细文档！

10.2.1  Minimum Router Configuration ..............................  140
10.2.2  Address and Address Mask Initialization ...................  141
10.2.3  Network Booting using BOOTP and TFTP ......................  142
10.3  Operation and Maintenance ...................................  143
10.3.1  Introduction ..............................................  143
10.3.2  Out Of Band Access ........................................  144
10.3.2  Router O&M Functions ......................................  144
10.3.2.1  Maintenance - Hardware Diagnosis ........................  144
10.3.2.2  Control - Dumping and Rebooting .........................  145
10.3.2.3  Control - Configuring the Router ........................  145
10.3.2.4  Netbooting of System Software ...........................  146
10.3.2.5  Detecting and responding to misconfiguration ............  146
10.3.2.6  Minimizing Disruption ...................................  147
10.3.2.7  Control - Troubleshooting Problems ......................  148
10.4  Security Considerations .....................................  149
10.4.1  Auditing and Audit Trails .................................  149
10.4.2  Configuration Control .....................................  150
11.  REFERENCES ...................................................  152
APPENDIX  A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ................  162
APPENDIX  B. GLOSSARY .............................................  164
APPENDIX  C. FUTURE DIRECTIONS ....................................  169
APPENDIX D.  Multicast Routing Protocols ..........................  172
D.1  Introduction .................................................  172
D.2  Distance Vector Multicast Routing Protocol - DVMRP ...........  172
D.3  Multicast Extensions to OSPF - MOSPF .........................  173
APPENDIX E  Additional Next-Hop Selection Algorithms ..............  174
E.1. Some Historical Perspective ..................................  174
E.2. Additional Pruning Rules .....................................  176
E.3  Some Route Lookup Algorithms .................................  177
E.3.1 The Revised Classic Algorithm ...............................  178
E.3.2 The Variant Router Requirements Algorithm ...................  179
E.3.3 The OSPF Algorithm ..........................................  179
E.3.4 The Integrated IS-IS Algorithm ..............................  180
Security Considerations ...........................................  182
Acknowledgments ...................................................  183
Editor's Address ..................................................  186










Almquist & Kastenholz                                          [Page vi]

RFC 1716          Towards Requirements for IP Routers      November 1994


0.  PREFACE

This document is a snapshot of the work of the Router Requirements
working group as of November 1991.  At that time, the working group had
essentially finished its task.  There were some final technical matters
to be nailed down, and a great deal of editing needed to be done in
order to get the document ready for publication.  Unfortunately, these
tasks were never completed.

At the request of the Internet Area Director, the current editor took
the last draft of the document and, after consulting the mailing list
archives, meeting minutes, notes, and other members of the working
group, edited the document to its current form.  This effort included
the following tasks: 1) Deleting all the parenthetical material (such as
editor's comments). Useful information was turned into DISCUSSION
sections, the rest was deleted.  2) Completing the tasks listed in the
last draft's To be Done sections. As a part of this task, a new "to be
done" list was developed and included as an appendix to the current
document.  3) Rolling Philip Almquist's "Ruminations on the Next Hop"
and "Ruminations on Route Leaking" into this document.  These represent
significant work and should be kept.  4) Fulfilling the last intents of
the working group as determined from the archival material.  The intent
of this effort was to get the document into a form suitable for
publication as an Historical RFC so that the significant work which went
into the creation of this document would be preserved.

The content and form of this document are due, in large part, to the
working group's chair, and document's original editor and author: Philip
Almquist.  Without his efforts, this document would not exist.



















Almquist & Kastenholz                                           [Page 1]

RFC 1716          Towards Requirements for IP Routers      November 1994


1.  INTRODUCTION

The goal of this work is to replace RFC-1009, Requirements for Internet
Gateways ([INTRO:1]) with a new document.

This memo is an intermediate step toward that goal. It defines and
discusses requirements for devices which 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 has to modify the IP header and to
strip off and replace the Link Layer framing.

The authors of this memo recognize, as should its readers, that many
routers support multiple protocol suites, and that 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 final version of 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 are required to implement forwarding
algorithms and Internet hosts do not require forwarding capabilities.
Any Internet host acting as a router must adhere to the requirements
contained in the final version of this memo.


Almquist & Kastenholz                                           [Page 2]

RFC 1716          Towards Requirements for IP Routers      November 1994


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 the final version of this document.

A good-faith implementation of the protocols produced after careful
reading of the RFCs, with some interaction with the Internet technical
community, and that follows good communications software engineering
practices, should differ from the requirements of this memo in only
minor ways.  Thus, 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.  However, they were
included because some past implementation has made the wrong choice,
causing problems of interoperability, performance, and/or robustness.

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.





Almquist & Kastenholz                                           [Page 3]

RFC 1716          Towards Requirements for IP Routers      November 1994


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 between Chapter 7, which
      discusses the protocols which routers use to exchange routing
      information with each other, Chapter 8, which discusses network
      management, and Chapter 9, which 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 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].

      In general, 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


Almquist & Kastenholz                                           [Page 4]

RFC 1716          Towards Requirements for IP Routers      November 1994


      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