Network Working Group                                   F. Baker, Editor
Request for Comments: 1812                                 Cisco Systems
Obsoletes: 1716, 1009                                          June 1995
Category: Standards Track


                 Requirements for IP Version 4 Routers

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

PREFACE

   This document is an updated version of RFC 1716, the historical
   Router Requirements document.  That RFC preserved the significant
   work that went into the working group, but failed to adequately
   describe current technology for the IESG to consider it a current
   standard.

   The current editor had been asked to bring the document up to date,
   so that it is useful as a procurement specification and a guide to
   implementors.  In this, he stands squarely on the shoulders of those
   who have gone before him, and depends largely on expert contributors
   for text.  Any credit is theirs; the errors are his.

   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.  It is also largely due to the efforts of its
   previous editor, Frank Kastenholz.  Without their efforts, this
   document would not exist.

Table of Contents

   1. INTRODUCTION ........................................    6
   1.1 Reading this Document ..............................    8
   1.1.1 Organization .....................................    8
   1.1.2 Requirements .....................................    9
   1.1.3 Compliance .......................................   10
   1.2 Relationships to Other Standards ...................   11
   1.3 General Considerations .............................   12
   1.3.1 Continuing Internet Evolution ....................   12
   1.3.2 Robustness Principle .............................   13
   1.3.3 Error Logging ....................................   14

   1.3.4 Configuration ....................................   14
   1.4 Algorithms .........................................   16
   2. INTERNET ARCHITECTURE ...............................   16
   2.1 Introduction .......................................   16
   2.2 Elements of the Architecture .......................   17
   2.2.1 Protocol Layering ................................   17
   2.2.2 Networks .........................................   19
   2.2.3 Routers ..........................................   20
   2.2.4 Autonomous Systems ...............................   21
   2.2.5 Addressing Architecture ..........................   21
   2.2.5.1 Classical IP Addressing Architecture ...........   21
   2.2.5.2 Classless Inter Domain Routing (CIDR) ..........   23
   2.2.6 IP Multicasting ..................................   24
   2.2.7 Unnumbered Lines and Networks Prefixes ...........   25
   2.2.8 Notable Oddities .................................   26
   2.2.8.1 Embedded Routers ...............................   26
   2.2.8.2 Transparent Routers ............................   27
   2.3 Router Characteristics .............................   28
   2.4 Architectural Assumptions ..........................   31
   3. LINK LAYER ..........................................   32
   3.1 INTRODUCTION .......................................   32
   3.2 LINK/INTERNET LAYER INTERFACE ......................   33
   3.3 SPECIFIC ISSUES ....................................   34
   3.3.1 Trailer Encapsulation ............................   34
   3.3.2 Address Resolution Protocol - ARP ................   34
   3.3.3 Ethernet and 802.3 Coexistence ...................   35
   3.3.4 Maximum Transmission Unit - MTU ..................   35
   3.3.5 Point-to-Point Protocol - PPP ....................   35
   3.3.5.1 Introduction ...................................   36
   3.3.5.2 Link Control Protocol (LCP) Options ............   36
   3.3.5.3 IP Control Protocol (IPCP) Options .............   38
   3.3.6 Interface Testing ................................   38
   4. INTERNET LAYER - PROTOCOLS ..........................   39
   4.1 INTRODUCTION .......................................   39
   4.2 INTERNET PROTOCOL - IP .............................   39
   4.2.1 INTRODUCTION .....................................   39
   4.2.2 PROTOCOL WALK-THROUGH ............................   40
   4.2.2.1 Options: RFC 791 Section 3.2 ...................   40
   4.2.2.2 Addresses in Options: RFC 791 Section 3.1 ......   42
   4.2.2.3 Unused IP Header Bits: RFC 791 Section 3.1 .....   43
   4.2.2.4 Type of Service: RFC 791 Section 3.1 ...........   44
   4.2.2.5 Header Checksum: RFC 791 Section 3.1 ...........   44
   4.2.2.6 Unrecognized Header Options: RFC 791,
           Section 3.1 ....................................   44
   4.2.2.7 Fragmentation: RFC 791 Section 3.2 .............   45
   4.2.2.8 Reassembly: RFC 791 Section 3.2 ................   46
   4.2.2.9 Time to Live: RFC 791 Section 3.2 ..............   46
   4.2.2.10 Multi-subnet Broadcasts: RFC 922 ..............   47

   4.2.2.11 Addressing: RFC 791 Section 3.2 ...............   47
   4.2.3 SPECIFIC ISSUES ..................................   50
   4.2.3.1 IP Broadcast Addresses .........................   50
   4.2.3.2 IP Multicasting ................................   50
   4.2.3.3 Path MTU Discovery .............................   51
   4.2.3.4 Subnetting .....................................   51
   4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP ...........   52
   4.3.1 INTRODUCTION .....................................   52
   4.3.2 GENERAL ISSUES ...................................   53
   4.3.2.1 Unknown Message Types ..........................   53
   4.3.2.2 ICMP Message TTL ...............................   53
   4.3.2.3 Original Message Header ........................   53
   4.3.2.4 ICMP Message Source Address ....................   53
   4.3.2.5 TOS and Precedence .............................   54
   4.3.2.6 Source Route ...................................   54
   4.3.2.7 When Not to Send ICMP Errors ...................   55
   4.3.2.8 Rate Limiting ..................................   56
   4.3.3 SPECIFIC ISSUES ..................................   56
   4.3.3.1 Destination Unreachable ........................   56
   4.3.3.2 Redirect .......................................   57
   4.3.3.3 Source Quench ..................................   57
   4.3.3.4 Time Exceeded ..................................   58
   4.3.3.5 Parameter Problem ..............................   58
   4.3.3.6 Echo Request/Reply .............................   58
   4.3.3.7 Information Request/Reply ......................   59
   4.3.3.8 Timestamp and Timestamp Reply ..................   59
   4.3.3.9 Address Mask Request/Reply .....................   61
   4.3.3.10 Router Advertisement and Solicitations ........   62
   4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ..........   62
   5. INTERNET LAYER - FORWARDING .........................   63
   5.1 INTRODUCTION .......................................   63
   5.2 FORWARDING WALK-THROUGH ............................   63
   5.2.1 Forwarding Algorithm .............................   63
   5.2.1.1 General ........................................   64
   5.2.1.2 Unicast ........................................   64
   5.2.1.3 Multicast ......................................   65
   5.2.2 IP Header Validation .............................   67
   5.2.3 Local Delivery Decision ..........................   69
   5.2.4 Determining the Next Hop Address .................   71
   5.2.4.1 IP Destination Address .........................   72
   5.2.4.2 Local/Remote Decision ..........................   72
   5.2.4.3 Next Hop Address ...............................   74
   5.2.4.4 Administrative Preference ......................   77
   5.2.4.5 Load Splitting .................................   79
   5.2.5 Unused IP Header Bits: RFC-791 Section 3.1 .......   79
   5.2.6 Fragmentation and Reassembly:  RFC-791,
         Section 3.2 ......................................   80
   5.2.7 Internet Control Message Protocol - ICMP .........   80

   5.2.7.1 Destination Unreachable ........................   80
   5.2.7.2 Redirect .......................................   82
   5.2.7.3 Time Exceeded ..................................   84
   5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ........   84
   5.3 SPECIFIC ISSUES ....................................   85
   5.3.1 Time to Live (TTL) ...............................   85
   5.3.2 Type of Service (TOS) ............................   86
   5.3.3 IP Precedence ....................................   87
   5.3.3.1 Precedence-Ordered Queue Service ...............   88
   5.3.3.2 Lower Layer Precedence Mappings ................   89
   5.3.3.3 Precedence Handling For All Routers ............   90
   5.3.4 Forwarding of Link Layer Broadcasts ..............   92
   5.3.5 Forwarding of Internet Layer Broadcasts ..........   92
   5.3.5.1 Limited Broadcasts .............................   93
   5.3.5.2 Directed Broadcasts ............................   93
   5.3.5.3 All-subnets-directed Broadcasts ................   94
   5.3.5.4  Subnet-directed Broadcasts ....................   94
   5.3.6 Congestion Control ...............................   94
   5.3.7 Martian Address Filtering ........................   96
   5.3.8 Source Address Validation ........................   97
   5.3.9 Packet Filtering and Access Lists ................   97
   5.3.10 Multicast Routing ...............................   98
   5.3.11 Controls on Forwarding ..........................   98
   5.3.12 State Changes ...................................   99
   5.3.12.1 When a Router Ceases Forwarding ...............   99
   5.3.12.2 When a Router Starts Forwarding ...............  100
   5.3.12.3 When an Interface Fails or is Disabled ........  100
   5.3.12.4 When an Interface is Enabled ..................  100
   5.3.13 IP Options ......................................  101
   5.3.13.1 Unrecognized Options ..........................  101
   5.3.13.2 Security Option ...............................  101
   5.3.13.3 Stream Identifier Option ......................  101
   5.3.13.4 Source Route Options ..........................  101
   5.3.13.5 Record Route Option ...........................  102
   5.3.13.6 Timestamp Option ..............................  102
   6. TRANSPORT LAYER .....................................  103
   6.1 USER DATAGRAM PROTOCOL - UDP .......................  103
   6.2 TRANSMISSION CONTROL PROTOCOL - TCP ................  104
   7. APPLICATION LAYER - ROUTING PROTOCOLS ...............  106
   7.1 INTRODUCTION .......................................  106
   7.1.1 Routing Security Considerations ..................  106
   7.1.2 Precedence .......................................  107
   7.1.3 Message Validation ...............................  107
   7.2 INTERIOR GATEWAY PROTOCOLS .........................  107
   7.2.1 INTRODUCTION .....................................  107
   7.2.2 OPEN SHORTEST PATH FIRST - OSPF ..................  108
   7.2.3 INTERMEDIATE SYSTEM TO  INTERMEDIATE  SYSTEM  -
         DUAL IS-IS .......................................  108

   7.3  EXTERIOR GATEWAY PROTOCOLS ........................  109
   7.3.1  INTRODUCTION ....................................  109
   7.3.2 BORDER GATEWAY PROTOCOL - BGP ....................  109
   7.3.2.1 Introduction ...................................  109
   7.3.2.2 Protocol Walk-through ..........................  110
   7.3.3 INTER-AS ROUTING WITHOUT AN  EXTERIOR  PROTOCOL
         ..................................................  110
   7.4 STATIC ROUTING .....................................  111
   7.5 FILTERING OF ROUTING INFORMATION ...................  112
   7.5.1 Route Validation .................................  113
   7.5.2 Basic Route Filtering ............................  113
   7.5.3 Advanced Route Filtering .........................  114
   7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE ........  114
   8. APPLICATION LAYER - NETWORK  MANAGEMENT  PROTOCOLS
      .....................................................  115
   8.1 The Simple Network Management Protocol - SNMP ......  115
   8.1.1 SNMP Protocol Elements ...........................  115
   8.2 Community Table ....................................  116
   8.3 Standard MIBS ......................................  118
   8.4 Vendor Specific MIBS ...............................  119
   8.5 Saving Changes .....................................  120
   9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS .........  120
   9.1 BOOTP ..............................................  120
   9.1.1 Introduction .....................................  120
   9.1.2 BOOTP Relay Agents ...............................  121
   10. OPERATIONS AND MAINTENANCE .........................  122
   10.1 Introduction ......................................  122
   10.2 Router Initialization .............................  123
   10.2.1 Minimum Router Configuration ....................  123
   10.2.2 Address and Prefix Initialization ...............  124
   10.2.3 Network Booting using BOOTP and TFTP ............  125
   10.3 Operation and Maintenance .........................  126
   10.3.1 Introduction ....................................  126
   10.3.2 Out Of Band Access ..............................  127
   10.3.3 Router O&M Functions ............................  127
   10.3.3.1 Maintenance - Hardware Diagnosis ..............  127
   10.3.3.2 Control - Dumping and Rebooting ...............  127
   10.3.3.3 Control - Configuring the Router ..............  128
   10.3.3.4 Net Booting of System Software ................  128
   10.3.3.5 Detecting and responding to misconfiguration
            ...............................................  129
   10.3.3.6 Minimizing Disruption .........................  130
   10.3.3.7 Control - Troubleshooting Problems ............  130
   10.4 Security Considerations ...........................  131
   10.4.1 Auditing and Audit Trails .......................  131
   10.4.2 Configuration Control ...........................  132
   11. REFERENCES .........................................  133
   APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ......  145

   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.

  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.

  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

  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

     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
  (i.e., it fails to satisfy one or more of the Relevant MUST, MUST
  IMPLEMENT, or MUST NOT requirements).

  This specification occasionally indicates that an implementation
  SHOULD implement a management variable, and that it SHOULD have a
  certain default value.  An unconditionally compliant implementation

  implements the default behavior, and if there are other implemented
  behaviors implements the variable.  A conditionally compliant
  implementation clearly documents what the default setting of the
  variable is or, in the absence of the implementation of a variable,
  may be construed to be.  An implementation that both fails to
  implement the variable and chooses a different behavior is not
  compliant.

  For any of the SHOULD and SHOULD NOT requirements, a router may
  provide a configuration option that will cause the router to act other
  than as specified by the requirement.  Having such a configuration
  option does not void a router's claim to unconditional compliance if
  the option has a default setting, and that setting causes the router
  to operate in the required manner.

  Likewise, routers may provide, except where explicitly prohibited by
  this memo, options which cause them to violate MUST or MUST NOT
  requirements.  A router that provides such options is compliant
  (either fully or conditionally) if and only if each such option has a
  default setting that causes the router to conform to the requirements
  of this memo.  Please note that the authors of this memo, although
  aware of market realities, strongly recommend against provision of
  such options.  Requirements are labeled MUST or MUST NOT because
  experts in the field have judged them to be particularly important to
  interoperability or proper functioning in the Internet.  Vendors
  should weigh carefully the customer support costs of providing options
  that violate those rules.

  Of course, this memo is not a complete specification of an IP router,
  but rather is closer to what in the OSI world is called a profile.
  For example, this memo requires that a number of protocols be
  implemented.  Although most of the contents of their protocol
  specifications are not repeated in this memo, implementors are
  nonetheless required to implement the protocols according to those
  specifications.

1.2 Relationships to Other Standards

  There are several reference documents of interest in checking the
  status of protocol specifications and standardization:

    o INTERNET OFFICIAL PROTOCOL STANDARDS
       This document describes the Internet standards process and lists
       the standards status of the protocols.  As of this writing, the
       current version of this document is STD 1, RFC 1780, [ARCH:7].
       This document is periodically re-issued.  You should always
       consult an RFC repository and use the latest version of this
       document.

    o Assigned Numbers
       This document lists the assigned values of the parameters used in
       the various protocols.  For example, it lists IP protocol codes,
       TCP port numbers, Telnet Option Codes, ARP hardware types, and
       Terminal Type names.  As of this writing, the current version of
       this document is STD 2, RFC 1700, [INTRO:7].  This document is
       periodically re-issued.  You should always consult an RFC
       repository and use the latest version of this document.

    o Host Requirements
       This pair of documents reviews the specifications that apply to
       hosts and supplies guidance and clarification for any
       ambiguities.  Note that these requirements also apply to routers,
       except where otherwise specified in this memo.  As of this
       writing, the current versions of these documents are RFC 1122 and
       RFC 1123 (STD 3), [INTRO:2] and [INTRO:3].

    o Router Requirements (formerly Gateway Requirements)
       This memo.

   Note that these documents are revised and updated at different times;
   in case of differences between these documents, the most recent must
   prevail.

   These and other Internet protocol documents may be obtained from the:

                               The InterNIC
                              DS.INTERNIC.NET
                  InterNIC Directory and Database Service
                             info@internic.net
                              +1-908-668-6587
                       URL: http://ds.internic.net/

1.3 General Considerations

   There are several important lessons that vendors of Internet software
   have learned and which a new vendor should consider seriously.

1.3.1 Continuing Internet Evolution

   The enormous growth of the Internet has revealed problems of
   management and scaling in a large datagram based packet communication
   system.  These problems are being addressed, and as a result there
   will be continuing evolution of the specifications described in this
   memo.  New routing protocols, algorithms, and architectures are
   constantly being developed.  New internet layer protocols, and
   modifications to existing protocols, are also constantly being
   devised.  Routers play a crucial role in the Internet, and the number

   of routers deployed in the Internet is much smaller than the number
   of hosts.  Vendors should therefore expect that router standards will
   continue to evolve much more quickly than host standards.  These
   changes will be carefully planned and controlled since there is
   extensive participation in this planning by the vendors and by the
   organizations responsible for operation of the networks.

   Development, evolution, and revision are characteristic of computer
   network protocols today, and this situation will persist for some
   years.  A vendor who develops computer communications software for
   the Internet protocol suite (or any other protocol suite!) and then
   fails to maintain and update that software for changing
   specifications is going to leave a trail of unhappy customers.  The
   Internet is a large communication network, and the users are in
   constant contact through it.  Experience has shown that knowledge of
   deficiencies in vendor software propagates quickly through the
   Internet technical community.

1.3.2 Robustness Principle

   At every layer of the protocols, there is a general rule (from
   [TRANS:2] by Jon Postel) whose application can lead to enormous
   benefits in robustness and interoperability:

                      Be conservative in what you do,
                be liberal in what you accept from others.

   Software should be written to deal with every conceivable error, no
   matter how unlikely.  Eventually a packet will come in with that
   particular combination of errors and attributes, and unless the
   software is prepared, chaos can ensue.  It is best to assume that the
   network is filled with malevolent entities that will send packets
   designed to have the worst possible effect.  This assumption will
   lead to suitably protective design.  The most serious problems in the
   Internet have been caused by unforeseen mechanisms triggered by low
   probability events; mere human malice would never have taken so
   devious a course!

   Adaptability to change must be designed into all levels of router
   software.  As a simple example, consider a protocol specification
   that contains an enumeration of values for a particular header field
   - e.g., a type field, a port number, or an error code; this
   enumeration must be assumed to be incomplete.  If the protocol
   specification defines four possible error codes, the software must
   not break when a fifth code is defined.  An undefined code might be
   logged, but it must not cause a failure.

   The second part of the principal is almost as important: software on
   hosts or other routers may contain deficiencies that make it unwise
   to exploit legal but obscure protocol features.  It is unwise to
   stray far from the obvious and simple, lest untoward effects result
   elsewhere.  A corollary of this is watch out for misbehaving hosts;
   router software should be prepared to survive in the presence of
   misbehaving hosts.  An important function of routers in the Internet
   is to limit the amount of disruption such hosts can inflict on the
   shared communication facility.

1.3.3 Error Logging

   The Internet includes a great variety of systems, each implementing
   many protocols and protocol layers, and some of these contain bugs
   and misguided features in their Internet protocol software.  As a
   result of complexity, diversity, and distribution of function, the
   diagnosis of problems is often very difficult.

   Problem diagnosis will be aided if routers include a carefully
   designed facility for logging erroneous or strange events.  It is
   important to include as much diagnostic information as possible when
   an error is logged.  In particular, it is often useful to record the
   header(s) of a packet that caused an error.  However, care must be
   taken to ensure that error logging does not consume prohibitive
   amounts of resources or otherwise interfere with the operation of the
   router.

   There is a tendency for abnormal but harmless protocol events to
   overflow error logging files; this can be avoided by using a circular
   log, or by enabling logging only while diagnosing a known failure.
   It may be useful to filter and count duplicate successive messages.
   One strategy that seems to work well is to both:

   o Always count abnormalities and make such counts accessible through
      the management protocol (see Chapter 8); and
   o Allow the logging of a great variety of events to be selectively
      enabled.  For example, it might useful to be able to log
      everything or to log everything for host X.

   This topic is further discussed in [MGT:5].

1.3.4 Configuration

   In an ideal world, routers would be easy to configure, and perhaps
   even entirely self-configuring.  However, practical experience in the
   real world suggests that this is an impossible goal, and that many
   attempts by vendors to make configuration easy actually cause
   customers more grief than they prevent.  As an extreme example, a

   router designed to come up and start routing packets without
   requiring any configuration information at all would almost certainly
   choose some incorrect parameter, possibly causing serious problems on
   any networks unfortunate enough to be connected to it.

   Often this memo requires that a parameter be a configurable option.
   There are several reasons for this.  In a few cases there currently
   is some uncertainty or disagreement about the best value and it may
   be necessary to update the recommended value in the future.  In other
   cases, the value really depends on external factors - e.g., the
   distribution of its communication load, or the speeds and topology of
   nearby networks - and self-tuning algorithms are unavailable and may
   be insufficient.  In some cases, configurability is needed because of
   administrative requirements.

   Finally, some configuration options are required to communicate with
   obsolete or incorrect implementations of the protocols, distributed
   without sources, that persist in many parts of the Internet.  To make
   correct systems coexist with these faulty systems, administrators
   must occasionally misconfigure the correct systems.  This problem
   will correct itself gradually as the faulty systems are retired, but
   cannot be ignored by vendors.

   When we say that a parameter must be configurable, we do not intend
   to require that its value be explicitly read from a configuration
   file at every boot time.  For many parameters, there is one value
   that is appropriate for all but the most unusual situations.  In such
   cases, it is quite reasonable that the parameter default to that
   value if not explicitly set.

   This memo requires a particular value for such defaults in some
   cases.  The choice of default is a sensitive issue when the
   configuration item controls accommodation of existing, faulty,
   systems.  If the Internet is to converge successfully to complete
   interoperability, the default values built into implementations must
   implement the official protocol, not misconfigurations to accommodate
   faulty implementations.  Although marketing considerations have led
   some vendors to choose misconfiguration defaults, we urge vendors to
   choose defaults that will conform to the standard.

   Finally, we note that a vendor needs to provide adequate
   documentation on all configuration parameters, their limits and
   effects.

1.4 Algorithms

   In several places in this memo, specific algorithms that a router
   ought to follow are specified.  These algorithms are not, per se,
   required of the router.  A router need not implement each algorithm
   as it is written in this document.  Rather, an implementation must
   present a behavior to the external world that is the same as a
   strict, literal, implementation of the specified algorithm.

   Algorithms are described in a manner that differs from the way a good
   implementor would implement them.  For expository purposes, a style
   that emphasizes conciseness, clarity, and independence from
   implementation details has been chosen.  A good implementor will
   choose algorithms and implementation methods that produce the same
   results as these algorithms, but may be more efficient or less
   general.

   We note that the art of efficient router implementation is outside
   the scope of this memo.