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RFC 1716


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Towards Requirements for IP Routers

Part 1 of 6, p. 1 to 34
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Network Working Group                                P. Almquist, Author
Request for Comments: 1716                                    Consultant
Category: Informational                            F. Kastenholz, Editor
                                                      FTP Software, Inc.
                                                           November 1994


                  Towards Requirements for IP Routers

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

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Table of Contents


0.  PREFACE .......................................................    1
1.  INTRODUCTION ..................................................    2
1.1  Reading this Document ........................................    4
1.1.1  Organization ...............................................    4
1.1.2  Requirements ...............................................    5
1.1.3  Compliance .................................................    6
1.2  Relationships to Other Standards .............................    7
1.3  General Considerations .......................................    8
1.3.1  Continuing Internet Evolution ..............................    8
1.3.2  Robustness Principle .......................................    9
1.3.3  Error Logging ..............................................    9
1.3.4  Configuration ..............................................   10
1.4  Algorithms ...................................................   11
2.  INTERNET ARCHITECTURE .........................................   13
2.1  Introduction .................................................   13
2.2  Elements of the Architecture .................................   14
2.2.1  Protocol Layering ..........................................   14
2.2.2  Networks ...................................................   16
2.2.3  Routers ....................................................   17
2.2.4  Autonomous Systems .........................................   18
2.2.5  Addresses and Subnets ......................................   18
2.2.6  IP Multicasting ............................................   20
2.2.7  Unnumbered Lines and Networks and Subnets ..................   20
2.2.8  Notable Oddities ...........................................   22
2.2.8.1  Embedded Routers .........................................   22
2.2.8.2  Transparent Routers ......................................   23
2.3  Router Characteristics .......................................   24
2.4  Architectural Assumptions ....................................   27
3.  LINK LAYER ....................................................   29
3.1  INTRODUCTION .................................................   29
3.2  LINK/INTERNET LAYER INTERFACE ................................   29
3.3  SPECIFIC ISSUES ..............................................   30
3.3.1  Trailer Encapsulation ......................................   30
3.3.2  Address Resolution Protocol - ARP ..........................   31
3.3.3  Ethernet and 802.3 Coexistence .............................   31
3.3.4  Maximum Transmission Unit - MTU ............................   31
3.3.5  Point-to-Point Protocol - PPP ..............................   32
3.3.5.1  Introduction .............................................   32
3.3.5.2  Link Control Protocol (LCP) Options ......................   33
3.3.5.3  IP Control Protocol (ICP) Options ........................   34
3.3.6  Interface Testing ..........................................   35
4.  INTERNET LAYER - PROTOCOLS ....................................   36
4.1  INTRODUCTION .................................................   36
4.2  INTERNET PROTOCOL - IP .......................................   36

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4.2.1  INTRODUCTION ...............................................   36
4.2.2  PROTOCOL WALK-THROUGH ......................................   37
4.2.2.1  Options: RFC-791 Section 3.2 .............................   37
4.2.2.2  Addresses in Options: RFC-791 Section 3.1 ................   40
4.2.2.3  Unused IP Header Bits: RFC-791 Section 3.1 ...............   40
4.2.2.4  Type of Service: RFC-791 Section 3.1 .....................   41
4.2.2.5  Header Checksum: RFC-791 Section 3.1 .....................   41
4.2.2.6  Unrecognized Header Options: RFC-791 Section 3.1 .........   41
4.2.2.7  Fragmentation: RFC-791 Section 3.2 .......................   42
4.2.2.8  Reassembly: RFC-791 Section 3.2 ..........................   43
4.2.2.9  Time to Live: RFC-791 Section 3.2 ........................   43
4.2.2.10  Multi-subnet Broadcasts: RFC-922 ........................   43
4.2.2.11  Addressing: RFC-791 Section 3.2 .........................   43
4.2.3  SPECIFIC ISSUES ............................................   47
4.2.3.1  IP Broadcast Addresses ...................................   47
4.2.3.2  IP Multicasting ..........................................   48
4.2.3.3  Path MTU Discovery .......................................   48
4.2.3.4  Subnetting ...............................................   49
4.3  INTERNET CONTROL MESSAGE PROTOCOL - ICMP .....................   50
4.3.1  INTRODUCTION ...............................................   50
4.3.2  GENERAL ISSUES .............................................   50
4.3.2.1  Unknown Message Types ....................................   50
4.3.2.2  ICMP Message TTL .........................................   51
4.3.2.3  Original Message Header ..................................   51
4.3.2.4  ICMP Message Source Address ..............................   51
4.3.2.5  TOS and Precedence .......................................   51
4.3.2.6  Source Route .............................................   52
4.3.2.7  When Not to Send ICMP Errors .............................   53
4.3.2.8  Rate Limiting ............................................   54
4.3.3  SPECIFIC ISSUES ............................................   55
4.3.3.1  Destination Unreachable ..................................   55
4.3.3.2  Redirect .................................................   55
4.3.3.3  Source Quench ............................................   56
4.3.3.4  Time Exceeded ............................................   56
4.3.3.5  Parameter Problem ........................................   57
4.3.3.6  Echo Request/Reply .......................................   57
4.3.3.7  Information Request/Reply ................................   58
4.3.3.8  Timestamp and Timestamp Reply ............................   58
4.3.3.9  Address Mask Request/Reply ...............................   59
4.3.3.10  Router Advertisement and Solicitations ..................   61
4.4  INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ....................   61
5.  INTERNET LAYER - FORWARDING ...................................   62
5.1  INTRODUCTION .................................................   62
5.2  FORWARDING WALK-THROUGH ......................................   62
5.2.1  Forwarding Algorithm .......................................   62
5.2.1.1  General ..................................................   63
5.2.1.2  Unicast ..................................................   64

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5.2.1.3  Multicast ................................................   65
5.2.2  IP Header Validation .......................................   66
5.2.3  Local Delivery Decision ....................................   68
5.2.4  Determining the Next Hop Address ...........................   70
5.2.4.1  Immediate Destination Address ............................   71
5.2.4.2  Local/Remote Decision ....................................   71
5.2.4.3  Next Hop Address .........................................   72
5.2.4.4  Administrative Preference ................................   77
5.2.4.6  Load Splitting ...........................................   78
5.2.5  Unused IP Header Bits: RFC-791 Section 3.1 .................   79
5.2.6  Fragmentation and Reassembly: RFC-791 Section 3.2 ..........   79
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 ..............................................   84
5.3.1  Time to Live (TTL) .........................................   84
5.3.2  Type of Service (TOS) ......................................   85
5.3.3  IP Precedence ..............................................   87
5.3.3.1  Precedence-Ordered Queue Service .........................   88
5.3.3.2  Lower Layer Precedence Mappings ..........................   88
5.3.3.3  Precedence Handling For All Routers ......................   89
5.3.4  Forwarding of Link Layer Broadcasts ........................   92
5.3.5  Forwarding of Internet Layer Broadcasts ....................   92
5.3.5.1  Limited Broadcasts .......................................   94
5.3.5.2  Net-directed Broadcasts ..................................   94
5.3.5.3  All-subnets-directed Broadcasts ..........................   95
5.3.5.4  Subnet-directed Broadcasts ...............................   97
5.3.6  Congestion Control .........................................   97
5.3.7  Martian Address Filtering ..................................   99
5.3.8  Source Address Validation ..................................   99
5.3.9  Packet Filtering and Access Lists ..........................  100
5.3.10  Multicast Routing .........................................  101
5.3.11  Controls on Forwarding ....................................  101
5.3.12  State Changes .............................................  101
5.3.12.1  When a Router Ceases Forwarding .........................  102
5.3.12.2  When a Router Starts Forwarding .........................  102
5.3.12.3  When an Interface Fails or is Disabled ..................  103
5.3.12.4  When an Interface is Enabled ............................  103
5.3.13  IP Options ................................................  103
5.3.13.1  Unrecognized Options ....................................  103
5.3.13.2  Security Option .........................................  104
5.3.13.3  Stream Identifier Option ................................  104
5.3.13.4  Source Route Options ....................................  104
5.3.13.5  Record Route Option .....................................  104
5.3.13.6  Timestamp Option ........................................  105

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6.  TRANSPORT LAYER ...............................................  106
6.1  USER DATAGRAM PROTOCOL - UDP .................................  106
6.2  TRANSMISSION CONTROL PROTOCOL - TCP ..........................  106
7.  APPLICATION LAYER - ROUTING PROTOCOLS .........................  109
7.1  INTRODUCTION .................................................  109
7.1.1  Routing Security Considerations ............................  109
7.1.2  Precedence .................................................  110
7.2  INTERIOR GATEWAY PROTOCOLS ...................................  110
7.2.1  INTRODUCTION ...............................................  110
7.2.2  OPEN SHORTEST PATH FIRST - OSPF ............................  111
7.2.2.1  Introduction .............................................  111
7.2.2.2  Specific Issues ..........................................  111
7.2.2.3  New Version of OSPF ......................................  112
7.2.3  INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM -  DUAL  IS-IS
     ..............................................................  112
7.2.4  ROUTING INFORMATION PROTOCOL - RIP .........................  113
7.2.4.1  Introduction .............................................  113
7.2.4.2  Protocol Walk-Through ....................................  113
7.2.4.3  Specific Issues ..........................................  118
7.2.5  GATEWAY TO GATEWAY PROTOCOL - GGP ..........................  119
7.3  EXTERIOR GATEWAY PROTOCOLS ...................................  119
7.3.1  INTRODUCTION ...............................................  119
7.3.2  BORDER GATEWAY PROTOCOL - BGP ..............................  120
7.3.2.1  Introduction .............................................  120
7.3.2.2  Protocol Walk-through ....................................  120
7.3.3  EXTERIOR GATEWAY PROTOCOL - EGP ............................  121
7.3.3.1  Introduction .............................................  121
7.3.3.2  Protocol Walk-through ....................................  122
7.3.4  INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL ..............  124
7.4  STATIC ROUTING ...............................................  125
7.5  FILTERING OF ROUTING INFORMATION .............................  127
7.5.1  Route Validation ...........................................  127
7.5.2  Basic Route Filtering ......................................  127
7.5.3  Advanced Route Filtering ...................................  128
7.6  INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE ..................  129
8.  APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS ..............  131
8.1  The Simple Network Management Protocol - SNMP ................  131
8.1.1  SNMP Protocol Elements .....................................  131
8.2  Community Table ..............................................  132
8.3  Standard MIBS ................................................  133
8.4  Vendor Specific MIBS .........................................  134
8.5  Saving Changes ...............................................  135
9.  APPLICATION LAYER - MISCELLANEOUS PROTOCOLS ...................  137
9.1  BOOTP ........................................................  137
9.1.1  Introduction ...............................................  137
9.1.2  BOOTP Relay Agents .........................................  137
10.  OPERATIONS AND MAINTENANCE ...................................  139

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10.1  Introduction ................................................  139
10.2  Router Initialization .......................................  140
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

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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.

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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.

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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.

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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

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      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.

      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, but 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.

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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 of 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
      of 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).

      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 as long as the option has a default
      setting, and that leaving the option at its default setting causes
      the router to operate in a manner which conforms to the
      requirement.

      Likewise, routers may provide, except where explicitly prohibited
      by this memo, options which cause them to violate MUST or MUST NOT
      requirements.  A router which provides such options is compliant
      (either fully or conditionally) if and only if each such option
      has a default setting which 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

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      support costs of providing options which 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
   current 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 1610, [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, 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 (December, 1993) the current versions of these
        documents are RFC 1122 and RFC 1123, (STD 3) [INTRO:2], and
        [INTRO:3] respectively.

     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.

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

     The InterNIC
     DS.INTERNIC.NET
     InterNIC Directory and Database Service

     +1 (800) 444-4345 or +1 (619) 445-4600

     info@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 and additional
      internet-layer protocols are also constantly being devised.
      Because routers play such a crucial role in the Internet, and
      because the number of routers deployed in the Internet is much
      smaller than the number of hosts, vendors should 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.

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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; sooner or later a packet will come in with
      that particular combination of errors and attributes, and unless
      the software is prepared, chaos can ensue.  In general, 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 shows up.  An undefined code might be
      logged, but it must not cause a failure.

      The second part of the principle 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 misfeatures in their Internet protocol software.
      As a result of complexity, diversity, and distribution of
      function, the diagnosis of problems is often very difficult.

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      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
      in fact 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.

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      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 which produce the same
   results as these algorithms, but may be more efficient or less
   general.

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

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2.  INTERNET ARCHITECTURE

This chapter does not contain any requirements.  However, it does
contain useful background information on the general architecture of the
Internet and of routers.

General background and discussion on the Internet architecture and
supporting protocol suite can be found in the DDN Protocol Handbook
[ARCH:1]; for background see for example [ARCH:2], [ARCH:3], and
[ARCH:4].  The Internet architecture and protocols are also covered in
an ever-growing number of textbooks, such as [ARCH:5] and [ARCH:6].

2.1  Introduction

   The Internet system consists of a number of interconnected packet
   networks supporting communication among host computers using the
   Internet protocols.  These protocols include the Internet Protocol
   (IP), the Internet Control Message Protocol (ICMP), the Internet
   Group Management Protocol (IGMP), and a variety transport and
   application protocols that depend upon them.  As was described in
   Section [1.2], the Internet Engineering Steering Group periodically
   releases an Official Protocols memo listing all of the Internet
   protocols.

   All Internet protocols use IP as the basic data transport mechanism.
   IP is a datagram, or connectionless, internetwork service and
   includes provision for addressing, type-of-service specification,
   fragmentation and reassembly, and security.  ICMP and IGMP are
   considered integral parts of IP, although they are architecturally
   layered upon IP.  ICMP provides error reporting, flow control,
   first-hop router redirection, and other maintenance and control
   functions.  IGMP provides the mechanisms by which hosts and routers
   can join and leave IP multicast groups.

   Reliable data delivery is provided in the Internet protocol suite by
   Transport Layer protocols such as the Transmission Control Protocol
   (TCP), which provides end-end retransmission, resequencing and
   connection control.  Transport Layer connectionless service is
   provided by the User Datagram Protocol (UDP).

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2.2  Elements of the Architecture


2.2.1  Protocol Layering

      To communicate using the Internet system, a host must implement
      the layered set of protocols comprising the Internet protocol
      suite.  A host typically must implement at least one protocol from
      each layer.

      The protocol layers used in the Internet architecture are as
      follows [ARCH:7]:

      o  Application Layer
         The Application Layer is the top layer of the Internet protocol
         suite.  The Internet suite does not further subdivide the
         Application Layer, although some application layer protocols do
         contain some internal sub-layering.  The application layer of
         the Internet suite essentially combines the functions of the
         top two layers - Presentation and Application - of the OSI
         Reference Model [ARCH:8].  The Application Layer in the
         Internet protocol suite also includes some of the function
         relegated to the Session Layer in the OSI Reference Model.

         We distinguish two categories of application layer protocols:
         user protocols that provide service directly to users, and
         support protocols that provide common system functions.  The
         most common Internet user protocols are:
         - Telnet (remote login)
         - FTP (file transfer)
         - SMTP (electronic mail delivery)

         There are a number of other standardized user protocols and
         many private user protocols.

         Support protocols, used for host name mapping, booting, and
         management, include SNMP, BOOTP, TFTP, the Domain Name System
         (DNS) protocol, and a variety of routing protocols.

         Application Layer protocols relevant to routers are discussed
         in chapters 7, 8, and 9 of this memo.

      o  Transport Layer
         The Transport Layer provides end-to-end communication services.
         This layer is roughly equivalent to the Transport Layer in the
         OSI Reference Model, except that it also incorporates some of
         OSI's Session Layer establishment and destruction functions.

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         There are two primary Transport Layer protocols at present:
         - Transmission Control Protocol (TCP)
         - User Datagram Protocol (UDP)

         TCP is a reliable connection-oriented transport service that
         provides end-to-end reliability, resequencing, and flow
         control.  UDP is a connectionless (datagram) transport service.
         Other transport protocols have been developed by the research
         community, and the set of official Internet transport protocols
         may be expanded in the future.

         Transport Layer protocols relevant to routers are discussed in
         Chapter 6.

      o  Internet Layer
         All Internet transport protocols use the Internet Protocol (IP)
         to carry data from source host to destination host.  IP is a
         connectionless or datagram internetwork service, providing no
         end-to-end delivery guarantees. IP datagrams may arrive at the
         destination host damaged, duplicated, out of order, or not at
         all.  The layers above IP are responsible for reliable delivery
         service when it is required.  The IP protocol includes
         provision for addressing, type-of-service specification,
         fragmentation and reassembly, and security.

         The datagram or connectionless nature of IP is a fundamental
         and characteristic feature of the Internet architecture.

         The Internet Control Message Protocol (ICMP) is a control
         protocol that is considered to be an integral part of IP,
         although it is architecturally layered upon IP, i.e., it uses
         IP to carry its data end-to-end.  ICMP provides error
         reporting, congestion reporting, and first-hop router
         redirection.

         The Internet Group Management Protocol (IGMP) is an Internet
         layer protocol used for establishing dynamic host groups for IP
         multicasting.

         The Internet layer protocols IP, ICMP, and IGMP are discussed
         in chapter 4.

      o  Link Layer
         To communicate on its directly-connected network, a host must
         implement the communication protocol used to interface to that
         network.  We call this a Link Layer layer protocol.

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         Some older Internet documents refer to this layer as the
         Network Layer, but it is not the same as the Network Layer in
         the OSI Reference Model.

         This layer contains everything below the Internet Layer.

         Protocols in this Layer are generally outside the scope of
         Internet standardization; the Internet (intentionally) uses
         existing standards whenever possible.  Thus, Internet Link
         Layer standards usually address only address resolution and
         rules for transmitting IP packets over specific Link Layer
         protocols.  Internet Link Layer standards are discussed in
         chapter 3.

2.2.2  Networks

      The constituent networks of the Internet system are required to
      provide only packet (connectionless) transport.  According to the
      IP service specification, datagrams can be delivered out of order,
      be lost or duplicated, and/or contain errors.

      For reasonable performance of the protocols that use IP (e.g.,
      TCP), the loss rate of the network should be very low.  In
      networks providing connection-oriented service, the extra
      reliability provided by virtual circuits enhances the end-end
      robustness of the system, but is not necessary for Internet
      operation.

      Constituent networks may generally be divided into two classes:

        o  Local-Area Networks (LANs)
           LANs may have a variety of designs.  In general, a LAN will
           cover a small geographical area (e.g., a single building or
           plant site) and provide high bandwidth with low delays.  LANs
           may be passive (similar to Ethernet) or they may be active
           (such as ATM).

        o  Wide-Area Networks (WANs)
           Geographically-dispersed hosts and LANs are interconnected by
           wide-area networks, also called long-haul networks.  These
           networks may have a complex internal structure of lines and
           packet-switches, or they may be as simple as point-to-point
           lines.

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2.2.3  Routers

      In the Internet model, constituent networks are connected together
      by IP datagram forwarders which are called routers or IP routers.
      In this document, every use of the term router is equivalent to IP
      router.  Many older Internet documents refer to routers as
      gateways.

      Historically, routers have been realized with packet-switching
      software executing on a general-purpose CPU.  However, as custom
      hardware development becomes cheaper and as higher throughput is
      required, but special-purpose hardware is becoming increasingly
      common.  This specification applies to routers regardless of how
      they are implemented.

      A router is connected to two or more networks, appearing to each
      of these networks as a connected host.  Thus, it has (at least)
      one physical interface and (at least) one IP address on each of
      the connected networks (this ignores the concept of un-numbered
      links, which is discussed in section [2.2.7]).  Forwarding an IP
      datagram generally requires the router to choose the address of
      the next-hop router or (for the final hop) the destination host.
      This choice, called routing, depends upon a routing database
      within the router.  The routing database is also sometimes known
      as a routing table or forwarding table.

      The routing database should be maintained dynamically to reflect
      the current topology of the Internet system.  A router normally
      accomplishes this by participating in distributed routing and
      reachability algorithms with other routers.

      Routers provide datagram transport only, and they seek to minimize
      the state information necessary to sustain this service in the
      interest of routing flexibility and robustness.

      Packet switching devices may also operate at the Link Layer; such
      devices are usually called bridges. Network segments which are
      connected by bridges share the same IP network number, i.e., they
      logically form a single IP network.  These other devices are
      outside of the scope of this document.

      Another variation on the simple model of networks connected with
      routers sometimes occurs: a set of routers may be interconnected
      with only serial lines, to form a network in which the packet
      switching is performed at the Internetwork (IP) Layer rather than
      the Link Layer.

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2.2.4  Autonomous Systems

      For technical, managerial, and sometimes political reasons, the
      routers of the Internet system are grouped into collections called
      autonomous systems.  The routers included in a single autonomous
      system (AS) are expected to:

      o  Be under the control of a single operations and maintenance
         (O&M) organization;

      o  Employ common routing protocols among themselves, to
         dynamically maintain their routing databases.

      A number of different dynamic routing protocols have been
      developed (see Section [7.2]); the routing protocol within a
      single AS is generically called an interior gateway protocol or
      IGP.

      An IP datagram may have to traverse the routers of two or more ASs
      to reach its destination, and the ASs must provide each other with
      topology information to allow such forwarding.  An exterior
      gateway protocol (generally BGP or EGP) is used for this purpose.

2.2.5  Addresses and Subnets

      An IP datagram carries 32-bit source and destination addresses,
      each of which is partitioned into two parts - a constituent
      network number and a host number on that network.  Symbolically:

         IP-address  ::=  { <Network-number>, <Host-number> }

      To finally deliver the datagram, the last router in its path must
      map the Host-number (or rest) part of an IP address into the
      physical address of a host connection to the constituent network.

      This simple notion has been extended by the concept of subnets,
      which were introduced in order to allow arbitrary complexity of
      interconnected LAN structures within an organization, while
      insulating the Internet system against explosive growth in network
      numbers and routing complexity.  Subnets essentially provide a
      multi-level hierarchical routing structure for the Internet
      system.  The subnet extension, described in [INTERNET:2], is now a
      required part of the Internet architecture.  The basic idea is to
      partition the <Host-number> field into two parts: a subnet number,
      and a true host number on that subnet:

         IP-address  ::=

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           { <Network-number>, <Subnet-number>, <Host-number> }

      The interconnected physical networks within an organization will
      be given the same network number but different subnet numbers.
      The distinction between the subnets of such a subnetted network is
      normally not visible outside of that network.  Thus, routing in
      the rest of the Internet will be based only upon the <Network-
      number> part of the IP destination address; routers outside the
      network will combine <Subnet-number> and <Host-number> together to
      form an uninterpreted rest part of the 32-bit IP address.  Within
      the subnetted network, the routers must route on the basis of an
      extended network number:

         { <Network-number>, <Subnet-number> }

      Under certain circumstances, it may be desirable to support
      subnets of a particular network being interconnected only via a
      path which is not part of the subnetted network.  Even though many
      IGP's and no EGP's currently support this configuration
      effectively, routers need to be able to support this configuration
      of subnetting (see Section [4.2.3.4]).  In general, routers should
      not make assumptions about what are subnets and what are not, but
      simply ignore the concept of Class in networks, and treat each
      route as a { network, mask }-tuple.

      DISCUSSION:
         It is becoming clear that as the Internet grows larger and
         larger, the traditional uses of Class A, B, and C networks will
         be modified in order to achieve better use of IP's 32-bit
         address space.  Classless Interdomain Routing (CIDR)
         [INTERNET:15] is a method currently being deployed in the
         Internet backbones to achieve this added efficiency.  CIDR
         depends on the ability of assigning and routing to networks
         that are not based on Class A, B, or C networks.  Thus, routers
         should always treat a route as a network with a mask.

      Furthermore, for similar reasons, a subnetted network need not
      have a consistent subnet mask through all parts of the network.
      For example, one subnet may use an 8 bit subnet mask, another 10
      bit, and another 6 bit.  Routers need to be able to support this
      type of configuration (see Section [4.2.3.4]).

      The bit positions containing this extended network number are
      indicated by a 32-bit mask called the subnet mask; it is
      recommended but not required that the <Subnet-number> bits be
      contiguous and fall between the <Network-number> and the <Host-
      number> fields.  No subnet should be assigned the value zero or -1

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      (all one bits).

      Although the inventors of the subnet mechanism probably expected
      that each piece of an organization's network would have only a
      single subnet number, in practice it has often proven necessary or
      useful to have several subnets share a single physical cable.

      There are special considerations for the router when a connected
      network provides a broadcast or multicast capability; these will
      be discussed later.

2.2.6  IP Multicasting

      IP multicasting is an extension of Link Layer multicast to IP
      internets.  Using IP multicasts, a single datagram can be
      addressed to multiple hosts. This collection of hosts is called a
      multicast group.  Each multicast group is represented as a Class D
      IP address.  An IP datagram sent to the group is to be delivered
      to each group member with the same best-effort delivery as that
      provided for unicast IP traffic.  The sender of the datagram does
      not itself need to be a member of the destination group.

      The semantics of IP multicast group membership are defined in
      [INTERNET:4].  That document describes how hosts and routers join
      and leave multicast groups.  It also defines a protocol, the
      Internet Group Management Protocol (IGMP), that monitors IP
      multicast group membership.

      Forwarding of IP multicast datagrams is accomplished either
      through static routing information or via a multicast routing
      protocol.  Devices that forward IP multicast datagrams are called
      multicast routers. They may or may not also forward IP unicasts.
      In general, multicast datagrams are forwarded on the basis of both
      their source and destination addresses.  Forwarding of IP
      multicast packets is described in more detail in Section [5.2.1].
      Appendix D discusses multicast routing protocols.

2.2.7  Unnumbered Lines and Networks and Subnets

      Traditionally, each network interface on an IP host or router has
      its own IP address.  Over the years, people have observed that
      this can cause inefficient use of the scarce IP address space,
      since it forces allocation of an IP network number, or at least a
      subnet number, to every point-to-point link.

      To solve this problem, a number of people have proposed and
      implemented the concept of unnumbered serial lines.  An unnumbered

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      serial line does not have any IP network or subnet number
      associated with it.  As a consequence, the network interfaces
      connected to an unnumbered serial line do not have IP addresses.

      Because the IP architecture has traditionally assumed that all
      interfaces had IP addresses, these unnumbered interfaces cause
      some interesting dilemmas.  For example, some IP options (e.g.
      Record Route) specify that a router must insert the interface
      address into the option, but an unnumbered interface has no IP
      address.  Even more fundamental (as we shall see in chapter 5) is
      that routes contain the IP address of the next hop router.  A
      router expects that that IP address will be on an IP (sub)net that
      the router is connected to.  That assumption is of course violated
      if the only connection is an unnumbered serial line.

      To get around these difficulties, two schemes have been invented.
      The first scheme says that two routers connected by an unnumbered
      serial line aren't really two routers at all, but rather two
      half-routers which together make up a single (virtual) router.
      The unnumbered serial line is essentially considered to be an
      internal bus in the virtual router.  The two halves of the virtual
      router must coordinate their activities in such a way that they
      act exactly like a single router.

      This scheme fits in well with the IP architecture, but suffers
      from two important drawbacks.  The first is that, although it
      handles the common case of a single unnumbered serial line, it is
      not readily extensible to handle the case of a mesh of routers and
      unnumbered serial lines.  The second drawback is that the
      interactions between the half routers are necessarily complex and
      are not standardized, effectively precluding the connection of
      equipment from different vendors using unnumbered serial lines.

      Because of these drawbacks, this memo has adopted an alternative
      scheme, which has been invented multiple times but which is
      probably originally attributable to Phil Karn.  In this scheme, a
      router which has unnumbered serial lines also has a special IP
      address, called a router-id in this memo.  The router-id is one of
      the router's IP addresses (a router is required to have at least
      one IP address).  This router-id is used as if it is the IP
      address of all unnumbered interfaces.

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2.2.8  Notable Oddities


2.2.8.1  Embedded Routers

         A router may be a stand-alone computer system, dedicated to its
         IP router functions.  Alternatively, it is possible to embed
         router functions within a host operating system which supports
         connections to two or more networks.  The best-known example of
         an operating system with embedded router code is the Berkeley
         BSD system.  The embedded router feature seems to make
         internetting easy, but it has a number of hidden pitfalls:

         (1)  If a host has only a single constituent-network interface,
              it should not act as a router.

              For example, hosts with embedded router code that
              gratuitously forward broadcast packets or datagrams on the
              same net often cause packet avalanches.

         (2)  If a (multihomed) host acts as a router, it must implement
              ALL the relevant router requirements contained in this
              document.

              For example, the routing protocol issues and the router
              control and monitoring problems are as hard and important
              for embedded routers as for stand-alone routers.

              Since Internet router requirements and specifications may
              change independently of operating system changes, an
              administration that operates an embedded router in the
              Internet is strongly advised to have the ability to
              maintain and update the router code (e.g., this might
              require router code source).

         (3)  Once a host runs embedded router code, it becomes part of
              the Internet system.  Thus, errors in software or
              configuration can hinder communication between other
              hosts.  As a consequence, the host administrator must lose
              some autonomy.

              In many circumstances, a host administrator will need to
              disable router code embedded in the operating system, and
              any embedded router code must be organized so that it can
              be easily disabled.

         (4)  If a host running embedded router code is concurrently

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              used for other services, the O&M (Operation and
              Maintenance) requirements for the two modes of use may be
              in serious conflict.

              For example, router O&M will in many cases be performed
              remotely by an operations center; this may require
              privileged system access which the host administrator
              would not normally want to distribute.

2.2.8.2  Transparent Routers

         There are two basic models for interconnecting local-area
         networks and wide-area (or long-haul) networks in the Internet.
         In the first, the local-area network is assigned a network
         number and all routers in the Internet must know how to route
         to that network.  In the second, the local-area network shares
         (a small part of) the address space of the wide-area network.
         Routers that support this second model are called address
         sharing routers or transparent routers.  The focus of this memo
         is on routers that support the first model, but this is not
         intended to exclude the use of transparent routers.

         The basic idea of a transparent router is that the hosts on the
         local-area network behind such a router share the address space
         of the wide-area network in front of the router.  In certain
         situations this is a very useful approach and the limitations
         do not present significant drawbacks.

         The words in front and behind indicate one of the limitations
         of this approach: this model of interconnection is suitable
         only for a geographically (and topologically) limited stub
         environment.  It requires that there be some form of logical
         addressing in the network level addressing of the wide-area
         network.  All of the IP addresses in the local environment map
         to a few (usually one) physical address in the wide-area
         network.  This mapping occurs in a way consistent with the { IP
         address <-> network address } mapping used throughout the
         wide-area network.

         Multihoming is possible on one wide-area network, but may
         present routing problems if the interfaces are geographically
         or topologically separated.  Multihoming on two (or more)
         wide-area networks is a problem due to the confusion of
         addresses.

         The behavior that hosts see from other hosts in what is
         apparently the same network may differ if the transparent

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         router cannot fully emulate the normal wide-area network
         service.  For example, the ARPANET used a Link Layer protocol
         that provided a Destination Dead indication in response to an
         attempt to send to a host which was powered off.  However, if
         there were a transparent router between the ARPANET and an
         Ethernet, a host on the ARPANET would not receive a Destination
         Dead indication if it sent a datagram to a host that was
         powered off and was connected to the ARPANET via the
         transparent router instead of directly.

2.3  Router Characteristics

   An Internet router performs the following functions:

   (1)  Conforms to specific Internet protocols specified in this
        document, including the Internet Protocol (IP), Internet Control
        Message Protocol (ICMP), and others as necessary.

   (2)  Interfaces to two or more packet networks.  For each connected
        network the router must implement the functions required by that
        network.  These functions typically include:

        o  Encapsulating and decapsulating the IP datagrams with the
           connected network framing (e.g., an Ethernet header and
           checksum),

        o  Sending and receiving IP datagrams up to the maximum size
           supported by that network, this size is the network's Maximum
           Transmission Unit or MTU,

        o  Translating the IP destination address into an appropriate
           network-level address for the connected network (e.g., an
           Ethernet hardware address), if needed, and

        o  Responding to the network flow control and error indication,
           if any.

        See chapter 3 (Link Layer).

   (3)  Receives and forwards Internet datagrams.  Important issues in
        this process are buffer management, congestion control, and
        fairness.

        o  Recognizes various error conditions and generates ICMP error
           and information messages as required.

        o  Drops datagrams whose time-to-live fields have reached zero.

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        o  Fragments datagrams when necessary to fit into the MTU of the
           next network.

        See chapter 4 (Internet Layer - Protocols) and chapter 5
        (Internet Layer - Forwarding) for more information.

   (4)  Chooses a next-hop destination for each IP datagram, based on
        the information in its routing database.  See chapter 5
        (Internet Layer - Forwarding) for more information.

   (5)  (Usually) supports an interior gateway protocol (IGP) to carry
        out distributed routing and reachability algorithms with the
        other routers in the same autonomous system.  In addition, some
        routers will need to support an exterior gateway protocol (EGP)
        to exchange topological information with other autonomous
        systems.  See chapter 7 (Application Layer - Routing Protocols)
        for more information.

   (6)  Provides network management and system support facilities,
        including loading, debugging, status reporting, exception
        reporting and control.  See chapter 8 (Application Layer -
        Network Management Protocols) and chapter 10 (Operation and
        Maintenance) for more information.

   A router vendor will have many choices on power, complexity, and
   features for a particular router product.  It may be helpful to
   observe that the Internet system is neither homogeneous nor fully-
   connected.  For reasons of technology and geography it is growing
   into a global interconnect system plus a fringe of LANs around the
   edge. More and more these fringe LANs are becoming richly
   interconnected, thus making them less out on the fringe and more
   demanding on router requirements.

   o  The global interconnect system is comprised of a number of wide-
      area networks to which are attached routers of several Autonomous
      Systems (AS); there are relatively few hosts connected directly to
      the system.

   o  Most hosts are connected to LANs.  Many organizations have
      clusters of LANs interconnected by local routers.  Each such
      cluster is connected by routers at one or more points into the
      global interconnect system.  If it is connected at only one point,
      a LAN is known as a stub network.

   Routers in the global interconnect system generally require:

   o  Advanced Routing and Forwarding Algorithms

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      These routers need routing algorithms which are highly dynamic and
      also offer type-of-service routing.  Congestion is still not a
      completely resolved issue (see Section [5.3.6]).  Improvements in
      these areas are expected, as the research community is actively
      working on these issues.

   o  High Availability

      These routers need to be highly reliable, providing 24 hours a
      day, 7 days a week service.  Equipment and software faults can
      have a wide-spread (sometimes global) effect.  In case of failure,
      they must recover quickly.  In any environment, a router must be
      highly robust and able to operate, possibly in a degraded state,
      under conditions of extreme congestion or failure of network
      resources.

   o  Advanced O&M Features

      Internet routers normally operate in an unattended mode.  They
      will typically be operated remotely from a centralized monitoring
      center.  They need to provide sophisticated means for monitoring
      and measuring traffic and other events and for diagnosing faults.

   o  High Performance

      Long-haul lines in the Internet today are most frequently 56 Kbps,
      DS1 (1.4Mbps), and DS3 (45Mbps) speeds.  LANs are typically
      Ethernet (10Mbps) and, to a lesser degree, FDDI (100Mbps).
      However, network media technology is constantly advancing and even
      higher speeds are likely in the future.  Full-duplex operation is
      provided at all of these speeds.

   The requirements for routers used in the LAN fringe (e.g., campus
   networks) depend greatly on the demands of the local networks.  These
   may be high or medium-performance devices, probably competitively
   procured from several different vendors and operated by an internal
   organization (e.g., a campus computing center).  The design of these
   routers should emphasize low average latency and good burst
   performance, together with delay and type-of-service sensitive
   resource management. In this environment there may be less formal O&M
   but it will not be less important.  The need for the routing
   mechanism to be highly dynamic will become more important as networks
   become more complex and interconnected.  Users will demand more out
   of their local connections because of the speed of the global
   interconnects.

   As networks have grown, and as more networks have become old enough

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   that they are phasing out older equipment, it has become increasingly
   imperative that routers interoperate with routers from other vendors.

   Even though the Internet system is not fully interconnected, many
   parts of the system need to have redundant connectivity.  Rich
   connectivity allows reliable service despite failures of
   communication lines and routers, and it can also improve service by
   shortening Internet paths and by providing additional capacity.
   Unfortunately, this richer topology can make it much more difficult
   to choose the best path to a particular destination.

2.4  Architectural Assumptions

   The current Internet architecture is based on a set of assumptions
   about the communication system.  The assumptions most relevant to
   routers are as follows:

   o  The Internet is a network of networks.

      Each host is directly connected to some particular network(s); its
      connection to the Internet is only conceptual.  Two hosts on the
      same network communicate with each other using the same set of
      protocols that they would use to communicate with hosts on distant
      networks.

   o  Routers don't keep connection state information.

      To improve the robustness of the communication system, routers are
      designed to be stateless, forwarding each IP packet independently
      of other packets.  As a result, redundant paths can be exploited
      to provide robust service in spite of failures of intervening
      routers and networks.

      All state information required for end-to-end flow control and
      reliability is implemented in the hosts, in the transport layer or
      in application programs.  All connection control information is
      thus co-located with the end points of the communication, so it
      will be lost only if an end point fails.  Routers effect flow
      control only indirectly, by dropping packets or increasing network
      delay.

      Note that future protocol developments may well end up putting
      some more state into routers.  This is especially likely for
      resource reservation and flows.

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   o  Routing complexity should be in the routers.

      Routing is a complex and difficult problem, and ought to be
      performed by the routers, not the hosts.  An important objective
      is to insulate host software from changes caused by the inevitable
      evolution of the Internet routing architecture.

   o  The system must tolerate wide network variation.

      A basic objective of the Internet design is to tolerate a wide
      range of network characteristics - e.g., bandwidth, delay, packet
      loss, packet reordering, and maximum packet size.  Another
      objective is robustness against failure of individual networks,
      routers, and hosts, using whatever bandwidth is still available.
      Finally, the goal is full open system interconnection: an Internet
      router must be able to interoperate robustly and effectively with
      any other router or Internet host, across diverse Internet paths.

      Sometimes implementors have designed for less ambitious goals.
      For example, the LAN environment is typically much more benign
      than the Internet as a whole; LANs have low packet loss and delay
      and do not reorder packets.  Some vendors have fielded
      implementations that are adequate for a simple LAN environment,
      but work badly for general interoperation.  The vendor justifies
      such a product as being economical within the restricted LAN
      market.  However, isolated LANs seldom stay isolated for long;
      they are soon connected to each other, to organization-wide
      internets, and eventually to the global Internet system.  In the
      end, neither the customer nor the vendor is served by incomplete
      or substandard routers.

      The requirements spelled out in this document are designed for a
      full-function router.  It is intended that fully compliant routers
      will be usable in almost any part of the Internet.


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