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Versions: 00 01 02 RFC 1812

                        INTERNET DRAFT

            Requirements for IP Version 4 Routers


                        17 March 1995
                    Document Revision 2.05                      |
                 draft-ietf-rreq-cidr-02.txt                    |
                        Revision Date:
                           3/17/95                              |

                     Fred Baker (Editor)

                        Cisco Systems
                        519 Lado Drive
               Santa Barbara, California 93111

                        fred@cisco.com






Status of this Memo

This document is an Internet Draft.  Internet Drafts are
working documents of the Internet Engineering Task Force
(IETF), its Areas, and its Working Groups.  Note that other
groups may also distribute working documents as Internet
Drafts.

Internet Drafts are draft documents valid for a maximum of six
months.  Internet Drafts may be updated, replaced, or
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appropriate to use Internet Drafts as reference material or to
cite them other than as a ``working draft'' or ``work in
progress.'' Please check the 1id-abstracts.txt listing
contained in the internet-drafts Shadow Directories on
nic.ddn.mil, venera.isi.edu, nnsc.nsf.net, nic.nordu.net,
ftp.nisc.sri.com, or munnari.oz.au to learn the current status
of any Internet Draft.










Draft       Requirements for IP Version 4 Routers   March 1995




This is a working document only, it should neither be cited
nor quoted in any formal document.

This document will expire before 22 Sep. 1995.

Distribution of this document is unlimited.

Please send comments to The editor or the Router Requirements
Working Group (rreq@isi.edu).

If your comment pertains to a particular piece of text, please
remember to mention the section number.  This document is very
large and locating the text solely by context might not be
possible.  Please also mention the date of this draft           |
(3/17/95) and the revision level (2.05).
































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0. 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.





























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1. INTRODUCTION

The 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





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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:

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

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

+ There may be valid reasons why particular vendor products
   that are designed for restricted contexts might choose to





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   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





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      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:

      + "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.





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

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

      + "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.

      + "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.

      + "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.

      + "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





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      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.





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      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:

     + 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 [ARCH:7].  This document is periodically re-issued.
        You should always consult an RFC repository and use
        the latest version of this document.

     + 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,





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        Telnet Option Codes, ARP hardware types, and Terminal
        Type names.  As of this writing, the current version
        of this document is [INTRO:7].  This document is
        periodically re-issued.  You should always consult an
        RFC repository and use the latest version of this
        document.

     + 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 [INTRO:2], and
        [INTRO:3].

     + 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:
                  DDN Network Information Center
                     14200 Park Meadow Drive,
                             Suite 200
                            Chantilly,
                             VA 22021
                                USA

                        nic@ds.internic.net

                               (800)
                            365-3642 or                         |
                               (703)
                             802-4535


1.3 General Considerations

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





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   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:





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                  "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





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      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:
      + Always count abnormalities and make such counts
         accessible through the management protocol (see
         Chapter 8); and
      + 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





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      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.





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      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.























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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 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





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   User Datagram Protocol (UDP).


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]:

      + 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,





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         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.

      + 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.

         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.

      + 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





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         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 - 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.

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

         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 and "above" the Physical Layer (which is the
         media connectivity, normally electrical or optical,
         which encodes and transports messages).  Its
         responsibility is the correct delivery of messages,
         among which it does not differentiate.

         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.








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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:

        + Local-Area Networks (LANs)
           LANs may have a variety of designs.  LANs normally
           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).

        + 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.


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-





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

      A router connects to two or more logical interfaces,
      represented by IP subnets or unnumbered point to point
      lines (discussed in section [2.2.7]).  Thus, it has at
      least one physical interface.  Forwarding an IP datagram
      generally requires the router to choose the address and
      relevant interface of the next-hop router or (for the
      final hop) the destination host.  This choice, called
      "relaying" or "forwarding depends upon a route database
      within the router.  The route database is also called a
      routing table or forwarding table.  The term "router"
      derives from the process of building this route
      database; routing protocols and configuration interact
      in a process called "routing".

      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 that are connected by bridges share the
      same IP network prefix forming a single IP subnet.
      These other devices are outside the scope of this
      document.


2.2.4 Autonomous Systems

      An Autonomous System (AS) is a connected segment of a     |
      network topology that consists of a collection of         |





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      subnetworks (with hosts attached) interconnected by a     |
      set of routes.  The subnetworks and the routers are       |
      expected to be under the control of a single operations   |
      and maintenance (O&M) organization.  Within an AS         |
      routers may use one or more interior routing protocols,   |
      and sometimes several sets of metrics.  An AS is          |
      expected to present to other ASs an appearence of a       |
      coherent interior routing plan, and a consistent picture  |
      of the destinations reachable through the AS.  An AS is   |
      identified by an Autonomous System number.


      The concept of an AS plays an important role in the       |
      Internet routing (see Section 7.1).


2.2.5 Addressing Architecture

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

         IP-address ::= { <Network-prefix>, <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 to the host's Link Layer address.


2.2.5.1 Classical IP Addressing Architecture

         Although well documented elsewhere [INTERNET:2], it
         is useful to describe the historical use of the
         network prefix.  The language developed to describe
         it is used in this and other documents and permeates
         the thinking behind many protocols.

         The simplest classical network prefix is the Class A,
         B, C, D, or E network prefix.  These address ranges
         are discriminated by observing the values of the most
         significant bits of the address, and break the
         address into simple prefix and host number fields.
         This is described in [INTERNET:18].  In short, the





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         classification is:

              0xxx - Class A - general purpose unicast
              addresses with standard 8 bit prefix
              10xx - Class B - general purpose unicast
              addresses with standard 16 bit prefix
              110x - Class C - general purpose unicast
              addresses with standard 24 bit prefix
              1110 - Class D - IP Multicast Addresses - 28 bit
              prefix, non-aggregatable
              1111 - Class E - reserved for experimental use

         This simple notion has been extended by the concept
         of "subnets".  These were introduced to allow
         arbitrary complexity of interconnected LAN structures
         within an organization, while insulating the Internet
         system against explosive growth in assigned network
         prefixes and routing complexity.  Subnets provide a
         multi-level hierarchical routing structure for the
         Internet system.  The subnet extension, described in
         [INTERNET:2], is 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 ::=
              { <Network-number>, <Subnet-number>, <Host-
              number> }

         The interconnected physical networks within an
         organization use the same network prefix but
         different subnet numbers.  The distinction between
         the subnets of such a subnetted network is not
         normally visible outside of that network.  Thus,
         routing in the rest of the Internet uses only the
         <Network-prefix> part of the IP destination address.
         Routers outside the network treat <Network-prefix>
         and <Host-number> together as an uninterpreted "rest"
         part of the 32-bit IP address.  Within the subnetted
         network, the routers use the extended network prefix:

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

         The bit positions containing this extended network





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         number have historically been indicated by a 32-bit
         mask called the "subnet mask".  The <Subnet-number>
         bits SHOULD be contiguous and fall between the
         <Network-number> and the <Host-number> fields.  More
         up to date protocols do not refer to a subnet mask,
         but to a "prefix length"; the "prefix" portion of an
         address is that which would be selected by a subnet
         mask whose most significant bits are all ones and the
         rest are zeroes.  The length of the prefix equals the
         number of ones in the subnet mask.  This document
         assumes that all subnet masks are expressible as
         prefix lengths.

         The inventors of the subnet mechanism presumed 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.  For this
         reason, routers should be capable of configuring
         multiple subnets on the same physical interfaces, and
         treat them (from a routing or forwarding perspective)
         as though they were distinct physical interfaces.



2.2.5.2 Classless Inter Domain Routing (CIDR)

         The explosive growth of the Internet has forced a
         review of address assignment policies.  The
         traditional uses of general purpose (Class A, B, and
         C) networks have been modified to achieve better use
         of IP's 32-bit address space.  Classless Inter Domain
         Routing (CIDR) [INTERNET:15] is a method currently
         being deployed in the Internet backbones to achieve
         this added efficiency.  CIDR depends on deploying and
         routing to arbitrarily sized networks.  In this
         model, hosts and routers make no assumptions about
         the use of addressing in the internet.  The Class D
         (IP Multicast) and Class E (Experimental) address
         spaces are preserved, although this is primarily an
         assignment policy.

         By definition, CIDR comprises three elements:
           + topologically significant address assignment,





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           + routing protocols that are capable of aggregating
              network layer reachability information, and
           + consistent forwarding algorithm ("longest
              match").

         The use of networks and subnets is now historical,
         although the language used to describe them remains
         in current use.  They have been replaced by the more
         tractable concept of a "network prefix".  A network
         prefix is, by definition, a contiguous set of bits at
         the more significant end of the address that defines
         a set of systems; host numbers select among those
         systems.  There is no requirement that all the
         internet use network prefixes uniformly.  To collapse
         routing information, it is useful to divide the
         internet into addressing domains.  Within such a
         domain, detailed information is available about
         constituent networks; outside it, only the common
         network prefix is advertised.

         The classical IP addressing architecture used
         addresses and subnet masks to discriminate the host
         number from the network prefix.  With network
         prefixes, it is sufficient to indicate the number of
         bits in the prefix.  Both representations are in
         common use.  Architecturally correct subnet masks are
         capable of being represented using the prefix length
         description.  They comprise that subset of all
         possible bits patterns that have
           + a contiguous string of ones at the more
              significant end,
           + a contiguous string of zeros at the less
              significant end, and
           + no intervening bits.

         Routers SHOULD always treat a route as a network
         prefix, and SHOULD reject configuration and routing
         information inconsistent with that model.

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

         An effect of the use of CIDR is that the set of
         destinations associated with address prefixes in the
         routing table may exhibit subset relationship.  A





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         route describing a smaller set of destinations (a
         longer prefix) is said to be more specific than a
         route describing a larger set of destinations (a
         shorter prefix); similarly, a route describing a
         larger set of destinations (a shorter prefix) is said
         to be less specific than a route describing a smaller
         set of destinations (a longer prefix).  Routers must
         use the most specific matching route (the longest
         matching network prefix) when forwarding traffic.


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 without sending it to
      all.  In the extended case, these hosts may reside in
      different address domains.  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.  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.






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2.2.7 Unnumbered Lines and Networks Prefixes

      Traditionally, each network interface on an IP host or
      router has its own IP address.  This can cause
      inefficient use of the scarce IP address space, since it
      forces allocation of an IP network prefix to every
      point-to-point link.

      To solve this problem, a number of people have proposed
      and implemented the concept of "unnumbered point to
      point lines".  An unnumbered point to point line does
      not have any network prefix associated with it.  As a
      consequence, the network interfaces connected to an
      unnumbered point to point 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 this IP address will be
      on an IP (sub)net to which the router is connected.
      That assumption is of course violated if the only
      connection is an unnumbered point to point line.

      To get around these difficulties, two schemes have been
      conceived.  The first scheme says that two routers
      connected by an unnumbered point to point line are not
      really two routers at all, but rather two "half-routers"
      that together make up a single virtual router.  The
      unnumbered point to point 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 point to point line, it is not readily
      extensible to handle the case of a mesh of routers and





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      unnumbered point to point 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 point to point lines.

      Because of these drawbacks, this memo has adopted an
      alternate scheme, which has been invented multiple times
      but which is probably originally attributable to Phil
      Karn.  In this scheme, a router that has unnumbered
      point to point 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.


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 that 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 building a network 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 is
              subject to the requirements for routers
              contained in this document.





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              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.

              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 maintain and update the router code.
              This might require router source code.

         (3) When a host executes embedded router code, it
              becomes part of the Internet infrastructure.
              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.  For this reason, it should be
              straightforward to disable embedded router
              functionality.

         (4) When a host running embedded router code is
              concurrently used for other services, the
              Operation and Maintenance requirements for the
              two modes of use may conflict.

              For example, router O&M will in many cases be
              performed remotely by an operations center; this
              may require privileged system access that 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 prefix and all
         routers in the Internet must know how to route to





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         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.  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 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 that was off-line.
         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 for





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         Ethernet hosts.


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:

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

        + 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",

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

        + Responding to network flow control and error
           indications, 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.

        + Recognizes error conditions and generates ICMP error
           and information messages as required.

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





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        + 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.

   + The global interconnect system is composed 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.

   + 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





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      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:

   + Advanced Routing and Forwarding Algorithms

      These routers need routing algorithms that are highly
      dynamic, impose minimal processing and communication
      burdens, and 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.

   + 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.

   + 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.

   + High Performance

      Long-haul lines in the Internet today are most
      frequently full duplex 56 KBPS, DS1 (1.544 Mbps), or DS3
      (45 Mbps) speeds.  LANs, which are half duplex
      multiaccess media, are typically Ethernet (10Mbps) and,
      to a lesser degree, FDDI (100Mbps).  However, network
      media technology is constantly advancing and higher





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      speeds are likely in the future.

   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 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:

   + The Internet is a network of networks.

      Each host is directly connected to some particular
      network(s); its connection to the Internet is only





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      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.

   + Routers do not 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 control message
      flow 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 multicast routing, resource
      reservation, and flow based forwarding.

   + 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.

   + 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.





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      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 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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3. LINK LAYER

Although [INTRO:1] covers Link Layer standards (IP over
various link layers, ARP, etc.), this document anticipates
that Link-Layer material will be covered in a separate Link
Layer Requirements document.  A Link-Layer Requirements
document would be applicable to both hosts and routers.  Thus,
this document will not obsolete the parts of [INTRO:1] that
deal with link-layer issues.


3.1 INTRODUCTION

   Routers have essentially the same Link Layer protocol
   requirements as other sorts of Internet systems.  These
   requirements are given in chapter 3 of "Requirements for
   Internet Gateways" [INTRO:1].  A router MUST comply with
   its requirements and SHOULD comply with its
   recommendations.  Since some of the material in that
   document has become somewhat dated, some additional
   requirements and explanations are included below.

   DISCUSSION:
      It is expected that the Internet community will produce
      a "Requirements for Internet Link Layer" standard which
      will supersede both this chapter and the chapter
      entitled "INTERNET LAYER PROTOCOLS" in [INTRO:1].



3.2 LINK/INTERNET LAYER INTERFACE

   This document does not attempt to specify the interface
   between the Link Layer and the upper layers.  However, note
   well that other parts of this document, particularly
   chapter 5, require various sorts of information to be
   passed across this layer boundary.

   This section uses the following definitions:

   + Source physical address

      The source physical address is the Link Layer address of
      the host or router from which the packet was received.





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   + Destination physical address

      The destination physical address is the Link Layer
      address to which the packet was sent.

   The information that must pass from the Link Layer to the
   Internetwork Layer for each received packet is:

   (1) The IP packet [5.2.2],

   (2) The length of the data portion (i.e., not including the
        Link-Layer framing) of the Link Layer frame [5.2.2],

   (3) The identity of the physical interface from which the
        IP packet was received [5.2.3], and

   (4) The classification of the packet's destination physical
        address as a Link Layer unicast, broadcast, or
        multicast [4.3.2], [5.3.4].

   In addition, the Link Layer also should provide:

   (5) The source physical address.

   The information that must pass from the Internetwork Layer
   to the Link Layer for each transmitted packet is:

   (1) The IP packet [5.2.1]

   (2) The length of the IP packet [5.2.1]

   (3) The destination physical interface [5.2.1]

   (4) The next hop IP address [5.2.1]

   In addition, the Internetwork Layer also should provide:

   (5) The Link Layer priority value [5.3.3.2]

   The Link Layer must also notify the Internetwork Layer if
   the packet to be transmitted causes a Link Layer
   precedence-related error [5.3.3.3].







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3.3 SPECIFIC ISSUES



3.3.1 Trailer Encapsulation

      Routers that can connect to ten megabit Ethernets MAY be
      able to receive and forward Ethernet packets
      encapsulated using the trailer encapsulation described
      in [LINK:1].  However, a router SHOULD NOT originate
      trailer encapsulated packets.  A router MUST NOT
      originate trailer encapsulated packets without first
      verifying, using the mechanism described in [INTRO:2],
      that the immediate destination of the packet is willing
      and able to accept trailer-encapsulated packets.  A
      router SHOULD NOT agree (using these mechanisms) to
      accept trailer-encapsulated packets.


3.3.2 Address Resolution Protocol - ARP

      Routers that implement ARP MUST be compliant and SHOULD
      be unconditionally compliant with the requirements in
      [INTRO:2].

      The link layer MUST NOT report a Destination Unreachable
      error to IP solely because there is no ARP cache entry
      for a destination; it SHOULD queue up to a small number
      of datagrams breifly while performing the ARP
      request/reply sequence, and reply that the destination
      is unreachable to one of the queued datagrams only when
      this proves fruitless.

      A router MUST not believe any ARP reply that claims that
      the Link Layer address of another host or router is a
      broadcast or multicast address.


3.3.3 Ethernet and 802.3 Coexistence

      Routers that can connect to ten megabit Ethernets MUST
      be compliant and SHOULD be unconditionally compliant
      with the Ethernet requirements of [INTRO:2].






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3.3.4 Maximum Transmission Unit - MTU

      The MTU of each logical interface MUST be configurable
      within the range of legal MTUs for the interface.

      Many Link Layer protocols define a maximum frame size
      that may be sent.  In such cases, a router MUST NOT
      allow an MTU to be set which would allow sending of
      frames larger than those allowed by the Link Layer
      protocol.  However, a router SHOULD be willing to
      receive a packet as large as the maximum frame size even
      if that is larger than the MTU.

      DISCUSSION:
         Note that this is a stricter requirement than imposed
         on hosts by [INTRO:2], which requires that the MTU of
         each physical interface be configurable.

         If a network is using an MTU smaller than the maximum
         frame size for the Link Layer, a router may receive
         packets larger than the MTU from misconfigured and
         incompletely initialized hosts.  The Robustness
         Principle indicates that the router should
         successfully receive these packets if possible.



3.3.5 Point-to-Point Protocol - PPP

      Contrary to [INTRO:1], the Internet does have a standard
      point to point line protocol: the Point-to-Point
      Protocol (PPP), defined in [LINK:2], [LINK:3], [LINK:4],
      and [LINK:5].

      A "point to point interface" is any interface that is
      designed to send data over a point to point line.  Such
      interfaces include telephone, leased, dedicated or
      direct lines (either 2 or 4 wire), and may use point to
      point channels or virtual circuits of multiplexed
      interfaces such as ISDN.  They normally use a
      standardized modem or bit serial interface (such as RS-
      232, RS-449 or V.35), using either synchronous or
      asynchronous clocking.  Multiplexed interfaces often
      have special physical interfaces.





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      A "general purpose serial interface" uses the same
      physical media as a point to point line, but supports
      the use of link layer networks as well as point to point
      connectivity.  Link layer networks (such as X.25 or
      Frame Relay) use an alternative IP link layer
      specification.

      Routers that implement point to point or general purpose
      serial interfaces MUST IMPLEMENT PPP.

      PPP MUST be supported on all general purpose serial
      interfaces on a router.  The router MAY allow the line
      to be configured to use point to point line protocols
      other than PPP.  Point to point interfaces SHOULD either
      default to using PPP when enabled or require
      configuration of the link layer protocol before being
      enabled.  General purpose serial interfaces SHOULD
      require configuration of the link layer protocol before
      being enabled.


3.3.5.1 Introduction

         This section provides guidelines to router
         implementors so that they can ensure interoperability
         with other routers using PPP over either synchronous
         or asynchronous links.

         It is critical that an implementor understand the
         semantics of the option negotiation mechanism.
         Options are a means for a local device to indicate to
         a remote peer what the local device will accept from
         the remote peer, not what it wishes to send.  It is
         up to the remote peer to decide what is most
         convenient to send within the confines of the set of
         options that the local device has stated that it can
         accept.  Therefore it is perfectly acceptable and
         normal for a remote peer to ACK all the options
         indicated in an LCP Configuration Request (CR) even
         if the remote peer does not support any of those
         options.  Again, the options are simply a mechanism
         for either device to indicate to its peer what it
         will accept, not necessarily what it will send.






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3.3.5.2 Link Control Protocol (LCP) Options

         The PPP Link Control Protocol (LCP) offers a number
         of options that may be negotiated.  These options
         include (among others) address and control field
         compression, protocol field compression, asynchronous
         character map, Maximum Receive Unit (MRU), Link
         Quality Monitoring (LQM), magic number (for loopback
         detection), Password Authentication Protocol (PAP),
         Challenge Handshake Authentication Protocol (CHAP),
         and the 32-bit Frame Check Sequence (FCS).

         A router MAY use address/control field compression on
         either synchronous or asynchronous links.  A router
         MAY use protocol field compression on either
         synchronous or asynchronous links.  A router that
         indicates that it can accept these compressions MUST
         be able to accept uncompressed PPP header information
         also.

         DISCUSSION:
            These options control the appearance of the PPP
            header.  Normally the PPP header consists of the
            address, the control field, and the protocol
            field.  The address, on a point to point line, is
            0xFF, indicating "broadcast".  The control field
            is 0x03, indicating "Unnumbered Information." The
            Protocol Identifier is a two byte value indicating
            the contents of the data area of the frame.  If a
            system negotiates address and control field
            compression it indicates to its peer that it will
            accept PPP frames that have or do not have these
            fields at the front of the header.  It does not
            indicate that it will be sending frames with these
            fields removed.

            Protocol field compression, when negotiated,
            indicates that the system is willing to receive
            protocol fields compressed to one byte when this
            is legal.  There is no requirement that the sender
            do so.

            Use of address/control field compression is
            inconsistent with the use of numbered mode





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            (reliable) PPP.


         IMPLEMENTATION:
            Some hardware does not deal well with variable
            length header information.  In those cases it
            makes most sense for the remote peer to send the
            full PPP header.  Implementations may ensure this
            by not sending the address/control field and
            protocol field compression options to the remote
            peer.  Even if the remote peer has indicated an
            ability to receive compressed headers there is no
            requirement for the local router to send
            compressed headers.

         A router MUST negotiate the Asynchronous Control
         Character Map (ACCM) for asynchronous PPP links, but
         SHOULD NOT negotiate the ACCM for synchronous links.
         If a router receives an attempt to negotiate the ACCM
         over a synchronous link, it MUST ACKnowledge the
         option and then ignore it.

         DISCUSSION:
            There are implementations that offer both
            synchronous and asynchronous modes of operation
            and may use the same code to implement the option
            negotiation.  In this situation it is possible
            that one end or the other may send the ACCM option
            on a synchronous link.

         A router SHOULD properly negotiate the maximum
         receive unit (MRU).  Even if a system negotiates an
         MRU smaller than 1,500 bytes, it MUST be able to
         receive a 1,500 byte frame.

         A router SHOULD negotiate and enable the link quality
         monitoring (LQM) option.

         DISCUSSION:
            This memo does not specify a policy for deciding
            whether the link's quality is adequate.  However,
            it is important (see Section [3.3.6]) that a
            router disable failed links.






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         A router SHOULD implement and negotiate the magic
         number option for loopback detection.

         A router MAY support the authentication options (PAP
         - Password Authentication Protocol, and/or CHAP -
         Challenge Handshake Authentication Protocol).

         A router MUST support 16-bit CRC frame check sequence
         (FCS) and MAY support the 32-bit CRC.


3.3.5.3 IP Control Protocol (IPCP) Options

         A router MAY offer to perform IP address negotiation.
         A router MUST accept a refusal (REJect) to perform IP
         address negotiation from the peer.

         Routers operating at link speeds of 19,200 BPS or
         less SHOULD implement and offer to perform Van
         Jacobson header compression.  Routers that implement
         VJ compression SHOULD implement an administrative
         control enabling or disabling it.


3.3.6 Interface Testing

      A router MUST have a mechanism to allow routing software
      to determine whether a physical interface is available
      to send packets or not; on multiplexed interfaces where
      permanent virtual circuits are opened for limited sets
      of neighbors, the router must also be able to determine
      whether the virtual circuits are viable.  A router
      SHOULD have a mechanism to allow routing software to
      judge the quality of a physical interface.  A router
      MUST have a mechanism for informing the routing software
      when a physical interface becomes available or
      unavailable to send packets because of administrative
      action.  A router MUST have a mechanism for informing
      the routing software when it detects a Link level
      interface has become available or unavailable, for any
      reason.

      DISCUSSION:
         It is crucial that routers have workable mechanisms





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         for determining that their network connections are
         functioning properly.  Failure to detect link loss,
         or failure to take the proper actions when a problem
         is detected, can lead to black holes.

         The mechanisms available for detecting problems with
         network connections vary considerably, depending on
         the Link Layer protocols in use and the interface
         hardware.  The intent is to maximize the capability
         to detect failures within the Link-Layer constraints.







































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4. INTERNET LAYER - PROTOCOLS



4.1 INTRODUCTION

   This chapter and chapter 5 discuss the protocols used at
   the Internet Layer: IP, ICMP, and IGMP.  Since forwarding
   is obviously a crucial topic in a document discussing
   routers, chapter 5 limits itself to the aspects of the
   protocols that directly relate to forwarding.  The current
   chapter contains the remainder of the discussion of the
   Internet Layer protocols.


4.2 INTERNET PROTOCOL - IP



4.2.1 INTRODUCTION

      Routers MUST implement the IP protocol, as defined by
      [INTERNET:1].  They MUST also implement its mandatory
      extensions: subnets (defined in [INTERNET:2]), IP
      broadcast (defined in [INTERNET:3]), and Classless
      Inter-Domain Routing (CIDR, defined in [INTERNET:15]).

      Router implementors need not consider compliance with
      the section of [INTRO:2] entitled "Internet Protocol --
      IP," as that section is entirely duplicated or
      superseded in this document.  A router MUST be
      compliant, and SHOULD be unconditionally compliant, with
      the requirements of the section entitled "SPECIFIC
      ISSUES" relating to IP in [INTRO:2].

      In the following, the action specified in certain cases
      is to "silently discard" a received datagram.  This
      means that the datagram will be discarded without
      further processing and that the router will not send any
      ICMP error message (see Section [4.3]) as a result.
      However, for diagnosis of problems a router SHOULD
      provide the capability of logging the error (see Section
      [1.3.3]), including the contents of the silently
      discarded datagram, and SHOULD count datagrams





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      discarded.


4.2.2 PROTOCOL WALK-THROUGH

      RFC 791 [INTERNET:1] is the specification for the
      Internet Protocol.


4.2.2.1 Options: RFC 791 Section 3.2

         In datagrams received by the router itself, the IP
         layer MUST interpret IP options that it understands
         and preserve the rest unchanged for use by higher
         layer protocols.

         Higher layer protocols may require the ability to set
         IP options in datagrams they send or examine IP
         options in datagrams they receive.  Later sections of
         this document discuss specific IP option support
         required by higher layer protocols.

         DISCUSSION:
            Neither this memo nor [INTRO:2] define the order
            in which a receiver must process multiple options
            in the same IP header.  Hosts and routers
            originating datagrams containing multiple options
            must be aware that this introduces an ambiguity in
            the meaning of certain options when combined with
            a source-route option.

         Here are the requirements for specific IP options:

         (a) Security Option

              Some environments require the Security option in
              every packet originated or received.  Routers
              SHOULD IMPLEMENT the revised security option
              described in [INTERNET:5].

              DISCUSSION:
                 Note that the security options described in
                 [INTERNET:1] and RFC 1038 ([INTERNET:16]) are
                 obsolete.





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         (b) Stream Identifier Option

              This option is obsolete; routers SHOULD NOT
              place this option in a datagram that the router
              originates.  This option MUST be ignored in
              datagrams received by the router.

         (c) Source Route Options

              A router MUST be able to act as the final
              destination of a source route.  If a router
              receives a packet containing a completed source
              route, the packet has reached its final
              destination.  In such an option, the pointer
              points beyond the last field and the destination
              address in the IP header addresses the router.
              The option as received (the recorded route) MUST
              be passed up to the transport layer (or to ICMP
              message processing).

              In the general case, a correct response to a
              source-routed datagram traverses the same route.
              A router MUST provide a means whereby transport
              protocols and applications can reverse the
              source route in a received datagram.  This
              reversed source route MUST be inserted into
              datagrams they originate (see [INTRO:2] for
              details) when the router is unaware of policy
              constraints.  However, if the router is policy
              aware, it MAY select another path.

              Some applications in the router MAY require that
              the user be able to enter a source route.

              A router MUST NOT originate a datagram
              containing multiple source route options.  What
              a router should do if asked to forward a packet
              containing multiple source route options is
              described in Section [5.2.4.1].

              When a source route option is created (which
              would happen when the router is originating a
              source routed datagram or is inserting a source
              route option as a result of a special filter),





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              it MUST be correctly formed even if it is being
              created by reversing a recorded route that
              erroneously includes the source host (see case
              (B) in the discussion below).

              DISCUSSION:
                 Suppose a source routed datagram is to be
                 routed from source S to destination D via
                 routers G1, G2, Gn.  Source S constructs a
                 datagram with G1's IP address as its
                 destination address, and a source route
                 option to get the datagram the rest of the
                 way to its destination.  However, there is an
                 ambiguity in the specification over whether
                 the source route option in a datagram sent
                 out by S should be (A) or (B):

                 (A): {>>G2, G3, ... Gn, D} <--- CORRECT

                 (B): {S, >>G2, G3, ... Gn, D} <---- WRONG

                 (where >> represents the pointer).  If (A) is
                 sent, the datagram received at D will contain
                 the option: {G1, G2, ... Gn >>}, with S and D
                 as the IP source and destination addresses.
                 If (B) were sent, the datagram received at D
                 would again contain S and D as the same IP
                 source and destination addresses, but the
                 option would be: {S, G1, ...Gn >>}; i.e., the
                 originating host would be the first hop in
                 the route.

         (d) Record Route Option

              Routers MAY support the Record Route option in
              datagrams originated by the router.

         (e) Timestamp Option

              Routers MAY support the timestamp option in
              datagrams originated by the router.  The
              following rules apply:

              + When originating a datagram containing a





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                 Timestamp Option, a router MUST record a
                 timestamp in the option if

                 - Its Internet address fields are not pre-
                    specified or
                 - Its first pre-specified address is the IP
                    address of the logical interface over
                    which the datagram is being sent (or the
                    router's router-id if the datagram is
                    being sent over an unnumbered interface).

              + If the router itself receives a datagram
                 containing a Timestamp Option, the router
                 MUST insert the current time into the
                 Timestamp Option (if there is space in the
                 option to do so) before passing the option to
                 the transport layer or to ICMP for
                 processing.  If space is not present, the      |
                 router MUST increment the Overflow Count in    |
                 the option.

              + A timestamp value MUST follow the rules
                 defined in [INTRO:2].

              IMPLEMENTATION:
                 To maximize the utility of the timestamps
                 contained in the timestamp option, the
                 timestamp inserted should be, as nearly as
                 practical, the time at which the packet
                 arrived at the router.  For datagrams
                 originated by the router, the timestamp
                 inserted should be, as nearly as practical,
                 the time at which the datagram was passed to
                 the Link Layer for transmission.

                 The timestamp option permits the use of a
                 non-standard time clock, but the use of a
                 non-synchronized clock limits the utility of
                 the time stamp.  Therefore, routers are well
                 advised to implement the Network Time
                 Protocol for the purpose of synchronizing
                 their clocks.







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4.2.2.2 Addresses in Options: RFC 791 Section 3.1

         Routers are called upon to insert their address into
         Record Route, Strict Source and Record Route, Loose
         Source and Record Route, or Timestamp Options.  When
         a router inserts its address into such an option, it
         MUST use the IP address of the logical interface on
         which the packet is being sent.  Where this rule
         cannot be obeyed because the output interface has no
         IP address (i.e., is an unnumbered interface), the
         router MUST instead insert its "router-id".  The
         router's router-id is one of the router's IP
         addresses.  The Router ID may be specified on a
         system basis or on a per-link basis.  Which of the
         router's addresses is used as the router-id MUST NOT
         change (even across reboots) unless changed by the
         network manager.  Relevant management changes include
         reconfiguration of the router such that the IP
         address used as the router-id ceases to be one of the
         router's IP addresses.  Routers with multiple
         unnumbered interfaces MAY have multiple router-id's.
         Each unnumbered interface MUST be associated with a
         particular router-id.  This association MUST NOT
         change (even across reboots) without reconfiguration
         of the router.

         DISCUSSION:
            This specification does not allow for routers that
            do not have at least one IP address.  We do not
            view this as a serious limitation, since a router
            needs an IP address to meet the manageability
            requirements of Chapter [8] even if the router is
            connected only to point-to-point links.


         IMPLEMENTATION:
            One possible method of choosing the router-id that
            fulfills this requirement is to use the
            numerically smallest (or greatest) IP address
            (treating the address as a 32-bit integer) that is
            assigned to the router.








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4.2.2.3 Unused IP Header Bits: RFC 791 Section 3.1

         The IP header contains two reserved bits: one in the
         Type of Service byte and the other in the Flags
         field.  A router MUST NOT set either of these bits to
         one in datagrams originated by the router.  A router
         MUST NOT drop (refuse to receive or forward) a packet
         merely because one or more of these reserved bits has
         a non-zero value; i.e., the router MUST NOT check the
         values of thes bits.

         DISCUSSION:
            Future revisions to the IP protocol may make use
            of these unused bits.  These rules are intended to
            ensure that these revisions can be deployed
            without having to simultaneously upgrade all
            routers in the Internet.



4.2.2.4 Type of Service: RFC 791 Section 3.1

         The "Type-of-Service" byte in the IP header is
         divided into three sections: the Precedence field
         (high-order 3 bits), a field that is customarily
         called "Type of Service" or "TOS" (next 4 bits), and
         a reserved bit (the low order bit).

         Rules governing the reserved bit were described in
         Section [4.2.2.3].

         A more extensive discussion of the TOS field and its
         use can be found in [ROUTE:11].

         The description of the IP Precedence field is
         superseded by Section [5.3.3].  RFC 795, "Service
         Mappings", is obsolete and SHOULD NOT be implemented.


4.2.2.5 Header Checksum: RFC 791 Section 3.1

         As stated in Section [5.2.2], a router MUST verify
         the IP checksum of any packet that is received, and
         MUST discard messages containing invalid checksums.





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         The router MUST NOT provide a means to disable this
         checksum verification.

         A router MAY use incremental IP header checksum
         updating when the only change to the IP header is the
         time to live.  This will reduce the possibility of
         undetected corruption of the IP header by the router.
         See [INTERNET:6] for a discussion of incrementally
         updating the checksum.

         IMPLEMENTATION:
            A more extensive description of the IP checksum,
            including extensive implementation hints, can be
            found in [INTERNET:6] and [INTERNET:7].



4.2.2.6 Unrecognized Header Options: RFC 791 Section 3.1

         A router MUST ignore IP options which it does not
         recognize.  A corollary of this requirement is that a
         router MUST implement the End of Option List option
         and the No Operation option, since neither contains
         an explicit length.

         DISCUSSION:
            All future IP options will include an explicit
            length.



4.2.2.7 Fragmentation: RFC 791 Section 3.2

         Fragmentation, as described in [INTERNET:1], MUST be
         supported by a router.

         When a router fragments an IP datagram, it SHOULD
         minimize the number of fragments.  When a router
         fragments an IP datagram, it SHOULD send the
         fragments in order.  A fragmentation method that may
         generate one IP fragment that is significantly
         smaller than the other MAY cause the first IP
         fragment to be the smaller one.






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         DISCUSSION:
            There are several fragmentation techniques in
            common use in the Internet.  One involves
            splitting the IP datagram into IP fragments with
            the first being MTU sized, and the others being
            approximately the same size, smaller than the MTU.
            The reason for this is twofold.  The first IP
            fragment in the sequence will be the effective MTU
            of the current path between the hosts, and the
            following IP fragments are sized to minimize the
            further fragmentation of the IP datagram.  Another
            technique is to split the IP datagram into MTU
            sized IP fragments, with the last fragment being
            the only one smaller, as described in
            [INTERNET:1].

            A common trick used by some implementations of
            TCP/IP is to fragment an IP datagram into IP
            fragments that are no larger than 576 bytes when
            the IP datagram is to travel through a router.
            This is intended to allow the resulting IP
            fragments to pass the rest of the path without
            further fragmentation.  This would, though, create
            more of a load on the destination host, since it
            would have a larger number of IP fragments to
            reassemble into one IP datagram.  It would also
            not be efficient on networks where the MTU only
            changes once and stays much larger than 576 bytes.
            Examples include LAN networks such as an IEEE
            802.5 network with a MTU of 2048 or an Ethernet
            network with an MTU of 1500).

            One other fragmentation technique discussed was
            splitting the IP datagram into approximately equal
            sized IP fragments, with the size less than or
            equal to the next hop network's MTU.  This is
            intended to minimize the number of fragments that
            would result from additional fragmentation further
            down the path, and assure equal delay for each
            fragment.

            Routers SHOULD generate the least possible number
            of IP fragments.






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            Work with slow machines leads us to believe that
            if it is necessary to fragment messages, sending
            the small IP fragment first maximizes the chance
            of a host with a slow interface of receiving all
            the fragments.



4.2.2.8 Reassembly: RFC 791 Section 3.2

         As specified in the corresponding section of
         [INTRO:2], a router MUST support reassembly of
         datagrams that it delivers to itself.


4.2.2.9 Time to Live: RFC 791 Section 3.2

         Time to Live (TTL) handling for packets originated or
         received by the router is governed by [INTRO:2]; this
         section changes none of its stipulations.  However,
         since the remainder of the IP Protocol section of
         [INTRO:2] is rewritten, this section is as well.

         Note in particular that a router MUST NOT check the
         TTL of a packet except when forwarding it.

         A router MUST NOT originate or forward a datagram
         with a Time-to-Live (TTL) value of zero.

         A router MUST NOT discard a datagram just because it
         was received with TTL equal to zero or one; if it is
         to the router and otherwise valid, the router MUST
         attempt to receive it.

         On messages the router originates, the IP layer MUST
         provide a means for the transport layer to set the
         TTL field of every datagram that is sent.  When a
         fixed TTL value is used, it MUST be configurable.
         The number SHOULD exceed the typical internet
         diameter, and current wisdom suggests that it should
         exceed twice the internet diameter to allow for
         growth.  Current suggested values are normally posted
         in the Assigned Numbers RFC.  The TTL field has two
         functions: limit the lifetime of TCP segments (see





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         RFC 793 [TCP:1], p. 28), and terminate Internet
         routing loops.  Although TTL is a time in seconds, it
         also has some attributes of a hop-count, since each
         router is required to reduce the TTL field by at
         least one.

         TTL expiration is intended to cause datagrams to be
         discarded by routers, but not by the destination
         host.  Hosts that act as routers by forwarding
         datagrams must therefore follow the router's rules
         for TTL.

         A higher-layer protocol may want to set the TTL in
         order to implement an "expanding scope" search for
         some Internet resource.  This is used by some
         diagnostic tools, and is expected to be useful for
         locating the "nearest" server of a given class using
         IP multicasting, for example.  A particular transport
         protocol may also want to specify its own TTL bound
         on maximum datagram lifetime.

         A fixed default value must be at least big enough for
         the Internet "diameter," i.e., the longest possible
         path.  A reasonable value is about twice the
         diameter, to allow for continued Internet growth.  As
         of this writing, messages crossing the United States
         frequently traverse 15 to 20 routers; this argues for
         a default TTL value in excess of 40, and 64 is a
         common value.



4.2.2.10 Multi-subnet Broadcasts: RFC 922

         All-subnets broadcasts (called "multi-subnet
         broadcasts" in [INTERNET:3]) have been deprecated.
         See Section [5.3.5.3].


4.2.2.11 Addressing: RFC 791 Section 3.2

         As noted in 2.2.5.1, there are now five classes of IP
         addresses: Class A through Class E.  Class D
         addresses are used for IP multicasting [INTERNET:4],





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         while Class E addresses are reserved for experimental
         use.  The distinction between Class A, B, and C
         addresses is no longer important; they are used as
         generalized unicast network prefixes with only
         historical interest in their class.

         An IP multicast address is a 28-bit logical address
         that stands for a group of hosts, and may be either
         permanent or transient.  Permanent multicast
         addresses are allocated by the Internet Assigned
         Number Authority [INTRO:7], while transient addresses
         may be allocated dynamically to transient groups.
         Group membership is determined dynamically using IGMP
         [INTERNET:4].

         We now summarize the important special cases for
         general purpose unicast IP addresses, using the
         following notation for an IP address:

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

         and the notation "-1" for a field that contains all 1
         bits and the notation "0" for a field that contains
         all 0 bits.

         (a) { 0, 0 }

              This host on this network.  It MUST NOT be used
              as a source address by routers, except the
              router MAY use this as a source address as part
              of an initialization procedure (e.g., if the
              router is using BOOTP to load its configuration
              information).

              Incoming datagrams with a source address of { 0,
              0 } which are received for local delivery (see
              Section [5.2.3]), MUST be accepted if the router
              implements the associated protocol and that
              protocol clearly defines appropriate action to
              be taken.  Otherwise, a router MUST silently
              discard any locally-delivered datagram whose
              source address is { 0, 0 }.

              DISCUSSION:





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                 Some protocols define specific actions to
                 take in response to a received datagram whose
                 source address is { 0, 0 }.  Two examples are
                 BOOTP and ICMP Mask Request.  The proper
                 operation of these protocols often depends on
                 the ability to receive datagrams whose source
                 address is { 0, 0 }.  For most protocols,
                 however, it is best to ignore datagrams
                 having a source address of { 0, 0 } since
                 they were probably generated by a
                 misconfigured host or router.  Thus, if a
                 router knows how to deal with a given
                 datagram having a { 0, 0 } source address,
                 the router MUST accept it.  Otherwise, the
                 router MUST discard it.

              See also Section [4.2.3.1] for a non-standard
              use of { 0, 0 }.

         (b) { 0, <Host-number> }

              Specified host on this network.  It MUST NOT be
              sent by routers except that the router MAY use
              this as a source address as part of an
              initialization procedure by which the it learns
              its own IP address.

         (c) { -1, -1 }

              Limited broadcast.  It MUST NOT be used as a
              source address.

              A datagram with this destination address will be
              received by every host and router on the
              connected physical network, but will not be
              forwarded outside that network.

         (d) { <Network-prefix>, -1 }

              Directed Broadcast - a broadcast directed to the
              specified network prefix.  It MUST NOT be used
              as a source address.  A router MAY originate
              Network Directed Broadcast packets.  A router
              MUST receive Network Directed Broadcast packets;





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              however a router MAY have a configuration option
              to prevent reception of these packets.  Such an
              option MUST default to allowing reception.

         (e) { 127, <any> }

              Internal host loopback address.  Addresses of
              this form MUST NOT appear outside a host.

         The <Network-prefix> is administratively assigned so
         that its value will be unique in the routing domain
         to which the device is connected.

         IP addresses are not permitted to have the value 0 or
         -1 for the <Host-number> or <Network-prefix> fields
         except in the special cases listed above.  This
         implies that each of these fields will be at least
         two bits long.


         DISCUSSION:
            Previous versions of this document also noted that
            subnet numbers must be neither 0 nor -1, and must
            be at least two bits in length.  In a CIDR world,
            the subnet number is clearly an extension of the
            network prefix and cannot be interpreted without
            the remainder of the prefix.  This restriction of
            subnet numbers is therefore meaningless in view of
            CIDR and may be safely ignored.


         For further discussion of broadcast addresses, see
         Section [4.2.3.1].

         When a router originates any datagram, the IP source
         address MUST be one of its own IP addresses (but not
         a broadcast or multicast address).  The only
         exception is during initialization.

         For most purposes, a datagram addressed to a
         broadcast or multicast destination is processed as if
         it had been addressed to one of the router's IP
         addresses; that is to say:






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         + A router MUST receive and process normally any
            packets with a broadcast destination address.

         + A router MUST receive and process normally any
            packets sent to a multicast destination address
            that the router has asked to receive.

         The term "specific-destination address" means the
         equivalent local IP address of the host.  The
         specific-destination address is defined to be the
         destination address in the IP header unless the
         header contains a broadcast or multicast address, in
         which case the specific-destination is an IP address
         assigned to the physical interface on which the
         datagram arrived.

         A router MUST silently discard any received datagram
         containing an IP source address that is invalid by
         the rules of this section.  This validation could be
         done either by the IP layer or (when appropriate) by
         each protocol in the transport layer.  As with any
         datagram a router discards, the datagram discard
         SHOULD be counted.

         DISCUSSION:
            A misaddressed datagram might be caused by a Link
            Layer broadcast of a unicast datagram or by
            another router or host that is confused or
            misconfigured.



4.2.3 SPECIFIC ISSUES



4.2.3.1 IP Broadcast Addresses

         For historical reasons, there are a number of IP
         addresses (some standard and some not) which are used
         to indicate that an IP packet is an IP broadcast.  A
         router

         (1) MUST treat as IP broadcasts packets addressed to





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              255.255.255.255 or { <Network-prefix>, -1 }.

         (2) SHOULD silently discard on receipt (i.e., do not
              even deliver to applications in the router) any
              packet addressed to 0.0.0.0 or { <Network-
              prefix>, 0 }.  If these packets are not silently
              discarded, they MUST be treated as IP broadcasts
              (see Section [5.3.5]).  There MAY be a
              configuration option to allow receipt of these
              packets.  This option SHOULD default to
              discarding them.

         (3) SHOULD (by default) use the limited broadcast
              address (255.255.255.255) when originating an IP
              broadcast destined for a connected (sub)network
              (except when sending an ICMP Address Mask Reply,
              as discussed in Section [4.3.3.9]).  A router
              MUST receive limited broadcasts.

         (4) SHOULD NOT originate datagrams addressed to
              0.0.0.0 or { <Network-prefix>, 0 }.  There MAY
              be a configuration option to allow generation of
              these packets (instead of using the relevant
              "1s" format broadcast).  This option SHOULD
              default to not generating them.

         DISCUSSION:
            In the second bullet, the router obviously cannot
            recognize addresses of the form { <Network-
            prefix>, 0 } if the router has no interface to
            that network prefix.  In that case, the rules of
            the second bullet do not apply because, from the
            point of view of the router, the packet is not an
            IP broadcast packet.



4.2.3.2 IP Multicasting

         An IP router SHOULD satisfy the Host Requirements
         with respect to IP multicasting, as specified in
         [INTRO:2].  An IP router SHOULD support local IP
         multicasting on all connected networks.  When a
         mapping from IP multicast addresses to link-layer





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         addresses has been specified (see the various IP-
         over-xxx specifications), it SHOULD use that mapping,
         and MAY be configurable to use the link layer
         broadcast instead.  On point-to-point links and all
         other interfaces, multicasts are encapsulated as link
         layer broadcasts.  Support for local IP multicasting
         includes originating multicast datagrams, joining
         multicast groups and receiving multicast datagrams,
         and leaving multicast groups.  This implies support
         for all of [INTERNET:4] including IGMP (see Section
         [4.4]).

         DISCUSSION:
            Although [INTERNET:4] is entitled Host Extensions
            for IP Multicasting, it applies to all IP systems,
            both hosts and routers.  In particular, since
            routers may join multicast groups, it is correct
            for them to perform the "host" part of IGMP,
            reporting their group memberships to any multicast
            routers that may be present on their attached
            networks (whether or not they themselves are
            multicast routers).

            Some router protocols may specifically require
            support for IP multicasting (e.g., OSPF
            [ROUTE:1]), or may recommend it (e.g., ICMP Router
            Discovery [INTERNET:13]).



4.2.3.3 Path MTU Discovery

         To eliminate fragmentation or minimize it, it is
         desirable to know what is the path MTU along the path
         from the source to destination.  The path MTU is the
         minimum of the MTUs of each hop in the path.
         [INTERNET:14] describes a technique for dynamically
         discovering the maximum transmission unit (MTU) of an
         arbitrary internet path.  For a path that passes
         through a router that does not support [INTERNET:14],
         this technique might not discover the correct Path
         MTU, but it will always choose a Path MTU as accurate
         as, and in many cases more accurate than, the Path
         MTU that would be chosen by older techniques or the





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         current practice.

         When a router is originating an IP datagram, it
         SHOULD use the scheme described in [INTERNET:14] to
         limit the datagram's size.  If the router's route to
         the datagram's destination was learned from a routing
         protocol that provides Path MTU information, the
         scheme described in [INTERNET:14] is still used, but
         the Path MTU information from the routing protocol
         SHOULD be used as the initial guess as to the Path
         MTU and also as an upper bound on the Path MTU.


4.2.3.4 Subnetting

         Under certain circumstances, it may be desirable to
         support subnets of a particular network being
         interconnected only through a path that is not part
         of the subnetted network.  This is known as
         discontiguous subnetwork support.

         Routers MUST support discontiguous subnetworks.

         IMPLEMENTATION:
            In classical IP networks, this was very difficult
            to achieve; in CIDR networks, it is a natural by-
            product.  Therefore, a router SHOULD NOT make
            assumptions about subnet architecture, but SHOULD
            treat each route as a generalized network prefix.



         DISCUSSION:
            The Internet has been growing at a tremendous rate
            of late.  This has been placing severe strains on
            the IP addressing technology.  A major factor in
            this strain is the strict IP Address class
            boundaries.  These make it difficult to
            efficiently size network prefixes to their
            networks and aggregate several network prefixes
            into a single route advertisement.  By eliminating
            the strict class boundaries of the IP address and
            treating each route as a generalized network
            prefix, these strains may be greatly reduced.





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            The technology for currently doing this is
            Classless Inter Domain Routing (CIDR)
            [INTERNET:15].

         For similar reasons, an address block associated with
         a given network prefix could be subdivided into
         subblocks of different sizes, so that the network
         prefixes associated with the subblocks would have
         different length.  For example, within a block whose
         network prefix is 8 bits long, one subblock may have
         a 16 bit network prefix, another may have an 18 bit
         network prefix, and a third a 14 bit network prefix.

         Routers MUST support variable length network prefixes
         in both their interface configurations and their
         routing databases.


4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP



4.3.1 INTRODUCTION

      ICMP is an auxiliary protocol, which provides routing,
      diagnostic and error functionality for IP.  It is
      described in [INTERNET:8].  A router MUST support ICMP.

      ICMP messages are grouped in two classes that are
      discussed in the following sections:

      ICMP error messages:

      Destination Unreachable     Section 4.3.3.1
      Redirect                    Section 4.3.3.2
      Source Quench               Section 4.3.3.3
      Time Exceeded               Section 4.3.3.4
      Parameter Problem           Section 4.3.3.5

      ICMP query messages:
      Echo                        Section 4.3.3.6
      Information                 Section 4.3.3.7
      Timestamp                   Section 4.3.3.8
      Address Mask                Section 4.3.3.9





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      Router Discovery            Section 4.3.3.10


      General ICMP requirements and discussion are in the next
      section.


4.3.2 GENERAL ISSUES



4.3.2.1 Unknown Message Types

         If an ICMP message of unknown type is received, it
         MUST be passed to the ICMP user interface (if the
         router has one) or silently discarded (if the router
         does not have one).


4.3.2.2 ICMP Message TTL

         When originating an ICMP message, the router MUST
         initialize the TTL.  The TTL for ICMP responses must
         not be taken from the packet that triggered the
         response.


4.3.2.3 Original Message Header

         Historically, every ICMP error message has included
         the Internet header and at least the first 8 data
         bytes of the datagram that triggered the error.  This
         is no longer adequate, due to the use of IP-in-IP
         tunneling and other technologies.  Therefore, the
         ICMP datagram SHOULD contain as much of the original
         datagram as possible without the length of the ICMP
         datagram exceeding 576 bytes.  The returned IP header
         (and user data) MUST be identical to that which was
         received, except that the router is not required to
         undo any modifications to the IP header that are
         normally performed in forwarding that were performed
         before the error was detected (e.g., decrementing the
         TTL, or updating options).  Note that the
         requirements of Section [4.3.3.5] supersede this





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         requirement in some cases (i.e., for a Parameter
         Problem message, if the problem is in a modified
         field, the router must "undo" the modification).  See
         Section [4.3.3.5]).


4.3.2.4 ICMP Message Source Address

         Except where this document specifies otherwise, the
         IP source address in an ICMP message originated by
         the router MUST be one of the IP addresses associated
         with the physical interface over which the ICMP
         message is transmitted.  If the interface has no IP
         addresses associated with it, the router's router-id
         (see Section [5.2.5]) is used instead.


4.3.2.5 TOS and Precedence

         ICMP error messages SHOULD have their TOS bits set to
         the same value as the TOS bits in the packet that
         provoked the sending of the ICMP error message,
         unless setting them to that value would cause the
         ICMP error message to be immediately discarded
         because it could not be routed to its destination.
         Otherwise, ICMP error messages MUST be sent with a
         normal (i.e., zero) TOS.  An ICMP reply message
         SHOULD have its TOS bits set to the same value as the
         TOS bits in the ICMP request that provoked the reply.

         ICMP Source Quench error messages, if sent at all,
         MUST have their IP Precedence field set to the same
         value as the IP Precedence field in the packet that
         provoked the sending of the ICMP Source Quench
         message.  All other ICMP error messages (Destination
         Unreachable, Redirect, Time Exceeded, and Parameter
         Problem) SHOULD have their precedence value set to 6
         (INTERNETWORK CONTROL) or 7 (NETWORK CONTROL).  The
         IP Precedence value for these error messages MAY be
         settable.

         An ICMP reply message MUST have its IP Precedence
         field set to the same value as the IP Precedence
         field in the ICMP request that provoked the reply.





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4.3.2.6 Source Route

         If the packet which provokes the sending of an ICMP
         error message contains a source route option, the
         ICMP error message SHOULD also contain a source route
         option of the same type (strict or loose), created by
         reversing the portion before the pointer of the route
         recorded in the source route option of the original
         packet UNLESS the ICMP error message is an ICMP
         Parameter Problem complaining about a source route
         option in the original packet, or unless the router
         is aware of policy that would prevent the delivery of
         the ICMP error message.

         DISCUSSION:
            In environments which use the U.S.  Department of
            Defense security option (defined in [INTERNET:5]),
            ICMP messages may need to include a security
            option.  Detailed information on this topic should
            be available from the Defense Communications
            Agency.



4.3.2.7 When Not to Send ICMP Errors

         An ICMP error message MUST NOT be sent as the result
         of receiving:

         + An ICMP error message, or

         + A packet which fails the IP header validation tests
            described in Section [5.2.2] (except where that
            section specifically permits the sending of an
            ICMP error message), or

         + A packet destined to an IP broadcast or IP
            multicast address, or

         + A packet sent as a Link Layer broadcast or
            multicast, or

         + A packet whose source address has a network prefix
            of zero or is an invalid source address (as





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            defined in Section [5.3.7]), or

         + Any fragment of a datagram other then the first
            fragment (i.e., a packet for which the fragment
            offset in the IP header is nonzero).

         Furthermore, an ICMP error message MUST NOT be sent
         in any case where this memo states that a packet is
         to be "silently discarded".

         NOTE: THESE RESTRICTIONS TAKE PRECEDENCE OVER ANY
         REQUIREMENT ELSEWHERE IN THIS DOCUMENT FOR SENDING
         ICMP ERROR MESSAGES.

         DISCUSSION:
            These rules aim to prevent the "broadcast storms"
            that have resulted from routers or hosts returning
            ICMP error messages in response to broadcast
            packets.  For example, a broadcast UDP packet to a
            non-existent port could trigger a flood of ICMP
            Destination Unreachable datagrams from all devices
            that do not have a client for that destination
            port.  On a large Ethernet, the resulting
            collisions can render the network useless for a
            second or more.

            Every packet that is broadcast on the connected
            network should have a valid IP broadcast address
            as its IP destination (see Section [5.3.4] and
            [INTRO:2]).  However, some devices violate this
            rule.  To be certain to detect broadcast packets,
            therefore, routers are required to check for a
            link-layer broadcast as well as an IP-layer
            address.


         IMPLEMENTATION:
            This requires that the link layer inform the IP
            layer when a link-layer broadcast packet has been
            received; see Section [3.1].









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4.3.2.8 Rate Limiting

         A router which sends ICMP Source Quench messages MUST
         be able to limit the rate at which the messages can
         be generated.  A router SHOULD also be able to limit
         the rate at which it sends other sorts of ICMP error
         messages (Destination Unreachable, Redirect, Time
         Exceeded, Parameter Problem).  The rate limit
         parameters SHOULD be settable as part of the
         configuration of the router.  How the limits are
         applied (e.g., per router or per interface) is left
         to the implementor's discretion.

         DISCUSSION:
            Two problems for a router sending ICMP error
            message are:
            (1) The consumption of bandwidth on the reverse
                 path, and
            (2) The use of router resources (e.g., memory, CPU
                 time)

            To help solve these problems a router can limit
            the frequency with which it generates ICMP error
            messages.  For similar reasons, a router may limit
            the frequency at which some other sorts of
            messages, such as ICMP Echo Replies, are
            generated.


         IMPLEMENTATION:
            Various mechanisms have been used or proposed for
            limiting the rate at which ICMP messages are sent:

            (1) Count-based - for example, send an ICMP error
                 message for every N dropped packets overall
                 or per given source host.  This mechanism
                 might be appropriate for ICMP Source Quench,
                 if used, but probably not for other types of
                 ICMP messages.

            (2) Timer-based - for example, send an ICMP error
                 message to a given source host or overall at
                 most once per T milliseconds.






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            (3) Bandwidth-based - for example, limit the rate
                 at which ICMP messages are sent over a
                 particular interface to some fraction of the
                 attached network's bandwidth.



4.3.3 SPECIFIC ISSUES



4.3.3.1 Destination Unreachable

         If a route cannot forward a packet because it has no
         routes at all (including no default route) to the
         destination specified in the packet, then the router
         MUST generate a Destination Unreachable, Code 0
         (Network Unreachable) ICMP message.  If the router
         does have routes to the destination network specified
         in the packet but the TOS specified for the routes is
         neither the default TOS (0000) nor the TOS of the
         packet that the router is attempting to route, then
         the router MUST generate a Destination Unreachable,
         Code 11 (Network Unreachable for TOS) ICMP message.

         If a packet is to be forwarded to a host on a network
         that is directly connected to the router (i.e., the
         router is the last-hop router) and the router has
         ascertained that there is no path to the destination
         host then the router MUST generate a Destination
         Unreachable, Code 1 (Host Unreachable) ICMP message.
         If a packet is to be forwarded to a host that is on a
         network that is directly connected to the router and
         the router cannot forward the packet because no route
         to the destination has a TOS that is either equal to
         the TOS requested in the packet or is the default TOS
         (0000) then the router MUST generate a Destination
         Unreachable, Code 12 (Host Unreachable for TOS) ICMP
         message.

         DISCUSSION:
            The intent is that a router generates the
            "generic" host/network unreachable if it has no
            path at all (including default routes) to the





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            destination.  If the router has one or more paths
            to the destination, but none of those paths have
            an acceptable TOS, then the router generates the
            "unreachable for TOS" message.



4.3.3.2 Redirect

         The ICMP Redirect message is generated to inform a
         local host that it should use a different next hop
         router for certain traffic.

         Contrary to [INTRO:2], a router MAY ignore ICMP
         Redirects when choosing a path for a packet
         originated by the router if the router is running a
         routing protocol or if forwarding is enabled on the
         router and on the interface over which the packet is
         being sent.


4.3.3.3 Source Quench

         A router SHOULD NOT originate ICMP Source Quench
         messages.  As specified in Section [4.3.2], a router
         that does originate Source Quench messages MUST be
         able to limit the rate at which they are generated.

         DISCUSSION:
            Research seems to suggest that Source Quench
            consumes network bandwidth but is an ineffective
            (and unfair) antidote to congestion.  See, for
            example, [INTERNET:9] and [INTERNET:10].  Section
            [5.3.6] discusses the current thinking on how
            routers ought to deal with overload and network
            congestion.

         A router MAY ignore any ICMP Source Quench messages
         it receives.

         DISCUSSION:
            A router itself may receive a Source Quench as the
            result of originating a packet sent to another
            router or host.  Such datagrams might be, e.g., an





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            EGP update sent to another router, or a telnet
            stream sent to a host.  A mechanism has been
            proposed ([INTERNET:11], [INTERNET:12]) to make
            the IP layer respond directly to Source Quench by
            controlling the rate at which packets are sent,
            however, this proposal is currently experimental
            and not currently recommended.



4.3.3.4 Time Exceeded

         When a router is forwarding a packet and the TTL
         field of the packet is reduced to 0, the requirements
         of section [5.2.3.8] apply.

         When the router is reassembling a packet that is
         destined for the router, it is acting as an Internet
         host.  [INTRO:2]'s reassembly requirements therefore
         apply.

         When the router receives (i.e., is destined for the
         router) a Time Exceeded message, it MUST comply with
         [INTRO:2].


4.3.3.5 Parameter Problem

         A router MUST generate a Parameter Problem message
         for any error not specifically covered by another
         ICMP message.  The IP header field or IP option
         including the byte indicated by the pointer field
         MUST be included unchanged in the IP header returned
         with this ICMP message.  Section [4.3.2] defines an
         exception to this requirement.

         A new variant of the Parameter Problem message was
         defined in [INTRO:2]:
              Code 1 = required option is missing.

         DISCUSSION:
            This variant is currently in use in the military
            community for a missing security option.






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4.3.3.6 Echo Request/Reply

         A router MUST implement an ICMP Echo server function
         that receives Echo Requests sent to the router, and
         sends corresponding Echo Replies.  A router MUST be
         prepared to receive, reassemble and echo an ICMP Echo
         Request datagram at least as the maximum of 576 and
         the MTUs of all the connected networks.

         The Echo server function MAY choose not to respond to
         ICMP echo requests addressed to IP broadcast or IP
         multicast addresses.

         A router SHOULD have a configuration option that, if
         enabled, causes the router to silently ignore all
         ICMP echo requests; if provided, this option MUST
         default to allowing responses.

         DISCUSSION:
            The neutral provision about responding to
            broadcast and multicast Echo Requests derives from
            [INTRO:2]'s "Echo Request/Reply" section.

         As stated in Section [10.3.3], a router MUST also
         implement a user/application-layer interface for
         sending an Echo Request and receiving an Echo Reply,
         for diagnostic purposes.  All ICMP Echo Reply
         messages MUST be passed to this interface.

         The IP source address in an ICMP Echo Reply MUST be
         the same as the specific-destination address of the
         corresponding ICMP Echo Request message.

         Data received in an ICMP Echo Request MUST be
         entirely included in the resulting Echo Reply.

         If a Record Route and/or Timestamp option is received
         in an ICMP Echo Request, this option (these options)
         SHOULD be updated to include the current router and
         included in the IP header of the Echo Reply message,
         without "truncation".  Thus, the recorded route will
         be for the entire round trip.

         If a Source Route option is received in an ICMP Echo





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         Request, the return route MUST be reversed and used
         as a Source Route option for the Echo Reply message,
         unless the router is aware of policy that would
         prevent the delivery of the message.


4.3.3.7 Information Request/Reply

         A router SHOULD NOT originate or respond to these
         messages.

         DISCUSSION:
            The Information Request/Reply pair was intended to
            support self-configuring systems such as diskless
            workstations, to allow them to discover their IP
            network prefixes at boot time.  However, these
            messages are now obsolete.  The RARP and BOOTP
            protocols provide better mechanisms for a host to
            discover its own IP address.



4.3.3.8 Timestamp and Timestamp Reply

         A router MAY implement Timestamp and Timestamp Reply.
         If they are implemented then:

         + The ICMP Timestamp server function MUST return a
            Timestamp Reply to every Timestamp message that is
            received.  It SHOULD be designed for minimum
            variability in delay.

         + An ICMP Timestamp Request message to an IP
            broadcast or IP multicast address MAY be silently
            discarded.

         + The IP source address in an ICMP Timestamp Reply
            MUST be the same as the specific-destination
            address of the corresponding Timestamp Request
            message.

         + If a Source Route option is received in an ICMP
            Timestamp Request, the return route MUST be
            reversed and used as a Source Route option for the





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            Timestamp Reply message, unless the router is
            aware of policy that would prevent the delivery of
            the message.

         + If a Record Route and/or Timestamp option is
            received in a Timestamp Request, this (these)
            option(s) SHOULD be updated to include the current
            router and included in the IP header of the
            Timestamp Reply message.

         + If the router provides an application-layer
            interface for sending Timestamp Request messages
            then incoming Timestamp Reply messages MUST be
            passed up to the ICMP user interface.

         The preferred form for a timestamp value (the
         "standard value") is milliseconds since midnight,
         Universal Time.  However, it may be difficult to
         provide this value with millisecond resolution.  For
         example, many systems use clocks that update only at
         line frequency, 50 or 60 times per second.
         Therefore, some latitude is allowed in a "standard
         value":

         (a) A "standard value" MUST be updated at least 16
              times per second (i.e., at most the six low-
              order bits of the value may be undefined).

         (b) The accuracy of a "standard value" MUST
              approximate that of operator-set CPU clocks,
              i.e., correct within a few minutes.

         IMPLEMENTATION:
            To meet the second condition, a router may need to
            query some time server when the router is booted
            or restarted.  It is recommended that the UDP Time
            Server Protocol be used for this purpose.  A more
            advanced implementation would use the Network Time
            Protocol (NTP) to achieve nearly millisecond clock
            synchronization; however, this is not required.









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4.3.3.9 Address Mask Request/Reply

         A router MUST implement support for receiving ICMP
         Address Mask Request messages and responding with
         ICMP Address Mask Reply messages.  These messages are
         defined in [INTERNET:2].

         A router SHOULD have a configuration option for each
         logical interface specifying whether the router is
         allowed to answer Address Mask Requests for that
         interface; this option MUST default to allowing
         responses.  A router MUST NOT respond to an Address
         Mask Request before the router knows the correct
         address mask.

         A router MUST NOT respond to an Address Mask Request
         that has a source address of 0.0.0.0 and which
         arrives on a physical interface that has associated
         with it multiple logical interfaces and the address
         masks for those interfaces are not all the same.

         A router SHOULD examine all ICMP Address Mask Replies
         that it receives to determine whether the information
         it contains matches the router's knowledge of the
         address mask.  If the ICMP Address Mask Reply appears
         to be in error, the router SHOULD log the address
         mask and the sender's IP address.  A router MUST NOT
         use the contents of an ICMP Address Mask Reply to
         determine the correct address mask.

         Because hosts may not be able to learn the address
         mask if a router is down when the host boots up, a
         router MAY broadcast a gratuitous ICMP Address Mask
         Reply on each of its logical interfaces after it has
         configured its own address masks.  However, this
         feature can be dangerous in environments that use
         variable length address masks.  Therefore, if this
         feature is implemented, gratuitous Address Mask
         Replies MUST NOT be broadcast over any logical
         interface(s) which either:

         + Are not configured to send gratuitous Address Mask
            Replies.  Each logical interface MUST have a
            configuration parameter controlling this, and that





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            parameter MUST default to not sending the
            gratuitous Address Mask Replies.

         + Share subsuming (but not identical) network
            prefixes and physical interface.

         The { <Network-prefix>, -1 } form of the IP broadcast
         address MUST be used for broadcast Address Mask
         Replies.

         DISCUSSION:
            The ability to disable sending Address Mask
            Replies by routers is required at a few sites that
            intentionally lie to their hosts about the address
            mask.  The need for this is expected to go away as
            more and more hosts become compliant with the Host
            Requirements standards.

            The reason for both the second bullet above and
            the requirement about which IP broadcast address
            to use is to prevent problems when multiple IP
            network prefixes are in use on the same physical
            network.



4.3.3.10 Router Advertisement and Solicitations

         An IP router MUST support the router part of the ICMP
         Router Discovery Protocol [INTERNET:13] on all
         connected networks on which the router supports
         either IP multicast or IP broadcast addressing.  The
         implementation MUST include all the configuration
         variables specified for routers, with the specified
         defaults.

         DISCUSSION:
            Routers are not required to implement the host
            part of the ICMP Router Discovery Protocol, but
            might find it useful for operation while IP
            forwarding is disabled (i.e., when operating as a
            host).







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         DISCUSSION:
            We note that it is quite common for hosts to use    |
            RIP Version 1 as the "router discovery" protocol.
            Such hosts listen to RIP traffic and use and use
            information extracted from that traffic to
            discover routers and to make decisions as to which
            router to use as a first-hop router for a given
            destination.  While this behavior is discouraged,
            it is still common and implementors should be
            aware of it.



4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP

   IGMP [INTERNET:4] is a protocol used between hosts and
   multicast routers on a single physical network to establish
   hosts' membership in particular multicast groups.
   Multicast routers use this information, in conjunction with
   a multicast routing protocol, to support IP multicast
   forwarding across the Internet.

   A router SHOULD implement the host part of IGMP.


























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5. INTERNET LAYER - FORWARDING



5.1 INTRODUCTION

   This section describes the process of forwarding packets.


5.2 FORWARDING WALK-THROUGH

   There is no separate specification of the forwarding
   function in IP.  Instead, forwarding is covered by the
   protocol specifications for the internet layer protocols
   ([INTERNET:1], [INTERNET:2], [INTERNET:3], [INTERNET:8],
   and [ROUTE:11]).


5.2.1 Forwarding Algorithm

      Since none of the primary protocol documents describe
      the forwarding algorithm in any detail, we present it
      here.  This is just a general outline, and omits
      important details, such as handling of congestion, that
      are dealt with in later sections.

      It is not required that an implementation follow exactly
      the algorithms given in sections [5.2.1.1], [5.2.1.2],
      and [5.2.1.3].  Much of the challenge of writing router
      software is to maximize the rate at which the router can
      forward packets while still achieving the same effect of
      the algorithm.  Details of how to do that are beyond the
      scope of this document, in part because they are heavily
      dependent on the architecture of the router.  Instead,
      we merely point out the order dependencies among the
      steps:

      (1) A router MUST verify the IP header, as described in
           section [5.2.2], before performing any actions
           based on the contents of the header.  This allows
           the router to detect and discard bad packets before
           the expenditure of other resources.

      (2) Processing of certain IP options requires that the





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           router insert its IP address into the option.  As
           noted in Section [5.2.4], the address inserted MUST
           be the address of the logical interface on which
           the packet is sent or the router's router-id if the
           packet is sent over an unnumbered interface.  Thus,
           processing of these options cannot be completed
           until after the output interface is chosen.

      (3) The router cannot check and decrement the TTL before
           checking whether the packet should be delivered to
           the router itself, for reasons mentioned in Section
           [4.2.2.9].

      (4) More generally, when a packet is delivered locally
           to the router, its IP header MUST NOT be modified
           in any way (except that a router may be required to
           insert a timestamp into any Timestamp options in
           the IP header).  Thus, before the router determines
           whether the packet is to be delivered locally to
           the router, it cannot update the IP header in any
           way that it is not prepared to undo.


5.2.1.1 General

         This section covers the general forwarding algorithm.
         This algorithm applies to all forms of packets to be
         forwarded: unicast, multicast, and broadcast.


         (1) The router receives the IP packet (plus
              additional information about it, as described in
              Section [3.1]) from the Link Layer.

         (2) The router validates the IP header, as described
              in Section [5.2.2].  Note that IP reassembly is
              not done, except on IP fragments to be queued
              for local delivery in step (4).

         (3) The router performs most of the processing of any
              IP options.  As described in Section [5.2.4],
              some IP options require additional processing
              after the routing decision has been made.






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         (4) The router examines the destination IP address of
              the IP datagram, as described in Section
              [5.2.3], to determine how it should continue to
              process the IP datagram.  There are three
              possibilities:

              + The IP datagram is destined for the router,
                 and should be queued for local delivery,
                 doing reassembly if needed.

              + The IP datagram is not destined for the
                 router, and should be queued for forwarding.

              + The IP datagram should be queued for
                 forwarding, but (a copy) must also be queued
                 for local delivery.


5.2.1.2 Unicast

         Since the local delivery case is well covered by
         [INTRO:2], the following assumes that the IP datagram
         was queued for forwarding.  If the destination is an
         IP unicast address:

         (5) The forwarder determines the next hop IP address
              for the packet, usually by looking up the
              packet's destination in the router's routing
              table.  This procedure is described in more
              detail in Section [5.2.4].  This procedure also
              decides which network interface should be used
              to send the packet.

         (6) The forwarder verifies that forwarding the packet
              is permitted.  The source and destination
              addresses should be valid, as described in
              Section [5.3.7] and Section [5.3.4] If the
              router supports administrative constraints on
              forwarding, such as those described in Section
              [5.3.9], those constraints must be satisfied.

         (7) The forwarder decrements (by at least one) and
              checks the packet's TTL, as described in Section
              [5.3.1].





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         (8) The forwarder performs any IP option processing
              that could not be completed in step 3.

         (9) The forwarder performs any necessary IP
              fragmentation, as described in Section
              [4.2.2.7].  Since this step occurs after
              outbound interface selection (step 5), all
              fragments of the same datagram will be
              transmitted out the same interface.

         (10) The forwarder determines the Link Layer address
              of the packet's next hop.  The mechanisms for
              doing this are Link Layer-dependent (see chapter
              3).

         (11) The forwarder encapsulates the IP datagram (or
              each of the fragments thereof) in an appropriate
              Link Layer frame and queues it for output on the
              interface selected in step 5.

         (12) The forwarder sends an ICMP redirect if
              necessary, as described in Section [4.3.3.2].


5.2.1.3 Multicast

         If the destination is an IP multicast, the following
         steps are taken.

         Note that the main differences between the forwarding
         of IP unicasts and the forwarding of IP multicasts
         are

         + IP multicasts are usually forwarded based on both
            the datagram's source and destination IP
            addresses,

         + IP multicast uses an expanding ring search,

         + IP multicasts are forwarded as Link Level
            multicasts, and

         + ICMP errors are never sent in response to IP
            multicast datagrams.





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         Note that the forwarding of IP multicasts is still
         somewhat experimental.  As a result, the algorithm
         presented below is not mandatory, and is provided as
         an example only.

         (5a) Based on the IP source and destination addresses
              found in the datagram header, the router
              determines whether the datagram has been
              received on the proper interface for forwarding.
              If not, the datagram is dropped silently.  The
              method for determining the proper receiving
              interface depends on the multicast routing
              algorithm(s) in use.  In one of the simplest
              algorithms, reverse path forwarding (RPF), the
              proper interface is the one that would be used
              to forward unicasts back to the datagram source.

         (6a) Based on the IP source and destination addresses
              found in the datagram header, the router
              determines the datagram's outgoing interfaces.
              To implement IP multicast's expanding ring
              search (see [INTERNET:4]) a minimum TTL value is
              specified for each outgoing interface.  A copy
              of the multicast datagram is forwarded out each
              outgoing interface whose minimum TTL value is
              less than or equal to the TTL value in the
              datagram header, by separately applying the
              remaining steps on each such interface.

         (7a) The router decrements the packet's TTL by one.

         (8a) The forwarder performs any IP option processing
              that could not be completed in step (3).

         (9a) The forwarder performs any necessary IP
              fragmentation, as described in Section
              [4.2.2.7].

         (10a) The forwarder determines the Link Layer address
              to use in the Link Level encapsulation.  The
              mechanisms for doing this are Link Layer-
              dependent.  On LANs a Link Level multicast or
              broadcast is selected, as an algorithmic
              translation of the datagrams' IP multicast





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              address.  See the various IP-over-xxx
              specifications for more details.

         (11a) The forwarder encapsulates the packet (or each
              of the fragments thereof) in an appropriate Link
              Layer frame and queues it for output on the
              appropriate interface.


5.2.2 IP Header Validation

      Before a router can process any IP packet, it MUST
      perform a the following basic validity checks on the
      packet's IP header to ensure that the header is
      meaningful.  If the packet fails any of the following
      tests, it MUST be silently discarded, and the error
      SHOULD be logged.

      (1) The packet length reported by the Link Layer must be
           large enough to hold the minimum length legal IP
           datagram (20 bytes).

      (2) The IP checksum must be correct.

      (3) The IP version number must be 4.  If the version
           number is not 4 then the packet may be another
           version of IP, such as IPng or ST-II.

      (4) The IP header length field must be large enough to
           hold the minimum length legal IP datagram (20 bytes
           = 5 words).

      (5) The IP header length field must be large enough to
           hold the IP datagram header, whose length is
           specified in the IP header length field.

      A router MUST NOT have a configuration option that
      allows disabling any of these tests.

      If the packet passes the second and third tests, the IP
      header length field is at least 4, and both the IP total
      length field and the packet length reported by the Link
      Layer are at least 16 then, despite the above rule, the
      router MAY respond with an ICMP Parameter Problem





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      message, whose pointer points at the IP header length
      field (if it failed the fourth test) or the IP total
      length field (if it failed the fifth test).  However, it
      still MUST discard the packet and still SHOULD log the
      error.

      These rules (and this entire document) apply only to
      version 4 of the Internet Protocol.  These rules should
      not be construed as prohibiting routers from supporting
      other versions of IP.  Furthermore, if a router can
      truly classify a packet as being some other version of
      IP then it ought not treat that packet as an error
      packet within the context of this memo.

      IMPLEMENTATION:
         It is desirable for purposes of error reporting,
         though not always entirely possible, to determine why
         a header was invalid.  There are four possible
         reasons:

         + The Link Layer truncated the IP header

         + The datagram is using a version of IP other than
            the standard one (version 4).

         + The IP header has been corrupted in transit.

         + The sender generated an illegal IP header.

         It is probably desirable to perform the checks in the
         order listed, since we believe that this ordering is
         most likely to correctly categorize the cause of the
         error.  For purposes of error reporting, it may also
         be desirable to check if a packet that fails these
         tests has an IP version number indicating IPng or
         ST-II; these should be handled according to their
         respective specifications.

      Additionally, the router SHOULD verify that the packet
      length reported by the Link Layer is at least as large
      as the IP total length recorded in the packet's IP
      header.  If it appears that the packet has been
      truncated, the packet MUST be discarded, the error
      SHOULD be logged, and the router SHOULD respond with an





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      ICMP Parameter Problem message whose pointer points at
      the IP total length field.

      DISCUSSION:
         Because any higher layer protocol that concerns
         itself with data corruption will detect truncation of
         the packet data when it reaches its final
         destination, it is not absolutely necessary for
         routers to perform the check suggested above to
         maintain protocol correctness.  However, by making
         this check a router can simplify considerably the
         task of determining which hop in the path is
         truncating the packets.  It will also reduce the
         expenditure of resources "down-stream" from the
         router in that down-stream systems will not need to
         deal with the packet.

      Finally, if the destination address in the IP header is
      not one of the addresses of the router, the router
      SHOULD verify that the packet does not contain a Strict
      Source and Record Route option.  If a packet fails this
      test (if it contains a strict source route option), the
      router SHOULD log the error and SHOULD respond with an
      ICMP Parameter Problem error with the pointer pointing
      at the offending packet's IP destination address.

      DISCUSSION:
         Some people might suggest that the router should
         respond with a Bad Source Route message instead of a
         Parameter Problem message.  However, when a packet
         fails this test, it usually indicates a protocol
         error by the previous hop router, whereas Bad Source
         Route would suggest that the source host had
         requested a nonexistent or broken path through the
         network.



5.2.3 Local Delivery Decision

      When a router receives an IP packet, it must decide
      whether the packet is addressed to the router (and
      should be delivered locally) or the packet is addressed
      to another system (and should be handled by the





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      forwarder).  There is also a hybrid case, where certain
      IP broadcasts and IP multicasts are both delivered
      locally and forwarded.  A router MUST determine which of
      the these three cases applies using the following rules.


      + An unexpired source route option is one whose pointer
         value does not point past the last entry in the
         source route.  If the packet contains an unexpired
         source route option, the pointer in the option is
         advanced until either the pointer does point past the
         last address in the option or else the next address
         is not one of the router's own addresses.  In the
         latter (normal) case, the packet is forwarded (and
         not delivered locally) regardless of the rules below.

      + The packet is delivered locally and not considered for
         forwarding in the following cases:

         - The packet's destination address exactly matches
            one of the router's IP addresses,

         - The packet's destination address is a limited
            broadcast address ({-1, -1}), or

         - The packet's destination is an IP multicast address
            which is never forwarded (such as 224.0.0.1 or
            224.0.0.2) and (at least) one of the logical
            interfaces associated with the physical interface
            on which the packet arrived is a member of the
            destination multicast group.

      + The packet is passed to the forwarder AND delivered
         locally in the following cases:

         - The packet's destination address is an IP broadcast
            address that addresses at least one of the
            router's logical interfaces but does not address
            any of the logical interfaces associated with the
            physical interface on which the packet arrived

         - The packet's destination is an IP multicast address
            which is permitted to be forwarded (unlike
            224.0.0.1 and 224.0.0.2) and (at least) one of the





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            logical interfaces associated with the physical
            interface on which the packet arrived is a member
            of the destination multicast group.

      + The packet is delivered locally if the packet's
         destination address is an IP broadcast address (other
         than a limited broadcast address) that addresses at
         least one of the logical interfaces associated with
         the physical interface on which the packet arrived.
         The packet is ALSO passed to the forwarder unless the
         link on which the packet arrived uses an IP
         encapsulation that does not encapsulate broadcasts
         differently than unicasts (e.g., by using different
         Link Layer destination addresses).

      + The packet is passed to the forwarder in all other
         cases.

      DISCUSSION:
         The purpose of the requirement in the last sentence
         of the fourth bullet is to deal with a directed
         broadcast to another network prefix on the same
         physical cable.  Normally, this works as expected:
         the sender sends the broadcast to the router as a
         Link Layer unicast.  The router notes that it arrived
         as a unicast, and therefore must be destined for a
         different network prefix than the sender sent it on.
         Therefore, the router can safely send it as a Link
         Layer broadcast out the same (physical) interface
         over which it arrived.  However, if the router can't
         tell whether the packet was received as a Link Layer
         unicast, the sentence ensures that the router does
         the safe but wrong thing rather than the unsafe but
         right thing.


      IMPLEMENTATION:
         As described in Section [5.3.4], packets received as
         Link Layer broadcasts are generally not forwarded.
         It may be advantageous to avoid passing to the
         forwarder packets it would later discard because of
         the rules in that section.

         Some Link Layers (either because of the hardware or





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         because of special code in the drivers) can deliver
         to the router copies of all Link Layer broadcasts and
         multicasts it transmits.  Use of this feature can
         simplify the implementation of cases where a packet
         has to both be passed to the forwarder and delivered
         locally, since forwarding the packet will
         automatically cause the router to receive a copy of
         the packet that it can then deliver locally.  One
         must use care in these circumstances to prevent
         treating a received loop-back packet as a normal
         packet that was received (and then being subject to
         the rules of forwarding, etc.).

         Even without such a Link Layer, it is of course
         hardly necessary to make a copy of an entire packet
         to queue it both for forwarding and for local
         delivery, though care must be taken with fragments,
         since reassembly is performed on locally delivered
         packets but not on forwarded packets.  One simple
         scheme is to associate a flag with each packet on the
         router's output queue that indicates whether it
         should be queued for local delivery after it has been
         sent.


5.2.4 Determining the Next Hop Address

      When a router is going to forward a packet, it must
      determine whether it can send it directly to its
      destination, or whether it needs to pass it through
      another router.  If the latter, it needs to determine
      which router to use.  This section explains how these
      determinations are made.

      This section makes use of the following definitions:

      + "LSRR" - IP Loose Source and Record Route option

      + "SSRR" - IP Strict Source and Record Route option

      + "Source Route Option" - an LSRR or an SSRR

      + "Ultimate Destination Address" - where the packet is
         being sent to: the last address in the source route





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         of a source-routed packet, or the destination address
         in the IP header of a non-source-routed packet

      + "Adjacent" - reachable without going through any IP
         routers

      + "Next Hop Address" - the IP address of the adjacent
         host or router to which the packet should be sent
         next

      + "IP Destination Address" - the ultimate destination
         address, except in source routed packets, where it is
         the next address specified in the source route

      + Immediate Destination - the node, System, router,
         end-system, or whatever that is addressed by the IP
         Destination Address.


5.2.4.1 IP Destination Address

         If :

         + the destination address in the IP header is one of
            the addresses of the router,

         + the packet contains a Source Route Option, and

         + the pointer in the Source Route Option does not
            point past the end of the option,

         then the next IP Destination Address is the address
         pointed at by the pointer in that option.  If :

         + the destination address in the IP header is one of
            the addresses of the router,

         + the packet contains a Source Route Option, and

         + the pointer in the Source Route Option points past
            the end of the option,

         then the message is addressed to the system analyzing
         the message.





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         A router MUST use the IP Destination Address, not the
         Ultimate Destination Address (the last address in the
         source route option), when determining how to handle
         a packet.

         It is an error for more than one source route option
         to appear in a datagram.  If it receives such a
         datagram, it SHOULD discard the packet and reply with
         an ICMP Parameter Problem message whose pointer
         points at the beginning of the second source route
         option.


5.2.4.2 Local/Remote Decision

         After it has been determined that the IP packet needs
         to be forwarded according to the rules specified in
         Section [5.2.3], the following algorithm MUST be used
         to determine if the Immediate Destination is directly
         accessible (see [INTERNET:2]).

         (1) For each network interface that has not been
              assigned any IP address (the "unnumbered lines"
              as described in Section [2.2.7]), compare the
              router-id of the other end of the line to the IP
              Destination Address.  If they are exactly equal,
              the packet can be transmitted through this
              interface.

              DISCUSSION:
                 In other words, the router or host at the
                 remote end of the line is the destination of
                 the packet or is the next step in the source
                 route of a source routed packet.

         (2) If no network interface has been selected in the
              first step, for each IP address assigned to the
              router:
              (a) isolate the network prefix used by the
                   interface.

                   IMPLEMENTATION:
                      The result of this operation will
                      usually have been computed and saved





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                      during initialization.

              (b) Isolate the corresponding set of bits from
                   the IP Destination Address of the packet.
              (c) Compare the resulting network prefixes.  If
                   they are equal to each other, the packet
                   can be transmitted through the
                   corresponding network interface.

         (3) If the destination was neither the router-id of a
              neighbor on an unnumbered interface nor a member
              of a directly connected network prefix, the IP
              Destination is accessible only through some
              other router.  The selection of the router and
              the "next hop" IP address is described in
              Section [5.2.4.3].  In the case of a host that
              is not also a router, this may be the configured
              default router.  Ongoing work in the IETF
              [ARCH:9, NRHP] considers some cases such as when
              multiple IP (sub)networks are overlaid on the
              same link layer network.  Barring policy
              restrictions, hosts and routers using a common
              link layer network can directly communicate even
              if they are not in the same IP (sub)network, if
              there is adequate information present.  The Next
              Hop Routing Protocol (NHRP) enables IP entities
              to determine the "optimal" link layer address to
              be used to traverse such a link layer network
              towards a remote destination.

      (4) If the selected "next hop" is reachable through an
           interface configured to use NHRP, then the
           following additional steps apply:
             (a) Compare the IP Destination Address to the
                destination addresses in the NHRP cache.  If
                the address is in the cache, then send the
                datagram to the corresponding cached link
                layer address.
             (b) If the address is not in the cache, then
                construct an NHRP request packet containing
                the IP Destination Address.  This message is
                sent to the NHRP server configured for that
                interface.  This may be a logically separate
                process or entity in the router itself.





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             (c) The NHRP server will respond with the proper
                link layer address to use to transmit the
                datagram and subsequent datagrams to the same
                destination.  The system MAY transmit the
                datagram(s) to the traditional "next hop"
                router while awaiting the NHRP reply.


5.2.4.3 Next Hop Address


      The router applies the algorithm in the previous section  *
      to determine if the IP Destination Address is adjacent.
      If so, the next hop address is the same as the IP
      Destination Address.  Otherwise, the packet must be
      forwarded through another router to reach its Immediate
      Destination.  The selection of this router is the topic
      of this section.

      If the packet contains an SSRR, the router MUST discard
      the packet and reply with an ICMP Bad Source Route
      error.  Otherwise, the router looks up the IP
      Destination Address in its routing table to determine an
      appropriate next hop address.

      DISCUSSION:
         Per the IP specification, a Strict Source Route must
         specify a sequence of nodes through which the packet
         must traverse; the packet must go from one node of
         the source route to the next, traversing intermediate
         networks only.  Thus, if the router is not adjacent
         to the next step of the source route, the source
         route can not be fulfilled.  Therefore, the router
         rejects such with an ICMP Bad Source Route error.

      The goal of the next-hop selection process is to examine
      the entries in the router's Forwarding Information Base
      (FIB) and select the best route (if there is one) for
      the packet from those available in the FIB.

      Conceptually, any route lookup algorithm starts out with
      a set of candidate routes that consists of the entire
      contents of the FIB.  The algorithm consists of a series
      of steps that discard routes from the set.  These steps





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      are referred to as Pruning Rules.  Normally, when the
      algorithm terminates there is exactly one route
      remaining in the set.  If the set ever becomes empty,
      the packet is discarded because the destination is
      unreachable.  It is also possible for the algorithm to
      terminate when more than one route remains in the set.
      In this case, the router may arbitrarily discard all but
      one of them, or may perform "load-splitting" by choosing
      whichever of the routes has been least recently used.

      With the exception of rule 3 (Weak TOS), a router MUST
      use the following Pruning Rules when selecting a next
      hop for a packet.  If a router does consider TOS when
      making next-hop decisions, the Rule 3 must be applied in
      the order indicated below.  These rules MUST be
      (conceptually) applied to the FIB in the order that they
      are presented.  (For some historical perspective,
      additional pruning rules, and other common algorithms in
      use, see Appendix E.)

      DISCUSSION:
         Rule 3 is optional in that Section [5.3.2] says that
         a router only SHOULD consider TOS when making
         forwarding decisions.


      (1) Basic Match
           This rule discards any routes to destinations other
           than the IP Destination Address of the packet.  For
           example, if a packet's IP Destination Address is     |
           10.144.2.5, this step would discard a route to net
           128.12.0.0/16 but would retain any routes to the     |
           network prefixes 10.0.0.0/8 and 10.144.0.0/16, and
           any default routes.

           More precisely, we assume that each route has a
           destination attribute, called route.dest, and a
           corresponding prefix length, called route.length,
           to specify which bits of route.dest are
           significant.  The IP Destination Address of the
           packet being forwarded is ip.dest.  This rule
           discards all routes from the set of candidates
           except those for which the most significant
           route.length bits of route.dest and ip.dest are





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           equal.

           For example, if a packet's IP Destination Address    |
           is 10.144.2.5 and there are network prefixes         |
           10.144.1.0/24, 10.144.2.0/24, and 10.144.3.0/24,     |
           this rule would keep only 10.144.2.0/24; it is the
           only route whose prefix has the same value as the
           corresponding bits in the IP Destination Address of
           the packet.

      (2) Longest Match
           Longest Match is a refinement of Basic Match,
           described above.  After performing Basic Match
           pruning, the algorithm examines the remaining
           routes to determine which among them have the
           largest route.length values.  All except these are
           discarded.

           For example, if a packet's IP Destination Address    |
           is 10.144.2.5 and there are network prefixes         |
           10.144.2.0/24, 10.144.0.0/16, and 10.0.0.0/8, then   |
           this rule would keep only the first (10.144.2.0/24)
           because its prefix length is longest.

      (3) Weak TOS
           Each route has a type of service attribute, called
           route.tos, whose possible values are assumed to be
           identical to those used in the TOS field of the IP
           header.  Routing protocols that distribute TOS
           information fill in route.tos appropriately in
           routes they add to the FIB; routes from other
           routing protocols are treated as if they have the
           default TOS (0000).  The TOS field in the IP header
           of the packet being routed is called ip.tos.

           The set of candidate routes is examined to
           determine if it contains any routes for which
           route.tos = ip.tos.  If so, all routes except those
           for which route.tos = ip.tos are discarded.  If
           not, all routes except those for which route.tos =
           0000 are discarded from the set of candidate
           routes.

           Additional discussion of routing based on Weak TOS





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           may be found in [ROUTE:11].

           DISCUSSION:
              The effect of this rule is to select only those
              routes that have a TOS that matches the TOS
              requested in the packet.  If no such routes
              exist then routes with the default TOS are
              considered.  Routes with a non-default TOS that
              is not the TOS requested in the packet are never
              used, even if such routes are the only available
              routes that go to the packet's destination.

      (4) Best Metric
           Each route has a metric attribute, called
           route.metric, and a routing domain identifier,
           called route.domain.  Each member of the set of
           candidate routes is compared with each other member
           of the set.  If route.domain is equal for the two
           routes and route.metric is strictly "inferior" for
           one when compared with the other, then the one with
           the "inferior" metric is discarded from the set.
           The determination of "inferior" is usually by a
           simple arithmetic comparison, though some protocols
           may have structured metrics requiring more complex
           comparisons.

      (5) Vendor Policy
           Vendor Policy is sort of a catch-all to make up for
           the fact that the previously listed rules are often
           inadequate to choose from the possible routes.
           Vendor Policy pruning rules are extremely vendor-
           specific.  See section [5.2.4.4].

      This algorithm has two distinct disadvantages.
      Presumably, a router implementor might develop
      techniques to deal with these disadvantages and make
      them a part of the Vendor Policy pruning rule.

      (1) IS-IS and OSPF route classes are not directly
           handled.

      (2) Path properties other than type of service (e.g.,
           MTU) are ignored.






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      It is also worth noting a deficiency in the way that TOS
      is supported: routing protocols that support TOS are
      implicitly preferred when forwarding packets that have
      non-zero TOS values.

      The Basic Match and Longest Match pruning rules
      generalize the treatment of a number of particular types
      of routes.  These routes are selected in the following,
      decreasing, order of preference:

      (1) Host Route: This is a route to a specific end
           system.

      (2) Hierarchical Network Prefix Routes: This is a route
           to a particular network prefix.  Note that the FIB
           may contain several routes to network prefixes that
           subsume each other (one prefix is the other prefix
           with additional bits).  These are selected in order
           of decreasing prefix length.

      (5) Default Route: This is a route to all networks for
           which there are no explicit routes.  It is by
           definition the route whose prefix length is zero.

      If, after application of the pruning rules, the set of
      routes is empty (i.e., no routes were found), the packet
      MUST be discarded and an appropriate ICMP error
      generated (ICMP Bad Source Route if the IP Destination
      Address came from a source route option; otherwise,
      whichever of ICMP Destination Host Unreachable or
      Destination Network Unreachable is appropriate, as
      described in Section [4.3.3.1]).


5.2.4.4 Administrative Preference

         One suggested mechanism for the Vendor Policy Pruning
         Rule is to use administrative preference, which is a
         simple prioritization algorithm.  The idea is to
         manually prioritize the routes that one might need to
         select among.

         Each route has associated with it a "preference
         value", based on various attributes of the route





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         (specific mechanisms for assignment of preference
         values are suggested below).  This preference value
         is an integer in the range [0..255], with zero being
         the most preferred and 254 being the least preferred.
         255 is a special value that means that the route
         should never be used.  The first step in the Vendor
         Policy pruning rule discards all but the most
         preferable routes (and always discards routes whose
         preference value is 255).

         This policy is not "safe" in that it can easily be
         misused to create routing loops.  Since no protocol
         ensures that the preferences configured for a router
         is consistent with the preferences configured in its
         neighbors, network managers must exercise care in
         configuring preferences.

         + Address Match
            It is useful to be able to assign a single
            preference value to all routes (learned from the
            same routing domain) to any of a specified set of
            destinations, where the set of destinations is all
            destinations that match a specified network
            prefix.

         + Route Class
            For routing protocols which maintain the
            distinction, it is useful to be able to assign a
            single preference value to all routes (learned
            from the same routing domain) which have a
            particular route class (intra-area, inter-area,
            external with internal metrics, or external with
            external metrics).

         + Interface
            It is useful to be able to assign a single
            preference value to all routes (learned from a
            particular routing domain) that would cause
            packets to be routed out a particular logical
            interface on the router (logical interfaces
            generally map one-to-one onto the router's network
            interfaces, except that any network interface that
            has multiple IP addresses will have multiple
            logical interfaces associated with it).





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         + Source router
            It is useful to be able to assign a single
            preference value to all routes (learned from the
            same routing domain) that were learned from any of
            a set of routers, where the set of routers are
            those whose updates have a source address that
            match a specified network prefix.

         + Originating AS
            For routing protocols which provide the
            information, it is useful to be able to assign a
            single preference value to all routes (learned
            from a particular routing domain) which originated
            in another particular routing domain.  For BGP
            routes, the originating AS is the first AS listed
            in the route's AS_PATH attribute.  For OSPF
            external routes, the originating AS may be
            considered to be the low order 16 bits of the
            route's external route tag if the tag's Automatic
            bit is set and the tag's Path Length is not equal
            to 3.

         + External route tag
            It is useful to be able to assign a single
            preference value to all OSPF external routes
            (learned from the same routing domain) whose
            external route tags match any of a list of
            specified values.  Because the external route tag
            may contain a structured value, it may be useful
            to provide the ability to match particular
            subfields of the tag.

         + AS path
            It may be useful to be able to assign a single
            preference value to all BGP routes (learned from
            the same routing domain) whose AS path "matches"
            any of a set of specified values.  It is not yet
            clear exactly what kinds of matches are most
            useful.  A simple option would be to allow
            matching of all routes for which a particular AS
            number appears (or alternatively, does not appear)
            anywhere in the route's AS_PATH attribute.  A more
            general but somewhat more difficult alternative
            would be to allow matching all routes for which





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            the AS path matches a specified regular
            expression.


5.2.4.6 Load Splitting

         At the end of the Next-hop selection process,
         multiple routes may still remain.  A router has
         several options when this occurs.  It may arbitrarily
         discard some of the routes.  It may reduce the number
         of candidate routes by comparing metrics of routes
         from routing domains that are not considered
         equivalent.  It may retain more than one route and
         employ a "load-splitting" mechanism to divide traffic
         among them.  Perhaps the only thing that can be said
         about the relative merits of the options is that
         load-splitting is useful in some situations but not
         in others, so a wise implementor who implements
         load-splitting will also provide a way for the
         network manager to disable it.


5.2.5 Unused IP Header Bits: RFC-791 Section 3.1

      The IP header contains several reserved bits, in the
      Type of Service field and in the Flags field.  Routers
      MUST NOT drop packets merely because one or more of
      these reserved bits has a non-zero value.

      Routers MUST ignore and MUST pass through unchanged the
      values of these reserved bits.  If a router fragments a
      packet, it MUST copy these bits into each fragment.

      DISCUSSION:
         Future revisions to the IP protocol may make use of
         these unused bits.  These rules are intended to
         ensure that these revisions can be deployed without
         having to simultaneously upgrade all routers in the
         Internet.










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5.2.6 Fragmentation and Reassembly: RFC-791 Section 3.2

      As was discussed in Section [4.2.2.7], a router MUST
      support IP fragmentation.

      A router MUST NOT reassemble any datagram before
      forwarding it.


      DISCUSSION:
         A few people have suggested that there might be some
         topologies where reassembly of transit datagrams by
         routers might improve performance.  The fact that
         fragments may take different paths to the destination
         precludes safe use of such a feature.

         Nothing in this section should be construed to
         control or limit fragmentation or reassembly
         performed as a link layer function by the router.

         Similarly, if an IP datagram is encapsulated in
         another IP datagram (e.g., it is tunnelled), that
         datagram is in turn fragmented, the fragments must be
         reassembled in order to forward the original
         datagram.  This section does not preclude this.



5.2.7 Internet Control Message Protocol - ICMP

      General requirements for ICMP were discussed in Section
      [4.3].  This section discusses ICMP messages that are
      sent only by routers.


5.2.7.1 Destination Unreachable

         The ICMP Destination Unreachable message is sent by a
         router in response to a packet which it cannot
         forward because the destination (or next hop) is
         unreachable or a service is unavailable.  Examples of
         such cases include a message addressed to a host
         which is not there and therefore does not respond to
         ARP requests, and messages addressed to network





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         prefixes for which the router has no valid route.

         A router MUST be able to generate ICMP Destination
         Unreachable messages and SHOULD choose a response
         code that most closely matches the reason the message
         is being generated.

         The following codes are defined in [INTERNET:8] and
         [INTRO:2]:

         0 = Network Unreachable - generated by a router if a
              forwarding path (route) to the destination
              network is not available;

         1 = Host Unreachable - generated by a router if a
              forwarding path (route) to the destination host
              on a directly connected network is not available
              (does not respond to ARP);

         2 = Protocol Unreachable - generated if the transport
              protocol designated in a datagram is not
              supported in the transport layer of the final
              destination;

         3 = Port Unreachable - generated if the designated
              transport protocol (e.g., UDP) is unable to
              demultiplex the datagram in the transport layer
              of the final destination but has no protocol
              mechanism to inform the sender;

         4 = Fragmentation Needed and DF Set - generated if a
              router needs to fragment a datagram but cannot
              since the DF flag is set;

         5 = Source Route Failed - generated if a router
              cannot forward a packet to the next hop in a
              source route option;

         6 = Destination Network Unknown - This code SHOULD
              NOT be generated since it would imply on the
              part of the router that the destination network
              does not exist (net unreachable code 0 SHOULD be
              used in place of code 6);






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         7 = Destination Host Unknown - generated only when a
              router can determine (from link layer advice)
              that the destination host does not exist;

         11 = Network Unreachable For Type Of Service -
              generated by a router if a forwarding path
              (route) to the destination network with the
              requested or default TOS is not available;

         12 = Host Unreachable For Type Of Service - generated
              if a router cannot forward a packet because its
              route(s) to the destination do not match either
              the TOS requested in the datagram or the default
              TOS (0).

         The following additional codes are hereby defined:

         13 = Communication Administratively Prohibited -
              generated if a router cannot forward a packet
              due to administrative filtering;

         14 = Host Precedence Violation.  Sent by the first
              hop router to a host to indicate that a
              requested precedence is not permitted for the
              particular combination of source/destination
              host or network, upper layer protocol, and
              source/destination port;

         15 = Precedence cutoff in effect.  The network
              operators have imposed a minimum level of
              precedence required for operation, the datagram
              was sent with a precedence below this level;

         NOTE: [INTRO:2] defined Code 8 for "source host
         isolated".  Routers SHOULD NOT generate Code 8;
         whichever of Codes 0 (Network Unreachable) and 1
         (Host Unreachable) is appropriate SHOULD be used
         instead.  [INTRO:2] also defined Code 9 for
         communication with destination network
         administratively prohibited and Code 10 for
         communication with destination host administratively
         prohibited.  These codes were intended for use by
         end-to-end encryption devices used by U.S military
         agencies.  Routers SHOULD use the newly defined Code





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         13 (Communication Administratively Prohibited) if
         they administratively filter packets.

         Routers MAY have a configuration option that causes
         Code 13 (Communication Administratively Prohibited)
         messages not to be generated.  When this option is
         enabled, no ICMP error message is sent in response to
         a packet that is dropped because its forwarding is
         administratively prohibited.

         Similarly, routers MAY have a configuration option
         that causes Code 14 (Host Precedence Violation) and
         Code 15 (Precedence Cutoff in Effect) messages not to
         be generated.  When this option is enabled, no ICMP
         error message is sent in response to a packet that is
         dropped because of a precedence violation.

         Routers MUST use Host Unreachable or Destination Host
         Unknown codes whenever other hosts on the same
         destination network might be reachable; otherwise,
         the source host may erroneously conclude that all
         hosts on the network are unreachable, and that may
         not be the case.

         [INTERNET:14] describes a slight modification the
         form of Destination Unreachable messages containing
         Code 4 (Fragmentation needed and DF set).  A router
         MUST use this modified form when originating Code 4
         Destination Unreachable messages.


5.2.7.2 Redirect

         The ICMP Redirect message is generated to inform a
         local host the it should use a different next hop
         router for a certain class of traffic.

         Routers MUST NOT generate the Redirect for Network or
         Redirect for Network and Type of Service messages
         (Codes 0 and 2) specified in [INTERNET:8].  Routers
         MUST be able to generate the Redirect for Host
         message (Code 1) and SHOULD be able to generate the
         Redirect for Type of Service and Host message (Code
         3) specified in [INTERNET:8].





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         DISCUSSION:
            If the directly connected network is not subnetted
            (in the classical sense), a router can normally
            generate a network Redirect that applies to all
            hosts on a specified remote network.  Using a
            network rather than a host Redirect may economize
            slightly on network traffic and on host routing
            table storage.  However, the savings are not
            significant, and subnets create an ambiguity about
            the subnet mask to be used to interpret a network
            Redirect.  In a CIDR environment, it is difficult
            to specify precisely the cases in which network
            Redirects can be used.  Therefore, routers must
            send only host (or host and type of service)
            Redirects.

         A Code 3 (Redirect for Host and Type of Service)
         message is generated when the packet provoking the
         redirect has a destination for which the path chosen
         by the router would depend (in part) on the TOS
         requested.

         Routers that can generate Code 3 redirects (Host and
         Type of Service) MUST have a configuration option
         (which defaults to on) to enable Code 1 (Host)
         redirects to be substituted for Code 3 redirects.  A
         router MUST send a Code 1 Redirect in place of a Code
         3 Redirect if it has been configured to do so.

         If a router is not able to generate Code 3 Redirects
         then it MUST generate Code 1 Redirects in situations
         where a Code 3 Redirect is called for.

         Routers MUST NOT generate a Redirect Message unless
         all the following conditions are met:

         + The packet is being forwarded out the same physical
            interface that it was received from,

         + The IP source address in the packet is on the same
            Logical IP (sub)network as the next-hop IP
            address, and

         + The packet does not contain an IP source route





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            option.

         The source address used in the ICMP Redirect MUST
         belong to the same logical (sub)net as the
         destination address.

         A router using a routing protocol (other than static
         routes) MUST NOT consider paths learned from ICMP
         Redirects when forwarding a packet.  If a router is
         not using a routing protocol, a router MAY have a
         configuration that, if set, allows the router to
         consider routes learned through ICMP Redirects when
         forwarding packets.

         DISCUSSION:
            ICMP Redirect is a mechanism for routers to convey
            routing information to hosts.  Routers use other
            mechanisms to learn routing information, and
            therefore have no reason to obey redirects.
            Believing a redirect which contradicted the
            router's other information would likely create
            routing loops.

            On the other hand, when a router is not acting as
            a router, it MUST comply with the behavior
            required of a host.



5.2.7.3 Time Exceeded

         A router MUST generate a Time Exceeded message Code 0
         (In Transit) when it discards a packet due to an
         expired TTL field.  A router MAY have a per-interface
         option to disable origination of these messages on
         that interface, but that option MUST default to
         allowing the messages to be originated.


5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP

      IGMP [INTERNET:4] is a protocol used between hosts and
      multicast routers on a single physical network to
      establish hosts' membership in particular multicast





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      groups.  Multicast routers use this information, in
      conjunction with a multicast routing protocol, to
      support IP multicast forwarding across the Internet.

      A router SHOULD implement the multicast router part of
      IGMP.


5.3 SPECIFIC ISSUES



5.3.1 Time to Live (TTL)

      The Time-to-Live (TTL) field of the IP header is defined
      to be a timer limiting the lifetime of a datagram.  It
      is an 8-bit field and the units are seconds.  Each
      router (or other module) that handles a packet MUST
      decrement the TTL by at least one, even if the elapsed
      time was much less than a second.  Since this is very
      often the case, the TTL is effectively a hop count limit
      on how far a datagram can propagate through the
      Internet.

      When a router forwards a packet, it MUST reduce the TTL
      by at least one.  If it holds a packet for more than one
      second, it MAY decrement the TTL by one for each second.

      If the TTL is reduced to zero (or less), the packet MUST
      be discarded, and if the destination is not a multicast
      address the router MUST send an ICMP Time Exceeded
      message, Code 0 (TTL Exceeded in Transit) message to the
      source.  Note that a router MUST NOT discard an IP
      unicast or broadcast packet with a non-zero TTL merely
      because it can predict that another router on the path
      to the packet's final destination will decrement the TTL
      to zero.  However, a router MAY do so for IP multicasts,
      in order to more efficiently implement IP multicast's
      expanding ring search algorithm (see [INTERNET:4]).

      DISCUSSION:
         The IP TTL is used, somewhat schizophrenically, as
         both a hop count limit and a time limit.  Its hop
         count function is critical to ensuring that routing





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         problems can't melt down the network by causing
         packets to loop infinitely in the network.  The time
         limit function is used by transport protocols such as
         TCP to ensure reliable data transfer.  Many current
         implementations treat TTL as a pure hop count, and in
         parts of the Internet community there is a strong
         sentiment that the time limit function should instead
         be performed by the transport protocols that need it.

         In this specification, we have reluctantly decided to
         follow the strong belief among the router vendors
         that the time limit function should be optional.
         They argued that implementation of the time limit
         function is difficult enough that it is currently not
         generally done.  They further pointed to the lack of
         documented cases where this shortcut has caused TCP
         to corrupt data (of course, we would expect the
         problems created to be rare and difficult to
         reproduce, so the lack of documented cases provides
         little reassurance that there haven't been a number
         of undocumented cases).

         IP multicast notions such as the expanding ring
         search may not work as expected unless the TTL is
         treated as a pure hop count.  The same thing is
         somewhat true of traceroute.

         ICMP Time Exceeded messages are required because the
         traceroute diagnostic tool depends on them.

         Thus, the tradeoff is between severely crippling, if
         not eliminating, two very useful tools and avoiding a
         very rare and transient data transport problem that
         may not occur at all.  We have chosen to preserve the
         tools.



5.3.2 Type of Service (TOS)

      The "Type-of-Service" byte in the IP header is divided
      into three sections: the Precedence field (high-order 3
      bits), a field that is customarily called "Type of
      Service" or "TOS" (next 4 bits), and a reserved bit (the





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      low order bit).  Rules governing the reserved bit were
      described in Section [4.2.2.3].  The Precedence field
      will be discussed in Section [5.3.3].  A more extensive
      discussion of the TOS field and its use can be found in
      [ROUTE:11].

      A router SHOULD consider the TOS field in a packet's IP
      header when deciding how to forward it.  The remainder
      of this section describes the rules that apply to
      routers that conform to this requirement.

      A router MUST maintain a TOS value for each route in its
      routing table.  Routes learned through a routing
      protocol that does not support TOS MUST be assigned a
      TOS of zero (the default TOS).

      To choose a route to a destination, a router MUST use an
      algorithm equivalent to the following:

      (1) The router locates in its routing table all
           available routes to the destination (see Section
           [5.2.4]).

      (2) If there are none, the router drops the packet
           because the destination is unreachable.  See
           section [5.2.4].

      (3) If one or more of those routes have a TOS that
           exactly matches the TOS specified in the packet,
           the router chooses the route with the best metric.

      (4) Otherwise, the router repeats the above step, except
           looking at routes whose TOS is zero.

      (5) If no route was chosen above, the router drops the
           packet because the destination is unreachable.  The
           router returns an ICMP Destination Unreachable
           error specifying the appropriate code: either
           Network Unreachable with Type of Service (code 11)
           or Host Unreachable with Type of Service (code 12).

      DISCUSSION:
         Although TOS has been little used in the past, its
         use by hosts is now mandated by the Requirements for





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         Internet Hosts RFCs ([INTRO:2] and [INTRO:3]).
         Support for TOS in routers may become a MUST in the
         future, but is a SHOULD for now until we get more
         experience with it and can better judge both its
         benefits and its costs.

         Various people have proposed that TOS should affect
         other aspects of the forwarding function.  For
         example:

         (1) A router could place packets that have the "Low
              Delay" bit set ahead of other packets in its
              output queues.

         (2) a router is forced to discard packets, it could
              try to avoid discarding those which have the
              "High Reliability" bit set.

         These ideas have been explored in more detail in
         [INTERNET:17] but we don't yet have enough experience
         with such schemes to make requirements in this area.



5.3.3 IP Precedence

      This section specifies requirements and guidelines for
      appropriate processing of the IP Precedence field in
      routers.  Precedence is a scheme for allocating
      resources in the network based on the relative
      importance of different traffic flows.  The IP
      specification defines specific values to be used in this
      field for various types of traffic.

      The basic mechanisms for precedence processing in a
      router are preferential resource allocation, including
      both precedence-ordered queue service and precedence-
      based congestion control, and selection of Link Layer
      priority features.  The router also selects the IP
      precedence for routing, management and control traffic
      it originates.  For a more extensive discussion of IP
      Precedence and its implementation see [FORWARD:6].

      Precedence-ordered queue service, as discussed in this





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      section, includes but is not limited to the queue for
      the forwarding process and queues for outgoing links.
      It is intended that a router supporting precedence
      should also use the precedence indication at whatever
      points in its processing are concerned with allocation
      of finite resources, such as packet buffers or Link
      Layer connections.  The set of such points is
      implementation-dependent.

      DISCUSSION:
         Although the Precedence field was originally provided
         for use in DOD systems where large traffic surges or
         major damage to the network are viewed as inherent
         threats, it has useful applications for many non-
         military IP networks.  Although the traffic handling
         capacity of networks has grown greatly in recent
         years, the traffic generating ability of the users
         has also grown, and network overload conditions still
         occur at times.  Since IP-based routing and
         management protocols have become more critical to the
         successful operation of the Internet, overloads
         present two additional risks to the network:

         (1) High delays may result in routing protocol
              packets being lost.  This may cause the routing
              protocol to falsely deduce a topology change and
              propagate this false information to other
              routers.  Not only can this cause routes to
              oscillate, but an extra processing burden may be
              placed on other routers.

         (2) High delays may interfere with the use of network
              management tools to analyze and perhaps correct
              or relieve the problem in the network that
              caused the overload condition to occur.

         Implementation and appropriate use of the Precedence
         mechanism alleviates both of these problems.











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5.3.3.1 Precedence-Ordered Queue Service

         Routers SHOULD implement precedence-ordered queue
         service.  Precedence-ordered queue service means that
         when a packet is selected for output on a (logical)
         link, the packet of highest precedence that has been
         queued for that link is sent.  Routers that implement
         precedence-ordered queue service MUST also have a
         configuration option to suppress precedence-ordered
         queue service in the Internet Layer.

         Any router MAY implement other policy-based
         throughput management procedures that result in other
         than strict precedence ordering, but it MUST be
         configurable to suppress them (i.e., use strict
         ordering).

         As detailed in Section [5.3.6], routers that
         implement precedence-ordered queue service discard
         low precedence packets before discarding high
         precedence packets for congestion control purposes.

         Preemption (interruption of processing or
         transmission of a packet) is not envisioned as a
         function of the Internet Layer.  Some protocols at
         other layers may provide preemption features.


5.3.3.2 Lower Layer Precedence Mappings

         Routers that implement precedence-ordered queuing
         MUST IMPLEMENT, and other routers SHOULD IMPLEMENT,
         Lower Layer Precedence Mapping.

         A router that implements Lower Layer Precedence
         Mapping:

         + MUST be able to map IP Precedence to Link Layer
            priority mechanisms for link layers that have such
            a feature defined.

         + MUST have a configuration option to select the Link
            Layer's default priority treatment for all IP
            traffic





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         + SHOULD be able to configure specific nonstandard
            mappings of IP precedence values to Link Layer
            priority values for each interface.

         DISCUSSION:
            Some research questions the workability of the
            priority features of some Link Layer protocols,
            and some networks may have faulty implementations
            of the link layer priority mechanism.  It seems
            prudent to provide an escape mechanism in case
            such problems show up in a network.

            On the other hand, there are proposals to use
            novel queuing strategies to implement special
            services such as multimedia bandwidth reservation
            or low-delay service.  Special services and
            queuing strategies to support them are current
            research subjects and are in the process of
            standardization.

            Implementors may wish to consider that correct
            link layer mapping of IP precedence is required by
            DOD policy for TCP/IP systems used on DOD
            networks.  Since these requirements are intended
            to encourage (but not force) the use of precedence
            features in the hope of providing better Internet
            service to all users, routers supporting
            precedence-ordered queue service should default to
            maintaining strict precedence ordering regardless
            of the type of service requested.



5.3.3.3 Precedence Handling For All Routers

         A router (whether or not it employs precedence-
         ordered queue service):

         (1) MUST accept and process incoming traffic of all
              precedence levels normally, unless it has been
              administratively configured to do otherwise.

         (2) MAY implement a validation filter to
              administratively restrict the use of precedence





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              levels by particular traffic sources.  If
              provided, this filter MUST NOT filter out or cut
              off the following sorts of ICMP error messages:
              Destination Unreachable, Redirect, Time
              Exceeded, and Parameter Problem.  If this filter
              is provided, the procedures required for packet
              filtering by addresses are required for this
              filter also.

              DISCUSSION:
                 Precedence filtering should be applicable to
                 specific source/destination IP Address pairs,
                 specific protocols, specific ports, and so
                 on.

              An ICMP Destination Unreachable message with
              code 14 SHOULD be sent when a packet is dropped
              by the validation filter, unless this has been
              suppressed by configuration choice.

         (3) MAY implement a cutoff function that allows the
              router to be set to refuse or drop traffic with
              precedence below a specified level.  This
              function may be activated by management actions
              or by some implementation dependent heuristics,
              but there MUST be a configuration option to
              disable any heuristic mechanism that operates
              without human intervention.  An ICMP Destination
              Unreachable message with code 15 SHOULD be sent
              when a packet is dropped by the cutoff function,
              unless this has been suppressed by configuration
              choice.

              A router MUST NOT refuse to forward datagrams
              with IP precedence of 6 (Internetwork Control)
              or 7 (Network Control) solely due to precedence
              cutoff.  However, other criteria may be used in
              conjunction with precedence cutoff to filter
              high precedence traffic.

              DISCUSSION:
                 Unrestricted precedence cutoff could result
                 in an unintentional cutoff of routing and
                 control traffic.  In the general case, host





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                 traffic should be restricted to a value of 5
                 (CRITIC/ECP) or below; this is not a
                 requirement and may not be correct in certain
                 systems.


         (4) MUST NOT change precedence settings on packets it
              did not originate.

         (5) SHOULD be able to configure distinct precedence
              values to be used for each routing or management
              protocol supported (except for those protocols,
              such as OSPF, which specify which precedence
              value must be used).

         (6) MAY be able to configure routing or management
              traffic precedence values independently for each
              peer address.

         (7) MUST respond appropriately to Link Layer
              precedence-related error indications where
              provided.  An ICMP Destination Unreachable
              message with code 15 SHOULD be sent when a
              packet is dropped because a link cannot accept
              it due to a precedence-related condition, unless
              this has been suppressed by configuration
              choice.

              DISCUSSION:
                 The precedence cutoff mechanism described in
                 (3) is somewhat controversial.  Depending on
                 the topological location of the area affected
                 by the cutoff, transit traffic may be
                 directed by routing protocols into the area
                 of the cutoff, where it will be dropped.
                 This is only a problem if another path that
                 is unaffected by the cutoff exists between
                 the communicating points.  Proposed ways of
                 avoiding this problem include providing some
                 minimum bandwidth to all precedence levels
                 even under overload conditions, or
                 propagating cutoff information in routing
                 protocols.  In the absence of a widely
                 accepted (and implemented) solution to this





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                 problem, great caution is recommended in
                 activating cutoff mechanisms in transit
                 networks.

                 A transport layer relay could legitimately
                 provide the function prohibited by (4) above.
                 Changing precedence levels may cause subtle
                 interactions with TCP and perhaps other
                 protocols; a correct design is a non-trivial
                 task.

                 The intent of (5) and (6) (and the discussion
                 of IP Precedence in ICMP messages in Section
                 [4.3.2]) is that the IP precedence bits
                 should be appropriately set, whether or not
                 this router acts upon those bits in any other
                 way.  We expect that in the future
                 specifications for routing protocols and
                 network management protocols will specify how
                 the IP Precedence should be set for messages
                 sent by those protocols.

                 The appropriate response for (7) depends on
                 the link layer protocol in use.  Typically,
                 the router should stop trying to send
                 "offensive" traffic to that destination for
                 some period of time, and should return an
                 ICMP Destination Unreachable message with
                 code 15 (service not available for precedence
                 requested) to the traffic source.  It also
                 should not try to reestablish a preempted
                 Link Layer connection for some time.



5.3.4 Forwarding of Link Layer Broadcasts

      The encapsulation of IP packets in most Link Layer
      protocols (except PPP) allows a receiver to distinguish
      broadcasts and multicasts from unicasts simply by
      examining the Link Layer protocol headers (most
      commonly, the Link Layer destination address).  The
      rules in this section that refer to "Link Layer
      broadcasts" apply only to Link Layer protocols that





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      allow broadcasts to be distinguished; likewise, the
      rules that refer to "Link Layer multicasts" apply only
      to Link Layer protocols that allow multicasts to be
      distinguished.

      A router MUST NOT forward any packet that the router
      received as a Link Layer broadcast, unless it is
      directed to an IP Multicast address.  In this latter
      case, one would presume that link layer broadcast was
      used due to the lack of an effective multicast service.

      A router MUST NOT forward any packet which the router
      received as a Link Layer multicast unless the packet's
      destination address is an IP multicast address.

      A router SHOULD silently discard a packet that is
      received via a Link Layer broadcast but does not specify
      an IP multicast or IP broadcast destination address.

      When a router sends a packet as a Link Layer broadcast,
      the IP destination address MUST be a legal IP broadcast
      or IP multicast address.


5.3.5 Forwarding of Internet Layer Broadcasts

      There are two major types of IP broadcast addresses;
      limited broadcast and directed broadcast.  In addition,
      there are three subtypes of directed broadcast: a
      broadcast directed to a specified network prefix, a
      broadcast directed to a specified subnetwork, and a
      broadcast directed to all subnets of a specified
      network.  Classification by a router of a broadcast into
      one of these categories depends on the broadcast address
      and on the router's understanding (if any) of the subnet
      structure of the destination network.  The same
      broadcast will be classified differently by different
      routers.

      A limited IP broadcast address is defined to be all-
      ones: { -1, -1 } or 255.255.255.255.

      A network-prefix-directed broadcast is composed of the    |
      network prefix of the IP address with a local part of     |





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      all-ones or { <Network-prefix>, -1 }.  For example, a
      Class A net broadcast address is net.255.255.255, a
      Class B net broadcast address is net.net.255.255 and a
      Class C net broadcast address is net.net.net.255 where
      "net" is a byte of the network address.

      The all-subnets-directed-broadcast is not well defined
      in a CIDR environment, and was deprecated in version 1
      of this memo.

      As was described in Section [4.2.3.1], a router may
      encounter certain non-standard IP broadcast addresses:

      + 0.0.0.0 is an obsolete form of the limited broadcast
         address

      + { <Network-prefix>, 0 } is an obsolete form of a
         network-prefix-directed broadcast address.

      As was described in that section, packets addressed to
      any of these addresses SHOULD be silently discarded, but
      if they are not, they MUST be treated according to the
      same rules that apply to packets addressed to the non-
      obsolete forms of the broadcast addresses described
      above.  These rules are described in the next few
      sections.


5.3.5.1 Limited Broadcasts

         Limited broadcasts MUST NOT be forwarded.  Limited
         broadcasts MUST NOT be discarded.  Limited broadcasts
         MAY be sent and SHOULD be sent instead of directed
         broadcasts where limited broadcasts will suffice.

         DISCUSSION:
            Some routers contain UDP servers which function by
            resending the requests (as unicasts or directed
            broadcasts) to other servers.  This requirement
            should not be interpreted as prohibiting such
            servers.  Note, however, that such servers can
            easily cause packet looping if misconfigured.
            Thus, providers of such servers would probably be
            well advised to document their setup carefully and





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            to consider carefully the TTL on packets that are
            sent.



5.3.5.2 Directed Broadcasts

         A router MUST classify as network-prefix-directed      |
         broadcasts all valid, directed broadcasts destined
         for a remote network or an attached nonsubnetted
         network.  Note that in view of CIDR, such appear to    |
         be host addresses within the network prefix; we
         preclude inspection of the host part of such network
         prefixes.  Given a route and no overriding policy,     |
         then, a router MUST forward network-prefix-directed
         broadcasts.  Network-Prefix-Directed broadcasts MAY
         be sent.

         A router MAY have an option to disable receiving       |
         network-prefix-directed broadcasts on an interface     |
         and MUST have an option to disable forwarding          |
         network-prefix-directed broadcasts.  These options
         MUST default to permit receiving and forwarding
         network-prefix-directed broadcasts.

         DISCUSSION:
            There has been some debate about forwarding or not
            forwarding directed broadcasts.  In this memo we
            have made the forwarding decision depend on the
            router's knowledge of the destination network
            prefix.  Routers cannot determine that a message
            is unicast or directed broadcast apart from this
            knowledge.  The decision to forward or not forward
            the message is by definition only possible in the
            last hop router.



5.3.5.3 All-subnets-directed Broadcasts

         The first version of this memo described an algorithm
         for distributing a directed broadcast to all the
         subnets of a classical network number.  This
         algorithm was stated to be "broken," and certain





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         failure cases were specified.

         In a CIDR routing domain, wherein classical IP
         network numbers are meaningless, the concept of an
         all-subnets-directed-broadcast is also meaningless.
         To the knowledge of the working group, the facility
         was never implemented or deployed, and is now
         relegated to the dustbin of history.


5.3.5.4 Network-Prefix-Directed Broadcasts

         The first version of this memo spelled out procedures
         for dealing with network-prefix-directed-broadcasts.
         In a CIDR routing domain, these are indistinguishable
         from network-prefix-directed-broadcasts.  The two are
         therefore treated together in section [5.3.5.2
         Directed Broadcasts].


5.3.6 Congestion Control

      Congestion in a network is loosely defined as a
      condition where demand for resources (usually bandwidth
      or CPU time) exceeds capacity.  Congestion avoidance
      tries to prevent demand from exceeding capacity, while
      congestion recovery tries to restore an operative state.
      It is possible for a router to contribute to both of
      these mechanisms.  A great deal of effort has been spent
      studying the problem.  The reader is encouraged to read
      [FORWARD:2] for a survey of the work.  Important papers
      on the subject include [FORWARD:3], [FORWARD:4],
      [FORWARD:5], [FORWARD:10], [FORWARD:11], [FORWARD:12],
      [FORWARD:13], [FORWARD:14], and [INTERNET:10], among
      others.

      The amount of storage that router should have available
      to handle peak instantaneous demand when hosts use
      reasonable congestion policies, such as described in
      [FORWARD:5], is a function of the product of the
      bandwidth of the link times the path delay of the flows
      using the link, and therefore storage should increase as
      this Bandwidth*Delay product increases.  The exact
      function relating storage capacity to probability of





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      discard is not known.

      When a router receives a packet beyond its storage
      capacity it must (by definition, not by decree) discard
      it or some other packet or packets.  Which packet to
      discard is the subject of much study but, unfortunately,
      little agreement so far.  The best wisdom to date
      suggests discarding a packet from the data stream most
      heavily using the link.  However, a number of additional
      factors may be relevant, including the precedence of the
      traffic, active bandwidth reservation, and the
      complexity associated with selecting that packet.

      A router MAY discard the packet it has just received;
      this is the simplest but not the best policy.  Ideally,
      the router should select a packet from one of the
      sessions most heavily abusing the link, given that the
      applicable Quality of Service policy permits this.  A
      recommended policy in datagram environments using FIFO
      queues is to discard a packet randomly selected from the
      queue (see [FORWARD:5]).  An equivalent algorithm in
      routers using fair queues is to discard from the longest  |
      queue or that using the greatest virtual time (see        |
      [FORWARD:13]).  A router MAY use these algorithms to
      determine which packet to discard.

      If a router implements a discard policy (such as Random
      Drop) under which it chooses a packet to discard from a
      pool of eligible packets:

      + If precedence-ordered queue service (described in
         Section [5.3.3.1]) is implemented and enabled, the
         router MUST NOT discard a packet whose IP precedence
         is higher than that of a packet that is not
         discarded.

      + A router MAY protect packets whose IP headers request
         the "maximize reliability" TOS, except where doing so
         would be in violation of the previous rule.

      + A router MAY protect fragmented IP packets, on the
         theory that dropping a fragment of a datagram may
         increase congestion by causing all fragments of the
         datagram to be retransmitted by the source.





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      + To help prevent routing perturbations or disruption of
         management functions, the router MAY protect packets
         used for routing control, link control, or network
         management from being discarded.  Dedicated routers
         (i.e., routers that are not also general purpose
         hosts, terminal servers, etc.) can achieve an
         approximation of this rule by protecting packets
         whose source or destination is the router itself.

      Advanced methods of congestion control include a notion
      of fairness, so that the 'user' that is penalized by
      losing a packet is the one that contributed the most to
      the congestion.  No matter what mechanism is implemented
      to deal with bandwidth congestion control, it is
      important that the CPU effort expended be sufficiently
      small that the router is not driven into CPU congestion
      also.

      As described in Section [4.3.3.3], this document
      recommends that a router SHOULD NOT send a Source Quench
      to the sender of the packet that it is discarding.  ICMP
      Source Quench is a very weak mechanism, so it is not
      necessary for a router to send it, and host software
      should not use it exclusively as an indicator of
      congestion.


5.3.7 Martian Address Filtering

      An IP source address is invalid if it is a special IP
      address, as defined in 4.2.2.11 or 5.3.7, or is not a
      unicast address.

      An IP destination address is invalid if it is among
      those defined as illegal destinations in 4.2.3.1, or is
      a Class E address (except 255.255.255.255).

      A router SHOULD NOT forward any packet that has an
      invalid IP source address or a source address on network
      0.  A router SHOULD NOT forward, except over a loopback
      interface, any packet that has a source address on
      network 127.  A router MAY have a switch that allows the
      network manager to disable these checks.  If such a
      switch is provided, it MUST default to performing the





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      checks.

      A router SHOULD NOT forward any packet that has an
      invalid IP destination address or a destination address
      on network 0.  A router SHOULD NOT forward, except over
      a loopback interface, any packet that has a destination
      address on network 127.  A router MAY have a switch that
      allows the network manager to disable these checks.  If
      such a switch is provided, it MUST default to performing
      the checks.

      If a router discards a packet because of these rules, it
      SHOULD log at least the IP source address, the IP
      destination address, and, if the problem was with the
      source address, the physical interface on which the
      packet was received and the Link Layer address of the
      host or router from which the packet was received.


5.3.8 Source Address Validation

      A router SHOULD IMPLEMENT the ability to filter traffic
      based on a comparison of the source address of a packet
      and the forwarding table for a logical interface on
      which the packet was received.  If this filtering is
      enabled, the router MUST silently discard a packet if
      the interface on which the packet was received is not
      the interface on which a packet would be forwarded to
      reach the address contained in the source address.  In
      simpler terms, if a router wouldn't route a packet
      containing this address through a particular interface,
      it shouldn't believe the address if it appears as a
      source address in a packet read from this interface.

      If this feature is implemented, it MUST be disabled by
      default.

      DISCUSSION:
         This feature can provide useful security improvements
         in some situations, but can erroneously discard valid
         packets in situations where paths are asymmetric.








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5.3.9 Packet Filtering and Access Lists

      As a means of providing security and/or limiting traffic
      through portions of a network a router SHOULD provide
      the ability to selectively forward (or filter) packets.
      If this capability is provided, filtering of packets
      SHOULD be configurable either to forward all packets or
      to selectively forward them based upon the source and
      destination prefixes, and MAY filter on other message
      attributes.  Each source and destination address SHOULD
      allow specification of an arbitrary prefix length.


      DISCUSSION:
         This feature can provide a measure of privacy, where
         systems outside a boundary are not permitted to
         exchange certain protocols with systems inside the
         boundary, or are limited as to which systems they may
         communicate with.  It can also help prevent certain
         classes of security breach, wherein a system outside
         a boundary masquerades as a system inside the
         boundary and mimics a session with it.


      If supported, a router SHOULD be configurable to allow
      one of an

      + Include list - specification of a list of message
         definitions to be forwarded, or an

      + Exclude list - specification of a list of message
         definitions NOT to be forwarded.

      A "message definition", in this context, specifies the
      source and destination network prefix, and may include
      other identifying information such as IP Protocol Type
      or TCP port number.

      A router MAY provide a configuration switch that allows
      a choice between specifying an include or an exclude
      list, or other equivalent controls.

      A value matching any address (e.g., a keyword "any", an
      address with a mask of all 0's, or a network prefix





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      whose length is zero) MUST be allowed as a source and/or
      destination address.

      In addition to address pairs, the router MAY allow any
      combination of transport and/or application protocol and
      source and destination ports to be specified.

      The router MUST allow packets to be silently discarded
      (i.e., discarded without an ICMP error message being
      sent).

      The router SHOULD allow an appropriate ICMP unreachable
      message to be sent when a packet is discarded.  The ICMP
      message SHOULD specify Communication Administratively
      Prohibited (code 13) as the reason for the destination
      being unreachable.

      The router SHOULD allow the sending of ICMP destination
      unreachable messages (code 13) to be configured for each
      combination of address pairs, protocol types, and ports
      it allows to be specified.

      The router SHOULD count and SHOULD allow selective
      logging of packets not forwarded.


5.3.10 Multicast Routing

      An IP router SHOULD support forwarding of IP multicast
      packets, based either on static multicast routes or on
      routes dynamically determined by a multicast routing
      protocol (e.g., DVMRP [ROUTE:9]).  A router that
      forwards IP multicast packets is called a multicast
      router.


5.3.11 Controls on Forwarding

      For each physical interface, a router SHOULD have a
      configuration option that specifies whether forwarding
      is enabled on that interface.  When forwarding on an
      interface is disabled, the router:

      + MUST silently discard any packets which are received





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         on that interface but are not addressed to the router

      + MUST NOT send packets out that interface, except for
         datagrams originated by the router

      + MUST NOT announce via any routing protocols the
         availability of paths through the interface

      DISCUSSION:
         This feature allows the network manager to
         essentially turn off an interface but leaves it
         accessible for network management.

         Ideally, this control would apply to logical rather
         than physical interfaces.  It cannot, because there
         is no known way for a router to determine which
         logical interface a packet arrived absent a one-to-
         one correspondence between logical and physical
         interfaces.



5.3.12 State Changes

      During router operation, interfaces may fail or be
      manually disabled, or may become available for use by
      the router.  Similarly, forwarding may be disabled for a
      particular interface or for the entire router or may be
      (re)enabled.  While such transitions are (usually)
      uncommon, it is important that routers handle them
      correctly.


5.3.12.1 When a Router Ceases Forwarding

         When a router ceases forwarding it MUST stop
         advertising all routes, except for third party
         routes.  It MAY continue to receive and use routes
         from other routers in its routing domains.  If the
         forwarding database is retained, the router MUST NOT
         cease timing the routes in the forwarding database.
         If routes that have been received from other routers
         are remembered, the router MUST NOT cease timing the
         routes that it has remembered.  It MUST discard any





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         routes whose timers expire while forwarding is
         disabled, just as it would do if forwarding were
         enabled.

         DISCUSSION:
            When a router ceases forwarding, it essentially
            ceases being a router.  It is still a host, and
            must follow all of the requirements of Host
            Requirements [INTRO:2].  The router may still be a
            passive member of one or more routing domains,
            however.  As such, it is allowed to maintain its
            forwarding database by listening to other routers
            in its routing domain.  It may not, however,
            advertise any of the routes in its forwarding
            database, since it itself is doing no forwarding.
            The only exception to this rule is when the router
            is advertising a route that uses only some other
            router, but which this router has been asked to
            advertise.

         A router MAY send ICMP destination unreachable (host
         unreachable) messages to the senders of packets that
         it is unable to forward.  It SHOULD NOT send ICMP
         redirect messages.

         DISCUSSION:
            Note that sending an ICMP destination unreachable
            (host unreachable) is a router action.  This
            message should not be sent by hosts.  This
            exception to the rules for hosts is allowed so
            that packets may be rerouted in the shortest
            possible time, and so that "black holes" are
            avoided.



5.3.12.2 When a Router Starts Forwarding

         When a router begins forwarding, it SHOULD expedite
         the sending of new routing information to all routers
         with which it normally exchanges routing information.








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5.3.12.3 When an Interface Fails or is Disabled

         If an interface fails or is disabled a router MUST
         remove and stop advertising all routes in its
         forwarding database that make use of that interface.
         It MUST disable all static routes that make use of
         that interface.  If other routes to the same
         destination and TOS are learned or remembered by the
         router, the router MUST choose the best alternate,
         and add it to its forwarding database.  The router
         SHOULD send ICMP destination unreachable or ICMP
         redirect messages, as appropriate, in reply to all
         packets that it is unable to forward due to the
         interface being unavailable.


5.3.12.4 When an Interface is Enabled

         If an interface that had not been available becomes
         available, a router MUST reenable any static routes
         that use that interface.  If routes that would use
         that interface are learned by the router, then these
         routes MUST be evaluated along with all the other
         learned routes, and the router MUST make a decision
         as to which routes should be placed in the forwarding
         database.  The implementor is referred to Chapter
         [7], "Application Layer - Routing Protocols" for
         further information on how this decision is made.

         A router SHOULD expedite the sending of new routing
         information to all routers with which it normally
         exchanges routing information.


5.3.13 IP Options

      Several options, such as Record Route and Timestamp,
      contain "slots" into which a router inserts its address
      when forwarding the packet.  However, each such option
      has a finite number of slots, and therefore a router may
      find that there is not free slot into which it can
      insert its address.  No requirement listed below should
      be construed as requiring a router to insert its address
      into an option that has no remaining slot to insert it





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      into.  Section [5.2.5] discusses how a router must
      choose which of its addresses to insert into an option.


5.3.13.1 Unrecognized Options

         Unrecognized IP options in forwarded packets MUST be
         passed through unchanged.


5.3.13.2 Security Option

         Some environments require the Security option in
         every packet; such a requirement is outside the scope
         of this document and the IP standard specification.
         Note, however, that the security options described in
         [INTERNET:1] and [INTERNET:16] are obsolete.  Routers
         SHOULD IMPLEMENT the revised security option
         described in [INTERNET:5].


         DISCUSSION:
            Routers intended for use in networks with multiple
            security levels should support packet filtering
            based on IPSO (RFC-1108) labels.  To implement
            this support, the router would need to permit the
            router administrator to configure both a lower
            sensitivity limit (e.g. Unclassified) and an upper
            sensitivity limit (e.g. Secret) on each interface.
            It is commonly but not always the case that the
            two limits are the same (e.g. a single-level
            interface).  Packets caught by an IPSO filter as
            being out of range should be silently dropped and
            a counter should note the number of packets
            dropped because of out of range IPSO labels.



5.3.13.3 Stream Identifier Option

         This option is obsolete.  If the Stream Identifier
         option is present in a packet forwarded by the
         router, the option MUST be ignored and passed through
         unchanged.





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5.3.13.4 Source Route Options

         A router MUST implement support for source route
         options in forwarded packets.  A router MAY implement
         a configuration option that, when enabled, causes all
         source-routed packets to be discarded.  However, such
         an option MUST NOT be enabled by default.

         DISCUSSION:
            The ability to source route datagrams through the
            Internet is important to various network
            diagnostic tools.  However, source routing may be
            used to bypass administrative and security
            controls within a network.  Specifically, those
            cases where manipulation of routing tables is used
            to provide administrative separation in lieu of
            other methods such as packet filtering may be
            vulnerable through source routed packets.

            EDITOR'S COMMENTS:
               Packet filtering can be defeated by source
               routing as well, if it is applied in any router
               except one on the final leg of the source
               routed path.  Neither route nor packet filters
               constitute a complete solution for security.



5.3.13.5 Record Route Option

         Routers MUST support the Record Route option in
         forwarded packets.

         A router MAY provide a configuration option that, if
         enabled, will cause the router to ignore (i.e., pass
         through unchanged) Record Route options in forwarded
         packets.  If provided, such an option MUST default to
         enabling the record-route.  This option should not
         affect the processing of Record Route options in
         datagrams received by the router itself (in
         particular, Record Route options in ICMP echo
         requests will still be processed according to Section
         [4.3.3.6]).






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         DISCUSSION:
            There are some people who believe that Record
            Route is a security problem because it discloses
            information about the topology of the network.
            Thus, this document allows it to be disabled.



5.3.13.6 Timestamp Option

         Routers MUST support the timestamp option in
         forwarded packets.  A timestamp value MUST follow the
         rules given [INTRO:2].

         If the flags field = 3 (timestamp and prespecified
         address), the router MUST add its timestamp if the
         next prespecified address matches any of the router's
         IP addresses.  It is not necessary that the
         prespecified address be either the address of the
         interface on which the packet arrived or the address
         of the interface over which it will be sent.

         IMPLEMENTATION:
            To maximize the utility of the timestamps
            contained in the timestamp option, it is suggested
            that the timestamp inserted be, as nearly as
            practical, the time at which the packet arrived at
            the router.  For datagrams originated by the
            router, the timestamp inserted should be, as
            nearly as practical, the time at which the
            datagram was passed to the network layer for
            transmission.

         A router MAY provide a configuration option which, if
         enabled, will cause the router to ignore (i.e., pass
         through unchanged) Timestamp options in forwarded
         datagrams when the flag word is set to zero
         (timestamps only) or one (timestamp and registering
         IP address).  If provided, such an option MUST
         default to off (that is, the router does not ignore
         the timestamp).  This option should not affect the
         processing of Timestamp options in datagrams received
         by the router itself (in particular, a router will
         insert timestamps into Timestamp options in datagrams





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         received by the router, and Timestamp options in ICMP
         echo requests will still be processed according to
         Section [4.3.3.6]).

         DISCUSSION:
            Like the Record Route option, the Timestamp option
            can reveal information about a network's topology.
            Some people consider this to be a security
            concern.








































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6. TRANSPORT LAYER

A router is not required to implement any Transport Layer
protocols except those required to support Application Layer
protocols supported by the router.  In practice, this means
that most routers implement both the Transmission Control
Protocol (TCP) and the User Datagram Protocol (UDP).


6.1 USER DATAGRAM PROTOCOL - UDP

   The User Datagram Protocol (UDP) is specified in [TRANS:1].

   A router that implements UDP MUST be compliant, and SHOULD
   be unconditionally compliant, with the requirements of
   [INTRO:2], except that:

   + This specification does not specify the interfaces
      between the various protocol layers.  Thus, a router's
      interfaces need not comply with [INTRO:2], except where
      compliance is required for proper functioning of
      Application Layer protocols supported by the router.

   + Contrary to [INTRO:2], an application SHOULD NOT disable
      generation of UDP checksums.


   DISCUSSION:
      Although a particular application protocol may require
      that UDP datagrams it receives must contain a UDP
      checksum, there is no general requirement that received
      UDP datagrams contain UDP checksums.  Of course, if a
      UDP checksum is present in a received datagram, the
      checksum must be verified and the datagram discarded if
      the checksum is incorrect.



6.2 TRANSMISSION CONTROL PROTOCOL - TCP

   The Transmission Control Protocol (TCP) is specified in
   [TRANS:2].

   A router that implements TCP MUST be compliant, and SHOULD





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   be unconditionally compliant, with the requirements of
   [INTRO:2], except that:

   + This specification does not specify the interfaces
      between the various protocol layers.  Thus, a router
      need not comply with the following requirements of
      [INTRO:2] (except of course where compliance is required
      for proper functioning of Application Layer protocols
      supported by the router):

      Use of Push: RFC-793 Section 2.8:
           "Passing a received PSH flag to the application
           layer is now OPTIONAL."

      Urgent Pointer: RFC-793 Section 3.1:
           "A TCP MUST inform the application layer
           asynchronously whenever it receives an Urgent
           pointer and there was previously no pending urgent
           data, or whenever the Urgent pointer advances in
           the data stream.  There MUST be a way for the
           application to learn how much urgent data remains
           to be read from the connection, or at least to
           determine whether or not more urgent data remains
           to be read."

      TCP Connection Failures:
           "An application MUST be able to set the value for
           R2 for a particular connection.  For example, an
           interactive application might set R2 to
           ``infinity,'' giving the user control over when to
           disconnect."

      TCP Multihoming:
           "If an application on a multihomed host does not
           specify the local IP address when actively opening
           a TCP connection, then the TCP MUST ask the IP
           layer to select a local IP address before sending
           the (first) SYN.  See the function GET_SRCADDR() in
           Section 3.4."

      IP Options:
           "An application MUST be able to specify a source
           route when it actively opens a TCP connection, and
           this MUST take precedence over a source route





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           received in a datagram."

   + For similar reasons, a router need not comply with any of
      the requirements of [INTRO:2].

   + The requirements concerning the Maximum Segment Size
      Option in [INTRO:2] are amended as follows: a router
      that implements the host portion of MTU discovery
      (discussed in Section [4.2.3.3] of this memo) uses 536
      as the default value of SendMSS only if the path MTU is
      unknown; if the path MTU is known, the default value for
      SendMSS is the path MTU - 40.

   + The requirements concerning the Maximum Segment Size
      Option in [INTRO:2] are amended as follows: ICMP
      Destination Unreachable codes 11 and 12 are additional
      soft error conditions.  Therefore, these message MUST
      NOT cause TCP to abort a connection.

   DISCUSSION:
      It should particularly be noted that a TCP
      implementation in a router must conform to the following
      requirements of [INTRO:2]:

      + Providing a configurable TTL.  [Time to Live: RFC-793
         Section 3.9]

      + Providing an interface to configure keep-alive
         behavior, if keep-alives are used at all.  [TCP
         Keep-Alives]

      + Providing an error reporting mechanism, and the
         ability to manage it.  [Asynchronous Reports]

      + Specifying type of service.  [Type-of-Service]

      The general paradigm applied is that if a particular
      interface is visible outside the router, then all
      requirements for the interface must be followed.  For
      example, if a router provides a telnet function, then it
      will be generating traffic, likely to be routed in the
      external networks.  Therefore, it must be able to set
      the type of service correctly or else the telnet traffic
      may not get through.





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7. APPLICATION LAYER - ROUTING PROTOCOLS



7.1 INTRODUCTION

   For technical, managerial, and sometimes political reasons,  |
   the Internet routing system consists of two components -     |
   interior routing and exterior routing.  The concept of an    |
   Autonomous System (AS), as define in Section 2.2.4 of this   |
   document, plays a key role in separating interior from an    |
   exterior routing, as this concept allows to deliniate the    |
   set of routers where a change from interior to exterior      |
   routing occurs.  An IP datagram may have to traverse the     |
   routers of two or more Autonomous Systems to reach its       |
   destination, and the Autonomous Systems must provide each    |
   other with topology information to allow such forwarding.
   Interior gateway protocols (IGPs) are used to distribute     |
   routing information within an AS (i.e., intra-AS routing).   |
   Exterior gateway protocols are used to exchange routing      |
   information among ASs (i.e., inter-AS routing).              |



7.1.1 Routing Security Considerations

      Routing is one of the few places where the Robustness
      Principle ("be liberal in what you accept") does not
      apply.  Routers should be relatively suspicious in
      accepting routing data from other routing systems.

      A router SHOULD provide the ability to rank routing
      information sources from "most trustworthy" to "least
      trustworthy" and to accept routing information about any
      particular destination from the most trustworthy sources
      first.  This was implicit in the original core/stub
      autonomous system routing model using EGP and various
      interior routing protocols.  It is even more important
      with the demise of a central, "trusted" core.

      A router SHOULD provide a mechanism to filter out
      "obviously invalid" routes (such as those for net 127).

      Routers MUST NOT by default redistribute routing data





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      they do not themselves use, trust or otherwise consider
      valid.  In rare cases, it may be necessary to
      redistribute suspicious information, but this should
      only happen under direct intercession by some human
      agency.

      Routers must be at least a little paranoid about
      accepting routing data from anyone, and must be
      especially careful when they distribute routing
      information provided to them by another party.  See
      below for specific guidelines.


7.1.2 Precedence

      Except where the specification for a particular routing
      protocol specifies otherwise, a router SHOULD set the IP
      Precedence value for IP datagrams carrying routing
      traffic it originates to 6 (INTERNETWORK CONTROL).

      DISCUSSION:
         Routing traffic with VERY FEW exceptions should be
         the highest precedence traffic on any network.  If a
         system's routing traffic can't get through, chances
         are nothing else will.



7.1.3 Message Validation

      Peer-to-peer authentication involves several tests.  The
      application of message passwords and explicit acceptable
      neighbor lists has in the past improved the robustness
      of the route database.  Routers SHOULD IMPLEMENT
      management controls that enable explicit listing of
      valid routing neighbors.  Routers SHOULD IMPLEMENT
      peer-to-peer authentication for those routing protocols
      that support them.


      Routers SHOULD validate routing neighbors based on their
      source address and the interface a message is received
      on; neighbors in a directly attached subnet SHOULD be
      restricted to communicate with the router via the





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      interface that subnet is posited on or via unnumbered
      interfaces.  Messages received on other interfaces
      SHOULD be silently discarded.


      DISCUSSION:
         Security breaches and numerous routing problems are
         avoided by this basic testing.



7.2 INTERIOR GATEWAY PROTOCOLS



7.2.1 INTRODUCTION

      An Interior Gateway Protocol (IGP) is used to distribute
      routing information between the various routers in a
      particular AS.  Independent of the algorithm used to
      implement a particular IGP, it should perform the
      following functions:

      (1) Respond quickly to changes in the internal topology
           of an AS

      (2) Provide a mechanism such that circuit flapping does
           not cause continuous routing updates

      (3) Provide quick convergence to loop-free routing

      (4) Utilize minimal bandwidth

      (5) Provide "equal cost" routes to enable "load-
           splitting"

      (6) Provide a means for authentication of routing
           updates

      Current IGPs used in the internet today are
      characterized as either being based on a distance-vector
      or a link-state algorithm.

      Several IGPs are detailed in this section, including





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      those most commonly used and some recently developed
      protocols that may be widely used in the future.
      Numerous other protocols intended for use in intra-AS
      routing exist in the Internet community.

      A router that implements any routing protocol (other
      than static routes) MUST IMPLEMENT OSPF (see Section
      [7.2.2]).  A router MAY implement additional IGPs.


7.2.2 OPEN SHORTEST PATH FIRST - OSPF

      Shortest Path First (SPF) based routing protocols are a
      class of link-state algorithms that are based on the
      shortest-path algorithm of Dijkstra.  Although SPF based
      algorithms have been around since the inception of the
      ARPANET, it is only recently that they have achieved
      popularity both inside both the IP and the OSI
      communities.  In an SPF based system, each router
      obtains the entire topology database through a process
      known as flooding.  Flooding insures a reliable transfer
      of the information.  Each router then runs the SPF
      algorithm on its database to build the IP routing table.
      The OSPF routing protocol is an implementation of an SPF
      algorithm.  The current version, OSPF version 2, is
      specified in [ROUTE:1].  Note that RFC-1131, which
      describes OSPF version 1, is obsolete.

      Note that to comply with Section [8.3] of this memo, a
      router that implements OSPF MUST implement the OSPF MIB
      [MGT:14].


7.2.3 INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM - DUAL IS-
IS

      The American National Standards Institute (ANSI) X3S3.3
      committee has defined an intra-domain routing protocol.
      This protocol is titled "Intermediate System to
      Intermediate System Routeing Exchange Protocol".

      Its application to an IP network has been defined in
      [ROUTE:2], and is referred to as Dual IS-IS (or
      sometimes as Integrated IS-IS).  IS-IS is based on a





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      link-state (SPF) routing algorithm and shares all the
      advantages for this class of protocols.


7.3  EXTERIOR GATEWAY PROTOCOLS




7.3.1  INTRODUCTION


      Exterior Gateway Protocols are utilized for inter-
      Autonomous System routing to exchange reachability
      information for a set of networks internal to a
      particular autonomous system to a neighboring autonomous
      system.

      The area of inter-AS routing is a current topic of
      research inside the Internet Engineering Task Force.
      The Exterior Gateway Protocol (EGP) described in Section
      [Appendix F.1] has traditionally been the inter-AS
      protocol of choice, but is now historical.  The Border
      Gateway Protocol (BGP) eliminates many of the
      restrictions and limitations of EGP, and is therefore
      growing rapidly in popularity.  A router is not required
      to implement any inter-AS routing protocol.  However, if
      a router does implement EGP it also MUST IMPLEMENT BGP.
      Although it was not designed as an exterior gateway
      protocol, RIP (described in Section [7.2.4]) is
      sometimes used for inter-AS routing.


7.3.2 BORDER GATEWAY PROTOCOL - BGP



7.3.2.1 Introduction

         The Border Gateway Protocol (BGP-4) is an inter-AS
         routing protocol that exchanges network reachability
         information with other BGP speakers.  The information
         for a network includes the complete list of ASs that
         traffic must transit to reach that network.  This





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         information can then be used to insure loop-free
         paths.  This information is sufficient to construct a
         graph of AS connectivity from which routing loops may
         be pruned and some policy decisions at the AS level
         may be enforced.

         BGP is defined by [ROUTE:4].  [ROUTE:5] specifies the
         proper usage of BGP in the Internet, and provides
         some useful implementation hints and guidelines.
         [ROUTE:12] and [ROUTE:13] provide additional useful
         information.

         To comply with Section [8.3] of this memo, a router
         that implements BGP is required to implement the BGP
         MIB [MGT:15].

         To characterize the set of policy decisions that can
         be enforced using BGP, one must focus on the rule
         that an AS advertises to its neighbor ASs only those
         routes that it itself uses.  This rule reflects the
         "hop-by-hop" routing paradigm generally used
         throughout the current Internet.  Note that some
         policies cannot be supported by the "hop-by-hop"
         routing paradigm and thus require techniques such as
         source routing to enforce.  For example, BGP does not
         enable one AS to send traffic to a neighbor AS
         intending that traffic take a different route from
         that taken by traffic originating in the neighbor AS.
         On the other hand, BGP can support any policy
         conforming to the "hop-by-hop" routing paradigm.

         Implementors of BGP are strongly encouraged to follow
         the recommendations outlined in Section 6 of
         [ROUTE:5].


7.3.2.2 Protocol Walk-through

         While BGP provides support for quite complex routing
         policies (as an example see Section 4.2 in
         [ROUTE:5]), it is not required for all BGP
         implementors to support such policies.  At a minimum,
         however, a BGP implementation:






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         (1) SHOULD allow an AS to control announcements of
              the BGP learned routes to adjacent AS's.
              Implementations SHOULD support such control with
              at least the granularity of a single network.
              Implementations SHOULD also support such control
              with the granularity of an autonomous system,
              where the autonomous system may be either the
              autonomous system that originated the route, or
              the autonomous system that advertised the route
              to the local system (adjacent autonomous
              system).

         (2) SHOULD allow an AS to prefer a particular path to
              a destination (when more than one path is
              available).  Such function SHOULD be implemented
              by allowing system administrator to assign
              "weights" to Autonomous Systems, and making
              route selection process to select a route with
              the lowest "weight" (where "weight" of a route
              is defined as a sum of "weights" of all AS's in
              the AS_PATH path attribute associated with that
              route).

         (3) SHOULD allow an AS to ignore routes with certain
              AS's in the AS_PATH path attribute.  Such
              function can be implemented by using technique
              outlined in (2), and by assigning "infinity" as
              "weights" for such AS's.  The route selection
              process must ignore routes that have "weight"
              equal to "infinity".


7.3.3 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL

      It is possible to exchange routing information between
      two autonomous systems or routing domains without using
      a standard exterior routing protocol between two
      separate, standard interior routing protocols.  The most
      common way of doing this is to run both interior
      protocols independently in one of the border routers
      with an exchange of route information between the two
      processes.

      As with the exchange of information from an EGP to an





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      IGP, without appropriate controls these exchanges of
      routing information between two IGPs in a single router
      are subject to creation of routing loops.


7.4 STATIC ROUTING

   Static routing provides a means of explicitly defining the
   next hop from a router for a particular destination.  A
   router SHOULD provide a means for defining a static route
   to a destination, where the destination is defined by a
   network prefix.  The mechanism SHOULD also allow for a
   metric to be specified for each static route.

   A router that supports a dynamic routing protocol MUST
   allow static routes to be defined with any metric valid for
   the routing protocol used.  The router MUST provide the
   ability for the user to specify a list of static routes
   that may or may not be propagated through the routing
   protocol.  In addition, a router SHOULD support the
   following additional information if it supports a routing
   protocol that could make use of the information.  They are:

   + TOS,

   + Subnet Mask, or

   + Prefix Length, or

   + A metric specific to a given routing protocol that can
      import the route.

   DISCUSSION:
      We intend that one needs to support only the things
      useful to the given routing protocol.  The need for TOS
      should not require the vendor to implement the other
      parts if they are not used.

   Whether a router prefers a static route over a dynamic
   route (or vice versa) or whether the associated metrics are
   used to choose between conflicting static and dynamic
   routes SHOULD be configurable for each static route.

   A router MUST allow a metric to be assigned to a static





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   route for each routing domain that it supports.  Each such
   metric MUST be explicitly assigned to a specific routing
   domain.  For example:

        route 10.0.0.0/8 via 192.0.2.3 rip metric 3             |

        route 10.21.0.0/16 via 192.0.2.4 ospf inter-area        |
        metric 27

        route 10.22.0.0/16 via 192.0.2.5 egp 123 metric 99      |

   DISCUSSION:
      It has been suggested that, ideally, static routes
      should have preference values rather than metrics (since
      metrics can only be compared with metrics of other
      routes in the same routing domain, the metric of a
      static route could only be compared with metrics of
      other static routes).  This is contrary to some current
      implementations, where static routes really do have
      metrics, and those metrics are used to determine whether
      a particular dynamic route overrides the static route to
      the same destination.  Thus, this document uses the term
      metric rather than preference.

      This technique essentially makes the static route into a
      RIP route, or an OSPF route (or whatever, depending on
      the domain of the metric).  Thus, the route lookup
      algorithm of that domain applies.  However, this is NOT
      route leaking, in that coercing a static route into a
      dynamic routing domain does not authorize the router to
      redistribute the route into the dynamic routing domain.

      For static routes not put into a specific routing
      domain, the route lookup algorithm is:

      (1) Basic match

      (2) Longest match

      (3) Weak TOS (if TOS supported)

      (4) Best metric (where metric are implementation-
           defined)






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      The last step may not be necessary, but it's useful in
      the case where you want to have a primary static route
      over one interface and a secondary static route over an
      alternate interface, with failover to the alternate path
      if the interface for the primary route fails.



7.5 FILTERING OF ROUTING INFORMATION

   Each router within a network makes forwarding decisions
   based upon information contained within its forwarding
   database.  In a simple network the contents of the database
   may be configured statically.  As the network grows more
   complex, the need for dynamic updating of the forwarding
   database becomes critical to the efficient operation of the
   network.

   If the data flow through a network is to be as efficient as
   possible, it is necessary to provide a mechanism for
   controlling the propagation of the information a router
   uses to build its forwarding database.  This control takes
   the form of choosing which sources of routing information
   should be trusted and selecting which pieces of the
   information to believe.  The resulting forwarding database
   is a filtered version of the available routing information.

   In addition to efficiency, controlling the propagation of
   routing information can reduce instability by preventing
   the spread of incorrect or bad routing information.

   In some cases local policy may require that complete
   routing information not be widely propagated.

   These filtering requirements apply only to non-SPF-based
   protocols (and therefore not at all to routers which don't
   implement any distance vector protocols).


7.5.1 Route Validation

      A router SHOULD log as an error any routing update
      advertising a route that violates the specifications of
      this memo, unless the routing protocol from which the





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      update was received uses those values to encode special
      routes (such as default routes).


7.5.2 Basic Route Filtering

      Filtering of routing information allows control of paths
      used by a router to forward packets it receives.  A
      router should be selective in which sources of routing
      information it listens to and what routes it believes.
      Therefore, a router MUST provide the ability to specify:

      + On which logical interfaces routing information will
         be accepted and which routes will be accepted from
         each logical interface.

      + Whether all routes or only a default route is
         advertised on a logical interface.

      Some routing protocols do not recognize logical
      interfaces as a source of routing information.  In such
      cases the router MUST provide the ability to specify

      + from which other routers routing information will be
         accepted.

      For example, assume a router connecting one or more leaf
      networks to the main portion or backbone of a larger
      network.  Since each of the leaf networks has only one
      path in and out, the router can simply send a default
      route to them.  It advertises the leaf networks to the
      main network.


7.5.3 Advanced Route Filtering

      As the topology of a network grows more complex, the
      need for more complex route filtering arises.
      Therefore, a router SHOULD provide the ability to
      specify independently for each routing protocol:

      + Which logical interfaces or routers routing
         information (routes) will be accepted from and which
         routes will be believed from each other router or





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         logical interface,

      + Which routes will be sent via which logical
         interface(s), and

      + Which routers routing information will be sent to, if
         this is supported by the routing protocol in use.

      In many situations it is desirable to assign a
      reliability ordering to routing information received
      from another router instead of the simple believe or
      don't believe choice listed in the first bullet above.
      A router MAY provide the ability to specify:

      + A reliability or preference to be assigned to each
         route received.  A route with higher reliability will
         be chosen over one with lower reliability regardless
         of the routing metric associated with each route.

      If a router supports assignment of preferences, the
      router MUST NOT propagate any routes it does not prefer
      as first party information.  If the routing protocol
      being used to propagate the routes does not support
      distinguishing between first and third party
      information, the router MUST NOT propagate any routes it
      does not prefer.

      DISCUSSION:
         For example, assume a router receives a route to
         network C from router R and a route to the same
         network from router S.  If router R is considered
         more reliable than router S traffic destined for
         network C will be forwarded to router R regardless of
         the route received from router S.

      Routing information for routes which the router does not
      use (router S in the above example) MUST NOT be passed
      to any other router.


7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE

   Routers MUST be able to exchange routing information
   between separate IP interior routing protocols, if





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   independent IP routing processes can run in the same
   router.  Routers MUST provide some mechanism for avoiding
   routing loops when routers are configured for bi-
   directional exchange of routing information between two
   separate interior routing processes.  Routers MUST provide
   some priority mechanism for choosing routes from
   independent routing processes.  Routers SHOULD provide
   administrative control of IGP-IGP exchange when used across
   administrative boundaries.

   Routers SHOULD provide some mechanism for translating or
   transforming metrics on a per network basis.  Routers (or
   routing protocols) MAY allow for global preference of
   exterior routes imported into an IGP.

   DISCUSSION:
      Different IGPs use different metrics, requiring some
      translation technique when introducing information from
      one protocol into another protocol with a different form
      of metric.  Some IGPs can run multiple instances within
      the same router or set of routers.  In this case metric
      information can be preserved exactly or translated.

      There are at least two techniques for translation
      between different routing processes.  The static (or
      reachability) approach uses the existence of a route
      advertisement in one IGP to generate a route
      advertisement in the other IGP with a given metric.  The
      translation or tabular approach uses the metric in one
      IGP to create a metric in the other IGP through use of
      either a function (such as adding a constant) or a table
      lookup.

      Bi-directional exchange of routing information is
      dangerous without control mechanisms to limit feedback.
      This is the same problem that distance vector routing
      protocols must address with the split horizon technique
      and that EGP addresses with the third-party rule.
      Routing loops can be avoided explicitly through use of
      tables or lists of permitted/denied routes or implicitly
      through use of a split horizon rule, a no-third-party
      rule, or a route tagging mechanism.  Vendors are
      encouraged to use implicit techniques where possible to
      make administration easier for network operators.





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8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS

Note that this chapter supersedes any requirements stated
under "REMOTE MANAGEMENT" in [INTRO:3].


8.1 The Simple Network Management Protocol - SNMP



8.1.1 SNMP Protocol Elements

      Routers MUST be manageable by SNMP [MGT:3].  The SNMP
      MUST operate using UDP/IP as its transport and network
      protocols.  Others MAY be supported (e.g., see [MGT:25,
      MGT:26, MGT:27, and MGT:28]).  SNMP management
      operations MUST operate as if the SNMP was implemented
      on the router itself.  Specifically, management
      operations MUST be effected by sending SNMP management
      requests to any of the IP addresses assigned to any of
      the router's interfaces.  The actual management
      operation may be performed either by the router or by a
      proxy for the router.

      DISCUSSION:
         This wording is intended to allow management either
         by proxy, where the proxy device responds to SNMP
         packets that have one of the router's IP addresses in
         the packets destination address field, or the SNMP is
         implemented directly in the router itself and
         receives packets and responds to them in the proper
         manner.

         It is important that management operations can be
         sent to one of the router's IP Addresses.  In
         diagnosing network problems the only thing
         identifying the router that is available may be one
         of the router's IP address; obtained perhaps by
         looking through another router's routing table.

      All SNMP operations (get, get-next, get-response, set,
      and trap) MUST be implemented.

      Routers MUST provide a mechanism for rate-limiting the





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      generation of SNMP trap messages.  Routers MAY provide
      this mechanism through the algorithms for asynchronous
      alert management described in [MGT:5].

      DISCUSSION:
         Although there is general agreement about the need to
         rate-limit traps, there is not yet consensus on how
         this is best achieved.  The reference cited is
         considered experimental.



8.2 Community Table

   For the purposes of this specification, we assume that
   there is an abstract `community table' in the router.  This
   table contains several entries, each entry for a specific
   community and containing the parameters necessary to
   completely define the attributes of that community.  The
   actual implementation method of the abstract community
   table is, of course, implementation specific.

   A router's community table MUST allow for at least one
   entry and SHOULD allow for at least two entries.

   DISCUSSION:
      A community table with zero capacity is useless.  It
      means that the router will not recognize any communities
      and, therefore, all SNMP operations will be rejected.

      Therefore, one entry is the minimal useful size of the
      table.  Having two entries allows one entry to be
      limited to read-only access while the other would have
      write capabilities.

   Routers MUST allow the user to manually (i.e., without
   using SNMP) examine, add, delete and change entries in the
   SNMP community table.  The user MUST be able to set the
   community name or construct a MIB view.  The user MUST be
   able to configure communities as read-only (i.e., they do
   not allow SETs) or read-write (i.e., they do allow SETs).

   The user MUST be able to define at least one IP address to
   which notifications are sent for each community or MIB





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   view, if traps are used.  These addresses SHOULD be
   definable on a community or MIB view basis.  It SHOULD be
   possible to enable or disable notifications on a community
   or MIB view basis.

   A router SHOULD provide the ability to specify a list of
   valid network managers for any particular community.  If
   enabled, a router MUST validate the source address of the
   SNMP datagram against the list and MUST discard the
   datagram if its address does not appear.  If the datagram
   is discarded the router MUST take all actions appropriate
   to an SNMP authentication failure.

   DISCUSSION:
      This is a rather limited authentication system, but
      coupled with various forms of packet filtering may
      provide some small measure of increased security.

   The community table MUST be saved in non-volatile storage.

   The initial state of the community table SHOULD contain one
   entry, with the community name string "public" and read-
   only access.  The default state of this entry MUST NOT send
   traps.  If it is implemented, then this entry MUST remain
   in the community table until the administrator changes it
   or deletes it.

   DISCUSSION:
      By default, traps are not sent to this community.  Trap
      PDUs are sent to unicast IP addresses.  This address
      must be configured into the router in some manner.
      Before the configuration occurs, there is no such
      address, so to whom should the trap be sent?  Therefore
      trap sending to the "public" community defaults to be
      disabled.  This can, of course, be changed by an
      administrative operation once the router is operational.



8.3 Standard MIBS

   All MIBS relevant to a router's configuration are to be
   implemented.  To wit:






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   + The System, Interface, IP, ICMP, and UDP groups of MIB-II
      [MGT:2] MUST be implemented.

   + The Interface Extensions MIB [MGT:18] MUST be
      implemented.

   + The IP Forwarding Table MIB [MGT:20] MUST be implemented.

   + If the router implements TCP (e.g., for Telnet) then the
      TCP group of MIB-II [MGT:2] MUST be implemented.

   + If the router implements EGP then the EGP group of MIB-II
      [MGT:2] MUST be implemented.

   + If the router supports OSPF then the OSPF MIB [MGT:14]
      MUST be implemented.

   + If the router supports BGP then the BGP MIB [MGT:15] MUST
      be implemented.

   + If the router has Ethernet, 802.3, or StarLan interfaces
      then the Ethernet-Like MIB [MGT:6] MUST be implemented.

   + If the router has 802.4 interfaces then the 802.4 MIB
      [MGT:7] MUST be implemented.

   + If the router has 802.5 interfaces then the 802.5 MIB
      [MGT:8] MUST be implemented.

   + If the router has FDDI interfaces that implement ANSI SMT
      7.3 then the FDDI MIB [MGT:9] MUST be implemented.

   + If the router has FDDI interfaces that implement ANSI SMT
      6.2 then the FDDI MIB [MGT:29] MUST be implemented.

   + If the router has RS-232 interfaces then the RS-232
      [MGT:10] MIB MUST be implemented.

   + If the router has T1/DS1 interfaces then the T1/DS1 MIB
      [MGT:16] MUST be implemented.

   + If the router has T3/DS3 interfaces then the T3/DS3 MIB
      [MGT:17] MUST be implemented.






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   + If the router has SMDS interfaces then the SMDS Interface
      Protocol MIB [MGT:19] MUST be implemented.

   + If the router supports PPP over any of its interfaces
      then the PPP MIBs [MGT:11], [MGT:12], and [MGT:13] MUST
      be implemented.

   + If the router supports RIP Version 2 then the RIP Version
      2 MIB [MGT:21] MUST be implemented.

   + If the router supports X.25 over any of its interfaces
      then the X.25 MIBs [MGT:22, MGT:23 and MGT:24] MUST be
      implemented.


8.4 Vendor Specific MIBS

   The Internet Standard and Experimental MIBs do not cover
   the entire range of statistical, state, configuration and
   control information that may be available in a network
   element.  This information is, nevertheless, extremely
   useful.  Vendors of routers (and other network devices)
   generally have developed MIB extensions that cover this
   information.  These MIB extensions are called Vendor
   Specific MIBs.

   The Vendor Specific MIB for the router MUST provide access
   to all statistical, state, configuration, and control
   information that is not available through the Standard and
   Experimental MIBs that have been implemented.  This
   information MUST be available for both monitoring and
   control operations.

   DISCUSSION:
      The intent of this requirement is to provide the ability
      to do anything on the router through SNMP that can be
      done through a console, and vice versa.  A certain
      minimal amount of configuration is necessary before SNMP
      can operate (e.g., the router must have an IP address).
      This initial configuration can not be done through SNMP.
      However, once the initial configuration is done, full
      capabilities ought to be available through network
      management.






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   The vendor SHOULD make available the specifications for all
   Vendor Specific MIB variables.  These specifications MUST
   conform to the SMI [MGT:1] and the descriptions MUST be in
   the form specified in [MGT:4].

   DISCUSSION:
      Making the Vendor Specific MIB available to the user is
      necessary.  Without this information the users would not
      be able to configure their network management systems to
      be able to access the Vendor Specific parameters.  These
      parameters would then be useless.

      The format of the MIB specification is also specified.
      Parsers that read MIB specifications and generate the
      needed tables for the network management station are
      available.  These parsers generally understand only the
      standard MIB specification format.



8.5 Saving Changes

   Parameters altered by SNMP MAY be saved to non-volatile
   storage.

   DISCUSSION:
      Reasons why this "requirement" is a MAY:

      + The exact physical nature of non-volatile storage is
         not specified in this document.  Hence, parameters
         may be saved in NVRAM/EEPROM, local floppy or hard
         disk, or in some TFTP file server or BOOTP server,
         etc.  Suppose that this information is in a file that
         is retrieved through TFTP.  In that case, a change
         made to a configuration parameter on the router would
         need to be propagated back to the file server holding
         the configuration file.  Alternatively, the SNMP
         operation would need to be directed to the file
         server, and then the change somehow propagated to the
         router.  The answer to this problem does not seem
         obvious.

         This also places more requirements on the host
         holding the configuration information than just





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         having an available TFTP server, so much more that
         its probably unsafe for a vendor to assume that any
         potential customer will have a suitable host
         available.

      + The timing of committing changed parameters to non-
         volatile storage is still an issue for debate.  Some
         prefer to commit all changes immediately.  Others
         prefer to commit changes to non-volatile storage only
         upon an explicit command.







































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9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS

For all additional application protocols that a router
implements, the router MUST be compliant and SHOULD be
unconditionally compliant with the relevant requirements of
[INTRO:3].


9.1 BOOTP



9.1.1 Introduction

      The Bootstrap Protocol (BOOTP) is a UDP/IP-based
      protocol that allows a booting host to configure itself
      dynamically and without user supervision.  BOOTP
      provides a means to notify a host of its assigned IP
      address, the IP address of a boot server host, and the
      name of a file to be loaded into memory and executed
      ([APPL:1]).  Other configuration information such as the
      local prefix length or subnet mask, the local time
      offset, the addresses of default routers, and the
      addresses of various Internet servers can also be
      communicated to a host using BOOTP ([APPL:2]).


9.1.2 BOOTP Relay Agents

      In many cases, BOOTP clients and their associated BOOTP
      server(s) do not reside on the same IP (sub)network.  In
      such cases, a third-party agent is required to transfer
      BOOTP messages between clients and servers.  Such an
      agent was originally referred to as a "BOOTP forwarding
      agent." However, to avoid confusion with the IP
      forwarding function of a router, the name "BOOTP relay
      agent" has been adopted instead.

      DISCUSSION:
         A BOOTP relay agent performs a task that is distinct
         from a router's normal IP forwarding function.  While
         a router normally switches IP datagrams between
         networks more-or-less transparently, a BOOTP relay
         agent may more properly be thought to receive BOOTP





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         messages as a final destination and then generate new
         BOOTP messages as a result.  One should resist the
         notion of simply forwarding a BOOTP message "straight
         through like a regular packet."

      This relay-agent functionality is most conveniently
      located in the routers that interconnect the clients and
      servers (although it may alternatively be located in a
      host that is directly connected to the client (sub)net).

      A router MAY provide BOOTP relay-agent capability.  If
      it does, it MUST conform to the specifications in
      [APPL:3].

      Section [5.2.3] discussed the circumstances under which
      a packet is delivered locally (to the router).  All
      locally delivered UDP messages whose UDP destination
      port number is BOOTPS (67) are considered for special
      processing by the router's logical BOOTP relay agent.

      Sections [4.2.2.11] and [5.3.7] discussed invalid IP
      source addresses.  According to these rules, a router
      must not forward any received datagram whose IP source
      address is 0.0.0.0.  However, routers that support a
      BOOTP relay agent MUST accept for local delivery to the
      relay agent BOOTREQUEST messages whose IP source address
      is 0.0.0.0.






















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10. OPERATIONS AND MAINTENANCE

This chapter supersedes any requirements of [INTRO:3] relating
to "Extensions to the IP Module."

Facilities to support operation and maintenance (O&M)
activities form an essential part of any router
implementation.  Although these functions do not seem to
relate directly to interoperability, they are essential to the
network manager who must make the router interoperate and must
track down problems when it doesn't.  This chapter also
includes some discussion of router initialization and of
facilities to assist network managers in securing and
accounting for their networks.


10.1 Introduction

   The following kinds of activities are included under router
   O&M:

   + Diagnosing hardware problems in the router's processor,
      in its network interfaces, or in its connected networks,
      modems, or communication lines.

   + Installing new hardware

   + Installing new software.

   + Restarting or rebooting the router after a crash.

   + Configuring (or reconfiguring) the router.

   + Detecting and diagnosing Internet problems such as
      congestion, routing loops, bad IP addresses, black
      holes, packet avalanches, and misbehaved hosts.

   + Changing network topology, either temporarily (e.g., to
      bypass a communication line problem) or permanently.

   + Monitoring the status and performance of the routers and
      the connected networks.

   + Collecting traffic statistics for use in (Inter-)network





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      planning.

   + Coordinating the above activities with appropriate
      vendors and telecommunications specialists.

   Routers and their connected communication lines are often
   operated as a system by a centralized O&M organization.
   This organization may maintain a (Inter-)network operation
   center, or NOC, to carry out its O&M functions.  It is
   essential that routers support remote control and
   monitoring from such a NOC through an Internet path, since
   routers might not be connected to the same network as their
   NOC.  Since a network failure may temporarily preclude
   network access, many NOCs insist that routers be accessible
   for network management through an alternative means, often
   dial-up modems attached to console ports on the routers.

   Since an IP packet traversing an internet will often use
   routers under the control of more than one NOC, Internet
   problem diagnosis will often involve cooperation of
   personnel of more than one NOC.  In some cases, the same
   router may need to be monitored by more than one NOC, but
   only if necessary, because excessive monitoring could
   impact a router's performance.

   The tools available for monitoring at a NOC may cover a
   wide range of sophistication.  Current implementations
   include multi-window, dynamic displays of the entire router
   system.  The use of AI techniques for automatic problem
   diagnosis is proposed for the future.

   Router O&M facilities discussed here are only a part of the
   large and difficult problem of Internet management.  These
   problems encompass not only multiple management
   organizations, but also multiple protocol layers.  For
   example, at the current stage of evolution of the Internet
   architecture, there is a strong coupling between host TCP
   implementations and eventual IP-level congestion in the
   router system [OPER:1].  Therefore, diagnosis of congestion
   problems will sometimes require the monitoring of TCP
   statistics in hosts.  There are currently a number of R&D
   efforts in progress in the area of Internet management and
   more specifically router O&M.  These R&D efforts have
   already produced standards for router O&M.  This is also an





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   area in which vendor creativity can make a significant
   contribution.


10.2 Router Initialization



10.2.1 Minimum Router Configuration

      There exists a minimum set of conditions that must be
      satisfied before a router may forward packets.  A router
      MUST NOT enable forwarding on any physical interface
      unless either:

      (1) The router knows the IP address and associated
           subnet mask or network prefix length of at least
           one logical interface associated with that physical
           interface, or

      (2) The router knows that the interface is an unnumbered
           interface and knows its router-id.

      These parameters MUST be explicitly configured:

      + A router MUST NOT use factory-configured default
         values for its IP addresses, prefix lengths, or
         router-id, and

      + A router MUST NOT assume that an unconfigured
         interface is an unnumbered interface.

      DISCUSSION:
         There have been instances in which routers have been
         shipped with vendor-installed default addresses for
         interfaces.  In a few cases, this has resulted in
         routers advertising these default addresses into
         active networks.











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10.2.2 Address and Prefix Initialization

      A router MUST allow its IP addresses and their address
      masks or prefix lengths to be statically configured and
      saved in non-volatile storage.

      A router MAY obtain its IP addresses and their
      corresponding address masks dynamically as a side effect
      of the system initialization process (see Section
      10.2.3]);

      If the dynamic method is provided, the choice of method
      to be used in a particular router MUST be configurable.

      As was described in Section [4.2.2.11], IP addresses are
      not permitted to have the value 0 or -1 in the <Host-
      number> or <Network-prefix> fields.  Therefore, a router
      SHOULD NOT allow an IP address or address mask to be set
      to a value that would make any of the these fields above
      have the value zero or -1.

      DISCUSSION:
         It is possible using arbitrary address masks to
         create situations in which routing is ambiguous
         (i.e., two routes with different but equally specific
         subnet masks match a particular destination address).
         This is one of the strongest arguments for the use of
         network prefixes, and the reason the use of
         discontiguous subnet masks is not permitted.

      A router SHOULD make the following checks on any address
      mask it installs:

      + The mask is neither all ones nor all zeroes (the
         prefix length is neither zero nor 32).

      + The bits which correspond to the network prefix part
         of the address are all set to 1.

      + The bits that correspond to the network prefix are
         contiguous.


      DISCUSSION:





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         The masks associated with routes are also sometimes
         called "subnet masks", this test should not be
         applied to them.



10.2.3 Network Booting using BOOTP and TFTP

      There has been much discussion of how routers can and
      should be booted from the network.  These discussions
      have revolved around BOOTP and TFTP.  Currently, there
      are routers that boot with TFTP from the network.  There
      is no reason that BOOTP could not be used for locating
      the server that the boot image should be loaded from.

      BOOTP is a protocol used to boot end systems, and
      requires some stretching to accommodate its use with
      routers.  If a router is using BOOTP to locate the
      current boot host, it should send a BOOTP Request with
      its hardware address for its first interface, or, if it
      has been previously configured otherwise, with either
      another interface's hardware address, or another number
      to put in the hardware address field of the BOOTP
      packet.  This is to allow routers without hardware
      addresses (like synchronous line only routers) to use
      BOOTP for bootload discovery.  TFTP can then be used to
      retrieve the image found in the BOOTP Reply.  If there
      are no configured interfaces or numbers to use, a router
      MAY cycle through the interface hardware addresses it
      has until a match is found by the BOOTP server.

      A router SHOULD IMPLEMENT the ability to store
      parameters learned through BOOTP into local non-volatile
      storage.  A router MAY implement the ability to store a
      system image loaded over the network into local stable
      storage.

      A router MAY have a facility to allow a remote user to
      request that the router get a new boot image.
      Differentiation should be made between getting the new
      boot image from one of three locations: the one included
      in the request, from the last boot image server, and
      using BOOTP to locate a server.






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10.3 Operation and Maintenance



10.3.1 Introduction

      There is a range of possible models for performing O&M
      functions on a router.  At one extreme is the local-only
      model, under which the O&M functions can only be
      executed locally (e.g., from a terminal plugged into the
      router machine).  At the other extreme, the fully remote
      model allows only an absolute minimum of functions to be
      performed locally (e.g., forcing a boot), with most O&M
      being done remotely from the NOC.  There are
      intermediate models, such as one in which NOC personnel
      can log into the router as a host, using the Telnet
      protocol, to perform functions that can also be invoked
      locally.  The local-only model may be adequate in a few
      router installations, but remote operation from a NOC is
      normally required, and therefore remote O&M provisions
      are required for most routers.

      Remote O&M functions may be exercised through a control
      agent (program).  In the direct approach, the router
      would support remote O&M functions directly from the NOC
      using standard Internet protocols (e.g., SNMP, UDP or
      TCP); in the indirect approach, the control agent would
      support these protocols and control the router itself
      using proprietary protocols.  The direct approach is
      preferred, although either approach is acceptable.  The
      use of specialized host hardware and/or software
      requiring significant additional investment is
      discouraged; nevertheless, some vendors may elect to
      provide the control agent as an integrated part of the
      network in which the routers are a part.  If this is the
      case, it is required that a means be available to
      operate the control agent from a remote site using
      Internet protocols and paths and with equivalent
      functionality with respect to a local agent terminal.

      It is desirable that a control agent and any other NOC
      software tools that a vendor provides operate as user
      programs in a standard operating system.  The use of the
      standard Internet protocols UDP and TCP for





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      communicating with the routers should facilitate this.

      Remote router monitoring and (especially) remote router
      control present important access control problems that
      must be addressed.  Care must also be taken to ensure
      control of the use of router resources for these
      functions.  It is not desirable to let router monitoring
      take more than some limited fraction of the router CPU
      time, for example.  On the other hand, O&M functions
      must receive priority so they can be exercised when the
      router is congested, since often that is when O&M is
      most needed.


10.3.2 Out Of Band Access

      Routers MUST support Out-Of-Band (OOB) access.  OOB
      access SHOULD provide the same functionality as in-band
      access.  This access SHOULD implement access controls,
      to prevent unauthorized access.

      DISCUSSION:
         This Out-Of-Band access will allow the NOC a way to
         access isolated routers during times when network
         access is not available.

         Out-Of-Band access is an important management tool
         for the network administrator.  It allows the access
         of equipment independent of the network connections.
         There are many ways to achieve this access.
         Whichever one is used it is important that the access
         is independent of the network connections.  An
         example of Out-Of-Band access would be a serial port
         connected to a modem that provides dial up access to
         the router.

         It is important that the OOB access provides the same
         functionality as in-band access.  In-band access, or
         accessing equipment through the existing network
         connection, is limiting, because most of the time,
         administrators need to reach equipment to figure out
         why it is unreachable.  In band access is still very
         important for configuring a router, and for
         troubleshooting more subtle problems.





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10.3.2 Router O&M Functions



10.3.2.1 Maintenance - Hardware Diagnosis

         Each router SHOULD operate as a stand-alone device
         for the purposes of local hardware maintenance.
         Means SHOULD be available to run diagnostic programs
         at the router site using only on-site tools.  A
         router SHOULD be able to run diagnostics in case of a
         fault.  For suggested hardware and software
         diagnostics see Section [10.3.3].


10.3.2.2 Control - Dumping and Rebooting

         A router MUST include both in-band and out-of-band
         mechanisms to allow the network manager to reload,
         stop, and restart the router.  A router SHOULD also
         contain a mechanism (such as a watchdog timer) which
         will reboot the router automatically if it "hangs"
         due to a software or hardware fault.

         A router SHOULD IMPLEMENT a mechanism for dumping the
         contents of a router's memory (and/or other state
         useful for vendor debugging after a crash), and
         either saving them on a stable storage device local
         to the router or saving them on another host via an
         up-line dump mechanism such as TFTP (see [OPER:2],
         [INTRO:3]).


10.3.2.3 Control - Configuring the Router

         Every router has configuration parameters that may
         need to be set.  It SHOULD be possible to update the
         parameters without rebooting the router; at worst, a
         restart MAY be required.  There may be cases when it
         is not possible to change parameters without
         rebooting the router (for instance, changing the IP
         address of an interface).  In these cases, care
         should be taken to minimize disruption to the router
         and the surrounding network.





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         There SHOULD be a way to configure the router over
         the network either manually or automatically.  A
         router SHOULD be able to upload or download its
         parameters from a host or another router.  A means
         SHOULD be provided, either as an application program
         or a router function, to convert between the
         parameter format and a human-editable format.  A
         router SHOULD have some sort of stable storage for
         its configuration.  A router SHOULD NOT believe
         protocols such as RARP, ICMP Address Mask Reply, and
         MAY not believe BOOTP.

         DISCUSSION:
            It is necessary to note here that in the future
            RARP, ICMP Address Mask Reply, BOOTP and other
            mechanisms may be needed to allow a router to
            auto-configure.  Although routers may in the
            future be able to configure automatically, the
            intent here is to discourage this practice in a
            production environment until auto-configuration
            has been tested more thoroughly.  The intent is
            NOT to discourage auto-configuration all together.
            In cases where a router is expected to get its
            configuration automatically it may be wise to
            allow the router to believe these things as it
            comes up and then ignore them after it has gotten
            its configuration.



10.3.2.4 Net Booting of System Software

         A router SHOULD keep its system image in local non-
         volatile storage such as PROM, NVRAM, or disk.  It
         MAY also be able to load its system software over the
         network from a host or another router.

         A router that can keep its system image in local
         non-volatile storage MAY be configurable to boot its
         system image over the network.  A router that offers
         this option SHOULD be configurable to boot the system
         image in its non-volatile local storage if it is
         unable to boot its system image over the network.






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         DISCUSSION:
            It is important that the router be able to come up
            and run on its own.  NVRAM may be a particular
            solution for routers used in large networks, since
            changing PROMs can be quite time consuming for a
            network manager responsible for numerous or
            geographically dispersed routers.  It is important
            to be able to netboot the system image because
            there should be an easy way for a router to get a
            bug fix or new feature more quickly than getting
            PROMs installed.  Also if the router has NVRAM
            instead of PROMs, it will netboot the image and
            then put it in NVRAM.

            Routers SHOULD perform some basic consistency
            check on any image loaded, to detect and perhaps
            prevent incorrect images.

         A router MAY also be able to distinguish between
         different configurations based on which software it
         is running.  If configuration commands change from
         one software version to another, it would be helpful
         if the router could use the configuration that was
         compatible with the software.


10.3.2.5 Detecting and responding to misconfiguration

         There MUST be mechanisms for detecting and responding
         to misconfigurations.  If a command is executed
         incorrectly, the router SHOULD give an error message.
         The router SHOULD NOT accept a poorly formed command
         as if it were correct.

         DISCUSSION:
            There are cases where it is not possible to detect
            errors: the command is correctly formed, but
            incorrect with respect to the network.  This may
            be detected by the router, but may not be
            possible.

         Another form of misconfiguration is misconfiguration
         of the network to which the router is attached.  A
         router MAY detect misconfigurations in the network.





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         The router MAY log these findings to a file, either
         on the router or a host, so that the network manager
         will see that there are possible problems on the
         network.

         DISCUSSION:
            Examples of such misconfigurations might be
            another router with the same address as the one in
            question or a router with the wrong address mask.
            If a router detects such problems it is probably
            not the best idea for the router to try to fix the
            situation.  That could cause more harm than good.



10.3.2.6 Minimizing Disruption

         Changing the configuration of a router SHOULD have
         minimal affect on the network.  Routing tables SHOULD
         NOT be unnecessarily flushed when a simple change is
         made to the router.  If a router is running several
         routing protocols, stopping one routing protocol
         SHOULD NOT disrupt other routing protocols, except in
         the case where one network is learned by more than
         one routing protocol.

         DISCUSSION:
            It is the goal of a network manager to run a
            network so that users of the network get the best
            connectivity possible.  Reloading a router for
            simple configuration changes can cause disruptions
            in routing and ultimately cause disruptions to the
            network and its users.  If routing tables are
            unnecessarily flushed, for instance, the default
            route will be lost as well as specific routes to
            sites within the network.  This sort of disruption
            will cause significant downtime for the users.  It
            is the purpose of this section to point out that
            whenever possible, these disruptions should be
            avoided.









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10.3.2.7 Control - Troubleshooting Problems


         (1) A router MUST provide in-band network access, but
              (except as required by Section [8.2]) for
              security considerations this access SHOULD be
              disabled by default.  Vendors MUST document the
              default state of any in-band access.  This
              access SHOULD implement access controls, to
              prevent unauthorized access.

              DISCUSSION:
                 In-band access primarily refers to access
                 through the normal network protocols that may
                 or may not affect the permanent operational
                 state of the router.  This includes, but is
                 not limited to Telnet/RLOGIN console access
                 and SNMP operations.

                 This was a point of contention between the
                 "operational out of the box" and "secure out
                 of The box" contingents.  Any "automagic"
                 access to the router may introduce
                 insecurities, but it may be more important
                 for the customer to have a router that is
                 accessible over the network as soon as it is
                 plugged in.  At least one vendor supplies
                 routers without any external console access
                 and depends on being able to access the
                 router through the network to complete its
                 configuration.

                 It is the vendors call whether in-band access
                 is enabled by default; but it is also the
                 vendor's responsibility to make its customers
                 aware of possible insecurities.

         (2) A router MUST provide the ability to initiate an
              ICMP echo.  The following options SHOULD be
              implemented:

              + Choice of data patterns

              + Choice of packet size





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              + Record route

              and the following additional options MAY be
              implemented:

              + Loose source route

              + Strict source route

              + Timestamps

         (3) A router SHOULD provide the ability to initiate a
              traceroute.  If traceroute is provided, then the
              3rd party traceroute SHOULD be implemented.

         Each of the above three facilities (if implemented)
         SHOULD have access restrictions placed on it to
         prevent its abuse by unauthorized persons.


10.4 Security Considerations



10.4.1 Auditing and Audit Trails

      Auditing and billing are the bane of the network
      operator, but are the two features most requested by
      those in charge of network security and those who are
      responsible for paying the bills.  In the context of
      security, auditing is desirable if it helps you keep
      your network working and protects your resources from
      abuse, without costing you more than those resources are
      worth.

      (1) Configuration Changes

           Router SHOULD provide a method for auditing a
           configuration change of a router, even if it's
           something as simple as recording the operator's
           initials and time of change.

           DISCUSSION:
              Configuration change logging (who made a





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              configuration change, what was changed, and
              when) is very useful, especially when traffic is
              suddenly routed through Alaska on its way across
              town.  So is the ability to revert to a previous
              configuration.

      (2) Packet Accounting

           Vendors should strongly consider providing a system
           for tracking traffic levels between pairs of hosts
           or networks.  A mechanism for limiting the
           collection of this information to specific pairs of
           hosts or networks is also strongly encouraged.

           DISCUSSION:
              A "host traffic matrix" as described above can
              give the network operator a glimpse of traffic
              trends not apparent from other statistics.  It
              can also identify hosts or networks that are
              "probing" the structure of the attached networks
              - e.g., a single external host that tries to
              send packets to every IP address in the network
              address range for a connected network.

      (3) Security Auditing

           Routers MUST provide a method for auditing security
           related failures or violations to include:

           + Authorization Failures: bad passwords, invalid
              SNMP communities, invalid authorization tokens,

           + Violations of Policy Controls: Prohibited Source
              Routes, Filtered Destinations, and

           + Authorization Approvals: good passwords - Telnet
              in-band access, console access.

           Routers MUST provide a method of limiting or
           disabling such auditing but auditing SHOULD be on
           by default.  Possible methods for auditing include
           listing violations to a console if present, logging
           or counting them internally, or logging them to a
           remote security server through the SNMP trap





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           mechanism or the Unix logging mechanism as
           appropriate.  A router MUST implement at least one
           of these reporting mechanisms - it MAY implement
           more than one.


10.4.2 Configuration Control

      A vendor has a responsibility to use good configuration
      control practices in the creation of the
      software/firmware loads for their routers.  In
      particular, if a vendor makes updates and loads
      available for retrieval over the Internet, the vendor
      should also provide a way for the customer to confirm
      the load is a valid one, perhaps by the verification of
      a checksum over the load.

      DISCUSSION:
         Many vendors currently provide short notice updates
         of their software products through the Internet.
         This a good trend and should be encouraged, but
         provides a point of vulnerability in the
         configuration control process.

      If a vendor provides the ability for the customer to
      change the configuration parameters of a router
      remotely, for example through a Telnet session, the
      ability to do so SHOULD be configurable and SHOULD
      default to off.  The router SHOULD require a password or
      other valid authentication before permitting remote
      reconfiguration.

      DISCUSSION:
         Allowing your properly identified network operator to
         twiddle with your routers is necessary; allowing
         anyone else to do so is foolhardy.

      A router MUST NOT have undocumented "back door" access
      and "master passwords".  A vendor MUST ensure any such
      access added for purposes of debugging or product
      development are deleted before the product is
      distributed to its customers.

      DISCUSSION:





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         A vendor has a responsibility to its customers to
         ensure they are aware of the vulnerabilities present
         in its code by intention - e.g., in-band access.
         "Trap doors", "back doors" and "master passwords"
         intentional or unintentional can turn a relatively
         secure router into a major problem on an operational
         network.  The supposed operational benefits are not
         matched by the potential problems.









































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11. REFERENCES

Implementors should be aware that Internet protocol standards
are occasionally updated.  These references are current as of
this writing, but a cautious implementor will always check a
recent version of the RFC index to ensure that an RFC has not
been updated or superseded by another, more recent RFC.
Reference [INTRO:6] explains various ways to obtain a current
RFC index.

APPL:1.
     B.  Croft and J.  Gilmore, "Bootstrap Protocol (BOOTP),
     Request For Comments (RFC) 951, DDN Network Information
     Center, SRI International, Menlo Park, California, USA,
     September 1985.

APPL:2.
     S.  Alexander and R.  Droms, "DHCP Options and BOOTP
     Vendor Extensions", Request For Comments (RFC) 1533,
     October 1993.

APPL:3.
     W.  Wimer, "Clarifications and Extensions for the
     Bootstrap Protocol", Request For Comments (RFC) 1542,
     October 1993.

ARCH:1.
     "DDN Protocol Handbook, NIC-50004, NIC-50005, NIC-50006
     (three volumes), DDN Network Information Center, SRI
     International, Menlo Park, California, USA, December
     1985.

ARCH:2.
     V.  Cerf and R.  Kahn, "A Protocol for Packet Network
     Intercommunication," IEEE Transactions on Communication,
     May 1974.  Also included in [ARCH:1].

ARCH:3.
     J.  Postel, C.  Sunshine, and D.  Cohen, "The ARPA
     Internet Protocol," Computer Networks, volume 5, number
     4, July 1981.  Also included in [ARCH:1].

ARCH:4.
     B.  Leiner, J.  Postel, R.  Cole, and D.  Mills, "The





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     DARPA Internet Protocol Suite," Proceedings of INFOCOM
     '85, IEEE, Washington, DC, March 1985.  Also in: IEEE
     Communications Magazine, March 1985.  Also available from
     the Information Sciences Institute, University of
     Southern California as Technical Report ISI-RS-85-153.

ARCH:5.
     D.  Comer, "Internetworking With TCP/IP Volume 1:
     Principles, Protocols, and Architecture", Prentice Hall,
     Englewood Cliffs, NJ, 1991.

ARCH:6.
     W.  Stallings, "Handbook of Computer-Communications
     Standards Volume 3: The TCP/IP Protocol Suite",
     Macmillan, New York, NY, 1990.

ARCH:7.
     J.  Postel, "Internet Official Protocol Standards",
     Request For Comments (RFC) 1540, October 1993.

ARCH:8.
     "Information processing systems - Open Systems
     Interconnection - Basic Reference Model", ISO 7489,
     International Standards Organization, 1984.

ARCH:9
     R.  Braden, J.  Postel, Y.  Rekhter, "Internet
     Architecture Extensions for Shared Media", 05/20/1994

FORWARD:1.
     IETF CIP Working Group (C. Topolcic, Editor),
     "Experimental Internet Stream Protocol, Version 2 (ST-
     II)", Request For Comments (RFC) 1190, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, October 1990.

FORWARD:2.
     A.  Mankin and K.  Ramakrishnan, Editors, "Gateway
     Congestion Control Survey", Request For Comments (RFC)
     1254, DDN Network Information Center, SRI International,
     Menlo Park, California, USA, August 1991.

FORWARD:3.
     J.  Nagle, "On Packet Switches with Infinite Storage,"





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     IEEE Transactions on Communications, volume COM-35,
     number 4, April 1987.

FORWARD:4.
     R.  Jain, K.  Ramakrishnan, and D.  Chiu, "Congestion
     Avoidance in Computer Networks With a Connectionless
     Network Layer", Technical Report DEC-TR-506, Digital
     Equipment Corporation.

FORWARD:5.
     V.  Jacobson, "Congestion Avoidance and Control,"
     Proceedings of SIGCOMM '88, Association for Computing
     Machinery, August 1988.

FORWARD:6.
     W.  Barns, "Precedence and Priority Access Implementation
     for Department of Defense Data Networks", Technical
     Report MTR-91W00029, The Mitre Corporation, McLean,
     Virginia, USA, July 1991.

FORWARD:7
     Fang, Chen, Hutchins, "Simulation Results of TCP
     Performance over ATM with and without Flow Control",
     presentation to the ATM Forum, November 15, 1993.

FORWARD:8
     V.  Paxson, S.  Floyd "Wide Area Traffic: the Failure of
     Poisson Modeling", short version in SIGCOMM '94

FORWARD:9
     Leland, Taqqu, Willinger and Wilson, "On the Self-Similar
     Nature of Ethernet Traffic", Proceedings of SIGCOMM '93,
     September, 1993.

FORWARD:10
     S.  Keshav "A Control Theoretic Approach to Flow
     Control", SIGCOMM 91, pages 3-16

FORWARD:11
     K.K.  Ramakrishnan and R.  Jain, "A Binary Feedback
     Scheme for Congestion Avoidance in Computer Networks,"
     ACM Transactions of Computer Systems, volume 8, number 2,
     1980.






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FORWARD:12
     H.  Kanakia, P.  Mishara, and A.  Reibman].  An adaptive
     congestion control scheme for real-time packet video
     transport.  In Proceedings of ACM SIGCOMM 1994, pages
     20-31, San Francisco, California, September 1993.

FORWARD:13
     A.  Demers, S.  Keshav, S.  Shenker "Analysis and
     Simulation of a Fair Queuing Algorithm",
      93 pages 1-12

FORWARD:14
     D.  Clark, S.  Shenker , L.  Zhang, "Supporting Real-Time
     Applications in an Integrated Services Packet Network:
     Architecture and Mechanism", 92 pages 14-26

INTERNET:1.
     J.  Postel, "Internet Protocol", Request For Comments
     (RFC) 791, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, September
     1981.

INTERNET:2.
     J.  Mogul and J.  Postel, "Internet Standard Subnetting
     Procedure", Request For Comments (RFC) 950, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, August 1985.

INTERNET:3.
     J.  Mogul, "Broadcasting Internet Datagrams in the
     Presence of Subnets", Request For Comments (RFC) 922, DDN
     Network Information Center, SRI International, Menlo
     Park, California, USA, October 1984.

INTERNET:4.
     S.  Deering, "Host Extensions for IP Multicasting",
     Request For Comments (RFC) 1112, DDN Network Information
     Center, SRI International, Menlo Park, California, USA,
     August 1989.

INTERNET:5.
     S.  Kent, "U.S.  Department of Defense Security Options
     for the Internet Protocol", Request for Comments (RFC)
     1108, November 1991.





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INTERNET:6.
     R.  Braden, D.  Borman, and C.  Partridge, "Computing the
     Internet Checksum", Request For Comments (RFC) 1071, DDN
     Network Information Center, SRI International, Menlo
     Park, California, USA, September 1988.

INTERNET:7.
     T.  Mallory and A.  Kullberg, "Incremental Updating of
     the Internet Checksum", Request For Comments (RFC) 1141,   |
     DDN Network Information Center, SRI International, Menlo
     Park, California, USA, January 1990.

INTERNET:8.
     J.  Postel, "Internet Control Message Protocol", Request
     For Comments (RFC) 792, DDN Network Information Center,
     SRI International, Menlo Park, California, USA, September
     1981.

INTERNET:9.
     A.  Mankin, G.  Hollingsworth, G.  Reichlen, K.
     Thompson, R.  Wilder, and R.  Zahavi, "Evaluation of
     Internet Performance - FY89", Technical Report MTR-
     89W00216, MITRE Corporation, February, 1990.

INTERNET:10.
     G.  Finn, "A Connectionless Congestion Control
     Algorithm," Computer Communications Review, volume 19,
     number 5, Association for Computing Machinery, October
     1989.

INTERNET:11.
     W.  Prue, "The Source Quench Introduced Delay (SQuID)",
     Request For Comments (RFC) 1016, DDN Network Information
     Center, SRI International, J.  Postel, August 1987.

INTERNET:12.
     A.  McKenzie, "Some comments on SQuID", Request For
     Comments (RFC) 1018, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, August 1987.

INTERNET:13.
     S.  Deering, "ICMP Router Discovery Messages", Request
     For Comments (RFC) 1256, DDN Network Information Center,
     SRI International, Menlo Park, California, USA, September





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     1991.

INTERNET:14.
     J.  Mogul and S.  Deering, "Path MTU Discovery", Request
     For Comments (RFC) 1191, DDN Network Information Center,
     SRI International, Menlo Park, California, USA, November
     1990.

INTERNET:15
     V.  Fuller, T.  Li, J.  Yi, and K.  Varadhan, "Classless
     Inter-Domain Routing (CIDR): an Address Assignment and
     Aggregation Strategy" Request For Comments (RFC) 1519,
     DDN Network Information Center, SRI International Menlo
     Park, California, USA September 1993.

INTERNET:16
     M.  St.  Johns, "Draft Revised IP Security Option",
     Request for Comments 1038, January 1988.

INTERNET:17
     W.  Prue and J.  Postel, "Queuing Algorithm to Provide
     Type-of-service For IP Links", Request for Comments 1046,
     February 1988.

INTERNET:18
     J.  Postel, "Address Mappings ", Request for Comments
     796, September 1981.

INTRO:1.
     R.  Braden and J.  Postel, "Requirements for Internet
     Gateways", Request For Comments (RFC) 1009, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, June 1987.

INTRO:2.
     Internet Engineering Task Force (R. Braden, Editor),
     "Requirements for Internet Hosts - Communication Layers",
     Request For Comments (RFC) 1122, DDN Network Information
     Center, SRI International, Menlo Park, California, USA,
     October 1989.

INTRO:3.
     Internet Engineering Task Force (R. Braden, Editor),
     "Requirements for Internet Hosts - Application and





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     Support", Request For Comments (RFC) 1123, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, October 1989.

INTRO:4.
     D.  Clark, "Modularity and Efficiency in Protocol
     Implementations", Request For Comments (RFC) 817, DDN
     Network Information Center, SRI International, Menlo
     Park, California, USA, July 1982.

INTRO:5.
     D.  Clark, "The Structuring of Systems Using Upcalls,"
     Proceedings of 10th ACM SOSP, December 1985.

INTRO:6.
     O.  Jacobsen and J.  Postel, "Protocol Document Order
     Information", Request For Comments (RFC) 980, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, March 1986.

INTRO:7.
     J.  Reynolds and J.  Postel, "Assigned Numbers", Request
     For Comments (RFC) 1340, July 1992.  This document is
     periodically updated and reissued with a new number.  It
     is wise to verify occasionally that the version you have
     is still current.

INTRO:8.
     "DoD Trusted Computer System Evaluation Criteria", DoD
     publication 5200.28-STD, U.S.  Department of Defense,
     December 1985.

INTRO:9
     G.  Malkin and T.  LaQuey Parker, "Internet Users'
     Glossary", Request for Comments (RFC) 1392 (also FYI
     0018), Network Information Center, January 1993.

LINK:1.
     S.  Leffler and M.  Karels, "Trailer Encapsulations",
     Request For Comments (RFC) 893, DDN Network Information
     Center, SRI International, Menlo Park, California, USA,
     April 1984.

LINK:2





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     W.  Simpson, "The Point-to-Point Protocol (PPP) for the
     Transmission of Multi-protocol Datagrams over Point-to-
     Point Links", Request For Comments (RFC) 1331, May 1992.

LINK:3
     G.  McGregor, "The PPP Internet Protocol Control Protocol
     (IPCP)", Request For Comments (RFC) 1332, May 1992.

LINK:4
     B.  Lloyd, W.  Simpson, "PPP Authentication Protocols",
     Request For Comments (RFC) 1334, May 1992.

LINK:5
     W.  Simpson "PPP Link Quality Monitoring", Request For
     Comments (RFC) 1333, May 1992.

MGT:1.
     M.  Rose and K.  McCloghrie, "Structure and
     Identification of Management Information of TCP/IP-based
     Internets", Request For Comments (RFC) 1155, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, May 1990.

MGT:2.
     K.  McCloghrie and M.  Rose (Editors), "Management
     Information Base of TCP/IP-Based Internets: MIB-II",
     Request For Comments (RFC) 1213, DDN Network Information
     Center, SRI International, Menlo Park, California, USA,
     March 1991.

MGT:3.
     J.  Case, M.  Fedor, M.  Schoffstall, and J.  Davin,
     "Simple Network Management Protocol", Request For
     Comments (RFC) 1157, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, May 1990.

MGT:4.
     M.  Rose and K.  McCloghrie (Editors), "Towards Concise
     MIB Definitions", Request For Comments (RFC) 1212, DDN
     Network Information Center, SRI International, Menlo
     Park, California, USA, March 1991.

MGT:5.
     L.  Steinberg, "Techniques for Managing Asynchronously





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     Generated Alerts", Request for Comments (RFC) 1224, May
     1991.

MGT:6.
     F.  Kastenholz, "Definitions of Managed Objects for the
     Ethernet-like Interface Types", Request for Comments
     (RFC) 1398, January 1993.

MGT:7.
     R.  Fox and K.  McCloghrie, "IEEE 802.4 Token Bus MIB",
     Request for Comments (RFC) 1230, May 1991.

MGT:8.
     E.  Decker, R.  Fox and K.  McCloghrie, "IEEE 802.5 Token
     Ring MIB", Request for Comments (RFC) 1231, February
     1993.

MGT:9.
     J.  Case and A.  Rijsinghani, "FDDI Management
     Information Base", Request for Comments (RFC) 1512,
     September 1993.

MGT:10.
     B.  Stewart, "Definitions of Managed Objects for RS-232-
     like Hardware Devices", Request for Comments (RFC) 1317,
     April 1992.

MGT:11.
     F.  Kastenholz, " Definitions of Managed Objects for the
     Link Control Protocol of the Point-to-Point Protocol",
     Request For Comments (RFC) 1471 June 1992.

MGT:12.
     F.  Kastenholz, "The Definitions of Managed Objects for
     the Security Protocols of the Point-to-Point Protocol",
     Request For Comments (RFC) 1472 June 1992.

MGT:13.
     F.  Kastenholz, "The Definitions of Managed Objects for
     the IP Network Control Protocol of the Point-to-Point
     Protocol", Request For Comments (RFC) 1473 June 1992.

MGT:14.
     F.  Baker and R.  Coltun, "OSPF Version 2 Management





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     Information Base", Request For Comments (RFC) 1253,
     August 1991.

MGT:15.
     S.  Willis and J.  Burruss, "Definitions of Managed
     Objects for the Border Gateway Protocol (Version 3)",
     Request For Comments (RFC) 1269, October 1991.

MGT:16.
     F.  Baker, J.  Watt, "Definitions of Managed Objects for
     the DS1 and E1 Interface Types", Request For Comments
     (RFC) 1406, January 1993.

MGT:17.
     T.  Cox and K.  Tesink, "Definitions of Managed Objects
     for the DS3/E3 Interface Types", Request For Comments
     (RFC) 1407, January 1993.

MGT:18.
     K.  McCloghrie, "Extensions to the Generic-Interface
     MIB", Request For Comments (RFC) 1229, August 1992.

MGT:19.
     T.  Cox and K.  Tesink, "Definitions of Managed Objects
     for the SIP Interface Type", Request For Comments (RFC)
     1304, February 1992.

MGT:20
     F.  Baker, "IP Forwarding Table MIB", Request For
     Comments (RFC) 1354, July 1992.

MGT:21.
     G.  Malkin and F.  Baker, "RIP Version 2 MIB Extension",
     Request For Comments (RFC) 1389, January 1993.

MGT:22.
     D.  Throop, "SNMP MIB Extension for the X.25 Packet
     Layer", Request For Comments (RFC) 1382, November 1992.

MGT:23.
     D.  Throop and F.  Baker, "SNMP MIB Extension for X.25
     LAPB", Request For Comments (RFC) 1381, November 1992.

MGT:24.





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     D.  Throop and F.  Baker, "SNMP MIB Extension for
     MultiProtocol Interconnect over X.25", Request For
     Comments (RFC) 1461, May 1993.

MGT:25.
     M.  Rose, "SNMP over OSI", Request For Comments (RFC)
     1418, March 1993.

MGT:26.
     G.  Minshall and M.  Ritter, "SNMP over AppleTalk",
     Request For Comments (RFC) 1419, March 1993.

MGT:27.
     S.  Bostock, "SNMP over IPX", Request For Comments (RFC)
     1420, March 1993.

MGT:28.
     M.  Schoffstall, C.  Davin, M.  Fedor, J.  Case, "SNMP
     over Ethernet", Request For Comments (RFC) 1089, February
     1989.

MGT:29.
     J.  Case, "FDDI Management Information Base", Request For
     Comments (RFC) 1285, January 1992.

OPER:1.
     J.  Nagle, "Congestion Control in IP/TCP Internetworks",
     Request For Comments (RFC) 896, DDN Network Information
     Center, SRI International, Menlo Park, California, USA,
     January 1984.

OPER:2.
     K.R.  Sollins, "TFTP Protocol (revision 2)", Request For
     Comments (RFC) 1350, July 1992.

ROUTE:1.
     J.  Moy, "OSPF Version 2", Request For Comments (RFC)
     1247, DDN Network Information Center, SRI International,
     Menlo Park, California, USA, July 1991.

ROUTE:2.
     R.  Callon, "Use of OSI IS-IS for Routing in TCP/IP and
     Dual Environments", Request For Comments (RFC) 1195, DDN
     Network Information Center, SRI International, Menlo





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     Park, California, USA, December 1990.

ROUTE:3.
     C.  L.  Hedrick, "Routing Information Protocol", Request
     For Comments (RFC) 1058, DDN Network Information Center,
     SRI International, Menlo Park, California, USA, June
     1988.

ROUTE:4.
     K.  Lougheed and Y.  Rekhter, "A Border Gateway Protocol
     3 (BGP-3)", Request For Comments (RFC) 1267, October
     1991.

ROUTE:5.
     P.  Gross and Y.  Rekhter, "Application of the Border
     Gateway Protocol in the Internet", Request For Comments
     (RFC) 1268, October 1991.

ROUTE:6.
     D.  Mills, "Exterior Gateway Protocol Formal
     Specification", Request For Comments (RFC) 904, DDN
     Network Information Center, SRI International, Menlo
     Park, California, USA, April 1984.

ROUTE:7.
     E.  Rosen, "Exterior Gateway Protocol (EGP)", Request For
     Comments (RFC) 827, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, October 1982.

ROUTE:8.
     L.  Seamonson and E.  Rosen, ""STUB" Exterior Gateway
     Protocol", Request For Comments (RFC) 888, DDN Network
     Information Center, SRI International, Menlo Park,
     California, USA, January 1984.

ROUTE:9.
     D.  Waitzman, C.  Partridge, and S.  Deering, "Distance
     Vector Multicast Routing Protocol", Request For Comments
     (RFC) 1075, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, November
     1988.

ROUTE:10.
     S.  Deering, "Multicast Routing in Internetworks and





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     Extended LANs," Proceedings of '88, Association for
     Computing Machinery, August 1988.

ROUTE:11.
     P.  Almquist, "Type of Service in the Internet Protocol
     Suite", Request for Comments (RFC) 1349, July 1992.

ROUTE:12.
     Y.  Rekhter, "Experience with the BGP Protocol", Request
     For Comments (RFC) 1266, October 1991.

ROUTE:13.
     Y.  Rekhter, "BGP Protocol Analysis", Request For
     Comments (RFC) 1265, October 1991.

TRANS:1.
     J.  Postel, "User Datagram Protocol", Request For
     Comments (RFC) 768, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, August 1980.

TRANS:2.
     J.  Postel, "Transmission Control Protocol", Request For
     Comments (RFC) 793, DDN Network Information Center, SRI
     International, Menlo Park, California, USA, September
     1981.
























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APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS

Subject to restrictions given below, a host MAY be able to act
as an intermediate hop in a source route, forwarding a
source-routed datagram to the next specified hop.

However, in performing this router-like function, the host
MUST obey all the relevant rules for a router forwarding
source-routed datagrams [INTRO:2].  This includes the
following specific provisions:

(A) TTL
     The TTL field MUST be decremented and the datagram
     perhaps discarded as specified for a router in [INTRO:2].

(B) ICMP Destination Unreachable
     A host MUST be able to generate Destination Unreachable
     messages with the following codes:
     4 (Fragmentation Required but DF Set) when a source-
       routed datagram cannot be fragmented to fit into the
       target network;
     5 (Source Route Failed) when a source-routed datagram
       cannot be forwarded, e.g., because of a routing problem
       or because the next hop of a strict source route is not
       on a connected network.

(C) IP Source Address
     A source-routed datagram being forwarded MAY (and
     normally will) have a source address that is not one of
     the IP addresses of the forwarding host.

(D) Record Route Option
     A host that is forwarding a source-routed datagram
     containing a Record Route option MUST update that option,
     if it has room.

(E) Timestamp Option
     A host that is forwarding a source-routed datagram
     containing a Timestamp Option MUST add the current
     timestamp to that option, according to the rules for this
     option.

To define the rules restricting host forwarding of source-
routed datagrams, we use the term "local source-routing" if





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the next hop will be through the same physical interface
through which the datagram arrived; otherwise, it is "non-
local source-routing".

A host is permitted to perform local source-routing without
restriction.

A host that supports non-local source-routing MUST have a
configurable switch to disable forwarding, and this switch
MUST default to disabled.

The host MUST satisfy all router requirements for configurable
policy filters [INTRO:2] restricting non-local forwarding.

If a host receives a datagram with an incomplete source route
but does not forward it for some reason, the host SHOULD
return an ICMP Destination Unreachable (code 5, Source Route
Failed) message, unless the datagram was itself an ICMP error
message.






























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APPENDIX B. GLOSSARY


This Appendix defines specific terms used in this memo.  It
also defines some general purpose terms that may be of
interest.  See also [INTRO:9] for a more general set of
definitions.

Autonomous System (AS)                                          |
     An Autonomous System (AS) is a connected segment of a      |
     network topology that consists of a collection of          |
     subnetworks (with hosts attached) interconnected by a set  |
     of routes.  The subnetworks and the routers are expected   |
     to be under the control of a single operations and         |
     maintenance (O&M) organization.  Within an AS routers may  |
     use one or more interior routing protocols, and sometimes  |
     several sets of metrics.  An AS is expected to present to  |
     other ASs an appearence of a coherent interior routing     |
     plan, and a consistent picture of the destinations         |
     reachable through the AS.  An AS is identified by an       |
     Autonomous System number.
Connected Network
     A network prefix to which a router is interfaced is often
     known as a "local network" or the "subnetwork" of that
     router.  However, these terms can cause confusion, and
     therefore we use the term "Connected Network" in this
     memo.

Connected (Sub)Network
     A Connected (Sub)Network is an IP subnetwork to which a
     router is interfaced, or a connected network if the
     connected network is not subnetted.  See also Connected
     Network.

Datagram
     The unit transmitted between a pair of internet modules.
     Data, called datagrams, from sources to destinations.
     The Internet Protocol does not provide a reliable
     communication facility.  There are no acknowledgments
     either end-to-end or hop-by-hop.  There is no error no
     retransmissions.  There is no flow control.  See IP.

Default Route
     A routing table entry that is used to direct any data





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     addressed to any network prefixes not explicitly listed
     in the routing table.

Dense Mode
     In multicast forwarding, two paradigms are possible: in
     "Dense Mode" forwarding, a network multicast is forwarded
     as a data link layer multicast to all interfaces except
     that on which it was received, unless and until the
     router is instructed not to by a multicast routing
     neighbor.  See Sparse Mode.

EGP
     Exterior Gateway Protocol A protocol that distributes
     routing information to the gateways (routers) which
     connect autonomous systems.  See IGP.

EGP-2
     Exterior Gateway Protocol version 2 This is an EGP
     routing protocol developed to handle traffic between
     Autonomous Systems in the Internet.

Forwarder
     The logical entity within a router that is responsible
     for switching packets among the router's interfaces.  The
     Forwarder also makes the decisions to queue a packet for
     local delivery, to queue a packet for transmission out
     another interface, or both.

Forwarding
     Forwarding is the process a router goes through for each
     packet received by the router.  The packet may be
     consumed by the router, it may be output on one or more
     interfaces of the router, or both.  Forwarding includes
     the process of deciding what to do with the packet as
     well as queuing it up for (possible) output or internal
     consumption.

Forwarding Information Base (FIB)
     The table containing the information necessary to forward
     IP Datagrams, in this document, is called the Forwarding
     Information Base.  At minimum, this contains the
     interface identifier and next hop information for each
     reachable destination network prefix.






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Fragment
     An IP datagram that represents a portion of a higher
     layer's packet that was too large to be sent in its
     entirety over the output network.

General Purpose Serial Interface
     A physical medium capable of connecting exactly two
     systems, and therefore configurable as a point to point
     line, but also configurable to support link layer
     networking using protocols such as X.25 or Frame Relay.
     A link layer network connects another system to a switch,
     and a higher communication layer multiplexes virtual
     circuits on the connection.  See Point to Point Line.

IGP
     Interior Gateway Protocol A protocol that distributes
     routing information with an Autonomous System (AS).  See
     EGP.

Interface IP Address
     The IP Address and network prefix length that is assigned
     to a specific interface of a router.

Internet Address
     An assigned number that identifies a host in an internet.
     It has two parts: an IP address and a prefix length.  The
     prefix length indicates how many of the most specific
     bits of the address constitute the network prefix.

IP
     Internet Protocol The network layer protocol for the
     Internet.  It is a packet switching, datagram protocol
     defined in RFC 791.  IP does not provide a reliable
     communications facility; that is, there are no end-to-end
     of hop-by-hop acknowledgments.

IP Datagram
     An IP Datagram is the unit of end-to-end transmission in
     the Internet Protocol.  An IP Datagram consists of an IP
     header followed by all of higher-layer data (such as TCP,
     UDP, ICMP, and the like).  An IP Datagram is an IP header
     followed by a message.

     An IP Datagram is a complete IP end-to-end transmission





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     unit.  An IP Datagram is composed of one or more IP
     Fragments.

     In this memo, the unqualified term "Datagram" should be
     understood to refer to an IP Datagram.

IP Fragment
     An IP Fragment is a component of an IP Datagram.  An IP
     Fragment consists of an IP header followed by all or part
     of the higher-layer of the original IP Datagram.

     One or more IP Fragments comprises a single IP Datagram.

     In this memo, the unqualified term "Fragment" should be
     understood to refer to an IP Fragment.

IP Packet
     An IP Datagram or an IP Fragment.

     In this memo, the unqualified term "Packet" should
     generally be understood to refer to an IP Packet.

Logical [network] interface
     We define a logical [network] interface to be a logical
     path, distinguished by a unique IP address, to a
     connected network.

Martian Filtering
     A packet that contains an invalid source or destination
     address is considered to be "martian" and discarded.

MTU (Maximum Transmission Unit)
     The size of the largest packet that can be transmitted or
     received through a logical interface.  This size includes
     the IP header but does not include the size of any Link
     Layer headers or framing.

Multicast
     A packet that is destined for multiple hosts.  See
     "broadcast".

Multicast Address
     A special type of address that is recognizable by
     multiple hosts.





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     A Multicast Address is sometimes known as a Functional
     Address or a Group Address.

Network Prefix
     The portion of an IP Address that signifies a set of
     systems.  It is selected from the IP Address by logically
     ANDing a subnet mask with the address, or (equivalently)
     setting the bits of the address not among the most
     significant <prefix-length> bits of the address to zero.

Originate
     Packets can be transmitted by a router for one of two
     reasons: 1) the packet was received and is being
     forwarded or 2) the router itself created the packet for
     transmission (such as route advertisements).  Packets
     that the router creates for transmission are said to
     originate at the router.

Packet
     A packet is the unit of data passed across the interface
     between the Internet Layer and the Link Layer.  It
     includes an IP header and data.  A packet may be a
     complete IP datagram or a fragment of an IP datagram.

Path
     The sequence of routers and (sub-)networks that a packet
     traverses from a particular router to a particular
     destination host.  Note that a path is uni-directional;
     it is not unusual to have different paths in the two
     directions between a given host pair.

Physical Network
     A Physical Network is a network (or a piece of an
     internet) which is contiguous at the Link Layer.  Its
     internal structure (if any) is transparent to the
     Internet Layer.

     In this memo, several media components that are connected
     using devices such as bridges or repeaters are considered
     to be a single Physical Network since such devices are
     transparent to the IP.

Physical Network Interface
     This is a physical interface to a Connected Network and





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     has a (possibly unique) Link-Layer address.  Multiple
     Physical Network Interfaces on a single router may share
     the same Link-Layer address, but the address must be
     unique for different routers on the same Physical
     Network.

Point to Point Line
     A physical medium capable of connecting exactly two
     systems.  In this document, it is only used to refer to
     such a line when used to connect IP entities.  See
     General Purpose Serial Interface.

router
     A special-purpose dedicated computer that connects
     several networks.  Routers switch packets between these
     networks in a process known as forwarding.  This process
     may be repeated several times on a single packet by
     multiple routers until the packet can be delivered to the
     final destination - switching the packet from router to
     router to router...  until the packet gets to its
     destination.

RPF
     Reverse Path Forwarding - A method used to deduce the
     next hops for broadcast and multicast packets.

Silently Discard
     This memo specifies several cases where a router is to
     "Silently Discard" a received packet (or datagram).  This
     means that the router should discard the packet without
     further processing, and that the router will not send any
     ICMP error message (see Section [4.3.2]) as a result.
     However, for diagnosis of problems, the router should
     provide the capability of logging the error (see Section
     [1.3.3]), including the contents of the silently
     discarded packet, and should record the event in a
     statistics counter.

Silently Ignore
     A router is said to "Silently Ignore" an error or
     condition if it takes no action other than possibly
     generating an error report in an error log or through
     some network management protocol, and discarding, or
     ignoring, the source of the error.  In particular, the





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     router does NOT generate an ICMP error message.

Sparse Mode
     In multicast forwarding, two paradigms are possible: in
     "Sparse Mode" forwarding, a network layer multicast
     datagram is forwarded as a data link layer multicast
     frame to routers and hosts that have asked for it.  The
     initial forwarding state is the inverse of dense-mode in
     that it assumes no part  of the network wants the data.
     See Dense Mode.

Specific-destination address
     This is defined to be the destination address in the IP
     header unless the header contains an IP broadcast or IP
     multicast address, in which case the specific-destination
     is an IP address assigned to the physical interface on
     which the packet arrived.

subnet
     A portion of a network, which may be a physically
     independent network, which shares a network address with
     other portions of the network and is distinguished by a
     subnet number.  A subnet is to a network what a network
     is to an internet.

subnet number
     A part of the internet address that designates a subnet.
     It is ignored for the purposes internet routing, but is
     used for intranet routing.

TOS
     Type Of Service A field in the IP header that represents
     the degree of reliability expected from the network layer
     by the transport layer or application.

TTL
     Time To Live A field in the IP header that represents how
     long a packet is considered valid.  It is a combination
     "hop count" and "timer value".










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APPENDIX C. FUTURE DIRECTIONS

This appendix lists work that future revisions of this
document may wish to address.

In the preparation of Router Requirements, we stumbled across
several other architectural issues.  Each of these is dealt
with somewhat in the document, but still ought to be
classified as an open issue in the IP architecture.

Most of the he topics presented here generally indicate areas
where the technology is still relatively new and it is not
appropriate to develop specific requirements since the
community is still gaining operational experience.

Other topics represent areas of ongoing research and indicate
areas that the prudent developer would closely monitor.

(1) SNMP Version 2

(2) Additional SNMP MIBs                                        |

(7) More detailed requirements for leaking routes between       |
     routing protocols

(8) Router system security

(9) Routing protocol security

(10) Internetwork Protocol layer security.  There has been
     extensive work refining the security of IP since the
     original work writing this document.  This security work
     should be included in here.

(12) Load Splitting

(13) Sending fragments along different paths


(15) Multiple logical (sub)nets on the same wire.  Router
     Requirements does not require support for this.  We made
     some attempt to identify pieces of the architecture
     (e.g., forwarding of directed broadcasts and issuing of
     Redirects) where the wording of the rules has to be done





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     carefully to make "the right thing" happen, and tried to
     clearly distinguish logical interfaces from physical
     interfaces.  However, we did not study this issue in
     detail, and we are not at all confident that all the
     rules in the document are correct in the presence of
     multiple logical (sub)nets on the same wire.

(15) Congestion control and resource management.  On the
     advice of the IETF's experts (Mankin and Ramakrishnan) we
     deprecated (SHOULD NOT) Source Quench and said little
     else concrete (Section 5.3.6).

(16) Developing a Link-Layer requirements document that would
     be common for both routers and hosts.

(17) Developing a common PPP LQM algorithm.

(18) Investigate of other information (above and beyond
     section [3.2]) that passes between the layers, such as
     physical network MTU, mappings of IP precedence to Link
     Layer priority values, etc.

(19) Should the Link Layer notify IP if address resolution
     failed (just like it notifies IP when there is a Link
     Layer priority value problem)?

(20) Should all routers be required to implement a DNS
     resolver?

(21) Should a human user be able to use a host name anywhere
     you can use an IP address when configuring the router?
     Even in ping and traceroute?

(22) Almquist's draft ruminations on the next hop and
     ruminations on route leaking need to be reviewed, brought
     up to date, and published.

(23) Investigation is needed to determine if a redirect
     message for precedence is needed or not.  If not, are the
     type-of-service redirects acceptable?

(24) RIPv2 and RIP+CIDR and variable length network prefixes.

(25) BGP-4 CIDR is going to be important, and everyone is





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     betting on BGP-4.  We can't avoid mentioning it.
     Probably need to describe the differences between BGP-3
     and BGP-4, and explore upgrade issues...

(26) Loose Source Route Mobile IP and some multicasting may
     require this.  Perhaps it should be elevated to a SHOULD
     (per Fred Baker's Suggestion).










































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APPENDIX D. Multicast Routing Protocols

Multicasting is a relatively new technology within the
Internet Protocol family.  It is not widely deployed or
commonly in use yet.  Its importance, however, is expected to
grow over the coming years.

This Appendix describes some of the technologies being
investigated for routing multicasts through the Internet.

A diligent implementor will keep abreast of developments in
this area to properly develop multicast facilities.

This Appendix does not specify any standards or requirements.


D.1 Introduction

   Multicast routing protocols enable the forwarding of IP
   multicast datagrams throughout a TCP/IP internet.
   Generally these algorithms forward the datagram based on
   its source and destination addresses.  Additionally, the
   datagram may need to be forwarded to several multicast
   group members, at times requiring the datagram to be
   replicated and sent out multiple interfaces.

   The state of multicast routing protocols is less developed
   than the protocols available for the forwarding of IP
   unicasts.  Three experimental multicast routing protocols
   have been documented for TCP/IP.  Each uses the IGMP
   protocol (discussed in Section [4.4]) to monitor multicast
   group membership.


D.2 Distance Vector Multicast Routing Protocol - DVMRP

   DVMRP, documented in [ROUTE:9], is based on Distance Vector
   or Bellman-Ford technology.  It routes multicast datagrams
   only, and does so within a single Autonomous System.  DVMRP
   is an implementation of the Truncated Reverse Path
   Broadcasting algorithm described in [ROUTE:10].  In
   addition, it specifies the tunneling of IP multicasts
   through non-multicast-routing-capable IP domains.






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D.3 Multicast Extensions to OSPF - MOSPF

   MOSPF, currently under development, is a backward-
   compatible addition to OSPF that allows the forwarding of
   both IP multicasts and unicasts within an Autonomous
   System.  MOSPF routers can be mixed with OSPF routers
   within a routing domain, and they will interoperate in the
   forwarding of unicasts.  OSPF is a link-state or SPF-based
   protocol.  By adding link state advertisements that
   pinpoint group membership, MOSPF routers can calculate the
   path of a multicast datagram as a tree rooted at the
   datagram source.  Those branches that do not contain group
   members can then be discarded, eliminating unnecessary
   datagram forwarding hops.


D.4 Protocol Independent Multicast - PIM

   PIM, currently under development, is a multicast routing
   protocol that runs over an existing unicast infrastructure.
   PIM provides for both dense and sparse group membership.
   It is different from other protocols, since it uses an
   explicit join model for sparse groups.  Joining occurs on a
   shared tree and can switch to a per-source tree.  Where
   bandwidth is plentiful and group membership is dense,
   overhead can be reduced by flooding data out all links and
   later pruning exception cases where there are no group
   members.





















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APPENDIX E Additional Next-Hop Selection Algorithms

Section [5.2.4.3] specifies an algorithm that routers ought to
use when selecting a next-hop for a packet.

This appendix provides historical perspective for the next-hop
selection problem.  It also presents several additional
pruning rules and next-hop selection algorithms that might be
found in the Internet.

This appendix presents material drawn from an earlier,
unpublished, work by Philip Almquist; "Ruminations on the Next
Hop".

This Appendix does not specify any standards or requirements.


E.1. Some Historical Perspective

   It is useful to briefly review the history of the topic,
   beginning with what is sometimes called the "classic model"
   of how a router makes routing decisions.  This model
   predates IP.  In this model, a router speaks some single
   routing protocol such as RIP.  The protocol completely
   determines the contents of the router's Forwarding
   Information Base (FIB).  The route lookup algorithm is
   trivial: the router looks in the FIB for a route whose
   destination attribute exactly matches the network prefix
   portion of the destination address in the packet.  If one
   is found, it is used; if none is found, the destination is
   unreachable.  Because the routing protocol keeps at most
   one route to each destination, the problem of what to do
   when there are multiple routes that match the same
   destination cannot arise.

   Over the years, this classic model has been augmented in
   small ways.  With the deployment of default routes,
   subnets, and host routes, it became possible to have more
   than one routing table entry which in some sense matched
   the destination.  This was easily resolved by a consensus
   that there was a hierarchy of routes: host routes should be
   preferred over subnet routes, subnet routes over net
   routes, and net routes over default routes.






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   With the deployment of technologies supporting variable
   length subnet masks (variable length network prefixes), the
   general approach remained the same although its description
   became a little more complicated; network prefixes were
   introduced as a conscious simplification and regularization
   of the architecture.  We now say that each route to a
   network prefix route has a prefix length associated with
   it.  This prefix length indicates the number of bits in the
   prefix.  This may also be represented using the classical
   subnet mask.  A route cannot be used to route a packet
   unless each significant bit in the route's network prefix
   matches the corresponding bit in the packet's destination
   address.  Routes with more bits set in their masks are
   preferred over routes that have fewer bits set in their
   masks.  This is simply a generalization of the hierarchy of
   routes described above, and will be referred to for the
   rest of this memo as choosing a route by preferring longest
   match.

   Another way the classic model has been augmented is through
   a small amount of relaxation of the notion that a routing
   protocol has complete control over the contents of the
   routing table.  First, static routes were introduced.  For
   the first time, it was possible to simultaneously have two
   routes (one dynamic and one static) to the same
   destination.  When this happened, a router had to have a
   policy (in some cases configurable, and in other cases
   chosen by the author of the router's software) which
   determined whether the static route or the dynamic route
   was preferred.  However, this policy was only used as a
   tie-breaker when longest match didn't uniquely determine
   which route to use.  Thus, for example, a static default
   route would never be preferred over a dynamic net route
   even if the policy preferred static routes over dynamic
   routes.

   The classic model had to be further augmented when inter-
   domain routing protocols were invented.  Traditional
   routing protocols came to be called "interior gateway
   protocols" (IGPs), and at each Internet site there was a
   strange new beast called an "exterior gateway", a router
   that spoke EGP to several "BBN Core Gateways" (the routers
   that made up the Internet backbone at the time) at the same
   time as it spoke its IGP to the other routers at its site.





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   Both protocols wanted to determine the contents of the
   router's routing table.  Theoretically, this could result
   in a router having three routes (EGP, IGP, and static) to
   the same destination.  Because of the Internet topology at
   the time, it was resolved with little debate that routers
   would be best served by a policy of preferring IGP routes
   over EGP routes.  However, the sanctity of longest match
   remained unquestioned: a default route learned from the IGP
   would never be preferred over a net route from learned EGP.

   Although the Internet topology, and consequently routing in
   the Internet, have evolved considerably since then, this
   slightly augmented version of the classic model has
   survived intact to this day in the Internet (except that
   BGP has replaced EGP).  Conceptually (and often in
   implementation) each router has a routing table and one or
   more routing protocol processes.  Each of these processes
   can add any entry that it pleases, and can delete or modify
   any entry that it has created.  When routing a packet, the
   router picks the best route using longest match, augmented
   with a policy mechanism to break ties.  Although this
   augmented classic model has served us well, it has a number
   of shortcomings:

   + It ignores (although it could be augmented to consider)
      path characteristics such as quality of service and MTU.

   + It doesn't support routing protocols (such as OSPF and
      Integrated IS-IS) that require route lookup algorithms
      different than pure longest match.

   + There has not been a firm consensus on what the tie-
      breaking mechanism ought to be.  Tie-breaking mechanisms
      have often been found to be difficult if not impossible
      to configure in such a way that the router will always
      pick what the network manger considers to be the
      "correct" route.


E.2. Additional Pruning Rules

   Section [5.2.4.3] defined several pruning rules to use to
   select routes from the FIB.  There are other rules that
   could also be used.





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   + OSPF Route Class
      Routing protocols that have areas or make a distinction
      between internal and external routes divide their routes
      into classes by the type of information used to
      calculate the route.  A route is always chosen from the
      most preferred class unless none is available, in which
      case one is chosen from the second most preferred class,
      and so on.  In OSPF, the classes (in order from most
      preferred to least preferred) are intra-area, inter-
      area, type 1 external (external routes with internal
      metrics), and type 2 external.  As an additional
      wrinkle, a router is configured to know what addresses
      ought to be accessible using intra-area routes, and will
      not use inter- area or external routes to reach these
      destinations even when no intra-area route is available.

      More precisely, we assume that each route has a class
      attribute, called route.class, which is assigned by the
      routing protocol.  The set of candidate routes is
      examined to determine if it contains any for which
      route.class = intra-area.  If so, all routes except
      those for which route.class = intra-area are discarded.
      Otherwise, router checks whether the packet's
      destination falls within the address ranges configured
      for the local area.  If so, the entire set of candidate
      routes is deleted.  Otherwise, the set of candidate
      routes is examined to determine if it contains any for
      which route.class = inter-area.  If so, all routes
      except those for which route.class = inter-area are
      discarded.  Otherwise, the set of candidate routes is
      examined to determine if it contains any for which
      route.class = type 1 external.  If so, all routes except
      those for which route.class = type 1 external are
      discarded.

   + IS-IS Route Class
      IS-IS route classes work identically to OSPF's.
      However, the set of classes defined by Integrated IS-IS
      is different, such that there isn't a one-to-one mapping
      between IS-IS route classes and OSPF route classes.  The
      route classes used by Integrated IS-IS are (in order
      from most preferred to least preferred) intra-area,
      inter-area, and external.






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      The Integrated IS-IS internal class is equivalent to the
      OSPF internal class.  Likewise, the Integrated IS-IS
      external class is equivalent to OSPF's type 2 external
      class.  However, Integrated IS-IS does not make a
      distinction between inter-area routes and external
      routes with internal metrics - both are considered to be
      inter-area routes.  Thus, OSPF prefers true inter-area
      routes over external routes with internal metrics,
      whereas Integrated IS-IS gives the two types of routes
      equal preference.

   + IDPR Policy
      A specific case of Policy.  The IETF's Inter-domain
      Policy Routing Working Group is devising a routing
      protocol called Inter-Domain Policy Routing (IDPR) to
      support true policy-based routing in the Internet.
      Packets with certain combinations of header attributes
      (such as specific combinations of source and destination
      addresses or special IDPR source route options) are
      required to use routes provided by the IDPR protocol.
      Thus, unlike other Policy pruning rules, IDPR Policy
      would have to be applied before any other pruning rules
      except Basic Match.

      Specifically, IDPR Policy examines the packet being
      forwarded to ascertain if its attributes require that it
      be forwarded using policy-based routes.  If so, IDPR
      Policy deletes all routes not provided by the IDPR
      protocol.


E.3 Some Route Lookup Algorithms

   This section examines several route lookup algorithms that
   are in use or have been proposed.  Each is described by
   giving the sequence of pruning rules it uses.  The
   strengths and weaknesses of each algorithm are presented


E.3.1 The Revised Classic Algorithm

      The Revised Classic Algorithm is the form of the
      traditional algorithm that was discussed in Section
      [E.1].  The steps of this algorithm are:





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      1.  Basic match
      2.  Longest match
      3.  Best metric
      4.  Policy

      Some implementations omit the Policy step, since it is
      needed only when routes may have metrics that are not
      comparable (because they were learned from different
      routing domains).

      The advantages of this algorithm are:

      (1) It is widely implemented.

      (2) Except for the Policy step (which an implementor can
           choose to make arbitrarily complex) the algorithm
           is simple both to understand and to implement.

      Its disadvantages are:

      (1) It does not handle IS-IS or OSPF route classes, and
           therefore cannot be used for Integrated IS-IS or
           OSPF.

      (2) It does not handle TOS or other path attributes.

      (3) The policy mechanisms are not standardized in any
           way, and are therefore are often implementation-
           specific.  This causes extra work for implementors
           (who must invent appropriate policy mechanisms) and
           for users (who must learn how to use the
           mechanisms. This lack of a standardized mechanism
           also makes it difficult to build consistent
           configurations for routers from different vendors.
           This presents a significant practical deterrent to
           multi-vendor interoperability.

      (4) The proprietary policy mechanisms currently provided
           by vendors are often inadequate in complex parts of
           the Internet.

      (5) The algorithm has not been written down in any
           generally available document or standard.  It is,
           in effect, a part of the Internet Folklore.





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E.3.2 The Variant Router Requirements Algorithm

      Some Router Requirements Working Group members have
      proposed a slight variant of the algorithm described in
      the Section [5.2.4.3].  In this variant, matching the
      type of service requested is considered to be more
      important, rather than less important, than matching as
      much of the destination address as possible.  For
      example, this algorithm would prefer a default route
      that had the correct type of service over a network
      route that had the default type of service, whereas the
      algorithm in [5.2.4.3] would make the opposite choice.

      The steps of the algorithm are:
      1.  Basic match
      2.  Weak TOS
      3.  Longest match
      4.  Best metric
      5.  Policy

      Debate between the proponents of this algorithm and the
      regular Router Requirements Algorithm suggests that each
      side can show cases where its algorithm leads to
      simpler, more intuitive routing than the other's
      algorithm does.  This variant has the same set of
      advantages and disadvantages that the algorithm
      specified in [5.2.4.3] does, except that pruning on Weak
      TOS before pruning on Longest Match makes this algorithm
      less compatible with OSPF and Integrated IS-IS than the
      standard Router Requirements Algorithm.


E.3.3 The OSPF Algorithm

      OSPF uses an algorithm that is virtually identical to
      the Router Requirements Algorithm except for one crucial
      difference: OSPF considers OSPF route classes.

      The algorithm is:
      1.  Basic match
      2.  OSPF route class
      3.  Longest match
      4.  Weak TOS
      5.  Best metric





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      6.  Policy

      Type of service support is not always present.  If it is
      not present then, of course, the fourth step would be
      omitted

      This algorithm has some advantages over the Revised
      Classic Algorithm:

      (1) It supports type of service routing.

      (2) Its rules are written down, rather than merely being
           a part of the Internet folklore.

      (3) It (obviously) works with OSPF.

      However, this algorithm also retains some of the
      disadvantages of the Revised Classic Algorithm:

      (1) Path properties other than type of service (e.g.,
           MTU) are ignored.

      (2) As in the Revised Classic Algorithm, the details (or
           even the existence) of the Policy step are left to
           the discretion of the implementor.

      The OSPF Algorithm also has a further disadvantage
      (which is not shared by the Revised Classic Algorithm).
      OSPF internal (intra-area or inter-area) routes are
      always considered to be superior to routes learned from
      other routing protocols, even in cases where the OSPF
      route matches fewer bits of the destination address.
      This is a policy decision that is inappropriate in some
      networks.

      Finally, it is worth noting that the OSPF Algorithm's
      TOS support suffers from a deficiency in that routing
      protocols that support TOS are implicitly preferred when
      forwarding packets that have non-zero TOS values.  This
      may not be appropriate in some cases.









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E.3.4 The Integrated IS-IS Algorithm

      Integrated IS-IS uses an algorithm that is similar to
      but not quite identical to the OSPF Algorithm.
      Integrated IS-IS uses a different set of route classes,
      and differs slightly in its handling of type of service.
      The algorithm is:
      1.  Basic Match
      2.  IS-IS Route Classes
      3.  Longest Match
      4.  Weak TOS
      5.  Best Metric
      6.  Policy

      Although Integrated IS-IS uses Weak TOS, the protocol is
      only capable of carrying routes for a small specific
      subset of the possible values for the TOS field in the
      IP header.  Packets containing other values in the TOS
      field are routed using the default TOS.

      Type of service support is optional; if disabled, the
      fourth step would be omitted.  As in OSPF, the
      specification does not include the Policy step.

      This algorithm has some advantages over the Revised
      Classic Algorithm:
      (1) It supports type of service routing.
      (2) Its rules are written down, rather than merely being
           a part of the Internet folklore.
      (3) It (obviously) works with Integrated IS-IS.

      However, this algorithm also retains some of the
      disadvantages of the Revised Classic Algorithm:
      (1) Path properties other than type of service (e.g.,
           MTU) are ignored.
      (2) As in the Revised Classic Algorithm, the details (or
           even the existence) of the Policy step are left to
           the discretion of the implementor.
      (3) It doesn't work with OSPF because of the differences
           between IS-IS route classes and OSPF route classes.
           Also, because IS-IS supports only a subset of the
           possible TOS values, some obvious implementations
           of the Integrated IS-IS algorithm would not support
           OSPF's interpretation of TOS.





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      The Integrated IS-IS Algorithm also has a further
      disadvantage (which is not shared by the Revised Classic
      Algorithm): IS-IS internal (intra-area or inter-area)
      routes are always considered to be superior to routes
      learned from other routing protocols, even in cases
      where the IS-IS route matches fewer bits of the
      destination address and doesn't provide the requested
      type of service.  This is a policy decision that may not
      be appropriate in all cases.

      Finally, it is worth noting that the Integrated IS-IS
      Algorithm's TOS support suffers from the same deficiency
      noted for the OSPF Algorithm.




































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Security Considerations

Although the focus of this document is interoperability rather
than security, there are obviously many sections of this
document that have some ramifications on network security.

"Security" means different things to different people.
Security from a router's point of view is anything that helps
to keep its own networks operational and in addition helps to
keep the Internet as a whole healthy.  For the purposes of
this document, the security services we are concerned with are
"denial of service", "integrity", and "authentication" as it
applies to the first two.  "Privacy" as a security service is
important, but only peripherally a concern of a router - at
least as of the date of this document.

In several places in this document there are sections entitled
"...  Security Considerations".  These sections discuss
specific considerations that apply to the general topic under
discussion.

Rarely does this document say "do this and your router/network
will be secure".  More likely, it says "this is a good idea
and if you do it, it *may* improve the security of the
Internet and your local system in general."

Unfortunately, this is the state-of-the-art AT THIS TIME.  Few
if any of the network protocols a router is concerned with
have reasonable, built-in security features.  Industry and the
protocol designers have been and are continuing to struggle
with these issues.  There is progress, but only small baby
steps such as the peer-to-peer authentication available in the
BGP and OSPF routing protocols.

In particular, this document notes the current research into
developing and enhancing network security.  Specific areas of
research, development, and engineering that are underway as of
this writing (December 1993) are in IP Security, SNMP
Security, and common authentication technologies.

Notwithstanding all the above, there are things both vendors
and users can do to improve the security of their router.
Vendors should get a copy of "Trusted Computer System
Interpretation" [INTRO:8].  Even if a vendor decides not to





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submit their device for formal verification under these
guidelines, the publication provides excellent guidance on
general security design and practices for computing devices.














































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APPENDIX F: HISTORICAL ROUTING PROTOCOLS

Certain routing protocols are common in the Internet, but the
authors of this document cannot in good conscience recommend
their use.  This is not because they do not work correctly,
but because the characteristics of the Internet assumed in
their design (simple routing, no policy, a single "core
router" network under common administration, limited
complexity, or limited network diameter) are not attributes of
today's Internet.  Those parts of the Internet that still use
them are generally limited "fringe" domains with limited
complexity.

As a matter of good faith, collected wisdom concerning their
implementation is recorded in this section.


F.1 EXTERIOR GATEWAY PROTOCOL - EGP



F.1.1 Introduction

      The Exterior Gateway Protocol (EGP) specifies an EGP
      that is used to exchange reachability information
      between routers of the same or differing autonomous
      systems.  EGP is not considered a routing protocol since
      there is no standard interpretation (i.e. metric) for
      the distance fields in the EGP update message, so
      distances are comparable only among routers of the same
      AS.  It is however designed to provide high-quality
      reachability information, both about neighbor routers
      and about routes to non-neighbor routers.

      EGP is defined by [ROUTE:6].  An implementor almost
      certainly wants to read [ROUTE:7] and [ROUTE:8] as well,
      for they contain useful explanations and background
      material.

      DISCUSSION:
         The present EGP specification has serious
         limitations, most importantly a restriction that
         limits routers to advertising only those networks
         that are reachable from within the router's





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         autonomous system.  This restriction against
         propagating "third party" EGP information is to
         prevent long-lived routing loops.  This effectively
         limits EGP to a two-level hierarchy.

         RFC-975 is not a part of the EGP specification, and
         should be ignored.



F.1.2 Protocol Walk-through


      Indirect Neighbors: RFC-888, pp.  26

         An implementation of EGP MUST include indirect
         neighbor support.

      Polling Intervals: RFC-904, pp.  10

         The interval between Hello command retransmissions
         and the interval between Poll retransmissions SHOULD
         be configurable but there MUST be a minimum value
         defined.

         The interval at which an implementation will respond
         to Hello commands and Poll commands SHOULD be
         configurable but there MUST be a minimum value
         defined.

      Network Reachability: RFC-904, pp.  15

         An implementation MUST default to not providing the
         external list of routers in other autonomous systems;
         only the internal list of routers together with the
         nets that are reachable through those routers should
         be included in an Update Response/Indication packet.
         However, an implementation MAY elect to provide a
         configuration option enabling the external list to be
         provided.  An implementation MUST NOT include in the
         external list routers that were learned through the
         external list provided by a router in another
         autonomous system.  An implementation MUST NOT send a
         network back to the autonomous system from which it





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         is learned, i.e.  it MUST do split-horizon on an
         autonomous system level.

         If more than 255 internal or 255 external routers
         need to be specified in a Network Reachability
         update, the networks reachable from routers that can
         not be listed MUST be merged into the list for one of
         the listed routers.  Which of the listed routers is
         chosen for this purpose SHOULD be user configurable,
         but SHOULD default to the source address of the EGP
         update being generated.

         An EGP update contains a series of blocks of network
         numbers, where each block contains a list of network
         numbers reachable at a particular distance through a
         particular router.  If more than 255 networks are
         reachable at a particular distance through a
         particular router, they are split into multiple
         blocks (all of which have the same distance).
         Similarly, if more than 255 blocks are required to
         list the networks reachable through a particular
         router, the router's address is listed as many times
         as necessary to include all the blocks in the update.
      Unsolicited Updates: RFC-904, pp.  16

         If a network is shared with the peer, an
         implementation MUST send an unsolicited update upon
         entry to the Up state if the source network is the
         shared network.

      Neighbor Reachability: RFC-904, pp.  6, 13-15

         The table on page 6 that describes the values of j
         and k (the neighbor up and down thresholds) is
         incorrect.  It is reproduced correctly here:

            Name    Active  Passive Description
            -----------------------------------------------
             j         3       1    neighbor-up threshold
             k         1       0    neighbor-down threshold

         The value for k in passive mode also specified
         incorrectly in RFC-904, page 14 The values in
         parenthesis should read:





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            (j = 1, k = 0, and T3/T1 = 4)

         As an optimization, an implementation can refrain
         from sending a Hello command when a Poll is due.  If
         an implementation does so, it SHOULD provide a user
         configurable option to disable this optimization.

      Abort timer: RFC-904, pages 6, 12, 13

         An EGP implementation MUST include support for the
         abort timer (as documented in section 4.1.4 of RFC-
         904).  An implementation SHOULD use the abort timer
         in the Idle state to automatically issue a Start
         event to restart the protocol machine.  Recommended
         values are P4 for a critical error (Administratively
         prohibited, Protocol Violation and Parameter Problem)
         and P5 for all others.  The abort timer SHOULD NOT be
         started when a Stop event was manually initiated
         (such as through a network management protocol).

      Cease command received in Idle state: RFC-904, page 13

         When the EGP state machine is in the Idle state, it
         MUST reply to Cease commands with a Cease-ack
         response.

      Hello Polling Mode: RFC-904, page 11

         An EGP implementation MUST include support for both
         active and passive polling modes.

      Neighbor Acquisition Messages: RFC-904, page 18

         As noted the Hello and Poll Intervals should only be
         present in Request and Confirm messages.  Therefore
         the length of an EGP Neighbor Acquisition Message is
         14 bytes for a Request or Confirm message and 10
         bytes for a Refuse, Cease or Cease-ack message.
         Implementations MUST NOT send 14 bytes for Refuse,
         Cease or Cease-ack messages but MUST allow for
         implementations that send 14 bytes for these
         messages.

      Sequence Numbers: RFC-904, page 10





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         Response or indication packets received with a
         sequence number not equal to S MUST be discarded.
         The send sequence number S MUST be incremented just
         before the time a Poll command is sent and at no
         other times.


F.2 ROUTING INFORMATION PROTOCOL - RIP



F.2.1 Introduction

      RIP is specified in [ROUTE:3].  Although RIP is still
      quite important in the Internet, it is being replaced in
      sophisticated applications by more modern IGPs such as
      the ones described above.  A router implementing RIP
      SHOULD implement RIP Version 2 [ROUTE:?], as it supports
      CIDR routes.  If occasional access networking is in use,
      a router implementing RIP SHOULD implement Demand RIP
      [ROUTE:?].

      Another common use for RIP is as a "router discovery"
      protocol.  Section [4.3.3.10] briefly touches upon this
      subject.


F.2.2 Protocol Walk-Through


      Dealing with changes in topology: [ROUTE:3], pp.  11

           An implementation of RIP MUST provide a means for
           timing out routes.  Since messages are occasionally
           lost, implementations MUST NOT invalidate a route
           based on a single missed update.

           Implementations MUST by default wait six times the
           update interval before invalidating a route.  A
           router MAY have configuration options to alter this
           value.

           DISCUSSION:
              It is important to routing stability that all





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              routers in a RIP autonomous system use similar
              timeout value for invalidating routes, and
              therefore it is important that an implementation
              default to the timeout value specified in the
              RIP specification.  However, that timeout value
              is too conservative in environments where packet
              loss is reasonably rare.  In such an
              environment, a network manager may wish to be
              able to decrease the timeout period to promote
              faster recovery from failures.


           IMPLEMENTATION:
              There is a very simple mechanism that a router
              may use to meet the requirement to invalidate
              routes promptly after they time out.  Whenever
              the router scans the routing table to see if any
              routes have timed out, it also notes the age of
              the least recently updated route that has not
              yet timed out.  Subtracting this age from the
              timeout period gives the amount of time until
              the router again needs to scan the table for
              timed out routes.


      Split Horizon: [ROUTE:3], pp.  14-15

           An implementation of RIP MUST implement "split
           horizon", a scheme used for avoiding problems
           caused by including routes in updates sent to the
           router from which they were learned.

           An implementation of RIP SHOULD implement "Split
           horizon with poisoned reverse", a variant of split
           horizon that includes routes learned from a router
           sent to that router, but sets their metric to
           infinity.  Because of the routing overhead that may
           be incurred by implementing split horizon with
           poisoned reverse, implementations MAY include an
           option to select whether poisoned reverse is in
           effect.  An implementation SHOULD limit the time in
           which it sends reverse routes at an infinite
           metric.






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           IMPLEMENTATION:
              Each of the following algorithms can be used to
              limit the time for which poisoned reverse is
              applied to a route.  The first algorithm is more
              complex but does a more thorough job of limiting
              poisoned reverse to only those cases where it is
              necessary.

              The goal of both algorithms is to ensure that
              poison reverse is done for any destination whose
              route has changed in the last Route Lifetime
              (typically 180 seconds), unless it can be sure
              that the previous route used the same output
              interface.  The Route Lifetime is used because
              that is the amount of time RIP will keep around
              an old route before declaring it stale.

              The time intervals (and derived variables) used
              in the following algorithms are as follows:

              Tu The Update Timer; the number of seconds
                   between RIP updates.  This typically
                   defaults to 30 seconds.

              Rl The Route Lifetime, in seconds.  This is the
                   amount of time that a route is presumed to
                   be good, without requiring an update.  This
                   typically defaults to 180 seconds.

              Ul The Update Loss; the number of consecutive
                   updates that have to be lost or fail to
                   mention a route before RIP deletes the
                   route.  Ul is calculated to be (Rl/Tu)+1.
                   The "+1" is to account for the fact that
                   the first time the ifcounter is decremented
                   will be less than Tu seconds after it is
                   initialized.  Typically, Ul will be 7:
                   (180/30)+1.


              In The value to set ifcounter to when a
                   destination is newly learned.  This value
                   is Ul-4, where the "4" is RIP's garbage
                   collection timer/30





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              The first algorithm is:

              - Associated with each destination is a counter,
                 called the ifcounter below.  Poison reverse
                 is done for any route whose destination's
                 ifcounter is greater than zero.

              - After a regular (not triggered or in response
                 to a request) update is sent, all the non-
                 zero ifcounters are decremented by one.

              - When a route to a destination is created, its
                 ifcounter is set as follows:

                 - If the new route is superseding a valid
                    route, and the old route used a different
                    (logical) output interface, then the
                    ifcounter is set to Ul.

                 - If the new route is superseding a stale
                    route, and the old route used a different
                    (logical) output interface, then the
                    ifcounter is set to MAX(0, Ul -
                    INT(seconds that the route has been
                    stale/Ut).

                 - If there was no previous route to the
                    destination, the ifcounter is set to In.

                 - Otherwise, the ifcounter is set to zero

              - RIP also maintains a timer, called the
                 resettimer below.  Poison reverse is done on
                 all routes whenever resettimer has not
                 expired (regardless of the ifcounter values).

              - When RIP is started, restarted, reset, or
                 otherwise has its routing table cleared, it
                 sets the resettimer to go off in Rl seconds.

              The second algorithm is identical to the first
              except that:

              - The rules which set the ifcounter to non-zero





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                 values are changed to always set it to Rl/Tu,
                 and

              - The resettimer is eliminated.

         Triggered updates: [ROUTE:3], pp.  15-16; page 29

              Triggered updates (also called "flash updates")
              are a mechanism for immediately notifying a
              router's neighbors when the router adds or
              deletes routes or changes their metrics.  A
              router MUST send a triggered update when routes
              are deleted or their metrics are increased.  A
              router MAY send a triggered update when routes
              are added or their metrics decreased.

              Since triggered updates can cause excessive
              routing overhead, implementations MUST use the
              following mechanism to limit the frequency of
              triggered updates:

              (1) When a router sends a triggered update, it
                   sets a timer to a random time between one
                   and five seconds in the future.  The router
                   must not generate additional triggered
                   updates before this timer expires.

              (2) If the router would generate a triggered
                   update during this interval it sets a flag
                   indicating that a triggered update is
                   desired.  The router also logs the desired
                   triggered update.

              (3) When the triggered update timer expires, the
                   router checks the triggered update flag.
                   If the flag is set then the router sends a
                   single triggered update which includes all
                   the changes that were logged.  The router
                   then clears the flag and, since a triggered
                   update was sent, restarts this algorithm.

              (4) The flag is also cleared whenever a regular
                   update is sent.






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              Triggered updates SHOULD include all routes that
              have changed since the most recent regular
              (non-triggered) update.  Triggered updates MUST
              NOT include routes that have not changed since
              the most recent regular update.

              DISCUSSION:
                 Sending all routes, whether they have changed
                 recently or not, is unacceptable in triggered
                 updates because the tremendous size of many
                 Internet routing tables could otherwise
                 result in considerable bandwidth being wasted
                 on triggered updates.

         Use of UDP: [ROUTE:3], pp.  18-19.

              RIP packets sent to an IP broadcast address
              SHOULD have their initial TTL set to one.

              Note that to comply with Section [6.1] of this
              memo, a router SHOULD use UDP checksums in RIP
              packets that it originates, MUST discard RIP
              packets received with invalid UDP checksums, but
              MUST NOT discard received RIP packets simply
              because they do not contain UDP checksums.

         Addressing Considerations: [ROUTE:3], pp.  22

              A RIP implementation SHOULD support host routes.
              If it does not, it MUST (as described on page 27
              of [ROUTE:3]) ignore host routes in received
              updates.  A router MAY log ignored hosts routes.

              The special address 0.0.0.0 is used to describe
              a default route.  A default route is used as the
              route of last resort (i.e., when a route to the
              specific net does not exist in the routing
              table).  The router MUST be able to create a RIP
              entry for the address 0.0.0.0.

         Input Processing - Response: [ROUTE:3], pp.  26

              When processing an update, the following
              validity checks MUST be performed:





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              + The response MUST be from UDP port 520.

              + The source address MUST be on a directly
                 connected subnet (or on a directly connected,
                 non-subnetted network) to be considered
                 valid.

              + The source address MUST NOT be one of the
                 router's addresses.

                 DISCUSSION:
                    Some networks, media, and interfaces allow
                    a sending node to receive packets that it
                    broadcasts.  A router must not accept its
                    own packets as valid routing updates and
                    process them.  The last requirement
                    prevents a router from accepting its own
                    routing updates and processing them (on
                    the assumption that they were sent by some
                    other router on the network).

              An implementation MUST NOT replace an existing
              route if the metric received is equal to the
              existing metric except in accordance with the
              following heuristic.

              An implementation MAY choose to implement the
              following heuristic to deal with the above
              situation.  Normally, it is useless to change
              the route to a network from one router to
              another if both are advertised at the same
              metric.  However, the route being advertised by
              one of the routers may be in the process of
              timing out.  Instead of waiting for the route to
              timeout, the new route can be used after a
              specified amount of time has elapsed.  If this
              heuristic is implemented, it MUST wait at least
              halfway to the expiration point before the new
              route is installed.










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F.2.3 Specific Issues


      RIP Shutdown

           An implementation of RIP SHOULD provide for a
           graceful shutdown using the following steps:

           (1) Input processing is terminated,

           (2) Four updates are generated at random intervals
                of between two and four seconds, These updates
                contain all routes that were previously
                announced, but with some metric changes.
                Routes that were being announced at a metric
                of infinity should continue to use this
                metric.  Routes that had been announced with a
                non-infinite metric should be announced with a
                metric of 15 (infinity - 1).

                DISCUSSION:
                   The metric used for the above really ought
                   to be 16 (infinity); setting it to 15 is a
                   kludge to avoid breaking certain old hosts
                   that wiretap the RIP protocol.  Such a host
                   will (erroneously) abort a TCP connection
                   if it tries to send a datagram on the
                   connection while the host has no route to
                   the destination (even if the period when
                   the host has no route lasts only a few
                   seconds while RIP chooses an alternate path
                   to the destination).

      RIP Split Horizon and Static Routes

           Split horizon SHOULD be applied to static routes by
           default.  An implementation SHOULD provide a way to
           specify, per static route, that split horizon
           should not be applied to this route.










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F.3 GATEWAY TO GATEWAY PROTOCOL - GGP                           |

      The Gateway to Gateway protocol is considered obsolete
      and SHOULD NOT be implemented.                            *













































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Acknowledgments


O that we now had here
But one ten thousand of those men in England
That do no work to-day!

What's he that wishes so?
My cousin Westmoreland? No, my fair cousin:
If we are mark'd to die, we are enow
To do our country loss; and if to live,
The fewer men, the greater share of honour.
God's will! I pray thee, wish not one man more.
By Jove, I am not covetous for gold,
Nor care I who doth feed upon my cost;
It yearns me not if men my garments wear;
Such outward things dwell not in my desires:
But if it be a sin to covet honour,
I am the most offending soul alive.
No, faith, my coz, wish not a man from England:
God's peace! I would not lose so great an honour
As one man more, methinks, would share from me
For the best hope I have. O, do not wish one more!
Rather proclaim it, Westmoreland, through my host,
That he which hath no stomach to this fight,
Let him depart; his passport shall be made
And crowns for convoy put into his purse:
We would not die in that man's company
That fears his fellowship to die with us.
This day is called the feast of Crispian:
He that outlives this day, and comes safe home,
Will stand a tip-toe when the day is named,
And rouse him at the name of Crispian.
He that shall live this day, and see old age,
Will yearly on the vigil feast his neighbours,
And say 'To-morrow is Saint Crispian:'
Then will he strip his sleeve and show his scars.
And say 'These wounds I had on Crispin's day.'
Old men forget: yet all shall be forgot,
But he'll remember with advantages
What feats he did that day: then shall our names.
Familiar in his mouth as household words
Harry the king, Bedford and Exeter,
Warwick and Talbot, Salisbury and Gloucester,





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Be in their flowing cups freshly remember'd.
This story shall the good man teach his son;
And Crispin Crispian shall ne'er go by,
From this day to the ending of the world,
But we in it shall be remember'd;
We few, we happy few, we band of brothers;
For he to-day that sheds his blood with me
Shall be my brother; be he ne'er so vile,
This day shall gentle his condition:
And gentlemen in England now a-bed
Shall think themselves accursed they were not here,
And hold their manhoods cheap whiles any speaks
That fought with us upon Saint Crispin's day.

This memo is a product of the IETF's Router Requirements
Working Group.  A memo such as this one is of necessity the
work of many more people than could be listed here.  A wide
variety of vendors, network managers, and other experts from
the Internet community graciously contributed their time and
wisdom to improve the quality of this memo.  The editor wishes
to extend sincere thanks to all of them.

The current editor also wishes to single out and extend his
heartfelt gratitude and appreciation to the original editor of
this document; Philip Almquist.  Without Philip's work, both
as the original editor and as the Chair of the working group,
this document would not have been produced.  He also wishes to
express deep and heartfelt gratitude to the previous editor,
Frank Kastenholz.  Frank changed the original document from a
collection of information to a useful description of IP
technology - in his words, a "snapshot" of the technology in
1991.  One can only hope that this snapshot, of the technology
in 1994, is as clear.

Philip Almquist, Jeffrey Burgan, Frank Kastenholz, and Cathy
Wittbrodt each wrote major chapters of this memo.  Others who
made major contributions to the document included Bill Barns,
Steve Deering, Kent England, Jim Forster, Martin Gross, Jeff
Honig, Steve Knowles, Yoni Malachi, Michael Reilly, and Walt
Wimer.

Additional text came from Andy Malis, Paul Traina, Art
Berggreen, John Cavanaugh, Ross Callon, John Lekashman, Brian
Lloyd, Gary Malkin, Milo Medin, John Moy, Craig Partridge,





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Stephanie Price, Yakov Rekhter, Steve Senum, Richard Smith,
Frank Solensky, Rich Woundy, and others who have been
inadvertently overlooked.

Some of the text in this memo has been (shamelessly)
plagiarized from earlier documents, most notably RFC-1122 by
Bob Braden and the Host Requirements Working Group, and RFC-
1009 by Bob Braden and Jon Postel.  The work of these earlier
authors is gratefully acknowledged.

Jim Forster was a co-chair of the Router Requirements Working
Group during its early meetings, and was instrumental in
getting the group off to a good start.  Jon Postel, Bob
Braden, and Walt Prue also contributed to the success by
providing a wealth of good advice before the group's first
meeting.  Later on, Phill Gross, Vint Cerf, and Noel Chiappa
all provided valuable advice and support.

Mike St.  Johns coordinated the Working Group's interactions
with the security community, and Frank Kastenholz coordinated
the Working Group's interactions with the network management
area.  Allison Mankin and K.K.  Ramakrishnan provided
expertise on the issues of congestion control and resource
allocation.

Many more people than could possibly be listed or credited
here participated in the deliberations of the Router
Requirements Working Group, either through electronic mail or
by attending meetings.  However, the efforts of Ross Callon
and Vince Fuller in sorting out the difficult issues of route
choice and route leaking are especially acknowledged.

The editor thanks his employer, Cisco Systems, for allowing
him to spend the time necessary to produce the 1994 snapshot.















IETF                  Exp. 22 Sep. 1995             [Page 227]


Draft       Requirements for IP Version 4 Routers   March 1995


Editor's Address

The address of the current editor of this document is
   Fred Baker
   Cisco Systems
   519 Lado Drive
   Santa Barbara, California 93111
   USA

   Phone:+1 805-681-0115

   EMail: fred@cisco.com





































IETF                  Exp. 22 Sep. 1995             [Page 228]


Draft       Requirements for IP Version 4 Routers   March 1995


Table of Contents


 Status of this Memo ....................................    i
 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 ...................    8
 1.3 General Considerations .............................    9
 1.3.1 Continuing Internet Evolution ....................   10
 1.3.2 Robustness Principle .............................   10
 1.3.3 Error Logging ....................................   11
 1.3.4 Configuration ....................................   12
 1.4 Algorithms .........................................   14
 2. INTERNET ARCHITECTURE ...............................   15
 2.1 Introduction .......................................   15
 2.2 Elements of the Architecture .......................   16
 2.2.1 Protocol Layering ................................   16
 2.2.2 Networks .........................................   19
 2.2.3 Routers ..........................................   19
 2.2.4 Autonomous Systems ...............................   20
 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 ..................................   25
 2.2.7 Unnumbered Lines and Networks Prefixes ...........   26
 2.2.8 Notable Oddities .................................   27
 2.2.8.1 Embedded Routers ...............................   27
 2.2.8.2 Transparent Routers ............................   28
 2.3 Router Characteristics .............................   30
 2.4 Architectural Assumptions ..........................   33
 3. LINK LAYER ..........................................   36
 3.1 INTRODUCTION .......................................   36
 3.2 LINK/INTERNET LAYER INTERFACE ......................   36
 3.3 SPECIFIC ISSUES ....................................   38
 3.3.1 Trailer Encapsulation ............................   38
 3.3.2 Address Resolution Protocol - ARP ................   38
 3.3.3 Ethernet and 802.3 Coexistence ...................   38
 3.3.4 Maximum Transmission Unit - MTU ..................   39
 3.3.5 Point-to-Point Protocol - PPP ....................   39
 3.3.5.1 Introduction ...................................   40





IETF                  Exp. 22 Sep. 1995              [Page ii]


Draft       Requirements for IP Version 4 Routers   March 1995


 3.3.5.2 Link Control Protocol (LCP) Options ............   41
 3.3.5.3 IP Control Protocol (IPCP) Options .............   43
 3.3.6 Interface Testing ................................   43
 4. INTERNET LAYER - PROTOCOLS ..........................   45
 4.1 INTRODUCTION .......................................   45
 4.2 INTERNET PROTOCOL - IP .............................   45
 4.2.1 INTRODUCTION .....................................   45
 4.2.2 PROTOCOL WALK-THROUGH ............................   46
 4.2.2.1 Options: RFC 791 Section 3.2 ...................   46
 4.2.2.2 Addresses in Options: RFC 791 Section 3.1 ......   50
 4.2.2.3 Unused IP Header Bits: RFC 791 Section 3.1 .....   51
 4.2.2.4 Type of Service: RFC 791 Section 3.1 ...........   51
 4.2.2.5 Header Checksum: RFC 791 Section 3.1 ...........   51
 4.2.2.6 Unrecognized Header Options: RFC 791  Section 
     3.1 ................................................   52
 4.2.2.7 Fragmentation: RFC 791 Section 3.2 .............   52
 4.2.2.8 Reassembly: RFC 791 Section 3.2 ................   54
 4.2.2.9 Time to Live: RFC 791 Section 3.2 ..............   54
 4.2.2.10 Multi-subnet Broadcasts: RFC 922 ..............   55
 4.2.2.11 Addressing: RFC 791 Section 3.2 ...............   55
 4.2.3 SPECIFIC ISSUES ..................................   59
 4.2.3.1 IP Broadcast Addresses .........................   59
 4.2.3.2 IP Multicasting ................................   60
 4.2.3.3 Path MTU Discovery .............................   61
 4.2.3.4 Subnetting .....................................   62
 4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP ...........   63
 4.3.1 INTRODUCTION .....................................   63
 4.3.2 GENERAL ISSUES ...................................   64
 4.3.2.1 Unknown Message Types ..........................   64
 4.3.2.2 ICMP Message TTL ...............................   64
 4.3.2.3 Original Message Header ........................   64
 4.3.2.4 ICMP Message Source Address ....................   65
 4.3.2.5 TOS and Precedence .............................   65
 4.3.2.6 Source Route ...................................   66
 4.3.2.7 When Not to Send ICMP Errors ...................   66
 4.3.2.8 Rate Limiting ..................................   68
 4.3.3 SPECIFIC ISSUES ..................................   69
 4.3.3.1 Destination Unreachable ........................   69
 4.3.3.2 Redirect .......................................   70
 4.3.3.3 Source Quench ..................................   70
 4.3.3.4 Time Exceeded ..................................   71
 4.3.3.5 Parameter Problem ..............................   71
 4.3.3.6 Echo Request/Reply .............................   72
 4.3.3.7 Information Request/Reply ......................   73





IETF                  Exp. 22 Sep. 1995             [Page iii]


Draft       Requirements for IP Version 4 Routers   March 1995


 4.3.3.8 Timestamp and Timestamp Reply ..................   73
 4.3.3.9 Address Mask Request/Reply .....................   75
 4.3.3.10 Router Advertisement and Solicitations ........   76
 4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ..........   77
 5. INTERNET LAYER - FORWARDING .........................   78
 5.1 INTRODUCTION .......................................   78
 5.2 FORWARDING WALK-THROUGH ............................   78
 5.2.1 Forwarding Algorithm .............................   78
 5.2.1.1 General ........................................   79
 5.2.1.2 Unicast ........................................   80
 5.2.1.3 Multicast ......................................   81
 5.2.2 IP Header Validation .............................   83
 5.2.3 Local Delivery Decision ..........................   85
 5.2.4 Determining the Next Hop Address .................   88
 5.2.4.1 IP Destination Address .........................   89
 5.2.4.2 Local/Remote Decision ..........................   90
 5.2.4.3 Next Hop Address ...............................   92
 5.2.4.4 Administrative Preference ......................   96
 5.2.4.6 Load Splitting .................................   99
 5.2.5 Unused IP Header Bits: RFC-791 Section 3.1 .......   99
 5.2.6 Fragmentation and Reassembly:  RFC-791  Section 
     3.2 ................................................  100
 5.2.7 Internet Control Message Protocol - ICMP .........  100
 5.2.7.1 Destination Unreachable ........................  100
 5.2.7.2 Redirect .......................................  103
 5.2.7.3 Time Exceeded ..................................  105
 5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ........  105
 5.3 SPECIFIC ISSUES ....................................  106
 5.3.1 Time to Live (TTL) ...............................  106
 5.3.2 Type of Service (TOS) ............................  107
 5.3.3 IP Precedence ....................................  109
 5.3.3.1 Precedence-Ordered Queue Service ...............  111
 5.3.3.2 Lower Layer Precedence Mappings ................  111
 5.3.3.3 Precedence Handling For All Routers ............  112
 5.3.4 Forwarding of Link Layer Broadcasts ..............  115
 5.3.5 Forwarding of Internet Layer Broadcasts ..........  116
 5.3.5.1 Limited Broadcasts .............................  117
 5.3.5.2 Directed Broadcasts ............................  118
 5.3.5.3 All-subnets-directed Broadcasts ................  118
 5.3.5.4 Network-Prefix-Directed Broadcasts .............  119
 5.3.6 Congestion Control ...............................  119
 5.3.7 Martian Address Filtering ........................  121
 5.3.8 Source Address Validation ........................  122
 5.3.9 Packet Filtering and Access Lists ................  123





IETF                  Exp. 22 Sep. 1995              [Page iv]


Draft       Requirements for IP Version 4 Routers   March 1995


 5.3.10 Multicast Routing ...............................  124
 5.3.11 Controls on Forwarding ..........................  124
 5.3.12 State Changes ...................................  125
 5.3.12.1 When a Router Ceases Forwarding ...............  125
 5.3.12.2 When a Router Starts Forwarding ...............  126
 5.3.12.3 When an Interface Fails or is Disabled ........  127
 5.3.12.4 When an Interface is Enabled ..................  127
 5.3.13 IP Options ......................................  127
 5.3.13.1 Unrecognized Options ..........................  128
 5.3.13.2 Security Option ...............................  128
 5.3.13.3 Stream Identifier Option ......................  128
 5.3.13.4 Source Route Options ..........................  129
 5.3.13.5 Record Route Option ...........................  129
 5.3.13.6 Timestamp Option ..............................  130
 6. TRANSPORT LAYER .....................................  132
 6.1 USER DATAGRAM PROTOCOL - UDP .......................  132
 6.2 TRANSMISSION CONTROL PROTOCOL - TCP ................  132
 7. APPLICATION LAYER - ROUTING PROTOCOLS ...............  135
 7.1 INTRODUCTION .......................................  135
 7.1.1 Routing Security Considerations ..................  135
 7.1.2 Precedence .......................................  136
 7.1.3 Message Validation ...............................  136
 7.2 INTERIOR GATEWAY PROTOCOLS .........................  137
 7.2.1 INTRODUCTION .....................................  137
 7.2.2 OPEN SHORTEST PATH FIRST - OSPF ..................  138
 7.2.3 INTERMEDIATE SYSTEM TO  INTERMEDIATE  SYSTEM  -
     DUAL IS-IS .........................................  138
 7.3  EXTERIOR GATEWAY PROTOCOLS ........................  139
 7.3.1  INTRODUCTION ....................................  139
 7.3.2 BORDER GATEWAY PROTOCOL - BGP ....................  139
 7.3.2.1 Introduction ...................................  139
 7.3.2.2 Protocol Walk-through ..........................  140
 7.3.3 INTER-AS ROUTING WITHOUT AN  EXTERIOR  PROTOCOL
     ....................................................  141
 7.4 STATIC ROUTING .....................................  142
 7.5 FILTERING OF ROUTING INFORMATION ...................  144
 7.5.1 Route Validation .................................  144
 7.5.2 Basic Route Filtering ............................  145
 7.5.3 Advanced Route Filtering .........................  145
 7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE ........  146
 8. APPLICATION LAYER - NETWORK  MANAGEMENT  PROTOCOLS
     ....................................................  148
 8.1 The Simple Network Management Protocol - SNMP ......  148
 8.1.1 SNMP Protocol Elements ...........................  148





IETF                  Exp. 22 Sep. 1995               [Page v]


Draft       Requirements for IP Version 4 Routers   March 1995


 8.2 Community Table ....................................  149
 8.3 Standard MIBS ......................................  150
 8.4 Vendor Specific MIBS ...............................  152
 8.5 Saving Changes .....................................  153
 9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS .........  155
 9.1 BOOTP ..............................................  155
 9.1.1 Introduction .....................................  155
 9.1.2 BOOTP Relay Agents ...............................  155
 10. OPERATIONS AND MAINTENANCE .........................  157
 10.1 Introduction ......................................  157
 10.2 Router Initialization .............................  159
 10.2.1 Minimum Router Configuration ....................  159
 10.2.2 Address and Prefix Initialization ...............  160
 10.2.3 Network Booting using BOOTP and TFTP ............  161
 10.3 Operation and Maintenance .........................  162
 10.3.1 Introduction ....................................  162
 10.3.2 Out Of Band Access ..............................  163
 10.3.2 Router O&M Functions ............................  164
 10.3.2.1 Maintenance - Hardware Diagnosis ..............  164
 10.3.2.2 Control - Dumping and Rebooting ...............  164
 10.3.2.3 Control - Configuring the Router ..............  164
 10.3.2.4 Net Booting of System Software ................  165
 10.3.2.5 Detecting and responding to misconfiguration
     ....................................................  166
 10.3.2.6 Minimizing Disruption .........................  167
 10.3.2.7 Control - Troubleshooting Problems ............  168
 10.4 Security Considerations ...........................  169
 10.4.1 Auditing and Audit Trails .......................  169
 10.4.2 Configuration Control ...........................  171
 11. REFERENCES .........................................  173
 APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ......  186
 APPENDIX B. GLOSSARY ...................................  188
 APPENDIX C. FUTURE DIRECTIONS ..........................  195
 APPENDIX D. Multicast Routing Protocols ................  198
 D.1 Introduction .......................................  198
 D.2 Distance  Vector  Multicast  Routing  Protocol  -
     DVMRP ..............................................  198
 D.3 Multicast Extensions to OSPF - MOSPF ...............  199
 D.4 Protocol Independent Multicast - PIM ...............  199
 APPENDIX E Additional Next-Hop  Selection  Algorithms
     ....................................................  200
 E.1. Some Historical Perspective .......................  200
 E.2. Additional Pruning Rules ..........................  202
 E.3 Some Route Lookup Algorithms .......................  204





IETF                  Exp. 22 Sep. 1995              [Page vi]


Draft       Requirements for IP Version 4 Routers   March 1995


 E.3.1 The Revised Classic Algorithm ....................  204
 E.3.2 The Variant Router Requirements Algorithm ........  206
 E.3.3 The OSPF Algorithm ...............................  206
 E.3.4 The Integrated IS-IS Algorithm ...................  208
 Security Considerations ................................  210
 APPENDIX F: HISTORICAL ROUTING PROTOCOLS ...............  212
 F.1 EXTERIOR GATEWAY PROTOCOL - EGP ....................  212
 F.1.1 Introduction .....................................  212
 F.1.2 Protocol Walk-through ............................  213
 F.2 ROUTING INFORMATION PROTOCOL - RIP .................  216
 F.2.1 Introduction .....................................  216
 F.2.2 Protocol Walk-Through ............................  216
 F.2.3 Specific Issues ..................................  223
 F.3 GATEWAY TO GATEWAY PROTOCOL - GGP ..................  224
 Acknowledgments ........................................  225
 Editor's Address .......................................  228

































IETF                  Exp. 22 Sep. 1995             [Page vii]


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