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      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.


Almquist & Kastenholz                                          [Page 10]

RFC 1716          Towards Requirements for IP Routers      November 1994


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

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

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

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

1.4  Algorithms

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

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


Almquist & Kastenholz                                          [Page 11]

RFC 1716          Towards Requirements for IP Routers      November 1994


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














































Almquist & Kastenholz                                          [Page 12]

RFC 1716          Towards Requirements for IP Routers      November 1994


2.  INTERNET ARCHITECTURE

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

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

2.1  Introduction

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

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

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









Almquist & Kastenholz                                          [Page 13]

RFC 1716          Towards Requirements for IP Routers      November 1994


2.2  Elements of the Architecture


2.2.1  Protocol Layering

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

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

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

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

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

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

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

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


Almquist & Kastenholz                                          [Page 14]

RFC 1716          Towards Requirements for IP Routers      November 1994


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

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

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

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

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

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

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

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

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


Almquist & Kastenholz                                          [Page 15]