are designed for restricted contexts might choose to use
        different specifications.

   However, the specifications of this document must be followed to meet
   the general goal of arbitrary host interoperation across the
   diversity and complexity of the Internet system.  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 document will be updated as required to provide
   additional clarifications or to include additional information in
   those areas in which specifications are still evolving.

   This introductory section begins with general advice to host software
   vendors, and then gives some guidance on reading the rest of the
   document.  Section 2 contains general requirements that may be
   applicable to all application and support protocols.  Sections 3, 4,
   and 5 contain the requirements on protocols for the three major
   applications: Telnet, file transfer, and electronic mail,
   respectively. Section 6 covers the support applications: the domain
   name system, system initialization, and management.  Finally, all
   references will be found in Section 7.

   1.1  The Internet Architecture

      For a brief introduction to the Internet architecture from a host
      viewpoint, see Section 1.1 of [INTRO:1].  That section also
      contains recommended references for general background on the
      Internet architecture.

   1.2  General Considerations

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

      1.2.1  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 document.  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 operations of the networks.




 



RFC1123                       INTRODUCTION                  October 1989


         Development, evolution, and revision are characteristic of
         computer network protocols today, and this situation will
         persist for some years.  A vendor who develops computer
         communication 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.

      1.2.2  Robustness Principle

         At every layer of the protocols, there is a general rule whose
         application can lead to enormous benefits in robustness and
         interoperability:

                "Be liberal in what you accept, and
                 conservative in what you send"

         Software should be written to deal with every conceivable
         error, no matter how unlikely; sooner or later a packet will
         come in with that particular combination of errors and
         attributes, and unless the software is prepared, chaos can
         ensue.  In general, it is best to assume that the network is
         filled with malevolent entities that will send in packets
         designed to have the worst possible effect.  This assumption
         will lead to suitable protective design, although the most
         serious problems in the Internet have been caused by
         unenvisaged 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
         Internet host 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.  Thus, if a protocol specification defines four
         possible error codes, the software must not break when a fifth