rfc1122.txt

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         gateway would, while still performing the application layer
         functions of a host.

         Such dual-purpose systems must follow the Gateway Requirements
         RFC [INTRO:2]  with respect to their gateway functions, and
         must follow the present document with respect to their host
         functions.  In all overlapping cases, the two specifications
         should be in agreement.

         There are varying opinions in the Internet community about
         embedded gateway functionality.  The main arguments are as
         follows:

         o    Pro: in a local network environment where networking is
              informal, or in isolated internets, it may be convenient
              and economical to use existing host systems as gateways.

              There is also an architectural argument for embedded
              gateway functionality: multihoming is much more common
              than originally foreseen, and multihoming forces a host to
              make routing decisions as if it were a gateway.  If the
              multihomed  host contains an embedded gateway, it will
              have full routing knowledge and as a result will be able
              to make more optimal routing decisions.

         o    Con: Gateway algorithms and protocols are still changing,
              and they will continue to change as the Internet system
              grows larger.  Attempting to include a general gateway
              function within the host IP layer will force host system
              maintainers to track these (more frequent) changes.  Also,
              a larger pool of gateway implementations will make
              coordinating the changes more difficult.  Finally, the
              complexity of a gateway IP layer is somewhat greater than
              that of a host, making the implementation and operation
              tasks more complex.

              In addition, the style of operation of some hosts is not
              appropriate for providing stable and robust gateway
              service.

         There is considerable merit in both of these viewpoints.  One
         conclusion can be drawn: an host administrator must have
         conscious control over whether or not a given host acts as a
         gateway.  See Section 3.1 for the detailed requirements.







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RFC1122                       INTRODUCTION                  October 1989


   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.

         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 [IP:1]:

                "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;



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RFC1122                       INTRODUCTION                  October 1989


         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
         code shows up.  An undefined code might be logged (see below),
         but it must not cause a failure.

         The second part of the principle is almost as important:
         software on other hosts 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"; host software should be prepared, not
         just to survive other misbehaving hosts, but also to cooperate
         to limit the amount of disruption such hosts can cause to the
         shared communication facility.

      1.2.3  Error Logging

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

         Problem diagnosis will be aided if host implementations include
         a carefully designed facility for logging erroneous or
         "strange" protocol 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
         host.

         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:
         (1) always count abnormalities and make such counts accessible
         through the management protocol (see [INTRO:1]); and (2) allow



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RFC1122                       INTRODUCTION                  October 1989


         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".

         Note that different managements may have differing policies
         about the amount of error logging that they want normally
         enabled in a host.  Some will say, "if it doesn't hurt me, I
         don't want to know about it", while others will want to take a
         more watchful and aggressive attitude about detecting and
         removing protocol abnormalities.

      1.2.4  Configuration

         It would be ideal if a host implementation of the Internet
         protocol suite could be entirely self-configuring.  This would
         allow the whole suite to be implemented in ROM or cast into
         silicon, it would simplify diskless workstations, and it would
         be an immense boon to harried LAN administrators as well as
         system vendors.  We have not reached this ideal; in fact, we
         are not even close.

         At many points in this document, you will find a requirement
         that a parameter be a configurable option.  There are several
         different reasons behind such requirements.  In a few cases,
         there is current 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 size of the host and 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 unfortunately persist in many
         parts of the Internet.  To make correct systems coexist with
         these faulty systems, administrators often have to "mis-
         configure" the correct systems.  This problem will correct
         itself gradually as the faulty systems are retired, but it
         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.  We recommend that
         implementors set up a default for each parameter, so a
         configuration file is only necessary to override those defaults



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RFC1122                       INTRODUCTION                  October 1989


         that are inappropriate in a particular installation.  Thus, the
         configurability requirement is an assurance that it will be
         POSSIBLE to override the default when necessary, even in a
         binary-only or ROM-based product.

         This document requires a particular value for such defaults in
         some cases.  The choice of default is a sensitive issue when
         the configuration item controls the accommodation to 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
         "mis-configurations" to accommodate faulty implementations.
         Although marketing considerations have led some vendors to
         choose mis-configuration 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.


   1.3  Reading this Document

      1.3.1  Organization

         Protocol layering, which is generally used as an organizing
         principle in implementing network software, has also been used
         to organize this document.  In describing the rules, we assume
         that an implementation does strictly mirror the layering of the
         protocols.  Thus, the following three major sections specify
         the requirements for the link layer, the internet layer, and
         the transport layer, respectively.  A companion RFC [INTRO:1]
         covers application level software.  This layerist organization
         was chosen for simplicity and clarity.

         However, strict layering is an imperfect model, both for the
         protocol suite and for recommended implementation approaches.
         Protocols in different layers interact in complex and sometimes
         subtle ways, and particular functions often involve multiple
         layers.  There are many design choices in an implementation,
         many of which involve creative "breaking" of strict layering.
         Every implementor is urged to read references [INTRO:7] and
         [INTRO:8].

         This document describes the conceptual service interface
         between layers using a functional ("procedure call") notation,
         like that used in the TCP specification [TCP:1].  A host
         implementation must support the logical information flow



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RFC1122                       INTRODUCTION                  October 1989