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




 



RFC1123                       INTRODUCTION                  October 1989


         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 user
         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 Section 6.3); and (2)
         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".

         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




 



RFC1123                       INTRODUCTION                  October 1989


         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-