RFC 2026               Internet Standards Process           October 1996


   An AS may describe particular methods of using a TS in a restricted
   "domain of applicability", such as Internet routers, terminal
   servers, Internet systems that interface to Ethernets, or datagram-
   based database servers.

   The broadest type of AS is a comprehensive conformance specification,
   commonly called a "requirements document", for a particular class of
   Internet systems, such as Internet routers or Internet hosts.

   An AS may not have a higher maturity level in the standards track
   than any standards-track TS on which the AS relies (see section 4.1).
   For example, a TS at Draft Standard level may be referenced by an AS
   at the Proposed Standard or Draft Standard level, but not by an AS at
   the Standard level.

3.3  Requirement Levels

   An AS shall apply one of the following "requirement levels" to each
   of the TSs to which it refers:

   (a)  Required:  Implementation of the referenced TS, as specified by
      the AS, is required to achieve minimal conformance.  For example,
      IP and ICMP must be implemented by all Internet systems using the
      TCP/IP Protocol Suite.

   (b)  Recommended:  Implementation of the referenced TS is not
      required for minimal conformance, but experience and/or generally
      accepted technical wisdom suggest its desirability in the domain
      of applicability of the AS.  Vendors are strongly encouraged to
      include the functions, features, and protocols of Recommended TSs
      in their products, and should omit them only if the omission is
      justified by some special circumstance. For example, the TELNET
      protocol should be implemented by all systems that would benefit
      from remote access.

   (c)  Elective:  Implementation of the referenced TS is optional
      within the domain of applicability of the AS;  that is, the AS
      creates no explicit necessity to apply the TS.  However, a
      particular vendor may decide to implement it, or a particular user
      may decide that it is a necessity in a specific environment.  For
      example, the DECNET MIB could be seen as valuable in an
      environment where the DECNET protocol is used.










 
RFC 2026               Internet Standards Process           October 1996


      As noted in section 4.1, there are TSs that are not in the
      standards track or that have been retired from the standards
      track, and are therefore not required, recommended, or elective.
      Two additional "requirement level" designations are available for
      these TSs:

   (d)  Limited Use:  The TS is considered to be appropriate for use
      only in limited or unique circumstances.  For example, the usage
      of a protocol with the "Experimental" designation should generally
      be limited to those actively involved with the experiment.

   (e)  Not Recommended:  A TS that is considered to be inappropriate
      for general use is labeled "Not Recommended". This may be because
      of its limited functionality, specialized nature, or historic
      status.

   Although TSs and ASs are conceptually separate, in practice a
   standards-track document may combine an AS and one or more related
   TSs.  For example, Technical Specifications that are developed
   specifically and exclusively for some particular domain of
   applicability, e.g., for mail server hosts, often contain within a
   single specification all of the relevant AS and TS information. In
   such cases, no useful purpose would be served by deliberately
   distributing the information among several documents just to preserve
   the formal AS/TS distinction.  However, a TS that is likely to apply
   to more than one domain of applicability should be developed in a
   modular fashion, to facilitate its incorporation by multiple ASs.

   The "Official Protocol Standards" RFC (STD1) lists a general
   requirement level for each TS, using the nomenclature defined in this
   section. This RFC is updated periodically.  In many cases, more
   detailed descriptions of the requirement levels of particular
   protocols and of individual features of the protocols will be found
   in appropriate ASs.