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Versions: 00 01 02 rfc2389                                              
FTPEXT Working Group                                          P. Hethmon
Internet Draft                                          Hethmon Brothers
Expiration Date: December 1997
                                                                  R. Elz
                                                 University of Melbourne

                                                               June 1997

      Feature negotiation mechanism for the File Transfer Protocol


Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).


   The File Transfer Protocol is, from time to time, extended with new
   commands, or facilities.  Implementations of the FTP protocol cannot
   be assumed to all immediately implement all newly defined mechanisms.
   This document provides a mechanism by which clients of the FTP
   protocol can discover which new features are supported by a
   particular FTP server.

   This draft extracts the FEAT and OPTS commands from the "mlst" draft,
   into this draft of their own.  The descriptions of those commands
   have been updated in an editorial way, no changes of substance have
   been made.  This paragraph will be deleted from the final version of
   this document.

Hethmon & Elz                                                   [Page 1]

Internet Draft       draft-ietf-ftpext-feat-00.txt             June 1997

Table of Contents

          Status of this Memo  .....................................   1
          Abstract  ................................................   1
    1     Introduction  ............................................   2
    2     Document Conventions  ....................................   2
    2.1   Basic Tokens  ............................................   3
    2.2   Server Replies  ..........................................   3
    3     Knowledge of Extra Capabilities - the FEAT Command  ......   3
    3.1   Feature (FEAT) Command Syntax  ...........................   4
    3.2   FEAT Command Responses  ..................................   4
    3.3   Rationale for FEAT  ......................................   5
    4     The OPTS Command  ........................................   6
    5     Security  ................................................   7
    6     References  ..............................................   7
          Acknowledgements  ........................................   7
          Editors' Addresses  ......................................   7

1. Introduction

   This document amends the File Transfer Protocol (FTP) [1].  Two new
   commands are added: "FEAT" and "OPTS".

   These commands allow a client to discover which optional commands a
   server supports, and how they are supported, and to select among
   various options that any FTP command may support.

2. Document Conventions

   This document makes use of the document conventions defined in BCP14
   [2].  That provides the interpretation of some capitalized words like
   MUST, SHOULD, etc.

   Terms defined in [1] will be used here as defined there.  These
   include ASCII, reply, server-FTP process, user-FTP process, server-
   PI, user-PI, and user.

   Syntax required is defined using the Augmented BNF defined in [3].
   Some general ABNF definitions are required throughout the document,
   those will be defined here.  At first reading, it may be wise to
   simply recall that these definitions exist here, and skip to the next

Hethmon & Elz                                                   [Page 2]

Internet Draft       draft-ietf-ftpext-feat-00.txt             June 1997

2.1. Basic Tokens

   This document imports the definitions given in Appendix A of [3].
   There definitions will be found for basic ABNF elements like ALPHA,
   DIGIT, SP, etc.  To that, the following terms are added for use in
   this document.

        PCHAR          = %x21-7e          ; a printing character, ! to ~
        TCHAR          = PCHAR / SP / %x08   ; printing plus white space

   The PCHAR and TCHAR types give basic character types from varying
   sub-sets of the ASCII character set for use in various commands and

        error-response = error-code SP *TCHAR CRLF
        error-code     = ("4" / "5") 2DIGIT

   Note that in ABNF, strings literals are case insensitive.  That
   convention is preserved in this document.  However note that ALPHA,
   in particular, is case sensitive, as are PCHAR and TCHAR.

2.2. Server Replies

   Section 4.2 of [1] defines the format and meaning of replies by the
   server-PI to FTP commands from the user-PI.  Those reply conventions
   are used here without change.  Implementors should note that the ABNF
   syntax (which was not used in [1]) in this document, and other FTP
   related documents, sometimes shows replies using the one line format.
   Unless otherwise explicitly stated, that is not intended to imply
   that multi-line responses are not permitted.  Implementors should
   assume that, unless stated to the contrary, any reply to any FTP
   command (including QUIT) may be of the multiline format described in

   Throughout this document, replies will be identified by the three
   digit code that is their first element.  Thus the term "500 Reply"
   means a reply from the server-PI using the three digit code "500".

3. Knowledge of Extra Capabilities - the FEAT Command

   It is not to be expected that all servers will necessarily support
   all of the new commands defined in all future amendments to the FTP
   protocol.  In order to permit clients to determine which new commands
   are supported by a particular server, without trying each possible
   command, one new command is added to the FTP command repertoire.
   This command requests the server to list all extension commands that
   it supports.

Hethmon & Elz                                                   [Page 3]

Internet Draft       draft-ietf-ftpext-feat-00.txt             June 1997

3.1. Feature (FEAT) Command Syntax

        feat            = "Feat" CRLF

   The FEAT command consists solely of the word "FEAT".  It has no
   parameters or arguments.

3.2. FEAT Command Responses

   Where a server-FTP process does not support the FEAT command, it will
   respond to the FEAT command with a 500 reply.  This is simply the
   normal "unrecognized command" reply that any unknown command would
   elicit.  Errors in the command syntax, such as giving parameters,
   will result in a 501 reply.

   Server-FTP processes that recognize the FEAT command, but implement
   no extended features, and therefore have nothing to report, SHOULD
   respond with the "no-features" 211 reply.  However, as this case is
   practically indistinguishable from a server-FTP that does not
   recognize the FEAT command, a 500 reply MAY also be used.  It is also
   possible to indicate that no features are implemented by returning a
   "feature-listing" 211 reply, with no features actually included,
   however server-FTP processes SHOULD NOT use this method.

   Replies to the FEAT command MUST comply with the following syntax.
   Text on the first line of the reply is free form, and not
   interpreted, but the syntax of other reply lines is precisely
   defined, and if present, MUST be exactly as specified.

        feat-response   = error-response / no-features / feature-listing
        no-features     = "211" SP *TCHAR CRLF
        feature-listing = "211-" *TCHAR CRLF
                          *( SP feature CRLF )
                          "211 End" CRLF
        feature         = feature-label [ SP parms ]
        feature-label   = 1*PCHAR
        parms           = 1*TCHAR

   Note that each feature line in the feature-listing begins with a
   space.  That space is not optional, nor does it indicate general
   white space.  This space guarantees that the feature line can never
   be misinterpreted as the end of the feature-listing, but is required
   even where there is no possibility of ambiguity.

   Each extension supported must be listed on a separate line to
   facilitate the possible inclusion of parameters supported by each
   extension command.  The feature-label to be used in the response to
   the FEAT command will be specified as each new feature is added to

Hethmon & Elz                                                   [Page 4]

Internet Draft       draft-ietf-ftpext-feat-00.txt             June 1997

   the FTP command set.  Often it will be the name of a new command
   added, however this is not required, in fact it is not required that
   a new feature actually add a new command.  Any parameters included
   are to be specified with the definition of the command concerned.
   That specification shall also specify how any parameters present are
   to be interpreted.

   The FEAT command itself is not included in the list of features
   supported, support for the FEAT command is indicated by return of a
   reply other than a 500 reply.

   A typical example reply to the FEAT command might be a multiline
   reply of the form:

        C> FEAT
        S> 211- Extensions supported:
        S>  MLST size*;create;modify*;perm;media-type
        S>  SIZE
        S>  MDTM
        S> 211 End

   The particular extensions shown here are simply examples of what may
   be defined in other places, no particular meaning should be
   attributed to them.

   FTP implementations which support extension commands beyond those
   commands described in RFC959 [1] MUST support FEAT.

3.3. Rationale for FEAT

   While not absolutely necessary, a standard mechanism for the server-
   PI to inform the user-PI of any features and extensions supported
   will help reduce unnecessary traffic between the user-PI and server-
   PI as more extensions may be introduced in the future.  If no
   mechanism existed for this, a user-FTP process would have to try each
   extension in turn resulting in a series of exchanges between the
   user-PI and server-PI.  Apart from being possibly wasteful, this
   procedure may not always be possible, as issuing of a command just to
   determine if it is supported or not may have some effect that is not

Hethmon & Elz                                                   [Page 5]

Internet Draft       draft-ietf-ftpext-feat-00.txt             June 1997

4. The OPTS Command

   The OPTS (options) command allows a user-PI to specify the desired
   behavior of a server-FTP process when another FTP command (the target
   command) is later issued.  The exact behavior, and syntax, will vary
   with the target command indicated, and will be specified with the
   definition of that command.  Where no OPTS behavior is defined for a
   particular command there are no options available for that command.

   Request Syntax:
        opts             = opts-cmd SP command-name
                               [ SP command-options ] CRLF
        opts-cmd         = "opts"
        command-name     = <any FTP command which allows option setting>
        command-options  = <format specified by individual FTP command>

   Response Syntax:
        opts-response    = opts-good / opts-bad
        opts-good        = "200" SP response-message CRLF
        opts-bad         = "451" SP response-message CRLF /
                           "501" SP response-message CRLF
        response-message = *TCHAR

   An "opts-good" response (200 reply) MUST be sent when the command-
   name specified in the OPTS command is recognized, and the command-
   options, if any, are recognized, and appropriate, An "opts-bad"
   response is sent in other cases.  A 501 reply is appropriate for any
   permanent error.  That is, for any case where simply repeating the
   command at some later time, without other changes of state, will also
   be an error.  A 451 reply should be sent where some temporary
   condition at the server, not related to the state of communications
   between user and server, prevents the command being accepted when
   issued, but where is repeated at some later time, a changed
   environment for the server-FTP process may permit the command to
   succeed.  If the OPTS command itself is not recognized, a 500 reply
   will, of course, result.

   The OPTS command MUST be implemented whenever the FEAT command is
   implemented.  Because of that, there is no indication in the list of
   features returned by FEAT to indicate that the OPTS command itself is
   supported.  Neither the FEAT command, nor the OPTS command, have any
   optional functionality, thus there are no "OPTS FEAT" or "OPTS OPTS"

Hethmon & Elz                                                   [Page 6]

Internet Draft       draft-ietf-ftpext-feat-00.txt             June 1997

5. Security

   This memo does not yet discuss security.  It is possible that no new
   security concerns are raised in this memo above what already exists
   within the FTP protocol.  However, the working group needs to
   consider this carefully.

6. References

   [1]  J. Postel, J. Reynolds, "File Transfer Protocol (FTP)",
        STD 9 (RFC 959), ISI, October 1985

   [2]  S. Bradner, "Key words for use in RFCs to Indicate
        Requirement Levels", BCP 14 (RFC 2119), Harvard University,
        March 1997

   [3]  D. Crocker, "Augmented BNF for Syntax Specifications: ABNF",
        Work In Progress <draft-ietf-drums-abnf-02.txt>, Internet
        Mail Consortium, March 1997.


   This protocol extension was developed in the FTPEXT Working Group of
   the IETF, and the members of that group are all acknowledged as its

Editors' Addresses

   Paul Hethmon
   Hethmon Brothers
   2305 Chukar Road
   Knoxville, TN 37923 USA

   Phone: 423-690-8990
   Email: phethmon@hethmon.com

   Robert Elz
   University of Melbourne
   Department of Computer Science
   Parkville, Vic   3052

   Email: kre@munnari.OZ.AU

Hethmon & Elz                                                   [Page 7]