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
draft-ietf-ftpext-feat-00.txt
Status of this Memo
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Abstract
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.
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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
section.
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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
responses.
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
[1].
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.
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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
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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
desired.
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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"
commands.
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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.
Acknowledgements
This protocol extension was developed in the FTPEXT Working Group of
the IETF, and the members of that group are all acknowledged as its
creators.
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
Australia
Email: kre@munnari.OZ.AU
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