FTPEXT Working Group R. Elz
Internet Draft University of Melbourne
Expiration Date: December 1997
P. Hethmon
Hethmon Brothers
June 1997
Extended Directory Listing and Restart Mechanism for FTP
draft-ietf-ftpext-mlst-01.txt
Status of this Memo
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Abstract
In order to overcome the problems caused by the undefined format of
the current FTP LIST command output, a new command is needed to
transfer standardized listing information from Server-FTP to Client-
FTP. Commands to enable this are defined in this document.
This proposal also extends the FTP protocol to allow character sets
other than US-ASCII[1] by allowing the transmission of 8-bit
characters and the recommended use of UTF-8[2] encoding.
Much implemented, but long undocumented, mechanisms to permit
restarts of interrupted data transfers in STREAM mode, are also
included here.
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This version contains changes as agreed at the 38th IETF, Memphis,
April 1997. In particular: the SIZE MDTM and modified REST draft has
been folded into this doc, (and the FEAT and OPTS commands have been
moved out to a separate draft). Almost everything that was case
dependent no longer is, MLST/MLSD no longer send replies on the
control connection but the ancient STAT command has been revitalised.
Files of type "link" are no longer reported, and the 'x' permission
indicator has gone to meet its maker. The draft has also be
editorially updated in some significant ways. 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 ............................................ 4
2 Document Conventions .................................... 4
2.1 Basic Tokens ............................................ 4
2.2 Pathnames ............................................... 5
2.3 Times ................................................... 6
2.4 Server Replies .......................................... 7
3 File Modification Time (MDTM) ........................... 7
3.1 Syntax .................................................. 8
3.2 Error responses ......................................... 8
3.3 FEAT response for MTDM .................................. 8
4 File SIZE ............................................... 9
4.1 Syntax .................................................. 9
4.2 Error responses ......................................... 9
4.3 FEAT response for SIZE .................................. 10
5 Restart of Interrupted Transfer (REST) .................. 10
5.1 Restarting in STREAM Mode ............................... 10
5.2 ERROR RECOVER AND RESTART ............................... 11
5.3 Syntax .................................................. 11
5.4 FEAT response for REST .................................. 12
6 Machine Parseable Listings (MLST and MLSD) .............. 13
6.1 Format of MLST Request .................................. 13
6.2 Format of MLST Response ................................. 14
6.3 Filename encoding ....................................... 15
6.4 Format of Facts ......................................... 16
6.5 Standard Facts .......................................... 16
6.6 FEAT response for MLST .................................. 23
6.7 OPTS parameters for MLST ................................ 23
7 Interpretation of STAT command output ................... 24
7.1 FEAT response for STAT .................................. 24
8 Impact On Other FTP Commands ............................ 24
8.1 Impact on Pathnames and Filenames ....................... 25
9 Character sets and Internationalisation ................. 25
10 Security ................................................ 25
11 References .............................................. 25
Acknowledgements ........................................ 26
Editors' Addresses ...................................... 27
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1. Introduction
This document amends the File Transfer Protocol (FTP) [6]. Four new
commands are added: "SIZE", "MDTM", "MLST", and "MLSD". Two existing
commands are modified, those are "REST" and "STAT". Of those, the
"SIZE" and "MDTM" commands, and the modifications to "REST" have been
in wide use for many years. The others are new.
These commands allow a client to restart an interrupted transfer in
transfer modes not previously supported in any documented way, and to
obtain a directory listing in a machine friendly, predictable,
format.
2. Document Conventions
This document makes use of the document conventions defined in
RFC2119 [10]. That provides the interpretation of capitalized
imperative words like MUST, SHOULD, etc.
This document also uses notation defined in STD 9 [6]. In
particular, the terms "reply", "user", "NVFS", "file", "pathname",
"FTP commands", "DTP", "user-FTP process", "user-PI", "user-DTP",
"server-FTP process", "server-PI", "server-DTP", "mode", "type",
"NVT", "control connection", "data connection", and "ASCII", are all
used here as defined there.
Syntax required is defined using the Augmented BNF defined in [9].
Some general ABNF definitions are required throughout the document,
those will be defined later in this section. At first reading, it
may be wise to simply recall that these definitions exist here, and
skip to the next section.
2.1. Basic Tokens
This document imports the definitions given in Appendix A of [9].
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
ltext = ALPHA / DIGIT / "," / "." / ":" / "!" /
"@" / "#" / "$" / "%" / "^" /
"&" / "(" / ")" / "-" / "_" /
"+" / "?" / "/" / "\" / "'" /
%x22 ; <"> -- double quote character
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The PCHAR, TCHAR, and ltext types give basic character types from
varying sub-sets of the ASCII character set for use in various
commands and responses.
token = 1*ltext
A "token" is a string whose precise meaning depends upon the context
in which it is used. In some cases it will be a value from a set of
possible values maintained elsewhere. In others it might be a string
invented by one party to an FTP conversation from whatever sources it
finds relevant.
error-response = error-code SP *TCHAR CRLF
error-code = ("4" / "5") 2DIGIT
Note that in ABNF, string literals are case insensitive. That
convention is preserved in this document. However note that ALPHA,
in particular, is case sensitive. That implies that a "token" is a
case sensitive value. That implication is correct.
2.2. Pathnames
Various FTP commands take pathnames as arguments, or return pathnames
in responses. When the MLST command is supported, as indicated in
the response to the FEAT command [11], pathnames are to be
transferred in one of the following two formats.
utf-8-name = <a UTF-8 encoded Unicode string>
raw = <any string not being a valid UTF-8 encoding>
Which format is used is at the option of the user-PI or server-PI
sending the pathname. UTF-8 encodings contain enough internal
structure that it is always, in practice, possible to determine
whether a UTF-8 or raw encoding has been used, in the cases where it
matters. Note that ASCII is a subset of UTF-8.
Unless otherwise specified, the pathname is terminated by the CRLF
that terminates the FTP command, or by the CRLF that ends a reply.
Any trailing spaces preceding that CRLF form part of the name.
Exactly one space will precede the pathname from the preceding syntax
element, any additional spaces form part of the pathname. See [4]
for a fuller explanation of the character encoding issues. All
implementations supporting MLST MUST support [4].
Implementations should also beware that the control connection uses
Telnet NVT conventions [12], and that the Telnet IAC character, if
part of a pathname, MUST be correctly escaped as defined by the
Telnet protocol.
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2.3. Times
The syntax of a time value is:
time-val = 12DIGIT [ "." 1*DIGIT ]
The leading, mandatory, twelve digits are to be interpreted as, in
order from the leftmost, four digits giving the year, with a range of
1000-9999, two digits giving the month of the year, with a range of
01-12, two digits giving the day of the month, with a range of 01-31,
two digits giving the hour of the day, with a range of 00-23, two
digits giving minutes past the hour, with a range of 00-59, and
finally, two digits giving seconds past the minute, with a range of
00-60 (with 60 being used only at a leap second). Years in the tenth
century, and earlier, cannot be expressed. This is not considered a
serious defect of the protocol.
[ Ed-Note: Should we permit 12*DIGIT (or maybe
12*13DIGIT) so times in the 101st century and beyond can
be represented? ]
The optional digits, which must be preceded by a period, give decimal
fractions of a second. These may be given to whatever precision is
appropriate to the circumstance, however implementations MUST NOT add
precision to time-vals where that precision does not exist in the
underlying value being transmitted.
Symbolically, a time-val may be viewed as
YYYYMMDDHHMMSS.sss
The "." and subsequent digits are optional.
Time values are always represented in UTC (GMT), and in the Gregorian
calendar regardless of what calendar may have been in use at the date
and time indicated at the location of the server-PI.
The technical differences between GMT, UTC, UT1, UT2, etc, are not
considered here. A server-FTP process should always use the same
time reference, so the times it returns will be consistent. Clients
are not expected to be time synchronised with the server, so the
possible difference in times that might be reported by the different
time standards is not considered important.
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2.4. Server Replies
Section 4.2 of [6] 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 [6]) 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
[6].
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. File Modification Time (MDTM)
The FTP command, MODIFICATION TIME (MDTM), can be used to determine
when a file in the server NVFS was last modified. This command has
existed in many FTP servers for many years, as an adjunct to the REST
command for STEAM mode, thus is widely available. However, where
supported the "mtime" fact which can be provided in the result from
the new MLST command is recommended as a superior alternative.
When attempting to restart a RETRieve, if the User FTP makes use of
the MDTM command, it can check and see if the modification time of
the source file is more recent than the modification time of the
partially transferred file. If it is, then most likely the source
file has changed and it would be unsafe to restart in the middle of
the file transfer.
When attempting to restart a STORe, the User FTP can use the MDTM
command to discover the modification time of the partially
transferred file. If it is older than the modification time of the
file that is about to be STORed, then most likely the source file has
changed and it would be unsafe to restart in the middle of the file
transfer.
Using MLST can provide this information, and much more, thus giving
an even better indication that a file has changed, and that
restarting a transfer would not give valid results.
Note that this is applicable to any RESTart attempt, regardless of
the mode of the file transfer.
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3.1. Syntax
The syntax for the MDTM command is:
mdtm = "MdTm" SP ( utf-8-name / raw ) CRLF
The server-PI will respond to the MDTM command with a 213 reply
giving the last modification time of the file whose pathname was
supplied, or an error response if the file does not exist, the
modification time is unavailable, or some other error has occurred.
mdtm-response = "213" SP time-val CRLF /
error-response
3.2. Error responses
Where the command is correctly parsed, but the modification time is
not available, either because the pathname identifies no existing
entity, or because the information is not available for the entity
named, then a 550 reply should be sent. Where the command cannot be
correctly parsed, a 500 or 501 reply should be sent, as specified in
[6].
3.3. FEAT response for MTDM
When replying to the FEAT command [11], a FTP server process that
supports the MTDM command MUST include a line containing the single
word "MDTM" (in upper case, and without the quotes). That is, the
response MUST be
C> FEAT
S> 211- <any descriptive text>
S> ...
S> MDTM
S> ...
S> 211 End
The ellipses indicate placeholders where other features may be
included, and are not required. The one space indentation of the
feature lines is mandatory.
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4. File SIZE
The FTP command, SIZE OF FILE (SIZE), is used to obtain the transfer
size of a file from the server-FTP process. That is, the exact
number of bytes (or octets) which would be transmitted over the data
connection should that file be transmitted. This value will change
depending on the current STRUcture, MODE and TYPE of the data
connection, or a data connection which would be created were one
created now. Thus, the result of the SIZE command is dependent on
the currently established STRU, MODE and TYPE parameters.
The SIZE command returns how many bytes would be transferred if the
file were to be transferred using the current transfer structure,
mode and type. This command is normally used in conjunction with the
RESTART (REST) command. The server-PI might need to read the
partially transferred file, do any appropriate conversion, and count
the number of bytes that would be generated when sending the file in
order to correctly respond to this command. Estimates of the file
transfer size MUST NOT be returned, only precise information is
acceptable.
4.1. Syntax
The syntax of the SIZE command is:
size = "Size" SP ( utf-8-name / raw ) CRLF
The server-PI will respond to the SIZE command with a 213 reply
giving the transfer size of the file whose pathname was supplied, or
an error response if the file does not exist, the size is
unavailable, or some other error has occurred. The value returned is
in a format suitable for use with the RESTART (REST) command for mode
STREAM, provided the transfer mode and type are not altered.
size-response = "213" SP 1*DIGIT CRLF /
error-response
4.2. Error responses
Where the command is correctly parsed, but the size is not available,
either because the pathname identifies no existing entity, or because
the entity named cannot be transferred in the current MODE and TYPE
(or at all), then a 550 reply should be sent. Where the command
cannot be correctly parsed, a 500 or 501 reply should be sent, as
specified in [6].
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4.3. FEAT response for SIZE
When replying to the FEAT command [11], a FTP server process that
supports the SIZE command MUST include a line containing the single
word "SIZE" (in upper case, and without the quotes). That is, the
response MUST be
C> FEAT
S> 211- <any descriptive text>
S> ...
S> SIZE
S> ...
S> 211 End
The ellipses indicate placeholders where other features may be
included, and are not required. The one space indentation of the
feature lines is mandatory.
5. Restart of Interrupted Transfer (REST)
To avoid having to resend the entire file if the file is only
partially transferred, both sides need some way to be able to agree
on where in the data stream to restart the data transfer.
The FTP specification [6] includes three modes of data transfer,
Stream, Block and Compressed. In Block and Compressed modes, the
data stream that is transferred over the data connection is
formatted, allowing the embedding of restart markers into the stream.
The sending DTP can include a restart marker with whatever
information it needs to be able to restart a file transfer at that
point. The receiving DTP can keep a list of these restart markers,
and correlate them with how the file is being saved. To restart the
file transfer, the receiver just sends back that last restart marker,
and both sides know how to resume the data transfer. Note that there
are some flaws in the description of the restart mechanism in RFC 959
[6], see section 4.1.3.4 of RFC 1123 [7] for the corrections.
5.1. Restarting in STREAM Mode
In Stream mode, the data connection contains just a stream of
unformatted bytes of data. Explicit restart markers thus cannot be
inserted into the data stream, they would be indistinguishable from
data. For this reason, the FTP specification [6] did not provide the
ability to do restarts in stream mode. However, there is not really
a need to have explicit restart markers in this case, as restart
markers can be implied by the byte offset into the data stream.
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If the data representation TYPE is IMAGE, and the STRUcture is File,
for many systems the file will be stored exactly in the same format
as it is sent across the data connection. It is then very easy for
the receiver to determine how much data was previously received, and
notify the sender the byte offset where the transfer should be
restarted. In other representation types and structures more effort
will be required, but it remains always possible to determine the
byte offset with finite, but perhaps non-negligible, effort. In the
worst case an FTP process may need to open a data connection to
itself, set the appropriate transfer type and structure, and actually
transmit the file, counting the transmitted bytes.
If the user-FTP process is intending to restart a retrieve, it will
directly calculate the restart marker, and send that information in
the RESTart command. However, if the user-FTP process is intending
to restart sending the file, it needs to be able to determine how
much data was previously sent, and correctly received and saved. A
new FTP command is needed to get this information. This is the
purpose of the SIZE command, as documented in section 4.
5.2. ERROR RECOVER AND RESTART
STREAM MODE transfers with FILE STRUcture may be restarted even
though no restart marker has been transferred in addition to the data
itself. This is done by perhaps using the SIZE command in
combination with the RESTART (REST) command.
When using TYPE ASCII or IMAGE, the SIZE command will return the
number of bytes that would actually be transferred if the file were
to be sent between the two systems. I.e. with type IMAGE, the SIZE
normally would be the number of octets in the file. With type ASCII,
the SIZE would be the number of characters in the file INCLUDING any
characters that would be inserted during the CR-LF expansion.
5.3. Syntax
The syntax for the REST command when the current transfer mode is
STREAM is:
rest = "Rest" SP 1*DIGIT CRLF
The numeric value gives the number of octets of the immediately
following transfer to not actually send, effectively causing the
transmission to be restarted at a later point. The server-PI will
respond to the REST command with a 350 reply, indicating that the
REST parameter has been saved, and that another command, which should
be either RETR or STOR, should then follow to complete the restart.
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rest-response = "350" SP *TCHAR CRLF /
error-response
Server-FTP processes may permit transfer commands other than RETR and
STOR, such as APPE and STOU, to complete a restart, however, this is
not recommended. STOU (store unique) is undefined in this usage, as
storing the remainder of a file into a unique filename is rarely
going to be useful. If APPE (append) is permitted, it MUST act
identically to STOR when a restart marker has been set. That is, in
both cases, octets from the data connection are placed into the file
at the location indicated by the restart marker value.
An error-response will follow a REST command only when the server
does not implement the command, or the restart marker value is
syntactically invalid for the current transfer mode. That is, in
STREAM mode, if something other than one or more digits appears in
the parameter to the REST command. Any other errors, including such
problems as restart marker out of range, should be reported when the
following transfer command is issued.
5.4. FEAT response for REST
Where a server-FTP process supports RESTart in STREAM mode, as
specified here, it MUST include in the response to the FEAT command
[11], a line containing exactly the string "REST STREAM" (in upper
case, and without the quotes). Where REST is not supported at all,
or supported only in block or compressed modes, the REST command MUST
NOT be included in the FEAT response. Where required, the response
MUST be
C> FEAT
S> 211- <any descriptive text>
S> ...
S> REST STREAM
S> ...
S> 211 End
The ellipses indicate placeholders where other features may be
included, and are not required. The one space indentation of the
feature lines is mandatory.
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6. Machine Parseable Listings (MLST and MLSD)
The MLST and MLSD commands are intended to standardize the file and
directory information returned by the Server-FTP process. These
commands differ from the LIST and NLST commands in that the format of
the replies is strictly defined although extensible.
Two commands are defined, MLST which provides data about exactly the
object named on its command line, and no others. MLSD on the other
hand will list the contents of a directory if a directory is named,
otherwise it is identical to MLST. In either case, if no object is
named, the current directory is assumed. That will cause MLST to
send a one line response, and MLSD to list the contents of the
current directory.
[ Ed-Note: An alternative would be to have MLSD fail if
the argument is not a directory. Which is better? ]
In the sequel only MLST will be described, other than as previously
mentioned, MLSD is identical.
The MLST and MLSD commands also extend the FTP protocol as presented
in RFC 959 [6] and RFC 1123 [7] to allow that transmission of 8-bit
data. Note this is not specifying character sets which are 8-bit,
but specifying that FTP implementations are to specifically allow the
transmission and reception of 8-bit bytes over the control
connection. That is, all 256 possible octet values are permitted.
The MLST command allows both UTF-8/Unicode and "raw" forms as
arguments, and in responses.
6.1. Format of MLST Request
The MLST and MLSD commands each allow a single optional argument.
This argument may be either a directory name or a filename. If a
directory name is given then MLSD must return a listing of the
contents of the named directory. If the argument is not a directory,
and the command is MLSD, and in all cases for MLST, then only a
single fact line containing the information about the named file or
directory shall be returned.
If no argument is given then MLSD must return a listing of the
contents of the current working directory, and MLST must return a
listing giving information about the current working directory
itself.
If the Client-FTP sends an invalid argument, the Server-FTP MUST
reply with an error code of 501.
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The syntax for the MLST command is:
mlst = "MLst" [ SP ( utf-8-name / raw ) ] CRLF
utf-8-name = <a UTF-8 encoded Unicode string>
raw = <any string not being a valid UTF-8 encoding>
6.2. Format of MLST Response
The format of a response to the MLST command is as follows:
mlst-response = initial-response final-response
initial-response = "150" [ SP response-message ] CRLF /
error-response
response-message = *TCHAR
final-response = "226" SP response-message CRLF
data-response = *( entry CRLF )
entry = [ facts ] SP ( utf-8-name / raw )
facts = fact *( ";" fact )
fact = factname "=" value
factname = 1*ltext
value = 1*ltext
end-token = "End"
Upon receipt of a MLST or MLSD command, the server will verify the
parameter, and if invalid return an error-response. If valid, the
server will open a data connection as indicated in section 3.2 of
RFC959. If that fails, the server will return an error-response. If
all is OK, the server will return the initial-response, send the
appropriate data-response over the new data connection, close that
connection, and then send the final-response.
The data connection opened for a MLST or MLSD response shall be
connection as if the "TYPE L 8", "MODE S", and "STRU F" commands had
been given, whatever FTP transfer type, mode and structure had
actually been set, and without causing those settings to be altered
for future commands. That is, this transfer type shall be set for
the duration of the data connection established for this command
only. While the content of the data sent can be viewed as a series
of lines, implementations should note that there is no maximum line
length defined. Implementations should be prepared to deal with
arbitrarily long lines.
The facts part of the specification would contain a series of "file
facts" about the file/directory named on the same line. Typical
information to be presented would include file size, last
modification time, creation time, a unique identifier, and a
file/directory flag.
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The complete format for a successful reply to the MLST command would
be:
facts SP utf-8-name CRLF
facts SP utf-8-name CRLF
facts SP utf-8-name CRLF
...
Note that the format is intended for machine processing, not human
viewing, and as such the format is very rigid. Implementations must
not vary the format by, for example, inserting extra spaces for
readability, or inserting blank lines, or in any other way alter this
format. Exactly one space is always required after the set of facts
(which may be empty). More spaces may be present on a line if, and
only if, the file name presented contains significant spaces. The
set of facts must not contain any spaces anywhere inside it.
6.3. Filename encoding
A FTP implementation using the MLST command must be 8-bit clean.
This is necessary in order to transmit UTF-8 encoded filenames. This
specification recommends the use of UTF-8 encoded filenames. FTP
implementations SHOULD use UTF-8 whenever possible to encourage the
maximum interoperability.
Filenames are not restricted to UTF-8, however treatment of arbitrary
character encodings is not specified by this standard. Applications
are encouraged to treat non-UTF-8 encodings of filenames as octet
sequences.
Note that this encoding is unrelated to that of the contents of the
file, even if the file contains character data.
Further information about filename encoding for FTP may be found in
"Internationalization of the File Transfer Protocol" [4].
6.3.1. Notes about the Filename
The filename returned in the MLST response should be the same name as
was specified in the MLST command. Filenames returned in the output
from an MLSD command should be unqualified names within the directory
named. That is, the directory named in the MLSD command should not
appear as a component of the filenames returned.
If the server-FTP process is able, it MAY return in the MLSD
response, an entry whose type is "cdir", which names the directory
from which the contents of the listing were obtained. Where more
than one name exists, multiple of these entries may be returned. The
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server MUST return type "cdir" names in a format such that if the
user-PI takes a name of type "cdir", and appends a name of type which
is not "cdir", and which appeared in the same MLSD response as the
type=cdir name, with no intervening separators, then a valid pathname
will be produced, using which the user-PI can reference the file
indicated from its current working directory.
Alternatively, the user-PI can issue a CWD command ([6]) giving the
name of type "cdir", and from that point reference the files returned
in the MLSD response from which the cdir was obtained by using the
filename components of the listing. Once having attempted any CWD
command however, it is no longer guaranteed that a file can be
referenced by the combination of type "cdir" and other names, whether
using CWD or name concatenation.
6.3.2. Examples
Once upon a (future) time, examples existed here.
6.4. Format of Facts
The "facts" for a file in a reply to a MLST command consist of
information about that file. The facts are a series of keyword=value
pairs separated by a semi-colon (";") character. The complete series
of facts may not contain the space character.
A sample of a typical series of facts would be: (spread over two
lines for presentation only)
size=4161;lang=en-us;modify=19970214165800;create=19961001124534;
type=file;x.myfact=foo,bar
6.5. Standard Facts
This document defines a standard set of facts as follows:
size -- Size in bytes
modify -- Last modification time
create -- Creation time
type -- Entry type
unique -- Unique id of file/directory
perm -- File permissions, whether read, write, execute is
allowed for the login id.
lang -- Language of the filename per IANA[5] registry.
media-type -- MIME media-type of file contents per IANA registry.
charset -- Character set per IANA registry (if not UTF-8)
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Fact names are case-insensitive. Size, size, SIZE, and SiZe are the
same fact.
Further operating system specific keywords could be specified by
using the IANA operating system name as a prefix (examples only):
OS/2.ea -- OS/2 extended attributes
MACOS.rf -- MacIntosh resource forks
UNIX.mode -- Unix file modes (permissions)
Implementations may define keywords for experimental, or private use.
All such keywords MUST begin with the two character sequence "x.".
As type names are case independent, "x." and "X." are equivalent.
For example:
x.ver -- Version information
x.desc -- File description
x.type -- File type
6.5.1. The type Fact
The type fact needs a special description. Part of the problem with
current practices is deciding when a file is a directory. If it is a
directory, is it the current directory, a regular directory, or a
parent directory? The MLST specification makes this unambiguous
using the type fact. The type fact given specifies information about
the object listed on the same line of the MLST response.
Five values are possible for the type fact:
file -- a file entry
cdir -- the current directory
pdir -- the parent directory
dir -- a directory or sub-directory
OSname=type -- an OS or file system dependent file type
The syntax is defined to be:
type-fact = type-label "=" type-val
type-label = "Type"
type-val = "File" / "cdir" / "pdir" / "dir" /
sys-depend-type
sys-depend-type = "OS" 1*ltext "=" 1*ltext
6.5.1.1. type=file
The presence of the type=file fact indicates the listed entry is a
file containing non-system data. That is, it may be transferred from
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one system to another of quite different characteristics, and perhaps
still be meaningful.
6.5.1.2. type=cdir
The type=cdir fact indicates the listed entry is the pathname of the
directory whose contents are listed. The value of this entry (the
filename part) plus the value of a type=file entry from the same MLSD
listing together should represent a complete pathname suitable for a
RETR command. The value for the type=cdir entry should include any
necessary system delimiters used between path components. An example
would be the forward slash "/" on a UNIX(TM) system, or a back slash
"\" on an OS/2 or Windows system.
6.5.1.3. type=dir
If present, the type=dir entry is the name of a directory. When
executed with the current directory in the same place in the NVFS as
it was when the MLST or MLSD command was issued, a CWD with its
argument being the formed by appending the name with type=pdir to a
name with type-cdir should succeed (assuming the user has the
appropriate access rights).
6.5.1.4. type=pdir
If present, which will occur only in the response to a MLSD command,
the type=pdir entry represents a pathname of the parent directory of
the listed directory. As well as having the properties of a
type=dir, a CWD command with the appropriate value should change the
user to the parent directory of the listed directory. User-FTP
processes should note not all responses will include this
information.
6.5.2. The unique Fact
The unique fact is used to present a unique identifier for a file or
directory in the NVFS accessed via a server-FTP process. The value
of this fact should be the same for any number of filenames that
refer to the same underlying file. The fact should have different
values for names which reference distinct files. The mapping between
files, and unique fact tokens should be maintained, and remain
consistent, for at least the lifetime of the control connection from
user-PI to server-PI.
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unique-fact = "Unique" "=" token
This fact would be expected to be used by Server-FTPs whose host
system allows things such as symbolic links so that the same file may
be represented in more than one directory on the server. The value
of the unique fact should be considered an opaque string for
comparison purposes. The only conclusion that should be drawn is
that if two different names each have the same value for the unique
fact, they refer to the same underlying object.
6.5.3. The modify Fact
The modify fact is used to determine the last time the content of the
file (or directory) indicated was modified. Any change of substance
to the file should cause this value to alter. Changes which merely
alter access rights, and similar attributes, which do not affect the
content or meaning of the data contained in a file, should not cause
this value to alter.
For directories, this value should alter whenever a change occurs to
the directory such that different filenames would (or might) be
included in MLSD output of that directory.
modify-fact = "Modify" "=" time-val
6.5.4. The create Fact
The create fact indicates when a file, or directory, was first
created. Exactly what "creation" is for this purpose is not
specified here, and may vary from server to server. About all that
can be said about the value returned is that it can never indicate a
later time than the mtime fact.
create-fact = "Create" "=" time-val
Implementation Note: Implementors of this fact on UNIX(TM) systems
should note that the unix "stat" "st_ctime" field does not give
creation time, and that unix filesystems do not record creation
time at all. Unix (and POSIX) implementations will normally not
include this fact.
6.5.5. The perm Fact
The perm fact is used to indicate access rights the current FTP user
has over the object listed. Its value is always an unordered
sequence of alphabetic characters.
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perm-fact = "Perm" "=" pvals
pvals = *ALPHA
There are ten permission indicators currently defined. Many are
meaningful only when used with a particular type of object. The ten
characters currently defined are:
a c d e f l m p r w
[ Ed-Note: should these be case dependent or independent
? ]
The "a" permission applies to objects of type=file, and indicates
that the APPE (append) command may be applied to the file named.
The "c" permission applies to objects of type=dir (and type=pdir,
type=cdir). It indicates that files may be created in the directory
named. That is, that a STOU command is likely to succeed, and that
STOR and APPE commands might succeed if the file named did not
previously exist, but is to be created in the directory object that
has the "c" permission. It also indicates that the RNTO command is
likely to succeed for names in the directory.
The "d" permission applies to all types. For type=file it indicates
that the file may be deleted, that is, that the DELE command may be
applied to it. For the directory types it indicates that (some)
files in the directory may be deleted.
The "e" permission applies to the directory types. When set on an
object of type=dir, type=cdir, or type=pdir it indicates that a CWD
command naming the object should succeed, and the user should be able
to enter the directory named. For type=pdir it also indicates that
the CDUP command should succeed.
The "f" permission for objects indicates that the object named may be
renamed - that is, may be the object of an RNFR command.
The "l" permission applies to the directory file types, and indicates
that the listing commands, LIST, NLST, and MLSD may be applied to the
directory in question, and that MLST, LIST, NLST, and STAT may be
applied to objects in the directory.
The "m" permission applies to directory types, and indicates that the
MKD command may be used to create a new directory within the
directory under consideration.
The "p" permission applies to directory types, and indicates that the
RMD command may be used to remove (purge) the directory named.
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The "r" permission applies to type=file objects, and indicates that
the RETR command may be applied to that object.
The "w" permission applies to type=file objects, and indicates that
the STOR command may be applied to the object named.
Note: That a permission indicator is set can never imply that the
appropriate command is guaranteed to work - just that it might.
Other system specific limitations, such as limitations on
available space for storing files, may cause an operation to
fail, where the permission flags may have indicated that it was
likely to succeed. The permissions are a guide only.
Implementation note: The permissions are described here as they apply
to FTP commands. They may not map easily into particular
permissions available on the server's operating system. Servers
are expected to synthesize these permission bits from the
permission information available from operating system. For
example, to correctly determine whether the "p" permission bit
should be set on a directory for a server running on the
UNIX(TM) operating system, the server should check that the
directory named is empty, and that the user has write permission
on both the directory under consideration, and its parent
directory.
Some systems may have more specific permissions than those
listed here, such systems should map those to the flags defined
as best they are able. Other systems may have only more broad
access controls. They will generally have just a few possible
permutations of permission flags, however they should attempt to
correctly represent what is permitted.
6.5.6. The lang Fact
The lang fact describes the natural language of the filename for use
in display purposes. Values used here should be taken from the
language registry of the IANA.
lang-fact = "Lang" "=" token
Server-FTP implementations MUST NOT guess language values. Language
values must be determined in an unambiguous way such as filesystem
tagging of language or by user configuration.
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6.5.7. The size Fact
The size should always reflect the approximate size of the file.
This should be as accurate as the server can make it, without going
to extraordinary lengths, such as reading the entire file. The size
is expressed in units of octets.
Given limitations in some systems, Client-FTP implementations must
understand this size may not be precise and may change between the
time of a MLST and RETR operation.
Clients that need highly accurate size information for some
particular reason should use the SIZE command as defined in section
4. The most common need for this accuracy is likely to be in
conjunction with the REST command described in section 5. The size
fact, on the other hand, should be used for purposes such as
indicating to a human user the approximate size of the file to be
transferred, and perhaps to give an idea of expected transfer
completion time.
size-fact = "Size" "=" 1*DIGIT
6.5.8. The media-type Fact
The media-type fact represents the IANA media type of the file. The
list of values used must follow the guidelines set by the IANA
registry.
media-type = "Media-Type" "=" <per IANA guidelines>
Server-FTP implementations MUST NOT guess media type values. Media
type values must be determined in an unambiguous way such as
filesystem tagging of media-type or by user configuration.
6.5.9. The charset Fact
The charset fact represents the IANA character set name for the
encoded names in a MLST response. The default character set is UTF-8
unless specified otherwise. FTP implementations SHOULD use UTF-8 if
possible to encourage maximum interoperability.
charset-type = "Charset" "=" token
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6.6. FEAT response for MLST
When responding to the FEAT command, a server-FTP process that
supports MLST, and related commands, MLSD, and the modified STAT,
plus internationalisation of pathnames, MUST indicate that this
support exists. It does this by including a MLST feature line. As
well as indicating the basic support, the MLST feature line indicates
which MLST facts are available from the server, and which of those
will be returned if no subsequent "OPTS MLST" command is sent.
mlst-feat = SP "MLST" [SP factlist] CRLF
factlist = factname ["*"] *( ";" factname ["*"] )
The initial space shown in the mlst-feat response is that required by
the FEAT command, two spaces are not permitted. If no factlist is
given, then the server-FTP process is indicating that it supports
MLST, but implements no facts. Only pathnames can be returned. This
would be a minimal MLST implementation, and useless for most
practical purposes. Where the factlist is present, the factnames
included indicate the facts supported by the server. Where the
optional asterisk appears after a factname, that fact will be
included in MLST format responses, until an "OPTS MLST" is given to
alter the list of facts returned.
[ Ed-Note: Perhaps the sense of the "*" should be
reversed? That is, make the asterisk indicate those
facts not returned? ]
6.7. OPTS parameters for MLST
For the MLST command, the Client-FTP may specify a list of facts it
wishes to be returned in all subsequent MLST commands until another
OPTS MLST command is sent. The format is specified by:
mlst-opts = "OPTS" SP "MLST"
[ SP factname *(";" factname) ]
By sending the "OPTS MLST" command, the client requests the server to
include only the facts listed as arguments to the command in
subsequent output from MLST commands. Facts not included in the
"OPTS MLST" command must not be returned by the server. Facts that
are included should be returned for each entry returned from the MLST
command where they apply. Facts requested that are not supported, or
which are inappropriate to the file or directory being listed should
simply be omitted from the MLST output. This is not an error. Note
that where no factname arguments are present, the client is
requesting that only the file names be returned. In this case, and
in any other case where no facts are included in the result, the
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space that separates the fact names and their values from the file
name is still required. That is, the first character of the output
line will be a space, and the file name will start immediately
thereafter.
Note, there is no "OPTS MLSD" command, the fact names set with the
TS MLST" command apply to both MLST and MLSD commands, and to the
STAT command when used with a file name argument and no transfer in
progress.
7. Interpretation of STAT command output
Where a server-FTP process supports the MLST and MLST commands, the
format of the output from the STAT command ([6] section 4.1.3) when
given with the optional pathname argument, permitted only when there
is no file transfer operation in progress, is hereby specified to be
the same as that of the MLST command, with just the following
modifications, required because the output is sent over the control
connection.
More text comes here, eventually, setting out exactly what the STAT
response will look like. Most of it will be borrowed from the
original definition of MLST output, from the days when it was
permitted over the control connection.
7.1. FEAT response for STAT
There is no output in the FEAT command that specifically indicates
that the STAT command behaves as described above. Implementations
must infer this from support of the MLST command by the server, which
is indicated in the FEAT output.
8. Impact On Other FTP Commands
Along with the introduction of MLST, traditional FTP commands must be
extended to allow for the use of more than US-ASCII or EBCDIC
character sets. In general, the support of MLST requires support for
arbitrary character sets wherever filenames and directory names are
allowed. This applies equally to both arguments given to the
following commands and to the replies from them, as appropriate.
CWD
RETR
STOR
STOU
APPE
RNFR
RNTO
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DELE
RMD
MKD
PWD
STAT
The arguments to all of these commands should be processed the same
way that MLST commands and responses are processed with respect to
handling embedded CRs and NULs. See section 2.2.
8.1. Impact on Pathnames and Filenames
The design of MLST requires the Server-FTP to allow concatenation of
certain elements of a MLST response. Specifically, a typical
response would include an element which indicates the current
directory and one or more elements which are files in the indicated
directory. A Server-FTP must be able to accept a simple
concatenation of these two names even if the underlying operating
system does not accept a simple concatenation. The Server-FTP must
perform any translation of the concatenated name to local
equivalents.
9. Character sets and Internationalisation
This section will set out just what is going on with char sets, what
data is part of the protocol, and always appears exactly as is
specified (and could almost as easily be numbers, or any other kind
of encoding), and what is text for users, which should be able to
appear in their language of choice, or otherwise be handled in some
kind of rational way. That is, it will once it is written. This is
merely a placeholder.
10. 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.
11. References
[1] Coded Character Set--7-bit American Standard Code for Information
Interchange, ANSI X3.4-1986.
[2] F. Yergeau, "UTF-8, a transformation format of Unicode and ISO
10646", RFC 2044, Alis Technologies, October 1996.
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[3] D. Crocker, "Augmented BNF for Syntax Specifications: ABNF",
Work In Progress <draft-ietf-drums-abnf-02.txt>, Internet
Mail Consortium, March 1997.
[4] W. Curtin, "Internationalization of the File Transfer Protocol",
Work In Progress <draft-ietf-ftpext-itln-02.txt>, Defense
Information Systems Agency, November 1996.
[5] Internet Assigned Numbers Authority. http://www.isi.edu/div7/iana/
Email: iana@iana.org.
[6] J. Postel, J. Reynolds, "File Transfer Protocol (FTP)",
STD 9 (RFC 959), ISI, October 1985
[7] R. Braden, "Requirements for Internet Hosts -- Application
and Support", STD 3 (RFC 1123), October 1989
[8] ISO 3307 (need a citation for this please!)
[9] D. Crocker, "Augmented BNF for Syntax Specifications: ABNF",
Work In Progress <draft-ietf-drums-abnf-NN.txt>, Internet
Mail Consortium, March 1997.
[10] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14 (RFC 2119), Harvard University,
March 1997
[11] P. Hethmon, R. Elz, "Feature negotiation mechanism for the
File Transfer Protocol", Work in progress,
<draft-ietf-ftpext-feat-00.txt> Hethmon Bros, University of Melbourne,
June 1997.
[12] J. Postel, J. Reynolds, "Telnet protocol Specification"
STD 8 (RFC 854), ISI, May 1983
Acknowledgements
The following people have contributed to this document:
Alex Belits
D. J. Berstein
Martin J. Duerst
Mark Harris
Alun Jones
James Matthews
Keith Moore
and the entire FTPEXT working group of the IETF.
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The description of the modifications to the REST command and the MDTM
and SIZE commands comes from a set of modifications suggested for
RFC959 by Rick Adams in 1989. A draft containing just those
commands, edited by David Borman, has been merged with this document.
Editors' Addresses
Robert Elz
University of Melbourne
Department of Computer Science
Parkville, Vic 3052
Australia
Email: kre@munnari.OZ.AU
Paul Hethmon
Hethmon Brothers
2305 Chukar Road
Knoxville, TN 37923 USA
Phone: +1-423-690-8990
Email: phethmon@hethmon.com
Elz & Hethmon [Page 27]
