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Network Working Group                                           A. Bryan
Internet-Draft                                              T. Tsujikawa
Intended status: Standards Track                             D. Stenberg
Expires: June 23, 2011                                 December 20, 2010


          File Transfer Protocol RANG Command for Byte Ranges
                        draft-bryan-ftp-range-00

Abstract

   The File Transfer Protocol offers REST, to designate a starting point
   for a transfer, but does not currently offer any method to specify an
   end point.  This document specifies a new FTP RANG command to be used
   by clients to designate a start and end point for STREAM mode
   transfers with RETR.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on June 23, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Document Conventions . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Basic Tokens . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Server Replies . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Byte Ranges in STREAM Mode . . . . . . . . . . . . . . . . . .  4
   4.  The RANGe Command (RANG) . . . . . . . . . . . . . . . . . . .  5
     4.1.  FEAT Command Response for RANG Command . . . . . . . . . .  7
     4.2.  User-PI usage of RANG  . . . . . . . . . . . . . . . . . .  7
     4.3.  RANG Command Errors  . . . . . . . . . . . . . . . . . . .  8
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     7.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgements and Contributors . . . . . . . . . .  9
   Appendix B.  Document History  . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
































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1.  Introduction

   The File Transfer Protocol offers REST [RFC3659], to designate a
   starting point for a transfer, but does not currently offer any
   method to specify an end point.  This document specifies a new FTP
   RANG command to be used by clients to designate a start and end point
   for STREAM mode transfers with RETR.

   The current alternatives, without being able to specify an end point,
   are to issue an ABOR command or close the data connection.

   HTTP offers similar functionality with the Range: header in Section
   14.35 of [RFC2616], where a specific byte range can optionally be
   requested.

   [[ Discussion of this draft should take place on the FTPEXT2 Working
   Group mailing list, although this draft is not a WG item:
   ftpext@ietf.org. ]]


2.  Document Conventions

   This specification describes conformance of File Transfer Protocol
   Extension for RANG, and end point in a byte range.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, [RFC2119], as
   scoped to those conformance targets.

   This document also uses notation defined in STD 9, [RFC0959].  In
   particular, the terms or commands "reply", "user", "file", "FTP
   commands", "user-PI" (user protocol interpreter), "server-FTP
   process", "server-PI", "mode", "Image type", "Stream transfer mode",
   "type", "STOR", "RETR", and "ASCII", are all used here as defined
   there.  The command "REST" is used as defined in Section 5 of
   [RFC3659].

   In the examples of FTP dialogs presented in this document, lines that
   begin "C> " were sent over the control connection from the user-PI to
   the server-PI, and lines that begin "S> " were sent over the control
   connection from the server-PI to the user-PI.  In all cases, the
   prefixes shown above, including the one space, have been added for
   the purposes of this document, and are not a part of the data
   exchanged between client and server.

   Syntax required is defined using the Augmented BNF defined in
   [RFC5234].



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2.1.  Basic Tokens

   This document imports the core definitions given in Appendix B of
   [RFC5234].  There definitions will be found for basic ABNF elements
   like ALPHA, DIGIT, SP, etc.  To that, the following term is added for
   use in this document.


      TCHAR = VCHAR / SP / HTAB    ; visible plus white space

   The VCHAR (from [RFC5234]) and TCHAR rules give basic character types
   from varying sub-sets of the ASCII character set for use in various
   commands and responses.

   Note that in ABNF, string literals are case insensitive.  That
   convention is preserved in this document, and implies that FTP
   commands and parameters that are added by this specification have
   values that can be represented in any case.  That is, "RANG" is the
   same as "rang", "Rang", "RaNg", etc., and "ftp.example.com" is the
   same as "Ftp.Example.Com", "fTp.eXample.cOm", etc.

2.2.  Server Replies

   Section 4.2 of [RFC0959] 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.


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

   Implementers should note that the ABNF syntax (which was not used in
   [RFC0959]) used 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.  Implementers should assume that, unless
   stated to the contrary, any reply to any FTP command (including QUIT)
   can be of the multi-line format described in [RFC0959].

   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.  Byte Ranges in STREAM Mode

   In STREAM mode, the data connection contains just a stream of
   unformatted octets of data.  Explicit restart markers thus cannot be



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   inserted into the data stream, they would be indistinguishable from
   data.  For this reason, the FTP specification [RFC0959] 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 octet offset into the data
   stream.

   Because the data stream defines the file in STREAM mode, a different
   data stream would represent a different file.  Thus, an offset will
   always represent the same position within a file.  On the other hand,
   in other modes than STREAM, the same file can be transferred using
   quite different octet sequences and yet be reconstructed into the one
   identical file.  Thus an offset into the data stream in transfer
   modes other than STREAM would not give an unambiguous restart or end
   point.

   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 usually very
   easy for the receiver to determine how much data was previously
   received, and notify the sender of the 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 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 octets.

   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.  The
   purpose of the SIZE command, as documented in [RFC3659], is to get
   this information.


4.  The RANGe Command (RANG)

   A new command "RANG" is added to the FTP command set to allow the
   client to specify both an start point byte range and an end point
   byte range of a file from a server-FTP process.

   The syntax for the RANG command when the current transfer mode is
   STREAM is:






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      range-command = "RANG" SP start-point  SP end-point CRLF
      start-point = 1*DIGIT
      end-point = 1*DIGIT

   [NOTE: end-point is inclusive.]

   start-point gives the number of octets of the immediately-following
   transfer to not actually send, effectively causing the transmission
   to be started at a later point.  A value of zero effectively causes
   the transmission to be started at first byte. end-point gives the
   number of octets, counted from the beginning of the file, of the
   immediately-following transfer to stop sending at, effectively
   causing the transmission to be ended.  (That is, the end point is
   relative to the start of the file and not relative to the start
   point).  The server-PI will response to the RANG command with a 350
   reply, indicating that RANG parameters has been saved, and that
   another command, which should be RETR, should then follow to complete
   the ranged request.


      range-response = range-ok / error-response
      range-ok       = "350" SP *TCHAR CRLF

   The RANG command is intended to get partial data of a file.  When the
   RANG command is used with RETR, client bears the responsibility of
   merging the retrieved data with partially retrieved file.  It may
   choose to use the data obtained other than to complete an earlier
   transfer, or to re-retrieve data that had been retrieved before.

   The RANG command must be the last command issued before the data
   transfer command that is to cause a partial data transfer.  The
   effect of issuing a RANG command at any other time is undefined.  The
   server-PI may react to a badly positioned RANG command by issuing an
   error response to the following command, not being a restartable data
   transfer command, or it may save the start-point and/or end-point
   byte range value and apply it to the next data transfer command, or
   it may silently ignore the inappropriate restart attempt.  Because of
   this, a user-PI that has issued a RANG command, but that has not
   successfully transmitted the following data transfer command for any
   reason, should send another RANG command before the next data
   transfer command.  If that transfer is not to be restarted, then
   "REST 0" should be issued.

   An error response will follow a RANG command only when the server
   does not implement the command, or when command syntax is invalid.
   Any other errors, including such problems as start-point and/or end-
   point byte range out of range, should be reported when the following
   transfer command is issued.  Such errors will cause that transfer



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   request to be rejected with an error indicating the invalid restart
   attempt.

4.1.  FEAT Command Response for RANG Command

   When replying to the FEAT command [RFC2389], a server-FTP process
   that supports the RANG command, as specified here, MUST include, a
   line containing exactly the string "RANG STREAM".  This string is
   case insensitive, and MAY be sent in any mixture of upper or lower
   case, however it SHOULD be sent in upper case.  That is, the response
   SHOULD be:


      C> FEAT
      S> 211-Extensions supported:
      S>  ...
      S>  RANG STREAM
      S>  ...
      S> 211 END

   The ellipses indicate place holders where other features may be
   included, and are not required.  The one-space indentation of the
   feature lines is mandatory [RFC2389].


      range-feat = SP "RANG" SP "STREAM"

4.2.  User-PI usage of RANG

   The user-PI issues the FEAT command to query the server-PI if it
   supports the RANG command.  In this example, the server-PI also
   supports REST.


      C> FEAT
      S> 211-Extensions supported:
      S>  ...
      S>  REST STREAM
      S>  RANG STREAM
      S>  ...
      S> 211 END

   Assume that the transfer of a largish file has previously been
   interrupted after 802816 octets had been received, that the tranfer
   should stop at octet 1000000 of the file, that the previous transfer
   was with TYPE=I, and that it has been verified that the file on the
   server has not since changed.




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   C> TYPE I
   S> 200 Type set to I.
   C> PORT 127,0,0,1,15,107
   S> 200 PORT command successful.
   C> RANG 802816 1000000
   S> 350 Restarting at 802816. End Byte range at 1000000. Send RETRIEVE
   C> RETR cap60.pl198.tar
   S> 150 Opening BINARY mode data connection
   [...]
   S> 226 Transfer complete.

   In the above example, data is sent from offset 802816 to, and
   including, offset 1000000.

4.3.  RANG Command Errors

   The server-PI SHOULD reply with a 500 reply if the RANG command is
   unrecognized or unimplemented.

   The server-PI SHOULD reply with a 550 reply if the RANG command is
   used on a file that can not be found.

   The server-PI SHOULD reply with a 551 reply if the server-PI is
   unable to deliver the file with TYPE I and MODE S.

   The server-PI SHOULD reply with a 552 reply if the user is not
   allowed to use the RANG command.

   The server-PI SHOULD reply with a 553 reply if the user requests the
   RANG of a directory, which is not allowed.

   The server-PI SHOULD reply with a 5yz reply if the specified end
   point is larger than the actual file or the end point is before the
   start point.


5.  IANA Considerations

   This new command is added to the "FTP Commands and Extensions"
   registry created by [RFC5797].

   Command Name: RANG

   Description: End point byte range (for STREAM mode).

   FEAT String: RANG

   Command Type: Service execution/parameter setting



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   Conformance Requirements: Optional

   Reference: This specification


6.  Security Considerations

   This memo does not directly concern security.  It is not believed
   that any of the mechanisms documented here impact in any particular
   way upon the security of FTP.


7.  References

7.1.  Normative References

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 0959, October 1985.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2389]  Hethmon, P. and R. Elz, "Feature negotiation mechanism for
              the File Transfer Protocol", RFC 2389, August 1998.

   [RFC3659]  Hethmon, P., "Extensions to FTP", RFC 3659, March 2007.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

7.2.  Informative References

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC5797]  Klensin, J. and A. Hoenes, "FTP Command and Extension
              Registry", RFC 5797, March 2010.


Appendix A.  Acknowledgements and Contributors

   Thanks to ...

   Portions of [RFC3659] were wholly reused in this document.






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Appendix B.  Document History

   [[ to be removed by the RFC editor before publication as an RFC. ]]

   Known issues concerning this draft:
   o  Expand current alternatives (ABOR, close data connection) and why
      they are not optimal.

   draft-bryan-ftp-range-00 : December 13, 2010.
   o  Initial draft.


Authors' Addresses

   Anthony Bryan
   Pompano Beach, FL
   USA

   Email: anthonybryan@gmail.com
   URI:   http://www.metalinker.org


   Tatsuhiro Tsujikawa
   Shiga
   Japan

   Email: tatsuhiro.t@gmail.com
   URI:   http://aria2.sourceforge.net


   Daniel Stenberg

   Email: daniel@haxx.se
   URI:   http://www.haxx.se/

















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