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rfc:rfc2347

Network Working Group G. Malkin Request for Commments: 2347 Bay Networks Updates: 1350 A. Harkin Obsoletes: 1782 Hewlett Packard Co. Category: Standards Track May 1998

                       TFTP Option Extension

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

 The Trivial File Transfer Protocol [1] is a simple, lock-step, file
 transfer protocol which allows a client to get or put a file onto a
 remote host.  This document describes a simple extension to TFTP to
 allow option negotiation prior to the file transfer.

Introduction

 The option negotiation mechanism proposed in this document is a
 backward-compatible extension to the TFTP protocol.  It allows file
 transfer options to be negotiated prior to the transfer using a
 mechanism which is consistent with TFTP's Request Packet format.  The
 mechanism is kept simple by enforcing a request-respond-acknowledge
 sequence, similar to the lock-step approach taken by TFTP itself.
 While the option negotiation mechanism is general purpose, in that
 many types of options may be negotiated, it was created to support
 the Blocksize option defined in [2].  Additional options are defined
 in [3].

Packet Formats

 TFTP options are appended to the Read Request and Write Request
 packets.  A new type of TFTP packet, the Option Acknowledgment
 (OACK), is used to acknowledge a client's option negotiation request.
 A new error code, 8, is hereby defined to indicate that a transfer

Malkin & Harkin Standards Track [Page 1] RFC 2347 TFTP Option Extension May 1998

 should be terminated due to option negotiation.
 Options are appended to a TFTP Read Request or Write Request packet
 as follows:
    +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+-->
    |  opc  |filename| 0 |  mode  | 0 |  opt1  | 0 | value1 | 0 | <
    +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+-->
     >-------+---+---~~---+---+
    <  optN  | 0 | valueN | 0 |
     >-------+---+---~~---+---+
    opc
       The opcode field contains either a 1, for Read Requests, or 2,
       for Write Requests, as defined in [1].
    filename
       The name of the file to be read or written, as defined in [1].
       This is a NULL-terminated field.
    mode
       The mode of the file transfer: "netascii", "octet", or "mail",
       as defined in [1].  This is a NULL-terminated field.
    opt1
       The first option, in case-insensitive ASCII (e.g., blksize).
       This is a NULL-terminated field.
    value1
       The value associated with the first option, in case-
       insensitive ASCII.  This is a NULL-terminated field.
    optN, valueN
       The final option/value pair.  Each NULL-terminated field is
       specified in case-insensitive ASCII.
 The options and values are all NULL-terminated, in keeping with the
 original request format.  If multiple options are to be negotiated,
 they are appended to each other.  The order in which options are
 specified is not significant.  The maximum size of a request packet
 is 512 octets.
 The OACK packet has the following format:

Malkin & Harkin Standards Track [Page 2] RFC 2347 TFTP Option Extension May 1998

    +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+
    |  opc  |  opt1  | 0 | value1 | 0 |  optN  | 0 | valueN | 0 |
    +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+
    opc
       The opcode field contains a 6, for Option Acknowledgment.
    opt1
       The first option acknowledgment, copied from the original
       request.
    value1
       The acknowledged value associated with the first option.  If
       and how this value may differ from the original request is
       detailed in the specification for the option.
    optN, valueN
       The final option/value acknowledgment pair.

Negotiation Protocol

 The client appends options at the end of the Read Request or Write
 request packet, as shown above.  Any number of options may be
 specified; however, an option may only be specified once.  The order
 of the options is not significant.
 If the server supports option negotiation, and it recognizes one or
 more of the options specified in the request packet, the server may
 respond with an Options Acknowledgment (OACK).  Each option the
 server recognizes, and accepts the value for, is included in the
 OACK.  Some options may allow alternate values to be proposed, but
 this is an option specific feature.  The server must not include in
 the OACK any option which had not been specifically requested by the
 client; that is, only the client may initiate option negotiation.
 Options which the server does not support should be omitted from the
 OACK; they should not cause an ERROR packet to be generated.  If the
 value of a supported option is invalid, the specification for that
 option will indicate whether the server should simply omit the option
 from the OACK, respond with an alternate value, or send an ERROR
 packet, with error code 8, to terminate the transfer.
 An option not acknowledged by the server must be ignored by the
 client and server as if it were never requested.  If multiple options
 were requested, the client must use those options which were
 acknowledged by the server and must not use those options which were
 not acknowledged by the server.

Malkin & Harkin Standards Track [Page 3] RFC 2347 TFTP Option Extension May 1998

 When the client appends options to the end of a Read Request packet,
 three possible responses may be returned by the server:
    OACK  - acknowledge of Read Request and the options;
    DATA  - acknowledge of Read Request, but not the options;
    ERROR - the request has been denied.
 When the client appends options to the end of a Write Request packet,
 three possible responses may be returned by the server:
    OACK  - acknowledge of Write Request and the options;
    ACK   - acknowledge of Write Request, but not the options;
    ERROR - the request has been denied.
 If a server implementation does not support option negotiation, it
 will likely ignore any options appended to the client's request.  In
 this case, the server will return a DATA packet for a Read Request
 and an ACK packet for a Write Request establishing normal TFTP data
 transfer.  In the event that a server returns an error for a request
 which carries an option, the client may attempt to repeat the request
 without appending any options.  This implementation option would
 handle servers which consider extraneous data in the request packet
 to be erroneous.
 Depending on the original transfer request there are two ways for a
 client to confirm acceptance of a server's OACK.  If the transfer was
 initiated with a Read Request, then an ACK (with the data block
 number set to 0) is sent by the client to confirm the values in the
 server's OACK packet.  If the transfer was initiated with a Write
 Request, then the client begins the transfer with the first DATA
 packet, using the negotiated values.  If the client rejects the OACK,
 then it sends an ERROR packet, with error code 8, to the server and
 the transfer is terminated.
 Once a client acknowledges an OACK, with an appropriate non-error
 response, that client has agreed to use only the options and values
 returned by the server.  Remember that the server cannot request an
 option; it can only respond to them.  If the client receives an OACK
 containing an unrequested option, it should respond with an ERROR
 packet, with error code 8, and terminate the transfer.

Malkin & Harkin Standards Track [Page 4] RFC 2347 TFTP Option Extension May 1998

Examples

 Read Request
    client                                           server
    -------------------------------------------------------
    |1|foofile|0|octet|0|blksize|0|1432|0|  -->               RRQ
                                  <--  |6|blksize|0|1432|0|   OACK
    |4|0|  -->                                                ACK
                           <--  |3|1| 1432 octets of data |   DATA
    |4|1|  -->                                                ACK
                           <--  |3|2| 1432 octets of data |   DATA
    |4|2|  -->                                                ACK
                           <--  |3|3|<1432 octets of data |   DATA
    |4|3|  -->                                                ACK
 Write Request
    client                                           server
    -------------------------------------------------------
    |2|barfile|0|octet|0|blksize|0|2048|0|  -->               RRQ
                                  <--  |6|blksize|0|2048|0|   OACK
    |3|1| 2048 octets of data |  -->                          DATA
                                                 <--  |4|1|   ACK
    |3|2| 2048 octets of data |  -->                          DATA
                                                 <--  |4|2|   ACK
    |3|3|<2048 octets of data |  -->                          DATA
                                                    <--  |4|3|   ACK

Security Considerations

 The basic TFTP protocol has no security mechanism.  This is why it
 has no rename, delete, or file overwrite capabilities.  This document
 does not add any security to TFTP; however, the specified extensions
 do not add any additional security risks.

References

 [1] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC 1350,
     October 1992.
 [2] Malkin, G., and A. Harkin, "TFTP Blocksize Option", RFC 2348,
     May 1998.
 [3] Malkin, G., and A. Harkin, "TFTP Timeout Interval and Transfer
     Size Options", RFC 2349, May 1998.

Malkin & Harkin Standards Track [Page 5] RFC 2347 TFTP Option Extension May 1998

Authors' Addresses

 Gary Scott Malkin
 Bay Networks
 8 Federal Street
 Billerica, MA  01821
 Phone:  (978) 916-4237
 EMail:  gmalkin@baynetworks.com
 Art Harkin
 Internet Services Project
 Information Networks Division
 19420 Homestead Road MS 43LN
 Cupertino, CA  95014
 Phone: (408) 447-3755
 EMail: ash@cup.hp.com

Malkin & Harkin Standards Track [Page 6] RFC 2347 TFTP Option Extension May 1998

Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Malkin & Harkin Standards Track [Page 7]

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