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rfc:rfc1782

Network Working Group G. Malkin Request for Comments: 1782 Xylogics, Inc. Updates: 1350 A. Harkin Category: Standards Track Hewlett Packard Co.

                                                            March 1995
                       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.

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 TFTPs 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].
 This document assumes reader familiarity with the TFTP specification
 [1] and its terminology.

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
 should be terminated due to option negotiation.

Malkin & Harkin [Page 1] RFC 1782 TFTP Option Extension March 1995

 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 |
     >-------+---+---~~---+---+
    The "0"s shown in these illustrations and the ones below are all
    all zero octets, i.e., NULL terminators for the preceeding
    fields.
    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 ASCII 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.

Malkin & Harkin [Page 2] RFC 1782 TFTP Option Extension March 1995

 The OACK packet has the following format:
    +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+
    |  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 must 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 [Page 3] RFC 1782 TFTP Option Extension March 1995

 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 [Page 4] RFC 1782 TFTP Option Extension March 1995

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

 Security issues are not discussed in this memo.

References

 [1] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC 1350,
     MIT, July 1992.
 [2] Malkin, G., and A. Harkin, "TFTP Blocksize Option", RFC 1783,
     Xylogics, Inc., Hewlett Packard Co., March 1995.
 [3] Malkin, G., and A. Harkin, A., "TFTP Timeout Interval and
     Transfer Size Options", RFC 1784, Xylogics, Inc., Hewlett Packard
     Co., March 1995.

Malkin & Harkin [Page 5] RFC 1782 TFTP Option Extension March 1995

Authors' Addresses

     Gary Scott Malkin
     Xylogics, Inc.
     53 Third Avenue
     Burlington, MA  01803
     Phone:  (617) 272-8140
     EMail:  gmalkin@xylogics.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 [Page 6]

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