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Network Working Group                                          G. MalkinRequest for Comments: 1782                                Xylogics, Inc.Updates:1350                                                  A. HarkinCategory: Standards Track                            Hewlett Packard Co.                                                              March 1995TFTP Option ExtensionStatus 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 1995Examples   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|   ACKSecurity 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 1995Authors' 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.comMalkin & Harkin                                                 [Page 6]