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309 lines
11 KiB
309 lines
11 KiB
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Network Working Group G. Malkin
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Request for Comments: 1782 Xylogics, Inc.
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Updates: 1350 A. Harkin
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Category: Standards Track Hewlett Packard Co.
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March 1995
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TFTP Option Extension
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Status of this Memo
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This document specifies an Internet standards track protocol for the
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Internet community, and requests discussion and suggestions for
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improvements. Please refer to the current edition of the "Internet
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Official Protocol Standards" (STD 1) for the standardization state
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and status of this protocol. Distribution of this memo is unlimited.
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Abstract
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The Trivial File Transfer Protocol [1] is a simple, lock-step, file
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transfer protocol which allows a client to get or put a file onto a
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remote host. This document describes a simple extension to TFTP to
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allow option negotiation prior to the file transfer.
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Introduction
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The option negotiation mechanism proposed in this document is a
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backward-compatible extension to the TFTP protocol. It allows file
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transfer options to be negotiated prior to the transfer using a
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mechanism which is consistent with TFTPs Request Packet format. The
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mechanism is kept simple by enforcing a request-respond-acknowledge
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sequence, similar to the lock-step approach taken by TFTP itself.
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While the option negotiation mechanism is general purpose, in that
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many types of options may be negotiated, it was created to support
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the Blocksize option defined in [2]. Additional options are defined
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in [3].
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This document assumes reader familiarity with the TFTP specification
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[1] and its terminology.
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Packet Formats
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TFTP options are appended to the Read Request and Write Request
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packets. A new type of TFTP packet, the Option Acknowledgment
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(OACK), is used to acknowledge a client's option negotiation request.
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A new error code, 8, is hereby defined to indicate that a transfer
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should be terminated due to option negotiation.
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Malkin & Harkin [Page 1]
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RFC 1782 TFTP Option Extension March 1995
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Options are appended to a TFTP Read Request or Write Request packet
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as follows:
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+-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+-->
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| opc |filename| 0 | mode | 0 | opt1 | 0 | value1 | 0 | <
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+-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+-->
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>-------+---+---~~---+---+
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< optN | 0 | valueN | 0 |
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>-------+---+---~~---+---+
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The "0"s shown in these illustrations and the ones below are all
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all zero octets, i.e., NULL terminators for the preceeding
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fields.
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opc
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The opcode field contains either a 1, for Read Requests, or 2,
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for Write Requests, as defined in [1].
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filename
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The name of the file to be read or written, as defined in [1].
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This is a NULL-terminated field.
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mode
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The mode of the file transfer: "netascii", "octet", or "mail",
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as defined in [1]. This is a NULL-terminated field.
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opt1
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The first option, in case-insensitive ASCII (e.g., "blksize").
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This is a NULL-terminated ASCII field.
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value1
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The value associated with the first option, in case-insensitive
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ASCII. This is a NULL-terminated field.
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optN, valueN
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The final option/value pair. Each NULL-terminated field is
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specified in case-insensitive ASCII.
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The options and values are all NULL-terminated, in keeping with the
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original request format. If multiple options are to be negotiated,
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they are appended to each other. The order in which options are
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specified is not significant. The maximum size of a request packet
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is 512 octets.
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Malkin & Harkin [Page 2]
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RFC 1782 TFTP Option Extension March 1995
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The OACK packet has the following format:
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+-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+
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| opc | opt1 | 0 | value1 | 0 | optN | 0 | valueN | 0 |
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+-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+
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opc
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The opcode field contains a 6, for Option Acknowledgment.
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opt1
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The first option acknowledgment, copied from the original
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request.
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value1
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The acknowledged value associated with the first option. If
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and how this value may differ from the original request is
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detailed in the specification for the option.
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optN, valueN
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The final option/value acknowledgment pair.
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Negotiation Protocol
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The client appends options at the end of the Read Request or Write
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request packet, as shown above. Any number of options may be
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specified; however, an option may only be specified once. The order
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of the options is not significant.
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If the server supports option negotiation, and it recognizes one or
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more of the options specified in the request packet, the server may
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respond with an Options Acknowledgment (OACK). Each option the
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server recognizes, and accepts the value for, is included in the
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OACK. Some options may allow alternate values to be proposed, but
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this is an option specific feature. The server must not include in
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the OACK any option which had not been specifically requested by the
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client; that is, only the client may initiate option negotiation.
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Options which the server does not support should be omitted from the
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OACK; they must not cause an ERROR packet to be generated. If the
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value of a supported option is invalid, the specification for that
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option will indicate whether the server should simply omit the option
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from the OACK, respond with an alternate value, or send an ERROR
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packet, with error code 8, to terminate the transfer.
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An option not acknowledged by the server must be ignored by the
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client and server as if it were never requested. If multiple options
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were requested, the client must use those options which were
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acknowledged by the server and must not use those options which were
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not acknowledged by the server.
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Malkin & Harkin [Page 3]
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RFC 1782 TFTP Option Extension March 1995
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When the client appends options to the end of a Read Request packet,
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three possible responses may be returned by the server:
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OACK - acknowledge of Read Request and the options;
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DATA - acknowledge of Read Request, but not the options;
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ERROR - the request has been denied.
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When the client appends options to the end of a Write Request packet,
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three possible responses may be returned by the server:
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OACK - acknowledge of Write Request and the options;
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ACK - acknowledge of Write Request, but not the options;
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ERROR - the request has been denied.
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If a server implementation does not support option negotiation, it
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will likely ignore any options appended to the client's request. In
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this case, the server will return a DATA packet for a Read Request
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and an ACK packet for a Write Request establishing normal TFTP data
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transfer. In the event that a server returns an error for a request
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which carries an option, the client may attempt to repeat the request
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without appending any options. This implementation option would
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handle servers which consider extraneous data in the request packet
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to be erroneous.
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Depending on the original transfer request there are two ways for a
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client to confirm acceptance of a server's OACK. If the transfer was
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initiated with a Read Request, then an ACK (with the data block
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number set to 0) is sent by the client to confirm the values in the
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server's OACK packet. If the transfer was initiated with a Write
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Request, then the client begins the transfer with the first DATA
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packet, using the negotiated values. If the client rejects the OACK,
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then it sends an ERROR packet, with error code 8, to the server and
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the transfer is terminated.
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Once a client acknowledges an OACK, with an appropriate non-error
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response, that client has agreed to use only the options and values
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returned by the server. Remember that the server cannot request an
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option; it can only respond to them. If the client receives an OACK
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containing an unrequested option, it should respond with an ERROR
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packet, with error code 8, and terminate the transfer.
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Malkin & Harkin [Page 4]
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RFC 1782 TFTP Option Extension March 1995
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Examples
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Read Request
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client server
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-------------------------------------------------------
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|1|foofile|0|octet|0|blksize|0|1432|0| --> RRQ
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<-- |6|blksize|0|1432|0| OACK
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|4|0| --> ACK
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<-- |3|1| 1432 octets of data | DATA
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|4|1| --> ACK
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<-- |3|2| 1432 octets of data | DATA
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|4|2| --> ACK
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<-- |3|3|<1432 octets of data | DATA
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|4|3| --> ACK
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Write Request
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client server
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-------------------------------------------------------
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|2|barfile|0|octet|0|blksize|0|2048|0| --> RRQ
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<-- |6|blksize|0|2048|0| OACK
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|3|1| 2048 octets of data | --> DATA
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<-- |4|1| ACK
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|3|2| 2048 octets of data | --> DATA
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<-- |4|2| ACK
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|3|3|<2048 octets of data | --> DATA
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<-- |4|3| ACK
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Security Considerations
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Security issues are not discussed in this memo.
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References
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[1] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC 1350,
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MIT, July 1992.
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[2] Malkin, G., and A. Harkin, "TFTP Blocksize Option", RFC 1783,
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Xylogics, Inc., Hewlett Packard Co., March 1995.
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[3] Malkin, G., and A. Harkin, A., "TFTP Timeout Interval and
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Transfer Size Options", RFC 1784, Xylogics, Inc., Hewlett Packard
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Co., March 1995.
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Malkin & Harkin [Page 5]
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RFC 1782 TFTP Option Extension March 1995
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Authors' Addresses
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Gary Scott Malkin
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Xylogics, Inc.
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53 Third Avenue
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Burlington, MA 01803
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Phone: (617) 272-8140
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EMail: [email protected]
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Art Harkin
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Internet Services Project
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Information Networks Division
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19420 Homestead Road MS 43LN
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Cupertino, CA 95014
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Phone: (408) 447-3755
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EMail: [email protected]
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Malkin & Harkin [Page 6]
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