RFC 2347
TFTP Option Extension
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
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:
+-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+
| 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.
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.
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.
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
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