TFTP Timeout/Size: TFTP Timeout Interval and Transfer Size Options
Two TFTP options for negotiating a timeout and learning a transfer's size.
Malkin, G. and A. Harkin, "TFTP Timeout Interval and Transfer Size Options", RFC 2349, DOI 10.17487/RFC2349, May 1998, <https://www.rfc-editor.org/info/rfc2349>.
@misc{rfc2349,
series = {Request for Comments},
number = 2349,
howpublished = {RFC 2349},
publisher = {RFC Editor},
doi = {10.17487/RFC2349},
url = {https://www.rfc-editor.org/info/rfc2349},
author = {G. Malkin and A. Harkin},
title = {{TFTP Timeout Interval and Transfer Size Options}},
pagetotal = 5,
year = 1998,
month = may,
} RFC text
Contents
Network Working Group G. Malkin
Request for Commments: 2349 Bay Networks
Updates: 1350 A. Harkin
Obsoletes: 1784 Hewlett Packard Co.
Category: Standards Track May 1998
TFTP Timeout Interval and Transfer Size Options
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 two TFTP options. The first allows the client
and server to negotiate the Timeout Interval. The second allows the
side receiving the file to determine the ultimate size of the
transfer before it begins. The TFTP Option Extension mechanism is
described in [2].
Timeout Interval Option Specification
The TFTP Read Request or Write Request packet is modified to include
the timeout option as follows:
+-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+
| opc |filename| 0 | mode | 0 | timeout| 0 | #secs | 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.
timeout
The Timeout Interval option, "timeout" (case in-sensitive).
This is a NULL-terminated field.
#secs
The number of seconds to wait before retransmitting, specified
in ASCII. Valid values range between "1" and "255" seconds,
inclusive. This is a NULL-terminated field.
For example:
+-------+--------+---+--------+---+--------+---+-------+---+
| 1 | foobar | 0 | octet | 0 | timeout| 0 | 1 | 0 |
+-------+--------+---+--------+---+--------+---+-------+---+
is a Read Request, for the file named "foobar", in octet (binary)
transfer mode, with a timeout interval of 1 second.
If the server is willing to accept the timeout option, it sends an
Option Acknowledgment (OACK) to the client. The specified timeout
value must match the value specified by the client.
Transfer Size Option Specification
The TFTP Read Request or Write Request packet is modified to include
the tsize option as follows:
+-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+
| opc |filename| 0 | mode | 0 | tsize | 0 | size | 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.
tsize
The Transfer Size option, "tsize" (case in-sensitive). This is
a NULL-terminated field.
size
The size of the file to be transfered. This is a NULL-
terminated field.
For example:
+-------+--------+---+--------+---+--------+---+--------+---+
| 2 | foobar | 0 | octet | 0 | tsize | 0 | 673312 | 0 |
+-------+--------+---+--------+---+--------+---+--------+---+
is a Write Request, with the 673312-octet file named "foobar", in
octet (binary) transfer mode.
In Read Request packets, a size of "0" is specified in the request
and the size of the file, in octets, is returned in the OACK. If the
file is too large for the client to handle, it may abort the transfer
with an Error packet (error code 3). In Write Request packets, the
size of the file, in octets, is specified in the request and echoed
back in the OACK. If the file is too large for the server to handle,
it may abort the transfer with an Error packet (error code 3).
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 92.
[2] Malkin, G., and A. Harkin, "TFTP Option Extension", RFC 2347,
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
Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
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kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
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ISO Plain Language
On this page
§1Two TFTP Options
The Trivial File Transfer Protocol (TFTP) lets a client get or put a file on a remote host. RFC 2349 describes two options: one for negotiating the timeout interval and one for telling the receiving side the transfer size before it starts.
The options are included in TFTP read-request or write-request packets, using the option extension mechanism described by RFC 2347.
The options add information to a TFTP file request. They let both sides agree on a timeout or learn the file size before transferring the file.
§2Negotiating the Timeout
The timeout option gives the number of seconds to wait before retransmitting. Its value is sent in ASCII and must be from 1 through 255 inclusive. The option name is case-insensitive.
If the server accepts the option, it replies with an Option Acknowledgment (OACK). The timeout value in the OACK must match the value sent by the client.
The client proposes how many seconds to wait before sending again. If the server accepts the option, its OACK repeats the same value.
§3Reporting the Transfer Size
The tsize option gives the file size in octets. In a read request, the client sends a size of zero and the server returns the actual size in the OACK. In a write request, the client sends the size and the server echoes it in the OACK.
If the file is too large for the receiving side, that side may abort with an Error packet using error code 3.
For a read request, the server reports the file size. For a write request, the client reports it. The receiver may stop the transfer if the file is too large.
- Read request: tsize = 0
- OACK: actual size
- Error code 3 (if too large)
§4Security Considerations
The RFC says basic TFTP has no security mechanism. The described options do not add security, but the RFC states that they do not add additional security risks.
These options do not add security to TFTP. The RFC says they also introduce no additional security risks.