TFTP Timeout/Size: TFTP Timeout Interval and Transfer Size Options
Two TFTP options: agree on the retransmission timeout, and learn the file size before the transfer starts.
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
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AI ELI5
On this page
§1Two Small Extensions
TFTP is a simple, lock-step protocol that lets a client get or put a file on a remote host. This RFC describes two options for it. The first lets the client and server negotiate the timeout interval. The second lets the side receiving the file learn the final size of the transfer before it begins.
Both use the TFTP option extension mechanism of RFC 2347: extra NULL-terminated name and value fields appended to the read request or write request.
Two little notes you can add to a file request: "wait this long before nagging me" and "the file is this big."
§2The Timeout Interval Option
After the mode, the request carries the option name timeout (case-insensitive) and a number of seconds in ASCII. Valid values are 1 to 255, inclusive. The number says how long to wait before retransmitting.
If the server accepts, it replies with an Option Acknowledgment (OACK), and the timeout value in it must match the value the client sent. The RFC's example is a read request for foobar in octet mode with a timeout of 1 second.
You say how long you will wait before asking again, and the other side has to agree to exactly that number.
§3The Transfer Size Option
The request carries tsize (case-insensitive) and a size in ASCII. In a read request the client sends a size of 0, and the server returns the real file size in octets in its OACK. In a write request the client states the file size in octets, and the server echoes it back in the OACK.
If the file is too large for the receiving side to handle, that side may abort the transfer with an ERROR packet using error code 3. The RFC's example is a write request for the 673312-octet file foobar.
Before the boxes arrive, you learn how many there will be. If your room is too small, you can say no right away.
- Read Request, tsize = 0The client asks for the file and sends a size of 0.
- OACK, tsize = file sizeThe server returns the file size in octets.
- ERROR code 3 (optional)The client may abort if the file is too large for it.
§4Security
The basic TFTP protocol has no security mechanism, which is why it has no rename, delete or file overwrite capabilities. This document adds no security to TFTP. The RFC states that the specified extensions do not add any additional security risks.
TFTP never had locks on its doors, and these notes do not add any, but they do not open any new doors either.