TFTP Timeout/Tsize: TFTP Timeout Interval and Transfer Size Options
Negotiate timeout and pre-transfer file size in TFTP.
Malkin, G. and A. Harkin, "TFTP Timeout Interval and Transfer Size Options", RFC 1784, DOI 10.17487/RFC1784, March 1995, <https://www.rfc-editor.org/info/rfc1784>.
@misc{rfc1784,
series = {Request for Comments},
number = 1784,
howpublished = {RFC 1784},
publisher = {RFC Editor},
doi = {10.17487/RFC1784},
url = {https://www.rfc-editor.org/info/rfc1784},
author = {G. Malkin and A. Harkin},
title = {{TFTP Timeout Interval and Transfer Size Options}},
pagetotal = 4,
year = 1995,
month = mar,
} RFC text
Contents
Network Working Group G. Malkin
Request for Comments: 1784 Xylogics, Inc.
Updates: 1350 A. Harkin
Category: Standards Track Hewlett Packard Co.
March 1995
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.
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].
This document assumes that the reader is familiar with the
terminology and notation of both [1] and [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 insensitive).
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" octets,
inclusive. This is a NULL-terminated field.
For example:
+-------+--------+---+--------+---+--------+---+--------+---+
| 1 | foobar | 0 | binary | 0 | timeout| 0 | 1 | 0 |
+-------+--------+---+--------+---+--------+---+--------+---+
is a Read Request, for the file named "foobar", in 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 insensitive). This is
a NULL-terminated field.
size
The size of the file to be transfered, specified as a
NULL-terminated ASCII string.
For example:
+-------+--------+---+--------+---+--------+---+--------+---+
| 2 | foobar | 0 | binary | 0 | tsize | 0 | 673312 | 0 |
+-------+--------+---+--------+---+--------+---+--------+---+
is a Write Request, with the 673312-octet file named "foobar", in
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
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 Option Extension", RFC 1782,
Xylogics, Inc., Hewlett Packard Co., March 1995.
Authors' 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.com
AI ELI5
On this page
§1TFTP Extensions for Flexibility
TFTP is simple and efficient for booting diskless machines, but it has limitations. The protocol uses a fixed retransmission timeout that may not suit all networks: satellite links need longer timeouts than LANs, and there is no standard way to determine file size before transfer begins. This RFC defines two options to address these issues.
Both options use the TFTP Option Extension mechanism, which allows client and server to negotiate optional parameters in the initial Read or Write Request and Option Acknowledgment (OACK) packets.
TFTP is simple, but sometimes you need more flexibility. Two new options let you pick the timeout and learn the file size upfront.
§2Timeout and Transfer Size Options
The timeout option ("timeout") specifies the number of seconds to wait before retransmitting (valid values 1 to 255). Both client and server must agree on the same timeout; the server's OACK must echo the client's proposed value. For example, a client might request a 1-second timeout on a fast LAN or 30 seconds for a slow wide-area link.
The tsize option ("tsize") negotiates file size. In Read Requests, the client sends tsize "0" and the server responds with the actual file size in bytes. In Write Requests, the client provides the file size, and the server echoes it back. If either side finds the size unacceptable (too large to store), they can abort with an ERROR packet (error code 3).
The timeout option is a number: how many seconds to wait for an answer before trying again. The tsize option lets you ask how big the file is before you start receiving it.
§3Transfer Control and Error Handling
Once the client and server agree on these options via OACK, the transfer proceeds using the negotiated timeout and the receiver knows the total size upfront. For tsize in Write requests, the client supplies the size and the server echoes it; if the server cannot accommodate the file, it aborts with error code 3 (Disk full). For tsize in Read requests, the server tells the client how large the file is; if the client cannot handle it, the client aborts with the same error code.
The TFTP Option Extension mechanism ensures both ends agree on all options before data transfer begins, avoiding compatibility problems and allowing graceful fallback if options are not supported.
Before sending data, the client and server agree on timeout and size. If the receiver does not have room for the file, it says so before wasting time and bandwidth.