TFTP Blocksize Option
Lets a TFTP client and server agree on a bigger block size than the default 512 octets.
Malkin, G. and A. Harkin, "TFTP Blocksize Option", RFC 2348, DOI 10.17487/RFC2348, May 1998, <https://www.rfc-editor.org/info/rfc2348>.
@misc{rfc2348,
series = {Request for Comments},
number = 2348,
howpublished = {RFC 2348},
publisher = {RFC Editor},
doi = {10.17487/RFC2348},
url = {https://www.rfc-editor.org/info/rfc2348},
author = {G. Malkin and A. Harkin},
title = {{TFTP Blocksize Option}},
pagetotal = 5,
year = 1998,
month = may,
} RFC text
Contents
Network Working Group G. Malkin
Request for Commments: 2348 Bay Networks
Updates: 1350 A. Harkin
Obsoletes: 1783 Hewlett Packard Co.
Category: Standards Track May 1998
TFTP Blocksize Option
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. One of its primary uses is the booting of diskless
nodes on a Local Area Network. TFTP is used because it is very
simple to implement in a small node's limited ROM space. However,
the choice of a 512-octet blocksize is not the most efficient for use
on a LAN whose MTU may 1500 octets or greater.
This document describes a TFTP option which allows the client and
server to negotiate a blocksize more applicable to the network
medium. The TFTP Option Extension mechanism is described in [2].
Blocksize Option Specification
The TFTP Read Request or Write Request packet is modified to include
the blocksize option as follows. Note that all fields except "opc"
are NULL-terminated.
+-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+
| opc |filename| 0 | mode | 0 | blksize| 0 | #octets| 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].
mode
The mode of the file transfer: "netascii", "octet", or "mail",
as defined in [1].
blksize
The Blocksize option, "blksize" (case in-sensitive).
#octets
The number of octets in a block, specified in ASCII. Valid
values range between "8" and "65464" octets, inclusive. The
blocksize refers to the number of data octets; it does not
include the four octets of TFTP header.
For example:
+-------+--------+---+--------+---+--------+---+--------+---+
| 1 | foobar | 0 | octet | 0 | blksize| 0 | 1428 | 0 |
+-------+--------+---+--------+---+--------+---+--------+---+
is a Read Request, for the file named "foobar", in octet (binary)
transfer mode, with a block size of 1428 octets (Ethernet MTU, less
the TFTP, UDP and IP header lengths).
If the server is willing to accept the blocksize option, it sends an
Option Acknowledgment (OACK) to the client. The specified value must
be less than or equal to the value specified by the client. The
client must then either use the size specified in the OACK, or send
an ERROR packet, with error code 8, to terminate the transfer.
The rules for determining the final packet are unchanged from [1].
The reception of a data packet with a data length less than the
negotiated blocksize is the final packet. If the blocksize is
greater than the amount of data to be transfered, the first packet is
the final packet. If the amount of data to be transfered is an
integral multiple of the blocksize, an extra data packet containing
no data is sent to end the transfer.
Proof of Concept
Performance tests were run on the prototype implementation using a
variety of block sizes. The tests were run on a lightly loaded
Ethernet, between two HP-UX 9000, in "octet" mode, on 2.25MB files.
The average (5x) transfer times for paths with (g-time) and without
(n-time) a intermediate gateway are graphed as follows:
|
37 + g
|
35 +
|
33 +
|
31 +
|
29 +
|
27 +
| g blocksize n-time g-time
25 + --------- ------ ------
s | n 512 23.85 37.05
e 23 + g 1024 16.15 25.65
c | 1428 13.70 23.10
o 21 + 2048 10.90 16.90
n | 4096 6.85 9.65
d 19 + 8192 4.90 6.15
s |
17 + g
| n
15 +
| n
13 +
|
11 + n
| g
9 +
|
7 + n
| g
5 + n
"
0 +------+------+--+---+------+------+---
512 1K | 2K 4K 8K
1428
blocksize (octets)
The comparisons between transfer times (without a gateway) between
the standard 512-octet blocksize and the negotiated blocksizes are:
1024 2x -32%
1428 2.8x -42%
2048 4x -54%
4096 8x -71%
8192 16x -80%
As was anticipated, the transfer time decreases with an increase in
blocksize. The reason for the reduction in time is the reduction in
the number of packets sent. For example, by increasing the blocksize
from 512 octets to 1024 octets, not only are the number of data
packets halved, but the number of acknowledgement packets is also
halved (along with the number of times the data transmitter must wait
for an ACK). A secondary effect is the efficiency gained by reducing
the per-packet framing and processing overhead.
Of course, if the blocksize exceeds the path MTU, IP fragmentation
and reassembly will begin to add more overhead. This will be more
noticable the greater the number of gateways in the path.
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 Option Extension", RFC 2347,
May 1998.
Authors' Addresses
Gary Scott Malkin
Bay Networks
8 Federal Street
Billerica, MA 10821
Phone: (978) 916-4237
EMail: gmalkin@baynetworks.com
Art Harkin
Networked Computing Division
Hewlett-Packard Company
19420 Homestead Road MS 43LN
Cupertino, CA 95014
Phone: (408) 447-3755
EMail: ash@cup.hp.com
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AI ELI5
On this page
§1Why Bigger Blocks
RFC 1350 defines TFTP, a simple, lock-step file transfer protocol. One of its main uses is booting diskless nodes on a local area network, and it is popular because it is small enough to fit in a tiny node's ROM space.
The catch is the fixed 512-octet block size. That is not the most efficient choice on a LAN whose MTU may be 1500 octets or greater. This RFC adds a TFTP option, built on the option extension mechanism of RFC 2347, so the client and server can negotiate a block size that suits the network medium.
Sending a big file in tiny envelopes takes a lot of trips. This note lets both sides agree to use bigger envelopes.
§2The Blocksize Option
The read request or write request packet gets two extra NULL-terminated fields after the mode: the option name blksize (case-insensitive) and the number of octets, written in ASCII. Valid values run from 8 to 65464 octets, inclusive. The block size counts data octets only, not the four octets of TFTP header.
The RFC's example is a read request (opcode 1) for the file foobar in octet mode with a block size of 1428, which is the Ethernet MTU less the TFTP, UDP and IP header lengths.
You write a note on the request: "please use pieces this big." The number can be small or very large, but not outside the allowed range.
§3Negotiating and Finishing
If the server accepts the option, it sends an Option Acknowledgment (OACK). The value in the OACK must be less than or equal to the value the client asked for. The client must then either use the size in the OACK or send an ERROR packet with error code 8 to end the transfer.
The rule for the final packet is unchanged from RFC 1350: a data packet shorter than the negotiated block size is the last one. If the file is smaller than the block size, the first packet is the last. If the file is an exact multiple of the block size, an extra data packet with no data ends the transfer.
You ask for big pieces and the other side can say "that big or smaller." You either accept or walk away. The last piece is always the one that is not full.
§4Measured Speedup and Security
The authors tested a prototype between two HP-UX 9000 machines in octet mode, using 2.25MB files on a lightly loaded Ethernet. Without a gateway, going from 512 to 8192 octets cut the transfer time by 80%. The gain comes mainly from sending fewer packets: doubling the block size halves both the data packets and the acknowledgements, and the sender waits for an ACK half as often. A smaller effect is less per-packet framing and processing overhead.
If the block size exceeds the path MTU, IP fragmentation and reassembly add overhead, and this is more noticeable the more gateways are on the path. On security, TFTP has none and this document adds none, but the RFC says the extensions add no additional security risks.
Fewer, bigger boxes means less waiting for "got it!" messages. But if a box is too big for the road, it has to be cut up and glued back, which slows things down.
| Block size (octets) | Time (seconds) | Change vs 512 |
|---|---|---|
| 512 | 23.85 | baseline |
| 1024 | 16.15 | -32% |
| 1428 | 13.70 | -42% |
| 2048 | 10.90 | -54% |
| 4096 | 6.85 | -71% |
| 8192 THIS RFC | 4.90 | -80% |