Network Working Group G. Malkin
Request for Comments:
1783 Xylogics, Inc.
Updates:
1350 A. Harkin
Category: Standards Track Hewlett Packard Co.
March 1995
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.
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-byte blocksize is not the most efficient for use
on a LAN whose MTU may 1500 bytes 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:
+-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+
| 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].
This is a NULL-terminated field.
The comparisons between transfer times (without a gateway) between
the standard 512-byte blocksize and the negotiated blocksizes are:
1024 2x -32%
1432 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 bytes to 1024 bytes, 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
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.