TFTP Blocksize Option
Author(s): A. Harkin, G. Malkin
replaced: RFC 1783
The Trivial File Transfer Protocol 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 a TFTP option which allows the client and server to...
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  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 . 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 . Malkin & Harkin Standards Track [Page 1] RFC 2348 TFTP Blocksize Option May 1998 filename The name of the file to be read or written, as defined in . mode The mode of the file transfer: "netascii", "octet", or "mail", as defined in . 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 . 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: Malkin & Harkin Standards Track [Page 2] RFC 2348 TFTP Blocksize Option May 1998 | 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% Malkin & Harkin Standards Track [Page 3] RFC 2348 TFTP Blocksize Option May 1998 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  Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC 1350, October 1992.  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: email@example.com Art Harkin Networked Computing Division Hewlett-Packard Company 19420 Homestead Road MS 43LN Cupertino, CA 95014 Phone: (408) 447-3755 EMail: firstname.lastname@example.org Malkin & Harkin Standards Track [Page 4] RFC 2348 TFTP Blocksize Option May 1998 Full Copyright Statement Copyright (C) The Internet Society (1998). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Malkin & Harkin Standards Track [Page 5]