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Network Working Group                                          G. MalkinRequest for Commments: 2348                                 Bay NetworksUpdates:1350                                                  A. HarkinObsoletes:1783                                      Hewlett Packard Co.Category: Standards Track                                       May 1998TFTP Blocksize OptionStatus 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].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 [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: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   [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.comMalkin & Harkin             Standards Track                     [Page 4]

RFC 2348                 TFTP Blocksize Option                  May 1998Full 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]