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rfc:rfc2348

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].

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.com

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]

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