TFTP Blocksize Option
RFC 2348

Document Type RFC - Draft Standard (May 1998; No errata)
Obsoletes RFC 1783
Updates RFC 1350
Authors Gary Malkin  , Art Harkin 
Last updated 2013-03-02
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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.


   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 |

         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

         The name of the file to be read or written, as defined in [1].

         The mode of the file transfer: "netascii", "octet", or "mail",
         as defined in [1].

         The Blocksize option, "blksize" (case in-sensitive).

         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
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