TFTP Blocksize Option
RFC 1783

Document Type RFC - Proposed Standard (March 1995; No errata)
Obsoleted by RFC 2348
Updates RFC 1350
Last updated 2013-03-02
Stream IETF
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IESG IESG state RFC 1783 (Proposed Standard)
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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.

Malkin & Harkin                                                 [Page 1]
RFC 1783                 TFTP Blocksize Option                March 1995

      mode
         The mode of the file transfer: "netascii", "octet", or "mail",
         as defined in [1].  This is a NULL-terminated field.

      blksize
         The Blocksize option, "blksize" (case insensitive).  This is a
         NULL-terminated field.

      #octets
         The number of octets in a block, specified in ASCII.  Valid
         values range between "8" and "65464" octets, inclusive.  This
         is a NULL-terminated field.

   For example:

      +-------+--------+---+--------+---+--------+---+--------+---+
      |   1   | foobar | 0 | binary | 0 | blksize| 0 |  1432  | 0 |
      +-------+--------+---+--------+---+--------+---+--------+---+

   is a Read Request, for the file named "foobar", in binary transfer
   mode, with a block size of 1432 bytes (Ethernet MTU, less the 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 size of the packet, the first packet is the final
   packet.  If 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.

Malkin & Harkin                                                 [Page 2]
RFC 1783                 TFTP Blocksize Option                March 1995

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:

           |
        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
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