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RFC 1783

Updates RFC 1350
Obsoleted by RFC 2348

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.


   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 |

         The opcode field contains either a 1, for Read Requests, or 2,
         for Write Requests, as defined in [1].

         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

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

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

         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
      c    |                             1432       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
                    blocksize (bytes)

Malkin & Harkin                                                 [Page 3]

RFC 1783                 TFTP Blocksize Option                March 1995

   The comparisons between transfer times (without a gateway) between
   the standard 512-byte blocksize and the negotiated blocksizes are:

      1024     2x   -32%
      1432   2.8x   -42%
      2048     4x   -54%
      4096     8x   -71%
      8192    16x   -80%

   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 bytes to 1024 bytes, 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

   Security issues are not discussed in this memo.


   [1] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC 1350,
       MIT, July 1992.

   [2] Malkin, G., and A. Harkin, "TFTP Option Extension", RFC 1782,
       Xylogics, Inc., Hewlett Packard Co., March 1995.

Malkin & Harkin                                                 [Page 4]

RFC 1783                 TFTP Blocksize Option                March 1995

Authors' Addresses

       Gary Scott Malkin
       Xylogics, Inc.
       53 Third Avenue
       Burlington, MA  01803

       Phone:  (617) 272-8140

       Art Harkin
       Internet Services Project
       Information Networks Division
       19420 Homestead Road MS 43LN
       Cupertino, CA  95014

       Phone: (408) 447-3755

Malkin & Harkin                                                 [Page 5]