rfc2348 - TFTP Blocksize Option
Description: TFTP Blocksize Option
Date: May 1998

Internet RFC/STD/FYI/BCP Archives

# RFC2348

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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.

    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].

          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:

               |
            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%

       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

    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.

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