TFTP Windowsize Option
draft-masotta-tftpexts-windowsize-opt-00

draft-masotta-tftpexts-windowsize-opt-00.txt        Patrick Masotta/Vercot
Intended status: Proposed Standard                    
INTERNET-DRAFT                                            March 2012
Expires: Aug 07, 2012

                         TFTP Windowsize Option

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 lock-step schema is not the most efficient for use
   on a LAN.

   This document describes a TFTP option which allows the client and
   server to negotiate a windowsize of consecutive blocks to send 
   as an alternative for replacing the lock-step single block schema. 
   The TFTP Option Extension mechanism is described in [2].

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   new BCP 42 obsoleting RFC 6195. Comments should be sent to the DNS
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Windowsize 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.

Patrick Masotta          Expires Aug 07, 2012                  [Page 1]
Internet Draft           TFTP Windowsize Option              March 2012

   +-------+---~~---+---+---~~---+---+-----~~-----+---+---~~---+---+
   |  opc  |filename| 0 |  mode  | 0 | windowsize | 0 | #blocks| 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].

      windowsize
         The Windowsize option, "windowsize" (case in-sensitive).

      #blocks
         The number of blocks in a window, specified in ASCII.  Valid
         values range between "1" and "65464" blocks, inclusive.  The
         windowsize refers to the number of consecutives blocks 
         transmited before stop and wait for the reception of the 
         ack of the last block transmited.

   For example:

      +-------+--------+---+--------+---+------------+---+------+---+
      |   1   | foobar | 0 | binary | 0 | windowsize | 0 |  16  | 0 |
      +-------+--------+---+--------+---+------------+---+------+---+

   is a Read Request, for the file named "foobar", in binary transfer
   mode, with a window size of 16 blocks (as blocksize is not defined
   the 512 Bytes per block default applies).

   If the server is willing to accept the windowsize 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 window with a number of blocks length less
   than the negotiated windowsize is the final window. If the 
   windowsize is greater than the amount of data to be transfered, 
   the first window is the final window. If the amount of data to be
   transfered is an integral multiple of the windowsize, an extra 
   data packet containing no data is sent to end the transfer.

Patrick Masotta          Expires Aug 07, 2012                  [Page 2]
Internet Draft           TFTP Windowsize Option             March 2012

Proof of Concept

   Performance tests were run on the prototype implementation using a
   variety of windowsizes and a fixed blocksize of 1456 bytes.  The 
   tests were run on a lightly loaded Gigabit Ethernet, between two
   Toshiba Tecra Core 2 Duo 2.2 Ghz, in "octet" mode, on 180MB files.

   Seconds |
           |
       300 +
           |                           windowsize   time(s)
           |                            ---------   ------
           |    x                          1         257
       250 +                               2         131
           |                               4          76
           |                               8          54
           |                              16          42
       200 +                              32          38
           |                              64          35
           |           
           |             
       150 +
           |                
           |          x
           |
       100 +                    
           |                           
           |                x
           |
        50 +                      x                           
           |                            x           
           |                                  x     x
           |                                        
         0 +----+-----+-----+-----+-----+-----+-----+--
                1     2     4     8    16    32   64 

                   windowsize (blocks of 1456 bytes)

Patrick Masotta          Expires Aug 07, 2012                   [Page 3]
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   The comparisons between transfer times (without a gateway) between
   the standard lock-step schema and the negotiated windowsizes are:

              1      -0%
              2     -49%
              4     -70%
              8     -79%
             16     -84%
             32     -85%
             64     -86%

   As was anticipated, the transfer time decreases with the use of a  
   windowed schema. The reason for the reduction in time is the 
   reduction in the number of the required synchronous acknowledgement
   exchanged.

Error Handling
   In case of an error detection the whole windowsize window is 
   retransmited.

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)", Request for
        Comments 1350 (STD 33), October 1992.

   [2]  Malkin, G., Harkin, A., "TFTP Option Extension", RFC 1782 March
        1995.

Authors' Addresses

   Patrick Masotta
   300 W 11th Avenue, #9A
   Denver, CO  80204

   EMail: masotta@vercot.com

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