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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.


 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]

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