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Network Working Group                                          G. MalkinRequest for Comments: 1783                                Xylogics, Inc.Updates:1350                                                  A. HarkinCategory: Standards Track                            Hewlett Packard Co.                                                              March 1995TFTP Blocksize OptionStatus 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 1995Proof 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                          1432                    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.References   [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 1995Authors' Addresses       Gary Scott Malkin       Xylogics, Inc.       53 Third Avenue       Burlington, MA  01803       Phone:  (617) 272-8140       EMail:  gmalkin@xylogics.com       Art Harkin       Internet Services Project       Information Networks Division       19420 Homestead Road MS 43LN       Cupertino, CA  95014       Phone: (408) 447-3755       EMail: ash@cup.hp.comMalkin & Harkin                                                 [Page 5]