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Internet Engineering Task Force (IETF)                        P. Masotta
Request for Comments: 7440                                         Serva
Category: Standards Track                                   January 2015
ISSN: 2070-1721

                         TFTP Windowsize Option


   The "Trivial File Transfer Protocol" (RFC 1350) is a simple,
   lockstep, file transfer protocol that allows a client to get or put a
   file onto a remote host.  One of its primary uses is in the early
   stages of nodes booting from a Local Area Network (LAN).  TFTP has
   been used for this application because it is very simple to
   implement.  The employment of a lockstep scheme limits throughput
   when used on a LAN.

   This document describes a TFTP option that allows the client and
   server to negotiate a window size of consecutive blocks to send as an
   alternative for replacing the single-block lockstep schema.  The TFTP
   option mechanism employed is described in "TFTP Option Extension"
   (RFC 2347).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 7440                 TFTP Windowsize Option             January 2015

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................3
   3. Windowsize Option Specification .................................3
   4. Traffic Flow and Error Handling .................................4
   5. Proof of Concept and Windowsize Evaluation ......................6
   6. Congestion and Error Control ....................................7
   7. Security Considerations .........................................8
   8. References ......................................................9
      8.1. Normative References .......................................9
   Author's Address ...................................................9

1.  Introduction

   TFTP is virtually unused for Internet transfers today, TFTP is still
   massively used in network boot/installation scenarios including EFI
   (Extensible Firmware Interface).  TFTP's inherently low transfer rate
   has been, so far, partially mitigated by the use of the blocksize
   negotiated extension [RFC2348].  Using this method, the original
   limitation of 512-byte blocks are, in practice, replaced in Ethernet
   environments by blocks no larger than 1468 Bytes to avoid IP block
   fragmentation.  This strategy produces insufficient results when
   transferring big files, for example, the initial ramdisk of Linux
   distributions or the PE images used in network installations by
   Microsoft WDS/MDT/SCCM.  Considering TFTP looks far from extinction
   today, this document presents a negotiated extension, under the terms
   of the "TFTP Option Extension" [RFC2347], that produces TFTP transfer
   rates comparable to those achieved by modern file transfer protocols.

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RFC 7440                 TFTP Windowsize Option             January 2015

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

   In this document, these words will appear with that interpretation
   only when in ALL CAPS.  Lowercase uses of these words are not to be
   interpreted as carrying the significance given in RFC 2119.

3.  Windowsize Option Specification

   The TFTP Read Request or Write Request packet is modified to include
   the windowsize option as follows.  Note that all fields except "opc"
   MUST be ASCII strings followed by a single-byte NULL character.

      2B     string   1B   string   1B     string     1B   string   1B
   |  opc  |filename|  0 |  mode  |  0 | windowsize |  0 | #blocks|  0 |

      The opcode field contains either a 1 for Read Requests or a 2 for
      Write Requests, as defined in [RFC1350].

      The name of the file to be read or written, as defined in

      The mode of the file transfer: "netascii", "octet", or "mail", as
      defined in [RFC1350].

      The windowsize option, "windowsize" (case insensitive).

      The base-10 ASCII string representation of the number of blocks in
      a window.  The valid values range MUST be between 1 and 65535
      blocks, inclusive.  The windowsize refers to the number of
      consecutive blocks transmitted before stopping and waiting for the
      reception of the acknowledgment of the last block transmitted.

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RFC 7440                 TFTP Windowsize Option             January 2015

   For example:

   |0x0001| foobar |0x00| octet |0x00| windowsize |0x00| 16 |0x00|

   is a Read Request for the file named "foobar" in octet transfer mode
   with a windowsize of 16 blocks (option blocksize is not negotiated in
   this example, the default of 512 Bytes per block 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
   [RFC1350] and [RFC2348].

   The reception of a data window with a number of blocks less than the
   negotiated windowsize is the final window.  If the windowsize is
   greater than the amount of data to be transferred, the first window
   is the final window.

4.  Traffic Flow and Error Handling

   The next diagram depicts a section of the traffic flow between the
   Data Sender (DSND) and the Data Receiver (DRCV) parties on a generic
   windowsize TFTP file transfer.

   The DSND MUST cyclically send to the DRCV the agreed windowsize
   consecutive data blocks before normally stopping and waiting for the
   ACK of the transferred window.  The DRCV MUST send to the DSND the
   ACK of the last data block of the window in order to confirm a
   successful data block window reception.

   In the case of an expected ACK not timely reaching the DSND
   (timeout), the last received ACK SHALL set the beginning of the next
   windowsize data block window to be sent.

   In the case of a data block sequence error, the DRCV SHOULD notify
   the DSND by sending an ACK corresponding to the last data block
   correctly received.  The notified DSND SHOULD send a new data block
   window whose beginning MUST be set based on the ACK received out of

   Traffic with windowsize = 1 MUST be equivalent to traffic specified
   by [RFC1350].

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RFC 7440                 TFTP Windowsize Option             January 2015

   For normative traffic not specifically addressed in this section,
   please refer to [RFC1350] and its updates.

         [ DRCV ]      <---traffic--->      [ DSND ]
           ACK#      ->               <-   Data Block#   window block#
                              <-           |DB n+01|          1
                              <-           |DB n+02|          2
                              <-           |DB n+03|          3
                              <-           |DB n+04|          4
         |ACK n+04|           ->
                              <-           |DB n+05|          1
                       Error |<-           |DB n+06|          2
                              <-           |DB n+07|          3
         |ACK n+05|           ->
                              <-           |DB n+06|          1
                              <-           |DB n+07|          2
                              <-           |DB n+08|          3
                              <-           |DB n+09|          4
         |ACK n+09|           ->
                              <-           |DB n+10|          1
                       Error |<-           |DB n+11|          2
                              <-           |DB n+12|          3
         |ACK n+10|           ->| Error
                              <-           |DB n+13|          4
                                          - timeout -
                              <-           |DB n+10|          1
                              <-           |DB n+11|          2
                              <-           |DB n+12|          3
                              <-           |DB n+13|          4
         |ACK n+13|           ->

                 Section of a Windowsize = 4 TFTP Transfer
                    Including Errors and Error Recovery

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RFC 7440                 TFTP Windowsize Option             January 2015

5.  Proof of Concept and Windowsize Evaluation

   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 laptops, in "octet" mode,
   transferring a 180 MByte file.

          300 +
      Seconds |                           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 (in Blocks of 1456 Bytes)

   Comparatively, the same 180 MB transfer performed over a drive mapped
   on Server Message Block (SMB) / Common Internet File System (CIFS) on
   the same scenario took 23 seconds.

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RFC 7440                 TFTP Windowsize Option             January 2015

   The comparison of transfer times (without a gateway) between the
   standard lockstep schema and the negotiated windowsizes are:

               Windowsize  |  Time Reduction (%)
               ----------     -----------------
                    1              -0%
                    2             -49%
                    4             -70%
                    8             -79%
                   16             -84%
                   32             -85%
                   64             -86%

   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 acknowledgements exchanged.

   The choice of appropriate windowsize values on a particular scenario
   depends on the underlying networking technology and topology, and
   likely other factors as well.  Operators SHOULD test various values
   and SHOULD be conservative when selecting a windowsize value because
   as the former table and chart shows, there is a point where the
   benefit of continuing to increase the windowsize is subject to
   diminishing returns.

6.  Congestion and Error Control

   From a congestion control (CC) standpoint, the number of blocks in a
   window does not pose an intrinsic threat to the ability of
   intermediate devices to signal congestion through drops.  The rate at
   which TFTP UDP datagrams are sent SHOULD follow the CC guidelines in
   Section 3.1 of [RFC5405].

   From an error control standpoint, while [RFC1350] and subsequent
   updates do not specify a circuit breaker (CB), existing
   implementations have always chosen to fail under certain
   circumstances.  Implementations SHOULD always set a maximum number of
   retries for datagram retransmissions, imposing an appropriate
   threshold on error recovery attempts, after which a transfer SHOULD
   always be aborted to prevent pathological retransmission conditions.

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RFC 7440                 TFTP Windowsize Option             January 2015

   An implementation example scaled for an Ethernet environment
   (1 Gbit/s, MTU=1500) would be to set:

   windowsize = 8
   blksize = 1456
   maximum retransmission attempts per block/window = 6
   timeout between retransmissions = 1 S
   minimum inter-packet delay = 80 uS

   Implementations might well choose other values based on expected
   and/or tested operating conditions.

7.  Security Considerations

   TFTP includes no login or access control mechanisms.  Care must be
   taken when using TFTP for file transfers where authentication, access
   control, confidentiality, or integrity checking are needed.  Note
   that those security services could be supplied above or below the
   layer at which TFTP runs.  Care must also be taken in the rights
   granted to a TFTP server process so as not to violate the security of
   the server's file system.  TFTP is often installed with controls such
   that only files that have public read access are available via TFTP.
   Also listing, deleting, renaming, and writing files via TFTP are
   typically disallowed.  TFTP file transfers are NOT RECOMMENDED where
   the inherent protocol limitations could raise insurmountable
   liability concerns.

   TFTP includes no protection against an on-path attacker; care must be
   taken in controlling windowsize values according to data sender, data
   receiver, and network environment capabilities.  TFTP service is
   frequently associated with bootstrap and initial provisioning
   activities; servers in such an environment are in a position to
   impose device or network specific throughput limitations as

   This document does not add any security controls to TFTP; however,
   the specified extension does not pose additional security risks

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RFC 7440                 TFTP Windowsize Option             January 2015

8.  References

8.1.  Normative References

   [RFC1350]   Sollins, K., "The TFTP Protocol (Revision 2)", STD 33,
               RFC 1350, July 1992,

   [RFC2347]   Malkin, G. and A. Harkin, "TFTP Option Extension", RFC
               2347, May 1998, <http://www.rfc-editor.org/info/rfc2347>.

   [RFC2348]   Malkin, G. and A. Harkin, "TFTP Blocksize Option", RFC
               2348, May 1998, <http://www.rfc-editor.org/info/rfc2348>.

   [RFC5405]   Eggert, L. and G. Fairhurst, "Unicast UDP Usage
               Guidelines for Application Designers", BCP 145, RFC 5405,
               November 2008, <http://www.rfc-editor.org/info/rfc5405>.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997,

Author's Address

   Patrick Masotta

   EMail: patrick.masotta.ietf@vercot.com
   URI:   http://www.vercot.com/~serva/

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