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TFTP Windowsize Option

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7440.
Author Patrick Masotta
Last updated 2018-12-20 (Latest revision 2014-10-16)
RFC stream Internet Engineering Task Force (IETF)
IETF conflict review conflict-review-masotta-tftpexts-windowsize-opt, conflict-review-masotta-tftpexts-windowsize-opt, conflict-review-masotta-tftpexts-windowsize-opt, conflict-review-masotta-tftpexts-windowsize-opt, conflict-review-masotta-tftpexts-windowsize-opt, conflict-review-masotta-tftpexts-windowsize-opt, conflict-review-masotta-tftpexts-windowsize-opt
Stream WG state Submitted to IESG for Publication
Document shepherd Joel Jaeggli
Shepherd write-up Show Last changed 2014-09-01
IESG IESG state RFC 7440 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Joel Jaeggli
Send notices to (None)
IANA IANA review state IANA OK - No Actions Needed
IANA action state No IANA Actions
INTERNET-DRAFT                                          Patrick Masotta
Intended status: Standard Track                                   Serva
Expires: Apr 16, 2015                                      Oct 16, 2014

                          TFTP Windowsize Option

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on Feb 16, 2014.

Copyright Notice

   Copyright (c) 2014 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
   ( in effect on the date of
   publication of this document. Please review these documents
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   described in the Simplified BSD License.

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   The Trivial File Transfer Protocol (RFC1350) 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 in the early stages of
   nodes booting from a Local Area Network. TFTP has been used for this
   application because it is very simple to implement. The Employment of
   a lock-step scheme limits throughput when used on a LAN.

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


   This documents and the information contained therein are provided on

Table of Contents

   1. Introduction...................................................3
   2. Conventions used in this document..............................3
   3. Windowsize Option Specification................................3
   4. Traffic Flow and Error Handling................................5
   5. Proof of Concept and Windowsize Evaluation.....................6
   6. Congestion and Error Control...................................8
   7. Security Considerations........................................9
   8. IANA Considerations............................................9
   9. References.....................................................9
      9.1. Normative References......................................9

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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). The TFTP protocol's inherently low
   transfer rate has been so far partially mitigated by the use of the
   blocksize negotiated extension [RFC2348]. This way 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 today far from
   extinction this draft presents a negotiated extension, under the
   terms of the TFTP Option Extension [RFC2347], that produces TFTP
   transfer rates comparable to those achieved today by modern file
   transfer protocols.

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
   RFC-2119 [RFC2119].

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

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

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     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 in-sensitive).

     The base-10 ASCII string representation of the number of blocks in
   a window. Valid values range MUST be between "1" and "65535" blocks,
   inclusive. The windowsize refers to the number of consecutives blocks
   transmitted before stop and wait for the reception of the
   acknowledgment of the last block transmitted.

   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 window-size of 16 blocks (option blocksize is not
   negotiated in this example, 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
   [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.

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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
   consecutives data blocks before to normally stop and wait 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 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 send.

   In case of 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 which
   beginning MUST be set based on the received out of sequence ACK.

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

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

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         [ 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

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.

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          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 an SMB/CIFS
   mapped drive on the same scenario took 23 seconds.

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

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

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   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 RFC 5405 [RFC5405].

   From an error control standpoint while RFC 1350 [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.

   An Implementation example scaled for an Ethernet environment
   (1 Gb/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.

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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 be also 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

   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

8. IANA Considerations

   This document has no actions for IANA.

9. References

9.1. Normative References

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

   [RFC2347] Malkin, G., Harkin, A., "TFTP Option Extension", RFC 2347
             May 1998.

   [RFC2348] Malkin, G., Harkin, A., "TFTP Blocksize option", RFC 2348
             May 1998.

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   [RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
             for Application Designers", BCP 145, RFC 5405, November

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

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Authors' Addresses

   Patrick Masotta
   300 W 11th Avenue,
   Denver, CO 80204

   Email: masotta[-at-]vercot[-dot-]com

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