TFTP Windowsize Option
draft-masotta-tftpexts-windowsize-opt-11

INTERNET-DRAFT                                          Patrick Masotta
Intended status: Standard Track                                   Serva
Expires: Feb 16, 2015                                      Aug 16, 2014

                          TFTP Windowsize Option
               draft-masotta-tftpexts-windowsize-opt-11.txt

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Abstract

   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
   (RFC2347).

Legal

   This documents and the information contained therein are provided on
   an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
   IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
   WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
   WARRANTY THAT THE USE OF THE INFORMATION THEREIN WILL NOT INFRINGE
   ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
   FOR A PARTICULAR PURPOSE.

Table of Contents

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

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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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "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 MUST be modified to
   include the windowsize option as follows.  Note that all fields
   except "opc" MUST be NULL-terminated.

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

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

   filename
     The name of the file to be read or written, as defined in
   [RFC1350].

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

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

   #blocks
     The number of blocks in a window MUST be specified in ASCII. 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:

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

   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 MUST
   send 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. 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].

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   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
   operating conditions.

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

   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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              ^
          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)

   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.

   Comparatively the same 180 MB transfer performed over an SMB/CIFS
   mapped drive on the same scenario took 23 seconds.

6. Error Handling

   In case of an error the last ACK received MUST set the beginning of
   the next windowsize window to send.

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          [Requester]<------------------->[Provider]
                         <-traffic->   transfer block#   window block#
                                             ...        (windowsize=4)
                              <-           | n+01 |           1
                              <-           | n+02 |           2
                              <-           | n+03 |           3
                              <-           | n+04 |           4
           |ACK n+04|         ->
                              <-           | n+05 |           1
                       Error |<-           | n+06 |           2
                              <-           | n+07 |           3
           |ACK n+05|         ->
                              <-           | n+06 |           1
                              <-           | n+07 |           2
                              <-           | n+08 |           3
                              <-           | n+09 |           4
           |ACK n+09|         ->
                              <-           | n+10 |           1
                       Error |<-           | n+11 |           2
                              <-           | n+12 |           3
           |ACK n+10|         ->| Error
                              <-           | n+13 |           4

                              <-           | n+10 |           1
                              <-           | n+11 |           2
                              <-           | n+12 |           3
                              <-           | n+13 |           4
           |ACK n+13|         ->

                                             ...

           Section of a transfer including errors and error recovery

7. Security Considerations

   TFTP includes no login or access control mechanisms. Care must 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. The use of TFTP is
   appropriate on networks where the inherent protocol limitations are
   not a liability.

   TFTP includes no protection against an on-path attacker, care must be
   taken in controlling windowsize values according to provider,
   requester, and network environment capabilities. TFTP service is

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   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
   appropriate.

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

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.

   [RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
             for Application Designers", BCP 145, RFC 5405, November
             2008.

   [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
   Serva
   300 W 11th Avenue,
   Denver, CO 80204

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

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