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RFC 3617 - Uniform Resource Identifier (URI) Scheme and Applicab


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Network Working Group                                            E. Lear
Request for Comments: 3617                                 Cisco Systems
Category: Informational                                     October 2003

              Uniform Resource Identifier (URI) Scheme and
                    Applicability Statement for the
                 Trivial File Transfer Protocol (TFTP)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   The Trivial File Transfer Protocol (TFTP) is a very simple TRIVIAL
   protocol that has been in use on the Internet for quite a long time.
   While this document discourages its continued use, largely due to
   security concerns, we do define a Uniform Resource Identifier (URI)
   scheme, as well as discuss the protocol's applicability.

1.  Introduction

   The Trivial File Transfer Protocol (TFTP) has been around for quite
   some time.  Its common uses are to initially configure devices or to
   load new versions of operating system code [1].  As devices begin to
   adopt use of Uniform Resource Identifiers (URIs) and Uniform Resource
   Locators (URLs), for completeness we specify a way to reference files
   that is still quite common.  Use of a URI is a convenient way to
   indicate underlying mechanism, server name or address, and file name.

   WHILE WE DEFINE THE TFTP URI TYPE, WE STRONGLY RECOMMEND AGAINST THE
   CONTINUED USE OF TFTP, FOR REASONS LISTED IN SECTION 5 (amongst
   others).  The definition of a URI merely allows tools that currently
   use protocols such as TFTP to have a standard name space and
   structure where one can understand the process used to resolve that
   name.  Indeed it is hoped that the definition of this URI will ease
   transition to modern file transfer mechanisms.

2.  Syntax of a TFTP URI

   A TFTP URI has the following ABNF syntax [2]:

   tftpURI         = "tftp://" host "/" file [ mode ]
   mode            = ";"  "mode=" ( "netascii" / "octet" )
   file            = *( unreserved / escaped )
   host            = <as specified by RFC 2732 [3]>
   unreserved      = <as specified in RFC 2396 [4]>
   escaped         = <as specified in RFC 2396>

   A TFTP URI specifies a file that is to be found or placed on a TFTP
   server.  The "mode" option is an option indicating how the file is to
   be transferred.  If left unspecified, the mode is assumed to be
   "octet".  A third "mail" mode was deprecated at the time RFC 1350 was
   adopted, and is not specified.

2.1.  Encoding Rules

   Aside from syntax as described above, the TFTP protocol does not
   specify length limits to either file names or file sizes.  In the
   case of file names, they may contain any character so long as those
   characters are properly escaped as described above.

3.  Semantics and Operations

   As previously stated the TFTP URI is a reference to a file.  The
   allowed operations on a TFTP URI are read and write.  When a TFTP URI
   is read the underlying mechanisms retrieve the named file via the
   TFTP protocol from the specified host with the optionally specified
   mode.  When a TFTP URI is written the underlying mechanisms transmit
   a file via TFTP to a specified server to either the specified file
   using the optionally specified mode.  No other operations are
   supported.

   Note that it is not possible to retrieve file size information prior
   to retrieval, nor is it possible to determine file existence or
   permissions prior to transfer.  Files transferred may or may not
   arrive intact, as there is no guarantee of reliability or even
   completeness.  See the TFTP standard for more details.  For more
   robust file transfer, consider using either FTP or HTTP [5, 6].

4. Examples

      tftp://example.com/myconfigurationfile;mode=netascii

   This example references file "myconfigurationfile" on server
   "example.com" and requests that the transfer occur in netascii mode.

      tftp://example.com/mystartupfile

   This file references file "mystartupfile" on server "example.com".
   The transfer should occur in octet mode, since no other mode was
   specified.

5.  Security Considerations and Concerns about TFTP's use

   Use of TFTP has been historically limited to those devices where a
   more full protocol stack is impractical due to either memory or CPU
   constraints.  While this still may be the case with a toaster, it is
   unlikely to be the case for even the simplest piece of network
   support hardware, such as simple routers or switches.  There are a
   myriad of reasons to use some protocol other than TFTP, only a few of
   which are listed below.

   TFTP has no mechanism for access control within the protocol, and
   there is no protection from a man in the middle attack.
   Implementations are left to their own devices in this area.  Because
   TFTP has no way to determine file sizes in advance, implementations
   should be prepared to properly check the bounds of transfers so that
   neither memory nor disk limitations are exceeded.

   TFTP is not well suited to large files for the following reasons.
   TFTP has no inherent integrity check.  There is no way to determine
   what one side sent is what the other received.  There is no way to
   restart TFTP transfers from anywhere other than the beginning.  TFTP
   is a lock step protocol.  Only one packet may be in flight at any one
   time.  There is no slow start or smart backoff mechanism in TFTP, but
   very simple timeouts.

   TFTP is not well suited to file transfers across administrative
   domains.  For one thing, TFTP utilizes UDP, and many NATs will not
   either support or allow TFTP transfers.  More likely firewalls will
   prohibit transfers.

   There are no caching semantics within TFTP.  There is no safe way to
   cache information using the TFTP protocol.

   In summary, use of TFTP is strongly discouraged except in the most
   limited of circumstances where memory and CPU are at the highest
   premium.

6.  IANA Considerations

   The IANA has registered the URL registration template found in
   Appendix A in accordance with RFC 2717 [7].

7.  Acknowledgments

   The author thanks Larry Masinter, Randy Presuhn, Phil Schafer, and
   Bill Fenner for their help in developing this document.

8.  Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

Appendix A. Registration Template

   URL scheme name: tftp
   URL scheme syntax: Section 2
   Character encoding considerations: Section 2
   Intended usage: Section 1
   Applications and/or protocols which use this scheme: [1]
   Interoperability considerations: None
   Security considerations: Section 5
   Relevant publications: [1]
   Contact: The author, Section 8
   Author/Change Controller: IESG

References

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

   [2]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [3]  Hinden, R., Carpenter, B. and L. Masinter, "Format for Literal
        IPv6 Addresses in URL's", RFC 2732, December 1999.

   [4]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
        Identifiers (URI): Generic Syntax", RFC 2396, August 1998.

   [5]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
        Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
        HTTP/1.1", RFC 2616, June 1999.

   [6]  Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9,
        RFC 959, October 1985.

   [7]  Petke, R. and I. King, "Registration Procedures for URL Scheme
        Names", BCP 35, RFC 2717, November 1999.

Author's Address

   Eliot Lear
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA 95134-1706

   Phone: +1 (408) 527 4020
   EMail: lear@cisco.com

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

 

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