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RFC 3617
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.
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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].
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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.
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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.
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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
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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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Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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