RFC2310 - The Safe Response Header Field

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　　Network Working Group K. Holtman
Request for Comments: 2310 TUE
Category: Experimental April 1998

The Safe Response Header Field

Status of this Memo

This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

This document defines a HTTP response header field called Safe, which
can be used to indicate that repeating a HTTP request is safe. Such
an indication will allow user agents to handle retries of some safe
requests, in particular safe POST requests, in a more user-friendly
way.

1 Introduction

This document defines a HTTP response header field called Safe, which
can be used to indicate that repeating a HTTP request is safe. Such
an indication will allow user agents to handle retries of some safe
requests, in particular safe POST requests, in a more user-friendly
way.

2 Terminology and Notation

This document uses the HTTP terminology and BNF notation defined in
[1]. It uses the key words in RFC2119 for defining the significance
of each particular requirement.

3 Rationale

According to Section 9.1.1 (Safe Methods) of the HTTP/1.1
specification [1], POST requests are assumed to be `unsafe' by
default. `Unsafe' means `causes side effects for which the user will
be held accountable'.

It is sometimes necessary for a user agent to repeat a POST request.
Examples of such cases are

- when retrying a POST request which gave an indeterminate error
result in the previous attempt
- when the user presses the RELOAD button while a POST result is
displayed
- when the history function is used to redisplay a POST result
which is no longer in the history buffer.

If the POST request is unsafe, HTTP requires explicit user
confirmation is before the request is repeated. The confirmation
dialog often takes the form of a `repost form data?' dialog box.
This dialog is confusing to many users, and slows down navigation in
any case.

If the repeated POST request is safe, the user-unfriendly
confirmation dialog can be omitted. However plain HTTP/1.1 [1] has
no mechanism by which agents can tell if POST requests are safe, and
they must be assumed unsafe by default. This document adds a
mechanism to HTTP, the Safe header field, for telling if a POST
request is safe.

Using the Safe header field, web applications which require the use
of a safe POST request, rather than a GET request, for the submission
of web forms, can be made more user-friendly. The use of a POST
request may be required for a number of reasons, including

- the contents of the form are potentially very large
- the form is used to upload a file (see [2])
- the application needs some internationalization features
(see [3]) which are only available if the form contents are
transmitted in a request body the information in the form cannot
be encoded in a GET request URL because of security
considerations.

4 The Safe response header field

The Safe response header field is defined as an addition to the
HTTP/1.x protocol suite.

The Safe response header field is used by origin servers to indicate
whether repeating the received HTTP request is safe in the sense of
Section 9.1.1 (Safe Methods) of the HTTP/1.1 specification [1]. For
the purpose of this specification, a HTTP request is considered to be
a repetition of a previous request if both requests

- are issued by the same user agent, and
- apply to the same resource, and
- have the same request method, and
- both have no request body, or both have request bodies which are
byte-wise identical after decoding of any content and transfer
codings.

The Safe header field has the following syntax.

Safe = "Safe" ":" safe-nature
safe-nature = "yes" | "no"

An example of the header field is:

Safe: yes

If a Safe header field is absent in the response, the corresponding
request MUST be considered unsafe, unless it is a GET or HEAD
request. As GET and HEAD requests are safe by definition, user
agents SHOULD ignore a `Safe: no' header field in GET and HEAD
responses.

If, according to a received Safe header field, the repeating of a
request is safe, the request MAY be repeated automatically without
asking for user confirmation.

5 Security Considerations

For a discussion of the security considerations connected to HTTP
form submission, see [1]. The Safe header field introduces no new
security risks.

The use of GET requests for form submission has some security risks
which are absent for submission with other HTTP methods. By taking
away a counter-incentive to the use of GET requests for form
submission, the Safe header field may improve overall security.

6 References

[1] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and
T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
2068, January 1997.

[2] Nebel, E., and L. Masinter, "Form-based File Upload in HTML",
RFC1867, November 1995.

[3] Yergeau, F., Nicol, G., Adams, G., and M. Duerst,
"Internationalization of the Hypertext Markup Language", RFC
2070, January 1997.

7 Author's Address

Koen Holtman
Technische Universiteit Eindhoven
Postbus 513
Kamer HG 6.57
5600 MB Eindhoven (The Netherlands)

EMail: koen@win.tue.nl

8. Full Copyright Statement

Copyright (C) The Internet Society (1998). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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