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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 RFC 2119 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'.

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

1. when retrying a POST request which gave an indeterminate error

result in the previous attempt

1. when the user presses the RELOAD button while a POST result is

displayed

1. 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

1. the contents of the form are potentially very large
2. the form is used to upload a file (see [2])
3. 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

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1. are issued by the same user agent, and
2. apply to the same resource, and
3. have the same request method, and
4. 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.

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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",
 RFC 1867, 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

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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
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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