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'.

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

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