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

    - 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





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    - 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.















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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: [email protected]





























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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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