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Network Working Group                                          K. HoltmanRequest for Comments: 2310                                            TUECategory: Experimental                                         April 1998The Safe Response Header FieldStatus 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 inRFC 2119 for defining the significance   of each particular requirement.3 Rationale   According toSection 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'.Holtman                       Experimental                      [Page 1]

RFC 2310             The Safe Response Header Field           April 1998   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 ofSection 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 requestsHoltman                       Experimental                      [Page 2]

RFC 2310             The Safe Response Header Field           April 1998     - 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.Holtman                       Experimental                      [Page 3]

RFC 2310             The Safe Response Header Field           April 19986 References   [1] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and   T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",RFC2068, 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",RFC2070, 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.nlHoltman                       Experimental                      [Page 4]

RFC 2310             The Safe Response Header Field           April 19988.  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.Holtman                       Experimental                      [Page 5]