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Versions: 00 01 02 RFC 2310

HTTP Working Group                                     Koen Holtman, TUE
Internet-Draft
Expires: January 29, 1998                                  July 29, 1997


                     The Safe Response Header Field

                     draft-holtman-http-safe-02.txt


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ABSTRACT

   This document proposes a HTTP response header field called Safe,
   which can be used to indicate that repeating the corresponding POST
   request is safe.  Such an indication will allow user agents to
   present services which use safe POSTs in a more user-friendly way.
   Improving the user-friendliness of safe POSTs is considered
   important, because web internationalization will depend for a large
   part on the use of safe POSTs.


1  Introduction

 This document proposes a HTTP response header field called Safe,
 which can be used to indicate that repeating the corresponding POST
 request is safe.  Such an indication will allow user agents to
 present services which use safe POSTs in a more user-friendly way.
 Improving the user-friendliness of safe POSTs is considered
 important, because web internationalization will depend for a large
 part on the use of safe POSTs.

2 Background

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

 If the POST request is unsafe, explicit user confirmation is
 necessary before the request is repeated.  User agents will repeat
 POST requests when the user presses the RELOAD button while a POST
 result is displayed, or when the history function is used to
 redisplay a POST result which is no longer in the history buffer.

 The necessary confirmation dialog often takes the form of a `repost
 form data?'  dialog box.  The dialog is confusing to many users, and
 slows down navigation in any case.

 In theory, if the repeated POST request is safe, the user-unfriendly
 confirmation dialog can be omitted.  However, up till now, HTTP has
 no mechanism by which agents can tell if POST requests are safe.
 This proposal adds such a mechanism.

 Many content authors have managed to avoid the confirmation dialog
 problem by using GETs for form submission instead of safe POSTs.
 However, this escape is not possible for forms

    a) which are (sometimes) used to submit large amounts of data
    b) which are (sometimes) used to submit data in a charset other
       than ISO-8859-1.

 Case b) will be the increasingly common; web internationalization [2]
 makes it necessary to use the POST method for form submission.

   Note: Actually, according to the authors of [2], web
   internationalization makes it necessary to use requests which
   request bodies.  This rules out the use of the only methods which
   are safe under HTTP/1.1: GET and HEAD.  A new GET-WITH-BODY method
   could be defined, but it would be unsafe by default under HTTP/1.1,
   so GET-WITH-HEAD would also need something like the Safe header.

 It is therefore considered important to eliminate the unnecessary
 confirmation dialogs for safe POSTs as soon as possible.  They are a
 counter-incentive to the upgrading of GET based forms services (like
 search engines) to internationalized POST based forms services.

3 The Safe response header

 This header is proposed as an addition to the HTTP/1.x suite.

 The Safe response header field indicates whether repeating the
 request in the corresponding request message is safe in the sense of
 Section 9.1.1 (Safe Methods) of the HTTP/1.1 specification [1].

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

 An example of the field is:

       Safe: yes

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

    Note: User agents can use the received information about safety
    when repeating an earlier request.  If the repeating the request
    is known to be safe, the request can be repeated automatically
    without asking for user confirmation.


4 Smooth upgrade path

 That the Safe header provides a smooth upgrade path; if a service
 switches from GETs to safe POSTs, existing browsers will still be
 able to access the service.  Its use requires little re-coding effort
 for service authors and user agent authors, and is optional in any
 case.


5 About syntax

 This document mainly intends to recommend a _mechanism_; the syntax
 of the corresponding header is considered less important.  One
 alternative to the addition of a Safe header would be the addition of
 a `safe' response directive to the Cache-Control header.


6 Alternatives to the Safe header

 A number of alternative ways to solve the confirmation dialog problem
 have been proposed.  These alternative solutions would make the
 introduction of the Safe header unnecessary.

6.1 GET-WITH-BODY

 If a new HTTP/1.x GET-WITH-BODY is defined, one would not need the
 Safe header anymore, one could simply define GET-WITH-BODY as always
 safe.  However, it has been shown that some ugly extensions to the
 HTML form syntax would be needed to provide the same level of
 downwards compatibility with existing browsers that Safe can provide,
 for example

  <form action="..." method=post preferred_method=get-with-body>
   ....
  </form>.

 One could say that a GET-WITH-BODY manages to keep HTTP clean at the
 cost of adding extensions to HTML.  The author of this draft prefers
 to keep HTML clean by adding the Safe extension to HTTP.

 Note that the Safe header does not block the introduction of a
 GET-WITH-BODY in the long run.

6.2 Link

 The need for a confirmation dialog can also be eliminated by offering
 the user agent an alternative to resubmitting the POST request
 altogether.  A POST result could for example have a Link header which
 would contain an URL from which the result can be retrieved again
 with a GET request.  However, this Link URL cannot be very long: it
 it were too long, request would fail due to user agent, proxy, or
 origin server limitations.  This length restriction would make the
 Link solution hard to use in the general case.

6.3 Conclusion

 The Safe header seems to be the best solution in the current
 framework.  Though the existence of alternative solutions like the
 ones above has been asserted, the author of this draft has not seen
 any work on getting alternative solutions standardized.

 The Safe header would eliminate a counter-incentive to web
 internationalization, and the author feels that these
 counter-incentives need to be eliminated as soon as possible.  It is
 therefore proposed to introduce the Safe header into HTTP/1.x without
 undue delay.

 The counter-incentive can be thought of as an interoperability
 problem between HTTP/1.1 [1] and [2].  These documents currently both
 have the Proposed Standard status.  It may be possible to add the
 Safe header to [1] before it goes to Draft Standard.


7 Security considerations

 This proposal adds no new HTTP security considerations.


8 References

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

   [2] Yergeau et al, Internationalization of the Hypertext Markup
       Language, Internet-Draft draft-ietf-html-i18n-05.txt, Network
       Working Group, August 7, 1996


9 Author's address

   Koen Holtman
   Technische Universiteit Eindhoven
   Postbus 513
   Kamer HG 6.57
   5600 MB Eindhoven (The Netherlands)
   Email: koen@win.tue.nl


Expires: January 29, 1998


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