    Internet Engineering Task Force (IETF)                        J. Reschke
    Request for Comments: 7538                                    greenbytes
    Obsoletes: 7238                                               April 2015
    Category: Standards Track
    ISSN: 2070-1721

      The Hypertext Transfer Protocol Status Code 308 (Permanent Redirect)

    Abstract

       This document specifies the additional Hypertext Transfer Protocol
       (HTTP) status code 308 (Permanent Redirect).

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc7538.

    Copyright Notice

       Copyright (c) 2015 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (http://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

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    RFC 7538                  HTTP Status Code 308                April 2015

    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
       2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   2
       3.  308 Permanent Redirect  . . . . . . . . . . . . . . . . . . .   3
       4.  Deployment Considerations . . . . . . . . . . . . . . . . . .   3
       5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
       6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
       7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
         7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
         7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
       Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
       Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

    1.  Introduction

       HTTP defines a set of status codes for the purpose of redirecting a
       request to a different URI ([RFC3986]).  The history of these status
       codes is summarized in Section 6.4 of [RFC7231], which also
       classifies the existing status codes into four categories.

       The first of these categories contains the status codes 301 (Moved
       Permanently), 302 (Found), and 307 (Temporary Redirect), which can be
       classified as below:

       +-------------------------------------------+-----------+-----------+
       |                                           | Permanent | Temporary |
       +-------------------------------------------+-----------+-----------+
       | Allows changing the request method from   | 301       | 302       |
       | POST to GET                               |           |           |
       | Does not allow changing the request       | -         | 307       |
       | method from POST to GET                   |           |           |
       +-------------------------------------------+-----------+-----------+

       Section 6.4.7 of [RFC7231] states that it does not define a permanent
       variant of status code 307; this specification adds the status code
       308, defining this missing variant (Section 3).

       This specification contains no technical changes from the
       Experimental RFC 7238, which it obsoletes.

    2.  Notational Conventions

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in [RFC2119].

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    RFC 7538                  HTTP Status Code 308                April 2015

    3.  308 Permanent Redirect

       The 308 (Permanent Redirect) status code indicates that the target
       resource has been assigned a new permanent URI and any future
       references to this resource ought to use one of the enclosed URIs.
       Clients with link editing capabilities ought to automatically re-link
       references to the effective request URI (Section 5.5 of [RFC7230]) to
       one or more of the new references sent by the server, where possible.

       The server SHOULD generate a Location header field ([RFC7231],
       Section 7.1.2) in the response containing a preferred URI reference
       for the new permanent URI.  The user agent MAY use the Location field
       value for automatic redirection.  The server's response payload
       usually contains a short hypertext note with a hyperlink to the new
       URI(s).

       A 308 response is cacheable by default; i.e., unless otherwise
       indicated by the method definition or explicit cache controls (see
       [RFC7234], Section 4.2.2).

          Note: This status code is similar to 301 (Moved Permanently)
          ([RFC7231], Section 6.4.2), except that it does not allow changing
          the request method from POST to GET.

    4.  Deployment Considerations

       Section 6 of [RFC7231] requires recipients to treat unknown 3xx
       status codes the same way as status code 300 (Multiple Choices)
       ([RFC7231], Section 6.4.1).  Thus, servers will not be able to rely
       on automatic redirection happening similar to status codes 301, 302,
       or 307.

       Therefore, the use of status code 308 is restricted to cases where
       the server has sufficient confidence in the client's understanding
       the new code or when a fallback to the semantics of status code 300
       is not problematic.  Server implementers are advised not to vary the
       status code based on characteristics of the request, such as the
       User-Agent header field ("User-Agent Sniffing") -- doing so usually
       results in code that is both hard to maintain and hard to debug and
       would also require special attention to caching (i.e., setting a
       "Vary" response header field, as defined in Section 7.1.4 of
       [RFC7231]).

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    RFC 7538                  HTTP Status Code 308                April 2015

       Note that many existing HTML-based user agents will emulate a refresh
       when encountering an HTML <meta> refresh directive ([HTML],
       Section 4.2.5.3).  This can be used as another fallback.  For
       example:

       Client request:

         GET / HTTP/1.1
         Host: example.com

       Server response:

         HTTP/1.1 308 Permanent Redirect
         Content-Type: text/html; charset=UTF-8
         Location: http://example.com/new
         Content-Length: 356

         <!DOCTYPE HTML>
         <html>
            <head>
               <title>Permanent Redirect</title>
               <meta http-equiv="refresh"
                     content="0; url=http://example.com/new">
            </head>
            <body>
               <p>
                  The document has been moved to
                  <a href="http://example.com/new"
                  >http://example.com/new</a>.
               </p>
            </body>
         </html>

    5.  Security Considerations

       All security considerations that apply to HTTP redirects apply to the
       308 status code as well (see Section 9 of [RFC7231]).

       Unsecured communication over the Internet is subject to man-in-the-
       middle modification of messages, including changing status codes or
       redirect targets.  Use of Transport Layer Security (TLS) is one way
       to mitigate those attacks.  See Section 9 of [RFC7230] for related
       attacks on authority and message integrity.

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    RFC 7538                  HTTP Status Code 308                April 2015

    6.  IANA Considerations

       The "Hypertext Transfer Protocol (HTTP) Status Code Registry"
       (defined in Section 8.2 of [RFC7231] and located at
       <http://www.iana.org/assignments/http-status-codes>) has been updated
       to reference this specification.

       +-------+--------------------+----------------------------------+
       | Value | Description        | Reference                        |
       +-------+--------------------+----------------------------------+
       | 308   | Permanent Redirect | Section 3 of this specification  |
       +-------+--------------------+----------------------------------+

    7.  References

    7.1.  Normative References

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997,
                  <http://www.rfc-editor.org/info/rfc2119>.

       [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
                  Resource Identifier (URI): Generic Syntax", STD 66, RFC
                  3986, January 2005,
                  <http://www.rfc-editor.org/info/rfc3986>.

       [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
                  Protocol (HTTP/1.1): Message Syntax and Routing", RFC
                  7230, June 2014, <http://www.rfc-editor.org/info/rfc7230>.

       [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
                  Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
                  June 2014, <http://www.rfc-editor.org/info/rfc7231>.

       [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
                  Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
                  RFC 7234, June 2014,
                  <http://www.rfc-editor.org/info/rfc7234>.

    7.2.  Informative References

       [HTML]     Hickson, I., Berjon, R., Faulkner, S., Leithead, T., Doyle
                  Navara, E., O'Connor, E., and S. Pfeiffer, "HTML5", W3C
                  Recommendation REC-html5-20141028, October 2014,
                  <http://www.w3.org/TR/2014/REC-html5-20141028/>.

                  Latest version available at <http://www.w3.org/TR/html5/>.

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    RFC 7538                  HTTP Status Code 308                April 2015

    Acknowledgements

       The definition for the new status code 308 reuses text from the
       HTTP/1.1 definitions of status codes 301 and 307.

       Furthermore, thanks to Ben Campbell, Cyrus Daboo, Adrian Farrell,
       Eran Hammer-Lahav, Bjoern Hoehrmann, Barry Leiba, Subramanian
       Moonesamy, Kathleen Moriarty, Peter Saint-Andre, Robert Sparks, and
       Roy Fielding for feedback on this document.

    Author's Address

       Julian F. Reschke
       greenbytes GmbH
       Hafenweg 16
       Muenster, NW  48155
       Germany

       EMail: julian.reschke@greenbytes.de
       URI:   http://greenbytes.de/tech/webdav/

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