HTTP Working Group                                                K. Oku
Internet-Draft                                            DeNA Co., Ltd.
Intended status: Experimental                             March 29,                               May 16, 2017
Expires: September 30, November 17, 2017

                An HTTP Status Code for Indicating Hints
                   draft-ietf-httpbis-early-hints-01
                   draft-ietf-httpbis-early-hints-02

Abstract

   This memo introduces an informational HTTP status code for HTTP that can be
   used for indicating hints to convey hints that help a client start making make preparations for
   processing the final response.

Note to Readers

   Discussion of this draft takes place on the HTTP working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/ .

   Working Group information can be found at https://httpwg.github.io/ ;
   source code and issues list for this draft can be found at
   https://github.com/httpwg/http-extensions/labels/early-hints .

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 30, November 17, 2017.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  103 Early Hints . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Changes . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Since draft-ietf-httpbis-early-hints-01 . . . . . . . . .   4
     6.2.  Since draft-ietf-httpbis-early-hints-00 . . . . . . . . .   4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   Most if not all of the web pages processed by a web browser

   It is common for HTTP responses to contain links to external
   resources that need to be fetched prior to their use; for example,
   rendering the documents.  Therefore, it is beneficial to send HTML by a Web browser.  Having such links available to the
   client as early as possible in order helps to minimize the time spent until
   the browser becomes possible to render the document.  Link header of
   type perceived latency.

   The "preload" ([Preload]) link relation can be used to indicate convey such
   links within in the response headers Link header field of an HTTP response.  However, it is
   not always possible for an origin server to send generate a response
   header block immediately after receiving a request.  In fact, it is often  For example, the contrary.  There are many deployments in which an
   origin server
   needs might need to query a database before generating a response.  It is also
   not unusual for an origin server to
   response, or it might delegate a request to an upstream HTTP server
   running at a distant location.

   The dilemma here is that even though it is preferable for an origin
   server to send some headers as soon as it receives a request, it
   cannot do so until the status code and the full headers of the final
   HTTP response is are determined.

   HTTP/2 ([RFC7540]) server push can be used as a solution to the this
   issue, but has its own limitations.  The resources responses that can be pushed
   using HTTP/2 are limited to those belonging to the same origin.
   Also, it is impossible to send only the links of the resources using HTTP/2 server push.  Sending
   Finally, sending HTTP responses for every resource is an inefficient
   way of using bandwidth, especially when a caching server exists as an
   intermediary.

   This memo defines a status code for sending an informational response
   ([RFC7231], section 6.2) that contains headers that are likely to be
   included in the final response.  A server can send the informational
   response containing some of the headers to help the client start
   making preparations for processing the final response, and then run
   time-consuming operations to generate the final response.  The
   informational response can also be used by an origin server to
   trigger HTTP/2 server push at an a caching intermediary.

1.1.  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].

2.  103 Early Hints

   This

   The 103 (Early Hints) informational status code indicates the client
   that the server is likely to send a final response with the headers
   included in the informational response.

   A server MUST NOT include Content-Length, Transfer-Encoding, or any
   hop-by-hop headers header fields ([RFC7230], section 6.1) in the informational
   response using the status code. a 103 (Early
   Hints) response.

   A client MAY speculatively evaluate the headers included in the
   informational a 103
   (Early Hints) response while waiting for the final response.  For
   example, a client may might recognize the link a Link header of field value
   containing the relation type preload "preload" and start fetching the target
   resource.

   However, the evaluation this MUST NOT affect how the final response is processed;
   when handling it, the client must MUST behave as if it had not seen the
   informational response.  A  In particular, a client MUST NOT process the
   headers included in the final response as if they belonged to the
   informational response. response, or vice versa.

   An intermediary MAY drop the informational response.  It MAY send
   HTTP/2 ([RFC7540]) push responses server pushes using the information found in the
   informational
   103 (Early Hints) response.

3.  Security Considerations

   Clients

   Some clients may have issues handling Early Hints, 103 (Early Hints), since
   informational
   response is responses are rarely used for in reply to requests not
   including an Expect header ([RFC7231], section 5.1.1).

   An

   In particular, an HTTP/1.1 client that mishandles the an informational
   response as a final response is likely to consider all the responses to
   the succeeding requests sent over the same connection to be part of
   the final response.  Such behavior may constitute a cross-origin
   information disclosure vulnerability in case the client multiplexes
   requests to different origins onto a single persistent connection.

   Therefore, a server might refrain from sending Early Hints over
   HTTP/1.1 unless when the client is known to handle informational
   responses correctly.

   HTTP/2 clients are less likely to suffer from incorrect framing since
   handling of the response headers does not affect how the end of the
   response body is determined.

4.  IANA Considerations

   If Early Hints is standardized, the

   The HTTP Status Codes Registry should will be updated with the following entries:
   entry:

   o  Code: 103

   o  Description: Early Hints

   o  Specification: this document [this document]

5.  Acknowledgements

   Thanks to Tatsuhiro Tsujikawa for coming up with the idea of sending
   the link headers using an informational response.

6.  Changes

6.1.  Since draft-ietf-httpbis-early-hints-01

   o  Editorial changes.

6.2.  Since draft-ietf-httpbis-early-hints-00

   o  Forbid processing the headers of a 103 response as part of the
      informational response.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, 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,
              DOI 10.17487/RFC7231, June 2014,
              <http://www.rfc-editor.org/info/rfc7231>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <http://www.rfc-editor.org/info/rfc7540>.

7.2.  Informative References

   [Preload]  Grigorik, I., "Preload", September 2016,
              <https://w3c.github.io/preload/>.

Author's Address

   Kazuho Oku
   DeNA Co., Ltd.

   Email: kazuhooku@gmail.com