HTTP Working Group                                                K. Oku
Internet-Draft                                                    Fastly
Intended status: Experimental                              July 11,                           October 28, 2017
Expires: January 12, May 1, 2018

                An HTTP Status Code for Indicating Hints


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

Note to Readers

   Discussion of this draft takes place on the HTTP working group
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  103 Early Hints . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5   6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5   6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5   6
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   5   6
   Appendix A.  Changes  . . . . . . . . . . . . . . . . . . . . . .   6
     A.1.  Since draft-ietf-httpbis-early-hints-03 draft-ietf-httpbis-early-hints-04 . . . . . . . . .   6
     A.2.  Since draft-ietf-httpbis-early-hints-02 draft-ietf-httpbis-early-hints-03 . . . . . . . . .   6   7
     A.3.  Since draft-ietf-httpbis-early-hints-01 draft-ietf-httpbis-early-hints-02 . . . . . . . . .   6   7
     A.4.  Since draft-ietf-httpbis-early-hints-01 . . . . . . . . .   7
     A.5.  Since draft-ietf-httpbis-early-hints-00 . . . . . . . . .   6   7
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .   6   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6   7

1.  Introduction

   It is common for HTTP responses to contain links to external
   resources that need to be fetched prior to their use; for example,
   rendering HTML by a Web browser.  Having such links available to the
   client as early as possible helps to minimize perceived latency.

   The "preload" ([Preload]) link relation can be used to convey such
   links in the Link header field of an HTTP response.  However, it is
   not always possible for an origin server to generate the header block
   of a final response immediately after receiving a request.  For
   example, the origin server might delegate a request to an upstream
   HTTP server running at a distant location, or the status code might
   depend on the result of a database query.

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

   HTTP/2 ([RFC7540]) server push can be used as a solution to this
   issue, accelerate the delivery of
   resources, but has its own limitations. only resources for which the server is authoritative.
   The responses other limitation of server push is that can the response will be pushed
   using HTTP/2 are limited to those belonging to
   transmitted regardless of whether the same origin.
   Also, it is impossible to send only client has the links using server push.
   Finally, sending HTTP responses for every resource is an inefficient
   way response cached.
   At the cost of using bandwidth, especially when a caching spending one extra round-trip compared to server exists as an
   intermediary. push
   in the worst case, delivering Link header fields in a timely fashion
   is more flexible and might consume less bandwidth.

   This memo defines a status code for sending an informational response
   ([RFC7231], Section 6.2) that contains header fields that are likely
   to be included in the final response.  A server can send the
   informational response containing some of the header fields 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 a caching

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  103 Early Hints

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

   Typically, a server will include the header fields sent in a 103
   (Early Hints) response in the final response as well.  However, there
   might be cases when this is not desirable, such as when the server
   learns that they are not correct before the final response is sent.

   A client can speculatively evaluate the header fields included in a
   103 (Early Hints) response while waiting for the final response.  For
   example, a client might recognize a Link header field value
   containing the relation type "preload" and start fetching the target
   resource.  However, these header fields only provide hints to the
   client; they do not replace the header fields on the final response.

   Aside from performance optimizations, such evaluation of the 103
   (Early Hints) response's header fields MUST NOT affect how the final
   response is processed.  A client MUST NOT interpret the 103 (Early
   Hints) response header fields as if they applied to the informational
   response itself (e.g., as metadata about the 103 (Early Hints)

   A server MAY use a 103 (Early Hints) response to indicate only some
   of the header fields that are expected to be found in the final
   response.  A client SHOULD NOT interpret the nonexistence of a header
   field in a 103 (Early Hints) response as a speculation that the
   header field is unlikely to be part of the final response.

   The following example illustrates a typical message exchange that
   involves a 103 (Early Hints) response.

   Client request:

     GET / HTTP/1.1

   Server response:

     HTTP/1.1 103 Early Hints
     Link: </style.css>; rel=preload; as=style
     Link: </script.js>; rel=preload; as=script

     HTTP/1.1 200 OK
     Date: Fri, 26 May 2017 10:02:11 GMT
     Content-Length: 1234
     Content-Type: text/html; charset=utf-8
     Link: </style.css>; rel=preload; as=style
     Link: </script.js>; rel=preload; as=script

     <!doctype html>
     [... rest of the response body is ommitted omitted from the example ...]

   As is the case with any informational response, a server might emit
   more than one 103 (Early Hints) response prior to sending a final
   response.  This can happen for example when a caching intermediary
   generates a 103 (Early Hints) response based on the header fields of
   a stale-cached response, then forwards a 103 (Early Hints) response
   and a final response that were sent from the origin server in
   response to a revalidation request.

   A server MAY emit multiple 103 (Early Hints) responses with
   additional header fields as new information becomes available while
   the request is being processed.  It does not need to repeat the
   fields that were already emitted, though it doesn't have to exclude
   them either.  The client can consider any combination of header
   fields received in multiple 103 (Early Hints) responses when
   anticipating the list of header fields expected in the final

   The following example illustrates a series of responses that a server
   might emit.  In the example, the server uses two 103 (Early Hints)
   responses to notify the client that it is likely to send three Link
   header fields in the final response.  Two of the three expected
   header fields are found in the final response.  The other header
   field is replaced by another Link header field that contains a
   different value.

     HTTP/1.1 103 Early Hints
     Link: </main.css>; rel=preload; as=style

     HTTP/1.1 103 Early Hints
     Link: </style.css>; rel=preload; as=style
     Link: </script.js>; rel=preload; as=script

     HTTP/1.1 200 OK
     Date: Fri, 26 May 2017 10:02:11 GMT
     Content-Length: 1234
     Content-Type: text/html; charset=utf-8
     Link: </main.css>; rel=preload; as=style
     Link: </newstyle.css>; rel=preload; as=style
     Link: </script.js>; rel=preload; as=script

     <!doctype html>
     [... rest of the response body is omitted from the example ...]

3.  Security Considerations

   Some clients might have issues handling 103 (Early Hints), since
   informational responses are rarely used in reply to requests not
   including an Expect header field ([RFC7231], Section 5.1.1).

   In particular, an HTTP/1.1 client that mishandles an informational
   response as a final response is likely to consider all responses to
   the succeeding requests sent over the same connection to be part of
   the final response.  Such behavior might 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 the client is known to handle informational responses

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

4.  IANA Considerations

   The HTTP Status Codes Registry will be updated with the following

   o  Code: 103

   o  Description: Early Hints

   o  Specification: [this document]

5.  References

5.1.  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,
              <>. <https://www.rfc-

   [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,

   [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,
              <>. <https://www.rfc-

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

5.2.  Informative References

   [Preload]  Grigorik, I., "Preload", n.d., <

Appendix A.  Changes

A.1.  Since draft-ietf-httpbis-early-hints-04

   o  Clarified that the server is allowed to add headers not found in a
      103 response to the final response.

   o  Clarify client's behavior when it receives more than one 103

A.2.  Since draft-ietf-httpbis-early-hints-03

   o  Removed statements that were either redundant or contradictory to

   o  Clarified what the server's expected behavior is.

   o  Explain that a server might want to send more than one 103

   o  Editorial Changes.


A.3.  Since draft-ietf-httpbis-early-hints-02

   o  Editorial changes.

   o  Added an example.


A.4.  Since draft-ietf-httpbis-early-hints-01

   o  Editorial changes.


A.5.  Since draft-ietf-httpbis-early-hints-00

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

Appendix B.  Acknowledgements

   Thanks to Tatsuhiro Tsujikawa for coming up with the idea of sending
   the Link header fields using an informational response.

Author's Address

   Kazuho Oku