An HTTP Status Code for Indicating Hints
RFC 8297

Internet Engineering Task Force (IETF)                            K. Oku
Request for Comments: 8297                                        Fastly
Category: Experimental                                     December 2017
ISSN: 2070-1721

                An HTTP Status Code for Indicating Hints

Abstract

   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.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  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).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8297.

Copyright Notice

   Copyright (c) 2017 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
   (https://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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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  HTTP Status Code 103: Early Hints . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   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 accelerate the delivery of
   resources, but only resources for which the server is authoritative.
   The other limitation of server push is that the response will be
   transmitted regardless of whether the client has the response cached.
   At the cost of spending one extra round trip compared to server 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

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   final response.  The informational response can also be used by an
   origin server to trigger HTTP/2 server push at a caching
   intermediary.

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  HTTP Status Code 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 the header fields in the 103 (Early Hints) response 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)
   response).

   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.

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   Client request:

     GET / HTTP/1.1
     Host: example.com

   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 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, and 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
   response.

   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.

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     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 a 103 (Early Hints) response,
   because 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 103 (Early Hints)
   responses over HTTP/1.1 unless 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 header fields does not affect how the end of
   the response body is determined.

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4.  IANA Considerations

   The following entry has been registered in the "HTTP Status Codes"
   registry:

   o  Code: 103

   o  Description: Early Hints

   o  Specification: RFC 8297 (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-editor.org/info/rfc2119>.

   [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-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,
              <https://www.rfc-editor.org/info/rfc7540>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

5.2.  Informative References

   [Preload]  Grigorik, I., Ed. and Y. Weiss, Ed., "Preload", W3C
              Candidate Recommendation, October 2017,
              <https://www.w3.org/TR/preload/>.

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Acknowledgements

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

   Mark Nottingham and Willy Tarreau provided substantial help in
   clarifying the semantics of the status code.

   Early stages of the author's work on this document was supported by
   DeNA Co., Ltd. during his employment there.

Author's Address

   Kazuho Oku
   Fastly

   Email: kazuhooku@gmail.com

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