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rfc:rfc8297

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.

Oku Experimental [Page 1] RFC 8297 Early Hints December 2017

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

Oku Experimental [Page 2] RFC 8297 Early Hints December 2017

 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.

Oku Experimental [Page 3] RFC 8297 Early Hints December 2017

 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.

Oku Experimental [Page 4] RFC 8297 Early Hints December 2017

   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.

Oku Experimental [Page 5] RFC 8297 Early Hints December 2017

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/>.

Oku Experimental [Page 6] RFC 8297 Early Hints December 2017

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

Oku Experimental [Page 7]

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