draft-ietf-httpbis-early-hints-04.txt   draft-ietf-httpbis-early-hints-05.txt 
HTTP Working Group K. Oku HTTP Working Group K. Oku
Internet-Draft Fastly Internet-Draft Fastly
Intended status: Experimental July 11, 2017 Intended status: Experimental October 28, 2017
Expires: January 12, 2018 Expires: May 1, 2018
An HTTP Status Code for Indicating Hints An HTTP Status Code for Indicating Hints
draft-ietf-httpbis-early-hints-04 draft-ietf-httpbis-early-hints-05
Abstract Abstract
This memo introduces an informational HTTP status code that can be This memo introduces an informational HTTP status code that can be
used to convey hints that help a client make preparations for used to convey hints that help a client make preparations for
processing the final response. processing the final response.
Note to Readers Note to Readers
Discussion of this draft takes place on the HTTP working group Discussion of this draft takes place on the HTTP working group
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 12, 2018. This Internet-Draft will expire on May 1, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. 103 Early Hints . . . . . . . . . . . . . . . . . . . . . . . 3 2. 103 Early Hints . . . . . . . . . . . . . . . . . . . . . . . 3
3. Security Considerations . . . . . . . . . . . . . . . . . . . 4 3. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Normative References . . . . . . . . . . . . . . . . . . 5 5.1. Normative References . . . . . . . . . . . . . . . . . . 6
5.2. Informative References . . . . . . . . . . . . . . . . . 5 5.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 6 Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 6
A.1. Since draft-ietf-httpbis-early-hints-03 . . . . . . . . . 6 A.1. Since draft-ietf-httpbis-early-hints-04 . . . . . . . . . 6
A.2. Since draft-ietf-httpbis-early-hints-02 . . . . . . . . . 6 A.2. Since draft-ietf-httpbis-early-hints-03 . . . . . . . . . 7
A.3. Since draft-ietf-httpbis-early-hints-01 . . . . . . . . . 6 A.3. Since draft-ietf-httpbis-early-hints-02 . . . . . . . . . 7
A.4. Since draft-ietf-httpbis-early-hints-00 . . . . . . . . . 6 A.4. Since draft-ietf-httpbis-early-hints-01 . . . . . . . . . 7
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 6 A.5. Since draft-ietf-httpbis-early-hints-00 . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction 1. Introduction
It is common for HTTP responses to contain links to external It is common for HTTP responses to contain links to external
resources that need to be fetched prior to their use; for example, resources that need to be fetched prior to their use; for example,
rendering HTML by a Web browser. Having such links available to the rendering HTML by a Web browser. Having such links available to the
client as early as possible helps to minimize perceived latency. client as early as possible helps to minimize perceived latency.
The "preload" ([Preload]) link relation can be used to convey such The "preload" ([Preload]) link relation can be used to convey such
links in the Link header field of an HTTP response. However, it is links in the Link header field of an HTTP response. However, it is
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of a final response immediately after receiving a request. For of a final response immediately after receiving a request. For
example, the origin server might delegate a request to an upstream example, the origin server might delegate a request to an upstream
HTTP server running at a distant location, or the status code might HTTP server running at a distant location, or the status code might
depend on the result of a database query. depend on the result of a database query.
The dilemma here is that even though it is preferable for an origin 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, 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 it cannot do so until the status code and the full header fields of
the final HTTP response are determined. the final HTTP response are determined.
HTTP/2 ([RFC7540]) server push can be used as a solution to this HTTP/2 ([RFC7540]) server push can accelerate the delivery of
issue, but has its own limitations. The responses that can be pushed resources, but only resources for which the server is authoritative.
using HTTP/2 are limited to those belonging to the same origin. The other limitation of server push is that the response will be
Also, it is impossible to send only the links using server push. transmitted regardless of whether the client has the response cached.
Finally, sending HTTP responses for every resource is an inefficient At the cost of spending one extra round-trip compared to server push
way of using bandwidth, especially when a caching server exists as an in the worst case, delivering Link header fields in a timely fashion
intermediary. is more flexible and might consume less bandwidth.
This memo defines a status code for sending an informational response This memo defines a status code for sending an informational response
([RFC7231], Section 6.2) that contains header fields that are likely ([RFC7231], Section 6.2) that contains header fields that are likely
to be included in the final response. A server can send the to be included in the final response. A server can send the
informational response containing some of the header fields to help informational response containing some of the header fields to help
the client start making preparations for processing the final the client start making preparations for processing the final
response, and then run time-consuming operations to generate the response, and then run time-consuming operations to generate the
final response. The informational response can also be used by an final response. The informational response can also be used by an
origin server to trigger HTTP/2 server push at a caching origin server to trigger HTTP/2 server push at a caching
intermediary. intermediary.
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resource. However, these header fields only provide hints to the resource. However, these header fields only provide hints to the
client; they do not replace the header fields on the final response. client; they do not replace the header fields on the final response.
Aside from performance optimizations, such evaluation of the 103 Aside from performance optimizations, such evaluation of the 103
(Early Hints) response's header fields MUST NOT affect how the final (Early Hints) response's header fields MUST NOT affect how the final
response is processed. A client MUST NOT interpret the 103 (Early response is processed. A client MUST NOT interpret the 103 (Early
Hints) response header fields as if they applied to the informational Hints) response header fields as if they applied to the informational
response itself (e.g., as metadata about the 103 (Early Hints) response itself (e.g., as metadata about the 103 (Early Hints)
response). 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 The following example illustrates a typical message exchange that
involves a 103 (Early Hints) response. involves a 103 (Early Hints) response.
Client request: Client request:
GET / HTTP/1.1 GET / HTTP/1.1
Host: example.com Host: example.com
Server response: Server response:
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Link: </script.js>; rel=preload; as=script Link: </script.js>; rel=preload; as=script
HTTP/1.1 200 OK HTTP/1.1 200 OK
Date: Fri, 26 May 2017 10:02:11 GMT Date: Fri, 26 May 2017 10:02:11 GMT
Content-Length: 1234 Content-Length: 1234
Content-Type: text/html; charset=utf-8 Content-Type: text/html; charset=utf-8
Link: </style.css>; rel=preload; as=style Link: </style.css>; rel=preload; as=style
Link: </script.js>; rel=preload; as=script Link: </script.js>; rel=preload; as=script
<!doctype html> <!doctype html>
[... rest of the response body is ommitted from the example ...] [... rest of the response body is omitted from the example ...]
As is the case with any informational response, a server might emit As is the case with any informational response, a server might emit
more than one 103 (Early Hints) response prior to sending a final more than one 103 (Early Hints) response prior to sending a final
response. This can happen for example when a caching intermediary response. This can happen for example when a caching intermediary
generates a 103 (Early Hints) response based on the header fields of generates a 103 (Early Hints) response based on the header fields of
a stale-cached response, then forwards a 103 (Early Hints) response a stale-cached response, then forwards a 103 (Early Hints) response
and a final response that were sent from the origin server in and a final response that were sent from the origin server in
response to a revalidation request. 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.
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 3. Security Considerations
Some clients might have issues handling 103 (Early Hints), since Some clients might have issues handling 103 (Early Hints), since
informational responses are rarely used in reply to requests not informational responses are rarely used in reply to requests not
including an Expect header field ([RFC7231], Section 5.1.1). including an Expect header field ([RFC7231], Section 5.1.1).
In particular, an HTTP/1.1 client that mishandles an informational In particular, an HTTP/1.1 client that mishandles an informational
response as a final response is likely to consider all responses to 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 succeeding requests sent over the same connection to be part of
the final response. Such behavior might constitute a cross-origin the final response. Such behavior might constitute a cross-origin
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o Description: Early Hints o Description: Early Hints
o Specification: [this document] o Specification: [this document]
5. References 5. References
5.1. Normative References 5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
<http://www.rfc-editor.org/info/rfc2119>. editor.org/info/rfc2119>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing", Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014, RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>. <https://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014, DOI 10.17487/RFC7231, June 2014, <https://www.rfc-
<http://www.rfc-editor.org/info/rfc7231>. editor.org/info/rfc7231>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540, Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015, DOI 10.17487/RFC7540, May 2015, <https://www.rfc-
<http://www.rfc-editor.org/info/rfc7540>. editor.org/info/rfc7540>.
5.2. Informative References 5.2. Informative References
[Preload] Grigorik, I., "Preload", n.d., <https://w3c.github.io/ [Preload] Grigorik, I., "Preload", n.d., <https://w3c.github.io/
preload/>. preload/>.
Appendix A. Changes Appendix A. Changes
A.1. Since draft-ietf-httpbis-early-hints-03 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
response.
A.2. Since draft-ietf-httpbis-early-hints-03
o Removed statements that were either redundant or contradictory to o Removed statements that were either redundant or contradictory to
RFC7230-7234. RFC7230-7234.
o Clarified what the server's expected behavior is. o Clarified what the server's expected behavior is.
o Explain that a server might want to send more than one 103 o Explain that a server might want to send more than one 103
response. response.
o Editorial Changes. o Editorial Changes.
A.2. Since draft-ietf-httpbis-early-hints-02 A.3. Since draft-ietf-httpbis-early-hints-02
o Editorial changes. o Editorial changes.
o Added an example. o Added an example.
A.3. Since draft-ietf-httpbis-early-hints-01 A.4. Since draft-ietf-httpbis-early-hints-01
o Editorial changes. o Editorial changes.
A.4. Since draft-ietf-httpbis-early-hints-00 A.5. Since draft-ietf-httpbis-early-hints-00
o Forbid processing the headers of a 103 response as part of the o Forbid processing the headers of a 103 response as part of the
informational response. informational response.
Appendix B. Acknowledgements Appendix B. Acknowledgements
Thanks to Tatsuhiro Tsujikawa for coming up with the idea of sending Thanks to Tatsuhiro Tsujikawa for coming up with the idea of sending
the Link header fields using an informational response. the Link header fields using an informational response.
Author's Address Author's Address
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