Early Hints (103): An HTTP Status Code for Indicating Hints
A provisional HTTP response that lets a server hand over hints while it is still working on the real answer.
Oku, K., "An HTTP Status Code for Indicating Hints", RFC 8297, DOI 10.17487/RFC8297, December 2017, <https://www.rfc-editor.org/info/rfc8297>.
@misc{rfc8297,
series = {Request for Comments},
number = 8297,
howpublished = {RFC 8297},
publisher = {RFC Editor},
doi = {10.17487/RFC8297},
url = {https://www.rfc-editor.org/info/rfc8297},
author = {K. Oku},
title = {{An HTTP Status Code for Indicating Hints}},
pagetotal = 7,
year = 2017,
month = dec,
} RFC text
Contents
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.
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
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.
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.
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.
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/>.
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
AI ELI5
On this page
§1The Waiting Problem
Many HTTP responses contain links to external resources that must be fetched before use, for example when a web browser renders HTML. Having those links as early as possible helps to minimize perceived latency.
The preload link relation can carry such links in the Link header field of a response. The catch is that an origin server cannot always build the header block of the final response immediately. It might delegate the request to an upstream HTTP server at a distant location, or the status code might depend on the result of a database query. The server would like to send some header fields at once, but it cannot until the status code and the full headers of the final response are settled.
The server knows what the web page will need long before the page is ready. It just has no polite way to say so until the whole answer is done.
§1.1Why Not Server Push?
HTTP/2 server push can also speed up delivery, but the RFC names two limits. It only works for resources the server is authoritative for, and the pushed response is sent whether or not the client already has it cached.
Delivering Link header fields early is more flexible and might consume less bandwidth, at the cost of spending one extra round trip compared to server push in the worst case. The informational response can also be used by an origin server to trigger HTTP/2 server push at a caching intermediary.
Pushing is like mailing someone a parcel they may already own. A hint is just a note that says what is coming, so they can say no thanks.
§2The 103 Status Code
The 103 (Early Hints) informational status code tells the client that the server is likely to send a final response with the header fields included in the informational response. A server will typically repeat those fields in the final response, though it might not if it learns before finishing that the hints were wrong.
A client can speculatively evaluate the header fields while it waits. For example, it might see a Link value with the relation type preload and start fetching the target. The fields are only hints and do not replace the header fields on the final response.
Apart from performance optimizations, evaluating a 103 response MUST NOT affect how the final response is processed. A client MUST NOT treat its header fields as metadata about the 103 response itself.
The note is only a hint. You may start setting the table, but you must still wait for the real meal and check what actually arrives.
§2.1Partial and Repeated Hints
A server MAY list only some of the header fields it expects in the final response. A client SHOULD NOT read the absence of a field from a 103 response as a sign that the field is unlikely to appear later.
A server might also send more than one 103 response before the final one. A server MAY emit several as new information becomes available while the request is processed. It does not need to repeat fields already sent, though it may. The client can consider any combination of fields from multiple 103 responses when anticipating the final one. One case the RFC gives is a caching intermediary that sends a 103 based on a stale cached response and then forwards a 103 and a final response from the origin server for a revalidation request.
In the RFC's second example, two 103 responses announce three Link header fields. The final 200 response contains two of them, and the third is replaced by a Link with a different value.
The waiter may come back with more notes as the cook decides things. The notes can overlap, and the final plate may differ a little from what the notes said.
§2.2A Worked Exchange
In the RFC's basic example, the client sends GET / HTTP/1.1 with Host: example.com. The server first answers HTTP/1.1 103 Early Hints with two Link headers asking for preloading of /style.css as a style and /script.js as a script. It then sends HTTP/1.1 200 OK with the same two Link fields, a Content-Length of 1234 and the HTML body.
The 103 response arrives first, so the client can begin fetching the two files while the server finishes producing the page.
The first message says, you will need these two files. The second message is the real page, and it lists the same files again.
- GET / HTTP/1.1Host: example.com
- 103 Early HintsLink headers: /style.css and /script.js, rel=preload
- 200 OKSame Link headers plus the HTML body
§3Security Considerations
Some clients may mishandle a 103 response, because informational responses are rarely used in reply to requests that do not include an Expect header field.
The serious case is an HTTP/1.1 client that treats an informational response as a final response. It is then likely to consider all responses to later requests on the same connection to be part of the final response. This might be a cross-origin information disclosure vulnerability if the client multiplexes requests to different origins onto one persistent connection. For this reason a server might refrain from sending 103 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 the response header fields does not affect how the end of the response body is determined.
A confused listener might think the first note was the whole answer and then mix up the next conversation. So be careful whom you send notes to.
§4IANA Registration
The RFC registers one entry in the HTTP Status Codes registry: code 103, description Early Hints, specification RFC 8297.
The number 103 was written in the official list of HTTP codes with the name Early Hints.