HTTP D. Benjamin
Internet-Draft Google LLC
Updates: ietf-httpbis-client-hints (if August 27, 2020
approved)
Intended status: Experimental
Expires: February 28, 2021
Client Hint Reliability
draft-davidben-http-client-hint-reliability-01
Abstract
This document defines the Critical-CH HTTP response header, and the
ACCEPT_CH HTTP/2 frame to allow HTTP servers to reliably specify
their Client Hint preferences, with minimal performance overhead.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 2
3. The Critical-CH Response Header Field . . . . . . . . . . . . 3
3.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. The ACCEPT_CH HTTP/2 Frame . . . . . . . . . . . . . . . . . 5
4.1. Processing ACCEPT_CH Frames . . . . . . . . . . . . . . . 6
4.2. Interaction with Critical-CH . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
[I-D.ietf-httpbis-client-hints] defines a response header, Accept-CH,
for servers to advertise a set of request headers used for proactive
content negotiation. This allows user agents to send request headers
only when used, improving their performance overhead as well as
reducing passive fingerprinting surface.
However, on the first HTTP request to a server, the user agent will
not have received the Accept-CH header and may not take the server
preferences into account. More generally, the server's configuration
may have changed since the most recent HTTP request to the server.
This document defines a pair of mechanisms to resolve this:
1. an HTTP response header, Critical-CH, for the server to instruct
the user agent to retry the request
2. an alternate delivery mechanism for Accept-CH in HTTP/2
[RFC7540], which can avoid the performance hit of a retry in most
cases
2. Conventions and Definitions
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.
This document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234].
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3. The Critical-CH Response Header Field
When a user agent requests a resource based on a missing or outdated
Accept-CH value, it may not send a desired request header field.
Neither user agent nor server has enough information to reliably and
efficiently recover from this situation. The server can observe that
the header is missing, but the user agent may not have supported the
header, or may have chosen not to send it. Triggering a new request
in these cases would risk an infinite loop or an unnecessary round-
trip.
Conversely, the user agent can observe that a request header appears
in the Accept-CH (Section 3.1 of [I-D.ietf-httpbis-client-hints]) and
Vary (Section 7.1.4 of [RFC7231]) response header fields. However,
retrying based on this information would waste resources if the
resource only used the Client Hint as an optional optimization.
This document introduces critical Client Hints. These are the Client
Hints which meaningfully change the resulting resource. For example,
a server may use the Device-Memory Client Hint [DEVICE-MEMORY] to
select simple and complex variants of a resource to different user
agents. Such a resource should be fetched consistently across page
loads to avoid jarring user-visible switches.
The server specifies critical Client Hints with the Critical-CH
response header field. It is a Structured Header
[I-D.ietf-httpbis-header-structure] whose value MUST be an sf-list
(Section 3.1 of [I-D.ietf-httpbis-header-structure]) whose members
are tokens (Section 3.3.4 of [I-D.ietf-httpbis-header-structure]).
Its ABNF is:
Critical-CH = sf-list
For example:
Critical-CH: Sec-CH-Example, Sec-CH-Example-2
Each token listed in the Critical-CH header SHOULD additionally be
present in the Accept-CH and Vary response headers.
When a user agent receives an HTTP response containing a Critical-CH
header, it first processes the Accept-CH header as described in
Section 3.1 of [I-D.ietf-httpbis-client-hints]. It then performs the
following steps:
1. If the request did not use a safe method (Section 4.2.1 of
[RFC7231]), ignore the Critical-CH header and continue processing
the response as usual.
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2. If the response was already the result of a retry, ignore the
Critical-CH header and continue processing the response as usual.
3. Determine the Client Hints that would have been sent given the
updated Accept-CH value, incorporating the user agent's local
policy and user preferences. See also Section 2.1 of
[I-D.ietf-httpbis-client-hints].
4. Compare this result to the Client Hints which were sent. If any
Client Hint listed in the Critical-CH header was not previously
sent and would now have been sent, retry the request with the new
preferences. Otherwise, continue processing the response as
usual.
Note this procedure does not cause the user agent to send Client
Hints it would not otherwise send.
3.1. Example
For example, if the user agent loads https://example.com with no
knowledge of the server's Accept-CH preferences, it may send the
following response:
GET / HTTP/1.1
Host: example.com
HTTP/1.1 200 OK
Content-Type: text/html
Accept-CH: Sec-CH-Example, Sec-CH-Example-2
Vary: Sec-CH-Example
Critical-CH: Sec-CH-Example
In this example, the server, across the whole origin, uses both Sec-
CH-Example and Sec-CH-Example-2 Client Hints. However, this resource
only uses Sec-CH-Example, which it considers critical.
The user agent now processes the Accept-CH header and determines it
would have sent both headers. Sec-CH-Example is listed in Critical-
CH, so the user agent retries the request, and receives a more
specific response.
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GET / HTTP/1.1
Host: example.com
Sec-CH-Example: 1
Sec-CH-Example-2: 2
HTTP/1.1 200 OK
Content-Type: text/html
Accept-CH: Sec-CH-Example, Sec-CH-Example-2
Vary: Sec-CH-Example
Critical-CH: Sec-CH-Example
4. The ACCEPT_CH HTTP/2 Frame
While Critical-CH header provides reliability, it requires a retry on
some requests. This document additionally introduces the ACCEPT_CH
HTTP/2 frame as an optimization so the server's Client Hint
preferences are usually available before the client's first request.
[[TODO: Alternatively, is it time to revive draft-bishop-httpbis-
extended-settings?]]
The ACCEPT_CH frame type is TBD (decimal TBD) and contains one or
more entries, each consisting of a pair of length-delimited strings:
+-------------------------------+
| Origin-Len (16) |
+-------------------------------+-------------------------------+
| Origin ...
+-------------------------------+-------------------------------+
| Accept-CH-Len (16) |
+-------------------------------+-------------------------------+
| Accept-CH-Value ...
+---------------------------------------------------------------+
The fields are defined as follows:
Origin-Len: An unsigned, 16-bit integer indicating the length, in
octets, of the Origin field.
Origin: A sequence of characters containing the ASCII serialization
of an origin (Section 6.2 of [RFC6454]) that the sender is
providing an Accept-CH value for.
Accept-CH-Len: An unsigned, 16-bit integer indicating the length, in
octets, of the Accept-CH-Value field.
Accept-CH-Value: A sequence of characters containing the Accept-CH
value for the corresponding origin. This value MUST satisfy the
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Accept-CH ABNF defined in Section 3.1 of
[I-D.ietf-httpbis-client-hints].
HTTP/2 Servers which request Client Hints SHOULD send an ACCEPT_CH
frame as early as possible. Connections using TLS [RFC8446] which
negotiate the Application Layer Protocol Settings (ALPS)
[I-D.vvv-tls-alps] extension SHOULD include the ACCEPT_CH frame in
the ALPS value as described in [I-D.vvv-httpbis-alps]. This ensures
the information is available to the user agent when it makes the
first request.
User agents MUST NOT send ACCEPT_CH frames. Servers which receive an
ACCEPT_CH frame MUST respond with a connection error (Section 5.4.1
of [RFC7540]) of type PROTOCOL_ERROR.
ACCEPT_CH frames always apply to a single connection, never a single
stream. The identifier in the ACCEPT_CH frame MUST be zero. The
flags field of an ACCEPT_CH field is unused and MUST be zero. If a
user agent receives an ACCEPT_CH frame whose stream identifier or
flags field is non-zero, it MUST respond with a connection error of
type PROTOCOL_ERROR.
4.1. Processing ACCEPT_CH Frames
The user agent remembers the most recently received ACCEPT_CH frame
for each connection. When it receives a new ACCEPT_CH frame, either
in application data or ALPS, it overwrites this value. As this is an
optimization, the user agent MAY bound the size and ignore or forget
entries to reduce resource usage.
When the user agent makes an HTTP request to a particular origin over
an HTTP/2 connection, it looks up the origin in the remembered
ACCEPT_CH, if any. If it finds a match, it determines additional
Client Hints to send, incorporating its local policy and user
preferences. See Section 2.1 of [I-D.ietf-httpbis-client-hints].
If there are additional Client Hints, the user agent restarts the
request with updated headers. The connection has already been
established, so this restart does not incur any additional network
latency. Note it may result in a different secondary HTTP cache key
(see Section 4.1 of [RFC7234]) and select a different cached
response. If the new cached response does not need revalidation, it
may not use the connection at all.
User agents MUST NOT process Client Hint preferences in ACCEPT_CH
frames corresponding to origins for which the connection is not
authoritative. Note the procedure above implicitly satisfies this by
deferring processing to after the connection has been chosen for a
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corresponding request. Unauthoritative origins and other unmatched
entries are ignored.
[[TODO: Some variations on this behavior we could choose instead:
o Do new ACCEPT_CH frames override the whole set or implement some
kind of update? Overriding the whole set seems simplest and most
consistent with an EXTENDED_SETTINGS variant.
o Should the user agent reject the ACCEPT_CH frame if there are
unexpected origins in there? Deferring avoids needing to worry
about this, and ignoring the unused ones may interact better with
secondary certs.
o Should ACCEPT_CH frames be deferred or just written to the cache
when received? Deferred simplifies reasoning about bad origins,
predictive connections, etc., but means interactions between
ACCEPT_CH and Accept-CH are more complex (see below).
o How should ACCEPT_CH and Accept-CH interact? The document
currently proposes unioning them, which is easy. Accept-CH first
would work, but unnecessarily ignore newer connection-level
ACCEPT_CHs. ACCEPT_CH would not; a stale connection-level
preference would get stuck. Whichever is received earlier would
also work, but requires tracking timestamps if deferred (see
above).]]
4.2. Interaction with Critical-CH
The ACCEPT_CH frame avoids a round-trip, so relying on it over
Critical-CH would be preferable. However, this is not always
possible:
o The server may be running older software without support for
ACCEPT_CH or ALPS.
o The server's Accept-CH preferences may change while existing
connections are open. Those connections will have outdated
ACCEPT_CH frames. While the server could send a new one, the
frame may not arrive in time for the next request. Moreover, if
the HTTP serving frontend is an intermediary like a CDN, it may
not be proactively notified of origin server changes.
o HTTP/2 allows connection reuse across multiple origins
(Section 9.1.1 of [RFC7540]). Some origins may not be listed in
the ACCEPT_CH frame, particularly if the server used a wildcard
X.509 certificate.
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Thus this document defines both mechanisms. Critical-CH provides
reliable Client Hint delivery, while the ACCEPT_CH frame avoids the
retry in most cases.
5. Security Considerations
Request header fields may expose sensitive information about the
user's environment. Section 4.1 of [I-D.ietf-httpbis-client-hints]
discusses some of these considerations. The document augments the
capabilities of Client Hints, but does not change these
considerations. The procedure described in Section 3 does not result
in the user agent sending request headers it otherwise would not
have.
The ACCEPT_CH frame does introduce a new way for HTTP/2 connections
to make assertions about origins they are not authoritative for, but
the procedure in Section 4.1 defers processing until after the user
agent has decided to use the connection for a particular request
(Section 9.1.1 of [RFC7540]). The user agent will thus only
information in an ACCEPT_CH frame if it considers the connection
authoritative for the origin.
6. IANA Considerations
This specification adds an entry to the "HTTP/2 Frame Type" registry
with the following parameters:
o Frame Type: ACCEPT_CH
o Code: TBD
o Allowed in ALPS: Yes
o Reference: [[this document]]
[[TODO: As of writing, the HTTP/2 Frame Type registry does not
include an Allowed in ALPS column. [I-D.vvv-httpbis-alps], as of
writing, will add it. This document should be updated as that design
evolves.]]
7. References
7.1. Normative References
[I-D.ietf-httpbis-client-hints]
Grigorik, I. and Y. Weiss, "HTTP Client Hints", draft-
ietf-httpbis-client-hints-15 (work in progress), July
2020.
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[]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", draft-ietf-httpbis-header-structure-19 (work in
progress), June 2020.
[I-D.vvv-httpbis-alps]
Vasiliev, V., "Using TLS Application-Layer Protocol
Settings (ALPS) in HTTP", draft-vvv-httpbis-alps-00 (work
in progress), July 2020.
[I-D.vvv-tls-alps]
Vasiliev, V., "TLS Application-Layer Protocol Settings
Extension", draft-vvv-tls-alps-00 (work in progress), June
2020.
[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>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>.
[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>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[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>.
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[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
7.2. Informative References
[DEVICE-MEMORY]
Panicker, S., "Device Memory", n.d.,
<https://w3c.github.io/device-memory/>.
Acknowledgments
This document has benefited from contributions and suggestions from
Ilya Grigorik, Nick Harper, Matt Menke, Aaron Tagliaboschi, Victor
Vasiliev, Yoav Weiss, and others.
Author's Address
David Benjamin
Google LLC
Email: davidben@google.com
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