Client Hint Reliability

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Table of Contents

1. Introduction

[RFC8942] 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] and HTTP/3 [I-D.ietf-quic-http], 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].¶

This document uses the variable-length integer encoding and frame diagram format from [RFC9000].¶

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When a user agent requests a resource based on a missing or outdated Accept-CH value, it may not send a desired request header field.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 [RFC8942]) 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 [RFC8941] whose value MUST be an sf-list (Section 3.1 of [RFC8941]) whose members are tokens (Section 3.3.4 of [RFC8941]). 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 [RFC8942]. 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.¶
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 [RFC8942].¶
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.¶

  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

  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 Frame

While Critical-CH header provides reliability, it requires a retry on some requests. This document additionally introduces the ACCEPT_CH HTTP/2 and HTTP/3 frames as an optimization so the server's Client Hint preferences are usually available before the client's first request.¶

HTTP/2 and HTTP/3 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.¶

[[TODO: Alternatively, is it time to revive draft-bishop-httpbis-extended-settings?]]¶

+-------------------------------+
|         Origin-Len (16)       |
+-------------------------------+-------------------------------+
|         Origin                                              ...
+-------------------------------+-------------------------------+
|         Value-Len (16)        |
+-------------------------------+-------------------------------+
|         Value                                               ...
+---------------------------------------------------------------+

HTTP/3 ACCEPT_CH Entry {
  Origin Length (i),
  Origin (..)
  Value Length (i),
  Value (..),
}

HTTP/3 ACCEPT_CH Frame {
  Type (i) = TBD,
  Length (i),
  HTTP/3 ACCEPT_CH Entry (..) ...,
}

5. Security Considerations

Request header fields may expose sensitive information about the user's environment. Section 4.1 of [RFC8942] 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 or HTTP/3 connections to make assertions about origins they are not authoritative for, but the procedure in Section 4.3 defers processing until after the user agent has decided to use the connection for a particular request (Section 9.1.1 of [RFC7540] and Section 3.4 of [I-D.ietf-quic-http]). The user agent will thus only use information from 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 [RFC7540] with the following parameters:¶

• Frame Type: ACCEPT_CH¶
• Code: TBD¶
• Allowed in ALPS: Yes¶
• Reference: [[this document]]¶

This specification adds an entry to the "HTTP/3 Frame Type" registry [I-D.ietf-quic-http] with the following parameters:¶

• Frame Type: ACCEPT_CH¶
• Code: TBD¶
• Allowed in ALPS: Yes¶
• Reference: [[this document]]¶

[[TODO: As of writing, the Frame Type registries do not include Allowed in ALPS columns, but [I-D.vvv-httpbis-alps] adds them. This document should be updated as that design evolves.]]¶

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