Client Hint Reliability
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HTTP D. Benjamin Internet-Draft Google LLC Updates: ietf-httpbis-client-hints (if July 13, 2020 approved) Intended status: Experimental Expires: January 14, 2021 Client Hint Reliability draft-davidben-http-client-hint-reliability-00 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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on January 14, 2021. Copyright Notice Copyright (c) 2020 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. Benjamin Expires January 14, 2021 [Page 1] Internet-Draft Client Hint Reliability July 2020 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. Client Behavior . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . 9 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 clients 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 client 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 client 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]. Benjamin Expires January 14, 2021 [Page 2] Internet-Draft Client Hint Reliability July 2020 3. The Critical-CH Response Header Field When a client requests a resource based on a missing or outdated Accept-CH value, it may not send a desired request header field. Neither client nor server has enough information to reliably and efficiently recover from this situation. The server can observe that the header is missing, but the client 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 client 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 clients. 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. Benjamin Expires January 14, 2021 [Page 3] Internet-Draft Client Hint Reliability July 2020 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. Benjamin Expires January 14, 2021 [Page 4] Internet-Draft Client Hint Reliability July 2020 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 Benjamin Expires January 14, 2021 [Page 5] Internet-Draft Client Hint Reliability July 2020 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 client when it makes the first request. Clients 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 client 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. Client Behavior The client 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 client MAY bound the size and ignore or forget entries to reduce resource usage. When the client 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 treats the union of that value and any saved Accept-CH response header as the server's Client Hint preferences. It then proceeds as in Section 2.1 of [I-D.ietf-httpbis-client-hints]. Clients 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 corresponding request. Unauthoritative origins and other unmatched entries are ignored. [[TODO: Some variations on this behavior we could choose instead: Benjamin Expires January 14, 2021 [Page 6] Internet-Draft Client Hint Reliability July 2020 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 client 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 clients have existing connections open. In this case, the connection-level settings may be out of date. While the server could send a new ACCEPT_CH frame, the frame may not arrive in time for the client's 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. Thus this document defines both mechanisms. Critical-CH provides reliable Client Hint delivery, while the ACCEPT_CH frame avoids the retry in most cases. Benjamin Expires January 14, 2021 [Page 7] Internet-Draft Client Hint Reliability July 2020 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 client has decided to use the connection for a particular request (Section 9.1.1 of [RFC7540]). 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. [I-D.ietf-httpbis-header-structure] Nottingham, M. and P. Kamp, "Structured Field Values for HTTP", draft-ietf-httpbis-header-structure-19 (work in progress), June 2020. Benjamin Expires January 14, 2021 [Page 8] Internet-Draft Client Hint Reliability July 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>. [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>. [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/>. Benjamin Expires January 14, 2021 [Page 9] Internet-Draft Client Hint Reliability July 2020 Acknowledgments This document has benefited from contributions and suggestions from Ilya Grigorik, Nick Harper, Aaron Tagliaboschi, Victor Vasiliev, Yoav Weiss, and others. Author's Address David Benjamin Google LLC Email: firstname.lastname@example.org Benjamin Expires January 14, 2021 [Page 10]