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Internet-Draft This documents defines an HTTP Client Hint for sharing a client's rough location using the Geohash format. Discussion Venues Source for this draft and an issue tracker can be found at .

Introduction HTTP Client Hints defines a convention for HTTP headers to communicate optional information from clients to servers as hints. This can be done conditionally based on if a server claims supports for a particular hint. This document defines a client hint that can be used to send a location that the client wants to use for influencing server behavior. It uses the Geohash algorithm to encode latitude and longitude coordinates into an alphanumeric token that can be truncated to provide a less specific location. This header is intended to be used to provide rough geolocation hints to servers in situations where the server cannot directly ascertain the location of the client. For example, a client that is accessing a server through a proxy or a VPN might provide a rough hint to a server when looking up information that may vary depending on location. This document also defines a how forward proxies can use proxy status fields to inform clients about the result of their Geohash hints.

Requirements 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 when, and only when, they appear in all capitals, as shown here.

Geohash Header The "Sec-CH-Geohash" is an Item Structured Header . Its value MUST be a String, and MUST have at least 1 character and no more than 12 characters. The ABNF is: The string itself is an encoded Geohash, which uses the 32 different characters from the "Geohash alphabet" . The following example shows an encoding of the coordinates 57.64911,10.40744: Servers that can provide different content based on Geohash hints SHOULD include the headers in their "Accept-CH" list. Servers also SHOULD indicate for any cacheable content if the Geohash hint will influence the cached content, using the "Vary" header.

Server Behavior Upon receiving a Geohash Client Hint, a server can use the information to influence its behavior in various ways. The server can use the Geohash to determine the content of HTTP responses, as a replacement for inferring location from client IP addresses. If the server is acting as a forward proxy, such as a CONNECT proxy, it can use the Geohash to determine an appropriate geo-mapped IP address to use for outbound connections, or a client subnet to present in the EDNS0 Client Subnet extension for DNS queries .

Proxy Behavior If a proxy receiving the Geohash hint cannot respect the location indicated by the hint, it SHOULD include a Proxy-Status header in its response, with the "details" parameter containing the string "invalid geohash".

Security Considerations The use of the Geohash Client Hint MUST use the Sec- header prefix as recommended in . Client location can be used to fingerprint and tracker users, so clients MUST have a default policy around when to allow use of the Geohash Client Hint, as well as a default length of Geohash. Shorter, truncated Geohashes provide less specific locality. Servers MUST NOT use Geohash Client Hints for making security or access-control decisions, as the value can be spoofed by a client. The hint is intended only for use in optimizing behavior.

IANA Considerations

HTTP Headers This document registers the "Sec-CH-Geohash" header in the "Permanent Message Header Field Names" registry <>.

References Normative References HTTP defines proactive content negotiation to allow servers to select the appropriate response for a given request, based upon the user agent's characteristics, as expressed in request headers. In practice, user agents are often unwilling to send those request headers, because it is not clear whether they will be used, and sending them impacts both performance and privacy. This document defines an Accept-CH response header that servers can use to advertise their use of request headers for proactive content negotiation, along with a set of guidelines for the creation of such headers, colloquially known as "Client Hints." In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document describes a set of data types and associated algorithms that are intended to make it easier and safer to define and handle HTTP header and trailer fields, known as "Structured Fields", "Structured Headers", or "Structured Trailers". It is intended for use by specifications of new HTTP fields that wish to use a common syntax that is more restrictive than traditional HTTP field values. Fastly Google This document defines the Proxy-Status HTTP field to convey the details of intermediary response handling, including generated errors. Informative References The Domain Name System's wire protocol includes a number of fixed fields whose range has been or soon will be exhausted and does not allow requestors to advertise their capabilities to responders. This document describes backward-compatible mechanisms for allowing the protocol to grow. This document updates the Extension Mechanisms for DNS (EDNS(0)) specification (and obsoletes RFC 2671) based on feedback from deployment experience in several implementations. It also obsoletes RFC 2673 ("Binary Labels in the Domain Name System") and adds considerations on the use of extended labels in the DNS. This document describes an Extension Mechanisms for DNS (EDNS0) option that is in active use to carry information about the network that originated a DNS query and the network for which the subsequent response can be cached. Since it has some known operational and privacy shortcomings, a revision will be worked through the IETF for improvement.