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Uniform Resource Identifier URI This document defines the 'acct' Uniform Resource Identifier (URI) scheme as a way to identify a user's account at a service provider, irrespective of the particular protocols that can be used to interact with the account.

Existing Uniform Resource Identifier (URI) schemes that enable interaction with, or that identify resources associated with, a user's account at a service provider are tied to particular services or application protocols. Two examples are the 'mailto' scheme (which enables interaction with a user's email account) and the 'http' scheme (which enables retrieval of web files controlled by a user or interaction with interfaces providing information about a user). However, there exists no URI scheme that generically identifies a user's account at a service provider without specifying a particular protocol to use when interacting with the account. This specification fills that gap.

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 .

During formalization of the WebFinger protocol , much discussion occurred regarding the appropriate URI scheme to include when specifying a user's account as a web link . Although both the 'mailto' and 'http' schemes were proposed, not all service providers offer email services or web interfaces on behalf of user accounts (e.g., a microblogging or instant messaging provider might not offer email services, or an enterprise might not offer HTTP interfaces to information about its employees). Therefore, the participants in the discussion recognized that it would be helpful to define a URI scheme that could be used to generically identify a user's account at a service provider, irrespective of the particular application protocols used to interact with the account. The result was the 'acct' URI scheme defined in this document. (Note that a user is not necessarily a human; it could be an automated application such as a bot, a role-based alias, etc. However, an 'acct' URI is always used to identify something that has an account at a service, not the service itself.)

The syntax of the 'acct' URI scheme is defined under of this document. Although 'acct' URIs take the form "user@host", the scheme is designed for the purpose of identification instead of interaction (regarding this distinction, see Section 1.2.2 of ). The "Internet resource" identified by an 'acct' URI is a user's account hosted at a service provider, where the service provider is typically associated with a DNS domain name. Thus, a particular 'acct' URI is formed by setting the "user" portion to the user's account name at the service provider and by setting the "host" portion to the DNS domain name of the service provider. Consider the case of a user with an account name of "foobar" on a microblogging service "status.example.net". It is taken as convention that the string "foobar@status.example.net" designates that account. This is expressed as a URI using the 'acct' scheme as "acct:foobar@status.example.net". A common scenario is for a user to register with a service provider using an identifier (such as an email address) that is associated with some other service provider. For example, a user with the email address "juliet@capulet.example" might register with a commerce website whose domain name is "shoppingsite.example". In order to use her email address as the localpart of the 'acct' URI, the at-sign character (U+0040) needs to be percent-encoded as described in . Thus, the resulting 'acct' URI would be "acct:juliet%40capulet.example@shoppingsite.example". It is not assumed that an entity will necessarily be able to interact with a user's account using any particular application protocol, such as email; to enable such interaction, an entity would need to use the appropriate URI scheme for such a protocol, such as the 'mailto' scheme. While it might be true that the 'acct' URI minus the scheme name (e.g., "user@example.com" derived from "acct:user@example.com") can be reached via email or some other application protocol, that fact would be purely contingent and dependent upon the deployment practices of the provider. Because an 'acct' URI enables abstract identification only and not interaction, this specification provides no method for dereferencing an 'acct' URI on its own, e.g., as the value of the 'href' attribute of an HTML anchor element. For example, there is no behavior specified in this document for an 'acct' URI used as follows:

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Any protocol that uses 'acct' URIs is responsible for specifying how an 'acct' URI is employed in the context of that protocol (in particular, how it is dereferenced or resolved; see ). As a concrete example, an "Account Information" application of the WebFinger protocol might take an 'acct' URI, resolve the host portion to find a WebFinger server, and then pass the 'acct' URI as a parameter in a WebFinger HTTP request for metadata (i.e., web links ) about the resource. For example:

The service retrieves the metadata associated with the account identified by that URI and then provides that metadata to the requesting entity in an HTTP response. If an application needs to compare two 'acct' URIs (e.g., for purposes of authentication and authorization), it MUST do so using case normalization and percent-encoding normalization as specified in Sections 6.2.2.1 and 6.2.2.2 of .

Because the 'acct' URI scheme does not directly enable interaction with a user's account at a service provider, direct security concerns are minimized. However, an 'acct' URI does provide proof of existence of the account; this implies that harvesting published 'acct' URIs could prove useful to spammers and similar attackers -- for example, if they can use an 'acct' URI to leverage more information about the account (e.g., via WebFinger) or if they can interact with protocol-specific URIs (such as 'mailto' URIs) whose user@host portion is the same as that of the 'acct' URI. In addition, protocols that make use of 'acct' URIs are responsible for defining security considerations related to such usage, e.g., the risks involved in dereferencing an 'acct' URI, the authentication and authorization methods that could be used to control access to personal data associated with a user's account at a service, and methods for ensuring the confidentiality of such information. The use of percent-encoding allows a wider range of characters in account names but introduces some additional risks. Implementers are advised to disallow percent-encoded characters or sequences that would (1) result in space, null, control, or other characters that are otherwise forbidden, (2) allow unauthorized access to private data, or (3) lead to other security vulnerabilities.

As specified in , the 'acct' URI scheme allows any character from the Unicode repertoire encoded as UTF-8 and then percent-encoded into valid ASCII . Before applying any percent-encoding, an application MUST ensure the following about the string that is used as input to the URI-construction process: The userpart consists only of Unicode code points that conform to the PRECIS IdentifierClass specified in . The host consists only of Unicode code points that conform to the rules specified in . Internationalized domain name (IDN) labels are encoded as A-labels .

In accordance with the guidelines and registration procedures for new URI schemes , this section provides the information needed to register the 'acct' URI scheme. acct permanent The 'acct' URI syntax is defined here in Augmented Backus-Naur Form (ABNF) , borrowing the 'host', 'pct-encoded', 'sub-delims', and 'unreserved' rules from :

The 'acct' URI scheme identifies accounts hosted at service providers. It is used only for identification, not interaction. A protocol that employs the 'acct' URI scheme is responsible for specifying how an 'acct' URI is dereferenced in the context of that protocol. There is no media type associated with the 'acct' URI scheme. See of this document. At the time of this writing, only the WebFinger protocol uses the 'acct' URI scheme. However, use is not restricted to the WebFinger protocol, and the scheme might be considered for use in other protocols. There are no known interoperability concerns related to use of the 'acct' URI scheme. See of this document. Peter Saint-Andre, peter@andyet.com This scheme is registered under the IETF tree. As such, the IETF maintains change control. None.

The Unicode Consortium

The 'acct' URI scheme was originally proposed during work on the WebFinger protocol; special thanks are due to Blaine Cook, Brad Fitzpatrick, and Eran Hammer-Lahav for their early work on the concept (which in turn was partially inspired by work on Extensible Resource Identifiers at OASIS). The scheme was first formally specified in ; the authors of that specification (Paul Jones, Gonzalo Salgueiro, and Joseph Smarr) are gratefully acknowledged. Thanks are also due to Stephane Bortzmeyer, Melvin Carvalho, Martin Duerst, Graham Klyne, Barry Leiba, Subramanian Moonesamy, Evan Prodromou, James Snell, and various participants in the IETF APPSAWG for their feedback. Meral Shirazipour completed a Gen-ART review. Dave Cridland completed an AppsDir review and is gratefully acknowledged for providing proposed text that was incorporated into Sections  and . IESG comments from Richard Barnes, Adrian Farrel, Stephen Farrell, Barry Leiba, Pete Resnick, and Sean Turner also led to improvements in the specification.