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The existing IMAP URL specification (RFC 5092) lists several <access> identifiers and <access> identifier prefixes, that can be used to restrict access to URLAUTH-generated URLs. However, these identifiers do not provide facilities for new services such as streaming. This document proposes a set of new <access> identifiers as well as an IANA mechanism to register new <access> identifiers for future applications.
This document updates RFC 5092.
1.
Introduction
2.
Conventions Used in this Document
3.
Additional Authorized Access Identifiers
3.1.
Existing Access Identifiers
3.2.
Requirement for Additional Access Identifiers
3.3.
Additional Access Identifier Specification
3.4.
Defining an access identifier
for Streaming
4.
Formal Syntax
5.
Acknowledgements
6.
IANA Considerations
6.1.
Access Identifier Registration Template
6.2.
Stream Application
Registration
6.3.
Submit Application
Registration
6.4.
User Application
Registration
6.5.
Authuser Application
Registration
6.6.
Anonymous Application
Registration
7.
Security Considerations
8.
References
8.1.
Normative References
8.2.
Informative References
§
Author's Address
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The IMAP URL specification [RFC5092] (Melnikov, A. and C. Newman, “IMAP URL Scheme,” November 2007.) provides a way to carry authorization information in IMAP URLs. Several authorization <access> identifiers are specified in the document, which allow URLAUTH-authorized URLs to be used only by anonymous users, authenticated users, or message submission entities. However there is no mechanism defined to create new <access> identifiers, and overloading the existing mechanisms has security as well as administrative implications.
This document describes a new <access> identifier "stream", to be used by message streaming entities (as described in [STREAMING] (Cook, N., “Streaming Internet Messaging Attachments,” Dec 2008.)), and defines an IANA registration template, which can be used to register new <access> identifiers for future applications. IANA definitions for the existing access identifiers and prefixes from RFC 5092 are also defined in this document - this document updates RFC 5092 and should be taken as the master in the event of any differences or discrepancies.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].
In examples, "C:" and "S:" indicate lines sent by the client and server respectively. If a single "C:" or "S:" label applies to multiple lines, then some of the line breaks between those lines are for editorial clarity only and may not be part of the actual protocol exchange.
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The IMAP URL specification, IMAPURL (Melnikov, A. and C. Newman, “IMAP URL Scheme,” November 2007.) [RFC5092], specifies the following authorized <access> identifiers:
Additionally the following <access> identifier prefixes are defined:
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The existing <access> identifiers are suitable for user-based authorization, but only the "submit+" <access> identifier prefix is suitable for entities acting on behalf of a user. Generic support for external entities acting on behalf of users is required for new services such as streaming (Cook, N., “Streaming Internet Messaging Attachments,” Dec 2008.) [STREAMING].
The "submit+" <access> identifier prefix is not suitable for use as a general mechanism to grant access to entities acting on behalf of users, for reasons that include:
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The previous section established that additional access identifiers are required to support applications, such as streaming (Cook, N., “Streaming Internet Messaging Attachments,” Dec 2008.) [STREAMING], that require entities to retrieve URLAUTH URLs on behalf of users. This section describes the scope and meaning of any additional <access> identifiers that are created.
Additional <access> identifiers MUST take one of two forms (Section 4 (Formal Syntax) gives the formal ABNF syntax):
In both cases, the semantics are the same as those for "submit+", i.e. the <access> identifier or <access> identifier prefix (which MUST be followed by a userid), indicates that only a userid authorized as an application entity for the specified application is permitted to use this URL. In the case of <access> identifier prefixes, the IMAP server SHALL NOT validate the specified userid but MUST validate that the IMAP session has an authorization identity that is authorized as an application entity for the specified application. The application entity itself MAY choose to perform validation on any specified userid before attempting to retrieve the URL.
The authorization granted by any <access> identifiers used as described above is self-describing, and so requires the IMAP server to provide an extensible mechanism for associating userids with new applications. For example, imagine a new application "foo" is created, which requires application entities to retrieve URLs on behalf of users. In this case, the IMAP server would need to provide a way to register a new application "foo", and to associate the set of userids to be used by those entities with the application "foo". Any attempt to retrieve URLs containing the <access> identifier "foo" would be checked for authorization against the list of userids associated with the application "foo".
Section 6 (IANA Considerations) provides the template required to register new <access> identifiers or prefixes with IANA.
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One application that makes use of URLAUTH-authorized URLs is that of streaming multimedia files received as internet messaging attachments. This application is described in [STREAMING] (Cook, N., “Streaming Internet Messaging Attachments,” Dec 2008.).
See Section 6.2 (Stream Application Registration) for the IANA registration template for the "stream" <access> identifier.
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The following syntax specification uses the Augmented Backus-Naur Form (ABNF) notation as specified in [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.).
Except as noted otherwise, all alphabetic characters are case-insensitive. The use of upper or lower case characters to define token strings is for editorial clarity only. Implementations MUST accept these strings in a case-insensitive fashion.
The ABNF specified below updates the formal syntax of <access> identifier as defined in IMAP URL (Melnikov, A. and C. Newman, “IMAP URL Scheme,” November 2007.) [RFC5092].
application = 1*(ALPHA/DIGIT) access =/ application / (application "+" enc-user)
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This document was inspired by discussions in the Lemonade Working Group.
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Access identifiers and prefixes MUST be specified in a standards track or IESG approved experimental RFC. This section gives the IAN registration entries for the existing access identifiers and prefixes from RFC 5092 as well as the entry for the "stream" application.
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To: iana@iana.org
Subject: IMAP URL Access Identifier Registration
Type: [Either "<access> identifier" or
"<access> identifier prefix"]
Application: [Name of the application, e.g. "stream"]
Description: [A description of the application and its use
of IMAP URLs]
RFC Number: [Number of the RFC that the application was
defined in ]
Contact: [email and/or physical address to contact for
additional information]
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To: iana@iana.org
Subject: IMAP URL Access Identifier Registration
Type: <access> identifier
Application: stream
Description: Used by SIP Media Servers to retrieve
attachments for streaming to email
clients
RFC Number: This RFC
Contact: Neil Cook mailto:neil.cook@noware.co.uk
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To: iana@iana.org
Subject: IMAP URL Access Identifier Registration
Type: <access> identifier prefix
Application: submit
Description: Used by message submission entities to
retrieve attachments to be included in
submitted messages
RFC Number: This RFC
Contact: Neil Cook mailto:neil.cook@noware.co.uk
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To: iana@iana.org Subject: IMAP URL Access Identifier Registration Type: <access> identifier prefix Application: user Description: Used to restrict access to to IMAP sessions that are logged in as the specified userid RFC Number: This RFC Contact: Neil Cook mailto:neil.cook@noware.co.uk
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To: iana@iana.org
Subject: IMAP URL Access Identifier Registration
Type: <access> identifier
Application: authuser
Description: Used to restrict access to to IMAP sessions
that are logged in as any non-anonymous
user of that IMAP server
RFC Number: This RFC
Contact: Neil Cook mailto:neil.cook@noware.co.uk
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To: iana@iana.org
Subject: IMAP URL Access Identifier Registration
Type: <access> identifier
Application: anonymous
Description: Indicates that use of this URL is
not restricted by session authorization
identity
RFC Number: This RFC
Contact: Neil Cook mailto:neil.cook@noware.co.uk
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The extension to <access> identifiers specified in this document provides a mechanism for extending the semantics of the "submit+" <access> prefix to arbitrary applications. The use of such additional <access> identifiers and prefixes is primarily for security purposes, i.e. to prevent the overloading of "submit+" as a generic mechanism to allow entities to retrieve IMAP URLs on behalf of userids. Other than this, the security implications are identical to those discussed in Section 10.1 of IMAPURL (Melnikov, A. and C. Newman, “IMAP URL Scheme,” November 2007.) [RFC5092].
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| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| [RFC5092] | Melnikov, A. and C. Newman, “IMAP URL Scheme,” RFC 5092, November 2007 (TXT). |
| [RFC5234] | Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” STD 68, RFC 5234, January 2008 (TXT). |
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| [STREAMING] | Cook, N., “Streaming Internet Messaging Attachments,” draft-ietf-lemonade-streaming-08.txt (Work in Progress) , Dec 2008. |
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| Neil L Cook | |
| Cloudmark | |
| Email: | neil.cook@noware.co.uk |