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PROPOSED STANDARD

Network Working Group                                            N. Cook
Request for Comments: 5593                                     Cloudmark
Updates: 5092                                                  June 2009
Category: Standards Track


               Internet Message Access Protocol (IMAP) -
                    URL Access Identifier Extension

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 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 in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   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.












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Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................2
   3. Additional Authorized Access Identifiers ........................3
      3.1. Existing Access Identifiers ................................3
      3.2. Requirement for Additional Access Identifiers ..............3
      3.3. Additional Access Identifier Specification .................4
      3.4. Defining an Access Identifier for Streaming ................5
   4. Formal Syntax ...................................................5
   5. Acknowledgements ................................................6
   6. IANA Considerations .............................................6
      6.1. Access Identifier Registration Template ....................7
      6.2. Stream Application Registration ............................7
      6.3. Submit Application Registration ............................8
      6.4. User Application Registration ..............................8
      6.5. Authuser Application Registration ..........................9
      6.6. Anonymous Application Registration .........................9
   7. Security Considerations .........................................9
   8. References .....................................................10
      8.1. Normative References ......................................10
      8.2. Informative References ....................................10

1.  Introduction

   The IMAP URL specification [RFC5092] provides a way to carry
   authorization information in IMAP URLs.  Several authorization
   <access> identifiers are specified in the document that 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]), 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.

2.  Conventions Used in This Document

   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 [RFC2119].



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   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.

3.  Additional Authorized Access Identifiers

3.1.  Existing Access Identifiers

   The IMAP URL specification [RFC5092] specifies the following
   authorized <access> identifiers:

   o  "authuser" - Indicates that use of this URL is limited to
      authenticated IMAP sessions that are logged in as any non-
      anonymous user.

   o  "anonymous" - Indicates that use of this URL is not restricted by
      session authorization identity.

   Additionally, the following <access> identifier prefixes are defined
   in [RFC5092]:

   o  "submit+" - Followed by a userid, indicates that only a userid
      authorized as a message submission entity on behalf of the
      specified userid is permitted to use this URL.

   o  "user+" - Followed by a userid, indicates that use of this URL is
      limited to IMAP sessions that are logged in as the specified
      userid.

3.2.  Requirement for Additional Access Identifiers

   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 [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:

   o  Security - The IMAP server maintains a list of submission server
      entities that are entitled to retrieve IMAP URLs specifying the
      "submit+" <access> identifier prefix.  If this list is extended to
      include the set of all external entities that could act on behalf
      of users, then the attack surface would be increased.



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   o  Administration - When URLAUTH-style IMAP URLs are presented to an
      IMAP server by entities acting on behalf of users, the server
      administrator has no way of determining the intended use of that
      URL from the server logs.

   o  Resourcing - Without a mechanism to distinguish between the
      application for which an IMAP URL is to be used, the IMAP server
      has no way to prioritize resources for particular applications.
      For example, the server could prioritize "submit+" URL fetch
      requests over other access identifiers.

3.3.  Additional Access Identifier Specification

   The previous section establishes that additional access identifiers
   are required to support applications, such as streaming [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
   gives the formal ABNF syntax):

   o  <access> identifier - The name of the application, e.g.,
      "exampleapp".

   o  <access> identifier prefix - The name of the application, e.g.,
      "exampleapp3", followed by a "+" and then a userid.  For example,
      consider "exampleapp3+testuser".

   Note that an <access> identifier name can also be registered as an
   <access> identifier prefix.  However, this would require 2 separate
   IANA registrations.

   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.










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   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 that the IMAP
   server provide an extensible mechanism for associating userids with
   new applications.  For example, imagine a new application, "foo", is
   created that requires application entities to retrieve URLs on behalf
   of users.  In this case, the IMAP server would need to provide a way
   to register the 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 provides the template required to register new <access>
   identifiers or prefixes with IANA.

3.4.  Defining an Access Identifier for Streaming

   One application that makes use of URLAUTH-authorized URLs is that of
   streaming multimedia files that are received as internet-messaging
   attachments.  This application is described in [STREAMING].

   See Section 6.2 for the IANA registration template for the "stream"
   <access> identifier.

4.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [RFC5234].

   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>
   identifiers as defined in IMAP URL [RFC5092].

   Copyright (c) 2009 IETF Trust and the persons identified as
   authors of the code.  All rights reserved.

   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions
   are met:





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   - Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.

   - Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in
     the documentation and/or other materials provided with the
     distribution.

   - Neither the name of Internet Society, IETF or IETF Trust, nor the
     names of specific contributors, may be used to endorse or promote
     products derived from this software without specific prior
     written permission.

   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
   A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT
   OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

         application = 1*(ALPHA/DIGIT)

         access      =/ application / (application "+" enc-user)

5.  Acknowledgements

   This document was inspired by discussions in the Lemonade Working
   Group.

6.  IANA Considerations

   IANA created a new registry for IMAP URLAUTH access identifiers and
   prefixes.

   Access identifiers and prefixes MUST be registered using the "IETF
   Review" policy [RFC5226].  This section gives the IANA 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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6.1.  Access Identifier Registration Template

   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 in which the application is
                   defined]

   Contact:       [Email and/or physical address to contact for
                   additional information]

6.2.  Stream Application Registration

   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:    RFC 5593

   Contact:       Neil Cook <neil.cook@noware.co.uk>
















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6.3.  Submit Application Registration

   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:    RFC 5593 and RFC 5092

   Contact:       Lemonade WG <lemonade@ietf.org>

6.4.  User Application Registration

   To: iana@iana.org
   Subject: IMAP URL Access Identifier Registration

   Type:          <access> identifier prefix

   Application:   user

   Description:   Used to restrict access to IMAP sessions
                  that are logged in as the specified userid

   RFC Number:    RFC 5593 and RFC 5092

   Contact:       Lemonade WG <lemonade@ietf.org>



















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6.5.  Authuser Application Registration

   To: iana@iana.org
   Subject: IMAP URL Access Identifier Registration

   Type:          <access> identifier

   Application:   authuser

   Description:   Used to restrict access to IMAP sessions
                  that are logged in as any non-anonymous
                  user of that IMAP server

   RFC Number:    RFC 5593 and RFC 5092

   Contact:       Lemonade WG <lemonade@ietf.org>

6.6.  Anonymous Application Registration

   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:    RFC 5593 and RFC 5092

   Contact:       Lemonade WG <lemonade@ietf.org>

7.  Security Considerations

   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 [RFC5092].







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8.  References

8.1.  Normative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5092]   Melnikov, A., Ed., and C. Newman, "IMAP URL Scheme", RFC
               5092, November 2007.

   [RFC5234]   Crocker, D., Ed., and P. Overell, "Augmented BNF for
               Syntax Specifications: ABNF", STD 68, RFC 5234, January
               2008.

8.2.  Informative References

   [RFC5226]   Narten, T. and H. Alvestrand, "Guidelines for Writing an
               IANA Considerations Section in RFCs", BCP 26, RFC 5226,
               May 2008.

   [STREAMING] Cook, N., "Streaming Internet Messaging Attachments",
               Work in Progress, May 2009.

Author's Address

   Neil L Cook
   Cloudmark

   EMail: neil.cook@noware.co.uk






















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