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Versions: (RFC 2192) 00 01 02 03 04 05 06 07 08 09 RFC 5092

INTERNET-DRAFT                                         A. Melnikov (Ed.)
Document: draft-ietf-lemonade-rfc2192bis-09.txt               Isode Ltd.
Expires: February 2008                                         C. Newman
Intended status: Standards Track                        Sun Microsystems
Obsoletes: RFC 2192 (if approved)                         August 8, 2007
Updates: RFC 4467


                            IMAP URL Scheme


Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts are draft documents valid for a maximum of six
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   at any time.  It is inappropriate to use Internet-Drafts as reference
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
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   A revised version of this draft document will be submitted to the RFC
   editor as a Proposed Standard for the Internet Community.  Discussion
   and suggestions for improvement are requested, and should be sent to
   the IMAPEXT Mailing list <ietf-imapext@imc.org>. Distribution of this
   draft is unlimited.


Copyright Notice

     Copyright (C) The IETF Trust (2007).








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Abstract

     IMAP (RFC 3501) is a rich protocol for accessing remote message
     stores.  It provides an ideal mechanism for accessing public mail-
     ing list archives as well as private and shared message stores.
     This document defines a URL scheme for referencing objects on an
     IMAP server.

     This document obsoletes RFC 2192. It also updates RFC 4467.


1. 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
     [KEYWORDS].

     This document references many productions from [URI-GEN]. When the
     document needs to emphasize IMAP URI-specific differences from
     [URI-GEN] (i.e. for parts of IMAP URIs which have more restricted
     syntax than generic URIs), it uses a non terminal i<foo> to define
     an IMAP specific version of the non terminal <foo> from [URI-GEN].

     Note that the ABNF syntax shown in section 11 is normative.  Sec-
     tions 2-6 may use a less formal syntax that does not necessarily
     match the normative ABNF shown in section 11. If there are any dif-
     ferences between syntax shown in sections 2-6 and section 11, then
     the syntax shown in the Section 11 must be treated as authorita-
     tive.  Non-syntax requirements included in sections 2-6 are, of
     course, normative.



2. Introduction

     The IMAP URL scheme is used to designate IMAP servers, mailboxes,
     messages, MIME bodies [MIME], and search programs on Internet hosts
     accessible using the IMAP protocol over TCP.

     The IMAP URL follows the common Internet scheme syntax as defined
     in [URI-GEN]. If :<port> is omitted, the port defaults to 143 (as
     defined in Section 2.1 of [IMAP4]).








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     An absolute IMAP URL takes one of the following forms:

         imap://<iserver>[/]

         imap://<iserver>/<enc-mailbox>[<uidvalidity>][?<enc-search>]

         imap://<iserver>/<enc-mailbox>[<uidvalidity>]<iuid>
          [<isection>][<ipartial>][<iurlauth>]

     The first form is used to refer to an IMAP server (see section 4),
     the second form refers to the contents of a mailbox or a set of
     messages resulting from a search (see section 5), and the final
     form refers to a specific message or message part, and possibly a
     byte range in that part (see section 6). If [URLAUTH] extension is
     supported, then the final form can have the <iurlauth> component
     (see section 6.1 for more details).

     The <iserver> component common to all types of absolute IMAP URLs
     has the following syntax expressed in ABNF [ABNF]:

          [iuserinfo "@"] host [ ":" port ]

     The <iserver> component is the same as "authority" defined in [URI-
     GEN].  The syntax and uses of the <iuserinfo> ("IMAP userinfo com-
     ponent") are described in details in section 3. Syntax of <host>
     and <port> is described in [URI-GEN].


3. IMAP userinfo component (iuserinfo)

     The <iuserinfo> component conforms to the generic syntax of <user-
     info> defined in [URI-GEN]. It has the following syntax expressed
     in ABNF [ABNF]:

          enc-user [iauth] / [enc-user] iauth

     The meaning of different parts is described in subsections of this
     section.


3.1. IMAP mailbox naming scope

     The "enc-user" part of the "iuserinfo" component, if present,
     denotes mailbox naming scope.  If it is absent, the IMAP URL can
     only reference mailboxes with globally unique names, i.e. mailboxes
     with names that don't change depending on the user the client
     authenticated as to the IMAP server. (Note, that not all IMAP
     implementations support globally unique names.)



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     For example, a personal mailbox described by the following URL
      <imap://michael@example.org/INBOX> is most likely be different
     from a personal mailbox described by
      <imap://bester@example.org/INBOX>, even though both URLs use the
     same mailbox name.


3.2. IMAP User Name and Authentication Mechanism

     The userinfo component (see [URI-GEN]) of an IMAP URI may contain
     an IMAP user name (a.k.a. authorization identity [SASL], "enc-
     user") and/or an authentication mechanism. (Note that the "enc-
     user" also defines a mailbox naming scope as described in section
     3.1).  The IMAP user name and the authentication mechanism are used
     in the "LOGIN" or "AUTHENTICATE" commands after making the connec-
     tion to the IMAP server.

     If no user name and no authentication mechanism is supplied, the
     client MUST authenticate as anonymous to the server. If the server
     advertises AUTH=ANONYMOUS IMAP capability, the client MUST use the
     AUTHENTICATE command with ANONYMOUS [ANONYMOUS] SASL mechanism.  If
     SASL ANONYMOUS is not available, the (case-insensitive) user name
     "anonymous" is used with the "LOGIN" command and the Internet e-
     mail address of the end user accessing the resource is supplied as
     the password. The latter option is given in order to provide for
     interoperability with deployed servers.

     Note that as described in RFC 3501, the LOGIN command MUST NOT be
     used when the IMAP server advertises the LOGINDISABLED capability.

     An authentication mechanism (as used by the IMAP AUTHENTICATE com-
     mand) can be expressed by adding ";AUTH=<enc-auth-type>" to the end
     of the user name in an IMAP URL.  When such an <enc-auth-type> is
     indicated, the client SHOULD request appropriate credentials from
     that mechanism and use the "AUTHENTICATE" command instead of the
     "LOGIN" command.  If no user name is specified, one MUST be
     obtained from the mechanism or requested from the user/configura-
     tion as appropriate.

     The string ";AUTH=*" indicates that the client SHOULD select an
     appropriate authentication mechanism. (Though the '*' character in
     this usage is not strictly a delimiter, it is being treated like a
     sub-delim [URI-GEN] in this instance.  It MUST NOT be percent
     encoded in this usage, as ";AUTH=%2A" will not match this produc-
     tion.)  It MAY use any mechanism listed in the response to the
     CAPABILITY command (or CAPABILITY response code) or use an out of
     band security service resulting in a PREAUTH connection.  If no
     user name is specified and no appropriate authentication mechanisms



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     are available, the client SHOULD fall back to anonymous login as
     described above.  The behavior prescribed in this section allows a
     URL which grants read-write access to authorized users, and read-
     only anonymous access to other users.

     If a user name is included with no authentication mechanism, then
     ";AUTH=*" is assumed.

     Clients must take care when resolving a URL which requires or
     requests any sort of authentication, since URLs can easily come
     from untrusted sources.  Supplying authentication credentials to
     the wrong server may compromise the security of the user's account,
     therefore the program resolving the URL should meet at least one of
     the following criteria in this case:

     (1) The URL comes from a trusted source, such as a referral server
     which the client has validated and trusts according to site policy.
     Note that user entry of the URL may or may not count as a trusted
     source, depending on the experience level of the user and site pol-
     icy.
     (2) Explicit local site policy permits the client to connect to the
     server in the URL.  For example, a company example.com may have a
     site policy to trust all IMAP server names ending in example.com,
     whereas such a policy would be unwise for example.edu where random
     students can set up IMAP servers.
     (3) The user confirms that connecting to that domain name with the
     specified credentials and/or mechanism is permitted.  For example,
     when using LOGIN or SASL PLAIN with TLS, the IMAP URL client
     presents a dialog box "Is it OK to send your password to server
     "example.com"?  Please be aware the owners of example.com will be
     able to reuse your password to connect to other servers on your
     behalf."
     (4) A mechanism is used which validates the server before passing
     potentially compromising client credentials.  For example, a site
     has a designated TLS certificate used to certify site-trusted IMAP
     server certificates and this has been configured explicitly into
     the IMAP URL client. Another example is use of a SASL mechanism
     such as DIGEST-MD5 [DIGEST-MD5], which supports mutual authentica-
     tion.
     (5) An authentication mechanism is used which will not reveal any
     information to the server which could be used to compromise future
     connections. For example, SASL ANONYMOUS [ANONYMOUS] or GSSAPI
     [GSSAPI].

     URLs which do not include a user name, but include an authentica-
     tion mechanism (";AUTH=<mech>") must be treated with extra care,
     since for some <mech>s they are more likely to compromise the
     user's primary account.  A URL containing ";AUTH=*" must also be



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     treated with extra care since it might fall back on a weaker secu-
     rity mechanism.  Finally, clients are discouraged from using a
     plain text password as a fallback with ";AUTH=*" unless the connec-
     tion has strong encryption.

     A program interpreting IMAP URLs MAY cache open connections to an
     IMAP server for later re-use.  If a URL contains a user name, only
     connections authenticated as that user may be re-used.  If a URL
     does not contain a user name or authentication mechanism, then only
     an anonymous connection may be re-used.

     Note that if unsafe or reserved characters such as " " (space) or
     ";" are present in the user name or authentication mechanism, they
     MUST be percent-encoded as described in [URI-GEN].


3.3. Limitations of enc-user

     As per sections 3.1 and 3.2 of this document the IMAP URI enc-user
     has two purposes:
        1) It provides context for user-specific mailbox paths such
           as "INBOX" (section 3.1).
        2) It specifies that resolution of the URL requires logging in
           as that user and limits use of that URL to only that user
           (Section 3.2).
     An obvious limitation of using the same field for both purposes is
     that the URL can be resolved only by the mailbox owner.  In order
     to avoid this restriction, implementations should use globally
     unique mailbox names (see Section 3.1) whenever possible (*).

     The URLAUTH component overrides the second purpose of the enc-user
     in the IMAP URI and by default permits the URI to be resolved by
     any user permitted by the <access> identifier. URLAUTH and <access>
     identifier are described in section 6.1.

     (*) There is currently no general way in IMAP of learning a glob-
     ally unique name for a mailbox. However by looking at the NAMESPACE
     [NAMESPACE] command result it is possible to determine if a mailbox
     name is globally unique or not.


4. IMAP server

     An IMAP URL referring to an IMAP server has the following form:

         imap://<iserver>[/]

     This URL type is frequently used to describe a location of an IMAP



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     server, both in referrals and in configuration. It may optionally
     contain the "iuserinfo" component (see Sections 3 and 11).  A pro-
     gram interpreting this URL would issue the standard set of commands
     it uses to present a view of the content of the IMAP server, as
     visible to the user described by the "enc-user" part of the "iuser-
     info" component, if the "enc-user" part is specified.


5. Lists of messages

     An IMAP URL referring to a list of messages has the following form:

         imap://<iserver>/<enc-mailbox>[<uidvalidity>][?<enc-search>]

     The <enc-mailbox> field is used as the argument to the IMAP4
     "SELECT" or "EXAMINE" command.  Note that if unsafe or reserved
     characters such as " " (space), ";", or "?" are present in <enc-
     mailbox> they MUST be percent-encoded as described in [URI-GEN].

     The <uidvalidity> field is optional.  If it is present, it MUST be
     the same as the value of IMAP4 UIDVALIDITY response code at the
     time the URL was created.  This MUST be used by the program inter-
     preting the IMAP URL to determine if the URL is stale. If the IMAP
     URL is stale, then the program should behave as if the correspond-
     ing mailbox doesn't exist.

     Note that the <uidvalidity> field is a modifier to the <enc-mail-
     box>, i.e. it is considered a part of the last "component" (as used
     in [URI-GEN]) of the <enc-mailbox>. This is significant during rel-
     ative URI resolution.

     The "?<enc-search>" field is optional.  If it is not present, the
     program interpreting the URL will present the entire content of the
     mailbox.

     If the "?<enc-search>" field is present, the program interpreting
     the URL should use the contents of this field as arguments follow-
     ing an IMAP4 SEARCH command. These arguments are likely to contain
     unsafe characters such as " " (space) (which are likely to be
     present in the <enc-search>). If unsafe characters are present,
     they MUST be percent-encoded as described in [URI-GEN].

     Note that quoted strings and non-synchronizing literals [LITERAL+]
     are allowed in the <enc-search> content, however synchronizing lit-
     erals are not allowed, as their presence would effectively mean
     that the agent interpreting IMAP URLs needs to parse an <enc-
     search> content, find all synchronizing literals and perform proper
     command continuation request handling (see sections 4.3 and 7 of



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     [IMAP4]).


6. A specific message or message part

     An IMAP URL referring to a specific message or message part has the
     following form:

         imap://<iserver>/<enc-mailbox>[<uidvalidity>]<iuid>
         [<isection>][<ipartial>][<iurlauth>]

     The <enc-mailbox> and [uidvalidity] are as defined in section 5
     above.

     If <uidvalidity> is present in this form, it SHOULD be used by the
     program interpreting the URL to determine if the URL is stale.

     The <iuid> refers to an IMAP4 message UID, and SHOULD be used as
     the <set> argument to the IMAP4 "UID FETCH" command.

     The <isection> field is optional.  If not present, the URL refers
     to the entire Internet message as returned by the IMAP command "UID
     FETCH <uid> BODY.PEEK[]".  If present, the URL refers to the object
     returned by a "UID FETCH <uid> BODY.PEEK[<section>]" command.  The
     type of the object may be determined with a "UID FETCH <uid> BODYS-
     TRUCTURE" command and locating the appropriate part in the result-
     ing BODYSTRUCTURE.  Note that unsafe characters in [isection] MUST
     be percent-encoded as described in [URI-GEN].

     The <ipartial> field is optional. If present, it effectively
     appends "<<partial-range>>" to the end of the UID FETCH
     BODY.PEEK[<section>] command constructed as described in the previ-
     ous paragraph. In other words it allows the client to request a
     byte range of the message/message part.

     The <iurlauth> field is described in details in section 6.1.


6.1 URLAUTH authorized URL


     URLAUTH authorized URLs are only supported by an IMAP server adver-
     tising the URLAUTH IMAP capability [URLAUTH].


6.1.1. Concepts





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6.1.1.1. URLAUTH


     The URLAUTH is a component, appended at the end of a URL, which
     conveys authorization to access the data addressed by that URL.  It
     contains an authorized access identifier, an authorization mecha-
     nism name, and an authorization token.  The authorization token is
     generated from the URL, the authorized access identifer, authoriza-
     tion mechanism name, and a mailbox access key.

     (Note that this specification only allows for the URLAUTH component
     in IMAP URLs describing a message or its part.)


6.1.1.2. Mailbox Access Key


     The mailbox access key is a random string with at least 128 bits of
     entropy.  It is generated by software (not by the human user), and
     MUST be unpredictable.

     Each user has a table of mailboxes and an associated mailbox access
     key for each mailbox.  Consequently, the mailbox access key is per-
     user and per-mailbox.  In other words, two users sharing the same
     mailbox each have a different mailbox access key for that mailbox,
     and each mailbox accessed by a single user also has a different
     mailbox access key.


6.1.1.3. Authorized Access Identifier


     The authorized access identifier restricts use of the URLAUTH
     authorized URL to certain users authorized on the server, as
     described in section 6.1.2.


6.1.1.4. Authorization Mechanism


     The authorization mechanism is the algorithm by which the URLAUTH
     is generated and subsequently verified, using the mailbox access
     key.


6.1.1.5. Authorization Token





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     The authorization token is a deterministic string of at least 128
     bits which an entity with knowledge of the secret mailbox access
     key and URL authorization mechanism can use to verify the URL.


6.1.2. URLAUTH extensions to IMAP URL


     A specific message or message part IMAP URL can optionally contain
     ";EXPIRE=<datetime>" and/or ";URLAUTH=<access>:<mech>:<token>".

     When ";EXPIRE=<datetime>" is used, this indicates the latest date
     and time that the URL is valid.  After that date and time, the URL
     has expired and server implementations MUST reject the URL.  If
     ";EXPIRE=<datetime>" is not used, the URL has no expiration, but
     still can be revoked using the RESETKEY command [URLAUTH].

     The URLAUTH takes the form ";URLAUTH=<access>:<mech>:<token>", and
     MUST be at the end of the URL. It is composed of three parts.  The
     <access> portion provides the authorized access identifiers which
     may constrain the operations and users that are permitted to use
     this URL.  The <mech> portion provides the authorization mechanism
     used by the IMAP server to generate the authorization token that
     follows.  The <token> portion provides authorization token, which
     can be generated using the GENURLAUTH command [URLAUTH].

     The "submit+" <access> identifier prefix, 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.  The IMAP server does not validate the specified userid but
     does validate that the IMAP session has an authorization identity
     that is authorized as a message submission entity.  The authorized
     message submission entity MUST validate the userid prior to con-
     tacting the IMAP server.

     The "user+" <access> identifier prefix, followed by a userid, indi-
     cates that use of this URL is limited to IMAP sessions which are
     logged in as the specified userid (that is, have authorization
     identity as that userid).

        Note: if a SASL mechanism which provides both authorization and
        authentication identifiers is used to authenticate to the IMAP
        server, the "user+" access identifier MUST match the authoriza-
        tion identifier. If the SASL mechanism can't transport the
        authorization identifier, the "user+" access identifier MUST
        match the authorization identifier derived from the authentica-
        tion identifier (see [SASL]).




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     The "authuser" <access> identifier indicates that use of this URL
     is limited to authenticated IMAP sessions which are logged in as
     any non-anonymous user (that is, have authorization identity as a
     non-anonymous user user) of that IMAP server. To restate this: use
     of this type of URL is prohibited to anonymous IMAP sessions, i.e.
     any URLFETCH command containing this type of URL issued in an
     anonymous session MUST return NIL in the URLFETCH response.

     The "anonymous" <access> identifier indicates that use of this URL
     is not restricted by session authorization identity; that is, any
     IMAP session in authenticated or selected state (as defined in
     [IMAP4]), including anonymous sessions, may issue a URLFETCH
     [URLAUTH] using this URL.

     The authorization token is represented as an ASCII-encoded hexadec-
     imal string, which is used to authorize the URL.  The length and
     the calculation of the authorization token depends upon the mecha-
     nism used; but, in all cases, the authorization token is at least
     128 bits (and therefore at least 32 hexadecimal digits).

     Example:

      <imap://joe@example.com/INBOX/;uid=20/;section=1.2;urlauth=
      submit+fred:internal:91354a473744909de610943775f92038>



7. Relative IMAP URLs

     Relative IMAP URLs are permitted and are resolved according to the
     rules defined in [URI-GEN].  In particular in IMAP URLs, parameters
     (such as ";uid=" or ";section=") are treated as part of the normal
     path with respect to relative URL resolution.

     [URI-GEN] defines 4 forms of relative URLs: <inetwork-path>, <iab-
     solute-path>, <irelative-path>, <ipath-empty>. Their syntax is
     defined in section 11.

     A relative reference that begins with two slash characters is
     termed a network-path reference (<inetwork-path>); such references
     are rarely used, because in most cases they can be replaced with an
     equivalent absolute URL.  A relative reference that begins with a
     single slash character is termed an absolute-path reference (<iab-
     solute-path>, see also section 7.1).  A relative reference that
     does not begin with a slash character is termed a relative-path
     reference (<irelative-path>, see also section 7.2).  The final form
     is <ipath-empty>, which is "same-document reference" (see section
     4.4 of [URI-GEN]).



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     The following observations about relative URLs are important:

     The <iauth> grammar element (which is a part of <iuserinfo>, which
     is in its turn a part of <iserver>; see section 3) is considered
     part of the user name for purposes of resolving relative IMAP URLs.
     This means that unless a new user name/server specification is
     included in the relative URL, the authentication mechanism is
     inherited from the base IMAP URL.

     URLs always use "/" as the hierarchy delimiter for the purpose of
     resolving paths in relative URLs.  IMAP4 permits the use of any
     hierarchy delimiter in mailbox names.  For this reason, relative
     mailbox paths will only work if the mailbox uses "/" as the hierar-
     chy delimiter.  Relative URLs may be used on mailboxes which use
     other delimiters, but in that case, the entire mailbox name MUST be
     specified in the relative URL or inherited as a whole from the base
     URL.

     If an IMAP server allows for mailbox names starting with "./" or
     "../", ending with "/." or "/..", or containing sequences "/../" or
     "/./", then such mailbox names MUST be percent-encoded as described
     in [URI-GEN]. Otherwise they would be misinterpreted as dot-seg-
     ments (see Section 3.3 of [URI-GEN]), which are processed specially
     during relative path resolution process.


7.1. absolute-path References

     A relative reference that begins with a single slash character is
     termed an absolute-path reference (see section 4.2 of [URI-GEN]).
     If an IMAP server permits mailbox names with a leading "/", then
     the leading "/" MUST be percent-encoded as described in [URI-GEN].
     Otherwise the produced absolute-path reference URI will be misin-
     terpreted as a network-path reference [URI-GEN] described by <inet-
     work-path> non-terminal.


7.2. relative-path References

     A relative reference that does not begin with a slash character is
     termed a relative-path reference [URI-GEN]. Implementations MUST
     NOT generate or accept relative-path IMAP references.

     See also section 4.2 of [URI-GEN] for restrictions on relative-path
     references.


8. Internationalization Considerations



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     IMAP4 [IMAP4] section 5.1.3 includes a convention for encoding non-
     US-ASCII characters in IMAP mailbox names.  Because this convention
     is private to IMAP, it is necessary to convert IMAP's encoding to
     one that can be more easily interpreted by a URL display program.
     For this reason, IMAP's modified UTF-7 encoding for mailboxes MUST
     be converted to UTF-8 [UTF-8].  Since 8-bit octets are not permit-
     ted in URLs, the UTF-8 octets are percent-encoded as required by
     the URL specification [URI-GEN], section 2.1.  Sample code is
     included in Appendix A to demonstrate this conversion.

     IMAP usernames are UTF-8 strings and MUST be percent-encoded as
     required by the URL specification [URI-GEN], section 2.1.

     Also note that IMAP SEARCH criteria can contain non US-ASCII char-
     acters.  8-bit octets in those strings MUST be percent-encoded as
     required by the URL specification [URI-GEN], section 2.1.



9. Examples

     The following examples demonstrate how an IMAP4 client program
     might translate various IMAP4 URLs into a series of IMAP4 commands.
     Commands sent from the client to the server are prefixed with "C:",
     and responses sent from the server to the client are prefixed with
     "S:".

     The URL:

      <imap://minbari.example.org/gray-council;UIDVALIDITY=385759045/;
       UID=20/;PARTIAL=0.1024>

     may result in in the following client commands and server
     responses:

         <connect to minbari.example.org, port 143>
         S: * OK [CAPABILITY IMAP4rev1 STARTTLS AUTH=ANONYMOUS] Welcome
         C: A001 AUTHENTICATE ANONYMOUS
         S: +
         C: c2hlcmlkYW5AYmFieWxvbjUuZXhhbXBsZS5vcmc=
         S: A001 OK Welcome sheridan@babylon5.example.org
         C: A002 SELECT gray-council
         <client verifies the UIDVALIDITY matches>
         C: A003 UID FETCH 20 BODY.PEEK[]<0.1024>







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     The URL:

      <imap://psicorp.example.org/~peter/%E6%97%A5%E6%9C%AC%E8%AA%9E/
      %E5%8F%B0%E5%8C%97>

     May results in the following client commands:

         <connect to psicorp.example.org, port 143>
         S: * OK [CAPABILITY IMAP4rev1 STARTTLS AUTH=CRAM-MD5] Welcome
         C: A001 LOGIN ANONYMOUS bester@psycop.psicorp.example.org
         C: A002 SELECT ~peter/&ZeVnLIqe-/&U,BTFw-
         <commands the client uses for viewing the contents of
          the mailbox>

     The URL:

      <imap://;AUTH=GSSAPI@minbari.example.org/gray-council/;uid=20/
      ;section=1.2>

     May result in the following client commands:

         <connect to minbari.example.org, port 143>
         S: * OK Greetings
         C: A000 CAPABILITY
         S: * CAPABILITY IMAP4rev1 STARTTLS AUTH=GSSAPI
         S: A000 OK
         C: A001 AUTHENTICATE GSSAPI
         <authentication exchange>
         C: A002 SELECT gray-council
         C: A003 UID FETCH 20 BODY.PEEK[1.2]


     If the following relative URL is located in that body part:

      <;section=1.4>

     This could result in the following client commands:

         C: A004 UID FETCH 20 (BODY.PEEK[1.2.MIME]
               BODY.PEEK[1.MIME]
               BODY.PEEK[HEADER.FIELDS (Content-Location)])
         <Client looks for Content-Location headers in
          result.  If no such headers, then it does the following>
         C: A005 UID FETCH 20 BODY.PEEK[1.4]







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     The URL:

      <imap://;AUTH=*@minbari.example.org/gray%20council?
       SUBJECT%20shadows>

     Could result in the following:

         <connect to minbari.example.org, port 143>
         S: * OK Welcome
         C: A001 CAPABILITY
         S: * CAPABILITY IMAP4rev1 AUTH=DIGEST-MD5
         S: A001 OK
         C: A002 AUTHENTICATE DIGEST-MD5
         <authentication exchange>
         S: A002 OK user lennier authenticated
         C: A003 SELECT "gray council"
         ...
         C: A004 SEARCH SUBJECT shadows
         S: * SEARCH 8 10 13 14 15 16
         S: A004 OK SEARCH completed
         C: A005 FETCH 8,10,13:16 ALL
         ...
     NOTE: In this final example, the client has implementation depen-
     dent choices.  The authentication mechanism could be anything,
     including PREAUTH.  And the final FETCH command could fetch more or
     less information about the messages, depending on what it wishes to
     display to the user.

     The URL:

      <imap://john;AUTH=*@minbari.example.org/babylon5/personel?
       charset%20UTF-8%20SUBJECT%20%7B14+%7D%0D%0A%D0%98%D0%B2%
       D0%B0%D0%BD%D0%BE%D0%B2%D0%B0>

     shows that 8-bit data can be sent using non-synchronizing literals
     [LITERAL+]. This could result in the following:

         <connect to minbari.example.org, port 143>
         S: * OK Hi there
         C: A001 CAPABILITY
         S: * CAPABILITY IMAP4rev1 LITERAL+ AUTH=DIGEST-MD5
         S: A001 OK
         C: A002 AUTHENTICATE DIGEST-MD5
         <authentication exchange>
         S: A002 OK user john authenticated
         C: A003 SELECT babylon5/personel
         ...
         C: A004 SEARCH CHARSET UTF-8 SUBJECT {14+}



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         C: XXXXXXXXXXXXXX
         S: * SEARCH 7 10 12
         S: A004 OK SEARCH completed
         C: A005 FETCH 7,10,12 ALL
         ...

     Where XXXXXXXXXXXXXX is 14 bytes of UTF-8 encoded data as specified
     in the URL above.


9.1. Examples of relative URLs

     The following absolute-path reference

      </foo/;UID=20/..>

     is the same as

      </foo>

     I.e. both of them reference the mailbox "foo" located on the IMAP
     server described by the corresponding Base URI.

     The following relative-path reference

      <;UID=20>

     references a message with UID in the mailbox specified by the Base
     URI.

     The following edge case example demostrates that the ;UIDVALIDITY=
     modifier is a part of the mailbox name as far as relative URI reso-
     lution is concerned:

      <..;UIDVALIDITY=385759045/;UID=20>

     In this example ".." is not a dot-segment [URI-GEN].



10. Security Considerations

     Security considerations discussed in the IMAP specification [IMAP4]
     and the URI specification [URI-GEN] are relevant.  Security consid-
     erations related to authenticated URLs are discussed in section 3.2
     of this document.

     Many email clients store the plain text password for later use



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     after logging into an IMAP server.  Such clients MUST NOT use a
     stored password in response to an IMAP URL without explicit permis-
     sion from the user to supply that password to the specified host
     name.

     Clients resolving IMAP URLs that wish to achieve data confidential-
     ity and/or integrity SHOULD use the STARTTLS command (if supported
     by the server) before starting authentication, or use a SASL mecha-
     nism, such as GSSAPI, that provides a confidentiality security
     layer.


10.1. Security Consideration specific to URLAUTH authorized URL

     The "user+<userid>" access identifier limits resolution of that URL
     to a particular userid, whereas the "submit+<userid>" access iden-
     tifier is more general and simply requires that the session be
     authorized by a user that has been granted a "submit" role within
     the authentication system.  Use of either of these access identi-
     fiers makes it impossible for an attacker, spying on the session,
     to use the same URL, either directly or by submission to a message
     submission entity.

     The "authuser" and "anonymous" access identifiers do not have this
     level of protection.  These access identifiers are primarily useful
     for public export of data from an IMAP server, without requiring
     that it be copied to a web or anonymous FTP server.

     The decision to use the "authuser" access identifier should be made
     with caution.  An "authuser" access identifier can be used by any
     authorized user of the IMAP server; and therefore use of this
     access identifier should be limited to content which may be dis-
     closed to any authorized user of the IMAP server.

     The decision to use the "anonymous" access identifier should be
     made with extreme caution.  An "anonymous" access identifier can be
     used by anyone; and therefore use of this access identifier should
     be limited to content which may be disclosed to anyone.


11. ABNF for IMAP URL scheme

     Formal syntax is defined using ABNF [ABNF], extending the ABNF
     rules in section 9 of [IMAP4].  Elements not defined here can be
     found in the [ABNF], [IMAP4], [IMAPABNF] or [URI-GEN].  Strings are
     not case sensitive and free insertion of linear-white-space is not
     permitted.




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     sub-delims-sh = "!" / "$" / "'" / "(" / ")" /
                     "*" / "+" / ","
                        ;; Same as [URI-GEN] sub-delims,
                        ;; but without ";", "&" and "=".

     uchar            = unreserved / sub-delims-sh / pct-encoded

     achar            = uchar / "&" / "="
                        ;; Same as [URI-GEN] 'unreserved / sub-delims /
                        ;; pct-encoded', but ";" is disallowed.

     bchar            = achar / ":" / "@" / "/"

     enc-auth-type    = 1*achar
                     ; %-encoded version of [IMAP4] "auth-type"

     enc-mailbox      = 1*bchar
                    ; %-encoded version of [IMAP4] "mailbox"

     enc-search       = 1*bchar
                             ; %-encoded version of [IMAPABNF]
                             ; "search-program". Note that IMAP4
                             ; literals may not be used in
                             ; a "search-program", i.e. only
                             ; quoted or non-synchronizing
                             ; literals (if the server supports
                             ; LITERAL+ [LITERAL+]) are allowed.

     enc-section      = 1*bchar
                    ; %-encoded version of [IMAP4] "section-spec"

     enc-user         = 1*achar
                    ; %-encoded version of [IMAP4] authorization
                    ; identity or "userid".

     imapurl          = "imap://" iserver ipath-query
                 ; Defines an absolute IMAP URL

     ipath-query      = ["/" [ icommand ]]
                 ; Corresponds to "path-abempty [ "?" query ]"
                 ; in [URI-GEN]


     Generic syntax for relative URLs is defined in Section 4.2
     of [URI-GEN]. For ease of implementation, the relative
     IMAP URL syntax is defined below:

     imapurl-rel     = inetwork-path



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                       / iabsolute-path
                       / irelative-path
                       / ipath-empty

     inetwork-path   = "//" iserver ipath-query
                 ; Corresponds to '"//" authority path-abempty
                 ; [ "?" query ]' in [URI-GEN]

     iabsolute-path  = "/" [ icommand ]
                 ; icommand, if present, MUST NOT start with '/'.
                 ;
                 ; Corresponds to 'path-absolute [ "?" query ]'
                 ; in [URI-GEN]

     irelative-path  = imessagelist /
                       imsg-or-part
                 ; Corresponds to 'path-noscheme [ "?" query ]'
                 ; in [URI-GEN]

     imsg-or-part    = ( imailbox-ref "/" iuid-only ["/" isection-only]
                         ["/" ipartial-only] ) /
                       ( iuid-only ["/" isection-only]
                         ["/" ipartial-only] ) /
                       ( isection-only ["/" ipartial-only] ) /
                       ipartial-only

     ipath-empty     = 0<pchar>
                 ; Zero characters.
                 ; The same-document reference.

     The following 3 rules are only used in the presence of the IMAP
     [URLAUTH] extension:

     authimapurl     = "imap://" iserver "/" imessagepart
                       ; Same as "imapurl" when "[icommand]" is
                       ; "imessagepart"

     authimapurlfull = authimapurl iurlauth
                       ; Same as "imapurl" when "[icommand]" is
                       ; "imessagepart iurlauth"

     authimapurlrump = authimapurl iurlauth-rump


     enc-urlauth     = 32*HEXDIG

     iurlauth        = iurlauth-rump iua-verifier




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     iua-verifier    = ":" uauth-mechanism ":" enc-urlauth

     iurlauth-rump   = [expire] ";URLAUTH=" access

     access          = ("submit+" enc-user) / ("user+" enc-user) /
                         "authuser" / "anonymous"

     expire          = ";EXPIRE=" date-time
                           ; date-time is defined in [DATETIME]

     uauth-mechanism = "INTERNAL" / 1*(ALPHA / DIGIT / "-" / ".")
                          ; Case-insensitive.
                          ; New mechanisms MUST be registered with IANA.




     iauth            = ";AUTH=" ( "*" / enc-auth-type )

     icommand         = imessagelist /
                        imessagepart [iurlauth]

     imailbox-ref     = enc-mailbox [uidvalidity]

     imessagelist     = imailbox-ref [ "?" enc-search ]
                    ; "enc-search" is [URI-GEN] "query".

     imessagepart     = imailbox-ref iuid [isection] [ipartial]

     ipartial         = "/" ipartial-only

     ipartial-only    = ";PARTIAL=" partial-range

     isection         = "/" isection-only

     isection-only    = ";SECTION=" enc-section

     iserver          = [iuserinfo "@"] host [ ":" port ]
                             ; This is the same as "authority" defined
                             ; in [URI-GEN]. See [URI-GEN] for "host"
                             ; and "port" definitions.

     iuid             = "/" iuid-only

     iuid-only        = ";UID=" nz-number
                    ; See [IMAP4] for "nz-number" definition

     iuserinfo        = enc-user [iauth] / [enc-user] iauth



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                             ; conforms to the generic syntax of
                             ; "userinfo" as defined in [URI-GEN].

     partial-range    = number ["." nz-number]
                    ; partial FETCH. The first number is
                             ; the offset of the first byte,
                             ; the second number is the length of
                             ; the fragment.

     uidvalidity      = ";UIDVALIDITY=" nz-number
                    ; See [IMAP4] for "nz-number" definition


12. IANA Considerations

     IANA is requested to update "imap" definition in the "Uniform
     Resource Identifier scheme registry" to point to this document.

     The registration template (as per [URI-REG]) is specified in sec-
     tion 12.1 of this document.


12.1.  IANA Registration of imap: URI Scheme


     This section provides the information required to register the
     imap: URI scheme.


     URI scheme name: imap

     Status: permanent

     URI scheme syntax:
        See section 11 of [RFCXXXX].

     URI scheme semantics:

        The imap: URI scheme is used to designate IMAP servers, mail-
     boxes, messages, MIME bodies [MIME] and their parts, and search
     programs on Internet hosts accessible using the IMAP protocol.

     There is no MIME type associated with this URI.

     Encoding considerations:

        See Section 8 of [RFCXXXX].




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     Applications/protocols that use this URI scheme name:

        The imap: URI is intended to be used by applications that might
     need access to IMAP mailstore. Such applications may include (but
     not limited to) IMAP-capable web browsers; IMAP clients that wish
     to access a mailbox, message, or edit a message on the server using
     [CATENATE]; [SUBMIT] clients and servers that are requested to
     assemble a complete message on submission using [BURL].

     Interoperability considerations:

        A widely deployed IMAP client Netscape Mail (and possibly
     Mozilla/ Thubderbird/Seamonkey) use a different imap: scheme inter-
     nally.

     Security considerations:

        See Security Considerations (Section 10) of [RFCXXXX].

     Contact:

        Alexey Melnikov <alexey.melnikov@isode.com>

     Author/Change controller:

        IESG

     References:

        [RFCXXXX] and [IMAP4].



13. References


13.1. Normative References

     [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate Require-
     ment Levels", BCP 14, RFC 2119, Harvard University, March 1997.

     [IMAP4] Crispin, M., "Internet Message Access Protocol - Version
     4rev1", RFC 3501, University of Washington, March 2003.

     [IMAPABNF] Melnikov, A., and C. Daboo, "Collected extensions to
     IMAP4 ABNF", RFC 4466, April 2006.





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     [ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications:
     ABNF", RFC 4234, October 2005.

     [MIME] Freed, N., Borenstein, N., "Multipurpose Internet Mail
     Extensions", RFC 2045, November 1996.

     [URI-GEN] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
     Resource Identifier (URI): Generic Syntax", RFC 3986, January 2005.

     [UTF-8]  Yergeau, F., "UTF-8, a transformation format of ISO
     10646", STD 63, RFC 3629, November 2003.

     [NAMESPACE] Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342,
     May 1998.

     [LITERAL+] Myers, J., "IMAP4 non-synchronizing literals", RFC 2088,
     January 1997.

     [ANONYMOUS] Zeilenga, K. (Ed.), "Anonymous Simple Authentication
     and Security Layer (SASL) Mechanism", RFC 4505, June 2006.

     [DATETIME] Klyne, G., and Newman, C., "Date and Time on the Inter-
     net: Timestamps", RFC 3339, July 2002.

     [URLAUTH] Crispin, M., "Internet Message Access Protocol (IMAP) -
     URLAUTH Extension", RFC 4467, May 2006.


13.2. Informative References


     [SUBMIT] Gellens, R. and J. Klensin, "Message Submission for Mail",
     RFC 4409, April 2006.

     [BURL] Newman, C. "Message Submission BURL Extension", RFC 4468,
     May 2006.

     [CATENATE]  Resnick, P., "Internet Message Access Protocol (IMAP)
     CATENATE Extension", RFC 4469, April 2006.

     [SASL] Melnikov, A. and K. Zeilenga, "Simple Authentication and
     Security Layer (SASL)", RFC 4422, June 2006.

     [GSSAPI] Melnikov, A., "The Kerberos V5 ("GSSAPI") Simple Authenti-
     cation and Security Layer (SASL) Mechanism", RFC 4752, November
     2006.





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     [DIGEST-MD5] Leach, P. and C. Newman, "Using Digest Authentication
     as a SASL Mechanism", RFC 2831, May 2000.

     [URI-REG] Hansen, T., Hardie, T. and L. Masinter, "Guidelines and
     Registration Procedures for New URI Schemes", BCP 115, RFC 4395,
     February 2006.


14. Author's Address

     Chris Newman (Author/Editor)
     Sun Microsystems
     3401 Centrelake Dr., Suite 410
     Ontario, CA  91761
     EMail: chris.newman@sun.com

     Alexey Melnikov (Editor)
     Isode Limited
     5 Castle Business Village
     36 Station Road
     Hampton, Middlesex
     TW12 2BX, UK
     Email: Alexey.Melnikov@isode.com
     URI:   http://www.melnikov.ca/



Appendix A.  Sample code

Here is sample C source code to convert between URL paths and IMAP mail-
box names, taking into account mapping between IMAP's modified UTF-7
[IMAP4] and hex-encoded UTF-8 which is more appropriate for URLs.  This
code has not been rigorously tested nor does it necessarily behave rea-
sonably with invalid input, but it should serve as a useful example.
This code just converts the mailbox portion of the URL and does not deal
with parameters, query or server components of the URL.

/* Copyright (C) The IETF Trust (2007).  This version of
   sample C code is part of RFC XXXX; see the RFC itself
   for full legal notices.

   Regarding this sample C code (or any portion of it), the authors
   make no guarantees and are not responsible for any damage
   resulting from its use.  The authors grant irrevocable permission
   to anyone to use, modify, and distribute it in any way that does
   not diminish the rights of anyone else to use, modify, and
   distribute it, provided that redistributed derivative works do
   not contain misleading author or version information.



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   Derivative works need not be licensed under similar terms.
 */

#include <stdio.h>
#include <string.h>

/* hexadecimal lookup table */
static const char hex[] = "0123456789ABCDEF";

#define XX 127
/*
 * Table for decoding hexadecimal in %encoding
 */
static const char index_hex[256] = {
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
     0, 1, 2, 3,  4, 5, 6, 7,  8, 9,XX,XX, XX,XX,XX,XX,
    XX,10,11,12, 13,14,15,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,10,11,12, 13,14,15,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
    XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX, XX,XX,XX,XX,
};
#define HEXCHAR(c)  (index_hex[(unsigned char)(c)])

/* "gen-delims" excluding "/" but including "%" */
#define GENERAL_DELIMS_NO_SLASH     ":?#[]@" "%"

/* "gen-delims" (excluding "/", but including "%")
   plus subset of "sub-delims" */
#define GENERAL_UNSAFE_NO_SLASH     GENERAL_DELIMS_NO_SLASH ";&=+"
#define OTHER_UNSAFE                " \"<>\\^`{|}"

/* URL unsafe printable characters */
static const char mailbox_url_unsafe[] = GENERAL_UNSAFE_NO_SLASH
                                         OTHER_UNSAFE;

/* UTF7 modified base64 alphabet */
static const char base64chars[] =
  "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+,";



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#define UNDEFINED 64

/* UTF16 definitions */
#define UTF16MASK   0x03FFUL
#define UTF16SHIFT  10
#define UTF16BASE   0x10000UL
#define UTF16HIGHSTART   0xD800UL
#define UTF16HIGHEND     0xDBFFUL
#define UTF16LOSTART     0xDC00UL
#define UTF16LOEND  0xDFFFUL

/* Convert an IMAP mailbox to a URL path
 *  dst needs to have roughly 4 times the storage space of src
 *    Hex encoding can triple the size of the input
 *    UTF-7 can be slightly denser than UTF-8
 *     (worst case: 8 octets UTF-7 becomes 9 octets UTF-8)
 */
void MailboxToURL(char *dst, char *src)
{
    unsigned char c, i, bitcount;
    unsigned long ucs4, utf16, bitbuf;
    unsigned char base64[256], utf8[6];

    /* initialize modified base64 decoding table */
    memset(base64, UNDEFINED, sizeof (base64));
    for (i = 0; i < sizeof (base64chars); ++i) {
     base64[(int) base64chars[i]] = i;
    }

    /* loop until end of string */
    while (*src != '\0') {
     c = *src++;
     /* deal with literal characters and &- */
     if (c != '&' || *src == '-') {
         /* NB: There are no "URL safe" characters after the '~' */
         if (c < ' ' || c > '~' ||
             strchr(mailbox_url_unsafe, c) != NULL) {
          /* hex encode if necessary */
          dst[0] = '%';
          dst[1] = hex[c >> 4];
          dst[2] = hex[c & 0x0f];
          dst += 3;
         } else {
          /* encode literally */
          *dst++ = c;
         }
         /* skip over the '-' if this is an &- sequence */
         if (c == '&') ++src;



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     } else {
        /* convert modified UTF-7 -> UTF-16 -> UCS-4 -> UTF-8 -> HEX */
         bitbuf = 0;
         bitcount = 0;
         ucs4 = 0;
         while ((c = base64[(unsigned char) *src]) != UNDEFINED) {
          ++src;
          bitbuf = (bitbuf << 6) | c;
          bitcount += 6;
          /* enough bits for a UTF-16 character? */
          if (bitcount >= 16) {
              bitcount -= 16;
              utf16 = (bitcount ? bitbuf >> bitcount
                             : bitbuf) & 0xffff;
              /* convert UTF16 to UCS4 */
              if
                    (utf16 >= UTF16HIGHSTART && utf16 <= UTF16HIGHEND) {
               ucs4 = (utf16 - UTF16HIGHSTART) << UTF16SHIFT;
               continue;
              } else if
                    (utf16 >= UTF16LOSTART && utf16 <= UTF16LOEND) {
               ucs4 += utf16 - UTF16LOSTART + UTF16BASE;
              } else {
               ucs4 = utf16;
              }
              /* convert UTF-16 range of UCS4 to UTF-8 */
              if (ucs4 <= 0x7fUL) {
               utf8[0] = (unsigned char) ucs4;
               i = 1;
              } else if (ucs4 <= 0x7ffUL) {
               utf8[0] = 0xc0 | (unsigned char) (ucs4 >> 6);
               utf8[1] = 0x80 | (unsigned char) (ucs4 & 0x3f);
               i = 2;
              } else if (ucs4 <= 0xffffUL) {
               utf8[0] = 0xe0 | (unsigned char) (ucs4 >> 12);
               utf8[1] = 0x80 | (unsigned char) ((ucs4 >> 6) & 0x3f);
               utf8[2] = 0x80 | (unsigned char) (ucs4 & 0x3f);
               i = 3;
              } else {
               utf8[0] = 0xf0 | (unsigned char) (ucs4 >> 18);
               utf8[1] = 0x80 | (unsigned char) ((ucs4 >> 12) & 0x3f);
               utf8[2] = 0x80 | (unsigned char) ((ucs4 >> 6) & 0x3f);
               utf8[3] = 0x80 | (unsigned char) (ucs4 & 0x3f);
               i = 4;
              }
              /* convert utf8 to hex */
              for (c = 0; c < i; ++c) {
               dst[0] = '%';



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               dst[1] = hex[utf8[c] >> 4];
               dst[2] = hex[utf8[c] & 0x0f];
               dst += 3;
              }
          }
         }
         /* skip over trailing '-' in modified UTF-7 encoding */
         if (*src == '-') ++src;
     }
    }
    /* terminate destination string */
    *dst = '\0';
}

/* Convert hex coded UTF-8 URL path to modified UTF-7 IMAP mailbox
 *  dst should be about twice the length of src to deal with non-hex
 *  coded URLs
 */
int URLtoMailbox(char *dst, char *src)
{
    unsigned int utf8pos = 0;
    unsigned int utf8total, i, c, utf7mode, bitstogo, utf16flag;
    unsigned long ucs4 = 0, bitbuf = 0;

    utf7mode = 0; /* is the output UTF7 currently in base64 mode? */
    utf8total = 0; /* how many octets is the current input UTF-8 char;
                      0 == between characters */
    bitstogo = 0; /* bits that need to be encoded into base64; if
                     bitstogo != 0 then utf7mode == 1 */
    while ((c = (unsigned char)*src) != '\0') {
     ++src;
     /* undo hex-encoding */
     if (c == '%' && src[0] != '\0' && src[1] != '\0') {
         c = HEXCHAR(src[0]);
         i = HEXCHAR(src[1]);
         if (c == XX || i == XX) {
             return 0;
         } else {
             c = (char)((c << 4) | i);
         }
         src += 2;
     }
     /* normal character? */
     if (c >= ' ' && c <= '~') {
         /* switch out of UTF-7 mode */
         if (utf7mode) {
          if (bitstogo) {
          *dst++ = base64chars[(bitbuf << (6 - bitstogo)) & 0x3F];



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          }
          *dst++ = '-';
          utf7mode = 0;
          bitstogo = bitbuf = 0;
         }
         *dst++ = c;
         /* encode '&' as '&-' */
         if (c == '&') {
          *dst++ = '-';
         }
         continue;
     }
     /* switch to UTF-7 mode */
     if (!utf7mode) {
         *dst++ = '&';
         utf7mode = 1;
     }
     /* Encode US-ASCII characters as themselves */
     if (c < 0x80) {
         ucs4 = c;
         utf8total = 1;
     } else if (utf8total) {
         /* this is a subsequent octet of a multi-octet character */
         /* save UTF8 bits into UCS4 */
         ucs4 = (ucs4 << 6) | (c & 0x3FUL);
         if (++utf8pos < utf8total) {
          continue;
         }
     } else {
         /* this is the first octet of a multi-octet character */
         utf8pos = 1;
         if (c < 0xE0) {
          utf8total = 2;
          ucs4 = c & 0x1F;
         } else if (c < 0xF0) {
          utf8total = 3;
          ucs4 = c & 0x0F;
         } else {
          /* NOTE: can't convert UTF8 sequences longer than 4 */
          utf8total = 4;
          ucs4 = c & 0x03;
         }
         continue;
     }
     /* Finished with UTF-8 character. Make sure it isn't an
        overlong sequence. If it is, return failure. */
     if ((ucs4 < 0x80 && utf8total > 1) ||
         (ucs4 < 0x0800 && utf8total > 2) ||



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         (ucs4 < 0x00010000 && utf8total > 3) ||
         (ucs4 < 0x00200000 && utf8total > 4) ||
         (ucs4 < 0x04000000 && utf8total > 5) ||
         (ucs4 < 0x80000000 && utf8total > 6)) {
         return 0;
     }
     /* loop to split ucs4 into two utf16 chars if necessary */
     utf8total = 0;
     do {
         if (ucs4 >= UTF16BASE) {
                ucs4 -= UTF16BASE;
          bitbuf = (bitbuf << 16) | ((ucs4 >> UTF16SHIFT)
                            + UTF16HIGHSTART);
          ucs4 = (ucs4 & UTF16MASK) + UTF16LOSTART;
          utf16flag = 1;
         } else {
          bitbuf = (bitbuf << 16) | ucs4;
          utf16flag = 0;
         }
         bitstogo += 16;
         /* spew out base64 */
         while (bitstogo >= 6) {
          bitstogo -= 6;
          *dst++ = base64chars[(bitstogo ? (bitbuf >> bitstogo)
                               : bitbuf)
                         & 0x3F];
         }
     } while (utf16flag);
    }
    /* if in UTF-7 mode, finish in ASCII */
    if (utf7mode) {
     if (bitstogo) {
         *dst++ = base64chars[(bitbuf << (6 - bitstogo)) & 0x3F];
     }
     *dst++ = '-';
    }
    /* tie off string */
    *dst = '\0';
    return 1;
}


Appendix B.  List of changes since RFC 2192

     Updated boilerplate, list of editor's, etc.
     Updated references.
     Updated ABNF not to use _, to use SP instead of SPACE, etc.
     Updated example domains to use example.org.



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     Fixed ABNF error in "imessagelist" non-terminal.
     Updated ABNF, due to changes in RFC 3501, RFC 4466 and RFC 3986.
     Renamed "iuserauth" non-terminal to "iuserinfo".
     Clarified that the userinfo component describes both authorization
      identity and mailbox naming scope.
     Allow for non-synchronizing literals in "enc-search".
     Added "ipartial" specifier that denotes a partial FETCH.
     Moved URLAUTH text from RFC 4467 to this document.
     Updated ABNF for the whole server to allow missing trailing "/"
      (e.g. "imap://imap.example.com" is now valid and is the same as
       "imap://imap.example.com/")
     Clarified how relative-path references are constructed.
     Added more examples demonstrating relative-path references.
     Added rules for relative URLs and restructured ABNF as the result.
     Removed text on use of relative URLs in MHTML.
     Added examples demonstrating security considerations when resolving
      URLs.
     Recommend usage of STARTTLS/SASL security layer to protect
      confidential data.
     Removed some advices about connection reuse, which were incorrect.
     Removed URLs referencing a list of mailboxes, as this feature
      haven't seen any deployments.
     Clarified that user name "anonymous" is case-insensitive.


Appendix C.  List of changes since RFC 4467

     Renamed <mechanism> to <uauth-mechanism>.  Restructured ABNF.


Appendix D.  Acknowledgments

     Text describing URLAUTH was lifted from [URLAUTH] by Mark Crispin.

     Stephane H. Maes contributed some ideas to this document, he also
     co-edited early versions of this document.

     Editors would like to thank Mark Crispin, Ken Murchison, Ted
     Hardie, Zoltan Ordogh, Dave Cridland, Kjetil Torgrim Homme, Lisa
     Dusseault, Spencer Dawkins, Filip Navara and Shawn M. Emery for the
     time they devoted to reviewing of this document and/or for the com-
     ments received.









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Intellectual Property

     The IETF takes no position regarding the validity or scope of any
     Intellectual Property Rights or other rights that might be claimed
     to pertain to the implementation or use of the technology
     described in this document or the extent to which any license
     under such rights might or might not be available; nor does it
     represent that it has made any independent effort to identify any
     such rights.  Information on the procedures with respect to rights
     in RFC documents can be found in BCP 78 and BCP 79.

     Copies of IPR disclosures made to the IETF Secretariat and any
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     attempt made to obtain a general license or permission for the use
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     at http://www.ietf.org/ipr.

     The IETF invites any interested party to bring to its attention
     any copyrights, patents or patent applications, or other
     proprietary rights that may cover technology that may be required
     to implement this standard.  Please address the information to the
     IETF at ietf-ipr@ietf.org.

Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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