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

Network Working Group                                        A. Melnikov
Document: draft-ietf-lemonade-rfc2192bis-04.txt               Isode Ltd.
Expires: July 2007                                             C. Newman
Intended category: Standards Track                      Sun Microsystems
                                                              S. H. Maes
                                                      Oracle Corporation
                                                            January 2007


                            IMAP URL Scheme


Status of this Memo
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A revised version of this draft document will be submitted to the RFC
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Copyright Notice

     Copyright (C) The IETF Trust (2007).


Abstract





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     IMAP [IMAP4] 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.


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 [KEY-
     WORDS].


2. IMAP scheme

     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.

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

     An 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]

     The first form is used to refer to an IMAP server, the second form
     refers to the contents of a mailbox or a set of messages resulting
     from a search, and the final form refers to a specific message or
     message part, and possibly a byte range in that part.  Note that
     the syntax here is informal.  The authoritative formal syntax for
     IMAP URLs is defined in section 11. The partial specifier semantics
     conforms to [IMAP4] partial specifiers.



3. IMAP userinfo component


3.1. IMAP mailbox naming scope




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     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.)

     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 [URI-GEN] of an IMAP URI may contains 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).  They
     are used in the "LOGIN" or "AUTHENTICATE" commands after making the
     connection to the IMAP server.

     If no user name or authentication mechanism is supplied, the client
     must authenticate as anonymous to the server. If the server adver-
     tises AUTH=ANONYMOUS IMAP capability, the client MUST use the
     AUTHENTICATE command with ANONYMOUS [ANONYMOUS] SASL mechanism.  If
     SASL ANONYMOUS is not available, the user name "anonymous" is used
     with the "LOGIN" command and the password is supplied as the Inter-
     net e-mail address of the end user accessing the resource. The lat-
     ter option is given in order to provide for interoperability with
     deployed servers.

     If the URL doesn't supply a user name, the program interpreting the
     IMAP URL SHOULD request one from the user (if it is an interactive
     program) or configuration.

     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 can be expressed by adding ";AUTH=<enc-
     auth-type>" to the end of the user name.  When such an <enc-auth-
     type> is indicated, the client SHOULD request appropriate creden-
     tials from that mechanism and use the "AUTHENTICATE" command
     instead of the "LOGIN" command.  If no user name is specified, one
     SHOULD be obtained from the mechanism or requested from the user as
     appropriate.




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     The string ";AUTH=*" indicates that the client SHOULD select an
     appropriate authentication mechanism.  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 appro-
     priate authentication mechanisms are available, the client SHOULD
     fall back to anonymous login as described above.  This 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.

     Since URLs can easily come from untrusted sources, care must be
     taken when resolving a URL which requires or requests any sort of
     authentication.  If authentication credentials are supplied to the
     wrong server, it may compromise the security of the user's account.
     The program resolving the URL should make sure it meets 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 mechaninism
     such as DIGEST-MD5 [DIGEST-MD5], which supports mutual authentica-
     tion.
     (5) An authentication mechanism is used which will not reveal
     information to the server which could be used to compromise future
     connections. For example, SASL ANONYMOUS [ANONYMOUS] or GSSAPI
     [GSSAPI].



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     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
     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 " " or ";" are
     present in the user name or authentication mechanism, they MUST be
     encoded as described in [URI-GEN].


3.3. Limitations of enc-user

     As per sections 3.1 and 3.2 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 only be resolved by the mailbox owner.  In order
     to avoid this restrictions, 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 is described
     in section 6.1.



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     (*) 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
     server, both in referrals and in configuration.  A program inter-
     preting this URL would issue the standard set of commands it uses
     to present a view of the contents of an IMAP server.


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" command.  Note that if unsafe or reserved characters such
     as " ", ";", or "?" are present in <enc-mailbox> they MUST be
     encoded as described in [URI-GEN].

     The [uidvalidity] field is optional.  If it is present, it MUST be
     the argument to the IMAP4 UIDVALIDITY status response at the time
     the URL was created.  This SHOULD be used by the program interpret-
     ing the IMAP URL to determine if the URL is stale.

     The [?<enc-search>] field is optional.  If it is not present, the
     contents of the mailbox SHOULD be presented by the program inter-
     preting the URL.  If it is present, it SHOULD be used as the argu-
     ments following an IMAP4 SEARCH command with unsafe characters such
     as " " (which are likely to be present in the <enc-search>) encoded
     as described in [URI-GEN].  Note that quoted strings and non-syn-
     chronizing literals [LITERAL+] are allowed in the <enc-search> con-
     tent, however synchronizing literals 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 [IMAP4]).





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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]

     The <enc-mailbox> and [uidvalidity] are as defined 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 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.


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



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



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     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 currently 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


     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.





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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, indi-
     cates 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 mes-
     sage submission entity MUST validate the userid prior to contacting
     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]).

     The "authuser" access identifier indicates that use of this URL is
     limited to IMAP sessions which are logged in as an authorized user
     (that is, have authorization identity as an authorized user) of
     that IMAP server.  Use of this URL is prohibited to anonymous IMAP
     sessions.



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     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).


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
     are treated as part of the normal path with respect to relative URL
     resolution.

     The following observations are also important:

     The <iauth> grammar element is considered part of the user name for
     purposes of resolving relative IMAP URLs.  This means that unless a
     new login/server specification is included in the relative URL, the
     authentication mechanism is inherited from a 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 encoded as described in
     [URI-GEN]. Otherwise they would be misinterpreted as dot-segments
     (see Section 3.3 of [URI-GEN]), which are processed specially dur-
     ing 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 [URI-GEN].  If an IMAP server



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     permits mailbox names with a leading "/", then the leading "/" MUST
     be encoded as described in [URI-GEN].  Otherwise the produced abso-
     lute-path reference URI will be misinterpreted as a network-path
     reference [URI-GEN].


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 SHOULD
     NOT generate or accept relative-path IMAP references.

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


8. Multinational Considerations

     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 characters are not per-
     mitted in URLs, the UTF-8 characters are 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 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 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:".







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     The URL:

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

     Results in the following client commands:

         <connect to minbari.example.org, port 143>
         C: A001 LOGIN ANONYMOUS sheridan@babylon5.example.org
         C: A002 SELECT gray-council
         <client verifies the UIDVALIDITY matches>
         C: A003 UID FETCH 20 BODY.PEEK[]<0.1024>

     The URL:

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

     Results in the following client commands:

       <connect to psicorp.example.org, port 143>
       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 a mailbox>

     The URL:

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

     Results in the following client commands:

         <connect to minbari.example.org, port 143>
         C: A001 AUTHENTICATE GSSAPI
         <authentication exchange>
         C: A002 SELECT gray-council
         C: A003 UID FETCH 20 BODY.PEEK[1.2]

     <<Remove this example as Content-Location is not described
     anywhere?>>

     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]



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               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]

     The URL:

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

     Could result in the following:

         <connect to minbari.example.org, port 143>
         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>
         C: A001 CAPABILITY
         S: * CAPABILITY IMAP4rev1 LITERAL+ AUTH=DIGEST-MD5
         S: A001 OK
         C: A002 AUTHENTICATE DIGEST-MD5
         <authentication exchange>



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         S: A002 OK user john authenticated
         C: A003 SELECT babylon5/personel
         ...
         C: A004 SEARCH CHARSET UTF-8 SUBJECT {14+}
         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".

     The following relative-path reference

      <;UID=20>

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


      <..;UIDVALIDITY=385759045/;UID=20> <<The last one *does not* con-
     tain dot-segment?!>>


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
     of this document.

     Many email clients store the plain text password for later use
     after logging into an IMAP server.  Such clients MUST NOT use a
     stored password in response to an IMAP URL without explicit



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     permission from the user to supply that password to the specified
     host name.


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
      identifier 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
      identifiers 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 disclosed 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.  Many
      IMAP servers do not permit anonymous access; in the case of such
      servers the "anonymous" access identifer is equivalent to
      "authuser", but this MUST NOT be relied upon.











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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.


     sub-delims-sh = "!" / "$" / "'" / "(" / ")" /
                     "*" / "+" / ","
                        ;; <<same as sub-delims, but without ";", "&"
                        ;; and "=">>

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

     achar            = uchar / "&" / "="
                        ;; <<same as 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 ]]



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                 ; <<new production in this draft>>
                 ; 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
                       / 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 [iurlauth] )
                 ; Corresponds to 'path-noscheme [ "?" query ]'
                 ; in [URI-GEN]

     <<Is [iurlauth] allowed in relative URLs?>>


     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.
                 ; Corresponds to the relative form of "an IMAP server"
                 ; URL

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

     authimapurl     = "imap://" iserver "/" imessagepart



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                       ; Same as "imapurl" when "[icommand]" is
                       ; "imessagepart"

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

     authimapurlrump = authimapurl iurlauth-rump


     <<What is the best way of having 2 formats - the full IMAP URL as
     used in clients and the rump URL which is to be URLAUTHed to
     produce a full IMAP URL?>>

     enc-urlauth     = 32*HEXDIG

     iurlauth        = iurlauth-rump iua-verifier

     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 defined in [DATETIME]

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




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

     icommand         = imessagelist /
                        imessagepart [iurlauth]

     imailbox-ref     = enc-mailbox [uidvalidity]
                    ; <<new production in this draft>>

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

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



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     ipartial         = "/" ipartial-only

     ipartial-only    = ";PARTIAL=" partial-range
                    ; <<new production in this draft>>

     isection         = "/" isection-only

     isection-only    = ";SECTION=" enc-section
                    ; <<new production in this draft>>

     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
                 ; <<new rule in this draft>>
                    ; See [IMAP4] for "nz-number" definition

     iuserinfo        = enc-user [iauth] / [enc-user] iauth
                             ; conforms to the generic syntax of
                             ;  "userinfo" as defined in [URI-GEN].

     list-type        = "LIST" / "LSUB"

     partial-range    = number ["." nz-number]
                    ; partial fetch

     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 13.1 of this document.


12.1.  IANA Registration of imap: URI Scheme


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



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     URI scheme name: imap

     Status: permanent

     URI scheme syntax:
        See section 12 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 9 of [RFCXXXX].

     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 Mozilla/Thubderbird/Seamonkey use
     a different imap: scheme internally.

     Security considerations:

        See Security Considerations (Section 11) of [RFCXXXX].

     Contact:

        Alexey Melnikov <alexey.melnikov@isode.com>

     Author/Change controller:

        IESG

     References:

        [RFCXXXX] and [IMAP4].



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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.

     [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








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     [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.

     [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
     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/

     Stephane H. Maes (Editor)
     Oracle Corporation
     500 Oracle Parkway
     M/S 4op634
     Redwood Shores, CA 94065
     USA
     Phone: +1-650-607-6296



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     Email: stephane.maes@oracle.com



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.

<<Does the code comply with the new URI rules?>>

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

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

/* URL unsafe printable characters */
static char urlunsafe[] = " \"#%&+:;<=>?@[\\]^`{|}";

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



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    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[base64chars[i]] = i;
    }

    /* loop until end of string */
    while (*src != '\0') {
     c = *src++;
     /* deal with literal characters and &- */
     if (c != '&' || *src == '-') {
         if (c < ' ' || c > '~' || strchr(urlunsafe, 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;
     } 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;



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              } else {
               ucs4 = utf16;
              }
              /* convert UTF-16 range of UCS4 to UTF-8 */
              if (ucs4 <= 0x7fUL) {
               utf8[0] = ucs4;
               i = 1;
              } else if (ucs4 <= 0x7ffUL) {
               utf8[0] = 0xc0 | (ucs4 >> 6);
               utf8[1] = 0x80 | (ucs4 & 0x3f);
               i = 2;
              } else if (ucs4 <= 0xffffUL) {
               utf8[0] = 0xe0 | (ucs4 >> 12);
               utf8[1] = 0x80 | ((ucs4 >> 6) & 0x3f);
               utf8[2] = 0x80 | (ucs4 & 0x3f);
               i = 3;
              } else {
               utf8[0] = 0xf0 | (ucs4 >> 18);
               utf8[1] = 0x80 | ((ucs4 >> 12) & 0x3f);
               utf8[2] = 0x80 | ((ucs4 >> 6) & 0x3f);
               utf8[3] = 0x80 | (ucs4 & 0x3f);
               i = 4;
              }
              /* convert utf8 to hex */
              for (c = 0; c < i; ++c) {
               dst[0] = '%';
               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
 */
void URLtoMailbox(char *dst, char *src)
{
   unsigned int utf8pos, utf8total, i, c, utf7mode, bitstogo, utf16flag;
   unsigned long ucs4, bitbuf;



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   unsigned char hextab[256];

    /* initialize hex lookup table */
    memset(hextab, 0, sizeof (hextab));
    for (i = 0; i < sizeof (hex); ++i) {
     hextab[hex[i]] = i;
     if (isupper(hex[i])) hextab[tolower(hex[i])] = i;
    }

    utf7mode = 0;
    utf8total = 0;
    bitstogo = 0;
    while ((c = *src) != '\0') {
     ++src;
     /* undo hex-encoding */
     if (c == '%' && src[0] != '\0' && src[1] != '\0') {
         c = (hextab[src[0]] << 4) | hextab[src[1]];
         src += 2;
     }
     /* normal character? */
     if (c >= ' ' && c <= '~') {
         /* switch out of UTF-7 mode */
         if (utf7mode) {
          if (bitstogo) {
          *dst++ = base64chars[(bitbuf << (6 - bitstogo)) & 0x3F];
          }
          *dst++ = '-';
          utf7mode = 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) {
         /* save UTF8 bits into UCS4 */
         ucs4 = (ucs4 << 6) | (c & 0x3FUL);



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         if (++utf8pos < utf8total) {
          continue;
         }
     } else {
         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;
     }
     /* 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++ = '-';
    }



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    /* tie off string */
    *dst = '\0';
}


Appendix B.  List of changes since RFC 2192

     <<Needs checking and updating>>
     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.
     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.
     Clarified how relative-path references are constructed.
     Added more examples demonstrating relative-path references.
     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/")
     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.
     Removed some advices about connection reuse, which were incorrect.
     Removed URLs referencing a list of mailboxes.


Appendix C.  Acknowledgments

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

     Editors would like to thank Mark Crispin, Ken Murchison, Dave Crid-
     land for the time they devoted to reviewing of this document and/or
     for the comments 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
     assurances of licenses to be made available, or the result of an
     attempt made to obtain a general license or permission for the use
     of such proprietary rights by implementers or users of this
     specification can be obtained from the IETF on-line IPR repository
     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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