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Versions: 00 01 02 03 04 05 06 07 08 09 rfc5092                         
Network Working Group                                          C. Newman
Document: draft-ietf-lemonade-rfc2192bis-00.txt         Sun Microsystems
Expires: March 2006                                          A. Melnikov
Intended category: Standards Track                            Isode Ltd.
                                                              S. H. Maes
                                                      Oracle Corporation
                                                            October 2005

                            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.

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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 Internet Society (2005).


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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", "SHOULD", "SHOULD NOT", and "MAY"
     in this document are to be interpreted as defined in "Key words for
     use in RFCs to Indicate Requirement Levels" [KEYWORDS].

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] <<except that clear text passwords are not permit-
     ted>>.  If :<port> is omitted, the port defaults to 143.

     An IMAP URL takes one of the following forms:





     The first form is used to refer to an IMAP server, the second form
     refers to a list of mailboxes, the third form refers to the con-
     tents 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

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3.1. IMAP mailbox naming scope

     The "enc-user" part of the "iuserinfo" component, if present,
     denotes mailbox naming scope.  <<What if "enc-user" is not present
     in "iuserinfo"?>>

     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.

     <<Chris: whenever possible the shared folders name for a mailbox
     should be used to avoid mailbox naming scope issues.>>

3.2. IMAP User Name and Authentication Mechanism

     The userinfo component [URI-GEN] of an IMAP URI may contains an
     IMAP user Name (authorization identity, "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 connec-
     tion to the IMAP server.  If no user name or authentication mecha-
     nism is supplied, the user name "anonymous" is used with the
     "LOGIN" command and the password is supplied as the Internet e-mail
     address of the end user accessing the resource.  If the URL doesn't
     supply a user name, the program interpreting the IMAP URL SHOULD
     request one from the user if necessary.


     <<How LOGINDISABLES affects LOGIN>>

     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

     The string ";AUTH=*" indicates that the client SHOULD select an
     appropriate authentication mechanism.  It MAY use any mechanism
     listed in the CAPABILITY command or use an out of band security
     service resulting in a PREAUTH connection.  If no user name is
     specified and no appropriate authentication mechanisms are avail-
     able, the client SHOULD fall back to anonymous login as described
     above.  This allows a URL which grants read-write access to

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     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-
     (2) Explicit local site policy permits the client to connect to the
     server in the URL.  For example, if the client knows the site
     domain name, site policy may dictate that any hostname ending in
     that domain is trusted.
     (3) The user confirms that connecting to that domain name with the
     specified credentials and/or mechanism is permitted.
     (4) A mechanism is used which validates the server before passing
     potentially compromising client credentials.  <<e.g. TLS?>>
     (5) An authentication mechanism is used which will not reveal
     information to the server which could be used to compromise future

     URLs which do not include a user name must be treated with extra
     care, since 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 security mechanism.
     Finally, clients are discouraged from using a plain text password
     as a fallback with ";AUTH=*" unless the connection has strong
     encryption (<<e.g. a key length of greater than 56 bits>>).

     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.  If a URL contains an
     authentication mechanism without a user name, then any non-anony-
     mous 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].

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     <<URLAUTH overrides the authorization identity>>

4. IMAP server

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


     A program interpreting this URL would issue the standard set of
     commands it uses to present a view of the contents of an IMAP
     server.  This is likely to be semanticly equivalent to one of the
     following URLs:


     <<The program interpreting this URL SHOULD use the LSUB form if it
     supports mailbox subscriptions.>>

5. Lists of mailboxes

     An IMAP URL referring to a list of mailboxes has the following


     The <list-type> may be either "LIST" or "LSUB", and is case insen-
     sitive.  The field ";TYPE=<list-type>" MUST be included.

     The <enc-list-mailbox> is any argument suitable for the list-mail-
     box field of the IMAP [IMAP4] LIST or LSUB commands.  The field
     <enc-list-mailbox> may be omitted, in which case the program inter-
     preting the IMAP URL may use "*" or "%" as the <enc-list-mailbox>.
     The program SHOULD use "%" if it supports a hierarchical view, oth-
     erwise it SHOULD use "*".  <<In order to show all available mail-
     boxes the program SHOULD use the NAMESPACE [NAMESPACE] command to
     discover avialable namespaces.  After that it should issue a LIST
     command for each namespace.  This might be required, because, for
     example, some servers might not show shared mailboxes in a <LIST ""
     *> command.>>

     Note that if unsafe or reserved characters such as " " or "%" are
     present in <enc-list-mailbox> they MUST be encoded as described in
     [URI-GEN].  If the character "/" is present in enc-list-mailbox, it
     SHOULD NOT be encoded.

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6. Lists of messages

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


     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].  If the character "/" is present
     in enc-mailbox, it SHOULD NOT be encoded.

     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].

7. A specific message or message part

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


     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

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     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.

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8. Relative IMAP URLs

     Relative IMAP URLs are permitted and are resolved according to the
     rules defined in [URI-GEN] with one exception.  In IMAP URLs,
     parameters are treated as part of the normal path with respect to
     relative URL resolution.  This is believed to be the behavior of
     the installed base and is likely to be documented in a future revi-
     sion of the relative URL specification.

     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

     The base URL for a list of mailboxes or messages which was referred
     to by an IMAP URL is always the referring IMAP URL itself.  The
     base URL for a message or message part which was referred to by an
     IMAP URL may be more complicated to determine.  The program inter-
     preting the relative URL will have to check the headers of the MIME
     entity and any enclosing MIME entities in order to locate the "Con-
     tent-Base" and "Content-Location" headers.  These headers are used
     to determine the base URL as defined in <<[HTTP]>>.  For example,
     if the referring IMAP URL contains a "/;SECTION=1.2" parameter,
     then the MIME headers for section 1.2, for section 1, and for the
     enclosing message itself SHOULD be checked in that order for "Con-
     tent-Base" or "Content-Location" headers.

9. 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 [UTF8].  Since 8-bit characters are not per-
     mitted in URLs, the UTF-8 characters are encoded as required by the

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     URL specification [URI-GEN].  Sample code is included in Appendix A
     to demonstrate this conversion.

10. 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

     The URL:


     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:


     Results in the following client commands:

       <client requests password from user>
       <connect to minbari.example.org imap server, activate strong encryption>
       C: A001 LOGIN MICHAEL zipper
       C: A002 LIST "" users.*

     The URL:


     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>

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     The URL:


     Results in the following client commands:

         <connect to minbari.example.org, port 143>
         <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:


     This could result in the following client commands:

         C: A004 UID FETCH 20 (BODY.PEEK[1.2.MIME]
               BODY.PEEK[HEADER.FIELDS (Content-Base Content-Location)])
         <Client looks for Content-Base or 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:


     Could result in the following:

         <connect to minbari.example.org, port 143>
         C: A001 CAPABILITY
         S: A001 OK
         <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

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     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.

11. Security Considerations

     Security considerations discussed in the IMAP specification [IMAP4]
     and the URL specification <<[BASIC-URL]>> are relevant.  Security
     considerations related to authenticated URLs are discussed in sec-
     tion 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 permis-
     sion from the user to supply that password to the specified host

12. 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

     uchar            = unreserved | pct-encoded

     achar            = uchar / "&" / "="

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

     <<sub-delims  = "!" / "$" / "&" / "'" / "(" / ")"
                       / "*" / "+" / "," / ";" / "="
     pchar         = unreserved / pct-encoded / sub-delims / ":" / "@">>

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

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

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

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     enc-search       = 1*bchar
                    ; encoded version of [IMAPABNF] "search-program".
                       ; Note that IMAP4 literals may not be used
                             ; in a "search-program".

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

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

     imapurl          = "imap://" iserver "/" [ icommand ]

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

     icommand         = imailboxlist / imessagelist / imessagepart

     imailboxlist     = [enc-list-mailbox] ";TYPE=" list-type

     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]

     ipartial         = "/;PARTIAL=" partial-range

     isection         = "/;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             = "/;UID=" nz-number
                    ; 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    = nz-number ["." nz-number]
                    ; partial fetch

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

13. Normative References

     <<[BASIC-URL] Berners-Lee, Masinter, McCahill, "Uniform Resource
     Locators (URL)", RFC 1738, CERN, Xerox Corporation, University of
     Minnesota, December 1994.>>

     [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate Require-
     ment Levels", 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", work in progress, draft-melnikov-imap-ext-abnf-XX.txt.

     <<[HTTP] Fielding, Gettys, Mogul, Frystyk, Berners-Lee, "Hypertext
     Transfer Protocol -- HTTP/1.1", RFC 2068, UC Irvine, DEC, MIT/LCS,
     January 1997.>>

     <<[ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifica-
     tions: ABNF", RFC 2234, Internet Mail Consortium, Demon Internet
     Ltd, November 1997.>>

     [MIME] Freed, N., Borenstein, N., "Multipurpose Internet Mail
     Extensions", RFC 2045, Innosoft, First Virtual, 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.

14. Author's Address

     Chris Newman
     Sun Microsystems
     1050 Lakes Drive
     West Covina, CA 91790 USA
     EMail: chris.newman@sun.com

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     Alexey Melnikov
     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
     Oracle Corporation
     500 Oracle Parkway
     M/S 4op634
     Redwood Shores, CA 94065
     Phone: +1-650-607-6296
     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 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[] =
#define UNDEFINED 64

/* UTF16 definitions */
#define UTF16MASK   0x03FFUL

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#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[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) {

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          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 */
                    (utf16 >= UTF16HIGHSTART && utf16 <= UTF16HIGHEND) {
               ucs4 = (utf16 - UTF16HIGHSTART) << UTF16SHIFT;
              } 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] = 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;

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         /* 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;
   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') {
     /* 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 == '&') {

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

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         /* 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';

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.
     Updated example domains to use example.org.
     Fixed ABNF error in "imessagelist" non-terminal.
     Updated ABNF, due to changes in RFC 3501, IMAPABNF and RFC 3986.
     Renamed "iuserauth" non-terminal to "iuserinfo".
     Clarified that the userinfo component describes both authorization
     identity and mailbox naming scope.
     Added "ipartial" specifier that denotes a partial fetch.

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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 Internet Society (2005).

   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


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

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