    Network Working Group                                           A. Young
    Request for Comments: 1798                              ISODE Consortium
    Category: Standards Track                                      June 1995

           Connection-less Lightweight X.500 Directory Access Protocol

    Status of this Memo

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

    X.500

       The protocol described in this document is designed to provide access
       to the Directory while not incurring the resource requirements of the
       Directory Access Protocol (DAP) [3].  In particular, it is aimed at
       avoiding the elapsed time that is associated with connection-oriented
       communication and it facilitates use of the Directory in a manner
       analagous to the DNS [5,6].  It is specifically targeted at simple
       lookup applications that require to read a small number of attribute
       values from a single entry.  It is intended to be a complement to DAP
       and LDAP [4].  The protocol specification draws heavily on that of
       LDAP.

    1.  Background

       The Directory can be used as a repository for many kinds of
       information.  The full power of DAP is unnecessary for applications
       that require simple read access to a few attribute values.
       Applications addressing is a good example of this type of use where
       an application entity needs to determine the Presentation Address
       (PA) of a peer entity given that peer's Application Entity Title
       (AET). If the AET is a Directory Name (DN) then the required result
       can be obtained from the PA attribute of the Directory entry
       identified by the AET.  This is very similar to DNS.

    Young                       Standards Track                     [Page 1]

    RFC 1798                         CLDAP                         June 1995

       Use of DAP to achieve this functionality involves a significant
       number of network exchanges:

          ___________________________________________________________
         |_#_|______Client_(DUA)________DAP________Server_(DSA)_____|
         |  1|  N-Connect.request       ->                          |
         |  2|                          <-    N-Connect.response    |
         |  3|  T-Connect.request       ->                          |
         |  4|                          <-    T-Connect.response    |
         |   |  S-Connect.request,                                  |
         |   |  P-Connect.request,                                  |
         |   |  A-Associate.request,                                |
         |  5|  DAP-Bind.request        ->                          |
         |   |                                S-Connect.response,   |
         |   |                                P-Connect.response,   |
         |   |                                A-Associate.response, |
         |  6|                          <-    DAP-Bind.response     |
         |  7|  DAP-Read.request        ->                          |
         |  8|                          <-    DAP-Read.response     |
         |   |  S-Release.request,                                  |
         |   |  P-Release.request,                                  |
         |   |  A-Release.request,                                  |
         |  9|  DAP-Unbind.request      ->                          |
         |   |                                S-Release.response,   |
         |   |                                P-Release.response,   |
         |   |                                A-Release.response,   |
         | 10|                          <-    DAP-Unbind.response   |
         |   |  T-Disconnect.request,                               |
         | 11|  N-Disconnect.request    ->                          |
         |   |                                T-Disconnect.response,|
         | 12|                          <-    N-Disconnect.response |
         |___|______________________________________________________|

    Young                       Standards Track                     [Page 2]

    RFC 1798                         CLDAP                         June 1995

       This is 10 packets before the application can continue, given that it
       can probably do so after issuing the T-Disconnect.request.  (Some
       minor variations arise depending upon the class of Network and
       Transport service that is being used; for example use of TP4 over
       CLNS reduces the packet count by two.) LDAP is no better in the case
       where the LDAP server uses full DAP to communicate with the
       Directory:

      ____________________________________________________________________
     |__#_|___Client_____LDAP_____LDAP_server______DAP_________DSA_______|
     |  1 |  TCP SYN      ->                                             |
     |  2 |               <-    TCP SYN ACK                              |
     |  3 |  BindReq      ->                                             |
     |  4 |                     N-Connect.req      ->                    |
     |  5 |                                        <-    N-Connect.res   |
     |  6 |                     T-Connect.req      ->                    |
     |  7 |                                        <-    T-Connect.res   |
     |  8 |                     DAP-Bind.req       ->                    |
     |  9 |                                        <-    DAP-Bind.res    |
     | 10 |               <-    BindRes                                  |
     | 11 |  SearchReq    ->                                             |
     | 12 |                     DAP-Search.req     ->                    |
     | 13 |                                        <-    DAP-Search.res  |
     | 14 |               <-    SearchRes                                |
     | 15 |  TCP FIN      ->                                             |
     | 16 |                     DAP-Unbind.req     ->                    |
     | 17 |                                        <-    DAP-Unbind.res  |
     | 18 |                     N-Disconnect.req   ->                    |
     | 19 |                                        <-    N-Disconnect.res|
     |____|______________________________________________________________|

    Young                       Standards Track                     [Page 3]

    RFC 1798                         CLDAP                         June 1995

       Here there are 14 packets before the application can continue.  Even
       if the LDAP server is on the same host as the DSA (so packet delay is
       negligible), or if the DSA supports LDAP directly, then there are
       still 6 packets.

                      ____________________________________
                     | #|   Client     LDAP   LDAP server|
                     |__|________________________________|
                     | 1|  TCP SYN      ->               |
                     | 2|               <-    TCP SYN ACK|
                     | 3|  BindReq      ->               |
                     | 4|               <-    BindRes    |
                     | 5|  SearchReq    ->               |
                     |_6|_______________<-____SearchRes__|

       This protocol provides for simple access to the Directory where the
       delays inherent in the above exchanges are unacceptable and where the
       additional functionality provided by connection-mode operation is not
       required.

    2.  Protocol Model

       CLDAP is based directly on LDAP [4] and inherits many of the key
       aspects of the LDAP protocol:

       - -  Many protocol data elements are encoding as ordinary strings
            (e.g., Distinguished Names).

       - -  A lightweight BER encoding is used to encode all protocol
            elements.

       It is different to LDAP in that:

       - -  Protocol elements are carried directly over UDP or other
            connection-less transport, bypassing much of the
            session/presentation overhead and that of connections (LDAP uses
            a connection-mode transport service).

       - -  A restricted set of operations is available.

       The definitions of most protocol elements are inherited from LDAP.

       The general model adopted by this protocol is one of clients
       performing protocol operations against servers. In this model, this
       is accomplished by a client transmitting a protocol request
       describing the operation to be performed to a server, which is then
       responsible for performing the necessary operations on the Directory.

    Young                       Standards Track                     [Page 4]

    RFC 1798                         CLDAP                         June 1995

       Upon completion of the necessary operations, the server returns a
       response containing any results or errors to the requesting client.

       Note that, although servers are required to return responses whenever
       such responses are defined in the protocol, there is no requirement
       for synchronous behaviour on the part of either client or server
       implementations: requests and responses for multiple operations may
       be exchanged by client and servers in any order, as long as servers
       eventually send a response for every request that requires one.

       Also, because the protocol is implemented over a connection-less
       transport service clients must be prepared for either requests or
       responses to be lost.  Clients should use a retry mechanism with
       timeouts in order to achieve the desired level of reliability.  For
       example, a client might send off a request and wait for two seconds.
       If no reply is forthcoming, the request is sent again and the client
       waits four seconds.  If there is still no reply, the client sends it
       again and waits eight seconds, and so on, until some maximun time.
       Such algorithms are widely used in other datagram-based protocol
       implementations, such as the DNS.  It is not appropriate to mandate a
       specific algorithm as this will depend upon the requirments and
       operational environment of individual CLDAP client implementations.

       It is not required that a client abandon any requests to which no
       response has been received and for which a reply is no longer
       required (because the request has been timed out), but they may do
       so.

       Consistent with the model of servers performing protocol operations
       on behalf of clients, it is also to be noted that protocol servers
       are expected to handle referrals without resorting to the return of
       such referrals to the client. This protocol makes no provisions for
       the return of referrals to clients, as the model is one of servers
       ensuring the performance of all necessary operations in the
       Directory, with only final results or errors being returned by
       servers to clients.

       Note that this protocol can be mapped to a strict subset of the
       Directory abstract service, so it can be cleanly provided by the DAP.

    3.  Mapping Onto Transport Services

       This protocol is designed to run over connection-less transports,
       with all 8 bits in an octet being significant in the data stream.
       Specifications for two underlying services are defined here, though
       others are also possible.

    Young                       Standards Track                     [Page 5]

    RFC 1798                         CLDAP                         June 1995

    3.1.  User Datagram Protocol (UDP)

       The CLDAPMessage PDUs are mapped directly onto UDP datagrams.  Only
       one request may be sent in a single datagram. Only one response may
       be sent in a single datagram.  Server implementations running over
       the UDP should provide a protocol listener on port 389.

    3.2.  Connection-less Transport Service (CLTS)

       Each LDAPMessage PDU is mapped directly onto T-Unit-Data.

    4.  Elements of Protocol

       CLDAP messages are defined by the following ASN.1:

        CLDAPMessage ::= SEQUENCE {
            messageID       MessageID,
            user            LDAPDN,         -- on request only --
            protocolOp      CHOICE {
                            searchRequest   SearchRequest,
                            searchResponse  SEQUENCE OF
                                                SearchResponse,
                            abandonRequest  AbandonRequest
            }
        }

       where MessageID, LDAPDN, SearchRequest, SearchResponse and
       AbandonRequest are defined in the LDAP protocol.

       The 'user' element is supplied only on requests (it should be zero
       length and is ignored in responses). It may be used for logging
       purposes but it is not required that a CLDAP server implementation
       apply any particular semantics to this field.

       Editorial note:
           There has been some discussion about the desirability of
           authentication with CLDAP requests and the addition of the fields
           necessary to support this. This might take the form of a clear
           text password (which would go against the current IAB drive to
           remove such things from protocols) or some arbitrary credentials.
           Such a field is not included.  It is felt that, in general,
           authentication would incur sufficient overhead to negate the
           advantages of the connectionless basis of CLDAP. If an
           application requires authenticated access to the Directory then
           CLDAP is not an appropriate protocol.

    Young                       Standards Track                     [Page 6]

    RFC 1798                         CLDAP                         June 1995

       Within a searchResponse all but the last SearchResponse has choice
       'entry' and the last SearchResponse has choice 'resultCode'.  Within
       a searchResponse, as an encoding optimisation, the value of the
       objectName LDAP DN may use a trailing '*' character to refer to the
       baseObject of the corresponding searchRequest.  For example, if the
       baseObject is specified as "o=UofM, c=US", then the following
       objectName LDAPDNs in a response would have the indicated meanings

              objectName returned   actual LDAPDN denoted
              ____________________________________________________
              "*"                   "o=UofM, c=US"
              "cn=Babs Jensen, *"   "cn=Babs Jensen, o=UofM, c=US"

    4.1.  Errors

    The following error code is added to the LDAPResult.resultCode
    enumeration of [4]:

                                 resultsTooLarge              (70),

       This error is returned when the LDAPMessage PDU containing the
       results of an operation are too large to be sent in a single
       datagram.

    4.2.  Example

       A simple lookup can be performed in 4 packets. This is reduced to 2
       if either the DSA implements the CLDAP protocol, the CLDAP server has
       a cache of the desired results, or the CLDAP server and DSA are co-
       located such that there is insignificant delay between them.

        _______________________________________________________________
       |_#|___Client_____CLDAP____CLDAP_server____DAP________DSA______|
       | 1|  SearchReq    ->                                          |
       | 2|                      DAP-Search.req   ->                  |
       | 3|                                       <-    DAP-Search.res|
       | 4|               <-     SearchRes                            |
       |__|___________________________________________________________|

    5.  Implementation Considerations

       The following subsections provide guidance on the implementation of
       clients and servers using the CLDAP protocol.

    Young                       Standards Track                     [Page 7]

    RFC 1798                         CLDAP                         June 1995

    5.1.  Server Implementations

       Given that the goal of this protocol is to minimise the elapsed time
       between making a Directory request and receiving the response, a
       server which uses DAP to access the directory should use techniques
       that assist in this.

       - -  A server should remain bound to the Directory during reasonably
            long idle periods or should remain bound permanently.

       - -  Cacheing of results is highly desirable but this must be
            tempered by the need to provide up-to-date results given the
            lack of a cache invalidation protocol in DAP (either implicit
            via timers or explicit) and the lack of a dontUseCopy service
            control in the protocol.

       Of course these issues are irrelevant if the CLDAP protocol is
       directly supported by a DSA.

    5.2.  Client Implementations

       For simple lookup applications, use of a retry algorithm with
       multiple servers similar to that commonly used in DNS stub resolver
       implementations is recommended.  The location of a CLDAP server or
       servers may be better specified using IP addresses (simple or
       broadcast) rather than names that must first be looked up in another
       directory such as DNS.

    6.  Security Considerations

       This protocol provides no facilities for authentication. It is
       expected that servers will bind to the Directory either anonymously
       or using simple authentication without a password.

    7.  Bibliography

       [1] The Directory: Overview of Concepts, Models and Service.  CCITT
           Recommendation X.500, 1988.

       [2] The Directory: Models.  CCITT Recommendation X.501 ISO/IEC JTC
           1/SC21; International Standard 9594-2, 1988.

       [3] The Directory: Abstract Service Definition.  CCITT Recommendation
           X.511, ISO/IEC JTC 1/SC21; International Standard 9594-3, 1988.

       [4] Yeong, W., Howes, T., and S. Kille, "X.500 Lightweight Directory
           Access Protocol", RFC 1487, Performance Systems International,
           University of Michigan, ISODE Consortium, July 1993.

    Young                       Standards Track                     [Page 8]

    RFC 1798                         CLDAP                         June 1995

       [5] Mockapetris, P., "Domain Names - Implementation and
           Specification", STD 13, RFC 1035, USC/Information Sciences
           Institute, November 1987.

       [6] Mockapetris, P., "Domain Names - Concepts and Facilities", STD
           13, RFC 1034, USC/Information Sciences Institute, November 1987.

    8.  Acknowledgements

       Many thanks to Tim Howes and Steve Kille for their detailed comments
       and to other members of the working group.

       This work was initiated by the Union Bank of Switzerland.

    9.  Author's Address

       Alan Young
       ISODE Consortium
       The Dome, The Square
       RICHMOND
       GB - TW9 1DT

       Phone: +44 81 332 9091
       EMail: A.Young@isode.com
       X.400:    i=A; s=Young; o=ISODE Consortium; p=ISODE; a=MAILNET; c=FI

    Young                       Standards Track                     [Page 9]