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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.


 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

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:
   |  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

|#_|_Client_LDAP_LDAP_serverDAP_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    ->               |
 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

2. Protocol Model

 CLDAP is based directly on LDAP [4] and inherits many of the key
 aspects of the LDAP protocol:
  1. - Many protocol data elements are encoding as ordinary strings

(e.g., Distinguished Names).

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


 It is different to LDAP in that:
  1. - 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).
  1. - 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
 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
                      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

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.
 | 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.
  1. - A server should remain bound to the Directory during reasonably

long idle periods or should remain bound permanently.

  1. - 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
 GB - TW9 1DT
 Phone: +44 81 332 9091
 X.400:    i=A; s=Young; o=ISODE Consortium; p=ISODE; a=MAILNET; c=FI

Young Standards Track [Page 9]

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