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IETF ASID Working Group                                       Tim Howes
INTERNET-DRAFT                            Netscape Communications Corp.
                                                              Mark Wahl
                                                   Critical Angle, Inc.
                                                               May 1997


         Referrals and Knowledge References in LDAP Directories
                <draft-ietf-asid-ldapv3-referral-00.txt>



1.  Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are working docu-
ments of the Internet Engineering Task Force (IETF), its areas, and its
working groups.  Note that other groups may also distribute working
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as ``work in progress.''

To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow
Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe),
ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

Distribution of this memo is unlimited. Editorial comments should be
sent to the author (howes@netscape.com). Technical discussion should
take place on the IETF ASID mailing list (ietf-asid@umich.edu).

2.  Abstract

This document defines a "ref" attribute and associated "referral" object
class for representing generic knowledge information in LDAP directories
[LDAP].  The attribute uses URIs [RFC1738] to represent knowledge, ena-
bling LDAP and non-LDAP services alike to be referenced.  The object
class can be used to construct entries in an LDAP directory containing
references to other directories or services. This document also defines
procedures directory servers should follow when supporting these schema
elements.

3.  Background and intended usage

The broadening of interest in LDAP directories beyond their use as front
ends to X.500 directories has created a need to represent knowledge



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information in a more general way. Knowledge information is information
about one or more servers maintained in another server, used to link
servers and services together.

This document defines a general method of representing knowledge infor-
mation in LDAP directories, based on URIs.

The key words "MUST", "SHOULD", and "MAY" used in this document are to
be interpreted as described in [BRADNER97].

4.  The ref attribute type

This section defines the ref attribute type for holding general
knowledge reference information.

( 2.16.840.1.113730.3.1.34 NAME 'ref' DESC 'URL reference'
  EQUALITY caseExactIA5Match SYNTAX 'IA5String' USAGE dSAOperation )

The ref attribute type has caseExactString syntax.  The ref attribute is
multivalued. Values placed in the attribute MUST conform to the specifi-
cation given for the labeledURI attribute defined in [RFC2079].  The
labeledURI specification defines a format that is a URI, optionally fol-
lowed by whitespace and a label. This document does not make use of the
label portion of the syntax. Future documents MAY enable new functional-
ity by imposing additional structure on the label portion of the syntax
as it appears in the ref attribute.

5.  Use of the ref attribute

Three uses for the ref attribute are defined in this document.  Other
uses of the ref attribute MAY be defined in subsequent documents, or by
bilateral agreement between cooperating clients and servers.

Except when the manageDsaIT control (documented in section 7 of this
document) is present in the operation request, the ref attribute is not
visible to clients, except as its value is returned in referrals or con-
tinuation references.

If the manageDsaIT control is not set, and the entry named in a request
contains the ref attribute, and the entry is not the root DSE, the
server returns an LDAPResult with the resultCode field set to "referral"
and the referral field set to contain the value(s) of the ref attribute.

If the manageDsaIT control is not set, and an entry containing the ref
attribute is otherwise in the scope of a one level or subtree search
request, the server returns a SearchResultReference for each such entry
containing the value(s) of the entry's ref attribute.




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When the manageDsaIT control is present in a request, the server will
treat an entry containing the ref attribute as an ordinary entry, and
the ref attribute as an ordinary attribute, and the server will not
return referrals or continuation refernences corresponding to ref attri-
butes.

The following sections define three uses for the ref attribute.

5.1.  Named reference

This use of the ref attribute is similar to the subordinate reference
concept found in X.500 [X500]. It is used to facilitate distributed name
resolution or search across multiple servers. The ref attribute appears
in an entry named in the referencing server. The value of the ref attri-
bute points to the corresponding entry maintained in the referenced
server.

While the distinguished name in a value of the ref attribute is typi-
cally that of an entry in a naming context below the naming context held
by the referencing server, it is permitted to be the distinguished name
of any entry.  If the ref attribute is multi-valued all the DNs in the
values of the ref attribute SHOULD have the same value.  It is the
responsibility of clients to not loop repeatedly if a naming loop is
present in the directory.  Administrators SHOULD avoid configuring nam-
ing loops using referrals.

Clients SHOULD perform at least simple "depth-of-referral count" loop
detection by incrementing a counter each time a new set of referrals is
received. Clients MAY perform more sophisticated loop detection, for
example not chasing the same URI twice.

If an entry containing the ref attribute is immediately subordinate to
the base object named in a one level search request, then the referring
server MUST include a scope of "base" in any LDAP URIs returned in the
corresponding SearchResultReference.

5.1.1.  Examples

A multi-valued ref attribute MAY be used to indicate different locations
for the same resource. An example configuration illustrating the use of
the ref attribute in this capacity is provided below.

       |------------------------------------------------------------|
       |                    Server A                                |
       | dn: o=abc,c=us                dn: o=xyz,c=us               |
       | ref: ldap://hostB/o=abc,c=us  ref: ldap://hostD/o=xyz,c=us |
       | ref: ldap://hostC/o=abc,c=us  objectclass: referral        |
       | objectclass: referral                                      |



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       |____________________________________________________________|

  |---------------------| |---------------------| |---------------------|
  |       Server B      | |       Server D      | |      Server C       |
  | dn: o=abc,c=us      | | dn: o=xyz,c=us      | | dn: o=abc,c=us      |
  | o: abc              | | o: xyz              | | o: abc              |
  | other attributes... | | other attributes... | | other attributes... |
  |_____________________| |_____________________| |_____________________|

In this example, Server A holds references for two entries:
"o=abc,c=us" and o=xyz,c=us". For the "o=abc,c=us" entry, Server A holds
two references, one to Server B and one to Server C.  The entries refer-
enced are replicas of each other. For the "o=xyz,c=us" entry, Server A
holds a single reference to the entry contained in Server D.

In the following protocol interaction examples, the client has contacted
Server A.  Server A holds the naming context "c=us".

5.1.1.1.  Subtree search from a superior naming context

If a client requests a subtree search of "c=us", then in addition to any
entries in the "c=us" naming context which match the filter, Server A
will also return two continuation references. One of the continuation
references will be for "o=abc,c=us", and the other continuation refer-
ence will be for "o=xyz,c=us".

The order in which the continuation references are returned, and the
order of LDAP URI values in each continuation reference, are not stand-
ardized.  One possible response might be:

        ... SearchResultEntry responses ...

        SearchResultReference {
         ldap://hostB/o=abc,c=us
         ldap://hostC/o=abc,c=us
        }

        SearchResultReference {
         ldap://hostD/o=xyz,c=us
        }

        SearchResultDone "success"

5.1.1.2.  One level search from an immediately superior object

If the client requests a one level search of "c=us", then in addition to
any entries in the "c=us" naming context which match the filter, Server
A will also return two continuation references, as in the previous



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example.  One possible response might be:

        ... SearchResultEntry responses ...

        SearchResultReference {
         ldap://hostB/o=abc,c=us??base
         ldap://hostC/o=abc,c=us??base
        }

        SearchResultReference {
         ldap://hostD/o=xyz,c=us??base
        }

        SearchResultDone "success"

Note the inclusion of the "base" scope in the returned URL continuation
references. This is required to maintain the one-level search semantics.

5.1.1.3.  Other operations

If the client requests an operation in which the base or target entry
has a ref attribute, then the server returns an LDAPResult with the
resultCode field set to referral and the referral field set to the
value(s) of the ref attribute. If the operation is a search, the refer-
ring server does not return any SearchResultEntry or SearchResultRefer-
ence before the SearchResultDone.

For example, if the client had issued a subtree search of "o=abc,c=us",
the server would return

        SearchResultDone "referral" {
         ldap://hostB/o=abc,c=us
         ldap://hostC/o=abc,c=us
        }

Similarly, if the client had issued a modify of "o=xyz,c=us", the server
would return

        ModifyResponse "referral" {
         ldap://hostD/o=xyz,c=us
        }


5.2.  Superior Reference

This use of the ref attribute is similar to the superior reference con-
cept found in X.500 [X500].  An LDAP server's root DSE MAY contain the
"ref" attribute.  The values of the ref attribute in the root DSE that



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are LDAP URIs SHOULD NOT contain any dn part, just the host name and
optional port number.

When the server receives an operation for which the base or target entry
of the request is not contained in or subordinate to any naming context
held by the server, the server will return an LDAPResult with the
resultCode set to "referral", and with the referral field filled in
using the values from the "ref" attribute from the root DSE.

5.3.  Unnamed reference

This use of the ref attribute is similar to the nonspecific subordinate
reference concept found in X.500 [X500]. It goes beyond this concept to
facilitate distributed searching or indexing across multiple servers.
The ref attribute is used to name an entry in the referencing server.
The reference entry may contain other attributes used to select the
reference during searching.

A multi-valued ref attribute MAY indicate the locations of different
resources all associated with the same LDAP entity. The following exam-
ple illustrates the use of the ref attribute to indicate two unnamed
references.

   |------------------------------------------------------------------|
   |                       Server A                                   |
   | dn: ref=ldap://hostB/o=abc,c=us  dn: ref=ldap://hostC/o=xyz,c=us |
   | cn: babs                         cn: babs                        |
   | cn: gern                         cn: bob                         |
   | cn: bob                                                          |
   |__________________________________________________________________|

  |-----------------------------| |-----------------------------|
  |          Server B           | |          Server C           |
  | dn: o=abc,c=us              | | dn: o=xyz,c=us              |
  | o: abc                      | | o: xyz                      |
  | other attributes...         | | other attributes...         |
  |                             | |                             |
  | dn: cn=babs,o=abc,c=us      | | dn: cn=babs,o=xyz,c=us      |
  | cn: babs                    | | o: xyz                      |
  | other attributes...         | | other attributes...         |
  |                             | |                             |
  | dn: cn=gern,o=abc,c=us      | | dn: cn=bob,o=xyz,c=us       |
  | cn: gern                    | | cn: bob                     |
  | other attributes...         | | other attributes...         |
  |                             | |_____________________________|
  | dn: cn=bob,o=abc,c=us       |
  | cn: bob                     |
  | other attributes...         |



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  |_____________________________|

In this example Server A contains two unnamed references to servers B
and C. The unnamed reference entries have additional cn attribute values
which may be used during a search operation to select the reference for
return to a client.

6.  The referral object class

The referral object class is defined as follows.

( 2.16.840.1.113730.3.2.6 NAME 'referral' SUP top STRUCTURAL
  MAY ( referral $ * ) )

The referral object class is a subclass of top and may contain the
referral attribute. It is a structural object class. The referral object
class may also contain any other attribute, as indicated by the "*" in
the MAY portion of the definition. This is required to support the nam-
ing attributes used in the entry's distinguished name.

Servers MAY support the ref attribute through use of the referral object
class. Servers MAY also support the ref attribute as an operational
attribute in any entry, or through use of other object classes.

7.  The manageDsaIT control

A client MAY specify the following control when issuing a search, com-
pare, add, delete, modify, or modifyDN request.

The control type is 2.16.840.1.113730.3.4.2.  The control SHOULD be
marked as critical.  There is no value; the controlValue field is
absent.

This control causes entries with the "ref" attribute to be treated as
normal entries, allowing clients to read and modify these entries.

This control is not needed if the entry containing the referral attri-
bute is one used for directory administrative purposes, such as the root
DSE, or the server change log entries.  Operations on these entries
never cause referrals or continuation references to be returned.

8.  Security Considerations

This document defines mechanisms that can be used to "glue" LDAP (and
other) servers together. The information used to specify this glue
information should be protected from unauthorized modification.  If the
server topology information itself is not public information, the infor-
mation should be protected from unauthorized access as well.



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9.  References

[RFC1738]
    Berners-Lee, T., Masinter, L., and McCahill, M., "Uniform Resource
    Locators (URL)", RFC 1738, CERN, Xerox Corporation, University of
    Minnesota, December 1994,
    <URL:ftp://ds.internic.net/rfc/rfc1738.txt>

[LDAP]
    M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access Protocol
    (v3)", Internet Draft draft-ietf-asid-ldapv3-protocol-04.txt, March,
    1997.

[BRADNER97]
    S. Bradner, "Key Words for use in RFCs to Indicate Requirement Lev-
    els", Internet Draft, draft-bradner-key-words-03.txt, January 1997.

[X500]
    ITU-T Rec. X.501, "The Directory: Models", 1993.

[RFC2079]
    M. Smith, "Definition of an X.500 Attribute Type and an Object Class
    to Hold Uniform Resource Identifiers (URIs)", RFC 2079, January
    1997.

10.  Author's Address

Tim Howes
Netscape Communications Corp.
501 E. Middlefield Rd.
Mountain View, CA 94043
USA
EMail:  howes@netscape.com

Mark Wahl
Critical Angle Inc.
4815 W Braker Lane #502-385
Austin, TX 78759
USA
EMail:  M.Wahl@critical-angle.com











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