RFC2589 - Lightweight Directory Access Protocol (v3): Extensions for Dynamic Dir

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　　Network Working Group Y. Yaacovi
Request for Comments: 2589 Microsoft
Category: Standards Track M. Wahl
Innosoft International, Inc.
T. Genovese
Microsoft
May 1999

Lightweight Directory Access Protocol (v3):
Extensions for Dynamic Directory Services

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.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

1. Abstract

This document defines the requirements for dynamic directory services
and specifies the format of request and response extended operations
for supporting client-server interoperation in a dynamic directories
environment.

The Lightweight Directory Access Protocol (LDAP) [1] supports
lightweight access to static directory services, allowing relatively
fast search and update access. Static directory services store
information about people that persists in its accuracy and value over
a long period of time.

Dynamic directory services are different in that they store
information that only persists in its accuracy and value when it is
being periodically refreshed. This information is stored as dynamic
entries in the directory. A typical use will be a client or a person
that is either online - in which case it has an entry in the
directory, or is offline - in which case its entry disappears from
the directory. Though the protocol operations and attributes used by
dynamic directory services are similar to the ones used for static
directory services, clients that store dynamic information in the
directory need to periodically refresh this information, in order to
prevent it from disappearing. If dynamic entries are not refreshed

within a given timeout, they will be removed from the directory. For
example, this will happen if the client that set them goes offline.

A flow control mechanism from the server is also described that
allows a server to inform clients how often they should refresh their
presence.

2. Requirements

The protocol extensions must allow accessing dynamic information in a
directory in a standard LDAP manner, to allow clients to access
static and dynamic information in the same way.

By definition, dynamic entries are not persistent and clients may go
away gracefully or not. The proposed extensions must offer a way for
a server to tell if entries are still valid, and to do this in a way
that is scalable. There also must be a mechanism for clients to
reestablish their entry with the server.

There must be a way for clients to find out, in a standard LDAP
manner, if servers support the dynamic extensions.

Finally, to allow clients to broadly use the dynamic extensions, the
extensions need to be registered as standard LDAP extended
operations.

3. Description of Approach

The Lightweight Directory Access Protocol (LDAP) [1] permits
additional operation requests and responses to be added to the
protocol. This proposal takes advantage of these to support
directories which contain dynamic information in a manner which is
fully integrated with LDAP.

The approach described in this proposal defines dynamic entries in
order to allow implementing directories with dynamic information. An
implementation of dynamic directories, must be able to support
dynamic directory entries.

3.1. Dynamic Entries and the dynamicObject object class

A dynamic entry is an object in the directory tree which has a time-
to-live associated with it. This time-to-live is set when the entry
is created. The time-to-live is automatically decremented, and when
it expires the dynamic entry disappears. By invoking the refresh
extended operation (defined below) to re-set the time-to-live, a
client can cause the entry to remain present a while longer.

A dynamic entry is created by including the objectClass value given
in section 5 in the list of attributes when adding an entry. This
method is subject to standard access control restrictions.

The extended operation covered here, allows a client to refresh a
dynamic entry by invoking, at intervals, refresh operations
containing that entry's name. Dynamic entries will be treated the
same as non-dynamic entries when processing search, compare, add,
delete, modify and modifyDN operations. However if clients stop
sending refresh operations for an entry, then the server will
automatically and without notification remove that entry from the
directory. This removal will be treated the same as if the entry had
been deleted by an LDAP protocol operation.

There is no way to change a static entry into a dynamic one and
vice-versa. If the client is using Modify with an objectClass of
dynamicObject on a static entry, the server must return a service
error either "objectClassModsProhibited" (if the server does not
allow objectClass modifications at all) or "objectClassViolation" (if
the server does allow objectClass modifications in general).

A dynamic entry may be removed by the client using the delete
operation. This operation will be subject to access control
restrictions.

A non-dynamic entry cannot be added subordinate to a dynamic entry:
the server must return an appropriate update or service error if this
is attempted.

The support of dynamic attributes of an otherwise static object, are
outside the scope of this document.

3.2 Dynamic meetings (conferences)

The way dynamicObject is defined, it has a time-to-live associated
with it, and that's about it. Though the most common dynamic object
is a person object, there is no specific type associated with the
dynamicObject as defined here. By the use of the dynamic object's
attributes, one can make this object represent practically anything.

Specifically, Meetings (conferences) can be represented by dynamic
objects. While full-featured meeting support requires special
semantics and handling by the server (and is not in the scope of this
document), the extensions described here, provide basic meetings
support. A meeting object can be refreshed by the meeting
participants, and when it is not, the meeting entry disappears. The
one meeting type that is naturally supported by the dynamic
extensions is creator-owned meeting.

3.2.1 Creator-owned meetings

Creator-owned meetings are created by a client that sets the time-
to-live attribute for the entry, and it is this client's
responsibility to refresh the meeting entry, so that it will not
disappear. Others might join the meeting, by modifying the
appropriate attribute, but they are not allowed to refresh the entry.
When the client that created the entry goes away, it can delete the
meeting entry, or it might disappear when its time-to-live expires.
This is consistent with the common model for dynamicObject as
described above.

4. Protocol Additions

4.1 Refresh Request

Refresh is a protocol operation sent by a client to tell the server
that the client is still alive and the dynamic directory entry is
still accurate and valuable. The client sends a Refresh request
periodically based on the value of the client refresh period (CRP).
The server can request that the client change this value. As long as
the server receives a Refresh request within the timeout period, the
directory entry is guaranteed to persist on the server. Client
implementers should be aware that since the intervening network
between the client and server is unreliable, a Refresh request packet
may be delayed or lost while in transit. If this occurs, the entry
may disappear, and the client will need to detect this and re-add the
entry.

A client may request this operation by transmitting an LDAP PDU
containing an ExtendedRequest. An LDAP ExtendedRequest is defined as
follows:

ExtendedRequest ::= [APPLICATION 23] SEQUENCE {
requestName [0] LDAPOID,
requestValue [1] OCTET STRING OPTIONAL }

The requestName field must be set to the string
"1.3.6.1.4.1.1466.101.119.1".

The requestValue field will contain as a value the DER-encoding of
the following ASN.1 data type:

SEQUENCE {
entryName [0] LDAPDN,
requestTtl [1] INTEGER
}

The entryName field is the UTF-8 string representation of the name of
the dynamic entry [3]. This entry must already exist.

The requestTtl is a time in seconds (between 1 and 31557600) that the
client requests that the entry exists in the directory before being
automatically removed. Servers are not required to accept this value
and might return a different TTL value to the client. Clients must
be able to use this server-dictated value as their CRP.

4.2 Refresh Response

If a server implements this extension, then when the request is made
it will return an LDAP PDU containing an ExtendedResponse. An LDAP
ExtendedResponse is defined as follows:

ExtendedResponse ::= [APPLICATION 24] SEQUENCE {
COMPONENTS OF LDAPResult,
responseName [10] LDAPOID OPTIONAL,
response [11] OCTET STRING OPTIONAL }

The responseName field contains the same string as that present in
the request.

The response field will contain as a value the DER-encoding of the
following ASN.1 data type:

SEQUENCE {
responseTtl [1] INTEGER
}

The responseTtl field is the time in seconds which the server chooses
to have as the time-to-live field for that entry. It must not be any
smaller than that which the client requested, and it may be larger.
However, to allow servers to maintain a relatively accurate
directory, and to prevent clients from abusing the dynamic
extensions, servers are permitted to shorten a client-requested
time-to-live value, down to a minimum of 86400 seconds (one day).

If the operation was successful, the errorCode field in the
standardResponse part of an ExtendedResponse will be set to success.

In case of an error, the responseTtl field will have the value 0, and
the errorCode field will contain an appropriate value, as follows: If
the entry named by entryName could not be located, the errorCode
field will contain "noSuchObject". If the entry is not dynamic, the
errorCode field will contain "objectClassViolation". If the
requester does not have permission to refresh the entry, the

errorCode field will contain "insufficientAccessRights". If the
requestTtl field is too large, the errorCode field will contain
"sizeLimitExceeded".

If a server does not implement this extension, it will return an LDAP
PDU containing an ExtendedResponse, which contains only the
standardResponse element (the responseName and response elements will
be absent). The LDAPResult element will indicate the protocolError
result code.

This request is permitted to be invoked when LDAP is carried by a
connectionless transport.

When using a connection-oriented transport, there is no requirement
that this operation be on the same particular connection as any
other. A client may open multiple connections, or close and then
reopen a connection.

4.3 X.500/DAP Modify(97)

X.500/DAP servers can map the Refresh request and response operations
into the X.500/DAP Modify(97) operation.

5. Schema Additions

All dynamic entries must have the dynamicObject value in their
objectClass attribute. This object class is defined as follows
(using the ObjectClassDescription notation of [2]):

( 1.3.6.1.4.1.1466.101.119.2 NAME 'dynamicObject'
DESC 'This class, if present in an entry, indicates that this entry
has a limited lifetime and may disappear automatically when
its time-to-live has reached 0. There are no mandatory
attributes of this class, however if the client has not
supplied a value for the entryTtl attribute, the server will
provide one.'
SUP top AUXILIARY )

Furthermore, the dynamic entry must have the following operational
attribute. It is described using the AttributeTypeDescription
notation of [2]:

( 1.3.6.1.4.1.1466.101.119.3 NAME 'entryTtl'
DESC 'This operational attribute is maintained by the server and
appears to be present in every dynamic entry. The attribute
is not present when the entry does not contain the
dynamicObject object class. The value of this attribute is
the time in seconds that the entry will continue to exist

before disappearing from the directory. In the absence of
intervening refresh operations, the values returned by
reading the attribute in two successive searches are
guaranteed to be nonincreasing. The smallest permissible
value is 0, indicating that the entry may disappear without
warning. The attribute is marked NO-USER-MODIFICATION since
it may only be changed using the refresh operation.'
SYNTAX 1.3.6.1.4.1.1466.115.121.1.27 SINGLE-VALUE
NO-USER-MODIFICATION USAGE dSAOperation )

To allow servers to support dynamic entries in only a part of the
DIT, the following operational attribute is defined. It is
described using the AttributeTypeDescription notation of [2]:

( 1.3.6.1.4.1.1466.101.119.4 NAME 'dynamicSubtrees'
DESC 'This operational attribute is maintained by the server and is
present in the Root DSE, if the server supports the dynamic
extensions described in this memo. The attribute contains a
list of all the subtrees in this directory for which the
server supports the dynamic extensions.'
SYNTAX 1.3.6.1.4.1.1466.115.121.1.12 NO-USER-MODIFICATION
USAGE dSAOperation )

6. Client and Server Requirements

6.1 Client Requirements

Clients can find out if a server supports the dynamic extensions by
checking the supportedExtension field in the root DSE, to see if the
OBJECT IDENTIFIER described in section 4 is present. Since servers
may select to support the dynamic extensions in only some of the
subtrees of the DIT, clients must check the dynamicSubtrees
operational attribute in the root DSE to find out if the dynamic
extensions are supported on a specific subtree.

Once a dynamic entry has been created, clients are responsible for
invoking the refresh extended operation, in order to keep that entry
present in the directory.

Clients must not expect that a dynamic entry will be present in the
DIT after it has timed out, however it must not require that the
server remove the entry immediately (some servers may only process
timing out entries at intervals). If the client wishes to ensure the
entry does not exist it should issue a RemoveRequest for that entry.

Initially, a client needs to know how often it should send refresh
requests to the server. This value is defined as the CRP (Client
Refresh Period) and is set by the server based on the entryTtl.

Since the LDAP AddRequest operation is left unchanged and is not
modified in this proposal to return this value, a client must issue a
Refresh extended operation immediately after an Add that created a
dynamic entry. The Refresh Response will return the CRP (in
responseTtl) to the client.

Clients must not issue the refresh request for dynamic entries which
they have not created. If an anonymous client attempts to do so, a
server is permitted to return insufficientAccessRights (50) in the
RefreshResponse, enforcing the client to bind first. Please note that
servers which allow anonymous clients to create and refresh dynamic
entries will not be able to enforce the above.

Clients should always be ready to handle the case in which their
entry timed out. In such a case, the Refresh operation will fail
with an error code such as noSuchObject, and the client is expected
to re-create its entry.

Clients should be prepared to experience refresh operations failing
with protocolError, even though the add and any previous refresh
requests succeeded. This might happen if a proxy between the client
and the server goes down, and another proxy is used which does not
support the Refresh extended operation.

6.2 Server Requirements

Servers are responsible for removing dynamic entries when they time
out. Servers are not required to do this immediately.

Servers must enforce the structural rules listed in above section 3.

Servers must ensure that the operational attribute described in
section 5 is present in dynamic entries

Servers may permit anonymous users to refresh entries. However, to
eliminate the possibility of a malicious use of the Refresh
operation, servers may require the refreshing client to bind first. A
server implementation can achieve this by presenting ACLs on the
entryTtl attribute, and returning insufficientAccessRights (50) in
the RefreshResponse, if the client is not allowed to refresh the
entry. Doing this, though, might have performance implications on the
server and might impact the server's scalability.

Servers may require that a client which attempts to create a dynamic
entry have a remove permission for that entry.

Servers which implement the dynamic extensions must have the OBJECT
IDENTIFIER, described above in section 4 for the request and

response, present as a value of the supportedExtension field in the
root DSE. They must also have as values in the attributeTypes and
objectClasses attributes of their subschema subentries, the
AttributeTypeDescription and ObjectClassDescription from section 5.

Servers can limit the support of the dynamic extensions to only some
of the subtrees in the DIT. Servers indicate for which subtrees they
support the extensions, by specifying the OIDs for the supported
subtrees in the dynamicSubtrees attribute described in section 5. If
a server supports the dynamic extensions for all naming contexts it
holds, the dynamicSubtrees attribute may be absent.

7. Implementation issues

7.1 Storage of dynamic information

Dynamic information is expected to change very often. In addition,
Refresh requests are expected to arrive at the server very often.
Disk-based databases that static directory services often use are
likely inappropriate for storing dynamic information. We recommend
that server implementations store dynamic entries in memory
sufficient to avoid paging. This is not a requirement.

We expect LDAP servers to be able to store static and dynamic
entries. If an LDAP server does not support dynamic entries, it
should respond with an error code such as objectClassViolation.

7.2 Client refresh behavior

In some cases, the client might not get a Refresh response. This may
happen as a result of a server crash after receiving the Refresh
request, the TCP/IP socket timing out in the connection case, or the
UDP packet getting lost in the connection-less case.

It is recommended that in such a case, the client will retry the
Refresh operation immediately, and if this Refresh request does not
get a response as well, it will resort to its original Refresh cycle,
i.e. send a Refresh request at its Client Refresh Period (CRP).

7.3 Configuration of refresh times

We recommend that servers will provide administrators with the
ability to configure the default client refresh period (CRP), and
also a minimum and maximum CRP values. This, together with allowing
administrators to request that the server will not change the CRP
dynamically, will allow administrators to set CRP values which will
enforce a low refresh traffic, or - on the other extreme, an highly
up-to-date directory.

8. Replication

Replication is only partially addressed in this memo. There is a
separate effort in progress at the IETF on replication of static and
dynamic directories.

it is allowed to replicate a dynamic entry or a static entry with
dynamic attributes. Since the entryTtl is expressed as a relative
time (how many seconds till the entry will expire), replicating it
means that the replicated entry will be "off" by the replication
time.

9. Localization

The are no localization issues for this extended operation.

10. Security Considerations

Standard LDAP security rules and support apply for the extensions
described in this document, and there are no special security issues
for these extensions. Please note, though, that servers may permit
anonymous clients to refresh entries which they did not create.
Servers are also permitted to control a refresh access to an entry by
requiring clients to bind before issuing a RefreshRequest. This will
have implications on the server performance and scalability.

Also, Care should be taken in making use of information obtained from
directory servers that has been supplied by client, as it may now be
out of date. In many networks, for example, IP addresses are
automatically assigned when a host connects to the network, and may
be reassigned if that host later disconnects. An IP address obtained
from the directory may no longer be assigned to the host that placed
the address in the directory. This issue is not specific to LDAP or
dynamic directories.

11. Acknowledgments

Design ideas included in this document are based on those discussed
in ASID and other IETF Working Groups.

12. Authors' Addresses

Yoram Yaacovi
Microsoft
One Microsoft way
Redmond, WA 98052
USA

Phone: +1 206-936-9629
EMail: yoramy@microsoft.com

Mark Wahl
Innosoft International, Inc.
8911 Capital of Texas Hwy #4140
Austin, TX 78759
USA

Email: M.Wahl@innosoft.com

Tony Genovese
Microsoft
One Microsoft way
Redmond, WA 98052
USA

Phone: +1 206-703-0852
EMail: tonyg@microsoft.com

13. Bibliography

[1] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory Access
Protocol (Version 3)", RFC2251, December 1997.

[2] Wahl, M. Coulbeck, A., Howes, T. and S. Kille, "Lightweight
Directory Access Protocol (v3): Attribute Syntax Definitions",
RFC2252, December 1997.

[3] Wahl, M. and S. Kille, "Lightweight Directory Access Protocol
(v3): UTF-8 String Representation of Distinguished Names", RFC
2253, December 1997.

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