Attributes for Exchange 5.5 Mailboxes : USN-Source
This is the global Update Sequence Number (USN) at the creation time of the object.
USN-Source
| LDAP name | USN-Source |
| Data type | Integer |
| Multivalue (Array) | No |
| Heuristic | 12 => Replicated between site, operational attribute |
| Access category | System access only |
| Ex55 Database name | USN-Source |
If the creation of the object was initiated on another exchange server first, then its USN of the creation time is stored in this attribute.
Update Sequence Nunbers are used to handle the incremental directory replication between the Exchange servers. The USN is an integer value on each Exchange server, which is incremented every time an object is changed in the directory of this server. This technique gives you an numeric counter which specifies the creation and change time of objects. This helps to determine which objects are to be replkicated to other servers in the next replication cycle.
There are some other replication attributes which are in relation to USN-Changed: USN-Changed, USN-Created and Object-Version. All these attributes are not shown in the regular property tabs in the Exchange admin tool.
The USN-Source attribute is an operational attribute which is not automatically retrieved by a GetObject() or an GetInfo() call. You can load such attributes into the object property cache by explicitly calling GetInfoEx(). Read more about this technique in the topic 'Reading object attributes' here in the SelfADSI tutorial.