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Free/Busy Data in Exchange

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In today’s fast-paced market, availability of an individual is increasingly important. Thus presents the need to be able to schedule an appointment/meeting with a person even if they are not available. Exchange provides such a service by storing what is referred to as free/busy data. This information will indicate what requested time range is free, busy, and tentative for a particular user. It will also provide any out of office (oof) status as well.

Web Services Method

Free/Busy data can be obtained using the Availability service provided by the Exchange Server. This is simply a web service which obtains the free/busy data directly from a user’s calendar. Specifically, the GetUserAvailability operation will provide the free/busy state at specific level of detail.The response to this operation will vary. This will depend on the permissions granted to the user requesting the availability. If the user in question does have access to perform the search you will get a ResponseCode of “NoError” indicating success. When a user requests data through web services this data is actually obtained from the specified user’s calendar in question.

Due to the nature of the data in question being held in each individual’s calendar, modifying free/busy data is simply done by creating calendar items using the CreateItem operation or modifying the items using UpdateItem operation. Thus, any subsequent calls to GetUserAvailability will show the updated information back to the requesting party.

Public Folders-Based Method

In Exchange Server versions pre-2010, Exchange stored free/busy data in the public folders store. Inside of the NON_IPM_SUBTREE\SCHEDULE+ FREE BUSY folder in the public folders store there will be a message object for each user. The free/busy data in question is represented by a set of properties that are set on that user’s message object in this folder. The message object is referred to as the “free/busy message”. There MUST be only one free/busy message per user. The subject of the message (PidTagNormalizedSubject property) is ultimately what is used in uniquely identifying a user’s free/busy message. The PidTagNormalizedSubject property will be in the form of “USER-/CN=RECIPIENTS/CN=CRAZYIVAN”.

Finding free/busy information for a specific user in the public folders-based method requires a series of remote operations (rops) to traverse the public folders store, and ultimately using the RopFindRow request looking for the data in question in the PidTagNormalizedSubject property. This procedure is shown in more detail in Section 4.2 of [MS-OXOPFFB].

All users have access to the public folders store. Thus, a client can read and write free/busy data to another user’s free/busy message other than the user who has logged in. The client will issue a series of rops with the client ultimately issuing a RopSetProperties to transmit the availability data back to the server.

Please note the following: The public folders-based protocol is no longer a feature in Exchange Server 2010. This has been removed and the user must use Exchange Web Services.

Availability Web Service Protocol Specification References:
Information for the availability service is provided via our open specifications document: [MS-OXWAVLS].
Information for the security model used in the availability service, please reference the following document: [MS-OXCPERM].

Public Folder-Based Free/Busy Protocol Specification References:
Information for the public folder-based free/busy Protocol is provided by the specification document: [MS-OXOPFFB].