Incremental Change Synchronization

Incremental Change Synchronization

ICS provides a means for the client to replicate changes in a folder or hierarchy between two different points in time. This protocol is actively used by Outlook to synchronize changes between the Exchange server and the Outlook client. These changes include creating, modifying, and deleting folders and messages. The way these changes are synchronized is a game of hot potato between the client and the server. A state is kept, commonly referred to as the ICS state.

ICS State

The ICS state is a collection of 4 properties inside of an IDSET object.  The four properties contained inside the object are as follows:

  1. PidTagIdsetGiven
  2. PidTagCnsetSeen
  3. PidTagCnsetSeenFAI
  4. PidTagCnsetRead

This object of 4 properties, or ICS state, is transferred back and forth from client to server using a method that represents the IDSET data in a serialized stream of binary data. The specification document [MS-OXCFXICS] details how the IDSET object gets serialized into a binary representation.

The state itself is maintained by the client. In this case, the server does not retain the state. It relies on the client to upload its current state, and then the synchronization occurs from that point forward.

Obtain changes from the server

The client has to perform an ICS download to obtain the changes that are currently on the server.

The process begins as follows:

  1. The client creates a download context.

  2. The client uploads its state to the synchronization download context.
    RopSynchronizationUploadStateStreamContinue RopSynchronizationUploadStateStreamEnd

  3. The client gets the faster transfer stream (gets synchronization changes).

    This is a serialized representation of the changes in a folder or hierarchy as explained above.

  4. Release the synchronization object after downloading the entire stream.

You can also include all of this information in one RPC Call using EcDoRpcExt2 (defined in [MS-OXCRPC] ).

Push changes to the server

Once the client has changes that must be represented on the server, it then uploads its current ICS state, and continues to perform any of the relevant Import Rops against the server.

  1. Create synchronization upload context
  2. Upload ICS state to upload context
    RopSynchronizationUploadStateStreamContinue RopSynchronizationUploadStateStreamEnd
  3. Client will then issue any one of the following operations to the upload context

Bootstrap State (Empty State)

The bootstrap state is considered the initial state for a client that has never synchronized with the server. Since the client has never talked to the server, it does not have an ICS state. Therefore, it has nothing to pass to the server to indicate the enumeration progress of changes from the server. Thus, the client will upload something called an empty state. This empty state is zero-length byte arrays for the properties in question. This causes the server to send the current state to the client. This first synchronization is obviously the longest synchronization operation as it usually contains the most changes as the client does not have any data yet.

This is an overview of the ICS protocol; please stay tuned for further blogs regarding the Rop specific details involved, and for a posting about conflict detection.