In the method, a source base station sends first information to a target base station. The first information indicates a sequence number of a first data unit in a sequence of data units undelivered to the mobile station by the source base station. The source base station also sends the target base station second information. The second information indicates data units, after the first undelivered data unit in the sequence, that have been delivered to the mobile station by the source base station. The second information may further indicate portions of the first undelivered data unit that have been delivered to the mobile station by the source base station.