A primary network element of a high availability cluster periodically synchronizes sequence numbers with a backup network element of the cluster. The primary determines the periodicity of updates according to a window size and a scale factor. The backup uses probe packets after switchover to discover the current sequence number. In one example, the primary network element includes an interface to receive first and second packets of a packet flow, a control unit to determine whether a difference between sequence numbers of the first and second packets exceeds an update window size value associated with the packet flow that is based on a window size value and a window scale factor associated with the packet flow, and a backup network interface configured to send an update message comprising the second sequence number to the backup network element of the high-availability cluster.