A data packet is conveyed between servers connected to a packet network. A first server securely distributes a list of distinct numbers to one or more authorized receiving servers. Subsequently, upon receiving a packet to be transferred, the first server selects an unused number from the number list and writes the number into the packet before routing the packet to one or more of the authorized receiving servers. Upon receipt of the packet, an authorized receiving server checks that the number included in the packet is valid in that it is both contained in the latest number list and has not already been used in another packet. If valid, the receiving server determines a sequence number representative of the position of the number in the latest number list and sends an acknowledgement message to the originating server, including the determined sequence number. The originating server checks the sequence number to verify the authenticity of the acknowledgement message, re-sending the packet if invalidly acknowledged.