The reassembly timer in a TCP/IP receiver is dynamically set to avoid data corruption on fast networks caused by IP identification wrap-around. A sender generates IP packets with a special option specifying a reassembly time for fragments based on the subnet of the destination or on the calculated round-trip time of the connection. A receiver sets the reassembly timer to the reassembly time specified in the option. A sender can generate IP packets with an alternative option setting a time stamp for the generated packet. A receiver calculates a traversal time based on the difference between the arrival time of the fragment and this time stamp, and sets the reassembly timer based on this traversal time.