A packet generator includes a checksum calculator configured to distinguish, in a communication packet belonging to a sequence of packets, between (i) one or more constant values of a header of the packet, the one or more constant values remaining unchanged across the packets in the sequence, (ii) a payload of the packet, and (iii) one or more variable values of the header, the one or more variable values changing among the packets in the sequence, to determine a constant-values partial checksum calculated over the constant values of the header, to calculate a payload partial checksum over the payload, to calculate a final checksum value for the packet based on (i) the constant-values partial checksum, (ii) the payload partial checksum and (iii) the variable values of the header, and to insert the final checksum value into the packet. An egress interface transmits the packet over a network.