A packet modification system comprising: a Packet BUS, PBUS, ingress logic to receive packets at the packet modification system; a Transmit In Data First In First Out, FIFO, coupled with the PBUS ingress logic to store the packets received at the PBUS ingress logic 404; wherein the Transmit In Data FIFO stores the packets such that portions of the packets are passed in parallel to a modification engine via a plurality of FIFOs, with the outputs of the FIFOs coupled in parallel to the modification engine, the packets being copied into each of the plurality of FIFOs providing the modification engine with sliced portions of the packets in parallel; a modification engine to modify one or more sliced portions of the packets received by the packet modification system by selectively modifying one or more Quality of Service, QoS, fields corresponding to each of the packets as represented within the one or more sliced portions; wherein the modification engine selectively modifying the one or more QoS fields comprises applying an egress mark and/or an egress mask to the one or more sliced portions of the packets and computing a checksum based on the modification to the one or more QoS fields and modifying the checksum of the packet based on the computed checksum; and an assembler for assembling the one or more sliced portions modified with one or more unmodified sliced portions of the packets stored in the Transmit In Data FIFO to form assembled packets on an egress portion of the packet modification system; and wherein the egress portion of the packet modification system is to output the packets as modified by the modification engine. a Packet BUS, PBUS, ingress logic to receive packets at the packet modification system; a Transmit In Data First In First Out, FIFO, coupled with the PBUS ingress logic to store the packets received at the PBUS ingress logic 404; wherein the Transmit In Data FIFO stores the packets such that portions of the packets are passed in parallel to a modification engine via a plurality of FIFOs, with the outputs of the FIFOs coupled in parallel to the modification engine, the packets being copied into each of the plurality of FIFOs providing the modification engine with sliced portions of the packets in parallel; a modification engine to modify one or more sliced portions of the packets received by the packet modification system by selectively modifying one or more Quality of Service, QoS, fields corresponding to each of the packets as represented within the one or more sliced portions; wherein the modification engine selectively modifying the one or more QoS fields comprises applying an egress mark and/or an egress mask to the one or more sliced portions of the packets and computing a checksum based on the modification to the one or more QoS fields and modifying the checksum of the packet based on the computed checksum; and an assembler for assembling the one or more sliced portions modified with one or more unmodified sliced portions of the packets stored in the Transmit In Data FIFO to form assembled packets on an egress portion of the packet modification system; and wherein the egress portion of the packet modification system is to output the packets as modified by the modification engine.