A number of TCP/IP offload engines have been developed to reduce the CPU load of processing TCP/IP, but most of them are implemented in hardware. Although hardware- based TOEs have a high performance, they lack the flexibility to accept changes in the TCP/IP. To preserve flexibility, we implemented a software-based TOE, called HLTCP 100134 (High-performance Lightweight TCP/IP). The HL-TCP is a standalone TCP/ IP without an embedded OS. The TOE using the HL-TCP has features of a zero-copy sending mechanism and an efficient DMA mechanism for TCP retransmission. It also fully utilizes offload features in the Ethernet. Our experimental results show that the TOE using the HL-TCP can achieve a bandwidth of 453 Mbps with almost zero CPU utilization, compared with a general gigabit Ethernet, which has a CPU utilization of approximately 23%.