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We present results from a detailed energy measurement study of TCP. We focus on the node-level cost of the TCP protocol and obtain a breakdown of the energy cost of different TCP functions. We analyze the energy consumption of TCP on two platforms (laptop and iPAQ) and three operating systems (FreeBSD 4.2, 5 and Linux 2.4.7). Our results show that 60-70% of the energy cost (for transmission or reception) is accounted for by the kernel NIC (network interface card) copy operation. Of the remainder, /spl sim/15% is accounted for in the copy operation from user space to kernel space with the remaining 15% being accounted for by TCP processing costs. We then further analyze the 15% TCP processing cost and show that the cost of computing checksums accounts for 20-30% of TCP processing cost. Finally, we determine the processing costs of two primary TCP functions - timeouts and triple duplicate ACKs. Pulling all these costs together, we present techniques whereby energy savings of between 20%-30% in the computational cost of TCP can be achieved.