A transmission control protocol (TCP) session processing architecture for conducting numerous TCP sessions during testing of a network-under-test: multiple processor cores running, allocated to TCP session handling, and program instructions configured to distribute processing of each TCP session across multiple cores with a first set of cores allocated to handle TCP control, a second set of cores allocated to handle TCP packet transmission, and a third set of cores allocated to handle TCP packet receiving. The TCP session processing architecture also includes a shared memory accessible to the first, second and third sets of cores, that holds PCBs for each of numerous TCP sessions during the testing with update access controlled by an atomic spinlock processor instruction that each TCP state machine running on a core must engage to secure the update access to a respective PCB, in order to proceed with state processing of its respective TCP session.