TCP isn't a process, but a stack or subsystem inside the kernel. It doesn't have a "process id", as such, although kernel threads doing work as part of its operation may do. The work needed to perform the work of the TCP stack is spread across multiple kthreads, kernel stacks of userspace processes, and elsewhere.
To see kworkers on your machine, for example:
% pgrep -af kworker | head
Some of these are dedicated to particular kinds of work, and some are more generally scoped. Since the TCP stack involves not only the network, but also the CPU scheduler, memory, and other resources, many different parts of the kernel may eventually be abstractly involved in making forward progress in processing TCP packets.