Sockets and the Network Stack

Between the application's write and the wire sit kernel queues and protocol processing. We follow a send: the syscall copies bytes into the socket's send buffer, TCP segments them, the device driver queues them, and the NIC transmits — then trace the receive path in reverse, through the receive queue to recvfrom.

Topics covered:

• The socket: the file descriptor and its send and receive buffers
• Syscall boundaries: where the application copy ends and the kernel begins
• Segmentation and checksum offload to the NIC
• The receive path: interrupt, softirq, and the socket's receive queue
• Blocking vs. non-blocking: what EAGAIN and poll and epoll observe
• Why loopback traffic never touches a NIC at all