The Linux kernel network subsystem implements a sophisticated layered architecture that handles all network communication. At the top, user space applications communicate through system calls to the kernel's socket layer. The kernel then processes packets through multiple layers: the protocol layer handles TCP and UDP, the network layer manages IP routing, and the link layer interfaces with hardware drivers. This modular design allows for efficient packet processing and enables advanced features like filtering, quality of service, and network virtualization.
The network stack processes packets through distinct layers, each with specialized kernel modules. The socket layer manages socket buffers and protocol dispatch. The protocol layer handles TCP and UDP processing along with port management. The network layer performs IP routing and implements netfilter hooks for packet filtering. Finally, the link layer contains device drivers and queue discipline modules. As packets flow through these layers, each module transforms the data according to its specific protocol requirements.
The packet processing pipeline demonstrates the complete journey from hardware to application. When a network interface card receives a packet, it triggers an interrupt to signal the CPU. DMA operations transfer packet data to memory, where an sk_buff structure is allocated to manage the packet metadata. The packet then flows through protocol processing layers before being queued to the appropriate socket for application delivery. Modern kernels implement several optimizations including NAPI polling to reduce interrupt overhead, GRO and GSO for packet aggregation, multi-queue support for parallel processing, and CPU affinity to improve cache locality.
Advanced networking features extend the traditional kernel stack with high-performance capabilities. eBPF allows programmable packet processing within the kernel, enabling custom filtering and monitoring. XDP provides express data path processing at the driver level for ultra-low latency. DPDK bypasses the kernel entirely for maximum throughput in specialized applications. Container networking integrates with these technologies through virtual ethernet pairs and network namespaces. Best practices for network module development include profiling before optimization, choosing appropriate tools for each use case, continuous performance monitoring, and leveraging kernel tracing for debugging complex network issues.