Path of a packet through a kernel

The path of a packet through a kernel can be quite complex, involving multiple layers of processing and several different subsystems. In this answer, we will describe the basic steps that a packet might take as it travels through a typical kernel.

1. Receiving the Packet

   The first step in processing a packet is to receive it from the network interface. This typically involves a device driver that is specific to the particular interface being used. The driver reads the packet from the interface and copies it into a buffer in memory.

2. Protocol Decapsulation

   Once the packet has been received, the kernel must determine which protocol it is using. This is typically done by examining the protocol field in the packet header. The kernel then passes the packet to the appropriate protocol layer, which is responsible for decapsulating the packet and extracting any data that it contains.

3. Network Protocol Processing

   Once the packet has been decapsulated, the kernel must perform a series of processing steps that are specific to the network protocol being used. For example, if the packet is using the IP protocol, the kernel must perform IP fragmentation and reassembly if necessary, and it must also perform routing and forwarding based on the destination address in the IP header.

4. Transport Protocol Processing

   After the network protocol processing is complete, the kernel must pass the packet up to the transport layer. This layer is responsible for ensuring reliable delivery of the packet and providing services such as flow control and congestion avoidance. If the packet is using a connection-oriented transport protocol like TCP, the kernel must perform a three-way handshake to establish the connection.

5. Socket Layer Processing

   Once the transport protocol processing is complete, the packet is passed up to the socket layer. This layer is responsible for managing the sockets that applications use to communicate over the network. The socket layer checks to see if there is a socket that is bound to the destination address and port number in the packet header. If there is, the packet is passed up to the appropriate application.

6. Application Processing

   Finally, the application receives the packet and processes it. This typically involves interpreting the data that was sent in the packet and taking some action based on it. For example, if the packet contains an HTTP request, the application might parse the request and generate a response.

Overall, the path of a packet through a kernel can be quite complex, involving multiple layers of processing and several different subsystems. However, by following this basic sequence of steps, the kernel is able to reliably deliver packets to their intended destinations and provide a wide range of network services to applications.