Briefly describe the process of sending and receiving network data packets in Linux system

Linux server receives network data packets, what processing needs to be done, and how does it pass the data to the application process step by step? When an application process sends a data packet, how does Linux send the data packet out? Today we will talk about this topic.

Before it is ready to receive network data packets, Linux needs to do a lot of preparation work, such as: initialization of the network subsystem, registration of the protocol stack, initialization of the network card driver, starting the network card, etc. Only after these are ready, Only then can you actually start receiving network packets.

Network Protocol Stack

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Before introducing Linux to send and receive network data packets, let us first understand the Linux network protocol stack.

The International Organization for Standardization has formulated the Open System Interconnection Reference Model, which is the OSI network model. The model mainly has 7 layers, namely application layer, presentation layer, session layer, transport layer, Network layer, data link layer and physical layer.

Because the OSI model is too complex, it is only a conceptual and theoretical model with too many layers, which increases the complexity of network work, so it has not been applied on a large scale.
The most common one we use is the TCP/IP network model. The Linux system implements the network protocol stack according to this network model.

The TCP/IP network model has 4 layers, namely application layer, transport layer, network layer and network interface layer. The functions of each layer are as follows:

1, Application layer Corresponds to the upper layer of the OSI reference model, providing users with various services required, such as: FTP, Telnet, DNS, SMTP, etc.

2, Transport layer Corresponds to the transport layer of the OSI reference model, which provides end-to-end communication functions for application layer entities, ensuring the sequential transmission of data packets and the integrity of data. This layer defines two main protocols: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

3, Network layer Corresponds to the network layer of the OSI reference model and mainly solves host-to-host communication problems. It contains protocols designed for the logical transmission of data packets throughout the network. It focuses on reassigning an IP address to the host to complete the addressing of the host. It is also responsible for routing data packets in various networks. There are three main protocols in this layer: Internet Protocol (IP), Internet Group Management Protocol (IGMP), and Internet Control Message Protocol (ICMP).

4, Network interface layer Corresponds to the physical layer and data link layer in the OSI reference model. It is responsible for monitoring the exchange of data between the host and the network. In fact, TCP/IP itself does not define the protocol of this layer. Instead, each network participating in the interconnection uses its own physical layer and data link layer protocols, and then connects to the network access layer of TCP/IP. Address Resolution Protocol (ARP) works at this layer, the data link layer of the OSI reference model.

Receive network data packet

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After the network data packet reaches the network card, it is stored in the receiving queue of the network card in FIFO order. The network card writes the network packet to the specified memory address (Ring Buffer) through DMA technology.

Ring Buffer is created and initialized when the network card driver starts, and stores the descriptor of the sk_buff buffer (physical address and size, etc.).

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When the network packet arrives, get the pointed sk_buff descriptor from the Ring Buffer and write the data to the address through DMA. After the data in sk_buff is handed over to the upper protocol stack for processing, the description in the Ring Buffer is updated to the newly allocated sk_buff.

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Then the network card initiates a hardware interrupt to the CPU. When the CPU receives the hardware interrupt request, it finds the registered interrupt processing function according to the interrupt registry.

The hardware interrupt handling function will do the following things:

1. Shield the interruption of the network card

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The purpose is to prevent the CPU from being frequently interrupted and unable to handle other tasks. Masking interrupts tells the network card that it already knows that there is data in the memory. Next time it receives a data packet, it can just write to the memory directly without notifying the CPU.

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2. Initiate a soft interrupt and restore the interrupt just blocked

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After the ksoftirqd thread in the kernel receives the soft interrupt, it will call the corresponding soft interrupt processing function to poll and process the data, that is: obtain a data frame from the Ring Buffer, represented by sk_buff, and hand it over as a network packet The network protocol stack is processed layer by layer from bottom to top.

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The network protocol stack processes network packets as follows:

1. Network interface layer

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First, the network interface layer checks the legality and correctness of the message. If it is illegal or the message verification is incorrect, discard it. Otherwise, find out the type of upper layer protocol (IPv4 or IPv6) and remove the frame header and frame tail. , and then handed over to the upper layer, the network layer, for processing.

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2. Network layer

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The network layer takes out the IP header and determines the next direction of the network packet, whether it is forwarded or handed over to the upper layer. After confirming that the network packet is to be sent to the local machine, the type of the upper layer protocol (such as TCP or UDP) is taken out, the IP header is removed, and then handed over to the transport layer for processing.

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3. Transport layer

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After the transport layer takes out the TCP header or UDP header, it finds the corresponding Socket based on the four-tuple [source IP, source port, destination IP, destination port], and copies the data to the Socket's receive buffer.

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4. Application layer

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Finally, the application layer program calls the Socket interface to copy the data in the kernel's Socket receiving buffer to the application layer buffer.

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At this point, the receiving process of a network packet is over.

Send network packet

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After we understand the receiving process of network packets, it is easy to understand the sending process of network packets. The sending direction of network packets is exactly opposite to the receiving direction.

First, the application calls the Socket interface to send network packets. This is a system call that will fall from user mode into the socket layer of kernel mode.

The socket layer will apply for a kernel-mode sk_buff memory, copy the data to be sent by the user to the sk_buff memory, and add it to the Socket sending buffer to wait for processing by the network protocol stack.

Since network data packets are raw data when transmitted from the application to the kernel, the protocol stack must add communication conventions to the raw data to ensure that the data can be correctly recognized when it reaches the server. The network protocol stack takes out the data packet from the Socket sending buffer, and then processes it layer by layer from top to bottom according to the layers of the TCP/IP stack (transport layer, network layer, network interface layer). Each layer converts the protocol header Information is continuously inserted into data packets.

The protocol stack's processing flow for sending data packets is as follows:

1. Transport layer

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At the transport layer, the TCP header will be added to the server and a new copy of sk_buff will be copied. This is because sk_buff will be released when it reaches the network card and the transmission is completed, and the TCP protocol supports retransmission. To ensure Network packets are transmitted reliably. This sk_buff cannot be deleted before receiving the ACK from the other party.

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2. Network layer

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At the network layer, it mainly does the following work: selecting routes (confirming the IP of the next hop), filling in IP headers, netfilter filtering, and fragmenting data packets that exceed the MTU size. After processing these tasks, it will be handed over to the network interface layer for processing.

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3. Network interface layer

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The network interface layer will perform physical address addressing to find the MAC address of the next hop, fill in the frame header and frame trailer, and put it in the sending queue. Then trigger a soft interrupt to tell the network card driver: There are new network packets in the queue that need to be sent. When the driver receives the notification, it will read the network frame from the send packet queue through DMA, and write the data into the FIFO send queue of the network card through DMA.

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4. Network card equipment

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The network card device takes out the data packet from the FIFO transmission queue and sends it to the network; when the transmission is completed, the network card device will trigger a hard interrupt to release memory, mainly to release sk_buff memory and clean up RingBuffer memory. Finally, when the ACK response of this TCP message is received, the transport layer will release the original sk_buff.

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At this point, the process of sending a network packet is over.

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