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A network protocol is a collection of rules, standards, or conventions established for data exchange in a computer network. It specifies the formats in which information must be communicated and the meaning of those formats.

Most networks have a layered architecture, with each layer built on top of its lower layers, providing a certain service to the layers above it, while shielding the layers above from the details of how to implement that service. The rule by which layer n on one device communicates with layer n on another device is the layer n protocol. There are many protocols in each layer of the network, and the protocols of the same layer of the receiver and the sender must be consistent, otherwise one party will not be able to recognize the information sent by the other party. Network protocols enable various devices on a network to exchange information with each other.

ppp protocol (point-to-point protocol) belongs to the network interface layer and is mainly used for dial-up access to the Internet.) PPP has the ability to dynamically assign IP addresses, allowing IP addresses to be negotiated at connection time; error detection and correction capabilities, supporting data compression; and to establish point-to-point link communications, LCP packets must first be sent at each end of the PPP link to configure and test the data link. After the link is established and the optional functions required by LCP are selected, PPP must send NCP packets in order to select and configure one or more network layer protocols. Once each selected network layer protocol is configured, datagrams from each network layer protocol can be sent over the link PPP is character-oriented and HDLC is bit-oriented. The ppp frame format is as follows:

The first and last 7E are frame delimitation flags, and the value is fixed to 7E;FF is an address field, and since the endpoint of the point-to-point link is unique, this field is set to FF;03 (C) is a control field, which contains information such as frame type and sequence number;FCS is a frame check field, which is used to detect whether there is an error;

ppp protocol specifies LCP(Link Control Protocol) for establishing, configuring and testing ppp protocol; the data packet captured by wireshark is as follows:

HDLC protocol

HDLC is a typical representative of bit-oriented data link control protocol, which does not depend on any character coding set; data messages can be transmitted transparently, and the "0-bit insertion method" for transparent transmission is easy to implement by hardware; full-duplex communication has high data link transmission efficiency; all frames are CRC checked, and the information frames are sequentially numbered to prevent missed reception or retransmission, and the transmission reliability is high; transmission control functions are separated from processing functions, which has greater flexibility. SDLC/HDLC protocol stipulates that all information transmission must start with a flag character and end with the same character. This flag character is 0111110 and is called the flag field (F). From the start mark to the end mark, a complete unit of information is formed, which is called a frame. All information is transmitted in frames, and flag characters provide the boundaries of each frame. The receiver can establish frame synchronization by searching for "0111110" to discover the beginning and end of a frame.

Address field and control field

After the flag field, there may be an address field A (Address) and a control field C (Contra1). The address field is used to specify the address of the secondary station with which to communicate. The control field may specify several commands. The 1st or 1st and 2nd bits in the control field indicate the type of transmission frame, the 1st bit is "0" indicating that it is an information frame, the 1st and 2nd bits are "10" indicating a monitoring frame, and "11" indicating an unnumbered frame. SDLC specifies that the A and C fields are 8 bits wide. HDLC allows the A field to be any length and the C field to be 8 or 16 bits. The receiver must check the first bit of each address byte, and if it is a "0," it is followed by another address byte; if it is a "1," it is the last address byte. Similarly, if the first bit of the first byte of the control field is "0," there is a second byte of the control field, otherwise there is only one byte. Immediately following the information field is a two-byte frame check field, referred to as a FC (Frame Check) field, and a check sequence FSC (Frame Check Sequence).

The packet captured with Wireshark is shown below:

Ethernet II Protocol

Ethernet is the most common communication protocol standard adopted by the existing local area network. This protocol defines the cable types and signal processing methods used in a local area network. It consists of a 6-byte destination MAC address, a 6-byte source MAC address, a 2-byte type field (used to indicate the type of data encapsulated in the Frame), followed by 46-1500 bytes of data and a 4-byte frame check.

header 8 destination address 6 source address 6 ethertype 2 payload 46-1500 frame check sequence 4

Header: 8 bytes, the first 7 bytes alternating with 0s and 1s (10101010) are used to synchronize the receiving station, and a 10101011 byte indicates the start of the frame. The header provides receiver synchronization and frame definition services.

Destination address: 6 bytes, unicast multicast or broadcast. Unicast addresses are called personal, physical, hardware or MAC addresses, broadcast addresses are all 1s,0xFF.

Source address: 6 bytes. Indicates the unicast address of the sending node.

EtherType: 2 bytes that indicate the upper layer protocol contained within an Ethernet frame. i.e., the protocol identifier of the frame format. For IP messages, the field value is 0x0800. For ARP messages, the value of the EtherType field is 0x0806.

Payload: Consists of a protocol data unit PDU of an upper layer protocol. The maximum payload that can be sent is 1500 bytes. Due to the collision-detecting nature of Ethernet, the payload is at least 46 bytes. If the upper layer PDU is less than 46 bytes in length, it must be supplemented to 46 bytes.

Frame check sequence: 4 bytes. Verify bit integrity.

Destination: Destination address is C4: 01:20: E8: 00:00

SOURCE: Source address is c4:02:21:e8:00:00

type: IP protocol;

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