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Ppp protocol (point-to-point protocol) belongs to the network interface layer, which is mainly used for dial-up access to Internet. )

PPP has the ability to dynamically assign IP addresses, allowing IP addresses to be negotiated at connection time, error detection and correction capabilities, and supports data compression

In order to establish point-to-point link communication, each end of the PPP link must first send LCP packets to set up and test the data link.

After the link is established and the optional functions required by the LCP are selected, the PPP must send NCP packets to select and set one or more network layer protocols.

Once each selected network layer protocol is set, datagrams from each network layer protocol can be sent on the link. PPP is character-oriented and HDLC is bit-oriented.

The ppp frame format is as follows:

The first and last 7e is the frame delimiting mark, and the value is fixed to 7E. FF is the address field. Because the endpoint of the point-to-point link is unique, this field is set to FF.

03 (C) is the control field, which contains information such as frame type and sequence number, while FCS is the frame check field, which is used to detect whether there is an error.

The ppp protocol specifies the LCP (Link Control Protocol) used to establish, configure, and test the ppp protocol. The packets captured with wireshark are shown below:

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 be implemented in hardware.

Full-duplex communication has high data link transmission efficiency, all frames are checked by CRC, and information frames are sequentially numbered, which can prevent missed reception or retransmission and high transmission reliability.

The transmission control function is separated from the processing function, so it has great flexibility. The SDLC/HDLC protocol stipulates that all information transmission must begin with a flag character and end with the same character.

The flag character is 01111110, called the flag field (F). A complete unit of information is formed from the start flag to the end flag, called a frame (Frame).

All information is transmitted in the form of frames, and flag characters provide the boundary of each frame. The receiver can establish frame synchronization by searching for "01111110" to find out the beginning and end of the frame.

Address field and control field

After the flag field, you can have an address field A (Address) and a control field C (Contro1).

The address field is used to specify the address of the secondary station with which it communicates. The control field can specify several commands.

Bit 1 or bit 1 and bit 2 in the control field represent the type of transmission frame, bit 1 is "0" indicates information frame, bit 1 and bit 2 is "10" is monitoring frame, and "11" is unnumbered frame.

SDLC specifies that the widths of fields An and C are 8 bits. HDLC allows the A field to be of any length and the C field to be 8 or 16 bits. The receiver must check the first bit of each address byte

If 0, it is followed by another address byte; if 1, that byte is the last address byte.

Similarly, if the first bit of the first byte of the control field is "0", there is a second control field byte, otherwise there is only one byte

. Immediately after the information field is the two-byte frame check field, which is called the FC (Frame Check) field and the check sequence FSC (Frame check Sequence).

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