TCP/IP principle--Link layer

Link Layer Effect:

(1) Send and receive IP datagram for IP module;

(2) Send ARP request and accept ARP reply for ARP

(3) Send Rarp request and accept ARP reply for Rarp

Protocol: Ethernet and Slip protocol

A. Ethernet Protocol Data Encapsulation Format (frames):

Note:

1. Both the destination address and the source address are MAC addresses (48 bits), which is the same

2. In the 802 standard, the 2-byte length represents the byte length of the subsequent data portion (excluding CRC). The Type field in the Ethernet encapsulation format represents the subsequent data type to indicate whether the IP datagram is a arp/rarp ball or an answer.

3. In the 802 standard, Next is the 3-byte LLC and 5 own snap (Sub-Network Access Protocol).

LLC: Purpose of service access point (Destination service access points,
D S A P) and the value of the source service access point, SSAP, are set to 0 x a A. Ct R-L field
The value is set to 3.

SNAP: The first half is set to 0 and the second part is the same as the Ethernet Type field.

4.CRC: Cyclic redundancy check code

5. Data part Minimum length, Ethernet: 46byte;802 standard: 38byte

6. Ethernet and 8 0 2. 3 has a limit on the length of the data frame, the maximum value is 1 5 0 0, and 1 4 9, 2 bytes. This feature of the link layer is called the M T U, the maximum transmission unit.

B.slip (Serial line IP) frame format

Definition: It is a simple form of encapsulation of the I P datagram on a serial line for the connection between the serial port and the modem , and a wide area network can be established.

function: SL I p applies to every computer in the home almost all r S-2 3 2 serial port and High speed modem access I N T e R n e T.

Slip Frames:

0XDB represents ESC, but differs from ANSCII.

Defects:

1. Each end must know the other's I p address. There is no way to notify the other end of the I p address on this side.

2. There is no type field in the data frame (similar to the Type field in Ethernet)

3.S L I p does not include a test in the data frame and (similar to the C R C field in Ethernet).

C.PPP protocol

PPP: It is typically used to create a direct connection between two nodes and can provide connection authentication, transfer encryption, and compression.

Frame format:

1. Each frame starts and ends with 0x7e

2. Protocol field: 0x0021: The Information field is an IP datagram; 0xc021 indicates that the information field is a link control data; 0x8021 indicates that the information field is a network control data.

3.CRC is a cyclic redundancy check code

4. When the flag character 0x7e appears in the Information field, it needs to be escaped in two different cases.

Sync Link: Bit fill

Async Link: Special characters 0 x 7 D are used as escape characters.

TCP/IP principle--Link layer