First of all, Ethernet, IEEE802.3, PPP and HDLC are all data link layer protocols, but the latter three are not commonly used. Ethernet and IEEE802.3 belong to the Ethernet link layer protocol, and the most commonly used protocol at the data link layer is the Etnernet Ethernet protocol.
definition:
Ethernet protocol, used to implement link layer data transmission and address encapsulation
Brief history:
- Experimental Ethernet
From 1973 to 1980, it was the research and development stage of Experimental Ethernet. At this time, the communication line is made of coaxial cable, and the speed can reach 367k bytes/s.
- Ethernet Version 1
From 1980 to 1981, the first generation of Ethernet technology was jointly developed by Digital, Intel, and Xerox. The first generation of Ethernet technology was named after the initials of these three companies, that is, DIX, version 1.0. Ethernet can work on thicker coaxial cables at speeds up to 1.25M bytes/s.
- Ethernet Version 2
In 1982, DIX1.0 was replaced by DIX2.0, and its standard is still in use today, also known as Ethernet II.
- IEEE 802.3 Standard
In 1983, IEEE formulated the 802.3 Ethernet protocol in order to promote the standardization of protocols used by all network devices. The English name is IEEE 802.3 Standard.
Encapsulation of Ethernet data frames
From the picture above you can see the Ethernet II frame
Destination address: 6 bytes, the destination address field determines the receiver of the frame.
Source address: 6 bytes, the source address field identifies the sender of the frame.
When using the six-byte source address field, the first three bytes represent the address assigned by the IEEE to the manufacturer, which will be burned into the ROM of each network interface card. Manufacturers typically allocate the last three bytes to each of their network interface cards. In fact, the destination and source addresses are what we often call the MAC address, for example, 00:1A:A0:31:39:D4 is a MAC address.
Type: 2 bytes, used to identify the type of protocol used by the upper layer, such as IP protocol (0x0800), ARP (0x0806), etc.
Data: The minimum size of an Ethernet packet is 64 bytes, and if it is insufficient, it will be filled to 64 bytes. The maximum length of an Ethernet packet is 1518 bytes, and the data field length ranges from 46 to 1500,
Why is this? Because the minimum size of an Ethernet packet is 64 bytes, any insufficient data will be filled to 64 bytes. The other parts of the Ethernet frame format add up to 6+6+2+4=18 bytes,
So the minimum length of the data part is 64-18=46 bytes; and the maximum length of the Ethernet packet is 1518 bytes, so 1518-18=1500 bytes.
FCS: The frame check field, namely Frame Check Sequence, is used to save the CRC (cyclic redundancy check) check value.