Ethernet Protocol
Contents
Reference link
1. introduction
- Ethernet II (DIX: DEC, Intel and Xerox) and IEEE 802.3 are widely used
2. Ethernet Frame Overview
Before the frame – preamble (8 bytes)
- In Ethernet II, the preamble is 7 bytes of alternating 1s and 0s (10101010), followed by a single byte of 10101010 (0xAA).
- In IEEE 802.3, the preamble is 7 bytes of alternating 1s and 0s (10101010), followed by a single byte of 10101011 (0xAB)(start of frame).
Destination Address and src Address (6 bytes each)
Type / Length field – Ethernet II (Type) (2 bytes)
This field tells the receiver that payload this frame carries.
- example
- IP handler
- ARP handler
Data and Pad (46-1500 bytes)
1500 bytes is the maximum size of the data field
46 bytes is the minimum size of the data field (plus other fields of the frame = 64 bytes)
- If the data is less than 46 bytes, the frame is padded with zeros in the back to make it 46 bytes.
Checksum – CRC32 (4 bytes)
- 32-bit cyclic redundancy check (CRC) checksum
The Problem with the Type / Length Field
- If the data is A0, the zero in the front will be mistaken as padding.
There are still lots of details here, but we will not go into it.
3. How Ethernet Addresses Work
Unicast and Multicast Bits
- The first special bit indicates whether the address is a unicast or a multicast address.
- Unicast means that the address represents a single device.
- Multicast addresses represent a group of devices – such as all printers on the network, or all devices in the same local network.
FF:FF:FF:FF:FF:FF is a broadcast address. It is used to send a message to all devices on the network.
Globally Unique / Locally Administered Bit
- The second special bit indicates whether the address is indeed globally unique.
4.Why Does an Ethernet Frame Have a Minimum Length?
- How are Collisions Handled in Ethernet?
TODO