ETHERNET HEADER

ETHERNET HEADER

The preamble is 56 bits of alternating 1s and 0s that synchronizes communication on an Ethernet network. It is followed by an 8-bit start of frame delimiter (10101011) that indicates a valid frame is about to begin.Preamble and start frame delimiter (SFD), which are both part of the layer 1 Ethernet packet.

Ethertype/Length field provides a different function depending on the standard - Ethernet II or 802.3.

Ether Type :

-          Values of 1500 (0x05DC) and below indicate that it is used as the size of the payload in octets

-          Values of 1536 (0x0600) and above indicate that it is used to represent EtherType.

EtherType is used to indicate which protocol is encapsulated in the payload of an Ethernet Frame.

0800 - IPv4

0806 - ARP

8035 - RARP

8600 - IPv6

8100 - 802.1Q

With 802.3, the field identifies the length of the payload. The length of a frame is important – there is both a minimum and maximum frame size.

The absolute minimum frame size for Ethernet is 64 bytes (or 512 bits) including headers. A frame that is smaller than 64 bytes will be discarded as a runt. The required fields in an Ethernet header add up to 18 bytes – thus, the frame payload must be a minimum of 46 bytes, to equal the minimum

64-byte frame size. If the payload does not meet this minimum, the payload is padded with 0 bits until the minimum is met.

Note: If the optional 4-byte 802.1Q tag is used, the Ethernet header size will total 22 bytes, requiring a minimum payload of 42 bytes.

By default, the maximum frame size for Ethernet is 1518 bytes – 18 bytes of header fields, and 1500 bytes of payload - or 1522 bytes with the 802.1Q tag. A frame that is larger than the maximum will be discarded as a giant. With both runts and giants, the receiving host will not notify the sender that

the frame was dropped. Ethernet relies on higher-layer protocols, such as TCP, to provide retransmission of discarded frames.

Some Ethernet devices support jumbo frames of 9216 bytes, which provide less overhead due to fewer frames. Jumbo frames must be explicitly enabled on all devices in the traffic path to prevent the frames from being dropped. The 32-bit Cycle Redundancy Check (CRC) field is used for errordetection. A frame with an invalid CRC will be discarded by the receiving device. This field is a trailer, and not a header, as it follows the payload.

The 96-bit Interframe Gap is a required idle period between frame transmissions, allowing hosts time to prepare for the next frame.