Network Notes: Ethernet

published: 12th of July 2018

Overview

The original ethernet specification was owned by DEC, Intel and Xerox hence the standard being named "Ethernet (DIX)". The ethernet specification has been through a number of updates over the years beginning with the IEEE standard in the early 1980's and going through many revisions. Ethernet standards are covered by the IEEE working group here.

Frame Format

There are three main ethernet framing formats.

Ethernet II (DIX) and Revised IEEE 802.3 (1997)

Known as Ethernet II, this is the most common Ethernet frame format in use today. Standardized by the IEEE as the revised 802.3 (1997) standard.

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Original IEEE 802.3

The original IEEE specification was flawed. It was discovered that the 1 Byte DSAP and SSAP fields where too small to be of much use.

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IEEE 802.3 with SNAP Header

This frame format adds the SNAP header to overcome some of the limitations of the original IEEE frame format.

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Ethernet Header Fields

Field	Purpose
Preamble	Provides clocking synchronization for the transmitted signal.
Start of Frame Delimiter (SFD)	Same use as in the 8 byte preamble, it is simply moved into its own 1 byte field with the preamble being shortened to 7 bytes.
Destination MAC	Destination MAC address of the ethernet frame.
Source MAC	Source MAC address of the ethernet frame.
Type (DIX)	Inform the receiver of the upper layer protocol type that follows the ethernet header. Examples are 0x800 - IPv4 or 0x8100 - 802.1Q.
Length	The length in bytes of the data payload.
Control	Provides a mechanism for both connection-orientated and connectionless operations.
Organizationally Unique Identifier (SNAP)	Allows for the advertisement of the NICs device manufacturer OUI.
Type (SNAP)	Same function as the DIX type field. Overcomes the size limitation of the DSAP field.

Packet Captures

The ethernet frame format can be referenced in PCAPs found here.

Bibliography

1. Kocharians, N. and Paluch, P. (2014) CCIE Routing and Switching v5.0 Official Cert Guide, Volume 1, 5th Edition - Chapter 1: Ethernet Basics. Indianapolis: Cisco Press