Ethernet (IEEE 802.3)

Ethernet is the most common local area networking technology, and, with gigabit and 10 gigabit Ethernet, is also being used for metropolitan-area and wide-area networking.

It is specified by [http://standards.ieee.org/getieee802/802.3.html various IEEE 802.3 specifications].

Ethernet sends network packets from the sending host to one (["Unicast"]) or more (["Multicast"]/["Broadcast"]) receiving hosts.

A physical Ethernet packet will look like this:

Preamble

Destination MAC address

Source MAC address

Type/Length

User Data

Frame Check Sequence (FCS)

8

6

6

2

46 - 1500

4

As the Ethernet hardware filters the preamble, only the green fields are given to Ethereal or any other application. Most Ethernet interfaces also either don't supply the FCS to Ethereal or other applications or aren't configured by their driver to do so.

An Ethernet host is addressed by its Ethernet MAC address, a 6 byte number usually displayed as: 08:00:08:15:ca:fe (the delimiters vary, so you might see 08-00-08-15-ca-fe or the like). The first three bytes of the address are assigned to a specific vendor, see [http://www.iana.org/assignments/ethernet-numbers Ethernet numbers] at the ["IANA"] and [http://www.cavebear.com/CaveBear/Ethernet/vendor.html Michael A. Patton's list of vendor codes] for assigned vendor codes, and see [http://www.iana.org/assignments/ethernet-numbers Ethernet numbers] and [http://www.cavebear.com/CaveBear/Ethernet/multicast.html Michael A. Patton's list of multicast addresses], for assigned multicast addresses.

A destination MAC address of ff:ff:ff:ff:ff:ff indicates a ["Broadcast"], meaning the packet is send from one host to any other on that network.

XXX - also describe multicast.

The original DEC/Intel/Xerox Ethernet specification included a 16-bit type field to indicate what upper layer protocol should be used. Ethernet frames with less than 64 bytes of Ethernet header, user data, and FCS are padded to 64 bytes (which means 60 bytes of Ethernet header and user data), which means that if there's less than 64-(14+4) = 46 bytes of user data in the frame, extra user data is added to the frame. If the upper layer protocol implementation has to know exactly how much user data is in the packet, and expects the length of the Ethernet packet to indicate the amount of user data, it will not behave correctly with padded packets.

When constructing standards for LANs, the IEEE added a new header, the [http://standards.ieee.org/getieee802/802.2.html 802.2] LLC header, to packets in those LANs. It contained a destination "service access point", source "service access point", and packet type field, similar to the packet type field used in HDLC and HDLC-derived protocols such as X.25's LAPB; the destination service access point indicated the service to which the packet should be delivered, where a "service" is implemented as a protocol. (XXX - is the notion of service and protocol formalized in the OSI reference model? If so, we should perhaps have a page for the OSI model and describe that notion, and link to it.) I.e., it indicates the upper layer protocol that should be used.

This meant that the type field in Ethernet could be used for other purposes, if an 802.2 header appeared at the beginning of the user data, so the IEEE standard for Ethernet, IEEE 802.3, included after the source MAC address a 16-bit field indicating the length of the user data in the packet, for the benefit of protocols that couldn't infer the length of the user data from the length of the packet as received.

However, that standard also had to support the traditional use of that field as a type field. Ethernet packets could have no more than 1500 bytes of user data, so the field is interpreted as a length field if it has a value <= 1500 and a type field if it has a value > 1500. (XXX - the maximum length value is slightly smaller than the minimum type value.)

Therefore, if the type/length field has a value 1500 or lower, it's a length field, and is followed by an 802.2 header, otherwise it's a type field and is followed by the data for the upper layer protocol.

For a more detailed discussion of this, which mentions a third possibility used by NetWare, and mentions the SNAP header that can follow the 802.2 header, see [http://www.ee.siue.edu/~bnoble/comp/networks/frametypes.html Ethernet Frame Types: Provan's Definitive Answer], by Don Provan.

Some examples of values in the type/length field:

See [http://www.iana.org/assignments/ethernet-numbers Ethernet numbers] and [http://www.cavebear.com/CaveBear/Ethernet/type.html Michael A. Patton's list of Ethernet type codes] for a list of assigned Ethernet type codes.

Ethernet uses a CyclicRedundancyCheck (CRC) algorithm to detect transmission errors. The FrameCheckSequence field is filled (using a CRC) by the sending host. If the receiving host detects a wrong CRC, it will throw away that packet.

History

XXX - add a brief description of Ethernet history

Protocol dependencies

Example traffic

XXX - Add example traffic here (as plain text or Ethereal screenshot).

Ethereal

The Ethernet dissector is fully functional.

Preference Settings

(XXX add links to preference settings affecting how Ethernet is dissected).

Example capture file

XXX - Add a simple example capture file. Keep it short, it's also a good idea to gzip it to make it even smaller, as Ethereal can open gzipped files automatically.

Display Filter

A complete list of Ethernet display filter fields can be found in the [http://www.ethereal.com/docs/dfref/e/eth.html display filter reference]

Capture Filter

Discussion