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2009-12-09 11:30:02
II framing (also known as DIX Ethernet, named after the major participants in the framing of the : , and ) defines the two- field in an , preceded by destination and source , that identifies an the frame data.
For example, an EtherType value of 0x0800 signals that the contains an datagram. Likewise, an EtherType of 0x0806 indicates an frame, 0x8100 indicates an frame and 0x86DD indicates an frame.
As this industry-developed standard went through a formal standardization process, the EtherType field was changed to a (data) length field in the new 802.3 standard. (Original Ethernet packets define their length with the framing that surrounds it, rather than with an explicit length count.) Since the packet recipient still needs to know how to interpret the packet, the standard required an header to follow the length and specify the packet type. Many years later, the 802.3x-1997 standard, and later versions of the standard, formally approved of both types of framing. In practice, both formats are in wide use, with original Ethernet framing the most common in Ethernet local area networks, due to its simplicity and lower overhead.
In order to allow some packets using Ethernet v2 framing and some packets using the original version of 802.3 framing to be used on the same Ethernet segment, EtherType values must be greater than or equal to 1536 (0x0600). That value was chosen because the maximum length of the data field of an Ethernet 802.3 frame is 1500 bytes (0x05DC). Thus if the field's value is greater than or equal to 1536, the frame must be an Ethernet v2 frame, with that field being a type field. If it's less than or equal to 1500, it must be an IEEE 802.3 frame, with that field being a length field. Values between 1500 and 1536, exclusive, are undefined.
