EtherType

EtherType is a two-octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the payload of an Ethernet Frame. This field was first defined by the Ethernet II framing networking standard, and later adapted for the IEEE 802.3 Ethernet networking standard.

EtherType numbering generally starts from 0x0800. In modern implementations of Ethernet, the field within the Ethernet frame used to describe the EtherType also can be used to represent the size of the payload of the Ethernet Frame. Historically, depending on the type of Ethernet framing that was in use on an Ethernet segment, both interpretations were simultaneously valid, leading to ambiguity. Ethernet v2 framing considered these octets to represent EtherType while the original IEEE 802.3 framing considered these octets to represent the size of the payload in bytes. In order to allow packets using Ethernet v2 framing and packets using the IEEE 802.3 framing to be used on the same Ethernet segment, a unifying standard (IEEE 802.3x-1997) was introduced that required that EtherType values be greater than or equal to 1536 (0x0600). That value was chosen because the maximum length (MTU) of the data field of an Ethernet 802.3 frame was 1500 bytes (0x05DC). Thus, values of 1500 (0x05DC) and below for this field indicate that the field is used as the size of the payload of the Ethernet Frame while values of 1536 and above indicate that the field is in actually used to represent EtherType. The interpretation of values between 1500 and 1536, exclusive, are undefined.^[1] The size of the payload of non-standard jumbo frames, typically ~9000 Bytes long, fall within the range used by EtherType; this conflict is resolved by substituting the special EtherType value 0x8870 when a length would otherwise used.^[2] The network stack can replace this special EtherType with the actual length of the packet on receive, or when bridging to non-Ethernet networks like FDDI.

Ethernet Frame

With the advent of the IEEE 802 suite of standards, a SNAP header combined with an IEEE 802.2 LLC header is used to transmit the EtherType of a payload for IEEE 802 networks other than Ethernet, as well as for non-IEEE networks that use the IEEE 802.2 LLC header, such as FDDI. However, for Ethernet, the Ethernet II header is still used.

Note that even very well-known de facto uses of EtherTypes are not always recorded in the IEEE list of EtherType values. For example, EtherType 0x0806 (used by ARP) appears in the IEEE list only as "Symbolics, Inc., Protocol unavailable."

External links

• IEEE EtherType Registration Authority
• IEEE list of EtherType values
• Michael Patton's list of EtherType values
• http://www.iana.org/assignments/ethernet-numbers

References

1. ^ IEEE Std 802.3-2005, 3.2.6
2. ^ Kaplan et. al. (November 2001). "Extended Ethernet Frame Size Support". Internet Engineering Task Force. http://tools.ietf.org/html/draft-ietf-isis-ext-eth-01. Retrieved March 5, 2010. 
3. ^ Network Load Balancing Technical Overview. Retrieved 10/29/09