EtherType


EtherType is a two-octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the payload of the frame and is used at the receiving end by the data link layer to determine how the payload is processed. The same field is also used to indicate the size of some Ethernet frames.
The EtherType is also used as the basis of 802.1Q VLAN tagging, encapsulating packets from VLANs for transmission multiplexed with other VLAN traffic over an Ethernet trunk.
EtherType was first defined by the Ethernet II framing standard, and later adapted for the IEEE 802.3 standard. EtherTypes are assigned by the IEEE Registration Authority.

Overview

In modern implementations of Ethernet, the field within the Ethernet frame used to describe the EtherType can also 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 potential ambiguity. Ethernet II 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 Ethernet II and 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. That value was chosen because the maximum length of the data field of an Ethernet 802.3 frame is 1500 bytes. Thus, values of 1500 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 used to represent an EtherType. The interpretation of values 1501–1535, inclusive, is undefined.
The end of a frame is signaled by loss of carrier or by a special symbol or sequence in the line coding scheme for a particular Ethernet physical layer, so the length of the frame does not always need to be encoded as a value in the Ethernet frame. However, as the minimum payload of an Ethernet frame is 46 bytes, a protocol which uses EtherType must include its own length field if that is necessary for the recipient of the frame to determine the length of short packets for that protocol.

VLAN tagging

uses an 0x8100 EtherType value. The payload following includes a 16-bit tag control identifier followed by an Ethernet frame beginning with a second EtherType field for consumption by end stations. IEEE 802.1ad extends this tagging with further nested EtherType and TCI pairs.

Jumbo frames

The size of the payload of non-standard jumbo frames, typically ~9000 Bytes long, falls within the range used by EtherType, and cannot be used for indicating the length of such a frame. The proposition to resolve this conflict was to substitute the special EtherType value 0x8870 when a length would otherwise be used. However, the proposition was not accepted and it is defunct. The chair of IEEE 802.3 at the time, Geoff Thompson, responded to the draft outlining IEEE 802.3's official position and the reasons behind the position. The draft authors also responded to the chair's letter, but no subsequent answer from the IEEE 802.3 has been recorded.
While defunct, this draft was implemented and is used in Cisco routers in their IS-IS implementation.

Use beyond Ethernet

With the advent of the IEEE 802 suite of standards, a Subnetwork Access Protocol 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, Ethernet II framing is still used.

Registration

EtherTypes are assigned by the IEEE Registration Authority. Not all well-known uses of EtherTypes are recorded in the IEEE list of EtherType values. For example, EtherType 0x0800 does not appear in the IEEE list. The Internet Assigned Numbers Authority has a separate list of some EtherType registrations, compiled from several sources, including the IEEE Registration Authority's list and some other lists; that list includes 0800.

Examples

EtherTypeProtocol
0x0800Internet Protocol version 4
0x0806Address Resolution Protocol
0x0842Wake-on-LAN
0x22F0Audio Video Transport Protocol
0x22F3IETF TRILL Protocol
0x22EAStream Reservation Protocol
0x6002DEC MOP RC
0x6003DECnet Phase IV, DNA Routing
0x6004DEC LAT
0x8035Reverse Address Resolution Protocol
0x809BAppleTalk
0x80F3AppleTalk Address Resolution Protocol
0x8100VLAN-tagged frame and Shortest Path Bridging IEEE 802.1aq with NNI compatibility
0x8102Simple Loop Prevention Protocol
0x8103Virtual Link Aggregation Control Protocol
0x8137IPX
0x8204QNX Qnet
0x86DDInternet Protocol Version 6
0x8808Ethernet flow control
0x8809Ethernet Slow Protocols such as the Link Aggregation Control Protocol
0x8819CobraNet
0x8847MPLS unicast
0x8848MPLS multicast
0x8863PPPoE Discovery Stage
0x8864PPPoE Session Stage
0x887BHomePlug 1.0 MME
0x888EEAP over LAN
0x8892PROFINET Protocol
0x889AHyperSCSI
0x88A2ATA over Ethernet
0x88A4EtherCAT Protocol
0x88A8Service VLAN tag identifier on Q-in-Q tunnel.
0x88ABEthernet Powerlink
0x88B8GOOSE
0x88B9GSE Management Services
0x88BASV
0x88BF
0x88CCLink Layer Discovery Protocol
0x88CDSERCOS III
0x88DCWSMP, WAVE Short Message Protocol
0x88E3Media Redundancy Protocol
0x88E5MAC security
0x88E7Provider Backbone Bridges
0x88F7Precision Time Protocol over IEEE 802.3 Ethernet
0x88F8NC-SI
0x88FBParallel Redundancy Protocol
0x8902IEEE 802.1ag Connectivity Fault Management Protocol / ITU-T Recommendation Y.1731
0x8906Fibre Channel over Ethernet
0x8914FCoE Initialization Protocol
0x8915RDMA over Converged Ethernet
0x891DTTEthernet Protocol Control Frame
0x892FHigh-availability Seamless Redundancy
0x9000Ethernet Configuration Testing Protocol
0x9100VLAN-tagged frame with double tagging
0xF1C1Redundancy Tag