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
An Ethernet frame including the EtherType field. Each lower slot designates an octet; the EtherType is two octets long.

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 (MTU) 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.[1]

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 (if allowed) for that protocol.

VLAN tagging
Insertion of the 802.1Q VLAN tag (four octets) into an Ethernet-II frame, with a typical VLAN arrangement of a TPID EtherType value of 0x8100. A QinQ arrangement would add another four octets tag containing two octets TPID using various EtherType values.

802.1Q VLAN tagging uses an 0x8100 EtherType value. The payload following includes a 16-bit tag control identifier (TCI) followed by an Ethernet frame beginning with a second (original) 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.[2] However, the proposition (its use case was bigger packets for IS-IS) 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.[3]

While defunct, this draft was implemented and is used in Cisco routers in their IS-IS implementation (for IIH Hello packets padding).[4][5]

Use beyond Ethernet

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

Registration

EtherTypes are assigned by the IEEE Registration Authority.[6] Not all well-known uses of EtherTypes are recorded in the IEEE list of EtherType values. For example, EtherType 0x0800 (used by IPv4) does not appear in the IEEE list.[7] 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.[8]

Examples
EtherType values for some notable protocols[8]
EtherType
(hexadecimal)		Protocol
0x0800Internet Protocol version 4 (IPv4)
0x0806Address Resolution Protocol (ARP)
0x0842Wake-on-LAN[9]
0x22F0Audio Video Transport Protocol (AVTP)
0x22F3IETF TRILL Protocol
0x22EAStream Reservation Protocol
0x6002DEC MOP RC
0x6003DECnet Phase IV, DNA Routing
0x6004DEC LAT
0x8035Reverse Address Resolution Protocol (RARP)
0x809BAppleTalk (Ethertalk)
0x80F3AppleTalk Address Resolution Protocol (AARP)
0x8100VLAN-tagged frame (IEEE 802.1Q) and Shortest Path Bridging IEEE 802.1aq with NNI compatibility[10]
0x8102Simple Loop Prevention Protocol (SLPP)
0x8103Virtual Link Aggregation Control Protocol (VLACP)
0x8137IPX
0x8204QNX Qnet
0x86DDInternet Protocol Version 6 (IPv6)
0x8808Ethernet flow control
0x8809Ethernet Slow Protocols[11] such as the Link Aggregation Control Protocol (LACP)
0x8819CobraNet
0x8847MPLS unicast
0x8848MPLS multicast
0x8863PPPoE Discovery Stage
0x8864PPPoE Session Stage
0x887BHomePlug 1.0 MME
0x888EEAP over LAN (IEEE 802.1X)
0x8892PROFINET Protocol
0x889AHyperSCSI (SCSI over Ethernet)
0x88A2ATA over Ethernet
0x88A4EtherCAT Protocol
0x88A8Service VLAN tag identifier (S-Tag) on Q-in-Q tunnel.
0x88ABEthernet Powerlink
0x88B8GOOSE (Generic Object Oriented Substation event)
0x88B9GSE (Generic Substation Events) Management Services
0x88BASV (Sampled Value Transmission)
0x88BF MikroTik RoMON (unofficial)
0x88CCLink Layer Discovery Protocol (LLDP)
0x88CDSERCOS III
0x88E3Media Redundancy Protocol (IEC62439-2)
0x88E5IEEE 802.1AE MAC security (MACsec)
0x88E7Provider Backbone Bridges (PBB) (IEEE 802.1ah)
0x88F7Precision Time Protocol (PTP) over IEEE 802.3 Ethernet
0x88F8NC-SI
0x88FBParallel Redundancy Protocol (PRP)
0x8902IEEE 802.1ag Connectivity Fault Management (CFM) Protocol / ITU-T Recommendation Y.1731 (OAM)
0x8906Fibre Channel over Ethernet (FCoE)
0x8914FCoE Initialization Protocol
0x8915RDMA over Converged Ethernet (RoCE)
0x891DTTEthernet Protocol Control Frame (TTE)
0x893a1905.1 IEEE Protocol
0x892FHigh-availability Seamless Redundancy (HSR)
0x9000Ethernet Configuration Testing Protocol[12]
0x9100VLAN-tagged (IEEE 802.1Q) frame with double tagging
0xF1C1Redundancy Tag (IEEE 802.1CB Frame Replication and Elimination for Reliability)

See also

References
  1. IEEE Std 802.3-2005, 3.2.6
  2. "Extended Ethernet Frame Size Support". November 2001.
  3. Kaplan; et al. (2000-05-26). "Extended Ethernet Frame Size Support". Internet Engineering Task Force.
  4. "Techexams ccie/104431-is".
  5. "Gmane ISIS in SCAPY and Jumbo frames". Archived from the original on 2018-03-31. Retrieved 2017-05-09.
  6. Use of the IEEE Assigned Ethertype with IEEE Std 802.3 Local and Metropolitan Area Networks (PDF), retrieved 2017-12-30
  7. "Public EtherType list". IEEE. Retrieved 2018-09-08.
  8. "IEEE 802 Numbers". Internet Assigned Numbers Authority. 2015-10-06. Retrieved 2016-09-23.
  9. "WakeOnLAN". Retrieved 2018-10-16.
  10. "Configuration - Shortest Path Bridging MAC (SPBM)". Avaya. June 2012. p. 35. Retrieved 23 June 2017.
  11. "IEEE Std 802.3-2015". September 3, 2015. Annex 57A.
  12. "8. Ethernet Configuration Testing Protocol". The Ethernet, A Local Area Network Data Link and Physical Layer Specification Version 2.0 (PDF). November 1982.
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