EtherType

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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. The same field is also used to indicate the size of some Ethernet frames. EtherType was first defined by the Ethernet II framing standard, and later adapted for the IEEE 802.3 standard.

Overview[edit]

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 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 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 EtherType. The interpretation of values 1501–1535, inclusive, is undefined.[1]

VLAN tagging[edit]

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. Triple tagging QinQinQ has three four-octet tags besides the original 16-bit EtherType field.

With 802.1Q VLAN tagging and QinQ the sparse 16-bit EtherType is being completely used. The 16-bit EtherType not only tags the payload class, it also serves to help end any VLAN tagging or QinQ stacking. Via look-ahead peeking in streams, the 16-bit EtherType can help to confirm or package a QinQ 32+32+16=80-bit header between the 48-bit MAC addresses and the payload. Of those 80-bits only 32-bits are used for dynamic information. For a full 66-bit addressing system, 18 bits are needed beyond the MAC. Thus, additional EtherType values are required and used for Triple Tagging QinQinQ.

Inefficient and conservative use of a 16-bit Tag Protocol Identifier (TPID) on each four octets VLAN tag, followed by the trailing lone 16-bits creates a 48-bit signature that cannot easily be mistaken as part of the payload. Vendor implementations may avoid wasting bandwidth sending those 48-bits in proprietary link compression schemes. The EtherType usually does not contain any CRC or FCS information.

Jumbo frames[edit]

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 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.[2]

Use beyond Ethernet[edit]

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, the Ethernet II header is still used.

Examples[edit]

EtherType values for some notable protocols[3]
EtherType Protocol
0x0800 Internet Protocol version 4 (IPv4)
0x0806 Address Resolution Protocol (ARP)
0x0842 Wake-on-LAN[4]
0x22F3 IETF TRILL Protocol
0x6003 DECnet Phase IV
0x8035 Reverse Address Resolution Protocol
0x809B AppleTalk (Ethertalk)
0x80F3 AppleTalk Address Resolution Protocol (AARP)
0x8100 VLAN-tagged frame (IEEE 802.1Q) and Shortest Path Bridging IEEE 802.1aq[5]
0x8137 IPX
0x8204 QNX Qnet
0x86DD Internet Protocol Version 6 (IPv6)
0x8808 Ethernet flow control
0x8819 CobraNet
0x8847 MPLS unicast
0x8848 MPLS multicast
0x8863 PPPoE Discovery Stage
0x8864 PPPoE Session Stage
0x887B HomePlug 1.0 MME
0x888E EAP over LAN (IEEE 802.1X)
0x8892 PROFINET Protocol
0x889A HyperSCSI (SCSI over Ethernet)
0x88A2 ATA over Ethernet
0x88A4 EtherCAT Protocol
0x88A8 Provider Bridging (IEEE 802.1ad) & Shortest Path Bridging IEEE 802.1aq[5]
0x88AB Ethernet Powerlink[citation needed]
0x88B8 GOOSE (Generic Object Oriented Substation event)
0x88B9 GSE (Generic Substation Events) Management Services
0x88CC Link Layer Discovery Protocol (LLDP)
0x88CD SERCOS III
0x88E1 HomePlug AV MME[citation needed]
0x88E3 Media Redundancy Protocol (IEC62439-2)
0x88E5 MAC security (IEEE 802.1AE)
0x88E7 Provider Backbone Bridges (PBB) (IEEE 802.1ah)
0x88F7 Precision Time Protocol (PTP) over Ethernet (IEEE 1588)
0x88FB Parallel Redundancy Protocol (PRP)
0x8902 IEEE 802.1ag Connectivity Fault Management (CFM) Protocol / ITU-T Recommendation Y.1731 (OAM)
0x8906 Fibre Channel over Ethernet (FCoE)
0x8914 FCoE Initialization Protocol
0x8915 RDMA over Converged Ethernet (RoCE)
0x891D TTEthernet Protocol Control Frame (TTE)
0x892F High-availability Seamless Redundancy (HSR)
0x9000 Ethernet Configuration Testing Protocol[6]
0x9100 VLAN-tagged (IEEE 802.1Q) frame with double tagging

Not all well known de facto uses of EtherTypes are always recorded in the IEEE list of EtherType values.[7] For example, EtherType 0x0806 (used by ARP) appears in the IEEE list only as "Symbolics, Inc., Protocol unavailable."[7] However, the IEEE Registration Authority lists all the accepted EtherTypes, including the 0x0806.[3]

See also[edit]

References[edit]

  1. ^ IEEE Std 802.3-2005, 3.2.6
  2. ^ Kaplan; et al. (2000-05-26). "Extended Ethernet Frame Size Support". Internet Engineering Task Force. 
  3. ^ a b "IEEE 802 Numbers". Internet Assigned Numbers Authority. 2015-10-06. Retrieved 2016-09-23. 
  4. ^ "WakeOnLAN". Retrieved 2013-01-16. 
  5. ^ a b "Configuration - Shortest Path Bridging MAC (SPBM)". Avaya. June 2012. p. 35. Retrieved 7 July 2012. [dead link]
  6. ^ "Loop". wireshark.org. 
  7. ^ a b "Public EtherType list". IEEE. Retrieved 2016-09-23. 

External links[edit]