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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. This field is used by the data link layer to determine which protocol to hand over the payload to on the receiving end.

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 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 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 endstations. 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 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.[8]

Examples

EtherType values for some notable protocols[8]
EtherType Protocol
0x0800 Internet Protocol version 4 (IPv4)
0x0806 Address Resolution Protocol (ARP)
0x0842 Wake-on-LAN[9]
0x22F3 IETF TRILL Protocol
0x22EA Stream Reservation 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 with NNI compatibility[10]
0x8137 IPX
0x8204 QNX Qnet
0x86DD Internet Protocol Version 6 (IPv6)
0x8808 Ethernet flow control
0x8809 Ethernet Slow Protocols[11] such as the Link Aggregation Control Protocol
0x8819 CobraNet
0x8847 MPLS unicast
0x8848 MPLS multicast
0x8863 PPPoE Discovery Stage
0x8864 PPPoE Session Stage
0x886D Intel Advanced Networking Services [12]
0x8870 Jumbo Frames (Obsoleted draft-ietf-isis-ext-eth-01)
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[10]
0x88AB Ethernet Powerlink[citation needed]
0x88B8 GOOSE (Generic Object Oriented Substation event)
0x88B9 GSE (Generic Substation Events) Management Services
0x88BA SV (Sampled Value Transmission)
0x88CC Link Layer Discovery Protocol (LLDP)
0x88CD SERCOS III
0x88DC WSMP, WAVE Short Message Protocol
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)
0x88F8 NC-SI
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[13]
0x9100 VLAN-tagged (IEEE 802.1Q) frame with double tagging

See also

References

  1. ^ IEEE Std 802.3-2005, 3.2.6
  2. ^ "draft-ietf-isis-ext-eth".
  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".
  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. ^ a b "IEEE 802 Numbers". Internet Assigned Numbers Authority. 2015-10-06. Retrieved 2016-09-23.
  9. ^ "WakeOnLAN". Retrieved 2013-01-16.
  10. ^ a b "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. ^ "Intel® Advanced Network Services (Intel® ANS) Advanced Settings for Teams". Retrieved 2017-06-23.
  13. ^ "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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