EtherType

Posted 2020-08-01 CasonChan

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. 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

 

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 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 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]

Jumbo frames

The size of the payload of non-standard jumbo frames, typically ~9000 Bytes long, falls within the range used by EtherType, creating a conflict. The proposition to resolve this conflict was to substitute the special EtherType 0x8870 when a length would otherwise be used.^[2] 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.^[3] The draft authors also responded to the chair‘s letter, but no subsequent answer from the IEEE 802.3 has been recorded.^[4] Currently, the value used for Ethertype in jumbo frames is not clear. The IEEE Registration Authority lists all the Ethertype values registered, and no reference to jumbo frames is found in the list.^[5]

VLAN tagging

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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.

Examples

EtherType values for some notable protocols^[6]

EtherType	Protocol
0x0800	Internet Protocol version 4 (IPv4)
0x0806	Address Resolution Protocol (ARP)
0x0842	Wake-on-LAN[7]
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[8]
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
0x8870	Jumbo Frames (proposed)[2][3]
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[9]
0x88AB	Ethernet Powerlink[citation needed]
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[10]

Not all well known de facto uses of EtherTypes are always recorded in the IEEE list of EtherType values.^[11] For example, EtherType 0x0806 (used by ARP) appears in the IEEE list only as "Symbolics, Inc., Protocol unavailable."^[11] However, the IEEE Registration Authority lists all the accepted EtherTypes, including the 0x0806.^[5]

https://en.wikipedia.org/wiki/EtherType