Type / Length (2 bytes) (Ethernet)
Protocol: Ethernet Ethernet PCI
Field: type (EtherType) / length
Length: 2 bytes
Contents: upper layer protocol ID or data length indicator
Depending on the used Ethernet standard, this 2-byte field can be interpreted in two different ways:
Ethernet II
Ethernet II uses a type value in order to identify the upper layer protocol (as IP or IPX). The EtherType IDs are assigned by Xerox Corporation. A well-known EtherType value is 0x0800 for DoD IP.
EtherType values less than 1536 (0x0600) had been cancelled during IEEE 802.3 standardization; they were replaced by higher values above 1536. This had been necessary in order to enable Ethernet stations to identify the used Ethernet variant (DIX or IEEE standard).
See: Ethernet Type IDs
IEEE 802.3 - CSMA/CD
IEEE CSMA/CD uses a data length indicator (without padding bytes). Since Ethernet can only transmit up to 1500 bytes of user data, the length value is restricted to 1500. Values above 1536 (0x0600) are interpreted as DIX EtherType values.
IEEE CSMA/CD always uses LLC as upper layer protocol. Therefore, the EtherType field was obsolete. As a result, a length value is always followed by LLC.
There is only one exclusion: Novell used a frame type called 'frame=ethernet_802.3' which uses a length value with IPX following. LLC is used with 'frame=ethernet_802.2' and 'frame=ethernet_snap'.
Novell's 'frame=ethernet_802.3' was created in the early 1980s when Novell tried to implement the IEEE standards as soon as possible without LLC being standardized, yet. Novell adopted the length field in order to assure its customers that Novell would follow the future standards. LLC was introduced on NetWare LANs with Novell's NetWare 3.0 in 1989 using the ODI drivers. - In order to let receivers distinguish between lengh+LLC vs. length+IPX, the IPX checksum (first two bytes of IPX header) were always set to 0xFFFF.
NetWare
Novell's NetWare uses these Ethernet frame formats:
frame=ethernet_II Ethernet II Typ-Feld
frame=ethernet_802.3 Novell 802.3 Längen-Feld mit IPX
frame=ethernet_802.2 IEEE 802.2 Längen-Feld mit LLC
frame=ethernet_snap IEEE 802.3 Längen-Feld mit LLC+SNAP