Ethernet frame: de­f­i­n­i­tion and variants of the frame format

In an Ethernet network, devices share data using packets. They contain, among other things, the Ethernet frame, which is divided into several data sets. These records consist of binary code that provides important in­for­ma­tion, such as addresses, control in­for­ma­tion, payload data, and checksums.

Depending on the Ethernet standard, Ethernet frames are struc­tured dif­fer­ent­ly and may contain more or fewer data fields, depending on the network protocol.

In the OSI model the frame is on the data link layer and is re­spon­si­ble for the error-free trans­mis­sion and sep­a­ra­tion of the bit stream into blocks. The first version of Ethernet (Ethernet I) was still based on 16-bit data fields without defined bytes. Modern Ethernet frames were first used in the Ethernet II structure, before the IEEE (Institute of Elec­tri­cal and Elec­tron­ics Engineers) developed the standard protocol IEEE 802.3 (first IEEE 802.3raw) in 1983.

Following technical ad­vance­ments, the frame structure was adapted several times so that the frames could carry more defined data. With the IEEE 802.3 format, the basic MAC frame and the SNAP frame were created for the mul­ti­plex­ing process and for man­u­fac­tur­er-related iden­ti­fi­ca­tion data. For the de­vel­op­ment of the VLAN, the Ethernet II frame and the Ethernet IEEE 802.3 frame were developed as "tagged" variants. These tags contain control data that can assign the frame to a specific VLAN.

An Ethernet frame must be at least 64 bytes for collision detection to work, and can be a maximum of 1,518 bytes. The packet starts with a preamble that controls the syn­chro­niza­tion between sender and receiver and a "Start Frame Delimiter" (SFD) that defines the frame. Both values are bit sequences in the format “10101010 ...” in which the actual frame contains in­for­ma­tion about source and des­ti­na­tion addresses (MAC format), control in­for­ma­tion (in the case of Ethernet II the type field, later a length spec­i­fi­ca­tion), followed by the trans­mit­ted data record. A frame check sequence (FCS) is an error-detecting code that closes the frame (except for the preamble and SFD). The packet is completed by an "Inter Frame Gap," which defines a 9.6 μs trans­mis­sion pause.

Ethernet II uses the classic frame structure with a type field ("Type") which defines various protocols of the network layer. In the OSI model, the network layer is important for con­nect­ing and providing network addresses. The type field was replaced by a length spec­i­fi­ca­tion in later frame formats.

The Ethernet II frame was defined in 1982 and has formed the foun­da­tion of all sub­se­quent frame de­vel­op­ments. However, the format still enjoys great pop­u­lar­i­ty, primarily because it gives the data field the most space (up to 1,500 bytes).

This rough version of the 802.3 packet, given the un­for­tu­nate name "Ethernet 802.3," was brought out by Novell before wide­spread es­tab­lish­ment of IEEE 802.3 standards and the popular IPX/SPX protocol, un­for­tu­nate­ly leading to frequent confusion with the IEEE standard. Con­se­quent­ly, Novell added "raw" to the name. In contrast to the classic Ethernet II model, this frame defines an exact end to the bit sequence for the SFD. This iden­ti­fies the data packet as the 802.3 standard for the receiver. 802.3raw frames do not contain a protocol iden­ti­fi­er, as they are only usable for Novell IPX. In addition, the data to be trans­mit­ted is always prefixed with 2 bytes, which always consist of ones. This is the only way to dis­tin­guish a "raw" frame from other frames in the 802.3 family.

The IEEE 802.3raw frame can only be used for the IPX protocol, because the type field's protocol ID is missing. The name "IEEE 802.3raw" is also slightly mis­lead­ing, since Novell used the protocol name without involving the IEEE in the de­vel­op­ment of the frame. The use of this frame means extra work for the user, because com­pat­i­bil­i­ty issues can arise between devices. From 1993 onwards, Novell itself rec­om­mend­ed the "Ethernet 802.2" standard, which used the IEEE 802.3 frame, to avoid the like­li­hood of confusion with the "raw" frame.

This stan­dard­ized version of the Ethernet 802.3 frame can define up to 256 com­pat­i­ble protocols, with important protocol in­for­ma­tion in­te­grat­ed into the data field. In addition, the "Des­ti­na­tion Service Access Point" (DSAP) and "Source Service Access Point" (SSAP) are included. The new control field defines the "Logical Link" (LLC) of the protocol. This point ensures the trans­paren­cy of the media sharing pro­ce­dures and can control the data flow.

Ethernet IEEE 802.3 is by far the most popular and widely used LAN frame structure today. However, some networks and protocols require more space for specific in­for­ma­tion. Con­se­quent­ly, there are variants of the IEEE 802.3 frame that provide ad­di­tion­al data blocks for specific in­for­ma­tion, among them the SNAP extension and the VLAN tag.

The SNAP field (“Sub Network Access Protocol”) is useful for defining more than 256 protocols. To do this, 2 bytes are made available for the protocol number. In addition, the man­u­fac­tur­er can integrate a unique iden­ti­fi­er (3 bytes). Unlike its pre­de­ces­sors, SNAP also ensures backward com­pat­i­bil­i­ty with Ethernet II. DSAP, SSAP, and Control are firmly defined here.

With the newly added space for protocol in­for­ma­tion, IEEE 802.3 SNAP is extremely versatile and makes com­pat­i­bil­i­ty between numerous different protocols in a network possible. However, the space for the actual data is slightly less.

Tagged frames contain a so-called VLAN tag for them to be assigned to a virtual local area network (VLAN), which separates the network structure into physical and logical levels. This means that with the help of VLANs, sub­net­works can be im­ple­ment­ed without having to install hardware. The sub­net­work is then virtual and not phys­i­cal­ly realized. Iden­ti­fy­ing Ethernet frames within a VLAN requires the “Tag” field. On a physical level, VLANs work through switches.

In the OSI model, a VLAN works on the data link layer (layer 2) and controls the data flow control. With VLAN, networks can become more efficient by being divided into subnets. For the in­for­ma­tion that the switch handles, the tagged frames are re­spon­si­ble. In the Ethernet II frame, the “Tag” field is im­ple­ment­ed before the “Type” field and uses 4 bytes. This increases the minimum size of the Ethernet II frames by 4 bytes.

VLAN tags can also be installed in today's most popular IEEE 802.3 frame format. Within this frame, the “Tag” field uses 4 bytes and is im­ple­ment­ed before the length spec­i­fi­ca­tion. The minimum size of the frame is now increased from 4 bytes to 68 bytes.