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Ethernet packet. The SFD (start frame delimiter) marks the end of the packet preamble. It is immediately followed by the Ethernet frame, which starts with the destination MAC address. [1] In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport
In the OSI model of computer networking, a frame is the protocol data unit at the data link layer. Frames are the result of the final layer of encapsulation before the data is transmitted over the physical layer. [1] A frame is "the unit of transmission in a link layer protocol, and consists of a link layer header followed by a packet."
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 and is used at the receiving end by the data link layer to determine how the payload is processed. The same field is also used to indicate the size of some Ethernet frames.
A data unit at layer 2, the data link layer, is a frame. In layer 4, the transport layer, the data units are segments and datagrams. Thus, in the example of TCP/IP communication over Ethernet, a TCP segment is carried in one or more IP packets, which are each carried in one or more Ethernet frames.
Ethernet devices must allow a minimum idle period between transmission of Ethernet packets. [1] A brief recovery time between packets allows devices to prepare for reception of the next packet. While some physical layer variants literally transmit nothing during the idle period, most modern ones continue to transmit an idle pattern signal.
Individual frames are then "minor frames" within that superframe. Each frame contains a subframe ID (often a simple counter) which identifies its position within the superframe. A second frame synchronizer establishes superframe synchronization. This allows subcommutation, where some data is sent less frequently than every frame.
Each Ethernet frame must be processed as it passes through the network. Processing the contents of a single large frame is preferable to processing the same content broken up into smaller frames, as this makes better use of available CPU time by reducing interrupts.
This is often implemented as a lookup in the FIB of the source address of the packet. If the interface has no route to the source address, the packet is assumed to be part of a denial of service attack, using a spoofed source address, and the router discards the packet. When the router is multihomed, ingress filtering becomes more complex ...