up:: [[Computing MOC]] tags:: #note/develop #on/computing/networking # Ethernet Frames A *frame* is the Protocol Data Unit (PDU) for Layer 2 of the [[OSI Model]]. They are handled by switches. ## Format Frames are composed of the following contents: - Header - Destination MAC address (6 bytes) - Source MAC address (6 bytes) - Type/Length of the packet (2 bytes) - Packet - Trailer - Frame Check Sequence (4 bytes) In addition to the frame itself, a Preamble and a Start Frame Delimiter (SFD) are sent along with it. ### Frame Header - Preamble and Start Frame Delimiter (SFD) - Sent with each Ethernet frame, though not considered to be part of the frame - Allows the receiving device to synchronize its receiver clock and prep to receive the incoming frame - Preamble is 7 bytes; just a series of alternating 1s and 0s - SFD (Start Frame Delimiter) is 1 byte; signals that the preamble is done - Sequence is 10101011 - Belong to Layer 1 - Destination and source - Destination is the destination MAC address - Source is the source MAC address - Type/Length - 2 bytes - Used to either indicate the type of the encapsulated packet (eg., an IPv4 packet vs an IPv6 packet) or to indicate the length of the encapsulated packet (in bytes) - These days it's mostly used for indicating the type; using it for length is historical - Value of 1500 (decimal) or less means it indicates the length of the encapsulated packet - Value above 1536 (decimal) indicates the type of the encapsulated packet - This field is called the EtherType field - IPv4: 0x0800 - IPv6: 0x86DD ### Frame Trailer - Only field is the Frame Check Sequence (FCS) - 4 bytes - Used to detect corrupt data in a frame - Contains a checksum - Algorithm used to generate and check checksum is Cyclic Redundancy Check --- > [!createdat] Created at: [[2025-11-13]]