EMZETT.Footer
In short: An optional block of data at the end of a data packet, usually for checksums or markers indicating that the packet has ended.
In more detail: Not every protocol uses a footer — many make do with length information in the header. Where it does occur (e.g. in Ethernet frames as the “frame check sequence”), it usually serves error detection: the recipient checks whether the received data matches the checksum given in the footer.
In Depth
Frame check sequence in Ethernet
In the Ethernet frame, the footer is called the “frame check sequence” (FCS) and consists of a 4-byte CRC32 checksum (cyclic redundancy check) calculated over the entire frame content. Before sending, the sender calculates the checksum mathematically from the bits to be transmitted and appends the result; after receiving, the recipient performs exactly the same calculation again over the same data and compares its result with the value sent along:
Received frame → recalculate CRC → matches the footer? → yes: continue processing
→ no: discard frame
CRC32 reliably detects practically all random bit errors of a typical magnitude, but is NOT cryptographic protection — a targeted attacker could theoretically manipulate data in such a way that the checksum still matches. For real integrity and authenticity protection against malicious manipulation, cryptographic methods such as hash functions or digital signatures are needed, not a simple CRC.
Silent loss instead of an error message
If the two values don’t match, the frame was altered on the transmission path — usually through physical causes such as electromagnetic interference, a defective cable or signal attenuation. In this case the recipient simply discards the damaged frame immediately and silently, WITHOUT sending an error message back to the sender — at layer 2 there simply isn’t any mechanism provided for this. It’s entirely up to higher layers such as TCP to notice the missing frame indirectly (via an ACK that doesn’t arrive within the expected time) and trigger a retransmission — so the actual error handling doesn’t happen where the error was discovered, but several layers higher.
Why some protocols do without a footer
Many more modern protocols deliberately do without a separate footer at the end and instead put all the necessary metadata (including length and checksum) directly into the header at the beginning of the message. This has a practical advantage in processing: when reading the header, a recipient immediately knows how long the entire packet will be, and can reserve memory accordingly or recognise when the message has been received completely — instead of having to wait until the actual end to find a footer at all. This is especially relevant for streaming-style processing, in which data can ideally already be processed before the complete message has even fully arrived.