In networking, the header and trailer are control information added to a data packet for transmission; the header is placed at the beginning, and the trailer at the end, to ensure proper delivery and error checking.
Here's a breakdown:
Header
- Definition: The header is a block of data added to the beginning of a data packet. It contains information needed for routing and delivery of the packet to its destination.
- Purpose:
- Addressing: Contains source and destination addresses.
- Protocol Identification: Specifies the protocol being used (e.g., TCP, UDP, IP).
- Sequencing: Helps in reassembling packets at the destination.
- Quality of Service (QoS): Indicates priority or other service requirements.
- Hop Limit/Time to Live (TTL): Prevents packets from looping endlessly.
- Example: In an IP packet, the header includes source and destination IP addresses, protocol type, and TTL.
Trailer
- Definition: The trailer is a block of data added to the end of a data packet. It primarily serves for error detection.
- Purpose:
- Error Detection: Contains checksums or cyclic redundancy checks (CRCs) to verify data integrity.
- Frame Delimitation: Signals the end of a frame, particularly in protocols like Ethernet.
- Example: Ethernet frames use a Frame Check Sequence (FCS) in the trailer for error detection.
Header and Trailer Together
The header and trailer work together to ensure reliable data transmission. The header guides the packet to its destination, while the trailer verifies that the data arrived correctly. The actual data being transmitted is often called the "payload," and it resides between the header and the trailer. This process of adding headers and trailers is called encapsulation.
