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In telecommunications and computer networking, a network packet is a formatted unit of data carried by a packet-switched network. A packet consists of control information and user data; the latter is also known as the payload. Control information provides data for delivering the payload (e.g., source and destination network addresses, error detection codes, or sequencing information). Typically, control information is found in packet headers and trailers.
A network packet is the fundamental building block for packet-switched networks. When an item such as a file, e-mail message, voice or video stream is transmitted through the network, it is broken into chunks called packets that can be more efficiently moved through the network than one large block of data. Numerous standards cover the structure of packets, but typically packets are composed of three elements:
Header – contains information about the packet, including origin, destination, length and packet number.
Payload (or body) – contains the data that comprises the packet
Trailer – indicates the end of the packet and frequently include error detection and correction information
In a packet-switched network, the sending host computer packetizes the original item and each packet is routed through the network to its destination. Some networks used fixed length packets, typically 1024 bits, while others use variable length packets and include the packet length in the header.
Individual packets may take different routes to the destination and arrive at the destination out of order. The destination computer verifies the correctness of the data in each packet (using information in the trailer), reassembles the original item using the packet number information in the header, and presents the item to the receiving application or user.
This basic example includes the three most fundamental packet processing functions, packetization, routing, and assembly. Packet processing functions range from the simple to highly complex.