Packet framing

A packet consists of two kinds of data control information and user data also known as payload. The control information provides data the network needs to deliver the user data, for example: source and destination addresses, error detection codes like checksums, and sequencing information. Typically, control information is found in packet headers and trailers, with user data in between.

Different communications protocols use different conventions for distinguishing between the elements and for formatting the data. In Binary Synchronous Transmission, the packet is formatted in bit bytes, and special characters are used to delimit the different elements. Other protocols, like Ethernet, establish the start of the header and data elements by their location relative to the start of the packet. Some protocols format the information at a bit level instead of a byte level.

A good analogy is to consider a packet to be like a letter: the header is like the envelope, and the data area is whatever the person puts inside the envelope. A difference, however, is that some networks can break a larger packet into smaller packets when necessarynote that these smaller data elements are still formatted as packets.A network design can achieve two major results by using packets: error detection and multiple host addressing.It is more efficient and reliable to calculate a checksum or cyclic redundancy check over the contents of a packet than to check errors using character-by-character parity bit checking.The packet trailer often contains error checking data to detect errors that occur during transmission.
Modern networks usually connect three or more host computers together; in such cases the packet header generally contains addressing information so that the packet is received by the correct host computer. In complex networks constructed of multiple routing and switching nodes, like the ARPANETand the modern Internet, a series of packets sent from one host computer to another may follow different routes to reach the same destination. This technology is called packet switching.
In general, the term packet applies to any message formatted as a packet, while the term datagram is generally reserved for packets of an "unreliable" service. A reliable service is one that notifies the user if delivery fails, while an "unreliable" one does not notify the user if delivery fails. For example, IP provides an unreliable service. Together, TCP and IP provide a reliable service, whereas UDP and IP provide an unreliable one. All these protocols use packets, but UDP packets are generally called datagrams. When the ARPANET pioneered packet switching, it provided a reliable packet delivery procedure to its connected hosts via its. A host computer simply arranged the data in the correct packet format, inserted the address of the destination host computer, and sent the message across the interface to its connected IMP. Once the message was delivered to the destination host, an acknowledgement was delivered to the sending host. If the network could not deliver the message, it would send an error message back to the sending host.