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Note that some protocols also use a trailer at the end of the payload, such as Ethernet, which includes a trailing checksum.
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When a network protocol belonging to a higher layer is implemented and placed within the data or payload of a lower layer network protocol, the higher layer protocol is said to be encapsulated within the lower layer. As such, a network protocol, if encapsulated is both a payload and a header + data. It is the payload of the layer below it and it is itself a network protocol with separate header and data.
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