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From Citizendium, the Citizens' Compendium

In current usage, a packet is a unit of information transferred through a network by routers or virtual circuit switches, which forward the unit out whichever of their interfaces will take it one "hop" closer to the destination specified by the destination address in the packet header.

Packets, in routing systems, contain the complete information for the router both to forward them, and send error messages back to the sender. Thus, while a router does need to have a forwarding information base of destinations, it need not maintain knowledge (i.e., state) of the history of individual packets, or the context (e.g., a telephone call between a calling and called party.

In virtual circuit technologies, such as MPLS, Asynchronous Transfer Mode, and conventional telephony switching, the intermediate switching devices have to maintain state of a connection between the calling and called parties. It needs to look up, from a short address in each packet, the identity of the connection, the destination of that connection, and then look up the destination in the forwarding information base. This requires more processing at each switch.