Datagram

In simple 1594 terminology, a datagram is a self-contained entity of data that holds information to be forwarded to a specific destination. A datagram does not depend on previous exchanges between the transmitting source and the destination source. A datagram must be self-contained without reliance on other prior data exchanges because there is no connection of fixed time in between the two communicating points. This term is still used in common communication protocols such as UDP (User Datagram Protocol) and AppleTalk. It’s modern term, packet, is used in IP (Internet Protocol) and other communication standards related to the internet. IP provides an unreliable service and relies on TCP (Transmission Control Protocol) to provide reliable service - UDP uses IP to provide a reliable service. While all of these protocols use the packet term, UDP still tends to call them datagrams. Often associated with the same definition, a packet is essentially a collection of data formatted in a single unit while a datagram typically refers to packets of an unreliable service.