Data frames, packets, data segments

The data and control information in each layer of the OSI Reference Model is transmitted in many formats, including frames, packets, datagrams, segments, messages, elements, and data units. The information exchange occurs between the peer OSI layers, where the control information is attached to the data at each layer of the source machine, and each layer of the destination machine is analyzed with the information received, and the control information is removed from the data, the following is a description of each information unit:

Data frame: is an information unit whose starting and destination points are data link layers.
Packet (Packet): is also a unit of information, its starting and destination is the network layer.
Datagram (Datagram): typically refers to a unit of information at the starting point and destination using a network layer with no connected network service.
Segment (Segment): usually refers to the starting point and destination are the information units of the transport layer.
Message: A unit of information that is the starting point and destination that are above the network layer (often at the application level).

An element (cell) is a fixed-length information whose origin and destination are data-link layers. Elements are commonly used in exchange environments such as asynchronous transfer Mode (ATM) and switched multi-gigabit Data Service (SMDS) networks.

Data unit refers to a number of information units. The common data unit has the Service data unit (SDU), the Protocol Data Unit (PDU). SDU is the transmission of information between two layers on the same machine. The PDU is the sending of information on each layer of the transmitting machine to the corresponding layer on the receiving machine (for the same level of communication).

Packet (Packet): The basic unit of encapsulation, which traverses the decomposition surface of the network layer and the data link layer. Usually a packet is mapped to a frame, but there are exceptions: when the data link layer performs a split or a few packet to compose a frame.

The PDU on the data link layer is called frame;
The PDU on the network layer is called packet (packet);
TCP is called segment (data segment);
UDP is called datagram. (datagram)--a transmission unit (for example, IP) in the network layer. A datagram may be encapsulated into one or several packets, which is transmitted in the data link layer.

Data frames, packets, data segments