Protocol data unit

In telecommunications, a protocol data unit (PDU) is a single unit of information transmitted among peer entities of a computer network. It is composed of protocol-specific control information and user data. In the layered architectures of communication protocol stacks, each layer implements protocols tailored to the specific type or mode of data exchange.

For example, the Transmission Control Protocol (TCP) implements a connection-oriented transfer mode, and the PDU of this protocol is called a segment, while the User Datagram Protocol (UDP) uses datagrams as protocol data units for connectionless communication. A layer lower in the Internet protocol suite, at the Internet layer, the PDU is called a packet, irrespective of its payload type.

In the context of packet switching data networks, a protocol data unit (PDU) is best understood in relation to a service data unit (SDU).

The features or services of the network are implemented in distinct layers. The physical layer sends ones and zeros across a wire or fiber. The data link layer then organizes these ones and zeros into chunks of data and gets them safely to the right place on the wire. The network layer transmits the organized data over multiple connected networks, and the transport layer delivers the data to the right software application at the destination.^[1]

Protocol data units of the OSI model are:^[2]

• The Layer 4: transport layer PDU is the segment.
• The Layer 3: network layer PDU is the packet or the datagram.^[3]
• The Layer 2: data link layer PDU is the frame.
• The Layer 1: physical layer PDU is the bit or, more generally, symbol.

Given a context pertaining to a specific OSI layer, PDU is sometimes used as a synonym for its representation at that layer.

Protocol data units for the Internet protocol suite are:

• The transport layer PDU is the TCP segment for TCP, and the datagram for UDP
• The Internet layer PDU is the packet.
• The link layer PDU is the frame.

On TCP/IP over Ethernet, the data on the physical layer is carried in Ethernet frames.

• For IPX, the IPX packet is the network layer PDU for IPX, and can simultaneously be used in the transport layer as well.
• The SPX packet on top of IPX, is a transport layer PDU.
• The link-layer PDU is the frame.

The data link layer PDU in Asynchronous Transfer Mode (ATM) networks is called a cell.

A media access control protocol data unit (MAC PDU or MPDU) is a message that is exchanged between media access control (MAC) entities in a communication system based on the layered OSI model.^[4]

In systems where the MPDU may be larger than the MAC service data unit (MSDU), the MPDU may include multiple MSDUs as a result of packet aggregation. In systems where the MPDU is smaller than the MSDU, then one MSDU may generate multiple MPDUs as a result of packet segmentation.^[4]

• Distributed Interactive Simulation § Protocol data units
• IP fragmentation
• Smart card application protocol data unit
• Transaction Protocol Data Unit

• Alani, Mohammed M. (2014). Guide to OSI and TCP/IP Models. SpringerBriefs in Computer Science. doi:10.1007/978-3-319-05152-9. ISBN 978-3-319-05151-2. 
• Wetteroth, Debbra (27 November 2001) (in English). OSI Reference Model for Telecommunications (Illustrated ed.). McGraw-Hill Professional Publishing. ISBN 978-0-07-138041-6. 

• comp.protocols.iso FAQ (search for "PDU")
• protocol data unit (PDU)

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