What is a protocol data unit (PDU)?In networking, a protocol data unit (PDU) is the basic unit of exchange between entities that communicate using a specified networking protocol. When working with a multilayer protocol stack, like the TCP/IP networking suite, use of the correct PDU is important when discussing protocol interactions. Show
Networking protocols, especially those developed in the framework of a layered reference model, like the seven-layer Open Systems Interconnection (OSI) model or the five-layer TCP/IP model, define their units of data exchange between networked entities. For example, packet is the PDU for IP, which defines interactions between hosts identified by IP addresses at the network layer. When a host is connected to the internet through an Ethernet network, IP packets are incorporated into Ethernet frames to be transmitted between network interface cards. The PDUs most often carried within IP packets are datagrams when using the User Datagram Protocol (UDP) transport protocol or segments when using TCP. These two transport layer protocols mediate communication between processes (programs), and the datagram and segment are the units of exchange between processes running on the network hosts. While the PDU is the unit of exchange between communicating entities, the service data unit (SDU) is the unit of data to be delivered over data networks. The SDU can be thought of as the message to be delivered by the next lower protocol layer, which that lower layer protocol incorporates as the payload of its PDU. What does a PDU contain?The format and function of a protocol's PDU are set in the protocol specification. To be compliant with a protocol specification, a protocol implementation must be able to accept PDUs sent by other compliant implementations, as well as be able to compose PDUs that other implementations accept as valid. The PDU defines what a protocol message should look like, and PDUs typically consist of the following:
Protocols that define specific word lengths -- groupings of multiple bits or bytes used to expedite processing -- may require padding some sections of the header or PDU with bits with values of either zero or one. Many networking protocols call for the processing of network data in 32-bit groups, so if the PDU has a remainder of fewer than 32 bits, it is augmented with padding. How do PDUs work?PDUs enable the interoperable transmission of data across internetworks -- networks of networks -- through the use of encapsulation. Consider the following example:
In general, most of the functions assigned to the upper layers of the OSI model -- application layer, presentation layer and session layer -- are incorporated in the application layer of the TCP/IP model. Most application layer protocols use messages as their PDUs. For simplicity, the focus in this example is on how the file is sent from the server to Alice's client using the lower layers of the OSI model: the transport, network, data link and physical (hardware) layers. The file is passed into the lower layers of the protocol stack for transmission over the necessary data networks that separate Alice from the remote server. The process proceeds as follows:
The encapsulation process is reversed, known as decapsulation, when it arrives at a destination. When an Ethernet frame is sent to a local router, the router only needs to decapsulate the frame to determine the next internet layer hop. Once the packet arrives at Alice's host, the process is reversed:
This explanation is simplified for clarity. For example, most IP packets are no longer than 1,500 octets (bytes) because that is the maximum transmission unit length for many Ethernet networks. IP packets larger than that are likely to need to be fragmented during their internet transit, which is undesirable because it can slow transmissions significantly. PDUs for commonly used protocolsAlthough internet transmissions are all contained in IP packets, those packets also usually encapsulate other protocols, such as those used in the transport layer or the application layer. Further, IP packets are, in turn, encapsulated in data link layer protocols. The IP packet is an SDU for LAN technologies, such as those used in Ethernet or asynchronous transfer mode (ATM) networks. Commonly used PDUs are listed in the following table.
Multilayer protocols stacks use different protocols to communicate at different layers, all while enabling interoperability between systems using different software applications, operating systems and physical network types -- all capable of interoperating across geographic and technological boundaries. Find out more about how the TCP/IP and the OSI reference models differ. This was last updated in May 2022 Continue Reading About protocol data unit (PDU)
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Which section in an Ethernet frame contains the data from higher layers such as Internet Protocol?Each Ethernet frame starts with an Ethernet header, which contains destination and source MAC addresses as its first two fields. The middle section of the frame is payload data including any headers for other protocols (for example, Internet Protocol) carried in the frame.
Which section in an Ethernet frame contains the data from higher layers such as Internet Protocol IP and the transport and application layers preamble ethertype payload?The payload is also referred to as data.
Which part of an Ethernet frame describes the higher layer protocol that is encapsulated?If there is a match, the device accepts the frame. It can be a unicast, multicast, or broadcast address. This 6-byte field identifies the originating NIC or interface of the frame. This 2-byte field identifies the upper-layer protocol encapsulated in the Ethernet frame.
What are the contents of the data field in an Ethernet frame?In an Ethernet network, devices share data using packets. They contain, among other things, the Ethernet frame, which is divided into several data sets. These records consist of binary code that provides important information, such as addresses, control information, payload data, and checksums.
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