What Is a Protocol Data Unit?

A protocol data unit refers to a layered network structure, for example, in the Open System Interconnection (OSI) model, a protocol data unit (PDU) will be established at each layer of the transmission system.

A protocol data unit refers to a layered network structure, for example, in the Open System Interconnection (OSI) model, a protocol data unit (PDU) will be established at each layer of the transmission system.
Chinese name
Protocol data unit
Foreign name
Protocol Data Unit
Brief introduction
Refers to a hierarchical network structure
Abbreviation
PDU

Protocol data unit definition

In layered network structures, such as in the Open Systems Interconnection (OSI) model, protocol data units (PDUs) will be established at each layer of the transmission system. The PDU contains the information from the upper layer and the entity additional information of the current layer. This PDU will be transmitted to the next lower layer. The physical layer actually transmits these PDUs in the form of a framed bitstream, and these PDUs are built by higher layers of the protocol stack. The receiving system transmits these packets from bottom to top through the protocol stack, and separates the relevant information in the PDU at each layer of the protocol stack.
The information attached to each PDU by each layer is assigned to the peer layer of another system. This is the process of coordinating a communication session by the peer layer. The data segment is processed by stripping the header from the transport layer segment, performing protocol data detection to determine the data of the protocol segment as part of the data of the transport layer segment, and performing flag verification and stripping. Techniques are also provided for processing data segments in which a header portion of a protocol data unit is received. The received header portion is used to determine the number of bytes of data to be stored in the application space. Moreover, the received header portion is used to determine the next header portion of the next protocol data unit. Then, issue a peep command to get the next header part. Techniques are also provided for performing a cyclic redundancy check using the stored partial cyclic redundancy check summary and the remaining data.
Protocol data unit

Protocol Data Unit Classification

SNMP specifies five types of protocol data unit PDUs (ie, SNMP messages), which are used to exchange between management processes and agents.
  • get-request operation: extract one or more parameter values from the proxy process
  • get-next-request operation: extract the next parameter value immediately following the current parameter value from the proxy process
  • set-request operation: set one or more parameter values of the proxy process
  • get-response operation: One or more parameter values returned. This operation is issued by the agent process, it is the response operation of the first three operations.
  • Trap operation: A message sent by the agent process to notify the management process that something happened.
The first three operations are issued by the management process to the agent process, and the latter two operations are sent by the agent process to the management process. For simplicity, the first three operations will be called get, get-next, and set operations in the future.
Note: The agent process uses the well-known port 161 to receive get or set messages, and the management process uses the well-known port 162 to receive trap messages.

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