1. Definition
In a layered network structure, such as in the Open Systems Interconnection (OSI) model, protocol data units (PDUs) are established at each layer of the transmission system. The PDU contains information from the upper layer and additional information from the current layer’s entities, and it will be transmitted to the next lower layer. The physical layer actually transmits these PDUs in the form of a framed bitstream, which 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 layer of the PDU is assigned to an equivalent layer of another system, which is the process of coordinating a communication session between peer layers. By stripping the header from the transport layer segment, performing protocol data detection to determine the data of the protocol segment that is part of the transport layer segment, and performing flag verification and stripping, the data segment is processed. Simultaneously providing techniques for processing data segments, wherein the header portion of the protocol data unit is received. Use the received header portion 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 peek command to obtain the next header section. Additionally, provide techniques for performing cyclic redundancy check using stored partial cyclic redundancy check digests and remaining data.
2. Classification
SNMP specifies 5 types of protocol data units (PDU) (also known as SNMP messages) for 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: returns one or more parameter values. This operation is initiated by the proxy process and is a response operation to the first three operations.
- Trap operation: A message actively sent by the proxy process to notify the management process that something has happened.
The first three operations are issued by the management process to the proxy process, and the last two operations are issued by the proxy process to the management process. For simplicity, the first three operations will be called get, get next, and set operations in the future.