**Network Management Based on TCP/IP Consists of Three Components:**
1) A **Management Information Base (MIB)**. The MIB contains all the parameters of all agent processes that can be queried and modified. RFC 1213 [McCloghrie and Rose, 1991] defines the second edition of the MIB, known as MIB-II.
2) A **common set of structures and representation symbols** for the MIB. This is called the **Structure of Management Information (SMI)**, defined in RFC 1155 [Rose and McCloghrie, 1990]. For example, the SMI defines a "counter" as a non-negative integer with a range from 0 to 4,294,967,295. When the maximum value is reached, it resets to 0.
3) The **communication protocol** between the management process and the agent process is called the **Simple Network Management Protocol (SNMP)**, defined in RFC 1157 [Case et al., 1990]. SNMP includes the format of datagram exchange. Although various transport protocols can be used, UDP is most commonly used in SNMP.
**I. Overview of the SNMP Protocol**
The Simple Network Management Protocol (SNMP) is a set of network management protocols defined by the Internet Engineering Task Force (IETF). It is based on the Simple Gateway Monitor Protocol (SGMP). With SNMP, a management station can remotely manage all network devices that support this protocol, including monitoring network status, modifying device configurations, and receiving alerts about network events. Although originally developed for IP-based networks, SNMP has also been successfully applied in telephone network management as an industry standard.
**II. History of SNMP Development**
SNMP has evolved through several versions over time. The most widely used version today is SNMPv2.
In 1989, the first version, SNMPv1, was released. In 1991, RMON (Remote Network Monitoring) was introduced as a supplement to SNMP, extending its capabilities for managing LANs and attached devices. However, RMON does not modify the SNMP protocol or SMI itself; it enhances SNMP's ability to monitor subnets.
In 1993, an upgraded version of SNMPv1, known as SNMPv2, was introduced. It was officially released in 1995, adding new functions and specifying how SNMP could be used in OSI-based networks. That same year, RMON was expanded into RMONv2.
In 1998, SNMPv3 was released, introducing enhanced security features and defining the structure for future improvements. SNMPv3 can work with both SNMPv1 and SNMPv2.
**III. Working Principle of SNMP**
SNMP operates using a client/server model, specifically the **agent/management station model**. Network management is carried out through interactions between the management workstation and the SNMP agent. Each SNMP agent is responsible for responding to queries from the management station regarding MIB-defined information.
The management station and the agent use the MIB as a unified interface. The MIB defines managed objects in the device. Both the management station and the agent implement these MIB objects, allowing them to recognize each other’s data and communicate effectively. The management station requests data from the agent, which then converts the relevant state or parameter data from the managed device into the format defined by the MIB before returning it.
**IV. SNMP Message Types**
Five types of messages are defined in SNMP: **Get-Request**, **Get-Response**, **Get-Next-Request**, **Set-Request**, and **Trap**.
- **Get-Request**: Used by the management station to retrieve information from an SNMP agent.
- **Get-Response**: Sent by the agent in response to a Get-Request.
- **Get-Next-Request**: Used to query the next element in a table.
- **Set-Request**: Allows the management station to configure network devices remotely.
- **Trap**: Sent by the agent to the management station to notify about specific events, such as interface changes or IP address updates.
Among these, **Get-Request**, **Get-Next-Request**, and **Set-Request** are sent from the management station to port 161 of the agent. **Get-Response** and **Trap** are sent from the agent to the management station, with **Trap** going to port 162. All data is encapsulated in UDP.
**V. SNMP Message Format**
SNMP agents and management stations communicate via standardized messages. Each message is a separate datagram. SNMP uses **UDP** (User Datagram Protocol) at Layer 4 for connectionless communication. An SNMP message consists of two parts: an **SNMP header** and a **Protocol Data Unit (PDU)**.
In actual network environments, the length of an SNMP packet depends on the encoding method used. SNMP employs **BER (Basic Encoding Rules)** for encoding, and **ASN.1** syntax is used in official specifications. These concepts will be explored further in practice, so for now, just understanding their basic function is sufficient.
BER encoding rules define how ASN.1 objects are encoded as bitstreams for network transmission. They consist of three main components:
SNMP defines several data types, some of which have changed between versions. Here are some common types:
- **INTEGER**: A signed integer.
- **OCTET STRING**: A sequence of 0 or more 8-bit bytes.
- **DisplayString**: A string of ASCII characters, up to 255 characters in MIB-II.
- **NULL**: Indicates no value.
- **IpAddress**: A 4-byte OCTET STRING representing an IP address.
- **PhysicalAddress**: A 6-byte OCTET STRING for MAC addresses.
- **Counter**: A non-negative integer that increments up to 2^32 - 1 and then wraps around.
- **TimeTicks**: A counter incremented in 0.01-second intervals.
- **SEQUENCE**: Similar to a C structure.
- **SEQUENCE OF**: A vector type.
SNMP packets are encapsulated in UDP packets, which are then transmitted over IP networks. This results in a complete packet structure, as shown below:
The PDU type includes two bytes: the first indicates the real PDU type, and the second specifies the total number of bytes in the subsequent message. For SNMPv1, the PDU types include:
- **GetRequest**
- **GetNextRequest**
- **GetResponse**
- **SetRequest**
- **Trap**
Capturing and analyzing SNMP messages using tools like Wireshark helps understand their structure and behavior.
Analyzing captured SNMP messages against the previous structure provides deeper insights.
**SNMPv2 Trap Packet**
The format of an SNMPv2 trap packet is similar to normal SNMP request and response packets. Except for the trap type and packet length, which are in network byte order, all other fields are BER-encoded.
The original content of an SNMPv2 trap message is structured as follows:
To simplify analysis, we label each part as:
- **XX label type**
- **XX label length**
- **XX label real data**
This makes it easier to interpret raw data.
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