In this article in our series, we examine the Simple Network Management Protocol (SNMP). This article will focus specifically on the layered communication model used to exchange information. Our last article focused on the structure of SNMP messages, however an SNMP message is not sent by itself. It is wrapped in the User Datagram Protocol (UDP), which in turn is wrapped in the Internet Protocol (IP). These are commonly referred to as layers and are based on a four-layer model developed by the Department of Defense (you may recall the DoD origins of the Internet).
SNMP resides in what is called the Application layer, UDP resides in the Transport layer and IP resides in the Internet layer (somewhat obvious). The fourth layer is the Network Interface layer where the assembled packet is actually interfaced to some kind of transport media (i.e., twisted pair copper, RG58 co-axial or fiber). While this multi-layer model may seem a bit confusing, it effectively isolates the tasks of communication and ultimately assists in designing and implementing a network.
Traversing the Layers.
To illustrate the function of this layered model, let's look at a single SNMP GET request from the agent's perspective. The SNMP manager wants to know what the Agent's System Name is and prepares a GET message for the appropriate OID. It then passes the message to the UDP layer. The UDP layer adds a data block that identifies the manager port to which the response packet should be sent. It also picks the port on which it expects the SNMP agent to be listening for messages. The packet thus formed is then passed to the IP layer. Here a data block containing the IP and Media Access addresses of the manager and the agent is added. Then, the entire assembled packet gets passed to the Network Interface layer. The Network Interface layer verifies media access and availability and places the packet on the media for transport.
After working its way across bridges and through routers (the modern equivalent of over the rivers and through the woods) based on the IP information, the packet finally arrives at the agent. Here it passes through the same four layers in exactly the opposite order as it did at the manager. First, it is pulled off the media by the Network Interface layer. After confirming that the packet is intact and valid, the Network Interface layer simply passes it to the IP layer. The IP layer verifies the Media Access and IP address and passes it on to the UDP layer where the target port is checked for joined applications. If an application is listening at the target port, the packet is passed to the Application layer. If the listening application is the SNMP agent, the GET request is processed as we have discussed in previous articles. The agent response then follows the identical path in reverse to reach the manager.
An Aid for Troubleshooting.
Understanding this layered model makes it easier to troubleshoot communication problems. When there is a problem, you can simply trace it down, out one end, into, and up the other. LAN/WAN link and activity status indicators provide some visibility to the Network Interface layer. ICMP echo requests and responses (Pings) provide some information regarding the proper working of the IP layer. SNMP processing indicators can be used to verify the passage of the packet through the UDP layer and the working of the Application layer. Each step can be verified independently until all steps are working correctly for end-to-end communication.
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