Part 4: Understanding Packet Types and Structure

Previous Page: Part 3: Understanding the MIB (Management Information Base)
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Let's examine the communication between managers and agents. Basic serial telemetry protocols, like TBOS, are byte-oriented, with a single byte exchanged to communicate. Expanded serial telemetry protocols, like TABS, are packet oriented with packets of bytes exchanged to communicate. The packets contain header, data and checksum bytes. SNMP is also packet oriented with the following SNMP v1 packets (Protocol Data Units or PDUs) used to communicate:

• Get
• GetNext
• Set
• GetResponse
• Trap

The manager sends a Get or GetNext to read a variable or variables and the agent's response contains the requested information if managed. The manager sends a Set to change a variable or variables and the agent's response confirms the change if allowed. The agent sends a Trap when a specific event occurs.

The above illustration shows the packet formats. Each variable binding contains an identifier, a type and a value (if a Set or GetResponse). The agent checks each identifier against its MIB to determine whether the object is managed and changeable (if processing a Set). The manager uses its MIB to display the readable name of the variable and sometimes interpret its value.

Essential SNMP: What is UDP? UDP (User Datagram Protocol) is the IP transport layer protocol that supports SNMP messages. Unlike TCP, UDP is a connectionless protocol. A UDP host places messages on the network without first establishing a connection with the recipient. UDP does not guarantee message delivery, but it's a lightweight protocol that can transport a large number of status messages without using too many network resources.

Reality Check: 4 Signs You Need Protocol Mediation

1. You have a lot of non-SNMP equipment: Before planning your SNMP implementation, do a site survey and find out how much non-SNMP equipment you have in your network. Changing out a large number of non-SNMP devices can add hundreds of thousands of dollars to your project costs. Protocol mediation saves you money by keeping your non-SNMP equipment in place.
2. You want to gradually migrate from your old system: It's time to replace your older system - the master is starting to fail, and it's hard to get new remotes. But you can't afford a forklift swapout of your whole system. Protocol mediation lets you integrate your old remotes to an SNMP manager, so you can replace your legacy system step by step.
3. You inherited someone else's incompatible system: If your company has merged with another, you might find yourself responsible for a whole new network of incompatible equipment. You can't afford to replace this network, but you need to integrate it into your existing operations. Protocol mediation will merge your two networks at minimal expense.
4. You don't want to pay license fees: You may have to pay a separate license fee for every device you monitor with your SNMP manager. If you have a large network, that can get real expensive real fast. A protocol mediator can take input from your whole network and consolidate it to one SNMP input, at only one license fee.

Next Page: Part 5: Understanding Layered Communication
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