23.5 SIMPLE NETWORK MANAGEMENT PROTOCOL (SNMP)

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For any network, network management software is extremely important to provide reliable service, to tune the performance of the network, to upgrade the network based on traffic requirements, and to take care of topological changes. Network administrators have to obtain the information about the various network elements such as servers, routers, bridges, and so on. The network administrator should also be able to configure these network elements through simple commands.

Two types of network management are possible: centralized and decentralized, depending on the requirements of the network. In centralized network management, a central station carries out the entire management of the network, whereas in decentralized network management, the management functions are distributed.

Figure 23.4 shows a typical network with various elements for network management. Each subnetwork to be managed will have a management agent. These agents are controlled by a set of managers. These managers in turn will act as agents to a centralized management server. The agents can be different network elements such as a bridge, a router, or an end system. These agents run the agent software and the managers run the manager software.

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Figure 23.4: Network management using SNMP.

The key elements of network management are:

Management station: The management station provides the human machine interface (HMI) to monitor and control the network. It contains the set of applications for data analysis and fault recovery. It provides a graphical user interface to manage the network through simple commands or mouse-clicks. It has the complete database of network management information.

Agents: This is the software residing in the bridges, routers, hosts, and such for remote management. This software will send the necessary data based on the commands from the management station.

Management information base (MIB): Data variables (objects) represent one aspect of a managed agent. The collection of objects is the MIB. For example, for a packet switch, the buffer size, the number of packets dropped, the delay, and so on can be the variables.

Network management protocol: This is SNMP (Simple Network Management Protocol) in TCP/IP networks and CMIP (common management information protocol) in ISO/OSI networks. SNMP runs on the UDP and not TCP. Because the network management information is not of very high priority, UDP is used instead of TCP.

The present version of SNMP running on the Internet is SNMP Version 2. SNMP is capable of three operations: get, set, and notify. Using the get operation, the management station retrieves values of objects at the agent. Using the set operation, the management station sets values of objects at the agent. Using the notify operation, the agent notifies management station of any significant events.

In a network, there will be network management workstations, which collect the data and present it to the users, and SNMP agent packages that run on the elements to be managed. SNMP does not by itself provide network management, it provides the infrastructure for network management. That is, network management applications can be built using the components. Applications such as fault management, performance monitoring, accounting, and traffic analysis are outside the scope of SNMP.

SNMP v2 is used to exchange management information using a set of data types, structures, and so forth. As shown in Figure 23.4, there will be an MIB at each manager/agent that is used to do the network management through application software. An MIB is organized as a hierarchy of objects. The SNMP protocol runs as follows:

The manager issues a get request PDU (protocol data unit) with a list of object names for which values are requested.
The agent will send a response PDU with values of objects. Partial responses are allowed. Error codes are sent if an object name is incorrect.
A get next request PDU is sent if more object values are required. The next value in the hierarchy is sent back by the agent.
A get bulk request PDU (supported by SNMP v2) is used to avoid a large number of data exchanges. A PDU is sent with a request to send information about a set of variables, and the information is returned in one PDU. This strategy reduces the traffic on the network for network management.

SNMP provides the basic framework required to carry out network management by defining how the management information has to be organized and how the information has to be retrieved without dealing with the specific information to be handled. If the network administrator has to obtain the information on the number of packets dropped in a router, the object corresponding to this information has to be created in the router, through a get command from the manager, the information is obtained. Of course, network equipment vendors provide the necessary objects for network management, so the network administrator's job is only to write the graphical user interface to process the information and present it in a user-friendly manner.

The Internet is such a great platform for exciting applications thanks to the TCP/IP protocols and the application layer protocols described in this chapter.

Note

SNMP uses UDP and not TCP to reduce the communication overhead in the networks. Note that SNMP provides only the necessary framework for network management. To generate information such as traffic analysis, fault reports, and such is not in the scope of SNMP— these have to be done by application software.

Summary

This chapter presented the most important application layer protocols: Simple Mail Transfer Protocol (SMTP), Multipurpose Internet Mail Extension (MIME), Hypertext Transfer Protocol (HTTP), Lightweight Directory Access Protocol (LDAP), and Simple Network Management Protocol (SNMP). SMTP facilitates mail transfer but with the restriction that only ASCII text messages can be sent. The MIME protocol enhances the capability of SMTP by supporting multimedia. HTTP is a client/server protocol that allows sending a URL as a request and getting back the resource corresponding to that URL from the origin server. LDAP defines a standard mechanism for storing, retrieving, and modifying directory information. SNMP provides the necessary framework to manage computer networks by defining a simple protocol to obtain management information from network elements such as hosts, servers, routers, and bridges.

References

L.L. Peterson and B.S. Davie. Computer Networks: A Systems Approach, Morgan Kaufmann Publishers, 2000. Chapter 9 of this book deals with application layer protocols.
http://www.apache.org You can obtain the details of Apache HTTP server from this site.
http://www.ietf.org You can download the Requests for Comments (RFCs) referred to in this chapter from this site.
http://www.w3.org Web site of the World Wide Web Consortium.

Questions

Explain the operation of SMTP.
What is the protocol used for supporting multimedia e-mail content? Explain the protocol.
Explain how Web content is accessed through HTTP.
What is LDAP? In what way does it help end users in directory service?
Explain SNMP.

Exercises

1.	Study the FTP protocol used for file transfer.
2.	Interconnect two PCs using an RS232 link and implement SMTP to exchange mail between the two PCs.
3.	What are the problems associated with having proprietary directories? How does LDAP solve these problems?
4.	List some of the standard protocols for file transfer.
5.	List the network management information to be collected to monitor performance as well as to plan expansion.

Answers

1.	You can get the details of FTP from the RFC at http://www.ietf.org.
2.	When you interconnect two PCs using an RS232 link, the data is transferred character by character. You need to write the software to implement SMTP and then send the data via the RS232 port.
3.	When the information in the directory is stored using a proprietary mechanism, it is not possible to share the directory information among devices. For instance, the address directory stored in a digital diary cannot be transferred to the Outlook Express address book because the directory formats are different. LDAP solves this problem by defining the directory structure, the operations to be performed on the directories, and so on.
4.	The standard protocols for file transfer are FTP, TFTP, and FTAM.
5.	The network management software has to collect the status of various network elements to ensure the reliable operation of the network. In addition, traffic data has to be collected. The traffic data can be used to identify the busy routes and plan for expansion.

Projects

Develop an intranet messaging system. On the LAN, the server should have different folders for the users of the LAN. Any user should be able to log on to the server and check the mailbox for received messages and also send messages to others. Effectively, you need to implement a mailing system similar to Hotmail or Yahoo!.
Download Apache Web server software and port it onto your system.
Using an RDBMS package such as MS Access or MS SQL or Oracle, implement LDAP functionality.

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