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1.7 The Internet Network Management Framework

The three network management architectures discussed so far have one common denominator: their design was a formal process involving participants worldwide. More importantly, the network management architecture was put in place first, and vendors were then encouraged to implement it.

The Internet, however, was an operational worldwide internetwork, so its administrators couldn’t study network management for years before coming up with an implementation.

The Internet was derived from U.S. Government research that began in 1969. The objective was to develop communication technologies and protocols so that government organizations, defense contractors, and academic researchers using dissimilar computer systems could collaborate on projects. The result of the government’s research project was the Advanced Research Projects Agency network (ARPANET), which used packet switching technology to connect dissimilar systems. ARPANET came on-line in 1969 with nodes in four locations in the United States. From that humble beginning, the Internet now connects more than one million host computers worldwide.

By the late 1980s, the Internet Activities Board (IAB) realized that it needed a method to manage the growing Internet and the other attached networks. The board considered three proposals: the high-level entity management system (HEMS) [1-9]; an OSI-based system, utilizing CMIS and CMIP; and extensions to the existing Simple Gateway Monitoring Protocol (SGMP) [1-10] that the regional networks that made up the Internet were using.

The IAB decided to take a two-step approach to Internet management. Enhancements to the SGMP, which became known as the Simple Network Management Protocol (SNMP), would provide a short-term solution. The long-term solution would be based on the CMIS/CMIP architecture, and was called CMOT (CMIP over TCP/IP). RFC 1052 [1-11] summarizes these directives.

(Documentation for the Internet and its protocols is based on the Request for Comments (RFC) documents. Upon publication, these documents are given a number, such as 1052, that is used for identification. This text will make numerous references to RFCs. Appendix D gives complete details on how to obtain the RFCs and other pertinent Internet documentation.)

The long-term CMOT solution, however, has never received the widespread acceptance of SNMP, though it is still discussed with some network management architectures. CMOT is currently designated “historic,” meaning that more recent standards have superseded it or it is considered obsolete for some other reason. Nevertheless, Section 1.7.2 will discuss CMOT briefly.

1.7.1 SNMP, the Simple Network Management Protocol

SNMP is based on the manager/agent model (see Figure 1-4). SNMP is referred to as “simple” because the agent requires minimal software. Most of the processing power and data storage resides on the management system, while a complementary subset of those functions resides on the managed system.

To achieve its goal of being simple, SNMP includes a limited set of management commands and responses (see Figure 1-11a). The management system issues Get, GetNext, and Set messages to retrieve single or multiple object variables or to establish the value of a single variable. The managed system sends a Response message to complete the Get, GetNext, or Set. The managed system sends an event notification, called a trap, to the management system to identify the occurrence of conditions such as a threshold that exceeds a predetermined value.


Figure 1-11a.  SNMP architecture (©1990, IEEE)

SNMP assumes that the communication path is a connectionless communication subnetwork. In other words, no prearranged communication path is established prior to the transmission of data. As a result, SNMP makes no guarantees about the reliable delivery of the data; however, in practice most messages get through, and those that don’t can be retransmitted. Reviewing Figure 1-11a, the primary protocols that SNMP implements are the User Datagram Protocol (UDP) and the Internet Protocol (IP). SNMP also requires Data Link layer protocols, such as Ethernet or token ring, to implement the communication channel from the management to the managed system.

SNMP’s simplicity and connectionless communication also produce a degree of robustness. Neither the manager nor the agent relies on the other for its operation. Thus, a manager may continue to function even if a remote agent fails. When the agent resumes functioning, it can send a trap to the manager, notifying it of its change in operational status.

SNMP is defined in RFC 1157 [1-12]. For more information, refer to references [1-13] and [1-14].

1.7.2 CMIP over TCP/IP (CMOT)

As a result of its study of various Internetwork management strategies, in 1990 the IAB developed a strategy to implement CMOT [1-15]. Although the Internet Engineering Task Force (IETF, the standards setting body of the Internet) has designated this work as “historic,” a number of vendors have included CMOT in their architecture plans. Therefore, a brief discussion is in order.

Architecturally, CMOT fits the manager/agent paradigm (see Figure 1-11b). Unlike SNMP, which provides connectionless service using UDP/IP, however, CMOT uses an association-oriented communication mechanism and the TCP/IP protocol to assure reliable transport of data. To guarantee reliable transport, CMOT systems establish Application layer connections prior to transmitting management information. CMOT’s Application layer services are built on three OSI services: the Common Management Information Service Element (CMISE), the Remote Operation Service Element (ROSE), and the Association Control Service Element (ACSE). A Lightweight Presentation Protocol (LPP) provides Presentation layer services.


Figure 1-11b.  CMOT architecture (©1990, IEEE)

The CMOT model is also more rigorous than SNMP. Ben-Artzi’s paper [1-14] notes that the CMIP object definitions are more comprehensive and include attributes, events emitted, and imperative actions. As we will discuss, SNMP primarily defines objects by functional groups.

Nonetheless, SNMP is here today, and many vendors of both agent and management systems implement SNMP. As testimony to its widespread use, SNMP continues to be enhanced. In April 1993, RFCs 1443 to 1452 defined SNMP version 2. These RFCs were subsequently revised in January 1996, and published as RFCs 1901 to 1910. Chapter 5 will study the details of these enhancements.


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