Of Schema and Metaschema

As mentioned in the previous section, the model that underlies the management information in a MIB is specified in a MIB definition. Some people call the model the schema, reminiscent of a database schema that constitutes the definition of the database tables. The underlying "real world" that is being abstracted by the model is often called the domain because it constitutes the "subject domain" that the model is all about. In the example of the previous section, the domain of the model is that of TCP connections.

During runtime, the schema is instantiated in the device's MIB. For example, a specific MIB might contain at a certain point in time 18 TCP connection endpoints. Each of those TCP connection endpoints has particular values for the properties that are reflected in the MIB. For example, the MIB might contain the following management information about one of the TCP connection endpoints: TCP port number 189, the remote endpoint's IP address 247.168.3.17, the remote endpoint's port number 188, and the information that at this particular instance 452,895 packets have been sent and 38,657 packets have been received.

The schema that underlies the MIB remains constant over time. Regardless of when you ask the device, its MIB always represents information about TCP connections the same way, although, of course, the current values will vary. The information also is represented the same way in any other device, provided that it implements the same schema. In object-oriented parlance, the schema is the class, the MIB is the instance—ignoring for a moment that the schema need not be object-oriented. In fact, if the schema is object-oriented, it contains definitions of managed object classes, whereas the MIB contains managed objects that are instances of those managed object classes. The difference between the schema and its instantiation in a MIB is the same as the difference between a "BMW 3 series, 1996 model" and "the 3 series BMW with California license plate 3NAW875 and VIN# 1BAL44P4W9R355280, odometer reading 85667 and a dent on the left side of the rear bumper," or between "a penguin" and "my penguin, named Walter."

Confusingly, in network management, often both the schema and the particular instance on the device are called MIBs. In many cases, it is clear from the context what is meant, but sometimes it is not. As mentioned, a cleaner use of terms would be MIB (for the instance information) and MIB definition (for the model or schema).

Now we have established where the information in a MIB is defined, but an important part is still missing. The MIB definition itself needs to be specified using some specification language, sometimes also referred to as the metaschema. The term metaschema means "a schema of a schema," a definition of how to write and interpret model definitions. Figure 6-5 depicts the relationship between schema and metaschema, and model and domain.

Figure 6-5 Schema, Metaschema, Model, Domain, and MIB

Figure 6-5 Schema, Metaschema, Model, Domain, and MIB

Mib Schema Metaschema

Quite a few MIB specification languages exist. Each of those languages is generally used to define MIBs that are to be used in conjunction with a particular management protocol; for example, the following:

■ SMI and SMIv2 (Structure of Management Information versions 1 and 2), the MIB specification language that is used in conjunction with SNMP

■ Managed Object Format (MOF), a specification language that is used in conjunction with a management technology called Common Information Model (CIM)

■ Guidelines for the Definition of Managed Objects (GDMO), used in conjunction with the Common Management Information Protocol (CMIP), today of only limited commercial relevance

Perhaps surprisingly, given the popularity of the Web and web services, at this point in time, there is no well-established MIB specification language that is based on XML. However, there are quite a few proprietary management interfaces that are based on XML and have management information represented as XML documents. Some industry consortia—notably, the DSL Forum—have defined management information in XML for certain market segments. In addition, Netconf (discussed in Chapter 8) is an emerging management protocol standard that uses XML. Given these trends and the popularity of XML, it seems likely that standardized XML-based management metaschemata will emerge—for example, standard XML Schema Definitions (XSDs).

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