Previous | Table of Contents | Next |
This and the next three chapters explore the theory of network management and the components of version 1 of the Internet Network Management Framework. In this chapter, youll learn about the structure of management information (SMI), which defines the rules for identifying managed objects. SMI is described in RFC 1155 [2-1] and refined in RFC 1212 and RFC 1215. Chapter 3 discusses the management information bases (MIBs), examined in RFC 1213 [2-2], that describe the managed objects themselves. Chapter 4 discusses SNMP, which defines the mechanism by which managers and agents communicate, and is described in RFC 1157 [2-3].
We will revisit the SMI, MIB, and SNMP in Chapter 5 to compare and contrast SNMP with its intended successor, SNMP version 2, which is part of version 2 of the Internet Network Management Framework. Since this entire framework has been enhanced, there are changes to all elements of that framework: the SMI, the MIB, and the protocols. The migration from version 1 to version 2 has been a rather slow process, however, so it is very likely that SNMP version 1-based devices will be in service for some time. Therefore, if you are primarily interested in SNMP version 1, study Chapters 2, 3, and 4. To learn more about the enhancements with SNMP version 2, study Chapter 5. It is recommended that the section on SNMP version 1 be studied before moving to the section on version 2, because many of the underlying principles of version 2 assume a working knowledge of version 1.
For clarity, we will use the term SNMP to refer to SNMP version 1, and the term SNMPv2 to refer to SNMP version 2.
Chapter 6 studies the underlying communication protocols, such as the User Datagram Protocol (UDP) and the Internet Protocol (IP), that transport SNMP messages between the manager and agents. Chapter 7 puts the entire architecture together by presenting case studies that demonstrate real-world use of SNMP.
In the manager/agent paradigm for network management, managed network objects must be physically and logically accessible. The term physically accessible means that some entity must physically check the address, count the packets, or otherwise quantify the network management information. Logical accessibility means that management information must be stored somewhere and, therefore, that the information must be retrievable and modifiable. (SNMP actually performs the retrieval and modification.) The structure of management information (SMI) (RFC 1155 [2-1]) organizes, names, and describes information so that logical access can occur.
The SMI states that each managed object must have a name, a syntax, and an encoding. The name, an object identifier (OID), uniquely identifies the object. The syntax defines the data type, such as an integer or a string of octets. The encoding describes how the information associated with the managed objects is serialized for transmission between machines.
This chapter discusses how the SMI applies to SNMP. It begins by looking at the syntax (the Abstract Syntax Notation One, ASN.1, in Sections 2.3 and 2.4), the encoding (the Basic Encoding Rules, examined in Section 2.5), and finally the names (the object identifier, discussed in Section 2.6). In this way, the discussion moves from the abstract to the practical. Chapter 3 discusses how the MIBs use these names.
In terms of the ISO/OSI model [2-4], the ASN.1 syntax is a Presentation-layer (layer 6) function. Recall that the Presentation layer defines the format of the data stored within a host computer system.
In order for managers and agents to exchange data, both must understand it, regardless of the way either machine represents data internally. For this to occur, two items must be standardized: the abstract syntax and the transfer syntax. The abstract syntax defines specifications for data notation. The transfer syntax defines (transmittable) encodings for the elements in the abstract syntax.
The Internet SMI specifies that ASN.1 define the abstract syntax for messages; that is, ASN.1 defines the basic language elements and provides rules for combining elements into messages. The Basic Encoding Rules (BER) provide the transfer syntax. The BER are associated with the abstract syntax and provide bit-level communication between machines. Thus the SMI and SNMP use the ASN.1 formalizations (ISO 8824-1 [2-5]) and BER (ISO 8825-1 [2-6]) to define various aspects of the Internet network management framework. The specifics of ASN.1 are discussed in the next section.
Network administrators often criticize ASN.1 for its complexity. Some of their criticisms are fair: it is quite difficult to interpret the standard. However, ASN.1 has a straightforward objective. It is designed to define structured information (messages) in a machine-independent (or host-independent) fashion. To do this, ASN.1 defines basic data types, such as integers and strings, and new data types that are based on combinations of the basic ones. The BER then define the way the data is serialized for transmission.
ASN.1 defines data as a pattern of bits in computer memory, just as any high-level computer programming language defines data that the language manipulates as variables. The BER define a standard way to convert ASN.1 definitions into bit patterns for transmission, and then they actually transfer the data between computers. The BER are necessary because the ASN.1 description is human-readable and must be translated differently for each type of computer. The BER representation, however, is always the same for any ASN.1 description, regardless of the computers that send or receive that information. This assures communication between machines, regardless of their internal architecture.
The objective here is to describe ASN.1 to the level of detail necessary to apply it to network management and SNMP. (SNMP uses a subset of ASN.1 for the sake of simplicity.) For additional information, refer to Douglas Steedmans Abstract Syntax Notation One (ASN.1), the Tutorial and Reference [2-7] or Motteler and Sidhus Components of OSI: Abstract Syntax Notation One (ASN.1) [2-8].
ASN.1 uses some unique terms to define its procedures, including type definitions, value assignments, macro definitions and evocations, and module definitions. You need to understand these terms before the discussion can proceed. Moreover, ASN.1 specifies some words as keywords, or reserved character sequences. Keywords, such as INTEGER, OBJECT, and NULL, have special meanings and appear in uppercase letters.
Previous | Table of Contents | Next |