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2.5.3.9 Gauge Type Encoding

A Gauge type (also defined in the SMI) is a nonnegative integer that may increase or decrease, but latches at a maximum value of 4,294,967,295. The Gauge is not used frequently. MIB-II defines it for the ifSpeed, ifOutQLen, and tcpCurrEstab objects only. For example, Figure 2-13 assumes that the maximum output queue length of a particular interface is 32 packets. To encode this Gauge value, the Type field is set to 42H (application class, Primitive, Tag = 2). One octet encodes decimal 32; therefore, the Length field = 01H and the Value field contains 20H, the desired value of 32 decimal.

2.5.3.10 TimeTicks Type Encoding

The TimeTicks type (also defined in the SMI) contains a time-stamp that measures the elapsed time (in hundredths of a second) since some event. The sysUpTime object measures the time since the network management entity on a device was reinitialized. If the sysUpTime value for a particular device was 263,691,156 hundredths of a second (about 30 days), its value would be encoded as shown in Figure 2-14. The Type field would be set to 43H (application class, Primitive, Tag = 3). Four octets represent a Value equal to 263691156. Therefore, the Length field contains 04H. The four octets in the Value field contain the binary representation of the TimeTicks value.

2.5.3.11 Context-Specific Encodings for SNMP

The final class of encodings discussed in this chapter are the context-specific encodings, which are used within the context of SNMP. Five protocol data units (PDUs), which Chapter 4 discusses in greater detail, convey SNMP information. The PDUs are GetRequest, GetNextRequest, GetResponse, SetRequest, and Trap. These PDUs have tag numbers of 0 to 4, respectively. These encodings are all context-specific class (10) and constructed (P/C = 1). The Type fields thus have values ranging from A0 to A4H (see Figure 2-15). The Length and Value fields depend on the information conveyed.

2.6 Object Names

Each object, whether it’s a device or a characteristic of a device, must have a name by which it can be uniquely identified. That name is the object identifier. It is written as a sequence of integers separated by periods. For example, the sequence {1.3.6.1.2.1.1.1.0} specifies the system description, within the system group, of the mgmt subtree.


Figure 2-15.  Encoding for the context-specific types used with SNMP

Annexes B, C, D and E of ISO 8824-1 define the numerical sequences; they resemble a tree with a root and several directly attached branches, referred to as children (see Figure 2-16). These branches connect to other branches. You can use the structure of root, branches, subbranches, and leaves to diagram all of the objects within a particular MIB and their relationships.


Figure 2-16.  The root node and ITU-T-assigned OBJECT IDENTIFIER component values

The root does not need a designation, but a specific numeric value designates the three connected arcs, or branches. The International Telecommunications Union - Telecommunications Standards Sector (ITU-T) administers the branch labeled 0, the International Organization for Standardization (ISO) administers the branch labeled 1, and ISO and ITU-T jointly administer the third branch, labeled 2.

The ITU-T branch has five children: recommendation (0) identifies ITU-T recommendations; question (1) is used for ITU-T study groups; administration (2) identifies the values of the X.121 DCCs (Data Country Codes); network-operator (3) identifies the values of the X.121 Data Network Identification codes (DNICs); and identified-organization (4) identifies values assigned by the ITU Telecommunication Standardization Bureau (TSB).

The ISO branch (Figure 2-17) has three children: standard (0) designates international standards; member-body (2) is a three-digit numeric country code that ISO 3166 assigns to each member of ISO/IEC; and identified-organizations (3) have values of an international code designator (ICD), defined in ISO 6523. (Branch (1) was previously assigned to registration -authority (1), but it is no longer in use, per ISO 8834-1.)


Figure 2-17.  The root node and ISO assigned OBJECT IDENTIFIER component values

The U.S. Department of Defense is assigned to one of the children under 1.3, and is designated as 6. On this tree, the Internet community has designation 1.

To identify a particular position on the tree, you list the numeric values in a string, separated by periods. For example, to identify the position of the Internet subtree, you start at the root and move down until you reach position {1.3.6.1}.

At the Internet level (Figure 2-18), you begin to see details germane to network management and SNMP. The Internet subtree has seven branches:

  The directory (1) subtree, {internet 1} or {1.3.6.1.1}, is reserved for future use by the OSI directory within the Internet.
  The mgmt (2) subtree, {internet 2} or {1.3.6.1.2}, is managed by the Internet Assigned Numbers Authority, and includes the standard MIBs.
  The experimental (3) subtree, {internet 3} or {1.3.6.1.3}, is used for Internet experiments.
  The private (4) subtree, {internet 4} or {1.3.6.1.4}, allows vendors to register objects.
  The security (5) subtree, {internet 5} or {1.3.6.1.5} for security-related objects.
  The snmpV2 (6) subtree, {internet 6} or {1.3.6.1.6}, for SNMP version 2 objects.
  The mail (7) subtree, {internet 7} or {1.3.6.1.7}, for mail objects.


Figure 2-18.  Internet assigned OBJECT IDENTIFIER component values

The Internet Assigned Numbers Authority (IANA) administers these subtrees and publishes them in the current Assigned Numbers document (currently RFC 1700). Appendix F excerpts RFC 1700 for your reference.

Chapter 3 discusses various MIBs in detail. This chapter now looks at the structure of trees applicable to the Internet Standard Network Management Framework. The Internet Standard MIB is defined by {mgmt 1} or {1.3.6.1.2.1}. Under this tree are objects defined by MIB-II (RFC 1213), such as the remote network monitoring (RMON) MIB, (RFC 1757), and many others.

Let’s now return to the example given at the beginning of this section. Now that you know the identities of the individual tree structures, we can construct the following sequence:

     internet OBJECT IDENTIFIER ::= {iso org(3) dod(6) 1 }
     mgmt OBJECT IDENTIFIER ::= { internet 2 }
     mib OBJECT IDENTIFIER ::= { mgmt 1 }
     system OBJECT IDENTIFIER ::= { mib-2 1 }
     sysDescr OBJECT IDENTIFIER ::= { system 1 }


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