4.1.6. Mapping of the INDEX clause

The INDEX clause, which may be present only if that object type corresponds to a conceptual row, defines instance identification information for that object type. (Historically, each MIB definition contained a section entitled "Identification of OBJECT instances for use with the SNMP". By using the INDEX clause, this section need no longer occur as this clause concisely captures the precise semantics needed for instance identification.)

If the INDEX clause is not present, and the object type corresponds to a non-columnar object, then instances of the object are identified by appending a sub-identifier of zero to the name of that object. Further, note that if the MIB module does not contain a textual description of how instance identification information is derived for columnar objects, then the INDEX clause must be present.

To define the instance identification information, determine which object value(s) will unambiguously distinguish a conceptual row. The syntax of those objects indicate how to form the instance-identifier:

1. integer-valued: a single sub-identifier taking the integer value (this works only for non-negative integers);

2. string-valued, fixed-length strings: `n' sub-identifiers, where `n' is the length of the string (each octet of the string is encoded in a separate sub-identifier);

3. string-valued, variable-length strings: `n+1' sub- identifiers, where `n' is the length of the string (the first sub-identifier is `n' itself, following this, each octet of the string is encoded in a separate sub- identifier);

4. object identifier-valued: `n+1' sub-identifiers, where `n' is the number of sub-identifiers in the value (the first sub-identifier is `n' itself, following this, each sub-identifier in the value is copied);

5. NetworkAddress-valued: `n+1' sub-identifiers, where `n' depends on the kind of address being encoded (the first sub-identifier indicates the kind of address, value 1 indicates an IpAddress); or,

6. IpAddress-valued: 4 sub-identifiers, in the familiar a.b.c.d notation.

Note that if an "indextype" value is present (e.g., INTEGER rather than ifIndex), then a DESCRIPTION clause must be present; the text contained therein indicates the semantics of the "indextype" value.

By way of example, in the context of MIB-II [7], the following INDEX clauses might be present:

                 objects under         INDEX clause
               -----------------       ------------
               ifEntry                 { ifIndex }
               atEntry                 { atNetIfIndex,
                                         atNetAddress }
               ipAddrEntry             { ipAdEntAddr }
               ipRouteEntry            { ipRouteDest }
               ipNetToMediaEntry       { ipNetToMediaIfIndex,
                                         ipNetToMediaNetAddress }
               tcpConnEntry            { tcpConnLocalAddress,
                                         tcpConnLocalPort,
                                         tcpConnRemoteAddress,
                                         tcpConnRemotePort }
               udpEntry                { udpLocalAddress,
                                         udpLocalPort }
               egpNeighEntry           { egpNeighAddr }