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SNMP Version 1

SNMP version 1 (SNMPv1) is the initial implementation of the SNMP protocol. It is described in Request For Comments (RFC) 1157 and functions within the specifications of the Structure of Management Information (SMI). SNMPv1 operates over protocols such as User Datagram Protocol (UDP), Internet Protocol (IP), OSI Connectionless Network Service (CLNS), AppleTalk Datagram-Delivery Protocol (DDP) and Novell Internet Packet Exchange (IPX), SNMPv1 is widely used and is the de facto network-management protocol in the Internet community.

The Structure of Management Information (SMI) defines the rules for describing management information, using Abstract Syntax Notation One (ASN.1). The SNMPv1 SMI is defined in RFC 1155. The SMI makes three key specifications: ASN.1 data types, SMI-specific data types and SNMP MIB tables.

The SNMPv1 SMI specifies that all managed objects have a certain subset of Abstract Syntax Notation One (ASN.1) data types associated with them. Three ASN.1 data types are required: name syntax and encoding. The name serves as the object identifier (object ID). The syntax defines the data type of the object (for example, integer or string). The SMI uses a subset of the ASN.1 syntax definitions. The encoding data describes how information associated with a managed object is formatted as a series of data items for transmission over the network.

The SNMPv1 SMI specifies the use of a number of SMI-specific data types, which are divided into two categories: simple data types and application-wide data types. Three simple data types are defined in the SNMPv1 SMI, all of which are unique values: integers, octet strings, and object IDs. The integer data type is a signed integer in the range of -2,147,483,648 to 2,147,483,647. Octet strings are ordered sequences of 0 to 65,535 octets. Object IDs come from the set of all object identifiers allocated according to the rules specified in ASN.1.

Seven application-wide data types exist in the SNMPv1 SMI: network addresses, counters gauges, time ticks, opaques, integers and unsigned integers. Network addresses represent an address from a particular protocol family. SNMPv1 supports only 32-bit IP addresses. Counters are non-negative integers that increase until they reach a maximum value and then return zero. In SNMPv1, a 32 bit counter size is specified. Gauges are non-negative integers that can increase or decrease but that retain the maximum value reached. A time tick represents a hundredth of a second since some event. A opaque represents an arbitrary encoding that is used to pass arbitrary information strings that do not conform to the strict data typing used by the SMI. An integer represents signed integer-valued information. This data type redefines the integer data type, which has arbitrary precision in ASN.1 but bounded precision in the SMI. An unsigned integer represents unsigned integer-valued information and is useful when values are always non-negative. This data type redefines the integer data type, which has arbitrary precision in ASN.1 but bounded precision in the SMI.

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