If you have ever debugged a switch with snmpwalk, stared at an OID like 1.3.6.1.2.1.2.2.1.10, or wondered why every network device on Earth speaks the same dialect of dotted numbers, you have RFC 1155 to thank (or curse). It defines the Structure of Management Information, universally known as SMI, the type system and naming scheme that underpins SNMP and every MIB ever written.

The problem. By 1990 the IETF had agreed on SNMP as the management protocol of the moment, but a protocol is useless without an agreed vocabulary. How do you name "the number of octets received on interface 3" in a way that an HP printer, a Cisco router, and a Sun workstation all understand identically? You need: a global naming hierarchy, a small set of primitive data types, and rules for how new objects get defined and registered. RFC 1155 supplies all three.

Key design decisions.

• Borrow ASN.1, but only a slice of it. Rose and McCloghrie deliberately took a tiny subset of ISO's Abstract Syntax Notation One. Full ASN.1 was considered too baroque for embedded agents running on 16-bit CPUs with kilobytes of RAM. SMIv1 allows essentially: INTEGER, OCTET STRING, OBJECT IDENTIFIER, NULL, plus application types IpAddress, Counter, Gauge, TimeTicks, and Opaque. That's the entire universe SNMP agents need to encode.
• The OID tree as global namespace. Every managed object lives at a node in a hierarchical tree rooted at three children: ccitt(0), iso(1), and joint-iso-ccitt(2). Internet management lives under 1.3.6.1 — iso.org.dod.internet — a path whose existence is half historical artifact (the DoD branch was assigned during ARPANET days) and half political compromise with ISO.
• Counters wrap, gauges latch. A Counter is a non-negative integer that monotonically increases and wraps at 2^32. A Gauge latches at its maximum. These semantics are why interface counter math always involves modular arithmetic and why your monitoring system computes deltas, not absolute values.
• No structures, no enumerations of complex data. Tables are simulated by appending an index to an OID. There are no records, no nested types. This forced enormous simplicity on agents — and enormous awkwardness on MIB authors, who learned to encode IP-address-indexed tables by gluing four octets onto the end of an OID.

Why it still matters. SMIv1 was superseded by SMIv2 (RFC 2578) which added 64-bit counters and richer textual conventions, but the OID tree, the wire encoding (BER), and the overall philosophy are unchanged. Every Cisco IOS MIB, every Juniper Junos MIB, every UPS, PDU, printer, and access point you have ever polled traces its type definitions to this document. Even modern telemetry systems built on gNMI and YANG inherit the idea of a globally-rooted naming tree from SMI.

A bit of history. Marshall Rose was the dominant force in IETF network management in this era — he also wrote the original SNMP RFC (1157) and later the BEEP protocol. The "SMI" name and the type system were chosen partly to look like ISO CMIP work so that the two camps could eventually converge. They never did. SNMP won by being shippable on an Ethernet card with 8KB of code, and CMIP died in committee. RFC 1155 is a small, opinionated document — barely 22 pages — and it shaped four decades of operational monitoring.