SNMP

SNMP is the standard monitoring protocol of network and infrastructure equipment: UPSs, switches, generator controllers, air conditioning units, servers. inSCADA supports it natively, in the client role only.

Addressing is by OID (object identifier): a dot-separated numeric path defined in the device’s MIB.

Addressing

Level	What it carries
Connection	The agent’s address and port, the version, the security
Device	The polling interval
Frame	The set of OIDs asked for together
Variable	One OID and the type of its value

Connection

Parameter	Default	Description
Version	—	V1, V2C or V3
Timeout (ms)	2000	The request timeout (100 – 60000)
Retries	1	How many retries (0 – 10)
Max OIDs Per Request	20	The most OIDs asked for in one request (1 – 100)

What Max OIDs Per Request changes

SNMP can ask for several OIDs in one packet. A higher number means fewer round trips — but some older agents answer a large request with tooBig and return nothing at all. If no values arrive and the agent is old, try lowering this first.

Security — by version

v1 and v2c

A single community string is used. It is a password, and it travels unencrypted.

Parameter	Description
Community	The community string (max 255 characters)

v3

v3 uses user-based security and configures authentication and encryption separately.

Parameter	Description
Security Name	The v3 user name (max 32 characters)
Auth Protocol	NONE, MD5, SHA, SHA224, SHA256, SHA384, SHA512
Auth Passphrase	The authentication passphrase
Priv Protocol	NONE, DES, TRIPLE_DES, AES128, AES192, AES256
Priv Passphrase	The encryption passphrase
Context Name	The context name (max 32 characters)

A passphrase must be at least 8 characters — RFC 3414’s minimum.

None of the three secrets is shown back

The community string and the two v3 passphrases are stored encrypted, appear in responses only as whether they are set, and are kept out of exports and backups. Leaving one empty while editing keeps the stored value.

One consequence: an update that does not carry a secret is ordinary, so an empty field is not by itself an error. Whether the version’s secret is actually present is checked when the connection starts.

On v1 and v2c the community travels in clear

The community string is not encrypted and anyone listening on the network can read it. Do not use a community with write access on an untrusted network — where that is needed, move to V3 and turn on authentication and encryption.

Variable

Parameter	Description
OID	The dot-separated numeric OID (3 – 400 characters)
Type	The value’s SNMP type

Data types

Type	Description
INTEGER32	A 32-bit signed integer
GAUGE32	A 32-bit measure that can go up and down
COUNTER32 / COUNTER64	A counter that only increases
TIMETICKS	A duration in hundredths of a second
OCTET_STRING	A byte string or text
IP_ADDRESS	An IP address
OBJECT_IDENTIFIER	An OID as a value
TRUTH_VALUE	True or false

Do not confuse a counter with a gauge

COUNTER32 only increases and wraps when it overflows — you use the difference between two readings, not the present value (packets per second, say). GAUGE32 is the value at that moment (a temperature, a fill level). Putting a counter straight onto a trend gives a meaninglessly rising line that occasionally resets to zero.

Browsing OIDs

inSCADA can browse the agent’s OID tree, so while defining a variable you pick the OID from a list instead of typing it.

Related

• Connections — defining a connection