SNMP (Simple Network Management Protocol)

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SNMP(Simple Network Management Protocol)

• Jon Sevy
• Geometric and Intelligent Computing Laboratory
• Department of Mathematics and Computer Science
• Drexel University
• http//gicl.mcs.drexel.edu

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SNMP Overview

• Define mechanism for remote management of network
  devices (routers, bridges, etc.)
• Fundamental principle all device management done
  by simple variable value manipulation
• Approach
• standard means for specifying quantities
  recognized by devices
• protocol for requesting, returning, notifying of
  changes of values

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SNMP Message Protocol

• Messages use UDP, ports 161 (requests/responses)
  and 162 (notifications)
• Message types
• GetRequest request values of variables from
  device
• GetNextRequest request value of variable
  following the one supplied
• GetResponse return values
• SetRequest instruct device to set values of
  variables
• Trap from device - notify monitor / manager of
  value change

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SNMP Communities

• Define communities specifying access to
  specific variable sets (read-write, read only,
  none)
• Supply community name as password in each
  message
• unencrypted in version of SNMP still widely used!!

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SNMP Message Encoding

• Encode message as byte stream using ASN.1 BER
  (Abstract Syntax Notation 1 Basic Encoding Rules)
• Quantites encoded as Type,Length,Value triples
• Types
• Subset of basic ASN.1 types used in SNMP
  integer, octet string, object identifier
  (variable name), sequence
• SNMP-defined types gauge, counter, IP address,
  etc.
• Values
• weirdly encoded!! (see ASN.1 specs)

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SNMP SMI(Structure of Management Information)

• Variables recognized by device supplied in MIB
  (Management Information Base)
• text file giving variables and data structures
  defined using ASN.1
• standard variable sets often provided as RFCs
• device-specific sets provided by vendors
• Management stations parse MIBs to determine
  variables available for management
• obtain both data structure and management
  information

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Example portion of MIB (from RFC 1213)
-- the Interfaces group --
Implementation of the Interfaces group is
mandatory for -- all systems.
ifNumber OBJECT-TYPE SYNTAX
INTEGER ACCESS read-only
STATUS mandatory DESCRIPTION
"The number of network
interfaces (regardless of
their current state) present on this system."
interfaces 1
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ASN.1 Object Identifiers

• Variables identified by globally unique strings
  of digits
• ex 1.3.6.1.4.1.3.5.1.1
• name space is hierarchical tree on next slide
• in above, 1 stands for iso, 3 stands for org,
  6 stands for dod, 1 stands for internet, 4
  stands for private, etc.
• Variable names are aliases for digit strings
  (within MIB)
• From previous page ifNumber interfaces
  1
• interfaces was previously defined in MIB as
  1.3.6.1.2.1.2, so ifNumber 1.3.6.1.2.1.2.1

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Partial ASN.1 Object Identifier Tree(from ASN.1
Complete, J. Larmouth, Open Systems Solutions,
1999)
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Application GICL SNMP Monitor

• Java-based SNMP application
• Query devices for available MIB variables
• Set desired variable values
• Current status
• retrieve and display all values from device
• automatically build data structures to hold
  retrieved values
• Future work
• incorporate MIB information via MIB parser
• auto-generate GUI display
• implement device discovery

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References

• Internet Standards 15, 16 and 17
• ASN.1 Complete, J. Larmouth, Open Systems
  Solutions, 1999 (available online)
• SNMP a guide to network management, S. Feit,
  McGraw-Hill, 1995