Download SNMP Message Protocol

SNMP
Simple Network Management Protocol
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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:

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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:

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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 SMI
(Structure of Management Information)

Variables recognized by device supplied in MIB
(Management Information Base)

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text file giving variables and data structures defined
using ASN.1
standard variable sets often provided as RFC’s
device-specific sets provided by vendors
Management stations parse MIB’s 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
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ex: 1.3.6.1.4.1.3.5.1.1
name space is hierarchical; tree on next slide
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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)
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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
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SNMP Environment
MIB
Network
Manager
Gateway
Manager
process
SNMP
Manager
process
SNMP
Manager
process
SNMP
User
Proc.
UDP
UDP
UDP
TCP
IP
IP
IP
Network dependent
protocols
Network dependent
protocols
Network dependent
protocols
Host/User Station
Internetwork
FTP
etc.
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SNMP Specific Problems


Work on SNMP security took many many years to
finally result in a stable and accepted SNMPv3
specification
Inefficiency of SNMP for retrieving bulk MIB data
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SNMP Bulk Retrieval Problem

Problem:
 SNMP shows poor performance when retrieving
several thousands of MIB variables in a single
logical transaction

Reasons:
 Lack of flow control
 Bandwidth inefficiency due to OID naming
overhead
 High latency caused by a large number of
request/response interactions
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Proposal #1: SNMP over TCP

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SNMP over TCP gives flow and congestion control
for virtually no costs
Originator of a request/response transaction chooses
the transport for one or more complete transactions
SNMP engines can close TCP connections at any
point in time
SNMP engines may revert to SNMP over UDP when
needed
SNMP engines must perform packetizing and
connection management
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Proposal #2: Lossless Payload
Compression

Compress and wrap SNMP PDUs in a
CompressedPDU
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Each SNMP message is compressed and
decompressed by itself without any relation to other
SNMP messages (stateless compression)

The size of a compressed SNMP message must
never exceed the size of the uncompressed message
(non-expansion policy)
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Support for multiple compression algorithms

Negotiation of compression algorithm via MIB objects
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Proposal #2: Lossless Payload
Compression

Deflate:
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Deflate achieves high compression ratios of 80 %
on typical MIB-II data
Requires noticeable CPU resources on the
sending SNMP engine
Interactions with message size constraints make it
difficult to build response messages that send as
much data as possible
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Proposal #3: Get-Subtree / Linked
Responses
CG
CR
GetNext
(row by row)
CG
CR
GetNext
(rows in parallel)
CG
CR
GetBulk
CG
CR
GetSubTree
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Proposal #3: Get-Subtree / Linked
Responses
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The format of get-subtree PDUs is similar to the
format of the get-bulk PDU, except that there is no
max-repetitions parameter
Linked response PDUs are ordinary response PDUs
where the error-index contains a sequence number if
the error-status is noError
Measurements over the loopback interface on a
Linux box show that get-subtree is 4.5 times faster
compared to get-next walks and 2 times faster than
get-bulk walks
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End of First Lecture
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