SNMP(SIMPLE NETWORK MANAGEMENT
PROTOCOL)
A large
network can often get into various kinds of trouble due to routers (dropping
too many packets), hosts( going down) etc. One has to keep track of all these
occurence and adapt to such situations. A protocol has been defined. Under this
scheme all entities in the network belong to 4 classes:
1. Managed
Nodes
2. Management
Stations
3. Management
Information (called Object)
4. A
management protocol
The
managed nodes can be hosts,routers,bridges,printers or any other device capable
of communicating status information to others. To be managed directly by SNMP,
a node must be capable of running am SNMP management process, called SNMP
agent. Network management is done by management stations by exchanging
information with the nodes. These are basically general purpose computers
running special management software. The management stations polls the stations
periodically. Since SNMP uses unreliable service of UDP the polling is
essential to keep in touch with the nodes. Often the nodes send a trap message
indicating that it is going to go down. The management stations then periodically
checks (with an increased frequency) . This type of polling is called trap
directed polling. Often a group of nodes are represented by a single node which
communicates with the management stations. This type of node is called proxy
agent. The proxy agent can also server as a security arrangement. All the
variables in these schemes are called Objects. Each variable can be referenced
by a specific addressing scheme adopted by this system. The entire collection
of all objects is called Management Information Base (MIB). The addressing is
hierarchical as seen in the picture.
Internet
is addressed as 1.3.61. All the objects under this domain has this string at
the beginning. The information are exchanged in a standard and vendor-neutral
way . All the data are represented in Abstract Syntax Notation 1 (ASN.1). It is
similar to XDR as in RPC but it
have
widely different representation scheme. A part of it actually adopted in SNMP
and modified to form Structure Of Information Base. The Protocol specifies
various kinds of messages that can be exchanged between the managed nodes and
the management station.
Message : Description
1. Get_Request : Request the value for a
variable
2. Get_Response : Returns the value of the
variable asked for
3. Get_Next_Request : Request a variable next
to the previous one
4. Set_Request : Set the value of an Object.
5. Trap : Agent to manager Trap report
6. Get_bulk_request : Request a set of
variable of same type
7. Inform_Request : Exchange of MIB among
Management stations
The last
two options have been actually added in the SNMPv2. The fourth option need some
kind of authentication from the management station.
Addressing Example :
Following is an Example of the kind of address one
can refer to when fetching a value in the table :-
(20)
IP-Addr-Table = Sequence of IPAddr-Entry (1)
IPAddrEntry
= SEQUENCE {
IPADDENTRYADDR :
IPADDR (1)
Index : integer (2)
Netmask : IPAddr (3) }
So when
accessing the netmask of some IP-entity the variable name would be :
1.3.6.1.2.4.20 .1.3.key-value
Here
since Ip-address the unique key to index any member of the array the address
can be like :- 1.3.6.1.2.4.20.1.3.128.10.2.3
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