1. Extra Lecture and examples
SNMP
Extra Lecture and examples

2. Outline Reading Columnar Objects SNMP PDU messages GetRequest Example
GetNextRequest Example
Lexicographic Ordring- example
GetBulkRequest Example
SNMP Decentralized management
Message Encapsulation and UDP Port

3. Two Kinds of Managed Objects
Type-Specific Objects:
sysDescr OBJECT-TYPE
SYNTAX DisplayString (SIZE(0..255))
::= {system 1}
OID: mib-2.system.1.0
Columnar Objects
OID:
mib-2.interface.ifTable.ifEntry.ifDescr.2
mib-2.interface.ifTable.ifEntry.ifDescr.6
mib-2.interface.ifTable.ifEntry.ifType.2
mib-2.interface.ifTable.ifEntry.ifType.6

4. Columnar Objects
.ifTable.ifEntry.1 ( ) ifTable.ifEntry.2 ( )
.ifTable.ifEntry.3 ( )
ifIndex ifDescr ifType le
llc lo le

5. SNMP PDU messages . . . error-status error-index name value PDU type
request-id
variable-bindings
GetRequest, GetNextRequest, SetRequest
PDU type
request-id
error-status
error-index
variable-bindings
GetResponse
error-status INTEGER { noError (0), tooBig (1), noSuchName(2), badValue (3), readOnly (4), genErr (5) }, error-index INTEGER,
variable-bindings
name
value
var-bind 1
var-bind 2
var-bind n
. . .

6. GetRequest PDU Sender includes the following fields:
sysServices (7)
sysLocation (6)
sysDescr (1)
system
(mib-2 1)
sysObjectId
(2)
sysUpTime (3)
sysName (5)
sysContact (4)
Sender includes the following fields:
PDU Type
request-id
Variable-bindings
A list of object instances whose values are requested
SNMP dictates that a scalar object is identified by its OBJECT-IDENTIFIER concatenated with 0
e.g., sysDescr.0: distinguishes between the object type and an instance of the object

7. GetRequest PDU
.0 indicates that the scalar value
should be retrieved (scalar objects only)
Manager
Agent
Process
Process
GetRequest (sysDescr.0)
GetResponse (sysDescr .0= "SunOS" )
GetRequest (sysObjectID.0)
GetResponse ( sysObjectID.0=enterprises )
GetRequest (sysUpTime.0)
GetResponse (sysUpTime.0= )
GetRequest (sysContact.0)
GetResponse (sysContact.0=" ")
GetRequest (sysName.0)
GetResponse (sysName.0="noc1 ")
GetRequest (sysLocation.0)
GetResponse (sysLocation.0=" ")
GetRequest (sysServices.0)
GetResponse (sysServices.0=72)
A managed object should implement the system group. The manager by detecting the object, it will poll the new object to learn the values of objects in the system group
The manager could have used only one message to obtain the values of all objects under system group: using “variable binding list”

8. GetRequest PDU Get Request is atomic
Either all values (of all variables provided in the binding list) retrieved or none
error message is generated if at least one of the variables could not be found/returned; error- status:
noSuchName
tooBig
genErr
error-index: indicate the problem object (i.e., variable in binding list that caused the problem)
With SNMP, only leaf objects in the MIB can be retrieved
e.g. it is not possible to retrieve an entire row of a table by simply accessing the Entry Object (e.g., ipRouteEntry)
 the management stations has to include each object instance (in the row) in the binding list
By including the complete object identifier and respecting the rule of indexing!

9. GetRequest PDU ipRouteDest ipRouteMetric1 ipRouteNextHop
Index of table
GetRequest (ipRouteDest , ipRouteMetric , ipRouteNextHop )

10. GetNextRequest PDU PDU format: Difference:
sysServices (7)
sysLocation (6)
sysDescr (1)
system
(mib-2 1)
sysObjectId
(2)
sysUpTime (3)
sysName (5)
sysContact (4)
PDU format:
same as GetReqest
Difference:
each variable in the binding list refers to an object instance next in the lexicographic order
GetNextRequest (sysDescr.0)  return the value of the object instance of sysObjectId
Advantages:
Allows a network manager to discover a MIB structure dynamically
Efficient way for searching through tables whose entries are unknown

11. GetNextRequest PDU Error message: no object next to sysServices
Manager
Agent
Process
Process
GetRequest (sysDescr.0)
GetResponse (sysDescr .0= "SunOS" )
GetNextRequest (sysDescr.0)
GetResponse ( sysObjectID.0=enterprises )
GetNextRequest (sysObjectID.0)
GetResponse (sysUpTime.0= )
GetNextRequest (sysUpTime.0)
GetResponse (sysContact.0=" ")
GetNextRequest (sysContact.0)
GetResponse (sysName.0="noc1 ")
GetNextRequest (sysName.0)
GetResponse (sysLocation.0=" ")
GetNextRequest (sysLocation.0)
GetResponse (sysServices.0=72)
GetNextRequest (sysServices.0)
GetResponse (noSuchName)
Error message: no object next
to sysServices
Get-Next-Request Operation for System Group

12. Generalized Case
A sample MIB that contains both scalar values and aggregate objects
Retrieving scalar as well as aggregate objects using get- request and get-next-request T Z A B
1.1 E
2.1
3.1
1.2
2.2
3.2

13. Get-Next Request MIB Tree : ＊ In SNMP, Only leaf objects have values.1 2 3 4 5 6
MIB Tree : ＊
In SNMP,
Only leaf objects have
values.
:Non-Leaf Object
:Leaf Object

14. Generalized Case A Manager Agent Process Process B GetRequest ( A )
GetResponse ( A )
GetRequest ( B ) T
GetResponse ( B )
GetRequest (T.E.1.1)
GetResponse ( T.E.1.1 ) E
GetRequest (T.E.1.2)
GetResponse ( T.E.1.2 )
GetRequest (T.E.2.1)
T.E.1.1
T.E.2.1
T.E.3.1
GetResponse ( T.E.2.1 )
GetRequest (T.E.2.2)
GetResponse ( T.E.2.2 )
T.E.1.2
T.E.2.2
T.E.3.2
GetRequest (T.E.3.1 )
GetResponse ( T.E.3.1 )
GetRequest (T.E.3.2 ) Z
GetResponse ( T.E.3.2 )
GetRequest (Z )
GetResponse ( Z )

15. Generalized Case Observations:
1)- we need to know all the elements in the MIB, including the # of columns and rows in a table
2)- a MIB is traversed from top to bottom (i.e., from left to right in the tree structure)
3)- data in tables is retrieved by traversing all instances of a columnar object
NOTES:
1)- dynamic table: # rows may not be known to manager
A request to T.E.1.3 results in error message
3)- GetNextRequest could avoid this!
4)- A convention is required for the definition of the next object in a MIB
 SNMP uses lexicographic convention A B T E
T.E.1.1
T.E.2.1
T.E.3.1
T.E.1.2
T.E.2.2
T.E.3.2 Z

16. Lexicographic Convention
Procedure for ordering
Start with leftmost digit as first position
Before increasing the order in the first position, select the lowest digit in the second position
Continue the process till the lowest digit in the last position is captured
Increase the order in the last position until all the digits in the last position are captured
Move back to the last but one position and repeat the process
Continue advancing to the first position until all the numbers are ordered
Tree structure for the above process

17. Lexicographic Ordring- example
start
end 3 9 1 2 18 5 6 10 21 4
MIB example of lexicographic ordering

18. GetNextRequest PDU T.E.1.1 is next object to scalar B GetRequest ( A )
GetResponse ( A )
GetNextRequest ( A )
GetResponse ( B )
GetNextRequest ( B )
GetResponse ( T.E.1.1 )
GetNextRequest (T.E.1.1 )
GetResponse ( T.E.1.2 )
GetNextRequest (T.E.1.2 )
GetResponse ( T.E.2.1 )
GetNextRequest (T.E.2.1 )
GetResponse ( T.E.2.2 )
GetNextRequest (T.E.2.2 )
GetResponse ( T.E.3.1 )
GetNextRequest (T.E.3.1 )
GetResponse ( T.E.3.2 )
GetNextRequest (T.E.3.2 )
GetResponse ( Z )
GetNextRequest ( Z )
GetResponse ( noSuchName )
Manager
Process
Agent
T.E.1.1
T.E.2.1
T.E.3.1
T.E.1.2
T.E.2.2
T.E.3.2 E T Z A B
T.E.1.1 is next
object to scalar B

19. GetNextRequest PDU Advantages of Get-Next-Request
GetRequest ( A )
GetResponse ( A )
GetNextRequest ( A )
GetResponse ( B )
GetNextRequest ( B )
GetResponse ( T.E.1.1 )
GetNextRequest (T.E.1.1 )
GetResponse ( T.E.1.2 )
GetNextRequest (T.E.1.2 )
GetResponse ( T.E.2.1 )
GetNextRequest (T.E.2.1 )
GetResponse ( T.E.2.2 )
GetNextRequest (T.E.2.2 )
GetResponse ( T.E.3.1 )
GetNextRequest (T.E.3.1 )
GetResponse ( T.E.3.2 )
GetNextRequest (T.E.3.2 )
GetResponse ( Z )
GetNextRequest ( Z )
GetResponse ( noSuchName )
Manager
Process
Agent
Advantages of Get-Next-Request
1)- no need to know the object ID of the next entity to retrieve its value
2)- issues with dynamic table resolved
3)- allows NMS to discover the structure of a MIB view dynamically
4)- provides an efficient mechanism for searching a table whose entries are unknown

20. Additional Messages inform-request manager-to-manager message
The receiving manager responds with a response message
Enhances interoperability
get-bulk-request
transfer of large data, e.g. retrieval of table data
SNMPv2-trap
Similar to trap messages in SNMPv1

21. GetBulkRequest-PDU OperationZ A B
1.1 E
1.2
1.3
1.4
2.1
2.2
2.3
2.4
3.1
3.2
3.3
3.4
T.E.1.1
T.E.2.1
T.E.3.1
T.E.1.2
T.E.2.2
T.E.3.2 E T Z A B
T.E.1.3
T.E.2.3
T.E.3.3
T.E.1.4
T.E.2.4
T.E.3.4

22. GetBulkRequest-PDU Operation
GetRequest ( A,B )
GetNextRequest (T.E.1,T.E.2,T.E.3)
GetResponse (T.E.1.1,T.E.2.1,T.E.3.1)
GetNextRequest (T.E.1.1,T.E.2.1,T.E.3.1)
GetResponse (T.E.1.2,T.E.2.2,T.E.3.2)
GetResponse (T.E.1.3,T.E.2.3,T.E.3.3)
GetNextRequest (T.E.1.3,T.E.2.3,T.E.3.3)
GetResponse (T.E.1.4,T.E.2.4,T.E.3.4)
GetResponse (T.E.2.1,T.E.3.1,Z)
Manager
Process
Agent
GetResponse (A,B)
GetNextRequest (T.E.1.4,T.E.2.4,T.E.3.4)
GetNextRequest (T.E.1.2,T.E.2.2,T.E.3.2)

23. GetBulkRequest-PDU Operation
T.E.1.1
T.E.2.1
T.E.3.1
T.E.1.2
T.E.2.2
T.E.3.2 E T Z A B
T.E.1.3
T.E.2.3
T.E.3.3
T.E.1.4
T.E.2.4
T.E.3.4
2 non repetitive objects (A, B)
3 repetitive instances
Of the columnar object
T.E.1, T.E.2, T.E.3
Manager
Agent
Process
Process
GetBulkRequest ( 2,3,
A,B,T.E.1, T.E.2, T.E.3 )
Response ( A, B,
T.E.1.1, T.E.2.1, T.E.3.1
T.E.1.2, T.E.2.2, T.E.3.2
T.E.1.3, T.E.2.3, T.E.3.3 )
3 more rows
GetBulkRequest ( 0,3,
T.E.1.3, T.E.2.3, T.E.3.3 )
Response ( T.E.1.4, T.E.2.4, T.E.3.4, Z , "
endOfMibView" )
Z is next in the lexicographic order

24. GetBulkRequest-PDU Operation

25. SNMP Decentralized management
Agent
Element manager
MIB
SNMPv2
agent
MIB
SNMPv2
Manager/agent
Management server
MIB
Management Applications
SNMPv2
manager
MIB
SNMPv2
agent
MIB
SNMPv2
Manager/agent
MIB
SNMPv2
agent
SNMPv2 Configuration

26. Message Encapsulation
version
community
data
Communication between remote peer processes Message consists of :
Version identifier
Community name
Messages are exchanged in Protocol Data Unit PDU
Message encapsulated in UDP datagrams and transmitted
Loss of message  time out!
Like FTP, SNMP uses two well- known ports to operate:
UDP Port SNMP Messages
UDP Port SNMP Trap Messages
Size of SNMP message: bytes

27. Default UDP Ports for SNMP
Management
Station
Network Elements (NEs)
Manager
Agent
SNMP
SNMP
162
Any
161
Any
UDP
UDP IP IP
Ethernet
Ethernet