1. SNMP Communication and Functional Models
In the Name of the Most High
SNMP Communication and Functional Models by
Behzad Akbari
Fall 2008

2. Overview
• We have covered the organization and information models of SNMPv1.
• Here we will address the SNMPv1 communication and functional models
• SNMPv1 does not formally define a functional model
– What was the functional model?
• Deals with the user oriented requirements: (configuration, fault, performance, security, and accounting)
– The functions are actually built in the community based access policy of the SNMP administrative model

3. SNMP Architecture SNMP Manager SNMP Agent SNMP SNMP UDP UDP IP IP
Application
SNMP Agent
Application
Get-Request
GetNext-Request
Set-Request
Get-Response
Trap
Get-Request
GetNext-Request
Set-Request
Get-Response
Trap
SNMP
SNMP
UDP
UDP IP IP
Data link
Data link
Network

4. SNMP Messages Get-Request Get-Next-Request Set-Request Get-Response
Trap
Generic trap
Specific trap

5. SNMP Trap Messages
• Generic trap – coldStart – warmStart – linkDown – linkUp – authenticationfailure – egpNeighborLoss – enterpriseSpecific • Specific trap – for special measurements such as statistics • Time stamp: Time since last initialization

6. Administrative Model Based on community profile and policy
SNMP Entities:
SNMP application entities - Reside in management stations and network elements - Manager and agent
SNMP protocol entities - Communication processes (PDU handlers) - Peer processes that support application entities

7. SNMP Community Security in SNMPv1 is community-based
Authentication scheme in manager and agent
Community: Pairing of two application entities
Community name: String of octets
Two applications in the same community communicate with each other
Application could have multiple community names
Communication is not secured in SNMPv1 - no encryption

8. SNMP Community Community Relationship between an Agent and Managers.
Community Name
Used to validate the SNMP messages.
SNMP Password.
Default ‘Get’ community name: “public”.
Authentication Failure
Agent sends “Authentication Failure Trap” to Manager.

9. SNMP Community

10. Community Profile MIB view
An agent is programmed to view only a subset of managed objects of a network element
Access mode
Each community name is assigned an access mode:: read-only and read-write
Community profile = MIB view + access mode
Operations on an object determined by community profile and the access mode of the object
Total of four access privileges
Some objects, such as table and table entry are non-accessible

11. Community Profile
community

12. Access Policy Administration model is SNMP access policy
SNMP community paired with SNMP community profile is SNMP access policy

13. Access Policy

14. Generalized Administration Model

15. Proxy Access Policy

16. Protocol Entities

17. Default UDP Ports for SNMP
Management
Station
Network Elements (NEs)
Manager
Agent
SNMP
SNMP
162
Any
161
Any
UDP
UDP IP IP
Data link
Data link

18. Protocol Entities Protocol entities support application entities
Communication between remote peer processes
Message consists of
Version identifier
Community name
Protocol Data Unit
Message encapsulated and transmitted

19. SNMP Message SNMP Message
Version Identifier
Community Name
Protocol Data Unit
The length of SNMP messages should not exceed 484 octets.
Message ::=
SEQUENCE {
version INTEGER {version-1(0)},
community OCTET STRING,
data ANY }
Version
Community
SNMP PDU

20. SNMP PDUs

21. SNMP PDU Five SNMP PDUs: GetRquest ::= GetNextRequest ::=
PDU ::= SEQUENCE {
request-id INTEGER,
error-status INTEGER {
noError(0),
tooBig(1),
noSuchName(2),
badValue(3),
readOnly(4),
genErr(5)},
error-index INTEGER,
variable-bindings
SEQUENCE OF {
name ObjectName,
value ObjectSyntax }
Five SNMP PDUs:
GetRquest ::=
GetNextRequest ::=
GetResponse ::=
SetRequest ::=
Trap ::=
[0] PDU
[1] PDU
[2] PDU
[3] PDU
[4] Trap-PDU
PDU: Protocol Data Unit

22. SNMP PDU (cont.) . . . GetRequest, GetNextRequest, SetRequest PDU type
request-id
variable-bindings
GetResponse
PDU type
request-id
error-status
error-index
variable-bindings
variable-bindings
. . .
name
value
name
value
name
value

24. Trap-PDU Enterprise: Agent Address: Generic Trap: Specific Trap:
Type of Object generating trap.
Agent Address:
Address of object generating trap.
Generic Trap:
Generic trap type.
Specific Trap:
Enterprise specific trap.
Time Stamp:
Time elapsed between the last
initialization of the network entity and
the generation of the trap.
Variable Bindings
“Interesting” information
Trap-PDU ::= [4]
IMPLICIT SEQUENCE {
enterprise OBJECT IDENTIFIER,
agent-addr NetworkAddress,
generic-trap INTEGER {
coldStart(0),
warmStart(1),
linkDown(2),
linkUp(3),
authenticationFailure(4),
egpNeighborLoss(5),
enterpriseSpecific(6)},
specific-trap INTEGER,
time-stamp TimeTicks,
variable-bindings VarBindList }
PDU type
enterprise
agent-addr
generic-trap
specific-trap
time-stamp
variable-bindings

25. Trap Type

26. Generic Trap Example Enterprise: .1.3.6.1.4.1.311.1.1.3.1.1
Agent-Address:
Generic-Trap: 4
Specific-Trap: 0
Timestamp:
#VarBinds:

27. Enterprise-Specific Traps
Traps defined by enterprises
Identification of Enterprise-Specific Traps
Enterprise  Enterprise OID
Generic-Trap  6
Specific-Trap  an Integer

28. Enterprise Trap Example
Agent-Address:
Generic-Trap: 6
Specific-Trap: 4
Timestamp:
VariableBindings: (4)
: 02:18:25
: 14 :
(Info): Station 00092d Associated
: AssociationOK

29. Agent
Manager

30. Get-Next Request A B T E
1.1
1.2
2.1
2.2
3.1
3.2 Z

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

34. Get-Next Requests with Indices

35. SNMP Get-Request Example
>>snmpget -d
Transmitted 41 bytes to camry ( ) port 161:
Initial Timeout: 0.80 seconds
0: c a0 1a '.....public...
16: bc e 30 0c b
32:
0: SNMP MESSAGE (0x30): 39 bytes
2: INTEGER VERSION (0x2) 1 bytes: 0 (SNMPv1)
5: OCTET-STR COMMUNITY (0x4) 6 bytes: "public"
13: GET-REQUEST-PDU (0xa0): 26 bytes
15: INTEGER REQUEST-ID (0x2) 2 bytes: 6332
19: INTEGER ERROR-STATUS (0x2) 1 bytes: noError(0)
22: INTEGER ERROR-INDEX (0x2) 1 bytes: 0
25: SEQUENCE VARBIND-LIST (0x30): 14 bytes
27: SEQUENCE VARBIND (0x30): 12 bytes
29: OBJ-ID (0x6) 8 bytes:
39: NULL (0x5) 0 bytes

36. SNMP Get-Response Example
Received 69 bytes from port 161:
0: c a C.....public.6.
16: bc a b *0(..+
32: c e e 4d Sun SNM
48: e 74 2c e 57 2c 55 6c P Agent, SUNW,Ul
64: d tra
0: SNMP MESSAGE (0x30): 67 bytes
2: INTEGER VERSION (0x2) 1 bytes: 0 (SNMPv1)
5: OCTET-STR COMMUNITY (0x4) 6 bytes: "public"
13: RESPONSE-PDU (0xa2): 54 bytes
15: INTEGER REQUEST-ID (0x2) 2 bytes: 6332
19: INTEGER ERROR-STATUS (0x2) 1 bytes: noError(0)
22: INTEGER ERROR-INDEX (0x2) 1 bytes: 0
25: SEQUENCE VARBIND-LIST (0x30): 42 bytes
27: SEQUENCE VARBIND (0x30): 40 bytes
29: OBJ-ID (0x6) 8 bytes:
39: OCTET-STR (0x4) 28 bytes: "Sun SNMP Agent, SUNW,Ultra-1"
system.sysDescr.0 : DISPLAY STRING- (ascii): Sun SNMP Agent, SUNW,Ultra-1

37. SNMP-Walk - Use of SNMP Get-Next Request
system.sysDescr.0 : DISPLAY STRING- (ascii): Sun SNMP Agent, SUNW,Ultra-1
system.sysObjectID.0 : OBJECT IDENTIFIER: .iso.org.dod.internet.private.enterprises
system.sysUpTime.0 : Timeticks: ( ) 22 days, 22:36:39.58
system.sysContact.0 : DISPLAY STRING- (ascii):
system.sysName.0 : DISPLAY STRING- (ascii): camry
system.sysLocation.0 : DISPLAY STRING- (ascii): Information Technology Laboratory 3F
system.sysServices.0 : INTEGER: 72 ( )B

38. SNMP Trap Example Transmitted 64 bytes to 10.144.18.100 port 162:
0: 30 3e c a >.....public.1.
16: 09 2b a
32: f C
48: 04 2b Trap test
0: SNMP MESSAGE (0x30): 62 bytes
2: INTEGER VERSION (0x2) 1 bytes: 0 (SNMPv1)
5: OCTET-STR COMMUNITY (0x4) 6 bytes: "public"
13: V1-TRAP-PDU (0xa4): 49 bytes
15: OBJ-ID ENTERPRISE (0x6) 9 bytes:
26: IPADDRESS AGENT-ADDR (0x40) 4 bytes:
32: INTEGER GENERIC-TRAP (0x2) 1 bytes: 6
35: INTEGER SPECIFIC-TRAP (0x2) 3 bytes: 99999
40: TIMETICKS TIME-STAMP (0x43) 1 bytes: 0 (0x0)
43: SEQUENCE VARBIND-LIST (0x30): 19 bytes
45: SEQUENCE VARBIND (0x30): 17 bytes
47: OBJ-ID (0x6) 4 bytes:
53: OCTET-STR (0x4) 9 bytes: "Trap test"

39. Get System Information
Get “System Group” of MIB II
Use get_request or get_next_request
sysDescr
sysObjectID
sysUptime
sysContact
sysName
sysLocation

40. Get Interface Information
Get “Interface Group” of MIB II
Repeatedly Use “get_next_request”
Note: We don’t know the ifIndex values in ifTable.
First get the next object of .ifTable.ifEntry.0
Then repeatedly “get_next”
Until the whole subtree is visited.

41. Traffic Monitoring (C2 - C1 )  8  100% (t2 - t1)  Bandwidth
Get “ifInOctets” and “ifOutOctets” of MIB II Interface Group
t1: C1 t2: C2
(C2 - C1 )  8
 100%
Utilization (%) =
(t2 - t1)  Bandwidth

42. Internet Traffic of Sharif University

43. SNMP MIB Group
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