1. SNMP 訊息格式
IP 標頭
UDP 標頭
SNMP 訊息
SNMP 標頭
PDU 標頭
PDU 資料

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

3. SNMP 區域名稱 Community Relationship between an Agent and Managers.
Community Name
Used to validate the SNMP messages.
A device can participate in more than one community (password).
A community name (password) defines the scope of control that a manager has over elements in a community.
Default community name: “public”.

4. PDU Header
可分成兩種形式
針對 GetRequest , GetNextRequest , SetRequest , GetReponse
針對 Trap 而言
PDU 型態
要求代號
錯誤狀況
錯誤索引
PDU type
enterprise
agent-addr
generic-trap
specific-trap
time-stamp
variable-bindings

5. PDU 型態
五種 SNMP-PDUs 型態
SNMP-PDUs::= CHOICE { getrequest GetRequest-PDU,
get-next-request GetNextRequest-PDU,
get-response GetResponse-PDU,
set-request SetRequest-PDU,
trap Trap-PDU}

6. SNMP PDU Request-id Error-status Error-index 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 }

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

8. Trap PDU Enterprise: Agent Address: Generic Trap: Specific Trap:
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 }
Enterprise:
Type of Object generating trap.
Agent Address:
Address of object generating trap.
Generic Trap:
Generic trap type.
Specific Trap:
Enterprise specific trap.
Time Stamp:
表示該 Agent 啟動後，到發出 trap
的時間
Variable Bindings
“Interesting” information

9. PDU 資料 Variable binding VarBindList VarBindList::=SEQUENCE OF VarBind
VarBind::=SEQUENCE { name ObjectName,
value ObjectSyntax}
Object Name
Object Syntax
Object Name
Object Syntax

10. 完整 SNMP 訊息格式
SNMP 標頭
PDU 標頭
PDU 資料

11. BER 基本編碼方式 BER 主要是將 ASN.1所描述的網管物件轉換成 a string of octets 的格式，包含以下三個欄位：
Type : 表示該網管物件的資料型態
Length : 表示該網管物件內容的長度，以 Byte為單位
Value : 表示該網管物件內容值

12. BER 基本編碼方式(cont.) INTEGER (0x02) OCTET STRING (0x04)
OBJECT IDENTIFIER (0x06)
NULL (0x05)
SEQUENCE/SEQUENCE OF (Ox30)
GetRequest-PDU (0xA0)
GetNextRequest-PDU (0xA1)

13. BER 基本編碼方式(cont.) GetResponse-PDU (0xA2) SetRequest-PDU (0xA3)
Trap-PDU (0xA4)

14. Example 1
Question：Manager 利用 SNMP protocol 去詢問 Agent 的 sysName ( ) 經過 BER 的編碼過後的 SNMP Message
SNMP 標頭
PDU 標頭
PDU 資料

15. Example 1 (cont.)
0: C a0 1c 02
16:04 05 ae e 30 0c 06
32:08 2b
紅色：資料型態
黃色：長度
白色：內容
INTEGER (0x02)
OCTET STRING (0x04)
OBJECT IDENTIFIER (0x06)
NULL (0x05)
SEQUENCE/SEQUENCE OF (Ox30)
GetRequest-PDU (0xA0)

16. Object ID 編碼方式 針對 object ID 的前面兩個位數表示方法： 例如：1.3.6.1…………..
Content: the first two numbers, x and y, encoded as 40x+y in 1 byte
> 2B

17. Example 2
當 Agent 收到 Manager 傳送過來的 Request message 時，立刻作 get Response message 的動作，如下：
0:30 3e C a
16:04 05 ae
32:08 2b …
INTEGER (0x02)
OCTET STRING (0x04)
OBJECT IDENTIFIER (0x06)
NULL (0x05)
SEQUENCE/SEQUENCE OF (Ox30)
GetRequest-PDU (0xA0)

18. Example 3
30 2C
E F 70
A0 1E
D 3E
30 10
30 0E
06 0A 2B B 01
05 00
Message ::= SEQUENCE (0x30)
version INTEGER (0x02)
community OCTET STRING (0x04)
GetRequest-PDU ::= [0] ( = 0xA0)
request-id RequestID ::= INTEGER (0x02)
error-status ErrorStatus ::= INTEGER (0x02)
error-index ErrorIndex ::= INTEGER (0x02)
variable-bindings VarBindList ::= SEQUENCE OF (0x30)
VarBind ::= SEQUENCE (0x30)
name ObjectName ::= OBJECT IDENTIFIER (0x06)
value ObjectSyntax (NULL)

19. Simple Network Management Protocol (SNMP) 參考
RFC 1157 (SNMPv1), RFC 1215 (SNMPv1 Traps)
RFCs , (SNMPv2)
RFCs (SNMPv3)