556
NETWORK MANAGEMENT
at the management station. This poll-based approach increases network traffic, especially
on the backbone where some users report a 5% to 10% increase in traffic due to SNMP
network management packets (or messages). SNMP is a connectionless protocol that ini-
tially was designed to run over a UDP/IP stack (UDP stands for User Datagram Protocol).
Because of its design, it is traditionally high in overhead, with the ratio of overhead to
usable data running about ten bytes to one. A typical SNMP "message" (PDU) embedded
in a local network frame is shown in Figure 21.7.
Inside the frame in Figure 21.7 we find an IP (Internet Protocol; see Chapter 11)
datagram that has a header. The header has an IP destination address that directs the
datagram to the intended recipient. Following the IP header, there is the User Datagram
Protocol (UDP) that identifies the higher layer protocol process. This is the SNMP message
shown in Figure 21.7 as embedded in the UDP. The application here is typically for
LANs. If the IP is too long for one frame, it is fragmented (segmented) into one or more
additional frames.
Figure 21.8 shows a typical SNMP PDU structure that is valid for all messages but
the trap format. This structure is embedded as the "message" field of Figure 21.7. The
SNMP message itself is divided into two sections: (1) a version identifier plus community
Local
Network
Header
Local
Network
Trailer
IP
Header
UDP
Header
SNMP
Message
UDP Datagram
IP Datagram
Local Network Frame
Figure 21.7
An SNMP ``message'' embedded in a local network frame.
Version
PDU
Type
Request
ID
Community
Object 1, Value 1 Object 2, Value 2
GetRequest, GetNextRequest, GetResponse
or SetRequest PDU
SNMP Message
Error
Status
Error
Index
Variable Bindings
. . .
Figure 21.8
An SNMP PDU structure for GetRequest, GetNext-Request, GetResponse, and Set-
Response. (From Refs. 9, 13, and 14.)
Summary :
The SNMP message itself is divided into two sections: (1) a version identifier plus community Local Network Header Local Network Trailer IP Header UDP Header SNMP Message UDP Datagram IP Datagram Local Network Frame Figure 21.7 An SNMP ``message'' embedded in a local network frame.
Tags :
network,message,figure,datagram,header,udp,local,pdu,protocol,frame,217,embedded,management