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This section shows how the SNMP GetRequest and GetResponse PDUs fit within the structure of an Ethernet frame (review Figure 6-1). In this section we will examine the Ethernet frame format, the IP header, the UDP header, the SNMP PDUs, and the ASN.1 encoding of the variable bindings.
Trace 6.7a shows four layers of protocol operating in two Frames, 7 and 8. You can easily identify the Data Link Control (DLC) layer as Ethernet because of the Ethertype (or Type) field (review Figure 6-6). The next field, the IP Header, is 20 octets long, has routine service, and is not fragmented. The Protocol field identifies the next higher layer (UDP), while the source and destination addresses identify the origin and destination of this datagram.
The UDP header gives the source and destination port numbers. Note that the SunMgr assigns port number 3234 for SNMP, while the Retix bridge (GoldGate) uses the standard port number of 161 for SNMP.
The SNMP authentication header, containing the version number and community string, precede the SNMP PDU. We see the PDU identified (GetRequest) and the various error fields. Next comes the variable bindings, which consist of an object name and its value. The GetRequest PDU uses NULL for all the Value fields (Frame 7), while the GetResponse (Frame 8) contains the actual values retrieved. To review, you could return to Figure 3-2 and trace the subtree for the System group, verifying the accuracy of the OID designation {1.3.6.1.2.1....}.
Trace 6.7a. Using SNMP with Ethernet, IP, and UDP
Sniffer Network Analyzer data 10-Nov at 10:29:36 file GOLD_SYS.ENC Pg 1 --------------------------------- Frame 7 ------------------------------ DLC: ----- DLC Header ----- DLC: DLC: Frame 7 arrived at 10:29:37.30; frame size is 138 (008A hex) bytes DLC: Destination = Station Retix 034CF1, GoldGate DLC: Source = Station Sun 0900C8, SunMgr DLC: Ethertype = 0800 (IP) DLC: IP: ----- IP Header ----- IP: IP: Version = 4, header length = 20 bytes IP: Type of service = 00 IP: 000. .... = routine IP: ...0 .... = normal delay IP: .... 0... = normal throughput IP: .... .0.. = normal reliability IP: Total length = 124 bytes IP: Identification = 20055 IP: Flags = 0X IP: .0.. .... = may fragment IP: ..0. .... = last fragment IP: Fragment offset = 0 bytes IP: Time to live = 60 seconds/hops IP: Protocol = 17 (UDP) IP: Header checksum = A5C5 (correct) IP: Source address = [XXX.YYY.128.4] IP: Destination address = [XXX.YYY.1.10] IP: No options IP: UDP: ----- UDP Header ----- UDP: UDP: Source port = 3234 (SNMP) UDP: Destination port = 161 UDP: Length = 104 UDP: No checksum UDP: SNMP: ----- Simple Network Management Protocol (Version 1) ----- SNMP: SNMP: Version = 0 SNMP: Community = public SNMP: Command = Get request SNMP: Request ID = 0 SNMP: Error status = 0 (No error) SNMP: Error index = 0 SNMP: SNMP: Object = {1.3.6.1.2.1.1.3.0} (sysUpTime.0) SNMP: Value = NULL SNMP: SNMP: Object = {1.3.6.1.2.1.1.1.0} (sysDescr.0) SNMP: Value = NULL SNMP: SNMP: Object = {1.3.6.1.2.1.1.2.0} (sysObjectID.0) SNMP: Value = NULL SNMP: SNMP: Object = {1.3.6.1.2.1.1.3.0} (sysUpTime.0) SNMP: Value = NULL SNMP: SNMP: Object = {1.3.6.1.2.1.1.6.0} (system.6.0) SNMP: Value = NULL SNMP: ------------------------------- Frame 8 -------------------------------- DLC: ----- DLC Header ----- DLC: DLC: Frame 8 arrived at 10:29:37.33; frame size is 195 (00C3 hex) bytes DLC: Destination = Station Sun 0900C8, SunMgr DLC: Source = Station Retix 034CF1, GoldGate DLC: Ethertype = 0800 (IP) DLC: IP: ----- IP Header ----- IP: IP: Version = 4, header length = 20 bytes IP: Type of service = 00 IP: 000. .... = routine IP: ...0 .... = normal delay IP: .... 0... = normal throughput IP: .... .0.. = normal reliability IP: Total length = 181 bytes IP: Identification = 0 IP: Flags = 0X IP: .0.. .... = may fragment IP: ..0. .... = last fragment IP: Fragment offset = 0 bytes IP: Time to live = 16 seconds/hops IP: Protocol = 17 (UDP) IP: Header checksum = 1FE4 (correct) IP: Source address = [XXX.YYY.1.10] IP: Destination address = [XXX.YYY.128.4] IP: No options IP: UDP: ----- UDP Header ----- UDP: UDP: Source port = 161 (SNMP) UDP: Destination port = 3234 UDP: Length = 161 UDP: Checksum = 6417 (correct) UDP: SNMP: ----- Simple Network Management Protocol (Version 1) ----- SNMP: SNMP: Version = 0 SNMP: Community = public SNMP: Command = Get response SNMP: Request ID = 0 SNMP: Error status = 0 (No error) SNMP: Error index = 0 SNMP: SNMP: Object = {1.3.6.1.2.1.1.3.0} (sysUpTime.0) SNMP: Value = 240267300 hundredths of a second SNMP: SNMP: Object = {1.3.6.1.2.1.1.1.0} (sysDescr.0) SNMP: Value = Retix Local Ethernet Bridge Model 2265M SNMP: SNMP: Object = {1.3.6.1.2.1.1.2.0} (sysObjectID.0) SNMP: Value = {1.3.6.1.4.1.72.8.3} SNMP: SNMP: Object = {1.3.6.1.2.1.1.3.0} (sysUpTime.0) SNMP: Value = 240267300 hundredths of a second SNMP: SNMP: Object = {1.3.6.1.2.1.1.6.0} (system.6.0) SNMP: Value = SNMP:
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