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6.7 Using SNMP with UDP and IP

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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