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TCP/IP Monitoring by Trevor Eddolls

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TCP/IP Monitoring by Trevor Eddolls TCP/IP Monitoring By Trevor Eddolls TCP/IP stacks are NETSTAT ARP ALL|ipaddress displays ARP cache for the MAINFRAME-BASED ubiquitous, and are TCP/IP stack. used to allow access to mainframe data from other platforms and to NETSTAT DEV displays the status of the device(s) and link(s) transfer files around the network. for the TCP/IP stack. Accessing mainframe data can be achieved using a telnet service NETSTAT GATE|ROUTE displays routing information for the called TN3270E (in reality a standard 3270 screen protocol inside spe- TCP/IP stack. cial headers and trailers giving a telnet packet). There are TCP/IP NETSTAT HOME displays IP address(es) for the stack. stacks on the mainframe platform and on the other platforms (e.g. PCs and UNIX boxes) in the network and they handle the necessary hand- NETSTAT commands are probably most useful when you already shaking and verification to ensure the data is transferred successfully. have some idea of what the problem is and you are looking for more A socket interface is then used at the mainframe end to connect with information. Although NETSTAT commands indicate active ports, CICS, for example. they provide little information about non-TCP activity on the network Data transfer uses FTP. The mainframe and the other platforms use (such as UDP, ICMP, and OSPF) because these protocols are connec- the TCP/IP protocol to ensure that all the packets that are sent arrive tionless in nature. There is no way of showing connections being successfully. rejected because a server is down or simply unable to accept new con- When there is a problem with the network, it can be very hard to nections. Another difficulty with NETSTAT is that users must remem- identify. In fact, it can be hard deciding where to look first. There are ber the commands (or create a REXX EXEC containing them). There many ways of finding out where problems may have occurred and this is no simple GUI (Graphical User Interface). NETSTAT commands do article describes some of them. not form the basis for regular monitoring because of the potentially large amount of data that can be returned and the difficulty of doing COMMANDS anything with this data—like having to screen scrape it into another application. Let’s start with the simplest method, which is to use NETSTAT Another useful command is PING. This is used to test the connec- commands. The NETSTAT command is used to display the TCP/IP tion between the local server and a remote server. The name and pass- network protocol statistics and information. It can be used from word of the administrator client issuing this command must be defined Windows and UNIX platforms as well as from the mainframe. With on the remote server and the person issuing the command must have USS, the command is ONETSTAT. The following commands can be system privilege. used from TSO: To test the connection to a server called server1 the command would be: NETSTAT option {TCP procname} displays the network status ping server server1 of the local host. ? can be used for a list of all the options. From TSO: NETSTAT ALLCON|CONN displays port connections for the TCP/IP stack. PING hostname Technical Support | February 2005 ©2005 Technical Enterprises, Inc. Reproduction of this document without permission is prohibited. This would send an echo request to a host V TCPIP,{procname},Telnet,xxxx to SNMP public MIBs name or address to determine whether the perform a specified function for TELNET. z/OS extensions computer is accessible. OSA/ATM PING can also be used as a simple measure VTAM commands related to OSA cards OMPRoute of response time. PING sends out a block of that might be useful include: SLA subagent data using an ICMP echo request. The response time is how long it takes for that D NET,ID=name displays the network It makes use of the OSNMP and SNMPQE block of data to be returned. It is not a reliable named in the ID field. Additional address spaces. To get information from other measure of response time because most users parameters that may be added include: platforms, it makes use of agents. Both these are not using ICMP (which may travel over a ,SCOPE=ONLY|ACT|ALL|INACT,E things have a negative impact on performance. different route through the network compared which gives extended information about SNMP would seem like the ideal way to to TCP or UDP traffic), ICMP packets can be the node. monitor IP-related data. However, the infor- given different priorities by intermediate D NET,MAJNODES|APPLS shows the mation is retrieved by polling. This means routers, and users send and receive different status of all the active major nodes or there is always a delay between the informa- sized packets. Another problem is that sending applications. tion being stored in the MIB and being avail- a host of PINGs is a way to attack a stack, so D NET,PENDING lists all the nodes in able to the monitoring software. The second firewalls are often configured to block PINGs. pending states. problem is that every data item in the MIB has The traceroute command is used to identify V NET,ACT,ID=name activates the its own unique address or textual name. This the hops from one part of the network to the VTAM resource identified by the name. has to be supplied by the monitor in each GET next. With z/OS, the command is TRAC- V NET,INACT,ID=name inactivates the instruction. As each GET request requires one ERTE, with DOS it’s tracert, and with UNIX VTAM resource identified by the name. UDP datagram to be sent to the SNMP port it’s traceroute. From TSO you would enter: (161) on z/OS, and the response from the sub- SMF RECORDS agent is another UDP datagram, this adds to TRACERTE hostname the load on the network. SMF records can be used to provide infor- MVS or VTAM commands can be used to mation on mainframe performance, not just TRACES help diagnose TCP/IP performance problems. TCP/IP. With OS/390, type 118 SMF records z/OS console commands for TCP/IP include: provide data on TCP initiation, TCP termina- Packet traces can be very useful in identify- tion, ftp servers, ftp clients, and statistics. ing where a problem is. It works by allowing a D TCPIP to list the names and status of z/OS has type 119 SMF records, which pro- monitor product to be aware of all the IP pack- TCP/IP stacks. vides data on TCP initiation, TCP termination, ets flowing through the TCP/IP stack (and this D TCPIP,{procname},HELP to display ftp servers, ftp clients, statistics, servers, and includes UDP, ICMP, and OSPF packets). This a list of TCP/IP display options. interface. It also includes round-trip times and interface is event driven. The monitor builds a D TCPIP,{procname},Netstat, retransmissions for each TCP connection in complete picture of all IP-related activity, in ALLCONN|CONN to display socket the TCP termination record. Software can then real-time, by analyzing the packet headers, information for the TCP/IP stack. pick up the information from the SMF together with the maintenance of statistics D TCPIP,{procname},Netstat,ARP to records. The good news is that SMF exit- based on packet counts and lengths. display the contents of the ARP cache based monitoring forces the collection of There are two big drawbacks with this way for the TCP/IP stack. SMF records from TCP/IP. The downside is of monitoring. First, the amount of CPU time, D TCPIP,{procname},Netstat,DEVlinks that if the information is not needed there is paging rates, and virtual storage required can be to display device and link status for the an unnecessary system overhead with the large—and increases whenever the network TCP/IP stack. associated performance implication. traffic increases. Second, there can be a prob- D TCPIP,{procname},Netstat,HOME lem with selecting what data will be monitored. to display the IP address(es) for the SNMP TCP/IP stack. TCP1/IP MONITORING SOFT- D TCPIP,{procname},Netstat,ROUTE Most network hardware and software ven- WARE to display the routing table for the dors implement SNMP (Simple Network TCP/IP stack. Management Protocol) Management Inform- What’s needed is software that gives users V TCPIP,{procname},HELP to display ation Bases (MIBs) or diagnostic databases in real-time information about problems as they a list of TCP/IP vary options. their products. Various products provide are developing, and historical information for V TCPIP,{procname},PURGECACHE, generic access to SNMP MIBs. Usually a identifying trends. An easy to understand pic- linkname to purge the ARP cache for the vendor provides a product to access its own ture with drill-down ability would also be specified adapter (linkname is found from MIBs. Monitors accessing the data held in the nice. More over, the product should not dupli- the NETSTAT,DEVLINKS command). MIBs use the UDP-based SNMP protocol. cate information already on the system or V TCPIP,{procname},START|STOP, TCP/IP stack errors can often be found by consume too much valuable system resource devname to start or stop the device using SNMP. The z/OS MIB (1.2, 1.3, and acquiring and presenting the information. name identified in the NETSTAT DEV 1.4) has hundreds of variables. The major sec- And that’s what most of the third-party soft- output. tions in the z/OS MIB are: ware does.
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