Examples of managing TCP/IP

TCP/IP, like VTAM, has a number of commands available to monitor and change the environment.

TCP/IP provides two methods of issuing most commands, either through the z/OS console or from within a TSO or z/OS UNIX environment. However, the more powerful commands for controlling the TCP/IP environment are issued from the z/OS system console. This section focuses on console commands only. The TCP/IP commands are documented in z/OS IP System Administrator's Commands.

There are many commands for displaying and testing a TCP/IP network. Commands such as PING and TRACERTE are not covered here. They are effectively standard across all platforms and commonly used.

Examples of displaying TCP/IP

The DISPLAY TCPIP,,NETSTAT command is often used to monitor TCP connections. On a busy host, the command would need to be narrowed in focus to only display connections for a specific port. For example, a display of all TN3270 connections (port 23) can be found in Figure 1.

The complete command issued was:

D TCPIP,tcpprocname,NETSTAT,ALLCON,PORT=23

In this display, a network administrator can determine at a glance the number of connections and the state of each connection. If a problem is noted, the IP address of the remote host ("FOREIGN SOCKET") is readily available.

Figure 1. DISPLAY NETSTAT of port 23

 EZZ2500I NETSTAT CS V1R6 TCPIP 
 USER ID  CONN     LOCAL SOCKET           FOREIGN SOCKET        STATE  
 TCPIP    00000017 0.0.0.0..23            0.0.0.0..0            LISTEN 
 TCPIP    0000D4DE 192.168.246.24..23     192.168.6.251..2119   ESTBLSH
 TCPIP    00244B11 192.168.246.24..23     192.168.7.99..1499    CLOSWT
 3 OF 3 RECORDS DISPLAYED

On a regular basis, some automation programs check device or route status using the DISPLAY NETSTAT command. A network administrator might do the same manually, or might do so in response to a reported connectivity problem.

To check the status of all TCP/IP devices, the following command might be issued:

D TCPIP,tcpprocname,NETSTAT,DEV

Figure 2 shows the output from the command. Note the LNKSTATUS field, which describes whether the device is fully functional ("READY") or not. In addition, various statistics and counts can be found.

Figure 2. DISPLAY NETSTAT DEV command output

 DEVNAME: OSA100            DEVTYPE: MPCIPA                        
   DEVSTATUS: READY         CFGROUTER: NON  ACTROUTER: NON         
   LNKNAME: OSATR100          LNKTYPE: IPAQTR      LNKSTATUS: READY
     NETNUM: N/A  QUESIZE: N/A  SPEED: 0000000100                  
     MACADDRORDER: NON-CANONICAL    SRBRIDGINGCAPABILITY: YES      
     IPBROADCASTCAPABILITY: NO      ARPBROADCASTTYPE: ALL RINGS    
     ARPOFFLOAD: YES                ARPOFFLOADINFO: YES            
     ACTMTU: 17914                                                 
     READSTORAGE: GLOBAL (4096K)    INBPERF: BALANCED              
   BSD ROUTING PARAMETERS:                                         
     MTU SIZE: 00000             METRIC: 00                        
     DESTADDR: 0.0.0.0           SUBNETMASK: 255.255.255.128       
   MULTICAST SPECIFIC:                                             
     MULTICAST CAPABILITY: YES                                     
     GROUP             REFCNT                                      
     -----             ------                                      
     224.0.0.1         0000000001                                  
   LINK STATISTICS:                                                
     BYTESIN                           = 7721257                   
     INBOUND PACKETS                   = 44830                     
     INBOUND PACKETS IN ERROR          = 85896 
     INBOUND PACKETS DISCARDED         = 0  
     INBOUND PACKETS WITH NO PROTOCOL  = 0  
     BYTESOUT                          = 252
     OUTBOUND PACKETS                  = 3  
     OUTBOUND PACKETS IN ERROR         = 0  
     OUTBOUND PACKETS DISCARDED        = 0  
1 OF 1 RECORDS DISPLAYED

If a connectivity problem is apparent and the device appears to be in good working order, the next step would be to ensure that there were no routing problems with respect to (or from the perspective of) the z/OS host. Obviously, a routing problem could exist elsewhere in the network.

The command to display routes is:

D TCPIP,tcpprocname,NETSTAT,ROUTE

Figure 3 shows sample output from a NETSTAT ROUTE command. A network administrator would need to know which network is experiencing connectivity problems. The network administrator would then scan this table for the appropriate route taken to reach the network.

The flags field tells the network administrator information about the route. For example, in this display, the flags shown are:

U: The route is Up.
G: The route is a gateway (default) route.
S: The route was created from a static definition (not learned dynamically via RIP or OSPF, for example).
H: The route is a host route, with a network mask of 32 bits (that is, only one destination host is defined by this route).

Other route flags exist and can be found documented under the NETSTAT command in z/OS IP System Administrator's Commands.

Figure 3. DISPLAY NETSTAT ROUTE command

 EZZ2500I NETSTAT CS V1R6 TCPIP 278                              
 DESTINATION         GATEWAY          FLAGS     REFCNT  INTERFACE
 DEFAULT             192.168.246.1    UGS       000013  ETH1     
 DEFAULT             192.168.247.129  GS        000000  OSATR104 
 9.21.111.210/32     0.0.0.0          H         000000  SNA1     
 9.23.246.0/24       0.0.0.0          US        000000  ETH1     
 9.23.246.0/24       0.0.0.0          S         000000  ETH2     
 9.23.246.16/32      0.0.0.0          UH        000000  VLINK1   
 9.23.246.23/32      0.0.0.0          H         000000  ETH2     
 9.23.246.24/32      0.0.0.0          UH        000000  ETH1     
 9.23.247.128/25     0.0.0.0          US        000000  OSATR100 
 9.23.247.128/25     0.0.0.0          S         000000  OSATR104 
 9.23.247.130/32     0.0.0.0          H         000000  OSATR104 
 9.23.247.135/32     0.0.0.0          UH        000000  OSATR100 
 127.0.0.1/32        0.0.0.0          UH        000020  LOOPBACK 
 13 OF 13 RECORDS DISPLAYED

Examples of controlling TCP/IP

You can alter the TCP/IP configuration with the VARY TCPIP,,OBEYFILE command. The OBEYFILE command is a very powerful command, since it can change any aspect of the TCP/IP configuration. This is because the OBEYFILE command runs a process similar to the process that runs during TCP/IP startup. An OBEYFILE causes TCP/IP to read a configuration file in the same fashion as the TCP/IP PROFILE data set is read at startup.

A sample of an OBEYFILE command is as follows:

VARY TCPIP,tcpprocname,OBEYFILE,DSN=your.obey.file

Note: Unlike VTAM, of which only one copy can ever execute within a single LPAR, up to 8 TCP/IP stacks can be running concurrently within an LPAR. To identify which stack should be the target of a command, tcpprocname should be included in any TCP/IP command.

If tcpprocname is not coded (that is, there is nothing between the two commas), then the operand defaults to TCPIP.

In this example, your.obey.file might contain a new set of DEVICE and LINK statements, a HOME statement, or both.

Sometimes TCP/IP connections need to be cleared by issuing a VARY TCPIP,,DROP command. Before issuing the command, a network administrator would have discovered a connection that needed to end. Either the connection is an undesired connection entirely, or else it is in a state needing external intervention. Such a connection would be discovered using the DISPLAY NETSTAT command illustrated in Figure 1.

Note in this example that the last connection is in a state of CLOSWT. A CLOSWT connection is often an indication of a problem with a TCP/IP application. The application might have left this connection in this unusable state indefinitely. The DROP command can be used to cleanup (remove) such a connection. The command to remove the CLOSWT connection above would need to use the connection number to identify it on a DROP command, as follows:

VARY TCPIP,tcpprocname,DROP,CONN=244B11

This command would clean up the connection as far as the z/OS TCP/IP is concerned.

The VARY TCPIP,,STOP and VARY TCPIP,,START commands are used for stopping or starting TCP/IP devices. These commands are often the first set of commands issued when a network administrator discovers a device failure. For example, if a device problem was detected and the device was known to be inoperative, a VARY STOP followed by a VARY START would be attempted.

Note: OSA devices can be configured with an AUTORESTART parameter to cause TCP/IP to attempt to restart automatically a device under most circumstances.

A sample command would be:

VARY TCPIP,tcpprocname,START,OSA2380

This command causes TCP/IP to attempt to restart device OSA2380.