After enabling path MTU discovery, how can I display the MTU discovered for an EE connection?

(1) You can use the PING command to determine the path MTU size to a host. From the TSO environment, issue ping with the pmtu yes parameter. From the z/OS UNIX environment, issue ping with the -P yes parameter.

If the Ping command receives an ICMP/ICMPv6 error message indicating that an echo request packet requires fragmentation, and if the host returned the next-hop MTU size in the ICMP/ICMPv6 message, then this MTU size is displayed:

                    Next-hop MTU size is nnnnn

          

If path MTU discovery is enabled and has already determined an MTU value for the destination, and the length of the Ping echo request packet is larger than this MTU size, then the local TCP/IP stack will not send out the packet. In this case, Ping displays one of the local stack's IP addresses as the address of the host where fragmentation is needed, and the next-hop MTU value displayed by Ping is the current path MTU value to the destination. For Ping commands to IPv4 destinations, the Ping command processing will not cause path MTU discovery support to be triggered for the destination. For IPv4, only TCP processing causes path MTU discovery support to be triggered.

Here is an example of using the TSO PING command with PMTU yes:

 ping hostipv6 (count 4 pmtu yes length 3000
          CS V1R13: Pinging host hostipv6.raleigh.ibm.com
          at IPv6 address 50c9:c2d4:0:5:9:6b00:111a:1
          Ping #1 needs fragmentation at: hoste.test.ibm.com (50c9:c2d4:0:3:9:6b00:111a:250e)
            Next-hop MTU size is 1500
          Ping #2 needs fragmentation at: local.host (50c9:c2d4:0:6:1:6b00:111a:0001)
            Next-hop MTU size is 1500
          Ping #3 needs fragmentation at: local.host (50c9:c2d4:0:6:1:6b00:111a:0001)
            Next-hop MTU size is 1500
          Ping #4 needs fragmentation at: local.host (50c9:c2d4:0:6:1:6b00:111a:0001)
            Next-hop MTU size is 1500
                                                    

Here is an example of using the z/OS UNIX ping command with -P yes:

 ping -c 4 -l 3000 -P yes -n  hostipv6
          CS V1R13: Pinging host hostipv6.raleigh.ibm.com
          at IPv6 address 50c9:c2d4:0:5:9:6b00:111a:1
          Ping #1 needs fragmentation at: hoste.test.ibm.com (50c9:c2d4:0:3:9:6b00:111a:250e)
            Next-hop MTU size is 1500
          Ping #2 needs fragmentation at: local.host (50c9:c2d4:0:6:1:6b00:111a:0001)
            Next-hop MTU size is 1500
          Ping #3 needs fragmentation at: local.host (50c9:c2d4:0:6:1:6b00:111a:0001)
            Next-hop MTU size is 1500
          Ping #4 needs fragmentation at: local.host (50c9:c2d4:0:6:1:6b00:111a:0001)
            Next-hop MTU size is 1500

(2) For an active link or interface, TCP/IP reports the interface MTU value in the ActMtu field of the Netstat DEVLINKS/-d report.

The syntax of the command to be used is :
D TCPIP,tcpprocname,NETSTAT,DEV

The above command will show characteristics of all interfaces.

To limit the display to a single interface, use this command: D TCPIP,tcpprocname,NETSTAT,DEV,INTFN=interface_name