(1) .-PMTUD--=--TCPVALUE-----. >>-+------------------------+---------------------------------->< '-PMTUD--=--+-TCPVALUE-+-' '-NO-------'
Notes:
- PMTUD is meaningful only if the NODETYPE start option is also used.
A TCP/IP stack can use path MTU discovery to dynamically learn the maximum MTU for the total IP network connection. Use the PMTUD start option to enable and disable path MTU discovery for IPv4 Enterprise Extender (EE) connections and IPv6 EE connections. You can change the value of PMTUD with the MODIFY VTAMOPTS command while VTAM® is running.
- PMTUD=TCPVALUE
- Indicates that VTAM accepts
the TCP/IP stack (associated with EE) setting to control path MTU
discovery for both IPv4 and IPv6 EE connections.
- IPv4
- For IPv4 EE connections, the TCP/IP stack associated with EE is enabled to use path MTU discovery with the PATHMTUDISCOVERY operand on the IPCONFIG profile statement. For more information about the TCP/IP PATHMTUDISCOVERY setting, see z/OS Communications Server: IP Configuration Reference.
- IPv6
- For IPv6 EE connections, path MTU discovery is always enabled in the TCP/IP stack. The TCPVALUE setting indicates that VTAM accepts the TCP/IP stack (associated with EE) setting. Therefore, path MTU discovery is also enabled by default for IPv6 EE connections.
- PMTUD=NO
- Disables path MTU discovery for IPv4 EE connections and IPv6 EE connections.
Requirement: Path MTU discovery uses ICMP fragmentation-needed errors to detect the PMTUD for a path. If you use path MTU discovery, you must permit ICMP errors to flow to all hosts along the path of a connection. PATHMTUDISCOVERY does not function if a firewall blocks ICMP errors.
Tip: If you do not want path MTU discovery enabled
for EE, but you want to limit the maximum packet size that EE will
transmit, consider specifying the MTU operand available on the following
major nodes:
- For EE connection networks, you can define the MTU operand on the connection network GROUP definition statements in the EE XCA major major node.
- For dial-in EE connections with associated PUs that are dynamically created, you can define the MTU operand on the model major node (DYNTYPE=EE) PU definition statement.
- For predefined EE connections, you can define the MTU operand on the PU definition statement in the switched major node.