| statements: | PU |
| dependencies: | DYNTYPE=EE only |
| range: | 1024 - 32767 |
Defines the maximum transmission unit (MTU), in bytes,
that is used for dial-in Enterprise Extender connections with associated
PUs that are dynamically created using this model PU definition. This
MTU size is reduced by VTAM® to
account for header lengths such as the IP, UDP, and LLC headers that
are necessary for EE traffic. If MTU is not specified, VTAM uses the value learned from the TCP/IP
stack. This learned value is the minimum MTU size (minus IP and UDP
header sizes) associated with all the defined TCP/IP interfaces that
can use this local static VIPA. If an MTU size with a value is specified, VTAM uses the lesser of this mtu_size value
and the value learned from TCP/IP. If the TCP/IP stack presents a
value less than 768 bytes, VTAM sets
the MTU to 768 bytes because this is the smallest packet size allowed
by the HPR architecture. Typically, the MTU size for an EE connection
is constant after the EE connection is established. However, if the
MTU size of the TCP/IP stack changes, RTP pipes with endpoints on
the same node as the TCP/IP stack dynamically detect these changes
when their outbound packets are being transmitted. If the learned
value is less than the mtu_size value, VTAM uses the learned value. Reasons
for MTU size changes include the following information:
- New IP routes with different local MTU sizes become available
- Existing IP routes become unavailable
- Path MTU discovery is enabled for IPv4 or IPv6 EE connections (see PMTUD start option for more information), and path MTU changes are discovered in the IP network
You can enable path MTU discovery for IPv4 and IPv6 EE connections (see PMTUD start option for more information). If path MTU discovery is enabled, VTAM can dynamically learn about any MTU size changes that occur in the IP network. RTP pipes traversing Enterprise Extender connections can reduce their packet sizes to avoid IP fragmentation. In some environments, you might not be able to use path MTU discovery because firewalls must allow ICMP messages to be forwarded back to the host. In these cases, specify the MTU operand to control the maximum packet size that VTAM sends out on an Enterprise Extender connection.
Tips:
- If the mtu_size value is specified and IPv4 Enterprise Extender packets are being fragmented in the IP network, you might need to reduce the mtu_size value to account for optional headers that might be present.
- For EE connection networks, you can define the MTU parameter on the connection network GROUP definition statements in the EE XCA major node.
- For predefined Enterprise Extender connections, you can define the MTU parameter on the PU definition in the switched major node.