MTU


>>-+------------------+----------------------------------------><
   '-MTU--=--mtu_size-'

statements:

GROUP

dependencies:

Enterprise Extender connection network connections only

range:

1024 - 32767

Defines the maximum transmission unit (MTU), in bytes, that is used for Enterprise Extender connections associated with the connection network (CN) that is defined by this group. 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 are allowed for 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 in 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 size 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.

Result: If a predefined EE connection is established between two EE endpoints before the VRN connection is established (using the same VIPA addresses), the VRN connection uses the established predefined EE connection. In this case, to control the MTU size associated with the EE VRN, you must specify the MTU operand for the predefined EE connection (for example, on the switched PU definition).

Rule: When the MTU operand is specified on the GROUP statement, VNNAME and VNTYPE must also be specified on the GROUP statement. If the group is not defined as a connection network, the MTU operand cannot be specified on the GROUP definition statement.

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 dial-in Enterprise Extender connections with associated PUs that are dynamically created, you can define the MTU parameter in the model major node (DYNTYPE=EE) PU definition statement.
For predefined Enterprise Extender connections, you can define the MTU parameter on the PU definition in the switched major node.