Determining the maximum transmission unit

TCP/IP uses the maximum transmission unit (MTU) value to determine the largest sized frame to send. The MTU value that is in effect for a given outbound send is one of the following two values:

• Path MTU value

  TCP/IP automatically enables path MTU discovery for IPv6. If a packet is an IPv6 packet, or if a packet is an IPv4 packet and path MTU discovery is enabled, the path MTU value is used to determine the maximum size of the packet. Path MTU discovery initially sets the path MTU value to the actual route MTU value for the route. If packets require fragmentation to get to the final destination, path MTU discovery determines the path MTU value by repeatedly decreasing the value until it can send packets to the final destination without fragmentation.

  Guideline: You can enable path MTU discovery for IPv4 by configuring IPCONFIG PATHMTUDISCOVERY in the TCP/IP profile.
• Actual route MTU value

  The actual route MTU value is the lesser of the interface MTU value and the configured route MTU value. If path MTU discovery is not enabled, the actual route MTU value is used.

  • Interface MTU value

    The interface MTU value is a characteristic of an interface, and is either learned from the device during activation or is hardcoded based on the type of the physical device. For information about the interface MTU values that TCP/IP uses for the various network interface types supported by TCP/IP, see the summary of DEVICE and LINK statements and the summary of INTERFACE statements in z/OS Communications Server: IP Configuration Reference. For an IPAQENET6 interface, or for IPAQENET or IPAQIDIO interfaces defined with the INTERFACE statement, you can configure a lower interface MTU value using the MTU keyword on the INTERFACE statement.

    Restriction: You cannot modify the interface MTU value for IPAQENET or IPAQIDIO interfaces defined using the DEVICE, LINK, and HOME statements.
    Results:

    • The TCP/IP stack sets the interface MTU value to the lesser of the learned MTU value and the MTU value configured on the INTERFACE statement.
    • For an active link or interface, TCP/IP reports the interface MTU value in the ActMtu field of the Netstat DEVLINKS/-d report.
  • Configured route MTU value

    The configured route MTU value is the MTU size that is configured for a route.

    • Static route

      For a static route, you can specify the configured route MTU value in the TCP/IP profile on a ROUTE entry in a BEGINROUTES block or on a GATEWAY statement.

    • IPv4 dynamic route

      For IPv4 dynamic routes over an interface that are added by OMPROUTE, the configured route MTU value is the value of the MTU keyword specified on the RIP_INTERFACE, OSPF_INTERFACE or INTERFACE statement in the OMPROUTE configuration file for the outgoing interface of the route.

      Result: If you do not specify an MTU value for an interface in the OMPROUTE configuration file, OMPROUTE uses the value 576.
    • IPv6 dynamic route

      For IPv6 dynamic routes added by OMPROUTE, OMPROUTE learns the interface MTU value from TCP/IP; you cannot configure a route MTU value in the OMPROUTE configuration file.

      Result: For IPv6 dynamic routes that are learned by OMPROUTE, the configured route MTU size is the same as the interface MTU size.

These factors comprise a general set of rules for how TCP/IP determines the MTU, but there are some exceptions. For example, if an application uses the IPV6_USE_MIN_MTU socket option, TCP/IP sends outbound packets using the IPv6 minimum MTU value 1280.