Connection Handling in Mixed IPv4/IPv6 Environments

This document describes the type of connection used when the TCPI or TCPX ALLOWIP6 driver parameter is set to "Y" and the TCP/IP stack is enabled for IPv6.

Outgoing Connections

The type of outgoing connection used depends on the settings of other parameters.

  • If a value is specified for the V6IPADDR link parameter, the connection is attempted with IPv6.

  • If a value is specified for the INETADDR link parameter, the connection is attempted with IPv4.

  • If a value is specified for the ADJHOST link parameter, the connection attempt is based on whether the DNS name resolves to an IPv4 or and IPv6 address.

  • If values are specified for both the V6IPADDR and ADJHOST parameters, the V6IPADDR setting takes precedence. Likewise, if values are specified for both the INETADDR and ADJHOST parameters, the INETADDR setting takes precedence. The V6IPADDR and INETADDR parameters are mutually exclusive; they cannot both be specified in the same link.

Note:
If an IPv6 connection is attempted in a BS2000 environment, the target driver must have ALLOWIP6=Y to listen on the IPv6 address.

Incoming Connections

Links are searched for a match to the incoming connection request based on the following:

  • If V6IPADDR is specified, the link is selected if its connecting address matches the V6IPADDR setting.

  • If INETADDR is specified, the IPv4 address is typically delivered as an IPv4-mapped address. The link is selected if the IPv4 address in the mapped address matches the INETADDR setting.

    IPv4-mapped addresses are IPv4 addresses that are embedded within an IPv6-format address using the following form: 10 bytes of x'00', two bytes of x'FF', and the four-byte IPV4 address. For example: 

    ::FFFF:129.144.52.38

    Note:
    This form has the IPv4 address in dotted-decimal notation. This convention can be used in the V6IPADDR parameter.

    For more information about IPv4-mapped addresses, refer to your IBM documentation.

  • If ADJHOST is specified, the connecting IP address is resolved to a host name. The link is selected if the host name matches the ADJHOST setting.

  • If no matching link is found and the ACCEPTUI parameter is set to "Y", the connection is treated as unsolicited (a new link is created). Otherwise, the connection attempt is rejected.

Note:
If ALLOWIP6=Y is set in a BS2000 environment, it is listening on the IPv6 address. Connection using an IPv4 address cannot be made.