IUCV

IUCV (Inter User Communication Vehicle) is a data transfer mechanism in IBM VM line of operating systems.

IUCV allows establishing point to point communication channels, either between 2 virtual machines or between a virtual machine and hypervisor services. In effect, IUCV provides a form of message-based interaction between virtual machines that anticipated the client/server interaction between network connected physical machines that emerged later on distributed systems.

IUCV is implemented by CP (the VM hypervisor) and controls all aspects of session establishments, message passing and flow control.

Before a virtual machine can use the IUCV service, it must first indicate the address of an area within its address space where CP will be able to store information regarding pending information or status. Therefore, the DECLARE BUFFER method must be invoked first.

In IUCV terminology, the session between 2 end points is called a PATH. It is identified at each end by a Path ID which is only relevant to the virtual machine that owns the session end. A path is always a connected channel - meaning there are no connectionless paths.

To establish a path, the initiating virtual machine must invoke the CONNECT method, and specify the path target identity, which is either another virtual machine name or the name of a CP system service - which all start with the '*' character - which is not a valid character within a virtual machine name.

Provided the target has initialized IUCV itself, the target will be notified of the pending incoming path connection and may then either use the ACCEPT method - to complete path establishment - or the SEVER method - which effectively closes the pending path.

Once the path is established, messages may be passed between the two path endpoints.

IUCV Messages are bounded, that is, they have a beginning and an end. If more than one message is pending on a path for an endpoint, IUCV will not merge the messages.

Messages are sent on the path using the SEND method. The other end point can then receive the message using the RECEIVE method. If the original message also requested a reply, the receiving end point then use the REPLY method to send that reply.

Multiple messages may be made pending on a path. The number of messages allowed pending for a path is specified during path establishment but cannot exceed 65535. Attempting to send a message on a path which has reached its pending message limit will result in an error.

Data transfer may be temporarily suspended by using the QUIESCE method. While the path is suspended, no further message transfers are allowed on the path until the RESUME method is invoked by the virtual machine that initially suspended the path.

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