Ordering of requests using offset fields in a fibre channel input/output data routing system and method

A method for performing an input/output (I/O) operation between a host computer (102) and control-unit (118) comprises obtaining information about the I/O operation from a channel subsystem (128) in the host system, the channel having a local processor and memory. An Address Control Word (ACW) (Fig 11) is generated and specifying the memory location and having an expected offset filed (264, Fig 11) indicating a relative order of an expected data transfer request is stored in the local memory. An address control structure (Fig 5) for each data transfer in the I/O operation is generated and forwarded to the network interface between the channel subsystem and I/O device and specifies a location in the local memory of the ACW includes an offset field (180, Fig 5) indicating a relative order or a corresponding data transfer request relative to other data transfers in the plurality of consecutive data transfer requests. On receipt of an I/O command message via the network a data transfer request from the network interface is issued and includes addressing information. A data route in the channel compares the offset and expected offset fields to determine if the transfer request has been received in the correct order. If the request has been received in order, the corresponding ACW is accessed retrieved and used to route the data transfer request to the host memory (106) specified in the ACW. The expected offset field in the ACW is incremented by an amount corresponding to a size of the data transfer request.