Request/Response Types Carrying SCSI Payload

This request carries the SCSI CDB and all the other SCSI Execute Command [SAM2] procedure call IN arguments, such as task attributes, Expected Data Transfer Length for one or both transfer directions (the latter for bidirectional commands), and a task tag (as part of the I_T_L_x nexus). The I_T_L nexus is derived by the initiator and target from the LUN field in the request, and the I_T nexus is

implicit in the session identification.

In addition, the SCSI Command PDU carries information required for the proper operation of the iSCSI protocol -- the command sequence number (CmdSN) and the expected status sequence number (ExpStatSN) on the connection it is issued.

All or part of the SCSI output (write) data associated with the SCSI command may be sent as part of the SCSI Command PDU as a data

segment.

4.6.1.2. SCSI Response

The SCSI Response carries all the SCSI Execute Command procedure call (see [SAM2]) OUT arguments and the SCSI Execute Command procedure call return value.

The SCSI Response contains the residual counts from the operation, if any; an indication of whether the counts represent an overflow or an underflow; and the SCSI status if the status is valid or a response code (a non-zero return value for the Execute Command procedure call) if the status is not valid.

For a valid status that indicates that the command has been processed but resulted in an exception (e.g., a SCSI CHECK CONDITION), the PDU data segment contains the associated sense data. The use of

Autosense ([SAM2]) is REQUIRED by iSCSI.

Some data segment content may also be associated (in the data segment) with a non-zero response code.

In addition, the SCSI Response PDU carries information required for the proper operation of the iSCSI protocol:

- ExpDataSN - the number of Data-In PDUs that a target has sent (to enable the initiator to check that all have arrived) - StatSN - the status sequence number on this connection - ExpCmdSN - the next expected command sequence number at the target

- MaxCmdSN - the maximum CmdSN acceptable at the target from this initiator

4.6.1.3. Task Management Function Request

The Task Management Function Request provides an initiator with a way to explicitly control the execution of one or more SCSI tasks or iSCSI functions. The PDU carries a function identifier (i.e., which task management function to perform) and enough information to

unequivocally identify the task or task set on which to perform the action, even if the task(s) to act upon has not yet arrived or has been discarded due to an error.

The referenced tag identifies an individual task if the function refers to an individual task.

The I_T_L nexus identifies task sets. In iSCSI, the I_T_L nexus is identified by the LUN and the session identification (the session identifies an I_T nexus).

For task sets, the CmdSN of the Task Management Function Request helps identify the tasks upon which to act, namely all tasks associated with a LUN and having a CmdSN preceding the Task Management Function Request CmdSN.

For a task management function, the coordination between responses to the tasks affected and the Task Management Function Response is done by the target.

4.6.1.4. Task Management Function Response

The Task Management Function Response carries an indication of

function completion for a Task Management Function Request, including how it completed (response and qualifier) and additional information for failure responses.

After the Task Management Function Response indicates task management function completion, the initiator will not receive any additional responses from the affected tasks.

4.6.1.5. SCSI Data-Out and SCSI Data-In

SCSI Data-Out and SCSI Data-In are the main vehicles by which SCSI data payload is carried between the initiator and target. Data payload is associated with a specific SCSI command through the

Initiator Task Tag. For target convenience, outgoing solicited data also carries a Target Transfer Tag (copied from R2T) and the LUN.

Each PDU contains the payload length and the data offset relative to the buffer address contained in the SCSI Execute Command procedure call.

In each direction, the data transfer is split into "sequences". An end-of-sequence is indicated by the F bit.

An outgoing sequence is either unsolicited (only the first sequence can be unsolicited) or consists of all the Data-Out PDUs sent in response to an R2T.

Input sequences enable the switching of direction for bidirectional commands as required.

For input, the target may request positive acknowledgment of input data. This is limited to sessions that support error recovery and is implemented through the A bit in the SCSI Data-In PDU header.

Data-In and Data-Out PDUs also carry the DataSN to enable the initiator and target to detect missing PDUs (discarded due to an error).

In addition, the StatSN is carried by the Data-In PDUs.

To enable a SCSI command to be processed while involving a minimum number of messages, the last SCSI Data-In PDU passed for a command may also contain the status if the status indicates termination with no exceptions (no sense or response involved).

4.6.1.6. Ready To Transfer (R2T)

R2T is the mechanism by which the SCSI target "requests" the

initiator for output data. R2T specifies to the initiator the offset of the requested data relative to the buffer address from the Execute Command procedure call and the length of the solicited data.

To help the SCSI target associate the resulting Data-Out with an R2T, the R2T carries a Target Transfer Tag that will be copied by the initiator in the solicited SCSI Data-Out PDUs. There are no protocol-specific requirements with regard to the value of these tags, but it is assumed that together with the LUN, they will enable the target to associate data with an R2T.

R2T also carries information required for proper operation of the iSCSI protocol, such as:

- R2TSN (to enable an initiator to detect a missing R2T) - StatSN

- ExpCmdSN - MaxCmdSN

4.6.2. Requests/Responses Carrying SCSI and iSCSI Payload