iSCSI PDU Formats: iSCSI uses one general PDU format for many purposes. The specific format of an iSCSI PDU is determined by the type of PDU. RFC 7143 defines numerous PDU types to facilitate communication between initiators and targets.

The primary PDU types include:

• login request
• login response
• SCSI command
• SCSI response
• data-out
• data-in
• ready to transfer (R2T)
• selective negative acknowledgment (SNACK) request
• task management function (TMF) request
• TMF response, and reject.

All iSCSI PDUs begin with a basic header segment (BHS). The BHS may be followed by one or more additional header segments (AHS), a header-digest, a data segment, or a data-digest. The data-digest may be present only if the data segment is present.