HP-UX iSCSI Software Initiator Support Guide (Edition 7)

1.2 The iSCSI PDU
iSCSI initiators and targets communicate with messages known as “iSCSI Protocol Data Units”. An
iSCSI PDU has a Header and an optional Data Section.
Figure 2 iSCSI Protocol Data Unit (PDU)
iSCSI PDUs are transported in the TCP Segment Data Area of Ethernet Frames. The size of an iSCSI
PDU is not dictated by the capacity of the TCP segment data area and an iSCSI PDU does not
need to begin at a specific offset within a TCP segment data area. An iSCSI PDU can span multiple
TCP segment data areas, or several iSCSI PDUs can be contained in a single TCP segment data
area.
Figure 3 TCP Segment Data Area of an Ethernet Frame
1.3 iSCSI Layering
iSCSI constructs Protocol Data Units (PDUs) consisting of SCSI commands, SCSI data and SCSI
responses. iSCSI PDUs are inserted in the TCP segment data area of IP datagrams. The IP datagrams
are then moved through the TCP/IP stack and transported over the network, between the SCSI
services layer of host systems and the SCSI services layer of target storage devices.
The iSCSI protocol works seamlessly over TCP/IP networks, without requiring any changes to
TCP/IP protocols.
In the outbound direction (Initiator to Target):
1. The SCSI layer builds SCSI Command Descriptor Blocks (CDBs) and passes them to the iSCSI
layer (along with the rest of the command execution parameters).
2. The iSCSI layer builds iSCSI PDUs and relays them to one or more TCP connections.
3. The TCP connection(s) form an initiator-target “session” (I_T Nexus).
6 iSCSI Overview