HP-UX iSCSI Software Initiator Support Guide, HP-UX 11i v1 and 11i v2, July 2005

Chapter 1

iSCSI Overview

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).

In the inbound direction (Target to Initiator):

1. The iSCSI layer receives iSCSI PDUs on one or more TCP connections in a TCP/IP stream.

2. The iSCSI layer extracts the SCSI CDBs from the iSCSI PDUs and passes them to the SCSI layer.

Figure 1-4 iSCSI Layering