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

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 4 iSCSI Layering
1.4 iSCSI Session and TCP Connections
In an iSCSI session, communication between an initiator and a target occurs over one or more
TCP connections. The TCP connections carry control messages, data digests, SCSI commands,
parameters, and data, all encapsulated in iSCSI Protocol Data Units (iSCSI PDUs).
The TCP connections that link an initiator with a target, forming an iSCSI session, are comparable
to a SCSI I_T nexus.
An initiator sees one “target image” across all of the connections in a session. A target also sees
one “initiator image” across all of the connections in a session.
iSCSI defines two types of sessions:
• Normal Operational Session - a session in which SCSI commands, data and responses can
be transferred between an iSCSI initiator and an iSCSI target.
• Discovery Session - a session only opened for target discovery.
Figure 5 An iSCSI Session
1.4 iSCSI Session and TCP Connections 7