A Conceptual Overview of iSCSI

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entity. The iSCSI node is independent of the network portals providing access. The iSCSI name
uniquely identifies the iSCSI node.
A single iSCSI node within a network entity may be accessed through multiple network portals on
that network entity. An iSCSI node is identified by its worldwide unique name, and an iSCSI node is
defined independent of the network portals on the network entity.
A network portal for an iSCSI initiator node is identified by its IP address. Because the iSCSI
initiator will initiate I/O operations, the TCP port used by the initiator is chosen dynamically. The
initiator TCP port number may change on successive application opens. There is no dedicated
iSCSI initiator TCP port.
A network portal for an iSCSI target node is identified by its IP address and TCP port number. For
an initiator to connect to a target, the IP address and the TCP port number must be known. TCP
port 3260 is the default assigned TCP port for iSCSI targets; however, other ports may be used if
the target provides this flexibility.
Be careful not to associate a network portal with a physical network port. For a single physical port
MAC address, several different IP address/TCP port number pairs can be used to identify several
distinct network portals.
6. Sessions and Connectivity
At the SCSI level, for an initiator and target to communicate, a SCSI I-T Nexus must be established.
At the iSCSI level, a concept equivalent to the SCSI I-T Nexus, known as an iSCSI session, exists
between an iSCSI target and an iSCSI initiator. iSCSI session components on the initiator and
target must be connected to completely establish an iSCSI session. The iSCSI session provides
the mass storage transport for a SCSI I-T Nexus.
A TCP/IP connection ties the iSCSI initiator and target session components together. Network
portals identified by their IP address and TCP port numbers define the endpoints of a connection.
The network entity containing the iSCSI initiator node initiates the establishment of the TCP/IP
connection. The initiator IP address is known, and a TCP port is allocated dynamically, thus
defining the network portal for the initiator. A network portal of the network entity capable of
accessing the iSCSI target node is identified by its IP address and TCP port number. Through
TCP/IP, the connection is established. TCP/IP network connection establishment has not changed
for iSCSI. TCP/IP protocol permits connection establishment between 2 network entities. For
iSCSI, connection establishment is initiated by the network entity on behalf of an iSCSI initiator
node only. Network entities will never initiate establishment of a network connection on behalf of an
iSCSI target node.
An iSCSI session may have one or more connections between the session components on the
initiator and the target (Figure 6). The number of connections present is a function of how iSCSI is
implemented, and is not dictated by the iSCSI specification, except that at least one connection
must be established for a session. For multi-connection sessions, both session endpoints must
support the multi-connection functionality.