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iSCSI is a TCP/IP-based protocol for
establishing and managing connections between IP-based storage
devices, hosts and clients. The iSCSI protocol is being
standardized through the Internet Engineering Task Force (IETF).
iSCSI builds on the two most widely used protocols from the
storage and networking worlds.
- On the storage side, iSCSI uses the SCSI storage
commands used throughout all common storage
configurations.
- On the networking side, iSCSI uses IP and Ethernet, the
most widely used corporate networks. iSCSI is built on top
of Transmission Control Protocol (TCP)/IP to simplify the
protocol design and speed first implementations.
Together, this technological synergy enables block-level
storage data to be carried over TCP/IP networks.
iSCSI encapsulates SCSI packets in TCP for reliable transport
and routing using IP. As a result, standard SCSI commands can
pass between host systems (or "initiators") and
storage devices ("targets") over a standard
Ethernet/IP TCP/IP network infrastructure, rather than via
SCSI cabling or FC connections.
- iSCSI initiators are devices such as file servers that
exchange block-level data with iSCSI target devices and
encapsulate SCSI commands into TCP/IP for routing over an
IP network.
- iSCSI targets are devices that receive iSCSI commands
and exchange data across the IP network and/or through
internetworking equipment (such as storage routers and
switches). Examples of iSCSI targets include disk arrays,
RAID devices, and tape libraries.
Successful transporting and delivery of data from an initiator
to a target requires a structure that builds on the existing
SCSI and TCP/IP systems without disturbing them. The iSCSI
protocol is a multi-layered model that utilizes SCSI commands
and TCP/IP services to transport I/O block data over networks.
The layered model is used to ensure that each protocol layer
on the target side receives the exact object sent by the
corresponding layer on the initiator side. In other words, the
target application layer receives the same message sent by the
initiator application layer and the target's transport layer
receives the exact packet sent by the initiator's transport
layer.
An "iSCSI packet" contains SCSI
data and the iSCSI header, which is created by the iSCSI
initiator. The iSCSI header is used by the iSCSI target to
extract and store the I/O block data. While the iSCSI header
is the storage-related portion of the packet, other headers
provide information necessary for carrying out normal
networking functions. The IP header provides packet routing
information used for moving the messages across the network.
The TCP header contains the identification and control data
needed to guarantee message delivery to a desired destination.
For initiators and targets to communicate, they need to find
and identify each other. The process used by the initiator for
finding targets and establishing their address locations is
called Discovery. The initiator needs to establish three
elements for successful discovery: IP address, TCP port
number, and iSCSI target name. The initiator uses the IP
address and the TCP port to establish a TCP connection. The
iSCSI target name is then used to establish an iSCSI session
with the target.
- iSCSI is based on standards defined by the Internet
Engineering Task Force (IETF), the main standards
organization for the Internet.
- iSCSI provides the same reliability features as FC at a
fraction of FC's deployment and storage system costs.
- iSCSI can take advantage of existing Ethernet cabling
and network elements.
- iSCSI uses ubiquitous TCP/IP for global connectivity.
- iSCSI leverages the existing expertise of network
administrators, integrators, and support services.
- iSCSI offers the same performance class as FC, with
future 10 Gb Ethernet performance due to deliver 10 times
the performance of existing FC.
- iSCSI can natively span over LAN , MAN and WAN - the
most obvious protocol for implementing disaster recovery
solutions.
- iSCSI is being rapidly adopted by system, storage, and
network vendors.
For further information about iSCSI, visit the links
page.
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