The iSCSI protocol uses TCP for its data transfer. Its strength, compared to other network storage protocols, like Fibre Channel-SAN, is that all it needs to operate is the simple and ubiquitous ethernet interface (Gigabit ethernet interface). This in turn enables centralization of storage without all the usual in-compatibility problems associated with various nodes on the network.
Critics of iSCSI expect iSCSI to perform worse than Fibre Channel due to the protocol overhead TCP/IP adds to the communication between client and storage. However new techniques like TCP Offload Engine(TOE) help in reducing this overhead. A Gigabit Ethernet NIC with a TOE offloads the host CPU from the network related processing and speeds up the data transfer considerably.
iSCSI was ratified as an official standard by the Internet Engineering Task Force on February 11, 2003.