iSCSI is a block-level protocol that encapsulates SCSI commands into TCP/IP packets, and thereby leverages the investment in existing IP networks.
SCSI is a popular block transport command protocol that is used for high bandwidth transport of data between hosts and storage systems (e.g., disk, tape). Traditionally, SCSI commands have been transported over dedicated networks such as SCSI buses and Fiber Channel. With the emergence of Gigabit and 10 Gb/s Ethernet LANs, it is now feasible to transport SCSI commands over commodity networks and yet provide high throughput to bandwidth-intensive storage applications. To do so, iSCSI connects a SCSI initiator port on a host to a SCSI target port on a storage subsystem. For the sake of uniformity with NFS, we will refer to the initiator and the target as an iSCSI client and server, respectively.
Some of the salient features of iSCSI are as follows: (i) it uses the notion of a session between the client and the server to identify a communication stream between the two; (ii) it allows multiple connections to be multiplexed into a session; (iii) it supports advanced data integrity, authentication protocols as well as encryption (IPSEC)--these features are negotiated at session-startup time; and (iv) it supports advanced error recovery using explicit retransmission requests, markers and connection allegiance switching .