The SNIA (Storage Networking Industry Association) dictionary defines iSCSI (Internet Small Computer Systems Interface) as a transport protocol that allows the SCSI block protocol to be transmitted over a TCP-based IP network. This protocol is standardized by the Internet Engineering Task Force (IETF) and is detailed in RFC 7143 and RFC 7144.
iSCSI, much like Fibre Channel, can be employed to create a Storage Area Network (SAN). iSCSI traffic can be transmitted over a shared network or a dedicated storage network. However, iSCSI is not suitable for file access Network Attached Storage (NAS) or object storage access, as these utilize different transport protocols.
Transports for iSCSI
Various transport methods can be used for iSCSI, the most common being TCP/IP over Ethernet. Additionally, Remote Direct Memory Access (RDMA) can be utilized with iSER (iSCSI Extensions for RDMA). When using iSER, the transport can be either RoCE (RDMA over Converged Ethernet) or InfiniBand, with the underlying network being Ethernet for RoCE or InfiniBand for InfiniBand transport.
Compatibility and Usage
iSCSI is supported by all major operating systems and hypervisors and can run on both standard network cards and specialized network cards (such as those with TCP Offload Engine (TOE) capabilities). It is also compatible with nearly all enterprise storage arrays. Due to these characteristics, iSCSI has become popular for “Tier 2” applications that require good, but not the highest, block storage performance, as well as for storage shared by multiple hosts. It is also widely used by hyperscalers and large cloud service providers when a block storage solution running over Ethernet is needed.
Performance Enhancements
Performance of iSCSI can be enhanced using network adapters with iSCSI hardware offload capabilities or TOE. In the case of hardware offload, the network adapter offloads the iSCSI initiator function from the server CPU. In the case of TOE, the adapter offloads TCP processing from the server kernel and CPU.
The rapid development of higher Ethernet speeds such as 25G, 50G, and 100G, along with improved support for congestion management and traffic Quality of Service (QoS) on Ethernet switches, has significantly enhanced the performance, reliability, and predictability of iSCSI as a storage protocol. This improved version is sometimes referred to as iSCSI over Data Center Bridging (DCB).
Updated Standards
RFC 7143 and RFC 7144 have replaced RFC 3720, 3980, 4850, and 5048, and they update RFC 3721. For further information on how DCB improves iSCSI, you can visit the SNIA Blog: How DCB Makes iSCSI Better.
 dictionary defines iSCSI (Internet Small Computer Systems Interface) as a transport protocol that allows the SCSI block protocol to be transmitted over a TCP-based IP network.){kind=link}