Internet Small Computer System Interface (iSCSI) is essentially the SCSI command set for performing block I/O mapped to TCP/IP leveraging networking interfaces like Ethernet. Intended to be used for storage access, iSCSI is great for implementing tiered storage access using Ethernet and IP based networks from servers as an alternative to Fibre Channel or network attached storage (NAS) based file serving. iSCSI is also used by some for remote data access and movement (mirroring) applications as well.
iFCP and FCIP are intended for interconnecting storage area network (SAN) devices to support data movement and in the case of iFCP also provide SAN fabric segmentation which is a form of routing. iFCP supports movement and routing of data across different SAN segments where FCIP creates a tunnel between two sites in what ends up being a single fabric or segment unless a segment router is used. What the three (iSCSI, iFCP, FCIP) have in common is IP as a network transport protocol for moving block data over IP based networks.
There has debate of the pros and cons between when and where to use iFCP vs. FCIP and which is better. Most vendors support FCIP, while McData, via acquisitions of Nishan (iFCP) and CNT (FCIP), supports both. One thing to keep in mind is that even though different vendors support FCIP, I'm not aware of any vendor supporting an FCIP-based product that communicates with other vendors' FCIP-based product. Learn more about the differences, characteristics and when to use iSCSI, iFCP and FCIP in Chapter 4: "Storage and I/O networks" and Chapter 5: "Metropolitan and wide area storage networks" in my book "Resilient Storage Networks" (Elsevier) as well as in the SearchStorage tip "Bridging the gap".
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