IP for networked storage reduces acquisition, administrative, and support costs. The advent of 1 Gigabit ethernet and the soon arrival of 10 Gigabit ethernet (based in part on technology developped for Fibre Channel) allows using IP for high performance storage applications.
FCIP (Fibre Channel over IP) allows to encapsulate Fibre Channel frames within TCP/IP frames. The encapsulated Fibre Channel frames can then be send through an internet. FCIP establishes a tunnel between two Fibre Channel SAN islands. The two FCIP devices at each end of the tunnel have their own IP address. The existence of the channel is transparent to the Fibre Channel devices on any island. They only see additional Fibre Channel devices. The encapsulated Fibre Channel is sent by TCP do avoid out-of-order delivery.
Figure 1: FCIP Tunneling between separate Fibre Channel SANs
The quality of the IP link is an issue, because Fibre Channel has timeout values that might be too short for a low capacity WAN. Thus, a FCIP tunnel might collapse.The FCIP standard proposes two interfaces, E-Port and Fibre Channel Backbone (FCBB), which is still under development. If a fibre channel tunnel collapses, then the fibre channel switches connected to the E-Ports will disable the E-Ports, just as if the fibre channel cable between two fibre channel switches had been disconnected. The switches send a note to their relevant end nodes that the external devices are no longer available. After the FCIP device have recreated the tunnel between them, there is no automatic rocvery of the connection between the two remote Fibre Channel switches. The Fibre Channel switches need to be rebooted to discover the renewed tunnel connection.
The INternet Fibre Channel Protocol is a gateway to gateway protocol for providing Figre Channel fabric services to Fibre Channel end devices over a TCP/IP network. It replaces rather then connects native Fibre Channel switches. An iFCP implementation emulates an interface to an expected Fibre Channel switch or F-Port. At the iFCP layer, a Fibre Channel address is translated into an IP address.
Figure 2: iFCP Protocol
If FCIP is the least IP based protocol, then iSCSI sits at the opposite end of the spectrum. The iSCSI layer interfaces to the OS standard SCSI access model, but instead of using a SCSI bus, any SCSI traffic is converted to IP traffic and reconverted to SCSI at the other end. To maintain the high expectations that SCSI devices have on the bus (e.g. no traffic gets lost), iSCSI has internal measures such as monitoring of TCP connections and recovery from lost or corrupted commands to supplement the TCP control mechanisms.
|© 2003 Thomas Schwarz, S.J., COEN, SCU SCU COEN T. Schwarz COEN 180|