FC-4 and ULPs: application protocols
The layers FC-0 to FC-3 discussed previously serve solely to connect end devices together by means of a Fibre Channel network. However, the type of data that end devices exchange via Fibre Channel connections remains open. This is where the application protocols (upper layer protocols, ULPs) come into play. A specific Fibre Channel network can serve as a medium for several application protocols, for example, SCSI and IP. The task of the FC-4 protocol mappings is to map the application protocols onto the underlying Fibre Channel network. This means that the FC-4 protocol mappings support the API of existing protocols upwards in the direction of the operating system and realize these downwards in the direction of the medium via the Fibre Channel network The protocol mappings determine how the mechanisms of Fibre Channel are used in order to realize the application protocol by means of Fibre Channel. For example, they specify which service classes will be used and how the data flow in the application protocol will be projected onto the exchange sequence frame mechanism of Fibre Channel. This mapping of existing protocols aims to ease the transition to Fibre Channel networks: ideally, no further modifications are necessary to the operating system except for the installation of a new device driver.The application protocol for SCSI is called simply Fibre Channel Protocol (FCP). FCP maps the SCSI protocol onto the underlying Fibre Channel network. For the connection of storage devices to servers the SCSI cable is therefore replaced by a Fibre Channel network. The SCSI protocol operates as before via the new Fibre Channel medium to exchange data between server and storage. It is therefore precisely at this point that the transition from server-centric IT architecture to storage-centric IT-architecture takes place.
Thus it is here that the Fibre Channel network becomes a Fibre Channel SAN. The idea of the FCP protocol is that the system administrator merely installs a new
device driver on the server and this realizes the FCP protocol. The operating system recognizes storage devices connected via Fibre Channel as SCSI devices, which it addresses like 'normal' SCSI devices. This emulation of traditional SCSI devices should make it possible for Fibre Channel SANs to be simply and painlessly integrated into existing hardware and software. The FCP driver has to achieve a great deal: SCSI uses parallel cables; daisy chain connects several devices together via a SCSI bus. By contrast, in Fibre Channel the data transmission takes place serially. The parallel transmission via the SCSI bus must therefore
be serialized for the Fibre Channel SAN, so that the bits are transferred one after the other.Likewise, FCP must map the daisy chain of the SCSI bus onto the underlying Fibre Channel topology. For example, the scanning for devices on a SCSI bus or the arbitration of the SCSI bus requires a totally different logic compared to the same operations in a Fibre Channel network. A further application protocol is IPFC: IPFC uses a Fibre Channel connection between two servers as a medium for IP data traffic. To this end, IPFC defines how IP packetswill be transferred via a Fibre Channel network. Like all application protocols, IPFC
is realized as a device driver in the operating system. The connection into the local IP configuration takes place using 'ipconfig' or 'ifconfig'. The IPFC driver then addresses theFibre Channel host bus adapter card in order to transmit IP packets over Fibre Channel. The IP data traffic over Fibre Channel plays a less important role both in comparison to SCSI over Fibre Channel and in comparison to IP data traffic over Gigabit Ethernet. Fibre Connection (FICON) is a further important application protocol. FICON maps the ESCON protocol (Enterprise System Connection) used in the world of mainframes ontoFibre Channel networks. Using ESCON it has been possible to realize storage networks in the world of mainframes since the 1990s. Fibre Channel is therefore taking the old familiar storage networks from the world of mainframes into the Open System world (Unix, Windows NT/2000, OS/400, Novell, MacOS) and both worlds can even realize their storage networks on a common infrastructure. The Fibre Channel standard also defines a few more application protocols. Particularly worth a mention is the Virtual Interface Architecture (VIA, Section 3.7). VIA describes a very lightweight protocol that is tailored to the efficient communication within server clusters. With VIA it will in future be possible to construct systems of servers and storage devices in which the boundaries between servers and storage devices disappear to an ever greater degree.
No comments:
Post a Comment