FCoE Isn't Just Another Protocol

Posted by Howard Marks
October 05, 2009

My esteemed colleague George Crump views FCoE as just another protocol you can run in your data center if you think it fits your needs, as opposed to what he says is the near religious movement that some vendors would lead you to believe it is. As an old network guy, I can't call an application that requires as many changes to the network itself as FCoE does.

Not only does FCoE require the new and improved Ethernet with Data Center Bridging to eliminate packet losses due to congestion, it also requires that all FCoE traffic passes through the fiber channel forwarder (FCF) function of an FCoE switch. In fact, the designers of FCoE aren't so much trying to make fibre channel protocol run over Ethernet as they are trying to make Ethernet work like Fibre Channel.

Some of the differences between Ethernet and Fibre Channel are comparable to the debates that raged in the networking community in the 80s over smart vs. dumb networks. Smart networks like Frame Relay and SNA had every node in the network provide services like error checking and packet retransmission.  Dumb networks like Ethernet rely on the end nodes to provide those upper layer services while the network of routers and bridges is just in charge of transport. The dumb networks won, and carrier Ethernet is replacing Frame Relay and ATM just like IP replaced SNA.

So the FCF provides name services, makes devices log on to the network before they can send FCoE data and enforces FC zoning so servers can only see the other FCoE devices the SAN admin wants them to. iSCSI, by comparison, uses an iSNS server for naming and LUN masking to manage finding resources and limiting access. Network admins can get similar results to zoning and requiring device login with VLANs and/or switch ACLs.

So you can connect any two iSCSI devices directly, or across any Ethernet network, and they'll talk. You can even directly connect Fibre Channel devices. You can't connect FCoE devices directly, and even if your server and FCoE disk array are connected to the same end of row CEE/DCE switch, FCoE traffic will have to go up stream to an FCoE switch and back down.  

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