Distributed Computing with the CLAN Network

CLAN (Collapsed LAN) is a high performance user-level network targeted at the server room. It presents a simple low-level interface to applications: connection-oriented non-coherent shared memory for data transfer, and Tripwire, a user-level programmable CAM for synchronisation. This simple interface is implemented using only hardware state machines on the NIC, yet is flexible enough to support many different applications and communications paradigms. In this paper we describe the CLAN network, and contrast our design with other user-level networks whose network interface is sufficiently complicated that implementations require an embedded processor. These interfaces are often tailored to a particular class of applications, and layering disparate interfaces over the low-level network substantially degrades performance. We show how CLAN is used to support a number of standard transports and middleware: MPI, VIA, TCP/IP and CORBA. In each case we demonstrate performance that approaches the underlying network. For TCP/IP we present our initial results using an inkernel stack, and describe the architecture of our prototype Gigabit Ethernet/CLAN bridge, which demultiplexes Ethernet frames directly to user-level TCP/IP stacks via the CLAN network. For VIA we present a software implementation with better latency than a commercial VIA NIC implemented on ASIC technology. David and Kieran are both jointly funded by a Royal Commission for the Exhibition of 1851 Industrial Fellowship, and AT&T Laboratories-Cambridge.