How to use IEEE 1355 (HIC) as transport layer protocol for SCI Olav Lysne and Stein Gjessing Department of Informatics University of Oslo, Norway E-mail: Olav.Lysne@ifi.uio.no 1 Introduction HIC (Heterogeneous InterConnect) [7] is a technology that offers full duplex point to point serial links with speeds up to 1 Gbaud. This is an European technology that has been developed through the European ESPRIT projects OMI-HIC and OMI-MACRAM ´ E. These developments have achieved inter- national recognition through IEEE standardization. Nodes communicating over HIC may either be interconnected directly by the use of one HIC-link, or indirectly by a network of HIC-routers. The HIC routers forwards packets in a way referred to as wormhole routing. This is a technique that has roots in an early work by Kermani and Kleinrock [8]. The idea in wormhole routing is that instead of storing each packet of data completely in each routing node, the first part of each packet is sent on an outgoing link before the last part of the packet has arrived. In fact the routers need not have the buffer space to contain more than a small fraction of each packet. This means that when one output link is blocked a packet will become spread out across the network between its source and destination. There is no timesharing between packets on each physical link, thus a packet that occupies a link will block all other packets that needs the same link. This introduces the danger of deadlocks in that there may be circular dependencies between several packets, all waiting for each other to free occupied links. In the last decade there has been a lot of work addressing deadlock free wormhole routing, e.g. [4, 14, 3, 10, 5]. SCI [6] is an interface standard for multiprocessor systems with up to 64 K nodes. The standard supports coherent shared memory, and aims at facil- itating assembly of processors, memory, I/O and bus adaptor cards from 1