a second primary processor, and the latter advantage is only significant with active messages, where the on-board processor can handle the message and generate a reply. Using customized protocols also encourages user-level access to the hardware. This can also be done with commodity hardware. Unfortunately, user-level access protocols have the disadvantage of precluding clusters from transparently scaling from standard TCP and Ethernet on small-scale machines to alternative hardware such as Myrinet on big clusters.
5.3 TCP/IP
Parallel computers have traditionally utilized special high performance interprocessor communication networks that use custom protocols to transmit data between processing elements. In contrast, Beowulf clusters rely on commodity networks whose original design goals did not include serving as the interconnect for a commodity supercomputer. The use of commodity networks implies the use of commodity protocols when you are trying to keep costs down. Thanks to the tremendous growth of the Internet during the last decade of the 20th century, TCP/IP has become the de facto standard network communication protocol. Network software vendors have been forced to abandon their proprietary networking protocols in favor of the once obscure but now ubiquitous Internet Protocol and Transmission Control Protocol. Beowulfs naturally default to communicating with these protocols.
The IP protocol is conceptually divided into different logical layers that combine to form a protocol stack. The IP stack commonly supports two services, TCP (Transmission Control Protocol) a reliable, ordered byte stream service and UDP (User Datagram Protocol) an unacknowledged datagram service.
The IP layer is a route-able datagram layer. Data to be transferred is fragmented into datagrams individual packets of data. Packets are length-limited by the physical transport layer, and the IP layer contains the logic to fragment requests that are too large into multiple IP packets that are reassembled at the destination. Each datagram is individually route-able and contains a four byte IP address that specifies the destination host. This version of IP is called IPv4. A new version, called IPv6, will increase the address space available to IP applications. The four byte addresses used in IPv4 are too small for the total number of computers currently connected to the world's networks. This address depletion will be remedied by IPv6, which uses 16 bytes to represent host addresses.
Layered on top of IP is TCP, the most common IP service. TCP provides a reliable, sequenced byte stream service. While the underlying physical transport
