The Xpress transfer protocol (XTP)—a tutorial

Abstract

XTP is a reliable, real-time, lightweight transfer s layer protocol being developed by a group of researchers and developers coordinated by Protocol Engines Incorporated (PEI) . Unlike all previous transport layer protocols, XTP is being designed to be implemented in hardware a s a VLSI chip set . By streamlining the protocol, combining the transport and network layers and utilizing th e increased speed and parallelization possible with a VLSI implementation, XTP will be able to provide the endto-end data transmission rates demanded in high speed networks without compromising reliability an d functionality . This paper describes the operation of the XTP protocol and in particular, its error, flow and rate control, inter-networking addressing mechanisms and multicast support features, as defined in the XTP Protoco l Definition Revision 3 .4 . 11 1 Future computer networks will be characterized by high reliability and very high data transmission rates . Traditional transport layer protocols, such as TCP and TP4, which were designed in an era of relatively slow an d unreliable interconnected networks, may be poorly matched for the emerging environment . Although they contai n many necessary features, such as error detection, retransmission, flow control and data resequencing, they ar e deficient in many respects --they do not provide rate control and selective retransmission, reliable multicast i s not supported, their packet formats are complex and require extensive parsing due to variable header lengths an d support of complex modes . These protocols manage many timing events at both the sender and the receiver -fo r example, since the sender does not initiate receiver data acknowledgements, both the receiver and sender requir e an additional timer . The data transmission rates assumed are no longer valid and may limit the scalability of th e protocols -in TCP, for example, which was designed in an era of 56Kbps data transmission rates, the flo w window size is small, and based on 16 bit byte sequencing . Finally, the state machines for these transpor t protocols were intended for sequential rather than parallel execution . For example, the placement of the transport checksum field was considered arbitrary and so it was placed in the header .