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XTP Transport Layer Protocol is 'Appropriate for Real-Time Systems'
Publication: C²I² Press Release Issued: Date: 1997-07-24 Reporter: C²I² Systems

C²I² Press Release


The emerging Xpress Transport Protocol (XTP) standard is one of a small handful of Transport Layer protocols capable of providing network architects and designers with the necessary timeliness, throughput and reliability to build real-time, mission-critical distributed systems.

This is one of the conclusions contained in an extensive study of network strategies for real-time, mission-critical distributed systems by Richard Young, Managing Director of C²I² Systems, a pre-eminent Cape Town-based systems integration and networking company.

Young's aim was to analyse the information management requirements of typical next-generation distributed control systems, with a view to synthesising an optimal solution using distributed computing elements and local area networks (LANs).

The ideal solution had to provide the optimal performance, dependability, transparency and flexibility, as well as exhibit a high degree of integration across all its functional areas. Further, the optimal solution had to incorporate an open systems architecture and comply with international standards.

The study found that the most appropriate means to meet numerous, diverse and stringent user requirements is a distributed system architecture integrated by means of local area networks (LANs).

Such an architecture is best able to satisfy high-level (or allocated) requirements, such as system coherence, dependability and operability, as well as lower-level (or derived) requirements such as reliability, maintainability, reconfigurability and electromagnetic compatibility. 

Layer 4 of the OSI model is termed the Transport Layer (TL). The functions of the TL protocol are to enhance data communication reliability by providing end-to-end dataflow control and to optimise use of the network by providing special services.

The transport layer provides an interface between the higher application- orientated layers and the underlying network-dependent layers. It provides the higher layers with a reliable message transfer service that is independent of the underlying network type. The transport layer masks the detailed operation of the underlying network from the higher layers and provides the latter with a defined set of message transfer facilities.

The functions of the TL protocol are critical to the suitability of local area networks for real-time, mission-critical distributed systems as they determine such capabilities as end-to-end error, flow and rate control as well as special services which are of specific significance to mission-critical, real-time systems. Examples of such special services are multicast, timestamping, priority message scheduling and security.

To identify the most suitable TL protocol for distributed system architectures, Young undertook a detailed analysis of close to a dozen alternatives. Some of these, such as TCP/IP and NetBIOS, are commercially available technologies while others, such as XTP, are emerging.

Young concluded that none of the commercially available protocols are able to provide the hundreds of Mbit/s throughput and sub-millisecond latency required for critical real-time networking applications.

"Only emerging protocol standards such as XTP are able to meet the higher-order requirements of the future," Young commented. "Of these XTP has emerged as the leading contender for standardisation for real-time LANs, mainly because of its real-time capability and extended flexibility."

XTP, until recently referred to as the Xpress Transfer Protocol, is now a true ISO OSI transport layer protocol. It was originally designed to be a ‘silicon' protocol, i.e. implemented in hardware. However, owing to the dynamic nature of transport and network layer protocol definitions, each new delineation rendered all previous ‘silicon' versions obsolete. For this reason ‘silicon XTP' has proved to be commercially unsuccessful.

Another reason is that ‘silicon' protocols have been made all but obsolete by the advent of readily available processing power. This has meant that general purpose processors can easily supply the required protocol performance (at least for current data rates in the order of 100 Mbit/s) without requiring special-to-type devices such as RISC processors.

One of XTP's most important features as a real-time protocol is its orthogonal approach. This differentiates between policy regarding real-time LAN issues such as dataflow control and the mechanisms by which these are actually implemented, as well as how they interface to the user application. XTP provides the implementer with options for almost every protocol feature, including addressing, flow, rate and error control.

In addition, XTP offers a rich array of functional features that make it suitable for real-time, mission-critical distributed applications. These features include :

  • Alignment
  • Efficient Connection Management (requiring only 3 packets)
  • Maximum Transmission Unit Detection
  • Multilevel Priority Message Scheduling
  • Multicast Group Management
  • Out-of-Band Data
  • Rate and Burst Control
  • Reliable Datagram
  • Reliable Multicast
  • Selectable Error/Flow Control
  • Selective Acknowledgement
  • Selective Retransmission
  • Traffic Descriptors

"Many of the features being proposed in the next generation standard commercial transport protocols are already provided by XTP," Young concluded. "These include multicast, priority message scheduling, reservation mode and out-of-band data."

"Areas that require attention are latency control, jitter control, guaranteed quality of service (QoS) and security features. While progress is being made on the definition and design of next generation transport protocols, these efforts are still some way from completion and general acceptance."