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Xpress Transport Protocol 4.0

The Xpress Transport Protocol (XTP) is a next-generation communications protocol for Internet, intranet, real-time, multimedia, and command-and-control applications. It already contains many of the features that the Internet community expects to add to the Transmission Control Protocol (TCP) over the next few years: reliable transport multicast, multicast group management, transactions, reliable datagrams, rate and burst control, user-selectable error and flow control, selective retransmission, message-level priorities, fast connection setup and teardown, maximum transmission unit detection, and much, much more.

XTP does not replace either TCP or UDP; instead, it sits beside them and operates simultaneously with them. TCP and UDP operations are undisturbed when XTP's functionality is added to your operating system. XTP normally runs over raw IP, thus enabling it to run over any network; it can also run directly on top of the LLC of a LAN or the AAL of an ATM network.

Our customers have added XTP to their operating systems to achieve applications such as the following:

  • Reliable multicast of signal intelligence to 22 CRAYs in a C-130 aircraft
  • Support multiple multimedia stream distribution from a PC-based video file server
  • Multi-channel digital telephony using multicast distribution to remote receivers
  • Radar/sonar distribution over multiple FDDI rings in a shipboard self-defense system
  • Voice/video distribution in the Navy's shipboard Advanced Display System
  • Telemedicine support for ultrasound still images and video clips
  • Speedup of web browsers and web servers
  • Multimedia distribution aboard military aircraft and air-to-ground systems

In general, our customers report satisfaction with XTP's main enhancements over TCP and UDP:

  • Multicast allows one transmission to reach arbitrarily many receivers
  • Transport multicast reliability semantics are defined by the user but transparently implemented by XTP
  • Multicast group management semantics allow the application to know everything or nothing about the status of all receivers
  • Rate and burst control allow traffic shaping to avoid congestion
  • Selective retransmission improves performance over long-latency and/or noisy channels (e.g., satellites, WANs)
  • MTU discovery can eliminate segmentation and fragmentation
  • XTP supports high-security applications via unidirectional channels (no write-back)
Supported Systems
XTP is written in C++ for portability. The core implementations are designed for VxWorks (a real-time OS based on Unix) and Windows NT. Code size varies with the processor, and ranges from 64K to 256K. Either we or our customers can port the code to a particular combination of OS, processor, and network. Because XTP normally operates over raw IP, all networks (e.g., Ethernet, FDDI, ATM) are supported. The user interface is WinSock 2.

Measured Performance
Running on a pair of 133 MHz Intel Pentium processors, using the pSOS+ operating system and Rockwell FDDI interfaces:

  • Throughput: 92 Mbits/sec using reliable unicast with large (16KB) messages
  • Throughput: 96 Mbits/sec using unacknowledged multicast with large (16KB) messages
  • Latency: user memory to user memory latency is 220 µS for small (16-byte) messages
  • Multicast: reliable multicast outperforms reliable serial unicast for receiver groups of size two or larger

In high-latency and/or lossy environments, selective retransmission can dramatically reduce the system' s error repair time. For instance, by redirecting a Web server/browser to operate over XTP instead of TCP in a test network with an induced 15% packet loss rate, the load time for a 4 MB image is reduced from 1 minute to 4 seconds.

We are interested in special-purpose applications, including: teleconferencing and distance learning, digital multimedia servers and multimedia distribution, distributed simulation, algorithms for management of large multicast groups, satellite networks, and other network protocols (e.g., RTP, RSVP, NTP). Call us to discuss your special applications.

Network Xpress lnc.
NetX personnel have been involved with XTP design and development since 1988. We have implemented every revision of the XTP specification defined by the XTP Forum. We have made a name for ourselves as protocol providers for embedded real-time systems (e.g., command and control, multimedia). Our personnel and facility security clearances allow us to undertake classified work. Our code is currently operational in multiple military and commercial systems, ranging from ships to aircraft to file servers to Web servers.

More Information
For more information, call us at (804) 293-8066 or send email to We also offer tutorial information free of charge:

What is the Xpress Transport Protocol? - a high-level description of XTP that explains where it fits and what it accomplishes in the spectrum of communications protocols

Xpress Transport Protocol Version 4.0 - a more detailed tutorial on the protocol's functionality, performance, and applications that currently use it

Network XpressTelephone: (804) 293-8066
webmaster@netx.comFAX: (804) 293-8414