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Revision/Update Information: This manual supersedes TCP/IP Networking on OpenVMS Systems, Version 6.2.
Software Version: OpenVMS Alpha Version 7.1 OpenVMS VAX Version 7.1
Digital Equipment Corporation Maynard, Massachusetts
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© Digital Equipment Corporation 1996. All rights reserved.
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ZK6436
The OpenVMS documentation set is available on CD--ROM.
This manual is intended for anyone who is interested in using TCP/IP networking on either the OpenVMS Alpha or the OpenVMS VAX operating system.
Readers may be new to networking or may be familiar with the traditional DECnet networking interface on OpenVMS systems.
This manual contains the following chapters:
Refer to the following documents for more information about TCP/IP software features supported by OpenVMS.
See the appropriate vendor documentation for information about each of the layered TCP/IP software products that run on OpenVMS systems. These TCP/IP products are listed in Appendix A.
For additional information on the Open Systems Software Group (OSSG) products and services, access the Digital OpenVMS World Wide Web site. Use the following URL:
http://www.openvms.digital.com
Digital welcomes your comments on this manual.
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Use the following table to order additional documentation or information. If you need help deciding which documentation best meets your needs, call 800-DIGITAL (800-344-4825).
The name of the OpenVMS AXP operating system has been changed to the OpenVMS Alpha operating system. Any references to OpenVMS AXP or AXP are synonymous with OpenVMS Alpha or Alpha.
The following conventions are also used in this manual:
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An increasingly important aspect of using computers is the capability to communicate over open networks. Computer systems of similar or different design should be able to communicate with each other. In addition, various physical networks must be interconnected to form large networks called internets.
This chapter describes how Transmission Control Protocol/Internet
Protocol (TCP/IP) networking meets these needs and how OpenVMS systems
support TCP/IP networking.
The chapter also describes how TCP/IP internetworking capabilities have
made possible the global Internet, an openly accessible, worldwide
research and commercial network.
1.1 Overview
This section introduces the primary concepts and features that
characterize TCP/IP networking and summarizes networking capabilities
available on OpenVMS systems.
1.1.1 What Is TCP/IP Networking?
A network consists of two or more computer systems linked by communications software and hardware for the purpose of exchanging information and sharing resources. Data originated on one system can be routed through the network until it reaches its destination on another system. The design of communications software that supports the flow of information is based on networking protocols: services and rules for exchanging information between systems.
One of the most widely used networking software packages is TCP/IP. It is a common set, or "suite," of protocols that work together to provide the services necessary to interconnect computer systems and to interconnect networks. TCP/IP was designed to permit connection of computer systems of dissimilar design. TCP/IP application programs allow users to interact with remote processors. TCP/IP can also be used to communicate across many interconnected networks.
The main characteristics of TCP/IP are openness, interoperability, and
distributed client/server design.
1.1.1.1 Openness
The TCP/IP suite of protocols implements open networking standards that
support open system interconnection. An open system is one for which
the specifications are available to the public. Open specifications do
not rely on a particular technology or product. They allow users to
determine what open systems and other capabilities the user needs.
TCP/IP technology, which accommodates a variety of underlying network
technologies, permits connection of multiple kinds of computers in an
open network environment. See Section 2.1 for a comparison of TCP/IP
architecture with the Open Systems Interconnection (OSI) model.
1.1.1.2 Interoperability
In a TCP/IP networking environment multiple systems from various
vendors can work together, share data, and integrate applications. A
TCP/IP internet is useful for running application programs that carry
out tasks such as accessing remote resources. These applications can
interoperate with different applications running on other systems that
support TCP/IP standards. A user does not need to know about TCP/IP
software or data paths in order to run the applications. See
Chapter 3 for information about commonly used TCP/IP applications.
1.1.1.3 Distributed Client/Server Design
TCP/IP software supports the use of client/server configurations in a distributed networking environment. A distributed system stores data and information on many computers, instead of on just one computer. In the client/server model, two software programs running on separate computers work together: one program, called the client, makes use of resources supplied by the other program, called the server. A server can receive a request from a client anywhere in a TCP/IP network, accept the request, and return the results or data to the client. Server programs are application-level programs that can execute on one or more machines of any size, including PCs, in a TCP/IP network.
One example of a client/server model is a bank clerk who uses client
software running on a PC to enter deposit information about a
customer's account. The server software, running on the computer at the
bank's main branch, processes the request about the customer account
and returns the customer's balance to the client software. Chapter 3
describes commonly used TCP/IP applications that implement the
client/server model.
1.1.2 TCP/IP Standards
No single vendor or organization owns TCP/IP technologies. The Internet Architecture Board (IAB) currently oversees the TCP/IP standards. This group assigns network addresses and specifies the protocols that can be used on the Internet.
The Internet is based on numerous protocols and conventions. Each protocol is explained in a technical publication called a Request for Comment or RFC. RFCs are the working documents that the Internet community uses to develop and record technical information. RFCs document work on the Internet, proposals for new or revised protocols, and the TCP/IP protocol standards. The RFCs are numbered sequentially in the order in which they are written. Each new or revised RFC is given a new number; an index is available to help identify the latest version of a document. RFCs are publicly available and are stored electronically at many Internet computer sites.
As an example, TCP/IP protocols and additional services are implemented
in the UNIX operating system available in the University of
California's Berkeley Software Distribution (BSD). The
BSD UNIX software supports basic TCP/IP protocols and also supplies a
set of utilities for network services in addition to the standard
TCP/IP application programs. The Berkeley R commands and services are a
set of client and server facilities that allow you to access remote
resources in a TCP/IP network. Examples are the rlogin
and rcp commands described in Chapter 3.
1.1.3 Communicating Over TCP/IP Networks
Each end system connected to a TCP/IP network is called a host. Each host has a unique name and address. The local host is the system you are using, and the remote host is the system with which you are communicating. Hosts are connected by lines that carry information between the hosts. The line is the physical path over which data can pass from one host to another. (Examples of lines are telephone lines, fiber-optic cables, and satellites.)
A TCP/IP network is called a packet-switching network. Information is transmitted in small packets of data rather than as a continuous stream from host to host. For example, a file to be transmitted from one host to another is divided into many small packets that are sent across the network one at a time. Each packet contains information about the address of the destination host. At the destination, the packets are reassembled.
The packets that comprise the network traffic are combined (multiplexed) onto high-capacity machine interconnections for transmission across the network or internet. Because packets from different sources are mixed together, many users can use the same line simultaneously. Individual packets can take different paths to the destination. The basic unit of data transmitted by TCP/IP is called a datagram.
The process of directing a data message from a source host to a destination host is called routing. For hosts not directly connected to each other, data can be forwarded from the source to the destination through intervening hosts.
TCP/IP protocols are able to accommodate almost all underlying network hardware technologies. TCP/IP hosts can be connected to a local area network (LAN) or wide area network (WAN). A LAN is a network of systems in a specific geographic area, while a WAN allows long-distance communication. LAN interconnects include Ethernet cables, FDDI fiber-optic interconnects, and token ring networks. Hosts on a WAN connect through leased lines such as the U.S. telephone system or X.25 packet-switching data networks. Both LANs and WANs can be integrated in a single network.
Two or more networks can be interconnected using gateways to form an internet, as illustrated in Figure 1-1. A gateway is a computer that is connected to two networks and routes packets from one to the other. TCP/IP gateways route traffic from one physical network to another network, not to a destination host.
Figure 1-1 Two Networks Connected Through a TCP/IP Gateway
TCP/IP application programs permit users to access remote resources
over the network, as described in Chapter 3. A user at a TCP/IP host
can connect to a remote host and establish an interactive session.
Users can also transfer large quantities of data from remote hosts
(such as file servers). Another significant service is the TCP/IP mail
system, which allows users to send and receive electronic mail.
1.1.4 Using Firewalls to Protect a TCP/IP Network
Computer systems often contain sensitive data and critical processes that require protection from intruders. A firewall can be used to isolate a few critical systems or an entire internal network from the external world. A firewall system works by replacing an IP router with a host that does not forward packets, effectively severing the connection between networks.
The firewall host employs very strict security practices that resist any break-in attempts from intruders. The firewall system also provides:
OpenVMS systems support optional layered networking software products that permit users to communicate with other Digital systems and with non Digital systems in open, worldwide networks. Examples of layered networking software that can run concurrently on an OpenVMS system include TCP/IP networking products, DECnet-Plus software, and PATHWORKS software (supporting a broad range of network transports such as TCP/IP, DECnet, NetWare, and AppleTalk).
OpenVMS systems running TCP/IP layered products support TCP/IP connections to UNIX and other systems in open networking environments. The TCP/IP layered products enable OpenVMS users to perform the following operations:
Chapter 5 specifies DCL commands that support TCP/IP parameters and qualifiers. Appendix A lists TCP/IP layered products that run on OpenVMS systems. Other layered networking products can run concurrently with TCP/IP on OpenVMS systems to communicate with OpenVMS, other Digital systems, and systems supplied by other vendors.
In addition, TCP/IP, DECnet, NetWare, and AppleTalk network transports permit OpenVMS servers to communicate with personal computer and Macintosh clients in PATHWORKS environments.
Figure 1-2 shows an example of an open, multiprotocol network that includes OpenVMS systems, Digital UNIX systems, and TCP/IP networking software provided by other vendors. Wide area multiprotocol routers connect the separate LANs into a single network. A corporate gateway, running TCP/IP networking software, interconnects this network with the global Internet.
Figure 1-2 Multiprotocol Network Topology
TCP/IP protocols provide for the interconnection of networks and the
interconnection of computing systems in a network. This section
describes how TCP/IP protocols evolved to support interconnection of
networks, including the global Internet.
1.2.1 What Is Internetworking?
The technology called internetworking makes possible the interconnection of multiple diverse networks into a coordinated whole. Internetworking adds physical connections and a new set of conventions to permit networks with different underlying hardware technologies to form an internet.
An internet is a collection of packet-switching networks interconnected
by either gateways or routers; an internet supports protocols that
permit the diverse networks to function as a large, virtual network.
The largest internet, the global network formed by research,
commercial, and governmental organizations, is referred to as the
Internet.
1.2.2 Using TCP/IP for Internetworking
TCP/IP was developed as a result of research funded by the Defense Advanced Research Projects Agency (DARPA), an agency of the U.S. Department of Defense. The need to connect many computers with different hardware, operating systems, and networking technologies led to the development of the ARPANET, on which TCP/IP was first implemented. DARPA also made the TCP/IP implementation available to university researchers for use with UNIX operating systems.
The growing diversity of new networking technologies caused DARPA to study network connectivity, or internetworking. By 1983, the Department of Defense mandated that all computers connected to long-haul networks use TCP/IP. The success of the TCP/IP technology and internetworking has resulted in the enormous growth of the global Internet (see Section 1.2.3).
TCP/IP is widely used within organizations or industries to create internets that may or may not be connected to the global Internet. TCP/IP capabilities include the following:
The Internet is a worldwide network encompassing tens of thousands of individual networks linked together with a high-speed backbone network. The backbone network comprises telephone and fiber-optic links, lasers, microwaves, satellites, and other equipment connecting networks and computers throughout the world. The backbone network evolved in part from the high-speed network developed by the National Science Foundation (NSF), a U.S. government agency, to connect their supercomputer sites and make them accessible to scientists, researchers, and engineers.
Examples of the many kinds of organizations linked through the Internet are scientific research agencies, government laboratories, universities and other educational facilities, libraries and archives, commercial corporations, and groups of people with common interests. Some commercial companies act as Internet service providers, making access to the Internet available to organizations and individuals.
A great variety of information is accessible through the Internet, ranging from scientific and academic material to commercial product documentation to news databases and forums for exchanging opinions and ideas.
The Internet provides global connectivity; major Internet facilities are located on every continent and in most countries around the world. In many countries, the Internet is readily accessible to individuals as well as regional and local groups. In addition, gateways permit access to resources on many non-Internet networks. The Internet is expanding at a very rapid rate in terms of numbers of connections and the amount of message traffic.
Internet connectivity enables users to communicate with other connected hosts in distant countries as though the hosts were connected to the same local network. The design of the Internet allows an Internet user to access and retrieve vast amounts of information from anywhere in the world.
Two networking models support open systems interconnection. The first model, TCP/IP, is based on a suite of protocols in which each protocol solves a particular network communications problem. The second model, OSI, is based on international standards.
This chapter compares the models, and then provides an overview of
TCP/IP architectural design, layers and protocols, and the TCP/IP
naming mechanism.
2.1 Introduction to TCP/IP Architecture
Network software design is commonly based on a networking model made up of several layers that work together. Each layer is a group of related functions with its own characteristic protocols and purpose. The layers are built on top of one another so that each layer uses services provided by the layer beneath it. Information flows down through the layers of the sending host and up through the layers of the receiving host.
The architectural model on which networking implementations are based
defines the ways in which operating systems can communicate with each
other. Networking protocols, services, and interfaces allow systems
that implement the model to communicate.
2.1.1 OSI and TCP/IP Models
The OSI model is a layered architecture that interconnects systems from different vendors in an open systems network. The OSI model is based on a set of international standards developed by the International Organization for Standardization (ISO). The seven layers of the OSI model are shown in Figure 2-1. The lower layers (1 through 4) provide for reliable transfer of information between two communicating systems. The upper layers (5 through 7) provide services that enable user applications to communicate with each other.
The TCP/IP model can be used in a heterogeneous environment that has equipment from many different vendors. Layers in the TCP/IP model are also shown in Figure 2-1.
Figure 2-1 Comparison of Layers in the OSI and TCP/IP Models
6436P.HTM OSSG Documentation 26-NOV-1996 11:22:22.41
Copyright © Digital Equipment Corporation 1996. All Rights Reserved.