DIGITAL TCP/IP Services for OpenVMS

DIGITAL TCP/IP Services for OpenVMS

Concepts and Planning

October 1997

This book describes concepts and planning tasks to prepare you to use the DIGITAL TCP/IP Services for OpenVMS product.

Revision Information: This is a revised manual.

Operating Systems: OpenVMS Alpha Versions 6.2, 7.0, 7.1 OpenVMS VAX Versions 6.2, 7.0, 7.1

Software Version: DIGITAL TCP/IP Services
for OpenVMS Version 4.2

Digital Equipment Corporation Maynard, Massachusetts

October 1997

Digital Equipment Corporation makes no representations that the use of its products in the manner described in this publication will not infringe on existing or future patent rights, nor do the descriptions contained in this publication imply the granting of licenses to make, use, or sell equipment or software in accordance with the description.

Possession, use, or copying of the software described in this publication is authorized only pursuant to a valid written license from DIGITAL or an authorized sublicensor.

DIGITAL conducts its business in a manner that conserves the environment and protects the safety and health of its employees, customers, and the community.

© Digital Equipment Corporation 1997. All rights reserved.

The following are trademarks of Digital Equipment Corporation: ACMS, DECdtm, DDCMP, DEC, DECnet, DECNIS, DECserver, DECsystem, DECwindows, DIGITAL, DNA, InfoServer, LAT, OpenVMS, PATHWORKS, POLYCENTER, VAX, VAXstation, VMS, VMScluster, and the DIGITAL logo.

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This document is available on CD-ROM.



An open communications standard defined by the worldwide networking community, TCP/IP consists of numerous applications, routing, transport, and network management protocols. These protocols enable any connected host to communicate with any other connected host, without needing to know details about the other host or the intervening network topology. Computers and networks from different manufacturers running different operating systems can interoperate seamlessly.

This book describes DIGITAL's implementation of TCP/IP for the OpenVMS operating system --- DIGITAL TCP/IP Services for OpenVMS, also known as UCX. Chapters 1--4 describe the key concepts of TCP/IP as they relate to the protocols supported by the UCX product. Chapters 5 and 6 provide specific guidelines for system and network managers to plan an internet with the DIGITAL TCP/IP Services for OpenVMS product. The glossary at the end of this book defines many networking and TCP/IP terms used throughout the UCX document set.

Intended Audience

This book is for anyone familiar with OpenVMS, but new to the TCP/IP environment who needs an overview of the key components of the DIGITAL TCP/IP Services for OpenVMS product.

See the DIGITAL TCP/IP Services for OpenVMS User's Guide for more detailed user information and the DIGITAL TCP/IP Services for OpenVMS Management guide for more detailed system and network management information.

Document Structure

This book contains the following chapters, an appendix, and a glossary.

Guide to Documentation

Table 1 outlines the documents available to you with this version of DIGITAL TCP/IP Services for OpenVMS.

Table 1 DIGITAL TCP/IP Services for OpenVMS Documentation
Manual Contents
DIGITAL TCP/IP Services for OpenVMS Concepts and Planning This manual provides conceptual information about networking and the TCP/IP protocol. This manual also provides a description of DIGITAL's implementation of the Berkeley Internet Name Domain (BIND) service and the Network File System (NFS). It also outlines general planning issues to consider before configuring your system to use the DIGITAL TCP/IP Services for OpenVMS software.

This manual also describes the manuals in the documentation set and provides a glossary of terms and acronyms for the DIGITAL TCP/IP Services for OpenVMS software product. It also documents how to contact the InterNIC Registration Service to register domains and access Request for Comments (RFCs).

DIGITAL TCP/IP Services for OpenVMS Release Notes This text file describes new features and changes to the software including installation, upgrade, configuration, and compatibility information. These notes also describe new and existing software problems and restrictions, and software and documentation corrections.

Print this text file at the beginning of the installation procedure and read it before you install DIGITAL TCP/IP Services for OpenVMS.

DIGITAL TCP/IP Services for OpenVMS Installation and Configuration This manual explains how to install and configure the DIGITAL TCP/IP Services for OpenVMS layered application product.
DIGITAL TCP/IP Services for OpenVMS User's Guide This manual introduces the user services available with DIGITAL TCP/IP Services for OpenVMS such as remote file operations, E-mail, TELNET, TN3270, and network printing. This manual also explains how to use these services to communicate with systems on private internets or on the worldwide Internet.
DIGITAL TCP/IP Services for OpenVMS Management This manual describes how to manage a corporate internet that uses the DIGITAL TCP/IP Services for OpenVMS product. This manual also explains the kernel software, applications, and network services, from a system management perspective.

Use this manual with the DIGITAL TCP/IP Services for OpenVMS Management Command Reference manual.

DIGITAL TCP/IP Services for OpenVMS Management Command Reference This manual describes the DIGITAL TCP/IP Services for OpenVMS management commands.

Use this manual with the DIGITAL TCP/IP Services for OpenVMS Management manual.

DIGITAL TCP/IP Services for OpenVMS ONC RPC Programming This manual presents an overview of high-level programming using open network computing remote procedure calls (ONC RPC). This manual also describes the RPC programming interface and how to use the RPCGEN protocol compiler to create applications.
DIGITAL TCP/IP Services for OpenVMS System Services and C Socket Programming This manual describes how to use C socket and system services routines to build internet application programming interfaces (APIs).
DIGITAL TCP/IP Services for OpenVMS eSNMP Programming and Reference This manual describes the Extensible Simple Network Management Protocol (eSNMP), the eSNMP application programming interface (API), and how to build additional subagents to manage vendor-specific equipment.

For additional information about the DIGITAL TCP/IP Services for OpenVMS products and services, access the DIGITAL OpenVMS World Wide Web site at the following URL: 

You might also find the Internetworking with TCP/IP: Principles, Protocols, and Architecture, by Douglas Comer useful if you are looking for a comprehensive overview of the TCP/IP protocol suite.


This manual uses the following terminology:


For a complete list of acronyms used throughout this and other manuals in the DIGITAL TCP/IP Services for OpenVMS documentation set, see the glossary in this book.


All IP addresses in this book represent fictitious addresses. The following conventions apply to this book.
Convention Meaning
UPPERCASE TEXT Indicates names of OpenVMS and UCX commands, options, utilities, files, directories, hosts, and users.
lowercase special type Indicates UNIX system output or user input, commands, options, files, directories, utilities, hosts, and users.
italic type Indicates a variable.
[Return] Indicates that you press the Return key.
[Ctrl/] x Indicates that you press the Control key while you press the key noted by x.
[ ] In command format descriptions, indicates optional elements. You can enter as many as you want.
{ } In command format descriptions, indicates you must enter at least one listed element.

Reader's Comments

DIGITAL welcomes your comments on this manual or any of the DIGITAL TCP/IP Services for OpenVMS documents. Send us your comments through any of the following channels:
Fax 603 884-0120, Attention: OSSG Documentation, ZKO3-4/U08
Mail OSSG Documentation Group, ZKO3-4/U08
110 Spit Brook Rd.
Nashua, NH 03062-2698

How To Order Additional Documentation

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).

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Fax: 613-592-1946 Digital Equipment of Canada, Ltd.
Box 13000
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Attn: CICC
International --- --- Local DIGITAL subsidiary or
approved distributor
Internal Orders DTN: 261-2010
Fax: 800-741-6970 U.S. Software Supply Business
Digital Equipment Corporation
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Chapter 1
Introduction to DIGITAL TCP/IP Services for OpenVMS

The DIGITAL TCP/IP Services for OpenVMS product (also known as UCX) is the OpenVMS implementation of the industry-standard TCP/IP suite of communications protocols. With TCP/IP, heterogeneous networks can interconnect, making it possible for users to connect to remote hosts in many ways:

Internetworking with TCP/IP hides the hardware details of each individual network and allows computers to communicate independently of their physical network connections. TCP/IP provides both a standard transport mechanism and full-duplex, reliable, stream communication services for software applications.

The DIGITAL TCP/IP Services for OpenVMS product provides interoperability and resource sharing between OpenVMS systems, UNIX systems, and other systems that support the TCP/IP protocol suite and Sun Microsystems' Network File System (NFS). TCP/IP systems and other internet hosts share data and resources by using standard TCP/IP protocols over a number of network hardware configurations: Ethernet, Fiber Distributed Data Interface (FDDI), Token Ring, and asynchronous transfer mode (ATM).

1.1 TCP/IP Defined: Requests for Comments

TCP/IP evolved from the U.S. Government's need to connect many different networks regardless of their hardware architecture, operating system, or subnetwork technology. The resulting internetwork needed to be able to route data between networks, tolerate routing errors, and easily add new subnetworks. From a simple four-node entity in 1969 to today's Worldwide Internet connecting thousands of networks and millions of computers, TCP/IP has become the communications standard of the Internet.

TCP/IP is an open system interconnection. Although monitored by a number of organizations, no one entity owns TCP/IP; its specifications are publicly available and constantly growing as communications requirements evolve.

The process by which the specifications evolve is through a mechanism called Requests for Comments or, more commonly, RFCs. Basically, when someone has an idea for a new or improved capability for TCP/IP, he or she writes a proposal, posts it on the Internet, and requests comments from the networking community. After a review and revision cycle, working code is developed and an RFC becomes a standard protocol.

RFCs are available on the Internet from an organization called the Internet Network Information Center, or InterNIC. Appendix A discusses the InterNIC and explains how you can get copies of RFCs.

1.2 TCP/IP Architecture

The TCP/IP protocol suite is designed in a fashion similar to that of the OSI layered model. However, the TCP/IP protocol suite has four layers while the OSI model has seven layers. Figure 1-1 shows the relationship between the layers of the two models. As shown in the illustration, the OSI model's Session and Presentation layer functions are fulfilled by the TCP/IP Application layer protocols. Likewise, some of the functions of the OSI Physical layer are handled by the Network Interface layer and the hardware itself in the TCP/IP model.

Figure 1-2 and Table 1-1 outline the layers of the TCP/IP model. Sections 1.3 through 1.6 summarize the protocols.

Figure 1-1 Relationship Between TCP/IP and OSI Models

Figure 1-2 DIGITAL TCP/IP Protocol Architecture

Table 1-1 TCP/IP Network Architecture Description
Layer Function
Network Interface Transmits data across a single network. This layer also receives data routed from the Internet layer and transmits the data to its destination.
Internet Moves data around the internetwork. The Internet Protocol routes packets across networks independently of the network medium. It also encapsulates datagram headers, sends ICMP error and control messages, and maps ARP address conversions.
Transport Provides a flow of data between two hosts. The DIGITAL TCP/IP Services for OpenVMS product supports the two common transport protocols:
  • Transmission Control Protocol (TCP) provides a reliable data flow between two hosts.
  • User Datagram Protocol (UDP) provides a much simpler service to the Application layer than TCP but does not guarantee reliability.
Application Handles the details of the particular application, protocol, or user command; not concerned with the movement of data across the network. UCX supports the following TCP/IP applications, protocols, and user commands:

Remote Computing

  • TELNET for remote login to other hosts in the network.
  • Remote commands: RLOGIN for remote login, RSH for remote shell capabilities, REXEC to execute commands to a remote host, and RMT/RCD to read magtapes or CDs from remote hosts.
  • Finger utility to display user information.

File Transfer

  • File Transfer Protocol (FTP) to transfer files between hosts.
  • Trivial File Transfer Protocol (TFTP) to download and transfer files.

Resource Sharing

  • Line printer/line printer daemon (LPR/LPD) to provide printing services to local and remote hosts.
  • TELNET Print Symbiont (TELNETSYM) to provide remote printing using the TELNET protocol.
  • Network File System (NFS) and PC-NFSd to authenticate requests and access remote files.

Electronic Mail

  • Simple Mail Transfer Protocol (SMTP) for electronic mail.
  • Post Office Protocol (POP) for electronic mail for PC users.

Network Services

  • Simple Network Management Protocol (SNMP) to monitor and manage network devices (particularly routers and servers) from across an internetwork.
  • Network Time Protocol (NTP) to synchronize time between hosts in a TCP/IP network.
  • Berkeley Internet Name Domain (BIND), a distributed database system, to distribute and manage host information so that hosts do not need to know the address of every other host on the internet.
  • The Bootstrap Protocol (BOOTP) to answer bootstrap requests from remote devices.

1.3 Network Interface Layer Protocols

The Network Interface layer of the TCP/IP model (also called the Data Link layer) is responsible for properly sending and receiving communications signals between two communicating hosts through their network interfaces.

The network interface is a software component that communicates with the TCP/IP software and the network controller (the hardware connection between a computer system and a network). UCX supports multiple network interfaces for each physical network controller, which means that a single physical connection can have more than one IP address.

Individual host computers can be connected to many different types of networks such as Ethernet, FDDI, Token Ring, and asynchronous transfer mode (ATM). The Internet device driver, called the BG driver, is the software interface between the OpenVMS operating system and the network device controller for these hardware configurations.

Individual host computers can also connect with other hosts or networks over serial communications lines, which are most commonly telephone connections.

With TCP/IP --- as with any layered networking protocol --- each layer adds header information to the protocol data unit (PDU) from the layer above. Each packet contains a header from the Network Interface layer, followed by a header from the Internet layer, followed by a header from the Transport layer, followed by the application data.

At the Network Interface layer, standard encapsulation of IP packets are defined for the various hardware types. Ethernet, for example, uses the Ethernet frame standard to enclose the data being sent with header fields. Serial line connections use either Serial Line Internet Protocol (SLIP or CSLIP) or Point-to-Point Protocol (PPP).

Serial Line Internet Protocol

SLIP is a simple packet framing protocol. It defines a sequence of characters that frame IP packets on a serial line. It provides no mechanisms for addressing, packet type identification, error detection/correction, or compression. Although limited in scope, SLIP is easy to implement, but transmission speeds are relatively slow. CSLIP (Compressed SLIP) allows for faster transmission by compressing the TCP/IP headers.

For more information on SLIP and CSLIP, see the DIGITAL TCP/IP Services for OpenVMS Management guide.

Point-to-Point Protocol

PPP is more complex than SLIP and CSLIP, but it offers much greater functionality. As described in RFC 1331, PPP consists of three main components:

For more information on PPP, see the DIGITAL TCP/IP Services for OpenVMS Management guide.

1.4 Internet Layer Protocols

The Internet layer provides a connectionless packet delivery service using the Internet Protocol (IP). An IP datagram is a packet that has no delivery receipt and is called connectionless because IP does not maintain state information about successive datagrams. Each datagram is handled independently from all other datagrams.

The DIGITAL TCP/IP Services for OpenVMS product supports the following Internet layer protocols:

Internet Protocol

IP sends or routes data across the network from its source to its destination by means of internet addressing (an IP address). The IP address identifies the connection between the network controller of a node and the network cable. IP then receives data bits from the network hardware, assembles the bits into an IP datagram, and chooses the best route to send the packet to its destination. IP also fragments and reassembles packets during the routing process.

Routing Information Protocol

RIP enables gateways to exchange current routing information about hosts and directly connected networks.

Internet Control Message Protocol

ICMP provides a number of diagnostic functions and handles error and control messages. ICMP reports problems with data delivery to gateways and hosts.

Address Resolution Protocol

ARP dynamically maps an IP address to a physical hardware address of the broadcast medium such as Ethernet, FDDI, Token Ring, or PPP. ARP is limited to a single physical network and to networks that support hardware broadcast.

1.5 Transport Layer Protocols

The Transport layer protocols provide either connection-oriented or connectionless data stream transmission from one host to another. The Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) form the bridge between the Application layer functions and Internet layer protocols such as IP.

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