DIGITAL TCP/IP Services for OpenVMS

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Chapter 2
Configuring the Interface

OpenVMS systems running DIGITAL TCP/IP Services for OpenVMS communicate with other internet hosts over a variety of physical media1. Because TCP/IP is independent of the underlying physical network, IP addresses are implemented in the network software, not the network hardware.

This chapter reviews key concepts and explains how to configure the network interface and assign IP addresses, subnet masks, and broadcast addresses.


1 See the DIGITAL TCP/IP Services for OpenVMS Software Product Description for a complete list of supported media.

2.1 Reviewing Key Concepts

A network controller is the hardware connection between a computer system and a physical network. Controllers perform the packet channeling to and from the physical medium of your network, usually a cable.

The network interface is a logical network controller --- a software component that communicates with your network software and the network controller.

For each interface, you can enable or disable the interface; set the subnet mask; set protocols for the interface; and assign IP, Ethernet, and broadcast addresses.

2.2 Configuring Network Controllers

If you used TCPIP$CONFIG to configure your software, the product automatically recognizes network controllers at startup. If you need to add new network controllers to your system after installing and configuring the product, follow the installation and configuration instructions that come with your hardware, then rerun TCPIP$CONFIG. The TCP/IP Services software will recognize the new controller the next time the software starts up.


Hardware installation and configuration instructions are specific for the various network controllers. Be sure to read the instructions provided with your new hardware before installing.

To display current controller definitions, issue the LIST COMMUNICATION_CONTROLLER command. It lists the names and other information about the communication controllers known to TCP/IP Services. For example:

                    Communication Controller Configuration 
Controller:  LO   Internet Interface:  L 
                                Type:  LOCAL 
Controller:  XE   Internet Interface:  D 
                                Type:  CLUSTER ETHERNET 
Controller:  EF   Internet Interface:  F 
                                Type:  CLUSTER ETHERNET 
Controller:  ET   Internet Interface:  N 
                                Type:  CLUSTER ETHERNET 
Controller:  XQ   Internet Interface:  Q 
                                Type:  CLUSTER ETHERNET 
Controller:  ER   Internet Interface:  R 
                                Type:  CLUSTER ETHERNET 
Controller:  FW   Internet Interface:  W 
                                Type:  CLUSTER FDDI 
Controller:  FA   Internet Interface:  A 
                                Type:  CLUSTER FDDI 
Controller:  IR   Internet Interface:  R 
                                Type:  CLUSTER TOKEN_RING 
Controller:  SL   Internet Interface:  S 
                                Type:  SERIAL 
Controller:  EO   Internet Interface:  O 
                                Type:  CLUSTER ETHERNET 
Controller:  PP   Internet Interface:  P 
                         Description:  PPP 
                                Type:  Serial 
Controller:  CL   Internet Interface:  I 
                         Description:  ATM Classical IP 
                                Type:  FDDI 
Controller:  EL   Internet Interface:  L 
                         Description:  ATM Emulated LAN 
                                Type:  FDDI 

To manually add a new controller, issue the DEFINE COMMUNICATION_CONTROLLER command. To remove a controller definition, issue the DELETE COMMUNICATION_CONTROLLER command.

2.3 Configuring Network Interfaces

The TCP/IP Services product supports one local interface for loopbacks and one or more network interfaces for each network controller.

The configuration procedure initially configures your network interfaces. Use the following commands if you need to redefine an interface or configure serial lines. See Chapter 3 for more information about configuring serial lines.

To display information, use the SHOW INTERFACE command, and to disable an interface, use the SET NOINTERFACE command.


If you are redefining an existing interface, enter the SET NOINTERFACE command before you enter the SET INTERFACE command.

2.3.1 Specifying the Interface

On the SET INTERFACE command line, the interface parameter is the interface name for the communication controller you are configuring. Some examples are: FZ0, FZ1, FR1, IR1, EZ0, EX0, SL0, SL1, SL2.

The name of a network interface has two characters followed by the unit number of the communication controller, conforming to the following pattern:
Controller Unit Number
First controller 0
Second controller 1
Third controller 2

2.3.2 Specifying the Network Mask

An IP address consists of a network number and a host number. The network mask is the part of the host field of the IP address identified as the subnetwork. Every host on the same network must have the same subnetwork mask. To specify the network mask, use the /NETWORK_MASK qualifier. This is required if you use subnetworks.

The product calculates the default by setting:

You can also divide the host field into a site-specific subnetwork and host field. For more information about defining network interfaces for subnet routing, see Section 4.6.1.

2.4 Manually Configuring a Hardware Address

Network hosts require manual configuration of a hardware address for a remote IP address under the following conditions:

For example, to map the Ethernet address AA-02-04-05-06-07 of host ROOK, follow these steps:

  1. Add the hardware address to the ARP table. Enter:

    TCPIP> SET ARP AA-02-04-05-06-07 ROOK

  2. Specify the time interval for ARP to hold the information in its cache, for example, 30 minutes. Enter this information into dynamic memory and into the permanent configuration database. Enter:


    To set the maximum time that an unresolved query is kept in the ARP dynamic database, use the /INCOMPLETE_TIMER qualifier.

Chapter 3
Configuring Serial Lines

A serial connection is made between two systems using modems and telephone lines or other serial lines. DIGITAL TCP/IP Services for OpenVMS supports serial connections using the PPP (Point-to-Point Protocol) and SLIP (Serial Line IP) (SLIP) (including CSLIP) protocols1. You can use any standard OpenVMS terminal device as a PPP or SLIP line.

After establishing a PPP or SLIP connection between hosts, you can enter TCP/IP commands. If the remote system is configured as a gateway to a network, local users can also reach other systems on that network through the serial connection.

This chapter reviews key concepts and describes how to configure your network interface to use a telephone circuit or other serial line.


1 PPP is available for OpenVMS Alpha systems only.

3.1 Reviewing Key Concepts

If your OpenVMS system is part of a large network, you will probably use both PPP and SLIP for your serial connections. An internet standard, PPP is often preferred because it ensures interoperability between systems from a wide variety of vendors. PPP provides a way for your OpenVMS Alpha system to establish a dynamic IP network connection over a serial line without the extensive use of an additional router or server hardware.

However, SLIP has been in use for a longer period of time and thus is available for more kinds of hardware. SLIP is available for most terminal servers and in most PC implementations of TCP/IP. Because SLIP and PPP do not communicate with each other, hosts wanting to communicate must use the same protocol. For example, if your terminal server supports only SLIP, remote hosts that connect through this server must also use SLIP.

3.1.1 Uses for PPP and SLIP

One of the largest applications for IP over serial lines is dialup access. With this type of configuration, your OpenVMS host answers calls and establishes a connection initiated by a user on a client host. The client host may be another OpenVMS system, a UNIX system, or a PC. Or, users on your host can originate the dialup connection to a remote host or terminal server running the same protocol.

Dedicated serial lines running PPP or SLIP can also be used to connect separate LANs into a single WAN. In such a configuration, the host at each end of the serial connection is always the same; no other hosts are allowed to connect to either serial device.

3.1.2 Assigning an IP Address to Your PPP or SLIP Interface

Every network interface must have its own unique IP address. Interfaces cannot share IP addresses.

If you configure PPP interfaces for multiple remote hosts, the remote hosts can obtain their individual IP addresses from your host when they connect. Similarly, you can configure a PPP interface on your system without knowing your own IP address and obtain it when you connect to a remote system.

Before establishing SLIP communcation with a remote host, however, you must obtain the IP address for the host's serial interface and assign IP addresses for each interface you configure on the local host.

When using SLIP, consider placing each serial line in a separate subnetwork. You accomplish this by assigning the same subnet mask for the interfaces at either end of the link.

If you need to use an address in the same subnetwork as your site LAN, use the proxy Address Resolution Protocol (ARP) feature (see Section 3.3.4).

3.1.3 Serial Line Internet Protocol

SLIP sends a datagram across the serial line as a series of bytes. It uses the special characters described below to mark when a series of bytes should be grouped together, as follows:
Character Function Hex Value Decimal Values
END Marks the end of the datagram. When the receiving SLIP encounters the END character, it knows that it has a complete datagram. C0 192
ESC Used to "escape" the SLIP control characters. DB 219

SLIP starts by sending an END character. If END is encountered within the datagram as data, SLIP inserts an escape character, sending the two-character sequence DB DC instead. If the ESC character appears within the datagram as data, it is replaced with the two-character sequence DB DD. The datagram ends with the END character after the last byte in the packet is transmitted.

There is no standard SLIP specification, nor a defined maximum packet size for SLIP. DIGITAL's implementation of SLIP accepts 1006-byte datagrams and does not send more than 1006 bytes in a datagram.

Compressed SLIP provides header compression that is beneficial for small packets and low-speed serial links. Header compression improves packet throughput. You can enable CSLIP by means of the /COMPRESS qualifier when you enter a SET INTERFACE command. See Table 3-3 for more information.

3.1.4 Point-to-Point Protocol

PPP uses a frame format that includes a protocol field. The protocol field identifies the protocol (for example, IP, DECnet, or OSI) to be used for communication between the two hosts. PPP defines the network frame in a 5-byte header and 3-byte trailer. The PPP frame starts and ends in the control byte 7E hex (126 decimal). The address and control bytes are constant. The 2-byte protocol field indicates the contents of the PPP frame.

3.2 Setting Up a PPP Interface (Alpha Only)

Use the following commands to configure a PPP interface on an OpenVMS Alpha system:


If you enter a SHOW INTERFACE command, the address will not appear until a PPP connection is actually established.

Table 3-1 shows the command qualifiers used for configuring PPP interfaces.

Table 3-1 Configuring PPP Interfaces
Qualifier Description
/COMPRESS=[ON|OFF|AUTO] Optional. The default is ON. Use to negotiate header compression.
/DESTINATION=[ host_name
| IP_address]
Optional. The default is No destination host. If you do not specify the client host's address, PPP will obtain the correct address from the client host.

If the host is used as a dialup provider, this command is used to specify a unique IP address for a client. In this case, you must also specify your host address with the /HOST qualifier.

/HOST=[ host_name | IP_address] Required when setting up a host as a dialup provider; otherwise optional. Host name or IP address using the interface. If your host is multihomed, specify the unique IP address if the two IP addresses map to the same host name.
/NETWORK_MASK= IP_address Optional. The subnet mask of the local PPP interface in dotted-decimal notation.
/SERIAL_DEVICE= device Required for hard-wired or dedicated modem connections. Identifies the OpenVMS device name assigned to the PPP interface, for example, TTA1.

3.2.1 Setting Up Your Host for PPP Connections

In the client/server model for PPP connections, a host can function as a server, or dialup provider, to respond to incoming PPP connection requests. A host can also function as a client dialing in to a dialup provider.

Setting up an OpenVMS Alpha host as a PPP dialup provider or client involves the tasks listed in Table 3-2 in the order you should complete them. The tasks are explained in Sections through

Table 3-2 Set Up Tasks Required for an OpenVMS Alpha PPP Dialup Provider or Client
Step Task OpenVMS
Dialup Provider
1 Install the correct terminal driver. Yes Yes
2 Configure your modem. Yes Yes
3 Set up an asynchronous port for modem connections. Yes Yes
4 Configure an interface for a serial PPP connection. Yes Optional
5 Enable IP forwarding and dynamic routing, as appropriate. Yes No
6 Initiate a PPP connection. NETMBX and OPER privileges required. No Yes Installing the Terminal Driver

Confirm that the virtual terminal driver SYS$SYSTEM:SYS$TTDRIVER.STB is installed on your host. If it is not installed, run the System Management utility (SYSMAN), connect the device, and load the driver, as shown in the following example:


After you run SYSMAN, confirm that the VTA0 device was created. For more information about SYSMAN and its parameters, see the OpenVMS System Management Utilities Reference Manual: M-Z.

For OpenVMS Alpha Version 7.1, you must also install the ASNDRIVER remedial kit to prevent the system from crashing. To obtain the driver and associated corrections, access a remedial kit and accompanying cover letter from: Configuring the Modem

To configure the modem, follow these steps:

  1. Make sure the serial port and modem cable support modem control signals. (DIGITAL's BC22F cable is an example of such a cable.)
  2. Determine if there are any baud rate restrictions associated with your phone line or on your connecting cable (when using a null modem or modem eliminator).
  3. Adjust the settings on your modem to enable AT commands, as appropriate for your modem. Some modems require you to set DIP switches; others require you to specify software settings.
    Sample DIP switch configuration settings for U.S. Robotics Courier modems are as follows. In the samples:
    Dialup provider settings

    DTR normal                X**     DTR always on               
    Verbal result codes       X       Numeric results codes 
    Suppress result codes     X**     Display result codes        
    Echo offline commands     X       No echo offline commands 
    Auto answer on ring       X**     Suppress auto answer        
    Normal carrier detect     X**     Carrier detect override     
    Display all results codes X       Result codes orig. mode only 
    Disable AT command set            Enable AT command set       X 
    Disconnect with +++               No disconnect with +++      X 
    Load NVRAM defaults       X       Load &FO settings 

    Client settings (defaults)

    DTR normal                X     DTR always on 
    Verbal result codes       X     Numeric results codes 
    Suppress result codes           Display result codes        X 
    Echo offline commands     X     No echo offline commands 
    Auto answer on ring             Suppress auto answer        X 
    Normal carrier detect     X     Carrier detect override 
    Display all results codes X     Result codes orig. mode only 
    Disable AT command set          Enable AT command set       X** 
    Disconnect with +++       X     No disconnect with +++ 
    Load NVRAM defaults       X     Load &FO settings 

  4. If possible, also configure the modem so that it does not assert the Data Terminal Ready (DTR) signal until it asserts the Carrier Detect (CD) signal. This configuration ensures that the terminal driver does not drop the DTR signal prematurely.

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