Introduction to Telnet 3270

This section provides an introduction to Telnet 3270 (TN3270). It discusses why you use TN3270, provides some background information about TN3270, and discusses Cisco's product offerings for TN3270.

Why use Telnet 3270?

There are several key factors occurring that are determining the direction companies are taking in the networking arena. One of these factors is the advent of migrating from SNA networks to IP networks. As a part of this migration, companies are outfitting their end users with PCs that communicate via TCP/IP. As a result, they are replacing their SNA-based 3270 emulation packages with TN3270 clients and adding TN3270 servers to their networks. TN3270 is the method of transporting SNA 3270 data streams over an IP network. Some of the factors that are driving companies in this direction are:

The role of the mainframe has shifted from a central server for all application and data processing to an enterprise server.

: In the role of an enterprise server, the mainframe is now a peer with other servers, such as a UNIX or NT server. With the introduction of the mainframe operating system OpenEdition, the mainframe can now participate in a UNIX environment as a server. Because large companies continue to maintain more than 70 percent of their data on the mainframe, a method of transporting the SNA 3270 data stream over an IP network is necessary. In addition, network requirements such as providing a nondisruptive, redundant network must be met.

User requirements have expanded.

: Text-based applications are being replaced by applications with graphical user interfaces (GUIs). In addition, users require reduced screen input and enhanced application display methods, which can be provided by Web-based clients.

The technical skills of the networking workforce are becoming stronger in TCP/IP and weaker in SNA.

: The networks that are used and taught at the universities do not typically incorporate SNA. Students are taught TCP/IP and are expecting their future employers to have IP networks. Corporations are finding that while they can retrain their SNA network specialists to be IP specialists, it is much more difficult to teach SNA to an IP network specialist. SNA networking requires an understanding of historical hardware and software structures. Management of an SNA network requires mainframe knowledge, including dataset manipulation and operator console control. The combination of these factors makes it difficult for corporations to maintain existing mainframe applications and legacy SNA networks.

Corporations that maintain legacy SNA networks, often establish a separate department to create and support the Internet/intranet to satisfy requirements such as Internet access and e-mail. Maintaining two separate networks (and two separate network support departments) is not a cost-effective, long-term solution. Maintaining the mainframe applications and transporting them over the intranet is an ideal situation.

The TN3270 solution addresses all of these factors. Implementing the TN3270 server is simple. The difficulty in this migration path arises when you have to incorporate existing application and network restrictions into a new topology. It is important to understand the application and network requirements of your corporation before you decide which of the TN3270 server's features best meets these requirements and how it should be implemented.

Background

Before we discuss the variety of options available, it is important to understand the basic concepts of SNA 3270 and TN3270. If you are already familiar with these concepts, proceed to "Cisco Product Offerings."

What is a 3270 Data Stream?

3270 devices include a series of display devices, communications controllers, and printers that connect to an IBM mainframe. They support a special type of data stream, which is called the 3270 data stream. This data stream allows applications to include control characters that instruct the receiving device how to format or display the information. The control characters allow the application to use the entire screen, as opposed to a single command line, to display information and to receive input from various areas of the screen (called partitions).

What is SNA 3270?

SNA 3270 refers to 3270 data streams that are transported from the source to the destination using the SNA protocol. With SNA 3270, the data stream is transmitted over an LU-to-LU session between logical units. LU types 2 and 3 use only SNA 3270 data streams. LU types 6.1 and 6.2 use the SNA 3270 data stream in addition to other types of data streams.

With SNA 3270, the mainframe application creates a 3270 data stream using 3270 commands, the data stream is transported over an SNA network, and the data is displayed on a 3270-compliant device.

What is TN3270?

TN3270 is a protocol that defines how to transport 3270 data streams over a TCP/IP network. TN3270 was originally defined in RFC 1576 and is based on the Telnet protocol. The difference between a Telnet session and a TN3270 session is that a Telnet session uses the ASCII character set and sends a line of data at a time and a TN3270 session uses the EBCDIC character set and sends a block of data (a screen refresh) at a time.

The following elements work together to enable TN3270 communication:

The end stations run a TN3270 client. The client emulates a 3270 display device.
The client accesses a TN3270 server over an IP network.
The TN3270 server converts the TN3270 data stream to SNA 3270 and passes the data to the mainframe. The TN3270 server resides on either the mainframe, in which case it is considered an inboard server, or a front-end processor (FEP) such as the Cisco CIP and CPA, in which case it is considered an outboard server.
The mainframe provides Virtual Telecommunications Access Method (VTAM) and the mainframe application that the user is attempting to access.

Figure 1-1: Diagram of a TN3270 Session

In an SNA 3270 network, the user is known by the LU name. In a TCP/IP network, the user is known by an IP address. One of the tasks of the TN3270 server is to provide a correlation between these two addresses. This correlation allows the SNA applications to maintain the LU requirements and allows the network to use IP. Management of the LU-to-IP correlation is discussed in Chapter 5, Network Management.

What is TN3270E?

TN3270, as defined by RFC 1576, does not address several functions that are required to make the TN3270 server a viable migration solution. To address this problem, TN3270E was defined under RFC 2355 (which made obsolete RFC 1647). TN3270E added the following functions:

Emulation of 328x printers---All IBM-type printers accept both LU 1 and LU 3 printer streams because a printer may be shared between multiple IBM host applications, such as JES and CICS. The JES can be configured for LU 1 data streams and the CICS can be configured for LU 3 data streams. A TN3270 server cannot function properly with mainframe applications and 3270 printers if it does not allow a mix of LU 1 and LU 3 data streams.
Client request of a particular name---Many host applications behave differently depending on the network name of the terminal. In the case of printer emulation, many host applications use a method of predefining printer destinations. It is important that a Telnet client is allowed to request that a connection be associated with a specific 3270 device name.
3270 ATTN key---The 3270 ATTN key is interpreted by many host applications in an SNA environment as an indication that the user wants to interrupt the execution of the current process. The Telnet Interrupt Process (IP) command was defined for this purpose and was used in implementing support for the 3270 ATTN key. TN3270E supports the 3270 ATTN key in both the client and server:
- TN3270E clients allow a single key or a combination of keys to be mapped to the 3270 ATTN key. When the user presses the mapped keys, the client transmits a Telnet IP command to the server.
- TN3270E servers translate the IP command received from a TN3270E client and pass it to the host application as an ATTN key. In other words, the server representing a secondary LU (SLU) in an SNA session sends a SIGNAL RU command to the host application.
SNA positive/negative responses---A positive response indicates that the previously received data was successfully processed. A negative response is used to indicate that an error has occurred while processing the previously received data; this error may be caused by the host application building a 3270 data stream that contains an invalid command or by a mechanical error at the client side. Support for positive/negative responses is important in printer emulation, but it is also useful for some terminal applications.
Client access to bind information---The Bind image contains a detailed description of the session between the Telnet server and the host application. TN3270 provided no way for the client to access session information. Certain clients require access to the Bind information so that they can determine the format of the data they are to receive. For example, printer clients require access to Bind information to distinguish whether the data stream is LU 1 or LU 3.

Cisco Product Offerings

Cisco offers a TN3270 Server as well as a couple of TN3270 clients.

TN3270 Server on a CIP or CPA

The implementation of TN3270 server on a channel-attached router, using the CIP or CPA, provides an efficient method of removing the processing of TN3270 sessions from the costly mainframe cycles to a faster, more efficient router. The CIP can be installed in a Cisco 7000 or 7500 series router. The CPA can be installed in a Cisco 7200 series router.

The Cisco TN3270 Server feature implements RFC 2355 and allows TN3270 or TN3270E clients to access TN3270 host data using a channel-attached router that is connected directly to a host or remotely through an FEP. It provides mapping between SNA 3270 hosts and TN3270 clients connected through a TCP/IP network. TN3270 clients with LU 2.0 and TN3270E clients with LU 1, 2, and 3 are supported. (The server also supports printing as defined by RFC 1646.) From the perspective of an SNA 3270 host connected to the channel-attached router, the TN3270 server is an SNA device that supports physical units (PUs), each supporting up to 255 LUs. The SNA host is unaware of the existence of the TCP/IP extension on the implementation of these LUs.

The LUs implemented by TN3270 server are all dependent LUs. To route these dependent LU sessions to multiple VTAM hosts connected to the server in the channel-attached router (rather than on the VTAM hosts), a MiniDLUR with end node (EN) Dependent LU Requester (DLUR) function is implemented as part of the TN3270 server. The use of DLUR is optional so that the TN3270 Server can also be used for non-APPN capable VTAMs (versions older than VTAM 4.2). Because the TN3270 Server feature implements the function of DLUR, it can be configured to communicate with the primary server and, in case of failure, the backup Dependent LU Server (DLUS) residing on the host.

From the perspective of a TN3270 client, Cisco's TN3270 Server on a channel-attached router is a Telnet server that can support up to 16,000 concurrent Telnet sessions at 850 transactions per second. The server has been tested with up to 30,000 sessions at lower transactions per second. The server supports Telnet connection negotiation and data format as specified in RFC 1576 (traditional TN3270), RFC 1646 (printer extensions to RFC 1576), and RFC 2355 (TN3270E).

Implementing the TN3270 Server on the channel-attached router requires a minimum of two commands; the first command initiates the TN3270 Server and the second defines the PU. No LU definitions are created on the TN3270 Server. The function of the server is to provide the interface between the IP network and the SNA network. The TN3270 Server performs an exchange ID (XID) negotiation at the time of startup. At this point the details regarding the PU and LU activation are passed from VTAM to the TN3270 Server.

Table of Contents

Introduction to Telnet 3270