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SPECIFIC KERNEL ISSUES

Kernel applications affect a large organization in a variety of ways. They can affect future flexibility, network overhead, and administration time, to name just a few. When looking at stacks and deciding which kernel applications are important, managers should:

  Beware of stacks that force the organization into one standard over another, such as BOOTP vs. DHCP for dynamic address assignment. It is very expensive for a company that has standardized on BOOTP servers to replace all servers to support DHCP.
  Review the kernel applications that can affect network overhead. The way one stack handles handshaking with the host, for example, can significantly affect the number of packets put over both serial and Ethernet networks, and FTP and NFS caching reduces the overhead on serial lines.
  Look for stacks that provide dynamic link recovery and progressive acknowledgment, especially if the organization supports mobile and remote users.
  Determine how difficult it is to configure different desktops for different ways of connecting. Can the data center set up one client with an Ethernet connection and another with a serial PPP connection with little hassle? Configuration flexibility is an important kernel-level issue, especially when configuring large numbers of users, or remote users.
  Consider how security is implemented. This is one rapidly changing area that differs markedly from stack to stack. The most useful stack will support emerging security standards such as Kerberos, CHAP/PAP, IP-based security standards, and FTP firewall passthrough authentication. Is the NFS client implementation flexible? Does it provide easy access to NFS data and devices? Are NFS connection set-ups simple? Look for a stack that allows for tuning NFS parameters for better performance and that provides administration tools that help manage NFS connections.
  Determine whether kernel applications keep up with WinSock applications. For example, as multipoint data and video conferencing becomes more common, your stack will need to support IP multicasting, a kernel-based feature. Many free stacks are enhanced only when the operating systems they are embedded in are upgraded, thus limiting the timely implementation of new technologies.
  Evaluate network management tools. Does the stack come complete with diagnostic and monitoring tools, such as statistics for packet counts, trace utility, IP route tables, and ARP cache? SNMP MIB II agent and private MIB support are important, as well as event/fault logging, Finger client and server, and multimedia Ping. These allow for better administration and control of enterprisewide networks.

Other areas of kernel concern include memory resource management and scripting. TCP/IP implementations vary radically, with potential impact on an organization in a wide range of areas.

MATCHING REQUIREMENTS TO NEEDS

What people are finding in the real world of heterogeneous systems, enterprise-wide networks, and large corporate environments is that their requirements for a stack far outstrip what a free stack can provide. Many of these requirements are implemented within the architecture or the applications at the kernel level.

Enterprise customers are the market that looks for vendors who provide quality support, that take ownership of problems and respond quickly and helpfully. This means reliable support that does not require a credit card number or funneling the question through a single support contract. Support also means help for moving from proprietary transports to TCP/IP, from people who understand the desktop, networking, and host issues involved. If a PC’s network connection suddenly stops working, can the help desk get the information it needs to get it back on line quickly? The answer lies in the quality of the kernel applications — and in the quality of support provided.

MPTN ELIMINATES THE COMPROMISES OF TCP/IP IN AS/400 ENVIRONMENTS

The AS/400 world has been watching the proliferation of TCP/IP with great interest. A number of sites have been seduced by the promise of open networking with less expensive implementations and information sharing with other systems, across a common backbone network. Unfortunately, with the promise comes compromise.

Exhibit 2. Terminology
Basic Functions APPC TCP/IP

File Transfer SQL FTP
Terminal Services Display Station Pass Through Telnet
Printing Display Station Pass Through LPR/LPD
File Services Shared Folders NFS

The purpose of this chapter is twofold: To briefly illustrate the differences between TCP/IP and IBM’s proprietary networking technology, Advanced Peer-to-Peer Communication (APPC) and to illustrate the limitations of TCP/IP in an AS/400 environment, and suggest workarounds so the organization can get the benefits of TCP/IP without sacrificing the comprehensive nature of an APPC connection.

Exhibit 2 shows how TCP/IP and APPC connections are categorized into four critical areas. Because the table shows that TCP/IP and APPC provide similar functionality, it might be assumed that only the names of their respective functions are different. For example, TCP/IP provides terminal services with Telnet, whereas APPC provides terminal services with Display Station Pass Through (DSPT).

Limitations of TCP/IP

To better understand the compromise associated with TCP/IP, however, the data center manager needs to look beyond Exhibit 2 and explore the actual implementation of the functions. With TCP/IP connections, there are limitations with three of the four functions listed: file transfer, terminal services, and printing. Combined, these limitations make up the compromise in a TCP/IP connection. Although the constraints are subtle and in some instances pose no real problem, data center managers should consider them carefully before taking the plunge into TCP/IP.

File Transfer Limitation. In the AS/400 world, most file transfers incorporate SQL query capabilities. In the TCP/IP world, the common file transfer protocol, FTP, does not allow for SQL queries. This results in the inability to selectively download on a filed record basis. In addition, FTP does not handle packed fields or binary numbers appropriately. To circumvent these problems, managers need to send the output of a query to a disk file and transfer the resulting file to the PC. This workaround is not elegant, but it does accomplish the task at hand.

Terminal Services Limitation. Although this limitation is subtle, it can pose larger problems. The terminal services provided by Telnet do not allow for Device Name Mapping. Telnet has no method for assigning a specific device name to a TN5250session. Instead, sites are randomly assigned a device name from a pool of available names. Unfortunately, for those sites that require specific naming, there is no workaround at this time. This is the only noteworthy difference between terminal services provided by Telnet and terminal services provided by Display Station Pass Through.

Printing Limitation. In the APPC world, 3812-to-PCL conversion takes place at the PC. In the TCP/IP world, using LPR/LPD, 3812-to-PCL conversion occurs on the host. Because host CPU cycles are typically in short supply, having the AS/400 CPU perform printer conversions can bog down the system. With short print jobs, there is no noticeable performance degradation on the host. However, with large jobs or numerous print jobs, slightly slower performance can be expected from the AS/400. Although this limitation is also subtle, it’s a good idea to remember that AS/400 CPU cycles are being used for a task that used to be performed by the CPU of the PC, and manage your printing accordingly.

In spite of these limitations, TCP/IP offers both the benefits of open networking and a common protocol for sharing information with multi-vendor systems. The goal is to find a solution that capitalizes on the merits of TCP/IP, without trading off the functionality needed to maintain productivity. That solution can be found in Multi-Protocol Transport Networking. MPTN is an open architecture from IBM that enables integration of AS/400 systems into a multi-vendor connectivity environment. MPTN is available in Version 3 Release 1 of OS/400 as “AnyNet.”

The primary advantage of MPTN is that this new interface allows the use of APPC to communicate over TCP/IP networks. This means that the full capability of an APPC connection is available over the existing TCP/IP backbone. Effectively, this links a client PC directly to the AS/400, providing transparent access to functions such as AS/400 printing and file transfer. In essence, it enables the functions typically associated with DSPT, but over a TCP/IP connection. In contrast, TN5250 supports terminal functions via TCP/IP, printing is handled by LPR/LPD, and file transfer is handled by FTP.

Exhibit 3 shows a graphical representation of MPTN at the PC and the AS/400. MPTN can be thought of as a translation layer between APPC and TCP/IP. This translation layer must exist on both the client and host to accomplish the connection. As far as end users are concerned, they are communicating via APPC. To the network it just looks like another TCP/IP packet.

CONCLUSION

TCP/IP offers the benefits of open networking and connecting multi-vendor systems together with a single protocol. There has been a recent proliferation of free data communications protocol suites, with many distinct advantages. However, because an application is only as reliable as the protocol suite it runs on, several issues must be carefully considered before selecting a product.


Exhibit 3.  MPTN at the PC and the AS/400.

Data center managers should also examine architecture features, such as mobile connectivity, and applications, such as those critical to the smooth functioning of the enterprise, when they choose a TCP/IP stack for their complex organizations. For organizations with an AS/400, still more protocol limitations must be considered, such as file transfer, terminal services, and printing. Above all, the decision to use TCP/IP free software is the result of careful evaluation of organizations’ equipment and goals against the costs of sustaining such software.


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