Network Gateway Services John Enck

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1 Network Gateway Services John Enck Payoff In a TCP/IP network, a gateway has a specific purpose. The TCP/IP architecture even includes formal protocols that apply only to TCP/IP gateways. In a multivendor environment, however, a gateway can provide gateway tunneling/encapsulation services or translation/device emulation services. In a PC LAN, a gateway can provide network access for a remote PC over a dial-in circuit. These are all legitimate gateway services, though they are all quite different from one another. Introduction In the context of data communications, gateways are highly specialized devices that interconnect networks. With one exception, formal definitions for gateways are sketchy. For example, gateways can be composed of hardware, software, or combinations of both, and they provide functions ranging from simple protocol encapsulation to sophisticated protocol conversions. The exception is the Transmission Control Protocol/Internet Protocol (TCP/IP) environment. Unlike other network environments IBM's System Network Architecture (SNA),Digital's DECnet, and Novell's NetWare, for example in which the term gateway is ill-defined, a gateway in a TCP/IP network has a specific purpose and conforms to established rules. The architecture even includes formal protocols that apply only to TCP/IP gateways. The Big Picture The dictionary describes a gateway as a means of access. Although this definition embraces the spirit of network gateways, a more technical description for a gateway is a device or point-of-presence that interconnects dissimilar networks. The key concept is dissimilar networks, because networks can be dissimilar in a number of ways. For example, Ethernet and Token-Ring LANs are dissimilar. IBM System Network Architecture and TCP/IP networks are dissimilar. Wide-area Synchronous Data Link Control (SDLC) and metropolitan-area Fiber Distributed Data Interface (FDDI)networks are dissimilar. In truth, it is usually easier to find dissimilarities between networks than it is to find similarities. Gateways attempt to resolve most dissimilarities between networks, although rarely does a single gateway attempt to do it all. Network Functions of Gateways Many environments would benefit from the installation of a gateway. Although specific environments are too numerous to address individually, in general, gateways can be separated into groups that provide significantly different functions: Resource management. Transport. Intranetwork.

2 Interoperability. Resource Management Gateways in this category manage multiple, possibly concurrent access to a common network resource. For example, a modem gateway can be installed on a PC LAN that allows PCs to contend for access to dial-out services. Or a PC on a LAN can be dedicated as a remote control gateway to allow remote PCs to dial up and access all of the services of the LAN. Transport This kind of gateway carries data from one type of network over a second type of network. This is typically handled by embedding all of the information associated with one protocol inside another protocol. Two scenarios exemplify this approach: Tunneling. Traffic from one type of network can be routed through a different type of network. For example, IBM SNA traffic can be routed through atcp/ip network if one gateway inserts SNA protocol information from one SNA subnetwork into a TCP/IP network and another gateway extracts the System Network Architecture data from the TCP/IP network at a different geographical location and forwards it to another SNA subnetwork. This is sometimes referred to as tunneling. Encapsulation. To reduce the number of protocols required on a LAN, client-side software can embed foreign protocol information into the local protocol and route the traffic to a gateway, which can then extract the foreign protocol and forward it to the foreign network. For example, PC background software can embed Digital Local Area Transport (LAT) information into Novell Internet Packet EXchange (IPX) frames and route them to an IPX/LAT gateway. The gateway removes the Local Area Transport information and forwards it over a separate link to a LAT-oriented device (e.g., a Digital Equipment Corporation host computer). This technique is often called encapsulation. Intranetwork Internetwork gateways make connections within a single network architecture (e.g., SNA, TCP/IP, NetWare, DECnet). The connection allows traffic flow between multiple logical networks or between multiple physical networks (e.g., Ethernet and token ring). One of the confusing aspects of this category of gateways is that it overlaps with bridges and routers. Interoperability Gateways in this category translate network and application-oriented services between two (or more) types of networks to accommodate interoperability. These types of gateways require a high degree of sophistication because they must be able to translate between network protocols, network services, and data formats. For example, a gateway that accommodates exchange between SNA anddecnet must be fluent in the mail protocols and formats used in both network environments. Similarly, a gateway for file transfer and workstation access must translate between network file access services and also provide workstation emulation.

3 The TCP/IP Strategy Although it is possible to operate all the different kinds of gateways (resource management, transport, intranetwork, or interoperability) in a TCP/IP environment, the TCP/IP architecture depends on gateways to provide connections between different logical or physical networks. In terms of the gateway functions described,tcp/ip gateways fall into the intranetwork category. To better explain the role of TCP/IP gateways, the TCP/IP network address strategy must be demonstrated. 32-bit IP Addresses Under the TCP/IP architecture, each system in the network is assigned a four-byte (32-bit) address termed the IP address. Instead of being represented as hexadecimal values, however, these bytes are usually represented using the format w.x.y.z, where w, x, y, and z are replaced with a decimal number between 0 (hex 00) and 255 (hex FF). For example, is a valid IP address. The four-byte IP address is further broken down so that a portion of it identifies a logical network address and the rest of it identifies a system within that network. For example, identifies a specific system (address 12) within a specific network(address ). Similarly, address identifies system within network The IP address can be composed using one of three different formats, termed classes. Each class breaks the overall address down into network and host system components differently. Class A uses the format network.host.host.host, with the network component falling in the range between 0 and 127 (exclusive of 0 and 127) and the host component being greater than 0. For example, in the address , 64 identifies the network and identifies the host system. Class B uses the format network.network.host.host, with the first network component falling in the range from 128 to 191 (including 128and 191) and the host component being greater than 0. For example, in the address , identifies the network and 0.68identifies the host system. Class C uses the format network.network.network.host, with the first network component falling in the range from 192 to 223(including 192 and 223) and the host component being greater than 0.For example, in the address , identifies the network and 37 identifies the host system. All three address formats identify a particular host system within a specific network. A fourth type of address a Class D address is used for multicast addresses. Multicast addresses are intended for multiple systems that possibly reside in different networks; therefore Class D addresses have no network or host components. Class D addresses begin with a number in the range of 224 to 239 (inclusive of 224 and 239). If these four address classes are viewed as binary information, the high order bits of the first address byte determine the class type. Specifically, Class A addresses have the high order bit set to 0; in Class B the high order bits are set to 10; in Class C they are 110;and finally, in Class D the high order bits are These bit patterns then determine the acceptable address range for the first byte. TCP/IP Networks IP addresses not only define the boundaries for different physical networks, but in most implementations of TCP/IP software, these addresses create barriers between logical

4 networks. For example, a system at address usually cannot communicate with a system at address , even if the two systems are physically on the same cable segment. The safest assumption to make is that every system must go through a gateway to access a system residing in a different physical or logical network. This means, of course, that each system must be aware of the gateway(s) present in its local network. Conceptually, this poses no problem, because each gateway has an IP address for each network it handles therefore a gateway attached to three networks will have three different IP addresses. The logistics of informing a system about the presence of gateways depends on the protocol the host system uses to communicate with the gateways. Some protocols provide a means for gateways to announce themselves and the routes they support, whereas other protocols assume that the host system is explicitly informed of any gateways it is to use. For the sake of simplicity, this discussion focuses on the second case, where hosts must be explicitly informed. In most networks that involve cross-network traffic, each host system is configured to support a single, default gateway. Any traffic a system needs to route to any other network is delivered to that gateway, and the gateway forwards it as needed to reach the destination. If only one gateway is present in the network, then a single, default gateway connection can be defined for each host system to handle all of the cross-network traffic. If, however, the local network has multiple gateways and each gateway controls access to a different network (or set of networks),then the host systems must be made aware of each routing choice. Usually the configuration process begins by establishing a default gateway for each host system to handle any cross-network traffic not explicitly sent to other gateways. Establishing a default gateway is not absolutely necessary, but it does serve as a safety net to catch and report traffic that cannot be delivered because of missing or improper configuration information. After establishing a default gateway, every host system is then informed of each gateway and what specific systems or networks are obtainable through that gateway. Identifying gateways is a relatively simple process that associates target networks with gateway addresses. For example, a system residing in network may be given the following gateway assignments: To reach Use gateway all others (default gateway) Using this table, if a host system is trying to deliver a message to a system at address , it will route that message to gateway , because that gateway is responsible for all traffic to network Similarly, if the system needs to send a message to the host system at address , it will use gateway If, however, the system needs to send a message to address , it will use the default gateway ( ),because no gateway is explicitly configured to handle traffic for network Exactly how this information is configured at the host system level depends greatly on the operating system implementation. One common methodology used by many UNIX and UNIX-derived implementations is to use the route command to set up the routing table. For example, the previous routing table could be established using the following commands:

5 route route route route Regardless of how it is constructed, a simple list of networks and gateways provides a host system with all the information it needs to decide what gateway should handle its crossnetwork traffic. Unfortunately, the type of information required within the gateways as well as how that information is discovered is far more complex. TCP/IP Gateway Types In addition to the relationship that exists between host systems and the gateways that service them, gateways also have formal relationships with one another. This relationship allows gateways to share Routing Information and communicate changes that occur to therouting information. For example, if a gateway picks up a new network connection, it can share this information with other gateways. The protocol that a gateway uses to communicate with another gateway depends on the type of gateway it is. In the world of TCP/IP, all gateways are not equal and are, in fact, divided into three categories: interior gateways, noncore exterior gateways, and core exterior gateways. These gateways are shown in Exhibit 1. Types of TCP/IP Gateways Interior Gateways An interior gateway exists inside an autonomous, self-contained network. An autonomous network may be a small LAN, a series of interconnected LANs, a campuswide network, or anything in between. The defining aspect of an autonomous network is not the size; rather it is the fact that the network is fully functional in its own right it does not need external connections to be whole. The purpose of an interior gateway is to provide interconnections within the confines of the autonomous network. Noncore Exterior Gateways A noncore exterior gateway, on the other hand, provides a connection between two or more autonomous networks. In other words, an exterior gateway interfaces autonomous networks to the outside world. In the most simple case, exterior gateways can link up two autonomous networks. The more typical use, however, is to use exterior gateways to connect an autonomous network with the Internet. The Internet The difference between core and noncore exterior gateways comes into play on the Internet. Specifically, core exterior gateways provide interconnections within the framework of the Internet network. Unlike noncore exterior gateways, core gateways do not interface with autonomous networks they are dedicated to maintaining interconnections inside the Internet. When an autonomous network needs connectivity to the Internet, it uses a noncore exterior gateway that in turn interfaces to a core exterior gateway. The noncore gateways receive Internet Routing Information from the core gateway and makes those routes available to any interior gateways within the autonomous network. In simpler terms, when sending a message over the Internet, information will first pass from an interior gateway to a noncore exterior gateway, then from the noncore

6 exterior gateway to a core exterior gateway, which in turn routes the message through the Internet to another core gateway. At that point the message passes through another noncore external gateway and another internal gateway before reaching the destination host system. This message flow in shown in Exhibit 2. Traffic Flow Through TCP/IP Gateways TCP/IP Gateway Protocols As noted, the protocol used by a gateway to disseminate and maintain Routing Information is based on the type of gateway it is. The protocols used by the different types of gateways core exterior, noncore exterior, and interior are shown in Exhibit 3. TCP/IP Gateway Protocols Core exterior gateways use a gateway protocol known as SPREAD to talk with one another. This protocol is only used within the Internet. Because core exterior gateways also communicate with noncore exterior gateways, they support the same protocol used by noncore exterior gateways. A protocol known as the Exterior Gateway Protocol (EGP) can be used between any noncore exterior gateways to interconnect autonomous networks, or between a noncore gateway and a core exterior gateway to connect an autonomous network to the Internet. A noncore exterior gateway must also support at least one interior gateway protocol so it can communicate with interior gateways or host systems. Interior gateways support a variety of protocols because there are no formal Internet connectivity requirements within autonomus networks, therefore networking vendors can offer whatever protocol they happen to endorse. Some of the more common protocols are: HELLO. The HELLO protocol is one of the original interior gateway protocols used in TCP/IP networks. HELLO is different from the other interior gateway protocols in that it determines the best route for a message to take based on the estimated elapsed travel time. Although the HELLO protocol is still in use, it is not nearly as widespread asrouting Information Protocol. RIP. The RIP (RIP) was developed to provide routing services in Xerox Network System (XNS) networks. The XNS implementation was subsequently adapted for use in the TCP/IP and Novell NetWare environments. Routers using RIP broadcast their routing tables to one another on a regular basis. RIP operates as the Transmission Control Protocol level of TCP/IP and assumes that a path that passes through the fewest number of routers (has the fewest number of hops ) is the best path to take. OSPF. The Open Shortest Path First (OSPF) protocol was developed to replace RIP and offers several advantages over RIP. First, it operates at the IP level within TCP/IP, therefore it is capable of routing a wider variety of services than RIP. Second, OSPF is not as boisterous as RIP under OSPF, routing table updates get distributed to neighboring routers only. Finally, OSPF takes into account a number of factors to determine the best possible route, including line speeds, error rates, and current loading. Note that a gateway can fulfill both noncore exterior and interior roles there is no requirement that these functions must be provided by independent platforms. Many UNIX

7 systems, for example, include a program called gated that supports the EGP, HELLO, and RIP protocols, thereby enabling a host system to become an interior and a noncore exterior gateway. Conclusion Hardware or Software? Although the definitions of the various types of gateways describe key functions and specific protocols, the definitions do not dictate the characteristics of the system acting as a gateway. Is a TCP/IP gateway a dedicated piece of hardware or is it software running in a host system or server? The answer is simple a gateway can be either. It is possible to run dedicated gateway hardware or gateway software in an application host, or to do both. Another commonly asked question about gateways is this: how do gateway functions differ from the functions offered by network routers? The simple truth is that they do not differ. Gateways are called gateways in TCP/IP networks because they have been around long before routers came into popular use. In other words,tcp/ip gateways are gateways because that is the way it has always been. Author Biographies John Enck John Enck is a data communications and networking specialist with more than 15years of hands-on experience. He is currently employed as a network analyst with Forest Computer Inc., a manufacturer of multivendor gateways. He has also worked for Electronic Data Systems Corp.,Burroughs Corp., Illinois Law Enforcement Commission, and the US Department of Defense. He is the author of several networking books as well as articles for Data Communications Management, DEC Professional, HP Professional, MIDRANGE Systems, Network World, NEWS 3X/400,UniReview, and 3X/400 Information Management. He has a BA in computer science/communications from Antioch College.

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