One of the key trends in carrier services is reducing the load on switching devices (switches and routers). Switching capacity is the big limit in networking today. Optical fiber can carry data at enormous speeds, but building enough switching capacity to keep up with transmission lines is another matter.

There are two ways to reduce the load on switching devices. One is to use unreliable service. In other words, do not check for errors at every switching device along the route. Error checking is a complex process that consumes too much processing power within the switching device. It is better to do error checking once, at the receiving station, than on each switching device along the route.

The other way to reduce the load on switching devices is to use connection-oriented service instead of connectionless service. In connectionless service, each packet travels separately. This means that each switching device must decide, for every packet, the best way to route the packet. This places an enormous load on switching device. As a result, X.25, Frame Relay, and ATM rely on connection-oriented service. Before two stations begin to talk to one another, the network establishes a connection (preset route) between them. Once this route is established, switching devices along the route do not have to make complex routing decisions. They merely note the packet’s connection number, then send the packet out on the appropriate port.

What about routers? Routers offer unreliable service. They only check for errors in headers, and they only do that to protect the integrity of the network. However routers do not offer connection-oriented service. Instead, they offer connectionless service. When a series of packets passes through a routed internet, a routing decision is made for every packet at every router. This is the major reason why routers are extremely expensive. For a given traffic load, you need a much faster processor in the router than you would using connection-oriented service.

One solution is to make connectionless service available as an option on routers under appropriate conditions while still supporting the default of standard connectionless service. As noted in Module H, the new version of the Internet Protocol (IP Version 6) has the ability to deal with flows (connections). Once a flow is created, all packets with the same flow number are routed in the same way. Unfortunately, IPv6 did not fully specify how to implement flows. The most likely candidate standard, RSVP, has proven somewhat problematic.

To gain a competitive market advantage, several router vendors have created their own ways to add connection-oriented service to routers. In a typical operation, the router observes connectionless packets flowing through it. If it notes a persistent series of packets going to the same program on the same host computer, it creates a connection for this traffic. From that point on, packets in the series are not subjected to routing decisions at each router. They are merely sent out the port specified by the connection.

In effect, to use the terminology in Chapter 7, these switching routers set up switched virtual circuits. This makes them much more flexible than switches, which have traditionally used permanent virtual circuits which have to be established prior to the call. (Chapter 7 discusses these two concepts.) Because these routers then act like switches, this process is also called IP switching.

Not to be outdone, a number of switch vendors have added ways to handle IP traffic. Their argument is that switches are much less expensive than routers today. We will call their products routing switches.

In the Internet backbone, some of the fastest routers at the heart of the Internet are not routers themselves. Rather, they are ultra-high-speed ATM routing switches capable of handling IP. Quite simply, routers do not scale up sufficiently to handle the huge work loads place on Internet backbone routing points.

Unfortunately, we do not have firm standards either for switching routers or routing switches. In addition, several vendors have submitted competing standards for both switching routers and routing switches to the Internet Engineering Task Force.

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