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The alternative to connection-oriented switching is connectionless switching, sometimes called datagram service. In connectionless networks, no predetermined path goes from the source to the destination. Also, no call setup packet exists; all data packets are treated independently, and the switches perceive no relationship between them as they arrive hence the name connectionless. Every packet carries a complete destination address, since it cannot rely on the existence of a preestablished path created by a call setup packet. When a packet arrives at the ingress switch of a connectionless network, the switch examines the packet s destination address. Based on what it knows about the topology of the network, congestion, the cost of individual routes, distance (sometimes called hop count), and other factors that affect routing decisions, the switch will select an outbound route that optimizes whatever parameters the switch has been instructed to concern itself with. Each switch along the path does the same thing. For example, let s assume that the first packet of a message, upon arrival at the ingress switch, would normally be directed out physical
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Protocols
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port number seven, because, based upon current known network conditions, that port provides the shortest path (lowest hop count) to the destination. However, upon closer examination, the switch realizes that although port seven provides the shortest hop count, the route beyond the port is severely congested. As a result, the packet is routed out port 13, which results in a longer path but avoids the congestion. Because no preordained route through the network is used, the packet will simply have to get directions when it arrives at the next switch. Now the second packet of the message arrives. Because this is a connectionless environment, however, the switch does not realize that the packet is related to the packet that preceded it. The switch examines the destination address on the second packet and then proceeds to route the packet as it did with the preceding one. This time, however, upon examination of the network, the switch finds that port seven, the shortest path from the source to the destination, is no longer congested. It therefore transmits the packet on port 7, ensuring that packet two will in all likelihood arrive before packet one. Clearly, this poses a problem for message integrity and illustrates the criticality of the transport layer, which, you will recall, provides end-to-end message integrity by reassembling the message from a collection of out-of-order packets that arrive with varying degrees of delay because of the vagaries of connectionless networks. Connectionless service is often called unreliable because it fails to guarantee delay minimums, sequential deliveries, or, for that matter, any kind of delivery. This causes many people to question why network designers would rely on a technology that guarantees so little. The answer lies within the layered protocol model. Although connectionless networks do not guarantee sequential delivery or limits on delay, they will ultimately deliver the packets. Because they are not required to transmit along a fixed path, the switches in a connectionless network have the freedom to route around trouble spots by dynamically selecting alternate pathways, thus ensuring delivery, albeit somewhat unpredictably. If this process results in an out-of-order delivery, no problem: that s what the transport layer is for. Data communications is a team effort and requires the capabilities of many different layers to ensure the integrity and delivery of a message from the transmitter to the receiver. Thus, even an unreliable protocol has distinct advantages. An example of a well-known connectionless protocol is the Internet Protocol (IP). It relies on the Transmission Control Protocol (TCP), a transport layer protocol, to guarantee end-to-end correctness of the delivered message. Times will occur, however, when the foolproof capabilities of TCP and TCP-like protocols are considered overkill. For exam-
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