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In the network diagrammed in the previous section, there are no loops; graph theorists might describe this by saying the network graph is acyclic, or is a tree. In a loop-free network there is a unique path between any pair of nodes. The forwarding-table algorithm has only to make sure that every destination appears in the forwarding tables; the issue of choosing between alternative paths does not arise.
However, if there are no loops then there is no redundancy: any broken link will result in partitioning the network into two pieces that cannot communicate. All else being equal (which it is not, but never mind for now), redundancy is a good thing. However, once we start including redundancy, we have to make decisions among the multiple paths to a destination. Consider, for a moment, the following network:
Should S1 list S2 or S3 as the next_hop to B? Both paths A─S1─S2─S4─B and A─S1─S3─S4─B get there. There is no right answer. Even if one path is “faster” than the other, taking the slower path is not exactly wrong (especially if the slower path is, say, less expensive). Some sort of protocol must exist to provide a mechanism by which S1 can make the choice (though this mechanism might be as simple as choosing to route via the first path discovered to the given destination). We also want protocols to make sure that, if S1 reaches B via S2 and the S2─S4 link fails, then S1 will switch over to the still-working S1─S3─S4─B route.
As we shall see, many LANs (in particular Ethernet) prefer “tree” networks with no redundancy, while IP has complex protocols in support of redundancy (9 Routing-Update Algorithms).
1.5.1 Traffic Engineering
In some cases the decision above between routes A─S1─S2─S4─B and A─S1─S3─S4─B might be of material significance – perhaps the S2–S4 link is slower than the others, or is more congested. We will use the term traffic engineering to refer to any intentional selection of one route over another, or any elevation of the priority of one class of traffic. The route selection can either be directly intentional, through configuration, or can be implicit in the selection or tuning of algorithms that then make these route-selection choices automatically. As an example of the latter, the algorithms of 9.1 Distance-Vector Routing-Update Algorithm build forwarding tables on their own, but those tables are greatly influenced by the administrative assignment of link costs.
With pure datagram forwarding, used at either the LAN or the IP layer, the path taken by a packet is determined solely by its destination, and traffic engineering is limited to the choices made between alternative paths. We have already, however, suggested that datagram forwarding can be extended to take quality-of-service information into account; this may be used to have voice traffic – with its relatively low bandwidth but intolerance for delay – take an entirely different path than bulk file transfers. Alternatively, the network manager may simply assign voice traffic a higher priority, so it does not have to wait in queues behind file-transfer traffic.
The quality-of-service information may be set by the end-user, in which case an ISP may wish to recognize it only for designated users, which in turn means that the ISP will implicitly use the traffic source when making routing decisions. Alternatively, the quality-of-service information may be set by the ISP itself, based on its best guess as to the application; this means that the ISP may be using packet size, port number (1.12 Transport) and other contents as part of the routing decision. For some explicit mechanisms supporting this kind of routing, see 9.6 Routing on Other Attributes.
At the LAN layer, traffic-engineering mechanisms are historically limited, though see 2.8 Software-Defined Networking. At the IP layer, more strategies are available; see 20 Quality of Service.