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-ofservice 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 and other contents as part of the routing decision.