Chapter 5 Introducing SDN Control in MPLS Networks - High ...

183benefits of the map-abstraction before (Sec. 1.4.2). Here we focus on the benefits in thecontext of MPLS networks.Simplicity: The map abstraction helps simplify the implementation of controlprograms for the simple reason that they are no longer implemented as distributedapplications. When control programs are centralized and decoupled from the process ofstate collection and dissemination, they do not have to contend with subtle interactionswith the state distribution mechanisms; which can be complex and is often the source ofmany errors.Take the simple case of a network-function like IP based traffic-engineering on top ofplain-vanilla IP routing. Each router is a decision-maker and shortest-path decisions aremade on the basis of costs assigned to links. In most cases routing-protocols assign linkcosts taking the bandwidth of a link into account. But network operators have the abilityto configure and change the cost of any link, and often do so, to avail of somerudimentary IP traffic-engineering benefits (without invoking MPLS-TE and its complexcontrol plane).While changing link costs is simple, the effect it can have is far from trivial.Changing the cost of a link in one part of the network, potentially changes a lot ofshortest-paths that can in-turn affect traffic in a completely different part of the network.In principle it can be disruptive to many (or all) traffic flows. Worse, while the routingprotocolconverges, loops may get formed and packets may be lost. Thus changing linkcostsis considered just as disruptive as link-failures. But when decision making iscentralized and decoupled from the state-distribution mechanism, such a networkfunctioncan be a lot simpler. When link-costs change, the controller can simply beprogrammed to not re-route existing flows in the network, and use new shortest-pathsonly for new flows. Or it could selectively re-route existing flows. The choice lies withthe application-programmer (network-operator).As a more concrete example, in Sec. 5.3, we will show that centralization of controlfunctionsresults in simpler code. We have implemented nearly all the features of MPLS