Davide Cherubini - PhD Thesis - UniCA Eprints

6.4 LP models(a) Rerouting flow p → q(b) Rerouting flow q → pFigure 6.2: Rerouting flows in case of edge failure6.2(a) and 6.2(b).The survivability constraint on total flow traversing the directed link (i, j), afteran event of failure over a generic edge l, is:∑f∈Fx f,lij ·isf + ∑ flowij+ ∑ hh∈Nh∈N(x l +,lij·flow h l ++x l −,lij ·flow h l −) ≤ u max·c ij ∀(i, j) ≠ l + , l − ∈ A(6.50)In the first term the constant x f,lij specifies the share of flow f routed by IS-ISalong the link (i, j), when the edge l fails (calculated over the new graph obtainedremoving edge l), while the second term is the flow carried by explicit MPLS LSPalong link (i, j).In the third term, the constants x l +,lij and x l −,lij specify the share of MPLS trafficflowing through the link l, from p → q and from q → p respectively, that in caseof edge l failed, is rerouted along link (i, j) as shown in figure 6.2(a) and 6.2(b)respectively. As aforementioned, the solution obtained aims to guarantee 100%level of survivability in case of single link failure optimizing, automating, andspeeding up the traffic engineering decision process.Aggregated Flow DecompositionThe solution of the previously presented model gives as result the portion ofaggregated per source node flows traversing every link. These flows must be58