  Segment routing is an emerging technology to simplify traffic engineering
implementation in WANs. It expresses an end-to-end logical path as a sequence
of segments through a set of middlepoints. Traffic along each segment is routed
along shortest paths. In this paper, we study practical traffic engineering
(TE) with segment routing in SDN based WANs. We consider two common types of
TE, and show that the TE problem can be solved in weakly polynomial time when
the number of middlepoints is fixed and not part of the input. However, the
corresponding linear program is of large scale and computationally expensive.
For this reason, existing methods that work by taking each node as a candidate
middlepoint are inefficient. Motivated by this, we propose to select just a few
important nodes as middlepoints for all traffic. We use node centrality
concepts from graph theory, notably group shortest path centrality, for
middlepoint selection. Our performance evaluation using realistic topologies
and traffic traces shows that a small percentage of the most central nodes can
achieve good results with orders of magnitude lower runtime.
