Segment Routing (SR) leverages the source routing paradigm. A node steers a packet through a controlled set of instructions, called segments, by prepending the packet with an SR header. A segment can represent any instruction, topological or service-based. SR allows to enforce a flow through any topological path and service chain while maintaining per-flow state only at the ingress node to the SR domain. Segment Routing can be directly applied to the MPLS architecture with no change in the forwarding plane. This drafts describes how Segment Routing operates on top of the MPLS data plane.