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. A segment can have a local semantic to an SR node or global within an SR domain. 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. The Segment Routing architecture can be directly applied to the MPLS dataplane with no change on the forwarding plane. IGP-based segments require minor extension to the existing link-state routing protocols. Segment Routing can also be applied to IPv6 with a new type of routing extension header.