MPLS Segment Routing over IP

MPLS Segment Routing (SR-MPLS) is an MPLS data plane-based source routing paradigm in which the sender of a packet is allowed to partially or completely specify the route the packet takes through the network by imposing stacked MPLS labels on the packet. SR-MPLS can be leveraged to realize a source routing mechanism across MPLS, IPv4, and IPv6 data planes by using an MPLS label stack as a source routing instruction set while making no changes to SR-MPLS specifications and interworking with SR-MPLS implementations. This document describes how SR-MPLS capable routers and IP-only routers can seamlessly co- exist and interoperate through the use of SR-MPLS label stacks and IP encapsulation/tunneling such as MPLS-in- UDP as defined in RFC 7510.