MPLS: New foundation or next overlay network?

Like most major advances in networking technology, multiprotocol label switching (MPLS) was originally conceived to address a single, well-defined problem, then its mission ballooned as product developers anticipated more scenarios in which MPLS could play a part. MPLS is used in a handful of carrier backbones for its original purpose - to engineer IP traffic flows at Layer 3, rather than using ATM virtual circuits at Layer 2. There are also emerging roles for MPLS in IP-VPN services, legacy circuit emulation and transport, and in generalized Layer 1-3 carrier network management. Carriers looking to the future may have noticed that a circuit-oriented newcomer, dubbed Data over SONET (DoS), is sneaking up on MPLS's various value propositions. Given the persistence of layered architectures and multiple networks, it seems likely there will be room for both MPLS and the emerging DoS technologies.