UPDATE 5/14: Juniper says OpenContrail already has the correlated overlay-underlay visibility that Nuage is talking about. The physical-network information is collected via SNMP and sFlow, which suggests that OpenContrail's method, like Nuage's, is applicable to any physical underlay. More info here.

Nuage Networks has announced what it believes is the first software-defined networking (SDN) visibility platform that peers into any type of network underlay.

Visibility is crucial because the abstracted, virtualized nature of SDN-based networks can make it hard to find out what's happening to the equipment in the physical world.

Specifically, isolating the source of a network fault can be difficult. You'd want to find the physical source of the problem, though in order to fix it — or, even more importantly, to be able to explain just what happened.

Financial institutions and high-speed traders are particularly interested in that information, so it's no coincidence that Nuage's Virtualized Services Assurance Platform (VSAP) is being introduced today, during the Open Networking User Group (ONUG) conference in New York. The financial sector was the initial driver for ONUG, a group formed to articulate user demands, such as openness, to the vendor community.

Nuage particularly notes that VSAP comes with "the ability to do correlation between an overlay and any underlay, whether it's Arista, Cisco, HP, Dell, or, of course, our own," says Sunil Khandekar, CEO of Nuage (which is a subsidiary of Alcatel-Lucent).

Nuage does this by setting up an element called a route monitor. It behaves like a router, using routing protocols such as BGP and OSFP to peer with everything it can find in the underlay network. "It's as if it's one of the routers, except it's not advertising," meaning it's not telling the other routers it's available to process traffic, Khandekar says.

A separate element called a correlation engine then syncs up the information gathered from the overlay and underlay networks.

The result is that problems can be pinned down to a physical location — a particular rack or server, for instance.

Visibility tools such as VSAP aren't only useful for providing post-disaster audit trails. They can also be used to gauge a baseline for "normal" performance in the network or to test out what-if scenarios.

Nuage is at ONUG this week demonstrating VSAP on an Arista Networks-based underlay.