Northstar Multi-Layer PCE Demonstration and Interoperability
Oct 6, 2015
In a previous blog we were briefly introduced to a solution that enabled a packet-layer PCE to learn about the topology and link attributes of an under-lying transport layer network. This enables the packet-layer PCE to make more informed path computations by taking into account transport layer properties that would otherwise be hidden from the packet layer. The solution leverages a YANG data-model to exchange this topology information that is used to create an abstraction-layer network and derive the end-to-end “circuit” information. A view from the Northstar console of an example multi-layer network is shown in figure 1a and b below. This example network consists of several Juniper PTX and MX routers along with a 3rd party optical transport network.
As described in , the transport topology is built based on the exchange of a TE-topology data-model between controllers. This data-model consists of a set of nodes, links and their associated properties. A few salient aspects of the data-model are its technology-agnostic format, support for hierarchical and customized topologies and selective incremental updates. A brief example of the node & link properties that comprise the topology used in the above model are shown below. A few of the more important attributes are highlighted with bold text and their value is as follows:
The connectivity matrix enables the packet-layer PCE to build the end-to-end circuits by associating the matrix to each individual link present in the list of abstract-links.
The SRLG field is a dynamically populated, by the optical controller, set of attributes that can be used by the packet-layer PCE as a constraint to its path computation services for applications such as the diverse routing of LSPs.
The protection and restoration fields enable the under-lay controller to convey the protection status of abstract-links to either be used as a path computation constraint or a notification of an event within the under-lay network.
Multi-layer Visualization and Correlation:
This solution addresses several immediate use-cases, while opening up the door for many more opportunities in the future. Figures 2a and 2b, below, show two basic visualization examples. Figure 2a shows the correlation of client-layer links with their corresponding server-layer routing while figure 2b shows the SRLG attributes of the abstract-links within the transport-layer topology.
Figure 2a: client-layer link routing display
Figure 2b: SRLG display
While improved visualization, correlation and the communication of various topologic properties is certainly useful, the most powerful result of this solution are the ways in which the packet-layer PCE can leverage the new information in its application specific path computations. The packet-layer PCE now has access to new information which was previously unavailable.
Dynamic updates to SRLGs after optical restoration:
The first example, illustrated below in figures 3a, b, and c, is the ability for the packet-layer PCE to be informed that a failure has occurred in the transport-layer and that the transport-layer has restored the service. Furthermore, the transport-layer controller dynamically updates the SRLG attributes of the restored link thus enabling the packet-layer PCE to react if a constraint to one or more LSPs, under its control, has been violated. An example constraint may be a requirement for the diversity of 2 or more packet-layer LSPs. Figure 3a shows the initial network, figure 3b shows the failure indication captured on the PCE console, denoted by the dashed client-layer link, and figure 3c shows that the client-layer service has been restored and the updated SRLG attributes. The operator can then manually re-optimize the path of the effected LSPs or at the next periodic optimization interval; the PCE will dynamically optimize the network.
Figure 3b: Failure indication (----)
Figure 3c: Updated SRLGs
This solution has the ability to convey several new transport-layer attributes to the packet-layer, lending itself to be the foundation for many new types of PCE services not traditionally leveraged in modern IP/MPLS networks:
Attributes such as ‘protection’ flags can be used to route LSPs along links that are protected within the transport-layer
Abstract-link distance and/or delay can be used to aid in the routing of latency sensitive services
Attributes can be updated, just as SRLGs were shown to be updated above, when or if the transport-layer is repaired or re-optimized.
The technology agnostic, standardized data-model used to communicate this transport-layer information is easily extendable to accommodate new and emerging use-cases. The Northstar Controller leverages this information for traffic optimization and automation of traffic-engineering paths across the network, increasing network utilization and enabling a customized programmable networking experience.