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Northstar Multi-Layer PCE Demonstration and Interoperability
Oct 6, 2015

In a previous blog[1] 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[2] 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[3] 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.



                                       Figure 1a: Northstar Multi-Layer View collapsed links – LSP table



                                  Figure 1b: Northstar Multi-Layer View expanded links – SRLG table


As described in [1], 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.



                                                                           Figure3a: Steady-state



                                                                  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.


Please stop by the Juniper Networks, Inc. booth#73 at the SDN and OpenFlow World Congress October 12th to the 16th to see a working demonstration.