Using Zero Bandwidth(BW) LSPs for Optimal Network Utilization in a PCE Controlled Network
Mar 14, 2016
It can be challenging to obtain optimal bin packing without significant network disruption and/or bandwidth over-booking when RSVP-TE LSPs are reserving bandwidth in a network. A complex and involved process of make-before-break(MBB) LSP creation and LSP preemption is needed to optimize an existing network.
Here is one simple example of the aforementioned complexity.
The link between 18.104.22.168 and 22.214.171.124 is 20G
All the remaining links are 10G
The green LSP is a low priority LSP with a BW reservation of 9G that is signaled
The orange LSP is a high priority with a BW reservation of 5G that is signaled after the green LSP
The resulting paths are shown below in figure 1
Figure 1: Initial paths
Figure 2, below, illustrates the results one would expect after running a path optimization on a centralized TE controller such as Northstar.
Figure 2: Expected path optimization results
However, figure 3 below is what the result will be:
Figure 3: Actual path optimization results
Why is this? The green LSP will have a longer than expected path because when the TE controller computes a new path for the green LSP, the orange LSP is still occupying the RSVP BW on the link between 126.96.36.199 and 188.8.131.52, as such it will compute a path that does not result in a path set-up error.
PCE Zero-Bandwidth(BW) mode
In a PCE controlled environment, the PCE is not only synchronized with the run-time state of the network but also responsible for ensuring the network is not bandwidth over booked as a result of its path computation and path placement. In some regards, this is a duplication of responsibility; both the network and the PCE are ensuring there is no bandwidth over-subscription. The PCE is keeping track of the bandwidth for each LSP and is expected to be “optimizing” the network resources so there is very little need to duplicate the bandwidth reservation, via RSVP, in the network it self.
For each LSP, the PCE can maintain a BW local to the controller. This BW can be a static BW inputted by the operator/application or a near real-time BW can be learned either from an external analytics source or via the extensions defined in  or various combinations.
When a LSP is controlled or created by the PCE with a specified BW the BW specified in the PCinitiate/PCupdate message will be set to zero. The resulting LSP signaled via RSVP by the PCC will be a zero BW LSP.
Zero-BW-mode is enabled on Northstar by setting the 0_bw_signalling parameter to 1 in the dparam.txt file located at /u/wandl/db/misc/ directory. Figure 4 below illustrates a LSP operating in zero-bw-mode. As you can see in the diagram, Northstar is using a BW collected via auto-bandwidth statistics collection while the LSP signaled in the network is zero. This can be observed by looking at the BW and Live_BW columns in the Tunnels table.
Figure 4: Zero-BW-mode auto-BW LSPs
The result is that a PCE is free to route LSPs without considering the reservation style of the network or that a LSP set-up maybe rejected due to BW overbooking when a MBB operation is performed. This allows the PCE to optimize resource utilization more effectively and more aggressively thereby achieving greater network efficiency.
The solution also compliments workflow interaction with offline capacity planning tools, such as IP/MPLSview or Northstar-Planner, whereby a user may be re-sizing LSPs based on forecasted data and the LSP BW can be communicated/updated to Northstar-Operator.