The SDN-based IP+optical solution is able to avoid risks brought by shared risk link groups (SRLGs) when calculating and optimizing end-to-end multi-layer service paths.

First, lets have a look at the impact of SRLGs on path calculation.

An SRLG, as the name suggests, is a group of links sharing the same risk. If one of the links fails, other links in the group may also fail. For example, if multiple links share the same optical path, all links will be invalid after the optical path fails. This characteristic requires us to place the primary and backup paths in different SRLGs during service path planning. Placing a pair of primary and backup paths in the same SRLG is like putting all eggs in one basket. If the basket is dropped, all is lost.

SRLGs are given special consideration in optical network deployment. If the IP layer can obtain optical-layer SRLG information and have IP links planned based on SRLG constraints, IP link availability will be greatly improved.

Generally, more than one path meets a services requirements for bandwidth, delay, SRLG constraints, and other aspects. The IP layer uses routing protocols to dynamically calculate an optimal path based on path costs. The optical layer is like a hard pipe for IP paths. Sometimes, the optimal IP path cannot meet the SLA.

As shown in the following figure, P1 and P2 has two service paths in between, with the blue one being the primary service path and the green one the backup service path. After a fault occurs at the optical layer, the optical path corresponding to the backup service path reroutes. Without giving consideration to SRLG constraints, the backup service path may use the same optical path as the primary service path. If this is the case, IP services will fail to be protected.

The SDN-based IP+optical solution enables the IP layer to obtain optical-layer SRLG information and plan end-to-end IP paths based on SRLG constraints. In the preceding example, when SRLG constraints are taken into consideration, the calculated backup service path is P1 <-> P3 <-> P4 <-> P2, avoiding the risk of carrying the primary and backup service paths over the same optical path.

Next, lets have a look at how the SDN-based IP+optical solution avoids SRLG risks.

To avoid SRL risks, the IP layer must place the primary and backup paths into different SRLGs. Some dedicated IP links may share the same optical path, but their protection links must be carried over other optical paths.

On the network shown in the following figure, L11 and L12 belong to a link group named LG1 whereas L51 and L52 belong to another link group named LG5. LG5 provides protection for LG1. Member links in LG5 can share the same optical path, but they cannot share the same optical path as member links in LG1. In this case, you can create a path disjoint group named PD1 [LG1, LG5]. After calculating service paths based on service requirements, the IP and optical PCEs verify the calculation results based on path disjoint constraints, improving network reliability.

On actual networks, manually configuring end-to-end path disjoint groups is time-consuming. Configuring end-to-end path disjoint, such as distributing end-to-end primary and backup TE tunnel paths in different SRLGs, is almost impossible. In the SDN-based IP+optical solution, however, you can configure path disjoint constraints through the NetMatrix in advance. These constraints will then be automatically delivered to the optical PCE. After a service path calculation request is sent to the optical PCE, the optical PCE calculates paths based on specified path disjoint constraints.

Network administrators can dynamically change path disjoint constraints and associate these constraints with the path calculation process. This implementation enables network traffic optimization to be more flexible.

The preparatory work for SRLG path disjoint includes:

1.         Plan an appropriate SRLG for each link on the NetMatrix during network planning. 

2.         Ensure that network hardware resources are ready and basic hardware configurations are complete. 

       After the GMPLS UNI tunnel is established, the optical layer sends SRLG information to the IP layer. The IP layer then selects and protects paths based on received SRLG information.