4. Bandwidth-based Fast Reroute

4.1. Overview

The BBM solution described in Section 2 requires IGPs running on the control plane of the network device, to detect the link failures, determine remaining available bandwidth, re-compute new optimum paths, and finally install the new best paths to the forwarding plane. This may take some time (in the order of 500 ms) for the traffic to switch to a better path.

Also, even if regular FRR mechanism using LFA [RFC5286] and Remote-LFA [I-D.ietf-rtgwg-remote-lfa] has been deployed, the alternate paths chosen is not guaranteed to meet bandwidth constraints. Also, though, [RFC5286] does not specify anything, most LFA implementations in link-state protocols running on the network devices around the world, employs use of a single backup link. Also if there are multiple primary interfaces for a specific destinations, most implementations do not install a alternate path in the forwarding plane. So in the event of the primary link (or one of the multiple primary links) going down, traffic is either switched to a single interface, or not switched to any other link at all. In the first case, there is more likeliness of the single alternate path getting congested (as it might be already carrying some primary traffic for other destinations already). In the latter case, there is more likeliness of causing a congestion on the remaining primary links (e.g. for destination D1, if both L1 and L2 goes down R1 still keeps the traffic on L3 during local repair, trying to push 300G traffic on a single 100G link L5).

Service providers who have stringent bandwidth requirements would need the device to switch the traffic during local repair to multiple alternate paths that have bandwidth constraints satisfied. When the remaining primary OR alternate paths alone cannot satisfy bandwidth requirements, it will also be desirable, to redistribute the traffic over a combination of primary AND alternate paths, during local repair as well as next SPF computations in IGP.

This document proposes a solution the above problem, based on combination of BBM logic (referred to in Section 2) and protection using LFA [RFC5286] and Remote-LFA [I-D.ietf-rtgwg-remote-lfa]. It requires a group of primary links to be protected using multiple non-best feasible alternate paths. The same group of alternate links shall also be pre-installed in forwarding table to facilitate fast re-route (FRR). The details of the solution is specified in the following sub-sections.

4.2. Assumptions and Pre-requisites

Following are some of the assumptions that the solution proposed in this document is based on.

• The forwarding plane SHOULD be able handle multiple paths per route and let control plane set the preference for each path over the others. The forwarding machinery shall utilize this, to select a subset of preferred paths, and use them to forward actual traffic at any given point in time. Forwarding machinery SHOULD also load balance traffic with next-hops having same preference
• All the links attached to the network device are bundled to create one or more interface-group(s). Also a link MUST belong to one and only one interface-group.
• Loops are possible if protection is enabled on all three routes R1, R2 and R3 as shown in Figure 1. To avoid loops implementation MUST have downstream Path Criterion as explained in LFA [RFC5286]
• For each interface-group, operator MAY enable protection by configuring the following two parameters.
  • Minimum-bandwidth: When the remaining bandwidth goes below this the outgoing traffic can no more be carried entirely on this bundle. Some of it shall be distributed across links of other best/non-best interface-groups.
  • Restore-bandwidth: When the remaining bandwidth exceeds this, the outgoing traffic can entirely be back over the members of this bundle and there is no need to use any other backup for all destinations reachable over the links of the bundle.

4.3. Additional Configuration and Attributes

This document defines the following configuration parameters to be associated with each interface-group for facilitating Bandwidth-based Fast Re-Route. Implementations MUST allow operators to configure these parameters for each interface-group on a network device that implements this solution.

       IG1:
          Member-Links: L1,L2,L3
          Total-Available-BW: 200G,  Metric: 10
          Total-Available-BW: 100G,  Metric: 50
	  Default-Metric: 1000
          Protection: Enbaled
            Restore-Bandwidth:  200G
            Min-Bandwidth:      100G

       IG2:
          Member-Links: L4,L5,L6
          Total-Available-BW: 200G,  Metric: 10
          Total-Available-BW: 100G,  Metric: 50
	  Default-Metric: 1000
          Protection: Enbaled
            Restore-Bandwidth:  200G
            Min-Bandwidth:      100G
          

For each affected prefix (a prefix is affected if the fated link was 
one of the preferred active paths used for forwarding).
- Find the actual affected path, and mark it unusable.
- For all other paths downloaded from control-plane,
  - If the preference is same as that of the affected path, 
    - Modify its preference to value even lower than normal backup paths.
Finally, go through all remaining active paths
- Select a subset of paths (that share the same highest preference among all), 
- Use the selected subset of paths to actually forward traffic.
          

Step 1: 
=======
- For each interface-group "igp"
  - Update "igp.BW_Curr" by adding the bandwidths 
    of the individual active member interfaces.
  - Update "igp.BW_PostFail", assuming one of the active 
    member interfaces with highest bandwidth goes down next.

Step 2:
=======
- For each destination node "D" in the network (Pass-1)
  - Update D.Pri_BW_Restore and D.Pri_BW_Min from the SPF results.
  - Reset D.Pri_Nh_Count to 0.
  - For each corresponding primary path N,
    - Set "igp" -> Interface-group N belongs to.
    - If igp.BW_Curr > D.Min_BW
      - Set preference of N to Pri_Nh_Pref.
      - Increment the D.Pri_Nh_Count by 1.
    - Else 
      - Set preference of N to Bkup_Nh_Pref_Normal.

Step 3: 
=======
- For each destination node "D" in the network (Pass-2)
  - Update D.Pri_BW_Restore and D.Pri_BW_Min.
  - For each corresponding primary path N,
    - Set "Pri_Igp" -> Interface-group N belongs to.
    - If protection configured on "Pri_Igp"
      - If igp.Pri_BW_PostFail < D.Pri_BW_Restore, 
        OR igp.Pri_BW_PostFail ≤ D.Pri_BW_Min
        - For all backup paths M,
          - Set "Alt_Igp" -> Interface-group N belongs to.
          - Select M for installing in forwarding plane.
          - If D.Pri_Nh_Count == 0
            - If Alt_igp.BW_Curr ≥ D.Pri_BW_Restore, 
              AND Alt_Igp.BW_Curr > D.Pri_BW_Min
              - Set preference of M to Pri_Nh_Pref.
            - Else 
              - Set preference of M to Bkup_Nh_Pref_Normal.
          - Else 
            - If Alt_Igp.BW_Curr ≥ D.Pri_BW_Restore,
              AND Alt_Igp.BW_Curr > D.Pri_BW_Min
              - Set preference of M to Bkup_Nh_Pref_High.
            - Else 
              - Set preference of M to Bkup_Nh_Pref_Normal.