| Internet Engineering Task Force | S. Pallagatti, Ed. |
| Internet-Draft | P. Sarkar, Ed. |
| Intended status: Standards Track | H. Gredler |
| Expires: September 7, 2015 | Juniper Networks |
| March 06, 2015 |
IGP bandwidth based metric.
draft-spallagatti-rtgwg-bandwidth-based-metric-00
This document describes a method to group multiple interfaces and assign metric to that group based on the cumulative bandwidth of all the interfaces in that group. Each link in a group takes same group metric irrespective of its own bandwidth.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 [RFC2119].
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A low cost path is always preferred to carry traffic from source to destination. If a application is more interested in bandwidth than the cost itself and most preferred path does not satisfy bandwidth then this could potentially lead to congestion and packet loss for an application. Bandwidth critical applications needs minimum bandwidth to be satisfied even if traffic is carried over multiple alternative paths to reach a destination.
+--------+ +--------+
| |----------10---------| |
| R2 |----------10---------| R3 |
D1-----------| |----------10---------| |-----------D2
+--------+ +--------+
| | | | | |
| | | | | |
| | | +--------+ | | |
| | +--10---|L1 L4|---10---+ | |
| +----10---|L2 R1 L5|---10-----+ |
+-------10---|L3 L6|---10--------+
+--------+
Figure 1: Example Topology
Consider the topology as show in Figure 1. The device R1 uses a links L1, L2 and L3 to carry traffic to destination D1. Similarly it uses links L4, L5 and L6 for destination D2. However in the event of links L1 and/or L2 fails traffic is still forwarded on link L3 causing traffic congestion.
In such situations operators will prefer the traffic for destination D1 is forwarded on L4, L5 and L6, as there is lesser chance of congestion. Similarly when links L4 and L5 also fails, the operator will prefer the traffic for D1 is forwarded is switched back on link L3 again. This document proposes a method called Bandwidth Based Metrics (hereafter referred as BBM), which helps achieving this desired behavior.
BBM, on detecting a local link event, attempts to re-route traffic, based on remaining bandwidth across the links on the primary and alternate paths. When the remaining available bandwidth on the primary link(s) goes below a permissible limit (to be specified by the operator), traffic should be re-routed to one or more groups of alternative paths, and re-distributed onto multiple alternate paths with lesser likeliness of congesting them.
This document also specifies how to extend Fast Re-Route (FRR) for BBM to meet stringent re-convergence time constraints, and minimize traffic loss due to network congestion caused by standard FRR mechanisms.
BBM method proposed in this document requires grouping of all the local links (or interfaces) attached to a node into one or more locical bundles. Such a logical grouping of multiple local interfaces is called an interface-group, and needs to be provisioned manually by the operator on each node. While assigning the local interfaces to a interface-group, all links connecting the local node to the same one-hop neighbor, SHOULD be assigned to a single interface-group. In other words the number of interface-group to be created on a node SHOULD be at the least, the number of one-hop neighbor nodes the particular node is connected to.
In Figure 1 links L1, L2, and L3 connecting R1 and R2 can be grouped into a single interface group (say IG1) on both R1 and R2. Similarly links L4, L5 and L6 connecting R1 and R3 can be grouped into another single interface-group (say IG2) on both R1 and R3.
All the interfaces under a given interface-group shall share a metric that is proportionate to the cumulative bandwidth available using the individual links under the interface-group. Implementations SHOULD allow operator to specify what metric should be assoicated for a given total remaining available bandwith for each interface group. Implementations SHOULD also allow operator specify the default metric to be used for each interface-group.
In Figure 1, considering all the links L1 to L6 having bandwidth capacity of 100G each, and assigned into two interface-groups IG1 and IG2 (as shown in Section 2.1), following is an example of simple BBM config for each of these interface-group.
IG1:
Member-Links: L1,L2,L3
Total-Available-BW: 200G, Metric: 10
Total-Available-BW: 100G, Metric: 50
Default-Metric: 1000
IG2:
Member-Links: L4,L5,L6
Total-Available-BW: 200G, Metric: 10
Total-Available-BW: 100G, Metric: 50
Default-Metric: 1000
Figure 2: Example BBM Configuration
This document also defines the following attributes to be associated with each interface-group.
| Attribute | Value and Significance |
|---|---|
| Intf_List | The list of interfaces assigned to this group as per configuration. |
| BW_Curr | Total available bandwidth across all active member interfaces of this group. |
| BBM_Metric_Cfg_List | This is an array of "BBM_Metric_Cfg_Entry" (defined below). The key to the list is "bandwidth" and is always sorted in descending order (i.e. entries with higher "bandwidth" appears before entries with lower "bandwidth". |
| BBM_Metric_Cfg_Entry | This defines a single entry in "BBM_Metric_Cfg_List" array (defined above). It is a tuple ["Bandwidth", "Metric"], and defines the metric that should be associated with the individual interfaces of this group, when the total available bandwidth for the group matches "bandwidth" range specified in this entry. Refer to Table 2 for more details. |
| Default_Metric | The default metric as per configuration. Default metric will be assigned to all interfaces under this group if total available bandwidth for the group Does not match the "Bandwidth" range specified in any "BBM_Metric_Cfg_Entry" for this group. Refer to Table 2 for more details. |
Once a interface has been assigned to a interface-group, and the corresponding BBM metric configurations has been provisioned, metric to be associated with the member interfaces can be derived as follows:
Set Intf.Metric = 0
For (all BBM_Metric_Cfg_Entry in igp.BBM_Metric_Cfg_List
in descending order)
- If (igp.BW_Curr ≥
igp.BBM_Metric_Cfg_List.BBM_Metric_Cfg_Entry.Bandwidth)
- Set Intf.Metric =
igp.BBM_Metric_Cfg_List.BBM_Metric_Cfg_Entry.Metric.
If (Intf.Metric == 0)
- Set Intf.Metric = igp.Default_Metric.
Considering the BBM metric configurations for interface-group IG1 in Figure 2, Table 2 below shows how metric for individual interfaces of IG1 SHALL be computed at any point of time.
| Active-Links | Total-Available-BW | BBM-Metric | Remarks |
|---|---|---|---|
| L1, L2, L3 | 300G | 10 | Total-Avaiable-BW ≥ 200 |
| L1, L2, L3(down) | 200G | 10 | Total-Avaiable-BW ≥ 200 |
| L1, L2(down), L3(down) | 100G | 50 | 200 > Total-Avaiable-BW ≥ 100 |
Once the metric of individual interfaces are derived from the corresponding interface-group BBM configuration, the same are used in the local IGP SPF computations. In addition to using the metrics in SPF computations, the same are also advertised as the corresponding link cost (instead of the original cost associated with the individual links) in the locally-generated IGP link-state advertisements. This is done to eliminate any looping possible otherwise.
Considering the topology in Figure 1, Table 3 below shows how traffic for destination D1 shall be re-routed based on a series of events and BBM metric configurations as shown in Figure 2.
| Event | Interface-Group | Active-Links/Total-Available-BW | BBM-Metric | Shortest Path |
|---|---|---|---|---|
| Initially | IG1 | {L1, L2, L3} / 300G | 10 + Dopt(R1,D1) | YES |
| IG2 | {L4, L5, L6} / 300G | 20 + Dopt(R1,D1) | NO | |
| L1 goes down | IG1 | {L2, L3} / 200G | 10 + Dopt(R1,D1) | YES |
| IG2 | {L4, L5, L6} / 300G | 20 + Dopt(R1,D1) | NO | |
| L2 goes down | IG1 | {L2, L3} / 200G | 100 + Dopt(R1,D1) | NO |
| IG2 | {L5, L6} / 200G | 20 + Dopt(R1,D1) | YES | |
| L4 goes down | IG1 | {L3} / 100G | 100 + Dopt(R1,D1) | NO |
| IG2 | {L5, L6} / 200G | 20 + Dopt(R1,D1) | YES | |
| L5 goes down | IG1 | {L3} / 100G | 100 + Dopt(R1,D1) | YES |
| IG2 | {L6} / 100G | 110 + Dopt(R1,D1) | NO |
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 gauranteed 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 desireable, 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 pathss. 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.
Following are some of the assumptions that the solution proposed in this document is based on.
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.
| Attribute | Value and Significance |
|---|---|
| Min_BW | This is the minimum bandwidth below which outgoing traffic MUST not be carried on this interface-group. It needs to load-balance across links of best/non-best interface-groups as well. |
| Restore_BW | This is the bandwidth above which the outgoing traffic MUST entirely be carried over the members of this interface-group not needing to load-balance across member links of other non-best interface-groups, provided it provides a path with shortest metric. |
In Figure 1, considering all the links L1 to L6 having bandwidth capacity of 100G each, and assigned into two interface-groups IG1 and IG2 (as shown in Section 2.1), following is an example of simple BBM FRR config for each of these interface-group.
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
Figure 3: Example BBM FRR Configuration
This document defines the following attributes to be associated with each interface-group for facilitating Bandwidth-based Fast Re-Route.
| Attribute | Value and Significance |
|---|---|
| BW_PostFail | Cumulative bandwidth through all the remaining primary next-hops considering the primary next-hop with highest bandwidth goes down. |
| Metric_PostFail | Bandwidth based metric after a link goes down. |
This solution proposed in this document also requires IGPs to define and associated the following attributes for each destination node in the IGP link-state database.
| Attribute | Value and Significance |
|---|---|
| Pri_Nh_Count | Number of primary next-hops found for the destination. |
| Pri_BW_Curr | Cumulative bandwidth across all the remaining primary next-hops. |
| Pri_BW_PostFail | Cumulative bandwidth through all the remaining primary nexthops considering the primary nexthop with highest bandwidth goes down. |
| Pri_BW_Restore | Cumulative restore-bandwidth for all the interface-groups considered for primary nexthops. |
| Pri_BW_Min | Cumulative minimum-bandwidth for all the interface-groups considered for primary nexthops. |
Additionally, the solution proposed in this document also mandates, that the forwarding plane SHOULD implement the following enhanced local-repair logic, to facilitate BBM based fast-re-route, on detecting a link-down event.
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.
Figure 4: Enhanced Local Repair in Forwarding Plane
Like mentioned in Section 4.2 the solution proposed in this document relies on the preference-based local-repair logic implemented in forwarding-plane to facilitate fast re-route. This solution requires IGPs to indirectly influence the local-repair action taken by the forwarding-plane by choosing an suitable alternate path with an appropriate preference-value pre-computed and installed in the forwarding-plane, well ahead of the actual link failure event.
Table 7 below, specifies a set of well-defined path-preference types that this document proposes IGP to define and use while downloading any path for a given destination in the forwarding table.
| Preference-Type | Significance | Suggested Value |
|---|---|---|
| Pri_Nh_Pref | Preference type for normal primary paths. | 0x1 |
| Bkup_Nh_Pref_High | Preference type for paths, which are preferred, more than normal backup paths but less compared to normal primary paths. | 0xF100 |
| Bkup_Nh_Pref_Normal | Preference type for normal backup paths. | 0xFF00 |
Based on the above assumptions, additional configuration parameters and attributes the document proposes IGPs to implement the following logic for computing primary and alternate paths for each destination, and determine their corresponding path-preference value as well
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_Curr, D.Pri_BW_PostFail, 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 > igp.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_Curr, D.Pri_BW_PostFail, 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 D.Pri_BW_PostFail < D.Pri_BW_Restore,
OR Pri_BW_PostFail ≤ 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.
The BBM method proposed in this document does NOT ensure end to end bandwidth requirement. It, only ensures that the metric is altered only on local interfaces, based on the BBM metric configurations and remaining available bandwidth.
The solution proposed in this documents attempts to provide protection for single link failures only. It always assumes that link with the highest individual bandwidth capacity shall fail next. In case if any other link with lesser individual bandwidth capacity fails instead, the local repair action taken by the forwarding plane may not be exactly as expected, even though the forwarding plane will still take care of protecting the traffic.
Changes suggested in the draft does not raise any security concerns.
This draft does not have any request from IANA.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [I-D.ietf-rtgwg-remote-lfa] | Bryant, S., Filsfils, C., Previdi, S., Shand, M. and N. So, "Remote Loop-Free Alternate (LFA) Fast Re-Route (FRR)", Internet-Draft draft-ietf-rtgwg-remote-lfa-11, January 2015. |
| [RFC5286] | Atlas, A. and A. Zinin, "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, September 2008. |