RFC 7812
An Architecture for IP/LDP Fast Reroute Using Maximally Redundant Trees (MRT-FRR)
Pages: 44
Proposed Standard
Proposed Standard
Part 2 of 2 – Pages 23 to 44
10. Inter-area Forwarding Behavior
An ABR/LBR has two forwarding roles. First, it forwards traffic within areas. Second, it forwards traffic from one area into another. These same two roles apply for MRT transit traffic. Traffic on MRT-Red or MRT-Blue destined inside the area needs to stay on MRT-Red or MRT-Blue in that area. However, it is desirable for traffic leaving the area to also exit MRT-Red or MRT-Blue and return to shortest path forwarding. For unicast MRT-FRR, the need to stay on an MRT forwarding topology terminates at the ABR/LBR whose best route is via a different area/ level. It is highly desirable to go back to the default forwarding topology when leaving an area/level. There are three basic reasons for this. First, the default topology uses shortest paths; the packet will thus take the shortest possible route to the destination. Second, this allows a single router failure that manifests itself in multiple areas (as would be the case with an ABR/LBR failure) to be separately identified and repaired around. Third, the packet can be fast-rerouted again, if necessary, due to a second distinct failure in a different area. In OSPF, an ABR that receives a packet on MRT-Red or MRT-Blue towards destination Z should continue to forward the packet along MRT-Red or MRT-Blue only if the best route to Z is in the same OSPF area as the interface that the packet was received on. Otherwise, the packet should be removed from MRT-Red or MRT-Blue and forwarded on the shortest-path default forwarding topology. The above description applies to OSPF. The same essential behavior also applies to IS-IS if one substitutes IS-IS level for OSPF area. However, the analogy with OSPF is not exact. An interface in OSPF can only be in one area, whereas an interface in IS-IS can be in both Level-1 and Level-2. Therefore, to avoid confusion and address this difference, we explicitly describe the behavior for IS-IS in Appendix A. In the following sections, only the OSPF terminology is used.10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A
For LDP forwarding where a single label specifies (MT-ID, FEC), the ABR is responsible for advertising the proper label to each neighbor. Assume that an ABR has allocated three labels for a particular destination: L_primary, L_blue, and L_red. To those routers in the same area as the best route to the destination, the ABR advertises the following FEC-label bindings: L_primary for the default topology, L_blue for the MRT-Blue MT-ID, and L_red for the MRT-Red MT-ID, as expected. However, to routers in other areas, the ABR advertises the
following FEC-label bindings: L_primary for the default topology and L_primary for the Rainbow MRT MT-ID. Associating L_primary with the Rainbow MRT MT-ID causes the receiving routers to use L_primary for the MRT-Blue MT-ID and for the MRT-Red MT-ID. The ABR installs all next hops for the best area: primary next hops for L_primary, MRT-Blue next hops for L_blue, and MRT-Red next hops for L_red. Because the ABR advertised (Rainbow MRT MT-ID, FEC) with L_primary to neighbors not in the best area, packets from those neighbors will arrive at the ABR with a label L_primary and will be forwarded into the best area along the default topology. By controlling what labels are advertised, the ABR can thus enforce that packets exiting the area do so on the shortest-path default topology.10.1.1. Motivation for Creating the Rainbow-FEC
The desired forwarding behavior could be achieved in the above example without using the Rainbow-FEC. This could be done by having the ABR advertise the following FEC-label bindings to neighbors not in the best area: L1_primary for the default topology, L1_primary for the MRT-Blue MT-ID, and L1_primary for the MRT-Red MT-ID. Doing this would require machinery to spoof the labels used in FEC-label binding advertisements on a per-neighbor basis. Such label-spoofing machinery does not currently exist in most LDP implementations and doesn't have other obvious uses. Many existing LDP implementations do however have the ability to filter FEC-label binding advertisements on a per-neighbor basis. The Rainbow-FEC allows us to reuse the existing per-neighbor FEC filtering machinery to achieve the desired result. By introducing the Rainbow FEC, we can use per-neighbor FEC-filtering machinery to advertise the FEC-label binding for the Rainbow-FEC (and filter those for MRT-Blue and MRT-Red) to non-best-area neighbors of the ABR. An ABR may choose to either distribute the Rainbow-FEC or distribute separate MRT-Blue and MRT-Red advertisements. This is a local choice. A router that supports the MRT LDP Label Option 1A forwarding mechanism MUST be able to receive and correctly interpret the Rainbow-FEC.10.2. ABR Forwarding Behavior with IP Tunneling (Option 2)
If IP tunneling is used, then the ABR behavior is dependent upon the outermost IP address. If the outermost IP address is an MRT loopback address of the ABR, then the packet is decapsulated and forwarded based upon the inner IP address, which should go on the default SPT topology. If the outermost IP address is not an MRT loopback address of the ABR, then the packet is simply forwarded along the associated
forwarding topology. A PLR sending traffic to a destination outside its local area/level will pick the MRT and use the associated MRT loopback address of the selected ABR advertising the lowest cost to the external destination. Thus, for these two MRT forwarding mechanisms (MRT LDP Label Option 1A and IP tunneling Option 2), there is no need for additional computation or per-area forwarding state.10.3. ABR Forwarding Behavior with MRT LDP Label Option 1B
The other MRT forwarding mechanism described in Section 6 uses two labels: a topology-id label and a FEC-label. This mechanism would require that any router whose MRT-Red or MRT-Blue next hop is an ABR would need to determine whether the ABR would forward the packet out of the area/level. If so, then that router should pop off the topology-id label before forwarding the packet to the ABR. For example, in Figure 3, if node H fails, node E has to put traffic towards prefix p onto MRT-Red. But since node D knows that ABR1 will use a best route from another area, it is safe for D to pop the topology-id label and just forward the packet to ABR1 along the MRT- Red next hop. ABR1 will use the shortest path in Area 10. In all cases for IS-IS and most cases for OSPF, the penultimate router can determine what decision the adjacent ABR will make. The one case where it can't be determined is when two ASBRs are in different non-backbone areas attached to the same ABR, then the ASBR's Area ID may be needed for tie-breaking (prefer the route with the largest OSPF area ID), and the Area ID isn't announced as part of the ASBR LSA. In this one case, suboptimal forwarding along the MRT in the other area would happen. If that becomes a realistic deployment scenario, protocol extensions could be developed to address this issue.
+----[C]---- --[D]--[E] --[D]--[E] | \ / \ / \ p--[A] Area 10 [ABR1] Area 0 [H]--p +-[ABR1] Area 0 [H]-+ | / \ / | \ / | +----[B]---- --[F]--[G] | --[F]--[G] | | | | other | +----------[p]-------+ area (a) Example topology (b) Proxy node view in Area 0 nodes +----[C]<--- [D]->[E] V \ \ +-[A] Area 10 [ABR1] Area 0 [H]-+ | ^ / / | | +----[B]<--- [F]->[G] V | | +------------->[p]<--------------+ (c) rSPT towards destination p ->[D]->[E] -<[D]<-[E] / \ / \ [ABR1] Area 0 [H]-+ +-[ABR1] [H] / | | \ [F]->[G] V V -<[F]<-[G] | | | | [p]<------+ +--------->[p] (d) MRT-Blue in Area 0 (e) MRT-Red in Area 0 Figure 3: ABR Forwarding Behavior and MRTs11. Prefixes Multiply Attached to the MRT Island
How a computing router S determines its local MRT Island for each supported MRT profile is already discussed in Section 7. There are two types of prefixes or FECs that may be multiply attached to an MRT Island. The first type are multihomed prefixes that usually connect at a domain or protocol boundary. The second type represent routers that do not support the profile for the MRT Island.
The key difference is whether the traffic, once out of the MRT Island, might re-enter the MRT Island if a loop-free exit point is not selected. FRR using LFA has the useful property that it is able to protect multihomed prefixes against ABR failure. For instance, if a prefix from the backbone is available via both ABR A and ABR B, if A fails, then the traffic should be redirected to B. This can be accomplished with MRT FRR as well. If ASBR protection is desired, this has additional complexities if the ASBRs are in different areas. Similarly, protecting labeled BGP traffic in the event of an ASBR failure has additional complexities due to the per-ASBR label spaces involved. As discussed in [RFC5286], a multihomed prefix could be: o An out-of-area prefix announced by more than one ABR, o An AS-External route announced by two or more ASBRs, o A prefix with iBGP multipath to different ASBRs, o etc. See Appendix B for a discussion of a general issue with multihomed prefixes connected in two different areas. There are also two different approaches to protection. The first is tunnel endpoint selection where the PLR picks a router to tunnel to where that router is loop-free with respect to the failure-point. Conceptually, the set of candidate routers to provide LFAs expands to all routers that can be reached via an MRT alternate, attached to the prefix. The second is to use a proxy-node, which can be named via MPLS label or IP address, and pick the appropriate label or IP address to reach it on either MRT-Blue or MRT-Red as appropriate to avoid the failure point. A proxy-node can represent a destination prefix that can be attached to the MRT Island via at least two routers. It is termed a named proxy-node if there is a way that traffic can be encapsulated to reach specifically that proxy-node; this could be because there is an LDP FEC for the associated prefix or because MRT-Red and MRT-Blue IP addresses are advertised (in an as-yet undefined fashion) for that proxy-node. Traffic to a named proxy-node may take a different path than traffic to the attaching router; traffic is also explicitly forwarded from the attaching router along a predetermined interface towards the relevant prefixes.
For IP traffic, multihomed prefixes can use tunnel endpoint selection. For IP traffic that is destined to a router outside the MRT Island, if that router is the egress for a FEC advertised into the MRT Island, then the named proxy-node approach can be used. For LDP traffic, there is always a FEC advertised into the MRT Island. The named proxy-node approach should be used, unless the computing router S knows the label for the FEC at the selected tunnel endpoint. If a FEC is advertised from outside the MRT Island into the MRT Island and the forwarding mechanism specified in the profile includes LDP Label Option 1A, then the routers learning that FEC MUST also advertise labels for (MRT-Red, FEC) and (MRT-Blue, FEC) to neighbors inside the MRT Island. Any router receiving a FEC corresponding to a router outside the MRT Island or to a multihomed prefix MUST compute and install the transit MRT-Blue and MRT-Red next hops for that FEC. The FEC-label bindings for the topology-scoped FECs ((MT-ID 0, FEC), (MRT-Red, FEC), and (MRT-Blue, FEC)) MUST also be provided via LDP to neighbors inside the MRT Island.11.1. Protecting Multihomed Prefixes Using Tunnel Endpoint Selection
Tunnel endpoint selection is a local matter for a router in the MRT Island since it pertains to selecting and using an alternate and does not affect the transit MRT-Red and MRT-Blue forwarding topologies. Let the computing router be S and the next hop F be the node whose failure is to be avoided. Let the destination be prefix p. Have A be the router to which the prefix p is attached for S's shortest path to p. The candidates for tunnel endpoint selection are those to which the destination prefix is attached in the area/level. For a particular candidate B, it is necessary to determine if B is loop-free to reach p with respect to S and F for node-protection or at least with respect to S and the link (S, F) for link-protection. If B will always prefer to send traffic to p via a different area/level, then this is definitional. Otherwise, distance-based computations are necessary and an SPF from B's perspective may be necessary. The following equations give the checks needed; the rationale is similar to that given in [RFC5286]. In the inequalities below, D_opt(X,Y) means the shortest distance from node X to node Y, and D_opt(X,p) means the shortest distance from node X to prefix p. Loop-Free for S: D_opt(B, p) < D_opt(B, S) + D_opt(S, p) Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(F, p)
The latter is equivalent to the following, which avoids the need to
compute the shortest path from F to p.
Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(S, p) - D_opt(S, F)
Finally, the rules for Endpoint selection are given below. The basic
idea is to repair to the prefix-advertising router selected for the
shortest-path and only to select and tunnel to a different endpoint
if necessary (e.g., A=F or F is a cut-vertex or the link (S,F) is a
cut-link).
1. Does S have a node-protecting alternate to A? If so, select
that. Tunnel the packet to A along that alternate. For example,
if LDP is the forwarding mechanism, then push the label (MRT-Red,
A) or (MRT-Blue, A) onto the packet.
2. If not, then is there a router B that is loop-free to reach p
while avoiding both F and S? If so, select B as the endpoint.
Determine the MRT alternate to reach B while avoiding F. Tunnel
the packet to B along that alternate. For example, with LDP,
push the label (MRT-Red, B) or (MRT-Blue, B) onto the packet.
3. If not, then does S have a link-protecting alternate to A? If
so, select that.
4. If not, then is there a router B that is loop-free to reach p
while avoiding S and the link from S to F? If so, select B as
the endpoint and the MRT alternate for reaching B from S that
avoid the link (S,F).
The tunnel endpoint selected will receive a packet destined to itself
and, being the egress, will pop that MPLS label (or have signaled
Implicit Null) and forward based on what is underneath. This
suffices for IP traffic since the tunnel endpoint can use the IP
header of the original packet to continue forwarding the packet.
However, tunneling of LDP traffic requires targeted LDP sessions for
learning the FEC-label binding at the tunnel endpoint.
11.2. Protecting Multihomed Prefixes Using Named Proxy-Nodes
Instead, the named proxy-node method works with LDP traffic without
the need for targeted LDP sessions. It also has a clear advantage
over tunnel endpoint selection, in that it is possible to explicitly
forward from the MRT Island along an interface to a loop-free island
neighbor when that interface may not be a primary next hop.
A named proxy-node represents one or more destinations and, for LDP forwarding, has a FEC associated with it that is signaled into the MRT Island. Therefore, it is possible to explicitly label packets to go to (MRT-Red, FEC) or (MRT-Blue, FEC); at the border of the MRT Island, the label will swap to meaning (MT-ID 0, FEC). It would be possible to have named proxy-nodes for IP forwarding, but this would require extensions to signal two IP addresses to be associated with MRT-Red and MRT-Blue for the proxy-node. A named proxy-node can be uniquely represented by the two routers in the MRT Island to which it is connected. The extensions to signal such IP addresses will be defined elsewhere. The details of what label-bindings must be originated will be described in another document. Computing the MRT next hops to a named proxy-node and the MRT alternate for the computing router S to avoid a particular failure node F is straightforward. The details of the simple constant-time functions, Select_Proxy_Node_NHs() and Select_Alternates_Proxy_Node(), are given in [RFC7811]. A key point is that computing these MRT next hops and alternates can be done as new named proxy-nodes are added or removed without requiring a new MRT computation or impacting other existing MRT paths. This maps very well to, for example, how OSPFv2 (see [RFC2328], Section 16.5) does incremental updates for new summary-LSAs. The remaining question is how to attach the named proxy-node to the MRT Island; all the routers in the MRT Island MUST do this consistently. No more than two routers in the MRT Island can be selected; one should only be selected if there are no others that meet the necessary criteria. The named proxy-node is logically part of the area/level. There are two sources for candidate routers in the MRT Island to connect to the named proxy-node. The first set is made up of those routers in the MRT Island that are advertising the prefix; the named- proxy-cost assigned to each prefix-advertising router is the announced cost to the prefix. The second set is made up of those routers in the MRT Island that are connected to routers not in the MRT Island but in the same area/level; such routers will be defined as Island Border Routers (IBRs). The routers connected to the IBRs that are not in the MRT Island and are in the same area/level as the MRT Island are Island Neighbors (INs). Since packets sent to the named proxy-node along MRT-Red or MRT-Blue may come from any router inside the MRT Island, it is necessary that whatever router to which an IBR forwards the packet be loop-free with respect to the whole MRT Island for the destination. Thus, an IBR is a candidate router only if it possesses at least one IN whose shortest path to the prefix does not enter the MRT Island. A method
for identifying Loop-Free Island Neighbors (LFINs) is given in [RFC7811]. The named-proxy-cost assigned to each (IBR, IN) pair is cost(IBR, IN) + D_opt(IN, prefix). From the set of prefix-advertising routers and the set of IBRs with at least one LFIN, the two routers with the lowest named-proxy-cost are selected. Ties are broken based upon the lowest Router ID. For ease of discussion, the two selected routers will be referred to as proxy-node attachment routers. A proxy-node attachment router has a special forwarding role. When a packet is received destined to (MRT-Red, prefix) or (MRT-Blue, prefix), if the proxy-node attachment router is an IBR, it MUST swap to the shortest path forwarding topology (e.g., swap to the label for (MT-ID 0, prefix) or remove the outer IP encapsulation) and forward the packet to the IN whose cost was used in the selection. If the proxy-node attachment router is not an IBR, then the packet MUST be removed from the MRT forwarding topology and sent along the interface(s) that caused the router to advertise the prefix; this interface might be out of the area/level/AS.11.3. MRT Alternates for Destinations outside the MRT Island
A natural concern with new functionality is how to have it be useful when it is not deployed across an entire IGP area. In the case of MRT FRR, where it provides alternates when appropriate LFAs aren't available, there are also deployment scenarios where it may make sense to only enable some routers in an area with MRT FRR. A simple example of such a scenario would be a ring of six or more routers that is connected via two routers to the rest of the area. Destinations inside the local island can obviously use MRT alternates. Destinations outside the local island can be treated like a multihomed prefix and either endpoint selection or Named Proxy-Nodes can be used. Named proxy-nodes MUST be supported when LDP forwarding is supported and a label-binding for the destination is sent to an IBR. Naturally, there are more-complicated options to improve coverage, such as connecting multiple MRT Islands across tunnels, but the need for the additional complexity has not been justified.
12. Network Convergence and Preparing for the Next Failure
After a failure, MRT detours ensure that packets reach their intended destination while the IGP has not reconverged onto the new topology. As link-state updates reach the routers, the IGP process calculates the new shortest paths. Two things need attention: micro-loop prevention and MRT recalculation.12.1. Micro-loop Prevention and MRTs
A micro-loop is a transient packet-forwarding loop among two or more routers that can occur during convergence of IGP forwarding state. [RFC5715] discusses several techniques for preventing micro-loops. This section discusses how MRT-FRR relates to two of the micro-loop prevention techniques discussed in [RFC5715]: Nearside and Farside Tunneling. In Nearside Tunneling, a router (PLR) adjacent to a failure performs local repair and informs remote routers of the failure. The remote routers initially tunnel affected traffic to the nearest PLR, using tunnels that are unaffected by the failure. Once the forwarding state for normal shortest path routing has converged, the remote routers return the traffic to shortest path forwarding. MRT-FRR is relevant for Nearside Tunneling for the following reason. The process of tunneling traffic to the PLRs and waiting a sufficient amount of time for IGP forwarding state convergence with Nearside Tunneling means that traffic will generally rely on the local repair at the PLR for longer than it would in the absence of Nearside Tunneling. Since MRT-FRR provides 100% coverage for single link and node failure, it may be an attractive option to provide the local repair paths when Nearside Tunneling is deployed. MRT-FRR is also relevant for the Farside Tunneling micro-loop prevention technique. In Farside Tunneling, remote routers tunnel traffic affected by a failure to a node downstream of the failure with respect to traffic destination. This node can be viewed as being on the farside of the failure with respect to the node initiating the tunnel. Note that the discussion of Farside Tunneling in [RFC5715] focuses on the case where the farside node is immediately adjacent to a failed link or node. However, the farside node may be any node downstream of the failure with respect to traffic destination, including the destination itself. The tunneling mechanism used to reach the farside node must be unaffected by the failure. The alternative forwarding paths created by MRT-FRR have the potential to be used to forward traffic from the remote routers upstream of the failure all the way to the destination. In the event of failure, either the MRT-Red or MRT-Blue path from the remote upstream router to the destination is guaranteed to avoid a link
failure or inferred node failure. The MRT forwarding paths are also guaranteed to not be subject to micro-loops because they are locked to the topology before the failure. We note that the computations in [RFC7811] address the case of a PLR adjacent to a failure determining which choice of MRT-Red or MRT-Blue will avoid a failed link or node. More computation may be required for an arbitrary remote upstream router to determine whether to choose MRT-Red or MRT-Blue for a given destination and failure.12.2. MRT Recalculation for the Default MRT Profile
This section describes how the MRT recalculation SHOULD be performed for the Default MRT Profile. This is intended to support FRR applications. Other approaches are possible, but they are not specified in this document. When a failure event happens, traffic is put by the PLRs onto the MRT topologies. After that, each router recomputes its SPT and moves traffic over to that. Only after all the PLRs have switched to using their SPTs and traffic has drained from the MRT topologies should each router install the recomputed MRTs into the FIBs. At each router, therefore, the sequence is as follows: 1. Receive failure notification 2. Recompute SPT. 3. Install the new SPT in the FIB. 4. If the network was stable before the failure occurred, wait a configured (or advertised) period for all routers to be using their SPTs and traffic to drain from the MRTs. 5. Recompute MRTs. 6. Install new MRTs in the FIB. While the recomputed MRTs are not installed in the FIB, protection coverage is lowered. Therefore, it is important to recalculate the MRTs and install them quickly. New protocol extensions for advertising the time needed to recompute shortest path routes and install them in the FIB will be defined elsewhere.
13. Operational Considerations
The following aspects of MRT-FRR are useful to consider when deploying the technology in different operational environments and network topologies.13.1. Verifying Forwarding on MRT Paths
The forwarding paths created by MRT-FRR are not used by normal (non- FRR) traffic. They are only used to carry FRR traffic for a short period of time after a failure has been detected. It is RECOMMENDED that an operator proactively monitor the MRT forwarding paths in order to be certain that the paths will be able to carry FRR traffic when needed. Therefore, an implementation SHOULD provide an operator with the ability to test MRT paths with Operations, Administration, and Maintenance (OAM) traffic. For example, when MRT paths are realized using LDP labels distributed for topology-scoped FECs, an implementation can use the MPLS ping and traceroute as defined in [RFC4379] and extended in [RFC7307] for topology-scoped FECs.13.2. Traffic Capacity on Backup Paths
During a fast-reroute event initiated by a PLR in response to a network failure, the flow of traffic in the network will generally not be identical to the flow of traffic after the IGP forwarding state has converged, taking the failure into account. Therefore, even if a network has been engineered to have enough capacity on the appropriate links to carry all traffic after the IGP has converged after the failure, the network may still not have enough capacity on the appropriate links to carry the flow of traffic during a fast- reroute event. This can result in more traffic loss during the fast- reroute event than might otherwise be expected. Note that there are two somewhat distinct aspects to this phenomenon. The first is that the path from the PLR to the destination during the fast-reroute event may be different from the path after the IGP converges. In this case, any traffic for the destination that reaches the PLR during the fast-reroute event will follow a different path from the PLR to the destination than will be followed after IGP convergence. The second aspect is that the amount of traffic arriving at the PLR for affected destinations during the fast-reroute event may be larger than the amount of traffic arriving at the PLR for affected destinations after IGP convergence. Immediately after a failure, any non-PLR routers that were sending traffic to the PLR before the failure will continue sending traffic to the PLR, and that traffic will be carried over backup paths from the PLR to the destinations.
After IGP convergence, upstream non-PLR routers may direct some traffic away from the PLR. In order to reduce or eliminate the potential for transient traffic loss due to inadequate capacity during fast-reroute events, an operator can model the amount of traffic taking different paths during a fast-reroute event. If it is determined that there is not enough capacity to support a given fast-reroute event, the operator can address the issue either by augmenting capacity on certain links or modifying the backup paths themselves. The MRT Lowpoint algorithm produces a pair of diverse paths to each destination. These paths are generated by following the directed links on a common GADAG. The decision process for constructing the GADAG in the MRT Lowpoint algorithm takes into account individual IGP link metrics. At any given node, links are explored in order from lowest IGP metric to highest IGP metric. Additionally, the process for constructing the MRT-Red and Blue trees uses SPF traversals of the GADAG. Therefore, the IGP link metric values affect the computed backup paths. However, adjusting the IGP link metrics is not a generally applicable tool for modifying the MRT backup paths. Achieving a desired set of MRT backup paths by adjusting IGP metrics while at the same time maintaining the desired flow of traffic along the shortest paths is not possible in general. MRT-FRR allows an operator to exclude a link from the MRT Island, and thus the GADAG, by advertising it as MRT-Ineligible. Such a link will not be used on the MRT forwarding path for any destination. Advertising links as MRT-Ineligible is the main tool provided by MRT- FRR for keeping backup traffic off of lower bandwidth links during fast-reroute events. Note that all of the backup paths produced by the MRT Lowpoint algorithm are closely tied to the common GADAG computed as part of that algorithm. Therefore, it is generally not possible to modify a subset of paths without affecting other paths. This precludes more fine-grained modification of individual backup paths when using only paths computed by the MRT Lowpoint algorithm. However, it may be desirable to allow an operator to use MRT-FRR alternates together with alternates provided by other FRR technologies. A policy-based alternate selection process can allow an operator to select the best alternate from those provided by MRT and other FRR technologies. As a concrete example, it may be desirable to implement a policy where a downstream LFA (if it exists for a given failure mode and destination) is preferred over a given MRT alternate. This combination gives the operator the ability to affect where traffic flows during a fast-reroute event, while still
producing backup paths that use no additional labels for LDP traffic and will not loop under multiple failures. This and other choices of alternate selection policy can be evaluated in the context of their effect on fast-reroute traffic flow and available capacity, as well as other deployment considerations. Note that future documents may define MRT profiles in addition to the default profile defined here. Different MRT profiles will generally produce alternate paths with different properties. An implementation may allow an operator to use different MRT profiles instead of or in addition to the default profile.13.3. MRT IP Tunnel Loopback Address Management
As described in Section 6.1.2, if an implementation uses IP tunneling as the mechanism to realize MRT forwarding paths, each node must advertise an MRT-Red and an MRT-Blue loopback address. These IP addresses must be unique within the routing domain to the extent that they do not overlap with each other or with any other routing table entries. It is expected that operators will use existing tools and processes for managing infrastructure IP addresses to manage these additional MRT-related loopback addresses.13.4. MRT-FRR in a Network with Degraded Connectivity
Ideally, routers in a service provider network using MRT-FRR will be initially deployed in a 2-connected topology, allowing MRT-FRR to find completely diverse paths to all destinations. However, a network can differ from an ideal 2-connected topology for many possible reasons, including network failures and planned maintenance events. MRT-FRR is designed to continue to function properly when network connectivity is degraded. When a network contains cut-vertices or cut-links dividing the network into different 2-connected blocks, MRT-FRR will continue to provide completely diverse paths for destinations within the same block as the PLR. For a destination in a different block from the PLR, the redundant paths created by MRT- FRR will be link and node diverse within each block, and the paths will only share links and nodes that are cut-links or cut-vertices in the topology. If a network becomes partitioned with one set of routers having no connectivity to another set of routers, MRT-FRR will function independently in each set of connected routers, providing redundant paths to destinations in same set of connected routers as a given PLR.
13.5. Partial Deployment of MRT-FRR in a Network
A network operator may choose to deploy MRT-FRR only on a subset of routers in an IGP area. MRT-FRR is designed to accommodate this partial deployment scenario. Only routers that advertise support for a given MRT profile will be included in a given MRT Island. For a PLR within the MRT Island, MRT-FRR will create redundant forwarding paths to all destinations with the MRT Island using maximally redundant trees all the way to those destinations. For destinations outside of the MRT Island, MRT-FRR creates paths to the destination that use forwarding state created by MRT-FRR within the MRT Island and shortest path forwarding state outside of the MRT Island. The paths created by MRT-FRR to non-Island destinations are guaranteed to be diverse within the MRT Island (if topologically possible). However, the part of the paths outside of the MRT Island may not be diverse.14. IANA Considerations
IANA has created the "MRT Profile Identifier Registry". The range is 0 to 255. The Default MRT Profile defined in this document has value 0. Values 1-200 are allocated by Standards Action. Values 201-220 are for Experimental Use. Values 221-254 are for Private Use. Value 255 is reserved for future registry extension. (The allocation and use policies are described in [RFC5226].) The initial registry is shown below. Value Description Reference ------- ---------------------------------------- ------------ 0 Default MRT Profile RFC 7812 1-200 Unassigned 201-220 Experimental Use 221-254 Private Use 255 Reserved (for future registry extension) The "MRT Profile Identifier Registry" is a new registry in the IANA Matrix. Following existing conventions, http://www.iana.org/ protocols displays a new header: "Maximally Redundant Tree (MRT) Parameters". Under that header, there is an entry for "MRT Profile Identifier Registry", which links to the registry itself at http://www.iana.org/assignments/mrt-parameters.
15. Security Considerations
In general, MRT forwarding paths do not follow shortest paths. The transit forwarding state corresponding to the MRT paths is created during normal operations (before a failure occurs). Therefore, a malicious packet with an appropriate header injected into the network from a compromised location would be forwarded to a destination along a non-shortest path. When this technology is deployed, a network security design should not rely on assumptions about potentially malicious traffic only following shortest paths. It should be noted that the creation of non-shortest forwarding paths is not unique to MRT. MRT-FRR requires that routers advertise information used in the formation of MRT backup paths. While this document does not specify the protocol extensions used to advertise this information, we discuss security considerations related to the information itself. Injecting false MRT-related information could be used to direct some MRT backup paths over compromised transmission links. Combined with the ability to generate network failures, this could be used to send traffic over compromised transmission links during a fast-reroute event. In order to prevent this potential exploit, a receiving router needs to be able to authenticate MRT-related information that claims to have been advertised by another router.16. References
16.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>. [RFC7307] Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D. King, "LDP Extensions for Multi-Topology", RFC 7307, DOI 10.17487/RFC7307, July 2014, <http://www.rfc-editor.org/info/rfc7307>.
[RFC7811] Enyedi, G., Ed., Csaszar, A., Atlas, A., Ed., Bowers, C., and A. Gopalan, "An Algorithm for Computing IP/LDP Fast Reroute Using Maximally Redundant Trees (MRT-FRR)", RFC 7811, DOI 10.17487/RFC7811, June 2016, <http://www.rfc-editor.org/info/rfc7811>.16.2. Informative References
[EnyediThesis] Enyedi, G., "Novel Algorithms for IP Fast Reroute", Department of Telecommunications and Media Informatics, Budapest University of Technology and Economics Ph.D. Thesis, February 2011, <https://repozitorium.omikk.bme.hu/bitstream/ handle/10890/1040/ertekezes.pdf>. [LDP-MRT] Atlas, A., Tiruveedhula, K., Bowers, C., Tantsura, J., and IJ. Wijnands, "LDP Extensions to Support Maximally Redundant Trees", Work in Progress, draft-ietf-mpls-ldp- mrt-03, May 2016. [MRT-ARCH] Atlas, A., Kebler, R., Wijnands, IJ., Csaszar, A., and G. Enyedi, "An Architecture for Multicast Protection Using Maximally Redundant Trees", Work in Progress, draft-atlas- rtgwg-mrt-mc-arch-02, July 2013. [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/RFC2328, April 1998, <http://www.rfc-editor.org/info/rfc2328>. [RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures", RFC 4379, DOI 10.17487/RFC4379, February 2006, <http://www.rfc-editor.org/info/rfc4379>. [RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, DOI 10.17487/RFC5286, September 2008, <http://www.rfc-editor.org/info/rfc5286>. [RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream Label Assignment and Context-Specific Label Space", RFC 5331, DOI 10.17487/RFC5331, August 2008, <http://www.rfc-editor.org/info/rfc5331>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, <http://www.rfc-editor.org/info/rfc5340>. [RFC5443] Jork, M., Atlas, A., and L. Fang, "LDP IGP Synchronization", RFC 5443, DOI 10.17487/RFC5443, March 2009, <http://www.rfc-editor.org/info/rfc5443>. [RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC 5714, DOI 10.17487/RFC5714, January 2010, <http://www.rfc-editor.org/info/rfc5714>. [RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free Convergence", RFC 5715, DOI 10.17487/RFC5715, January 2010, <http://www.rfc-editor.org/info/rfc5715>. [RFC6976] Shand, M., Bryant, S., Previdi, S., Filsfils, C., Francois, P., and O. Bonaventure, "Framework for Loop-Free Convergence Using the Ordered Forwarding Information Base (oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July 2013, <http://www.rfc-editor.org/info/rfc6976>. [RFC6981] Bryant, S., Previdi, S., and M. Shand, "A Framework for IP and MPLS Fast Reroute Using Not-Via Addresses", RFC 6981, DOI 10.17487/RFC6981, August 2013, <http://www.rfc-editor.org/info/rfc6981>. [RFC6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D. McPherson, "OSPF Stub Router Advertisement", RFC 6987, DOI 10.17487/RFC6987, September 2013, <http://www.rfc-editor.org/info/rfc6987>. [RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N. So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)", RFC 7490, DOI 10.17487/RFC7490, April 2015, <http://www.rfc-editor.org/info/rfc7490>.
Appendix A. Inter-level Forwarding Behavior for IS-IS
In the description below, we use the terms "Level-1-only interface", "Level-2-only interface", and "Level-1-and-Level-2 interface" to mean an interface that has formed only a Level-1 adjacency, only a Level-2 adjacency, or both Level-1 and Level-2 adjacencies. Note that IS-IS also defines the concept of areas. A router is configured with an IS-IS area identifier, and a given router may be configured with multiple IS-IS area identifiers. For an IS-IS Level-1 adjacency to form between two routers, at least one IS-IS area identifier must match. IS-IS Level-2 adjacencies do not require any area identifiers to match. The behavior described below does not explicitly refer to IS-IS area identifiers. However, IS-IS area identifiers will indirectly affect the behavior by affecting the formation of Level-1 adjacencies. First, consider a packet destined to Z on MRT-Red or MRT-Blue received on a Level-1-only interface. If the best shortest path route to Z was learned from a Level-1 advertisement, then the packet should continue to be forwarded along MRT-Red or MRT-Blue. If, instead, the best route was learned from a Level-2 advertisement, then the packet should be removed from MRT-Red or MRT-Blue and forwarded on the shortest-path default forwarding topology. Now consider a packet destined to Z on MRT-Red or MRT-Blue received on a Level-2-only interface. If the best route to Z was learned from a Level-2 advertisement, then the packet should continue to be forwarded along MRT-Red or MRT-Blue. If, instead, the best route was learned from a Level-1 advertisement, then the packet should be removed from MRT-Red or MRT-Blue and forwarded on the shortest-path default forwarding topology. Finally, consider a packet destined to Z on MRT-Red or MRT-Blue received on a Level-1-and-Level-2 interface. This packet should continue to be forwarded along MRT-Red or MRT-Blue, regardless of which level the route was learned from. An implementation may simplify the decision-making process above by using the interface of the next hop for the route to Z to determine the level from which the best route to Z was learned. If the next hop points out a Level-1-only interface, then the route was learned from a Level-1 advertisement. If the next hop points out a Level- 2-only interface, then the route was learned from a Level-2 advertisement. A next hop that points out a Level-1-and-Level-2 interface does not provide enough information to determine the source of the best route. With this simplification, an implementation would need to continue forwarding along MRT-Red or MRT-Blue when the next- hop points out a Level-1-and-Level-2 interface. Therefore, a packet
on MRT-Red or MRT-Blue going from Level-1 to Level-2 (or vice versa) that traverses a Level-1-and-Level-2 interface in the process will remain on MRT-Red or MRT-Blue. This simplification may not always produce the optimal forwarding behavior, but it does not introduce interoperability problems. The packet will stay on an MRT backup path longer than necessary, but it will still reach its destination.Appendix B. General Issues with Area Abstraction
When a multihomed prefix is connected in two different areas, it may be impractical to protect them without adding the complexity of explicit tunneling. This is also a problem for LFA and Remote-LFA. 50 |----[ASBR Y]---[B]---[ABR 2]---[C] Backbone Area 0: | | ABR 1, ABR 2, C, D | | | | Area 20: A, ASBR X | | p ---[ASBR X]---[A]---[ABR 1]---[D] Area 10: B, ASBR Y 5 p is a Type 1 AS-external Figure 4: AS External Prefixes in Different Areas Consider the network in Figure 4 and assume there is a richer connective topology that isn't shown, where the same prefix is announced by ASBR X and ASBR Y, which are in different non-backbone areas. If the link from A to ASBR X fails, then an MRT alternate could forward the packet to ABR 1 and ABR 1 could forward it to D, but then D would find the shortest route is back via ABR 1 to Area 20. This problem occurs because the routers, including the ABR, in one area are not yet aware of the failure in a different area. The only way to get it from A to ASBR Y is to explicitly tunnel it to ASBR Y. If the traffic is unlabeled or the appropriate MPLS labels are known, then explicit tunneling MAY be used as long as the shortest path of the tunnel avoids the failure point. In that case, A must determine that it should use an explicit tunnel instead of an MRT alternate.
Acknowledgements
The authors would like to thank Mike Shand for his valuable review and contributions. The authors would like to thank Joel Halpern, Hannes Gredler, Ted Qian, Kishore Tiruveedhula, Shraddha Hegde, Santosh Esale, Nitin Bahadur, Harish Sitaraman, Raveendra Torvi, Anil Kumar SN, Bruno Decraene, Eric Wu, Janos Farkas, Rob Shakir, Stewart Bryant, and Alvaro Retana for their suggestions and review.Contributors
Robert Kebler Juniper Networks 10 Technology Park Drive Westford, MA 01886 United States Email: rkebler@juniper.net Andras Csaszar Ericsson Konyves Kalman krt 11 Budapest 1097 Hungary Email: Andras.Csaszar@ericsson.com Jeff Tantsura Ericsson 300 Holger Way San Jose, CA 95134 United States Email: jeff.tantsura@ericsson.com Russ White VCE Email: russw@riw.us
Authors' Addresses
Alia Atlas Juniper Networks 10 Technology Park Drive Westford, MA 01886 United States Email: akatlas@juniper.net Chris Bowers Juniper Networks 1194 N. Mathilda Ave. Sunnyvale, CA 94089 United States Email: cbowers@juniper.net Gabor Sandor Enyedi Ericsson Konyves Kalman krt 11. Budapest 1097 Hungary Email: Gabor.Sandor.Enyedi@ericsson.com