| Routing Area Working Group | A.K.A. Atlas, Ed. |
| Internet-Draft | R.K. Kebler |
| Intended status: Standards Track | Juniper Networks |
| Expires: January 13, 2014 | G.S.E. Enyedi |
| A.C. Császár | |
| J.T. Tantsura | |
| Ericsson | |
| M.K. Konstantynowicz | |
| Cisco Systems | |
| R.W. White | |
| VCE | |
| July 12, 2013 |
An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees
draft-ietf-rtgwg-mrt-frr-architecture-03
With increasing deployment of Loop-Free Alternates (LFA) [RFC5286], it is clear that a complete solution for IP and LDP Fast-Reroute is required. This specification provides that solution. IP/LDP Fast-Reroute with Maximally Redundant Trees (MRT-FRR) is a technology that gives link-protection and node-protection with 100% coverage in any network topology that is still connected after the failure.
MRT removes all need to engineer for coverage. MRT is also extremely computationally efficient. For any router in the network, the MRT computation is less than the LFA computation for a node with three or more neighbors.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 13, 2014.
Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
This document gives a complete solution for IP/LDP fast-reroute [RFC5714]. MRT-FRR creates two alternate trees separate from the primary next-hop forwarding used during stable operation. These two trees are maximally diverse from each other, providing link and node protection for 100% of paths and failures as long as the failure does not cut the network into multiple pieces. This document defines the architecture for IP/LDP fast-reroute with MRT. The associated protocol extensions are defined in [I-D.atlas-ospf-mrt] and [I-D.atlas-mpls-ldp-mrt]. The exact MRT algorithm is defined in [I-D.enyedi-rtgwg-mrt-frr-algorithm].
IP/LDP Fast-Reroute with MRT (MRT-FRR) uses two maximally diverse forwarding topologies to provide alternates. A primary next-hop should be on only one of the diverse forwarding topologies; thus, the other can be used to provide an alternate. Once traffic has been moved to one of MRTs, it is not subject to further repair actions. Thus, the traffic will not loop even if a worse failure (e.g. node) occurs when protection was only available for a simpler failure (e.g. link).
In addition to supporting IP and LDP unicast fast-reroute, the diverse forwarding topologies and guarantee of 100% coverage permit fast-reroute technology to be applied to multicast traffic as described in [I-D.atlas-rtgwg-mrt-mc-arch].
Other existing or proposed solutions are partial solutions or have significant issues, as described below.
Summary Comparison of IP/LDP FRR Methods
| Method | Coverage | Alternate Looping? | Computation (in SPFs) |
|---|---|---|---|
| MRT-FRR | 100% Link/Node | None | less than 3 |
| LFA | Partial Link/Node | Possible | per neighbor |
| Remote LFA | Partial Link/Node | Possible | per neighbor (link) or neighbor's neighbor (node) |
| Not-Via | 100% Link/Node | None | per link and node |
Fast-reroute is based upon the single failure assumption - that the time between single failures is long enough for a network to reconverge and start forwarding on the new shortest paths. That does not imply that the network will only experience one failure or change.
It is straightforward to analyze a particular network topology for coverage. However, a real network does not always have the same topology. For instance, maintenance events will take links or nodes out of use. Simply costing out a link can have a significant effect on what LFAs are available. Similarly, after a single failure has happened, the topology is changed and its associated coverage. Finally, many networks have new routers or links added and removed; each of those changes can have an effect on the coverage for topology-sensitive methods such as LFA and Remote LFA. If fast-reroute is important for the network services provided, then a method that guarantees 100% coverage is important to accomodate natural network topology changes.
Asymmetric link costs are also a common aspect of networks. There are at least three common causes for them. First, any broadcast interface is represented by a pseudo-node and has asymmetric link costs to and from that pseudo-node. Second, when routers come up or a link with LDP comes up, it is recommended in [RFC5443] and [RFC3137] that the link metric be raised to the maximum cost; this may not be symmetric and for [RFC3137] is not expected to be. Third, techniques such as IGP metric tuning for traffic-engineering can result in asymmetric link costs. A fast-reroute solution needs to handle network topologies with asymmetric link costs.
When a network needs to use a micro-loop prevention mechanism [RFC5715] such as Ordered FIB[I-D.ietf-rtgwg-ordered-fib] or Farside Tunneling[RFC5715], then the whole IGP area needs to have alternates available so that the micro-loop prevention mechanism, which requires slower network convergence, can take the necessary time without impacting traffic badly. Without complete coverage, traffic to the unprotected destinations will be dropped for significantly longer than with current convergence - where routers individually converge as fast as possible.
MRT-FRR supports partial deployment. As with many new features, the protocols (OSPF, LDP, ISIS) indicate their capability to support MRT. Inside the MRT-capable connected group of routers (referred to as an MRT Island), the MRTs are computed. Alternates to destinations outside the MRT Island are computed and depend upon the existence of a loop-free neighbor of the MRT Island for that destination.
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]
A pair of Maximally Redundant Trees are directed spanning trees that provide maximally disjoint paths towards their common root. Only links or nodes whose failure would partition the network (i.e. cut-links and cut-vertices) are shared between the trees. The algorithm to compute MRTs is given in [I-D.enyedi-rtgwg-mrt-frr-algorithm]. This algorithm can be computed in O(e + n log n); it is less than three SPFs. Modeling results comparing MRT alternates to the optimal are described in [I-D.enyedi-rtgwg-mrt-frr-algorithm]. This document describes how the MRTs can be used and not how to compute them.
MRT provides destination-based trees for each destination. Each router stores its normal primary next-hop(s) as well as MRT-Blue next-hop(s) and MRT-Red next-hop(s) toward each destination. The alternate will be selected between the MRT-Blue and MRT-Red.
The most important thing to understand about MRTs is that for each pair of destination-routed MRTs, there is a path from every node X to the destination D on the Blue MRT that is as disjoint as possible from the path on the Red MRT.
For example, in Figure 1, there is a network graph that is 2-connected in (a) and associated MRTs in (b) and (c). One can consider the paths from B to R; on the Blue MRT, the paths are B->F->D->E->R or B->C->D->E->R. On the Red MRT, the path is B->A->R. These are clearly link and node-disjoint. These MRTs are redundant trees because the paths are disjoint.
[E]---[D]---| [E]<--[D]<--| [E]-->[D]---|
| | | | ^ | | |
| | | V | | V V
[R] [F] [C] [R] [F] [C] [R] [F] [C]
| | | ^ ^ ^ | |
| | | | | | V |
[A]---[B]---| [A]-->[B]---| [A]<--[B]<--|
(a) (b) (c)
a 2-connected graph Blue MRT towards R Red MRT towards R
Figure 1: A 2-connected Network
By contrast, in Figure 2, the network in (a) is not 2-connected. If F, G or the link F<->G failed, then the network would be partitioned. It is clearly impossible to have two link-disjoint or node-disjoint paths from G, I or J to R. The MRTs given in (b) and (c) offer paths that are as disjoint as possible. For instance, the paths from B to R are the same as in Figure 1 and the path from G to R on the Blue MRT is G->F->D->E->R and on the Red MRT is G->F->B->A->R.
[E]---[D]---|
| | | |----[I]
| | | | |
[R]---[C] [F]---[G] |
| | | | |
| | | |----[J]
[A]---[B]---|
(a)
a non-2-connected graph
[E]<--[D]<--| [E]-->[D]
| ^ | [I] | |----[I]
V | | | V V ^
[R] [C] [F]<--[G] | [R]<--[C] [F]<--[G] |
^ ^ ^ V ^ | |
| | |----[J] | | [J]
[A]-->[B]---| [A]<--[B]<--|
(b) (c)
Blue MRT towards R Red MRT towards R
Figure 2: A non-2-connected network
In normal IGP routing, each router has its shortest-path-tree to all destinations. From the perspective of a particular destination, D, this looks like a reverse SPT (rSPT). To use maximally redundant trees, in addition, each destination D has two MRTs associated with it; by convention these will be called the MRT-Blue and MRT-Red. MRT-FRR is realized by using multi-topology forwarding. There is a MRT-Blue forwarding topology and a MRT-Red forwarding topology.
Any IP/LDP fast-reroute technique beyond LFA requires an additional dataplane procedure, such as an additional forwarding mechanism. The well-known options are multi-topology forwarding (used by MRT-FRR), tunneling (e.g. [I-D.ietf-rtgwg-ipfrr-notvia-addresses] or [I-D.ietf-rtgwg-remote-lfa]), and per-interface forwarding (e.g. Loop-Free Failure Insensitive Routing in [EnyediThesis]).
When there is a link or node failure affecting, but not partitioning, the network, each node will still have at least one path via one of the MRTs to reach the destination D. For example, in Figure 2, C would normally forward traffic to R across the C<->R link. If that C<->R link fails, then C could use the Blue MRT path C->D->E->R.
As is always the case with fast-reroute technologies, forwarding does not change until a local failure is detected. Packets are forwarded along the shortest path. The appropriate alternate to use is pre-computed. [I-D.enyedi-rtgwg-mrt-frr-algorithm] describes exactly how to determine whether the MRT-Blue next-hops or the MRT-Red next-hops should be the MRT alternate next-hops for a particular primary next-hop N to a particular destination D.
MRT alternates are always available to use. It is a local decision whether to use an MRT alternate, a Loop-Free Alternate or some other type of alternate.
As described in [RFC5286], when a worse failure than is anticipated happens, using LFAs that are not downstream neighbors can cause micro-looping. Section 1.1 of [RFC5286] gives an example of link-protecting alternates causing a loop on node failure. Even if a worse failure than anticipated happens, the use of MRT alternates will not cause looping. Therefore, while node-protecting LFAs may be preferred, the certainty that no alternate-induced looping will occur is an advantage of using MRT alternates when the available node-protecting LFA is not a downstream path.
With LFA, there is no need to tunnel unicast traffic, whether IP or LDP. The traffic is simply sent to an alternate. As mentioned earlier in Section 5, MRT needs multi-topology forwarding. Unfortunately, neither IP nor LDP provides extra bits for a packet to indicate its topology.
Once the MRTs are computed, the two sets of MRTs are seen by the forwarding plane as essentially two additional topologies. The same considerations apply for forwarding along the MRTs as for handling multiple topologies.
For LDP, it is very desirable to avoid tunneling because, for at least node protection, tunneling requires knowledge of remote LDP label mappings and thus requires targeted LDP sessions and the associated management complexity. There are two different mechanisms that can be used; Option A MUST be supported.
Note that with LDP unicast forwarding, regardless of whether topology-identification label or encoding topology in label is used, no additional loopbacks per router are required. This is because LDP labels are used on a hop-by-hop basis to identify MRT-blue and MRT-red forwading topologies.
For greatest hardware compatibility, routers implementing MRT LDP fast-reroute MUST support Option A of encoding the MT-ID in the labels. The extensions to indicate an MT-ID for a FEC are described in Section 3.2.1 of [I-D.ietf-mpls-ldp-multi-topology].
For IP, there is no currently practical alternative except tunneling to gain the bits needed to indicate the MRT-Blue or MRT-Red forwarding topology. The choice of tunnel egress MAY be flexible since any router closer to the destination than the next-hop can work. This architecture assumes that the original destination in the area is selected (see Section 10 for handling of multi-homed prefixes); another possible choice is the next-next-hop towards the destination. For LDP traffic, using the original destination simplifies MRT-FRR by avoiding the need for targeted LDP sessions to the next-next-hop. For IP, that consideration doesn't apply but consistency with LDP is RECOMMENDED. If the tunnel egress is the original destination router, then the traffic remains on the redundant tree with sub-optimal routing. Selection of the tunnel egress is a router-local decision.
There are three options available for marking IP packets with which MRT it should be forwarded in. For greatest hardware compatibility and ease in removing the MRT-topology marking at area/level boundaries, routers that support MPLS and implement IP MRT fast-reroute MUST support Option A - using an LDP label that indicates the destination and MT-ID.
For simplicity, the approach of defining a well-known profile is taken in [I-D.atlas-ospf-mrt]. The purpose of communicating support for MRT in the IGP is to indicate thatqq the MRT-Blue and MRT-Red forwarding topologies are created for transit traffic. This section describes the various options to be selected. The default MRT profile is described here and the signaling extensions for OSPF are given in [I-D.atlas-ospf-mrt].
For any MRT profile, the MRT Island is created by starting from the computing router. If the computing router supports the default MRT profile, add it to the MRT Island. Add a router to the MRT Island if the router supports the default MRT profile and is connected to the MRT Island via bidirectional links eligible for MRT.
If a router advertises support for multiple MRT profiles, then it MUST create the transit forwarding topologies for each of those, unless the profile specifies No Forwarding Mechanism (e.g. as might be done for a profile used only for multicast global protection). A router MUST NOT advertise multiple MRT profiles that overlap in their MRT-Red MT-ID or MRT-Blue MT-ID.
The MRT Profile also defines different behaviors such as how MRT recomputation is handled and how area/level boundaries are dealt with.
The following describes the aspects to be considered to define a profile to advertise. For some profiles, associated information may need to be distributed, such as GADAG Root Selection Priority, Red MRT Loopback Address, Blue MRT Loopback Address.
As with LFA, it is expected that OSPF Virtual Links will not be supported.
The protocol extensions for LDP are defined in [I-D.atlas-mpls-ldp-mrt]. A router must indicate that it has the ability to support MRT; having this explicit allows the use of MRT-specific processing, such as special handling of FECs sent with the Rainbow MRT MT-ID.
A FEC sent with the Rainbow MRT MT-ID indicates that the FEC applies to all the MRT-Blue and MRT-Red MT-IDs in supported MRT profiles as well as to the default shortest-path based MT-ID 0. The Rainbow MRT MT-ID is defined to provide an easy way to handle the special signaling that is needed at ABRs or LBRs. It avoids the problem of needing to signal different MPLS labels for the same FEC. Because the Rainbow MRT MT-ID is used only by ABRs/LBRs or the LDP egress, it is not MRT profile specific. The proposed experimental value is 3999 and the final value will be assigned by IANA and allocated from the LDP MT-ID space. The authoritative values are given in [I-D.atlas-mpls-ldp-mrt].
An ABR/LBR has two forwarding roles. First, it forwards traffic inside its area. 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 back to the 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 failures that might appear in multiple areas (e.g. ABR/LBR failures) to be separately identified and repaired around. Third, the packet can be fast-rerouted again, if necessary, due to a failure in a different area.
An ABR/LBR that receives a packet on MRT-Red or MRT-Blue towards a destination in another area/level should forward the packet in the area/level with the best route along MRT-Red or MRT-Blue. If the packet came from that area/level, this correctly avoids the failure. However, if the traffic came from a different area/level, the packet should be removed from MRT-Red or MRT-Blue and forwarded on the shortest-path default forwarding topology.
To avoid per-interface forwarding state for MRT-Red and MRT-Blue, the ABR/LBR needs to arrange that packets destined to a different area arrive at the ABR/LBR already not marked as MRT-Red or MRT-Blue.
For LDP forwarding where the MPLS label specifies (MT-ID, FEC), the ABR/LBR is responsible for advertising the proper label to each neighbor. Assume that an ABR/LBR has allocated three labels for a particular destination; those labels are L_primary, L_blue, and L_red. When the ABR/LBR advertises label bindings to routers in the area with the best route to the destination, the ABR/LBR provides L_primary for the default topology, L_blue for the MRT-Blue MT-ID and L_red for the MRT-Red MT-ID, exactly as expected. However, when the ABR/LBR advertises label bindings to routers in other areas, the ABR/LBR advertises L_primary for the Rainbow MRT MT-ID, which is then used for the default topology, for the MRT-Blue MT-ID and for the MRT-Red MT-ID.
The ABR/LBR installs all next-hops from 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/LBR 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/LBR 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/LBR can thus enforce that packets exiting the area do so on the shortest-path default topology.
If IP forwarding is used, then the ABR/LBR behavior is dependent upon the outermost IP address. If the outermost IP address is an MRT loopback address of the ABR/LBR, 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/LBR, 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/LBR connected to the external destination.
Thus, regardless of which of these two forwarding mechanisms are used, there is no need for additional computation or per-area forwarding state.
+----[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) Blue MRT in Area 0 (e) Red MRT in Area 0
Figure 3: ABR Forwarding Behavior and MRTs
The other forwarding mechanism described in Section 6 is using Topology-Identification Labels. This mechanism would require that any router whose MRT-Red or MRT-Blue next-hop is an ABR/LBR would need to determine whether the ABR/LBR would forward the packet out of the area/level. If so, then that router should pop off the topology-identification label before forwarding the packet to the ABR/LBR.
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 from another area, it is safe for D to pop the Topology-Identification 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 ISIS 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 OPSF area ID) and the Area ID isn't announced as part of the ASBR link-state advertisement (LSA). In this one case, suboptimal forwarding along the MRT in the other area would happen. If that becomes a realistic deployment scenario, OSPF extensions could be considered. This is not covered in [I-D.atlas-ospf-mrt].
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 multi-homed 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, remains in the same area/level and might reenter the MRT Island if a loop-free exit point is not selected.
One property of LFAs that is necessary to preserve is the ability to protect multi-homed 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 also be done for backups via MRT.
If ASBR protection is desired, this has additonal 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 multi-homed prefix could be:
There are also two different approaches to protection. The first is to do endpoint selection to pick 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, with an MRT alternate, attached to the prefix.
The second is to use a proxy-node, that 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, multi-homed prefixes can use 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 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, then the routers learning that FEC MUST also advertise labels for (MRT-Red, FEC) and (MRT-Blue, FEC) to neighbors inside the MRT Island. If the forwarding mechanism includes LDP, any router receiving a FEC corresponding to a router outside the MRT Island or to a multi-homed prefix MUST compute and install the transit MRT-Blue and MRT-Red next-hops for that FEC; the associated FECs ( (MT-ID 0, FEC), (MRT-Red, FEC), and (MRT-Blue, FEC)) MUST also be provided via LDP to neighbors inside the MRT Island.
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 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].
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).
The 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 where the MPLS labels understood by the endpoint router are not needed.
A clear advantage to using a named proxy-node is that it is possible to explicitly forward from the MRT Island along an interface to a loop-free island neighbor (LFIN) when that interface may not be a primary next-hop. For LDP traffic where the label indicates both the topology and the FEC, it is necessary to either use a named proxy-node or deal with learning remote MPLS labels.
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 are not defined in [I-D.atlas-ospf-mrt]. The details of what label-bindings must be originated are described in [I-D.atlas-mpls-ldp-mrt].
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 extremely straightforward. The details of the simple constant-time functions, Select_Proxy_Node_NHs() and Select_Alternates_Proxy_Node(), are given in [I-D.enyedi-rtgwg-mrt-frr-algorithm]. 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 [[RFC2328] Section 16.5] does incremental updates for new summary-LSAs.
The key 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 2 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 are those routers that are advertising the prefix; the cost assigned to each such router is the announced cost to the prefix. The second set are 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 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 regard to the whole MRT Island for the destination. Thus, an IBR is a candidate router only if it possesses at least one IN whose path to the prefix does not enter the MRT Island. The cost assigned to each (IBR, IN) pair is the D_opt(IN, prefix) plus Cost(IBR, IN).
From the set of prefix-advertising routers and the IBRs, the two lowest cost routers are selected and ties are broken based upon the lowest Router ID. For ease of discussion, such selected routers are proxy-node attachment routers and the two selected will be named A and B.
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 default 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 that caused the router to advertise the prefix; this interface might be out of the area/level/AS.
As discussed, the Island Neighbor needs to be loop-free with regard to the whole MRT Island for the destination. Conceptually, the cost of transiting the MRT Island should be regarded as 0. This can be done by collapsing the MRT Island into a single node, as seen in Figure 4, and then computing SPFs from each Island Neighbor and from the MRT Island itself.
[G]---[E]---(V)---(U)---(T)
| \ | | |
| \ | | |
| \ | | |
[H]---[F]---(R)---(S)----|
(1) Network Graph with Partial Deployment
[E],[F],[G],[H] : No support for MRT
(R),(S),(T),(U),(V): MRT Island - supports MRT
[G]---[E]----| |---(V)---(U)---(T)
| \ | | | | |
| \ | ( MRT Island ) [ proxy ] | |
| \ | | | | |
[H]---[F]----| |---(R)---(S)----|
(2) Graph for determining (3) Graph for MRT computation
loop-free neighbors
Figure 4: Computing alternates to destinations outside the MRT Island
The simple way to do this without manipulating the topology is to compute the SPFs from each IN and a node in the MRT Island (e.g. the GADAG root), but use a link metric of 0 for all links between routers in the MRT Island. The distances computed via SPF this way will be refered to as Dist_mrt0.
An IN is loop-free with respect to a destination D if: Dist_mrt0(IN, D) < Dist_mrt0(IN, MRT Island Router) + Dist_mrt0(MRT Island Router, D). Any router in the MRT Island can be used since the cost of transiting between MRT Island routers is 0. The GADAG Root is recommended for consistency.
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 6 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 multi-homed 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.
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 re-calculation.
As is well known[RFC5715], micro-loops can occur during IGP convergence; such loops can be local to the failure or remote from the failure. Managing micro-loops is an orthogonal issue to having alternates for local repair, such as MRT fast-reroute provides.
There are two possible micro-loop prevention mechanisms discussed in [RFC5715]. The first is Ordered FIB [I-D.ietf-rtgwg-ordered-fib]. The second is Farside Tunneling which requires tunnels or an alternate topology to reach routers on the farside of the failure.
Since MRTs provide an alternate topology through which traffic can be sent and which can be manipulated separately from the SPT, it is possible that MRTs could be used to support Farside Tunneling. Details of how to do so are outside the scope of this document.
Micro-loop mitigation mechanisms can also work when combined with MRT.
When a failure event happens, traffic is put by the PLRs onto the MRT topologies. After that, each router recomputes its shortest path tree (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:
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.
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 and Chris Bowers for their suggestions and review.
This doument includes no request to IANA.
This architecture is not currently believed to introduce new security concerns.
| [RFC5714] | Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC 5714, January 2010. |
| [RFC5286] | Atlas, A. and A. Zinin, "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, September 2008. |
| [I-D.enyedi-rtgwg-mrt-frr-algorithm] | Atlas, A., Envedi, G., Csaszar, A. and A. Gopalan, "Algorithms for computing Maximally Redundant Trees for IP/LDP Fast- Reroute", Internet-Draft draft-enyedi-rtgwg-mrt-frr-algorithm-02, October 2012. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC2328] | Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. |
| [RFC3137] | Retana, A., Nguyen, L., White, R., Zinin, A. and D. McPherson, "OSPF Stub Router Advertisement", RFC 3137, June 2001. |
| [RFC5443] | Jork, M., Atlas, A. and L. Fang, "LDP IGP Synchronization", RFC 5443, March 2009. |
| [RFC5715] | Shand, M. and S. Bryant, "A Framework for Loop-Free Convergence", RFC 5715, January 2010. |
| [RFC6571] | Filsfils, C., Francois, P., Shand, M., Decraene, B., Uttaro, J., Leymann, N. and M. Horneffer, "Loop-Free Alternate (LFA) Applicability in Service Provider (SP) Networks", RFC 6571, June 2012. |
| [I-D.atlas-rtgwg-mrt-mc-arch] | Atlas, A., Kebler, R., Wijnands, I., Csaszar, A. and G. Envedi, "An Architecture for Multicast Protection Using Maximally Redundant Trees", Internet-Draft draft-atlas-rtgwg-mrt-mc-arch-01, February 2013. |
| [I-D.ietf-mpls-ldp-multi-topology] | Zhao, Q., Fang, L., Zhou, C., Li, L. and K. Raza, "LDP Extensions for Multi Topology Routing", Internet-Draft draft-ietf-mpls-ldp-multi-topology-06, December 2012. |
| [I-D.bryant-ipfrr-tunnels] | Bryant, S., Filsfils, C., Previdi, S. and M. Shand, "IP Fast Reroute using tunnels", Internet-Draft draft-bryant-ipfrr-tunnels-03, November 2007. |
| [I-D.ietf-rtgwg-remote-lfa] | Bryant, S., Filsfils, C., Previdi, S., Shand, M. and S. Ning, "Remote LFA FRR", Internet-Draft draft-ietf-rtgwg-remote-lfa-01, December 2012. |
| [I-D.litkowski-rtgwg-node-protect-remote-lfa] | Litkowski, S., "Node protecting remote LFA", Internet-Draft draft-litkowski-rtgwg-node-protect-remote-lfa-00, April 2013. |
| [I-D.ietf-rtgwg-ipfrr-notvia-addresses] | Bryant, S., Previdi, S. and M. Shand, "A Framework for IP and MPLS Fast Reroute Using Not-via Addresses", Internet-Draft draft-ietf-rtgwg-ipfrr-notvia-addresses-10, December 2012. |
| [I-D.ietf-rtgwg-ordered-fib] | Shand, M., Bryant, S., Previdi, S., Filsfils, C., Francois, P. and O. Bonaventure, "Framework for Loop-free convergence using oFIB", Internet-Draft draft-ietf-rtgwg-ordered-fib-09, January 2013. |
| [I-D.atlas-ospf-mrt] | Atlas, A., Hegde, S., Chris, C. and J. Tantsura, "OSPF Extensions to Support Maximally Redundant Trees", Internet-Draft draft-atlas-ospf-mrt-00, July 2013. |
| [I-D.atlas-mpls-ldp-mrt] | Atlas, A., Tiruveedhula, K., Tantsura, J.T. and IJ.W. Wijnands, "LDP Extensions to Support Maximally Redundant Trees", Internet-Draft draft-atlas-mpls-ldp-mrt-00, July 2013. |
| [LightweightNotVia] | Enyedi, G.S.E., Retvari, G.R., Szilagyi, P.S. and A.C. Császár, "IP Fast ReRoute: Lightweight Not-Via without Additional Addresses", Proceedings of IEEE INFOCOM , 2009. |
| [LFARevisited] | Retvari, G.R., Tapolcai, J.T., Enyedi, G.S.E. and A.C. Császár, "IP Fast ReRoute: Loop Free Alternates Revisited", Proceedings of IEEE INFOCOM , 2011. |
| [EnyediThesis] | Enyedi, G.S.E., "Novel Algorithms for IP Fast Reroute", Department of Telecommunications and Media Informatics, Budapest University of Technology and Economics Ph.D. Thesis, February 2011. |
When a multi-homed 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 5: AS external prefixes in different areas
Consider the network in Figure 5 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.