--- 1/draft-ietf-rtgwg-mrt-frr-architecture-03.txt 2014-07-04 11:14:37.820318538 -0700 +++ 2/draft-ietf-rtgwg-mrt-frr-architecture-04.txt 2014-07-04 11:14:37.900320467 -0700 @@ -1,26 +1,27 @@ Routing Area Working Group A. Atlas, Ed. Internet-Draft R. Kebler -Intended status: Standards Track Juniper Networks -Expires: January 13, 2014 G. Enyedi +Intended status: Standards Track C. Bowers +Expires: January 5, 2015 Juniper Networks + G. Enyedi A. Csaszar J. Tantsura Ericsson M. Konstantynowicz Cisco Systems R. White VCE - July 12, 2013 + July 4, 2014 An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees - draft-ietf-rtgwg-mrt-frr-architecture-03 + draft-ietf-rtgwg-mrt-frr-architecture-04 Abstract 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. @@ -37,137 +38,176 @@ 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. + This Internet-Draft will expire on January 5, 2015. Copyright Notice - Copyright (c) 2013 IETF Trust and the persons identified as the + Copyright (c) 2014 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 - 1.1. Importance of 100% Coverage . . . . . . . . . . . . . . . 4 - 1.2. Partial Deployment and Backwards Compatibility . . . . . 5 + 1.1. Importance of 100% Coverage . . . . . . . . . . . . . . . 5 + 1.2. Partial Deployment and Backwards Compatibility . . . . . 6 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 6 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 - 4. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 7 - 5. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . 9 - 6. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . 10 - 6.1. LDP Unicast Forwarding - Avoid Tunneling . . . . . . . . 10 - 6.2. IP Unicast Traffic . . . . . . . . . . . . . . . . . . . 11 - 7. Protocol Extensions and Considerations: OSPF and ISIS . . . . 12 - 8. Protocol Extensions and considerations: LDP . . . . . . . . . 14 - 9. Inter-Area and ABR Forwarding Behavior . . . . . . . . . . . 15 - 10. Prefixes Multiply Attached to the MRT Island . . . . . . . . 18 - 10.1. Endpoint Selection . . . . . . . . . . . . . . . . . . . 19 - 10.2. Named Proxy-Nodes . . . . . . . . . . . . . . . . . . . 21 - 10.2.1. Computing if an Island Neighbor (IN) is loop-free . 22 - 10.3. MRT Alternates for Destinations Outside the MRT Island . 23 - 11. Network Convergence and Preparing for the Next Failure . . . 24 - 11.1. Micro-forwarding loop prevention and MRTs . . . . . . . 24 - 11.2. MRT Recalculation . . . . . . . . . . . . . . . . . . . 24 - 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25 - 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 - 14. Security Considerations . . . . . . . . . . . . . . . . . . . 25 - 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 - 15.1. Normative References . . . . . . . . . . . . . . . . . . 25 - 15.2. Informative References . . . . . . . . . . . . . . . . . 26 - Appendix A. General Issues with Area Abstraction . . . . . . . . 27 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 + 4. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 8 + 5. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . 10 + 6. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . 11 + 6.1. MRT Forwarding Mechanisms . . . . . . . . . . . . . . . . 11 + 6.1.1. MRT LDP labels . . . . . . . . . . . . . . . . . . . 11 + 6.1.1.1. Topology-scoped FEC encoded using a single label + (Option 1A) . . . . . . . . . . . . . . . . . . . 12 + 6.1.1.2. Topology and FEC encoded using a two label stack + (Option 1B) . . . . . . . . . . . . . . . . . . . 12 + 6.1.1.3. Compatibility of Option 1A and 1B . . . . . . . . 13 + 6.1.1.4. Mandatory support for MRT LDP Label option 1A . . 13 + 6.1.2. MRT IP tunnels (Options 2A and 2B) . . . . . . . . . 13 + 6.2. Forwarding LDP Unicast Traffic over MRT Paths . . . . . . 14 + 6.2.1. Forwarding LDP traffic using MRT LDP Labels (Option + 1A) . . . . . . . . . . . . . . . . . . . . . . . . . 14 + 6.2.2. Forwarding LDP traffic using MRT LDP Labels (Option + 1B) . . . . . . . . . . . . . . . . . . . . . . . . . 15 + 6.2.3. Other considerations for forwarding LDP traffic using + MRT LDP Labels . . . . . . . . . . . . . . . . . . . 15 + 6.3. Forwarding IP Unicast Traffic over MRT Paths . . . . . . 15 + 6.3.1. Tunneling IP traffic using MRT LDP Labels . . . . . . 16 + 6.3.1.1. Tunneling IP traffic using MRT LDP Labels (Option + 1A) . . . . . . . . . . . . . . . . . . . . . . . 16 + 6.3.1.2. Tunneling IP traffic using MRT LDP Labels (Option + 1B) . . . . . . . . . . . . . . . . . . . . . . . 16 + 6.3.2. Tunneling IP traffic using MRT IP Tunnels . . . . . . 17 + 6.3.3. Required support . . . . . . . . . . . . . . . . . . 17 + 7. MRT Island Formation . . . . . . . . . . . . . . . . . . . . 17 + 7.1. IGP Area or Level . . . . . . . . . . . . . . . . . . . . 17 + 7.2. Support for a specific MRT profile . . . . . . . . . . . 18 + 7.3. Excluding additional routers and interfaces from the MRT + Island . . . . . . . . . . . . . . . . . . . . . . . . . 18 + 7.3.1. Existing IGP exclusion mechanisms . . . . . . . . . . 18 + 7.3.2. MRT-specific exclusion mechanism . . . . . . . . . . 19 + 7.4. Connectivity . . . . . . . . . . . . . . . . . . . . . . 19 + 7.5. Example algorithm . . . . . . . . . . . . . . . . . . . . 19 + 8. MRT Profile . . . . . . . . . . . . . . . . . . . . . . . . . 19 + 8.1. MRT Profile Options . . . . . . . . . . . . . . . . . . . 19 + 8.2. Router-specific MRT paramaters . . . . . . . . . . . . . 20 + 8.3. Default MRT profile . . . . . . . . . . . . . . . . . . . 21 + 9. LDP signaling extensions and considerations . . . . . . . . . 22 + 10. Inter-area Forwarding Behavior . . . . . . . . . . . . . . . 22 + 10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A . . 23 + 10.1.1. Motivation for Creating the Rainbow-FEC . . . . . . 23 + 10.2. ABR Forwarding Behavior with IP Tunneling (option 2) . . 24 + 10.3. ABR Forwarding Behavior with LDP Label option 1B . . . . 24 + 11. Prefixes Multiply Attached to the MRT Island . . . . . . . . 26 + 11.1. Protecting Multi-Homed Prefixes using Tunnel Endpoint + Selection . . . . . . . . . . . . . . . . . . . . . . . 28 + 11.2. Protecting Multi-Homed Prefixes using Named Proxy-Nodes 29 + 11.2.1. Computing if an Island Neighbor (IN) is loop-free . 31 + 11.3. MRT Alternates for Destinations Outside the MRT Island . 32 + 12. Network Convergence and Preparing for the Next Failure . . . 33 + 12.1. Micro-forwarding loop prevention and MRTs . . . . . . . 33 + 12.2. MRT Recalculation . . . . . . . . . . . . . . . . . . . 33 + 13. Implementation Status . . . . . . . . . . . . . . . . . . . . 34 + 14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36 + 15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36 + 16. Security Considerations . . . . . . . . . . . . . . . . . . . 36 + 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 36 + 17.1. Normative References . . . . . . . . . . . . . . . . . . 36 + 17.2. Informative References . . . . . . . . . . . . . . . . . 37 + Appendix A. General Issues with Area Abstraction . . . . . . . . 39 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40 1. Introduction 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]. + [I-D.ietf-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 | Computation (in SPFs) | | | | Looping? | | - +-----------+---------------+---------------+-----------------------+ + +---------+-------------+-------------+-----------------------------+ | MRT-FRR | 100% | None | less than 3 | | | Link/Node | | | | | | | | | LFA | Partial | Possible | per neighbor | | | Link/Node | | | | | | | | - | Remote | Partial | Possible | per neighbor (link) | - | LFA | Link/Node | | or neighbor's | - | | | | neighbor (node) | + | Remote | Partial | Possible | per neighbor (link) or | + | LFA | Link/Node | | neighbor's neighbor (node) | | | | | | | Not-Via | 100% | None | per link and node | | | Link/Node | | | - +-----------+---------------+---------------+-----------------------+ + +---------+-------------+-------------+-----------------------------+ Table 1 Loop-Free Alternates (LFA): LFAs [RFC5286] provide limited topology-dependent coverage for link and node protection. Restrictions on choice of alternates can be relaxed to improve coverage, but this can cause forwarding loops if a worse failure is experienced than protected against. Augmenting a network to provide better coverage is NP-hard [LFARevisited]. [RFC6571] discusses the applicability of LFA to different topologies with a focus on common PoP architectures. Remote LFA: Remote LFAs [I-D.ietf-rtgwg-remote-lfa] improve coverage over LFAs for link protection but still cannot guarantee complete coverage. The trade-off of looping traffic to improve coverage is still made. Remote LFAs can provide node-protection - [I-D.litkowski-rtgwg-node-protect-remote-lfa] but not guaranteed - coverage and the computation required is quite high (an SPF per - neighbor's neighbor). [I-D.bryant-ipfrr-tunnels] describes + [I-D.psarkar-rtgwg-rlfa-node-protection] but not guaranteed + coverage and the computation required is quite high (an SPF for + each PQ-node evaluated). [I-D.bryant-ipfrr-tunnels] describes additional mechanisms to further improve coverage, at the cost of added complexity. Not-Via: Not-Via [I-D.ietf-rtgwg-ipfrr-notvia-addresses] is the only other solution that provides 100% coverage for link and node failures and does not have potential looping. However, the computation is very high (an SPF per failure point) and academic implementations [LightweightNotVia] have found the address management complexity to be high. @@ -200,22 +241,22 @@ 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 + adversely impacting traffic. 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. 1.2. Partial Deployment and Backwards Compatibility 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 @@ -256,22 +297,22 @@ Specifically, MRT-Blue is the increasing MRT where links in the GADAG are taken in the direction from a lower topologically ordered node to a higher one. Rainbow MRT: It is useful to have an MT-ID that refers to the multiple MRT topologies and to the default topology. This is referred to as the Rainbow MRT MT-ID and is used by LDP to reduce signaling and permit the same label to always be advertised to all peers for the same (MT-ID, Prefix). - MRT Island: From the computing router, the set of routers that - support a particular MRT profile and are connected. + MRT Island: The set of routers that support a particular MRT + profile and the links connecting them that support MRT. Island Border Router (IBR): A router in the MRT Island that is connected to a router not in the MRT Island and both routers are in a common area or level. Island Neighbor (IN): A router that is not in the MRT Island but is adjacent to an IBR and in the same area/level as the IBR. cut-link: A link whose removal partitions the network. A cut-link by definition must be connected between two cut-vertices. If @@ -305,29 +346,31 @@ named proxy-node: A proxy-node can represent a destination prefix that can be attached to the MRT Island via at least two routers. It is named 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 undefined fashion for that proxy-node. 4. Maximally Redundant Trees (MRT) - 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. + A pair of Maximally Redundant Trees is a pair of directed spanning + trees that provides 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.ietf-rtgwg-mrt-frr-algorithm]. This algorithm can be computed + in O(e + n log n); it is less than three SPFs. Modeling results + comparing the alternate path lengths obtained with MRT to other + approaches are described in [I-D.ietf-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. @@ -403,344 +446,716 @@ 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 + computed. [I-D.ietf-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. + hop to a particular destination. 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. 6. Unicast Forwarding with MRT Fast-Reroute - 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. + As mentioned before, MRT FRR 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 + indicate its topology. Once the MRTs are computed, the two sets of + MRTs can be used as two additional forwarding topologies. The same considerations apply for forwarding along the MRTs as for handling multiple topologies. -6.1. LDP Unicast Forwarding - Avoid Tunneling + There are three possible types of routers involved in forwarding a + packet along an MRT path. At the MRT ingress router, the packet + leaves the shortest path to the destination and follows an MRT path + to the destination. In a FRR application, the MRT ingress router is + the PLR. An MRT transit router takes a packet that arrives already + associated with the particular MRT, and forwards it on that same MRT. + In some situations (to be discussed later), the packet will need to + leave the MRT path and return to the shortest path. This takes place + at the MRT egress router. The MRT ingress and egress functionality + may depend on the underlying type of packet being forwarded (LDP or + IP). The MRT transit functionality is independent of the type of + packet being forwarded. We first consider several MRT transit + forwarding mechanisms. Then we look at how these forwarding + mechanisms can be applied to carrying LDP and IP traffic. - 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. +6.1. MRT Forwarding Mechanisms - 1. Option A - Encode MT-ID in Labels: In addition to sending a - single label for a FEC, a router would provide two additional - labels with the MT-IDs associated with the Blue MRT or Red MRT - forwarding topologies. This is very simple for hardware support. - It does reduce the label space for other uses. It also increases - the memory to store the labels and the communication required by - LDP. + The following options for MRT forwarding mechanisms are considered. - 2. Option B - Create Topology-Identification Labels: Use the label- - stacking ability of MPLS and specify only two additional labels - - one for each associated MRT color - by a new FEC type. When - sending a packet onto an MRT, first swap the LDP label and then - push the topology-identification label for that MRT color. When - receiving a packet with a topology-identification label, pop it - and use it to guide the next-hop selection in combination with - the next label in the stack; then swap the remaining label, if - appropriate, and push the topology-identification label for the - next-hop. This has minimal usage of additional labels, memory - and LDP communication. It does increase the size of packets and - the complexity of the required label operations and look-ups. - This can use the same mechanisms as are needed for context-aware - label spaces. + 1. MRT LDP Labels - 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. + A. Topology-scoped FEC encoded using a single label - 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]. + B. Topology and FEC encoded using a two label stack -6.2. IP Unicast Traffic + 2. MRT IP Tunnels - For IP, there is no currently practical alternative except tunneling - to gain the bits needed to indicate the MRT-Blue or MRT-Red + A. MRT IPv4 Tunnels + + B. MRT IPv6 Tunnels + +6.1.1. MRT LDP labels + + We consider two options for the MRT forwarding mechanisms using MRT + LDP labels. + +6.1.1.1. Topology-scoped FEC encoded using a single label (Option 1A) + + [I-D.ietf-mpls-ldp-multi-topology] provides a mechanism to distribute + FEC-Label bindings scoped to a given topology (represented by MT-ID). + To use multi-topology LDP to create MRT forwarding topologies, we + associate two MT-IDs with the MRT-Red and MRT-Blue forwarding + topologies, in addition to the default shortest path forwarding + topology with MT-ID=0. + + With this forwarding mechanism, a single label is distributed for + each topology-scoped FEC. For a given FEC in the default topology + (call it default-FEC-A), two additional topology-scoped FECs would be + created, corresponding to the Red and Blue MRT forwarding topologies + (call them red-FEC-A and blue-FEC-A). A router supporting this MRT + transit forwarding mechanism advertises a different FEC-label binding + for each of the three topology-scoped FECs. When a packet is + received with a label corresponding to red-FEC-A (for example), an + MRT transit router will determine the next-hop for the MRT-Red + forwarding topology for that FEC, swap the incoming label with the + outgoing label corresponding to red-FEC-A learned from the MRT-Red + next-hop router, and forward the packet. + + This forwarding mechanism has the useful property that the FEC + associated with the packet is maintained in the labels at each hop + along the MRT. We will take advantage of this property when + specifying how to carry LDP traffic on MRT paths using multi-topology + LDP labels. + + This approach is very simple for hardware to support. However, it + reduces the label space for other uses, and it increases the memory + needed to store the labels and the communication required by LDP to + distribute FEC-label bindings. + + This forwarding option uses the LDP signaling extensions described in + [I-D.ietf-mpls-ldp-multi-topology]. The MRT-specific LDP extensions + required to support this option are described in + [I-D.atlas-mpls-ldp-mrt]. + +6.1.1.2. Topology and FEC encoded using a two label stack (Option 1B) + + With this forwarding mechanism, a two label stack is used to encode + the topology and the FEC of the packet. The top label (topology-id + label) identifies the MRT forwarding topology, while the second label + (FEC label) identifies the FEC. The top label would be a new FEC + type with two values corresponding to MRT Red and Blue topologies. + + When an MRT transit router receives a packet with a topology-id + label, the router pops the top label and uses that it to guide the + next-hop selection in combination with the next label in the stack + (the FEC label). The router then swaps the FEC label, using the FEC- + label bindings learned through normal LDP mechanisms. The router + then pushes the topology-id label for the next-hop. + + As with Option 1A, this forwarding mechanism also has the useful + property that the FEC associated with the packet is maintained in the + labels at each hop along the MRT. + + This forwarding mechanism has minimal usage of additional labels, + memory and LDP communication. It does increase the size of packets + and the complexity of the required label operations and look-ups. + + This forwarding option is consistent with context-specific label + spaces, as described in [RFC 5331]. However, the precise LDP + behavior required to support this option for MRT has not been + specified. + +6.1.1.3. Compatibility of Option 1A and 1B + + In principle, MRT transit forwarding mechanisms 1A and 1B can coexist + in the same network, with a packet being forwarding along a single + MRT path using the single label of option 1A for some hops and the + two label stack of option 1B for other hops. + +6.1.1.4. Mandatory support for MRT LDP Label option 1A + + If a router supports a profile that includes the MRT LDP Label option + for MRT transit forwarding mechanism, then it MUST support option 1A, + which encodes topology-scoped FECs using a single label. + +6.1.2. MRT IP tunnels (Options 2A and 2B) + + IP tunneling can also be used as an MRT transit forwarding mechanism. + Each router supporting this MRT transit forwarding mechanism + announces two additional loopback addresses and their associated MRT + color. Those addresses are used as destination addresses for MRT- + blue and MRT-red IP tunnels respectively. The special loopback + addresses allow the transit nodes to identify the traffic as being + forwarded along either the MRT-blue or MRT-red topology to reach the + tunnel destination. Announcements of these two additional loopback + addresses per router with their MRT color requires IGP extensions, + which have not been defined. + + Either IPv4 (option 2A) or IPv6 (option 2B) can be used as the + tunneling mechanism. + + Note that the two forwarding mechanisms using LDP Label options do + not require additional loopbacks per router, as is required by the IP + tunneling mechanism. This is because LDP labels are used on a hop- + by-hop basis to identify MRT-blue and MRT-red forwarding topologies. + +6.2. Forwarding LDP Unicast Traffic over MRT Paths + + In the previous section, we examined several options for providing + MRT transit forwarding functionality, which is independent of the + type of traffic being carried. We now look at the MRT ingress + functionality, which will depend on the type of traffic being carried + (IP or LDP). We start by considering LDP traffic. + + We also simplify the initial discussion by assuming that the network + consists of a single IGP area, and that all routers in the network + participate in MRT. Other deployment scenarios that require MRT + egress functionality are considered later in this document. + + In principle, it is possible to carry LDP traffic in MRT IP tunnels. + However, for LDP traffic, it is very desirable to avoid tunneling. + Tunneling LDP traffic to a remote node requires knowledge of remote + FEC-label bindings so that the LDP traffic can continue to be + forwarded properly when it leaves the tunnel. This requires targeted + LDP sessions which can add management complexity. The two MRT LDP + Label forwarding mechanisms have the useful property that the FEC + associated with the packet is maintained in the labels at each hop + along the MRT, as long as an MRT to the originator of the FEC is + used. The MRT IP tunneling mechanism does not have this useful + property. Therefore, this document only considers the two MRT LDP + Label forwarding mechanisms for protecting LDP traffic with MRT fast- + reroute. + +6.2.1. Forwarding LDP traffic using MRT LDP Labels (Option 1A) + + The MRT LDP Label option 1A forwarding mechanism uses topology-scoped + FECs encoded using a single label as described in section + Section 6.1.1.1. When a PLR receives an LDP packet that needs to be + forwarded on the Red MRT (for example), it does a label swap + operation, replacing the usual LDP label for the FEC with the Red MRT + label for that FEC received from the next-hop router in the Red MRT + computed by the PLR. When the next-hop router in the Red MRT + receives the packet with the Red MRT label for the FEC, the MRT + transit forwarding functionality continues as described in + Section 6.1.1.1. In this way the original FEC associated with the + packet is maintained at each hop along the MRT. + +6.2.2. Forwarding LDP traffic using MRT LDP Labels (Option 1B) + + The MRT LDP Label option 1B forwarding mechanism encodes the topology + and the FEC using a two label stack as described in Section 6.1.1.2. + When a PLR receives an LDP packet that needs to be forwarded on the + Red MRT, it first does a normal LDP label swap operation, replacing + the incoming normal LDP label associated with a given FEC with the + outgoing normal LDP label for that FEC learned from the next-hop on + the Red MRT. In addition, the PLR pushes the topology-identification + label associated with the Red MRT, and forward the packet to the + appropriate next-hop on the Red MRT. When the next-hop router in the + Red MRT receives the packet with the Red MRT label for the FEC, the + MRT transit forwarding functionality continues as described in + Section 6.1.1.2. As with option 1A, the original FEC associated with + the packet is maintained at each hop along the MRT. + +6.2.3. Other considerations for forwarding LDP traffic using MRT LDP + Labels + + Note that forwarding LDP traffic using MRT LDP Labels requires that + an MRT to the originator of the FEC be used. For example, one might + find it desirable to have the PLR use an MRT to reach the primary + next-next-hop for the FEC, and then continue forwarding the LDP + packet along the shortest path tree from the primary next-next-hop. + However, this would require tunneling to the primary next-next-hop + and a targeted LDP session for the PLR to learn the FEC-label binding + for primary next-next-hop to correctly forward the packet. + + For greatest hardware compatibility, routers implementing MRT fast- + reroute of LDP traffic MUST support Option 1A of encoding the MT-ID + in the labels (See Section 9). + +6.3. Forwarding IP Unicast Traffic over MRT Paths + + For IP traffic, 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 + area is selected (see Section 11 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. + destination. As discussed in the previous section, for LDP traffic, + using the MRT to 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. However, consistency with + LDP is RECOMMENDED. - 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. + Some situations require tunneling IP traffic along an MRT to a tunnel + endpoint that is not the destination of the IP traffic. These + situations will be discussed in detail later. We note here that an + IP packet with a destination in a different IGP area/level from the + PLR should be tunneled on the MRT to the ABR/LBR on the shortest path + to the destination. For a destination outside of the PLR's MRT + Island, the packet should be tunneled on the MRT to a non-proxy-node + immediately before the named proxy-node on that particular color MRT. - 1. Tunnel IP packets via an LDP LSP. This has the advantage that - more installed routers can do line-rate encapsulation and - decapsulation. Also, no additional IP addresses would need to be - allocated or signaled. +6.3.1. Tunneling IP traffic using MRT LDP Labels - a. Option A - LDP Destination-Topology Label: Use a label that - indicates both destination and MRT. This method allows easy - tunneling to the next-next-hop as well as to the IGP-area - destination. For a proxy-node, the destination to use is the - non-proxy-node immediately before the proxy-node on that - particular color MRT. + An IP packet can be tunneled along an MRT path by pushing the + appropriate MRT LDP label(s). Tunneling using LDP labels, as opposed + to IP headers, has the the advantage that more installed routers can + do line-rate encapsulation and decapsulation using LDP than using IP. + Also, no additional IP addresses would need to be allocated or + signaled. - b. Option B - LDP Topology Label: Use a Topology-Identifier - label on top of the IP packet. This is very simple. If - tunneling to a next-next-hop is desired, then a two-deep - label stack can be used with [ Topology-ID label, Next-Next- - Hop Label ]. +6.3.1.1. Tunneling IP traffic using MRT LDP Labels (Option 1A) - 2. Tunnel IP packets in IP. Each router supporting this option - would announce two additional loopback addresses and their - associated MRT color. Those addresses are used as destination - addresses for MRT-blue and MRT-red IP tunnels respectively. They - allow the transit nodes to identify the traffic as being - forwarded along either MRT-blue or MRT-red tree topology to reach - the tunnel destination. Announcements of these two additional - loopback addresses per router with their MRT color requires IGP - extensions. + The MRT LDP Label option 1A forwarding mechanism uses topology-scoped + FECs encoded using a single label as described in section + Section 6.1.1.1. When a PLR receives an IP packet that needs to be + forwarded on the Red MRT to a particular tunnel endpoint, it does a + label push operation. The label pushed is the Red MRT label for a + FEC originated by the tunnel endpoint, learned from the next-hop on + the Red MRT. -7. Protocol Extensions and Considerations: OSPF and ISIS +6.3.1.2. Tunneling IP traffic using MRT LDP Labels (Option 1B) - 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]. + The MRT LDP Label option 1B forwarding mechanism encodes the topology + and the FEC using a two label stack as described in Section 6.1.1.2. + When a PLR receives an IP packet that needs to be forwarded on the + Red MRT to a particular tunnel endpoint, the PLR pushes two labels on + the IP packet. The first (inner) label is the normal LDP label + learned from the next-hop on the Red MRT, associated with a FEC + originated by the tunnel endpoint. The second (outer) label is the + topology-identification label associated with the Red 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. + For completeness, we note here a potential optimization. In order to + tunnel an IP packet over an MRT to the destination of the IP packet + (as opposed to an arbitrary tunnel endpoint), then we could just push + a topology-identification label directly onto the packet. An MRT + transit router would need to pop the topology-id label, do an IP + route lookup in the context of that topology-id , and push the + topology-id label. - 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. +6.3.2. Tunneling IP traffic using MRT IP Tunnels - The MRT Profile also defines different behaviors such as how MRT - recomputation is handled and how area/level boundaries are dealt - with. + In order to tunnel over the MRT to a particular tunnel endpoint, the + PLR encapsulates the original IP packet with an additional IP header + using the MRT-Blue or MRT-Red loopack address of the tunnel endpoint. - MRT Algorithm: MRT Lowpoint algorithm defined in - [I-D.enyedi-rtgwg-mrt-frr-algorithm]. +6.3.3. Required support - MRT-Red MT-ID: experimental 3997, final value assigned by IANA - allocated from the LDP MT-ID space + 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 tunneling of IP + traffic using MRT LDP Labels Option 1A (topology-scoped FEC encoded + using a single label). - MRT-Blue MT-ID: experimental 3998, final value assigned by IANA - allocated from the LDP MT-ID space +7. MRT Island Formation - GADAG Root Selection Priority: Among the routers in the MRT Island - and with the highest priority advertised, an implementation MUST - pick the router with the highest Router ID to be the GADAG root. + The purpose of communicating support for MRT in the IGP is to + indicate that the MRT-Blue and MRT-Red forwarding topologies are + created for transit traffic. The MRT architecture allows for + different, potentially incompatible options. In order to create + constistent MRT forwarding topologies, the routers participating in a + particular MRT Island need to use the same set of options. These + options are grouped into MRT profiles. In addition, the routers in + an MRT Island all need to use the same set of nodes and links within + the Island when computing the MRT forwarding topologies. This + section describes the information used by a router to determine the + nodes and links to include in a particular MRT Island. Some of this + information is shared among routers using the newly-defined IGP + signaling extensions for MRT described in [I-D.atlas-ospf-mrt] and + [I-D.li-isis-mrt]. Other information already exists in the IGPs and + can be used by MRT in Island formation, subject to the interpretation + defined here. - Forwarding Mechanisms: LDP + Deployment scenarios using multi-topology OSPF or IS-IS, or running + both ISIS and OSPF on the same routers is out of scope for this + specification. As with LFA, it is expected that OSPF Virtual Links + will not be supported. - Recalculation: Recalculation of MRTs SHOULD occur as described in - Section 11.2. This allows the MRT forwarding topologies to - support IP/LDP fast-reroute traffic. +7.1. IGP Area or Level - Area/Level Border Behavior: As described in Section 9, ABRs/LBRs - SHOULD ensure that traffic leaving the area also exits the MRT-Red - or MRT-Blue forwarding topology. + All links in an MRT Island MUST be bidirectional and belong to the + same IGP area or level. For ISIS, a link belonging to both level 1 + and level 2 would qualify to be in multiple MRT Islands. A given ABR + or LBR can belong to multiple MRT Islands, corresponding to the areas + or levels in which it participates. Inter-area forwarding behavior + is discussed in Section 10. - 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. +7.2. Support for a specific MRT profile + + All routers in an MRT Island MUST support the same MRT profile. A + router advertises support for a given MRT profile using the IGP + extensions defined in [I-D.atlas-ospf-mrt] and [I-D.li-isis-mrt] + using an 8-bit Profile ID value. A given router can support multiple + MRT profiles and participate in multiple MRT Islands. The options + that make up an MRT profile, as well as the default MRT profile, are + defined in Section 8. + +7.3. Excluding additional routers and interfaces from the MRT Island + + MRT takes into account existing IGP mechanisms for discouraging + traffic from using particular links and routers, and it introduces an + MRT-specific exclusion mechanism for links. + +7.3.1. Existing IGP exclusion mechanisms + + Mechanisms for discouraging traffic from using particular links + already exist in ISIS and OSPF. In ISIS, an interface configured + with a metric of 2^24-2 (0xFFFFFE) will only be used as a last + resort. (An interface configured with a metric of 2^24-1 (0xFFFFFF) + will not be advertised into the topology.) In OSPF, an interface + configured with a metric of 2^16-1 (0xFFFF) will only be used as a + last resort. These metrics can be configured manually to enforce + administrative policy, or they can be set in an automated manner as + with LDP IGP synchronization [RFC5443]. + + Mechanisms also exist in ISIS and OSPF to prevent transit traffic + from using a particular router. In ISIS, the overload bit is used + for this purpose. In OSPF, [RFC3137] specifies setting all outgoing + interface metrics to 0xFFFF to accomplish this. + + The following rules for MRT Island formation ensure that MRT FRR + protection traffic does not use a link or router that is discouraged + from carrying traffic by existing IGP mechanisms. + + 1. A bidirectional link MUST be excluded from an MRT Island if + either the forward or reverse cost on the link is 0xFFFFFE (for + ISIS) or 0xFFFF for OSPF. + + 2. A router MUST be excluded from an MRT Island if it is advertised + with the overload bit set (for ISIS), or it is advertised with + metric values of 0xFFFF on all of its outgoing interfaces (for + OSPF). + +7.3.2. MRT-specific exclusion mechanism + + This architecture also defines a means of excluding an otherwise + usable link from MRT Islands. [I-D.atlas-ospf-mrt] and + [I-D.li-isis-mrt] define the IGP extensions for OSPF and ISIS used to + advertise that a link is MRT-Ineligible. A link with either + interface advertised as MRT-Ineligible MUST be excluded from an MRT + Island. Note that an interface advertised as MRT-Ineligigle by a + router is ineligible with respect to all profiles advertised by that + router. + +7.4. Connectivity + + All of the routers in an MRT Island MUST be connected by + bidirectional links with other routers in the MRT Island. + Disconnected MRT Islands will operate independently of one another. + +7.5. Example algorithm + + An algorithm that allows a computing router to identify the routers + and links in the local MRT Island satisfying the above rules is given + in section 5.1 of [I-D.ietf-rtgwg-mrt-frr-algorithm]. + +8. MRT Profile + + An MRT Profile is a set of values and options related to MRT + behavior. The complete set of options is designated by the + corresponding 8-bit Profile ID value. + +8.1. MRT Profile Options + + Below is a description of the values and options that define an MRT + Profile. MRT Algorithm: This identifies the particular MRT algorithm used by the router for this profile. Algorithm transitions can be managed by advertising multiple MRT profiles. MRT-Red MT-ID: This specifies the MT-ID to be associated with the MRT-Red forwarding topology. It is needed for use in LDP signaling. All routers in the MRT Island MUST agree on a value. MRT-Blue MT-ID: This specifies the MT-ID to be associated with the MRT-Blue forwarding topology. It is needed for use in LDP signaling. All routers in the MRT Island MUST agree on a value. - GADAG Root Selection Priority: A MRT profile might specify this to - provide the network operator with a knob to force a particular - GADAG root selection. If not specified in the MRT profile, the - highest Router ID in the profile's MRT Island will be elected the - GADAG Root. If a GADAG Root Selection Priority is specified, then - the MRT profile must also specify how the GADAG Root is elected. + GADAG Root Selection Policy: This specifes the manner in which the + GADAG root is selected. All routers in the MRT island need to use + the same GADAG root in the calculations used construct the MRTs. + A valid GADAG Root Selection Policy MUST be such that each router + in the MRT island chooses the same GADAG root based on information + available to all routers in the MRT island. GADAG Root Selection + Priority values, advertised in the IGP as router-specific MRT + parameters, MAY be used in a GADAG Root Selection Policy. - Forwarding Mechanism: This specifies which forwarding mechanisms - the router supports for transit traffic. An MRT island must + MRT Forwarding Mechanism: This specifies which forwarding mechanism + the router uses to carry transit traffic along MRT paths. A + router which supports a specific MRT forwarding mechanism must program appropriate next-hops into the forwarding plane. The - known options are IPv4, IPv6, LDP, and None. If IPv4 is - supported, then both MRT-Red and MRT-Blue IPv4 Loopback Addresses - SHOULD be specified. If IPv6 is supported, both MRT-Red and MRT- - Blue IPv6 Loopback Addresses SHOULD be specified. If LDP is - supported, then LDP support and signaling extensions MUST be - supported. - - MRT-Red Loopback Address: This provides the router's loopback - address to reach the router via the MRT-Red forwarding topology. - It can, of course, be specified for both IPv4 and IPv6. - - MRT-Blue Loopback Address: This provides the router's loopback - address to reach the router via the MRT-Blue forwarding topology. - It can, of course, be specified for both IPv4 and IPv6. + current options are MRT LDP Labels, IPv4 Tunneling, IPv6 + Tunneling, and None. If the MRT LDP Labels option is supported, + then option 1A and the appropriate signaling extensions MUST be + supported. If IPv4 is supported, then both MRT-Red and MRT-Blue + IPv4 Loopback Addresses SHOULD be specified. If IPv6 is + supported, both MRT-Red and MRT-Blue IPv6 Loopback Addresses + SHOULD be specified. The None option in may be useful for + multicast global protection. Recalculation: As part of what process and timing should the new - MRTs be computed on a modified topology? Section 11.2 describes + MRTs be computed on a modified topology? Section 12.2 describes the minimum behavior required to support fast-reroute. Area/Level Border Behavior: Should inter-area traffic on the MRT- Blue or MRT-Red be put back onto the shortest path tree? Should it be swapped from MRT-Blue or MRT-Red in one area/level to MRT- Red or MRT-Blue in the next area/level to avoid the potential failure of an ABR? (See [I-D.atlas-rtgwg-mrt-mc-arch] for use- case details. Other Profile-Specific Behavior: Depending upon the use-case for the profile, there may be additional profile-specific behavior. - As with LFA, it is expected that OSPF Virtual Links will not be - supported. + 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 the None option for MRT Forwarding + Mechanism. A router MUST NOT advertise multiple MRT profiles that + overlap in their MRT-Red MT-ID or MRT-Blue MT-ID. + +8.2. Router-specific MRT paramaters + + For some profiles, additional router-specific MRT parameters may need + to be distributed via the IGP. While the set of options indicated by + the MRT Profile ID must be identical for all routers in an MRT + Island, these router-specific MRT parameters may differ between + routers in the same MRT island. Several such parameters are + described below. + + GADAG Root Selection Priority: A GADAG Root Selection Policy MAY + rely on the GADAG Root Selection Priority values advertised by + each router in the MRT island. A GADAG Root Selection Policy may + use the GADAG Root Selection Priority to allow network operators + to configure a parameter to ensure that the GADAG root is selected + from a particular subset of routers. An example of this use of + the GADAG Root Selection Priority value by the GADAG Root + Selection Policy is given in the Default MRT profile below. + + MRT-Red Loopback Address: This provides the router's loopback + address to reach the router via the MRT-Red forwarding topology. + It can be specified for either IPv4 and IPv6. + + MRT-Blue Loopback Address: This provides the router's loopback + address to reach the router via the MRT-Blue forwarding topology. + It can be specified for either IPv4 and IPv6. + + The extensions to OSPF and ISIS for advertising a router's GADAG Root + Selection Priority value are defined in [I-D.atlas-ospf-mrt] and + [I-D.li-isis-mrt]. IGP extensions for the advertising a router's + MRT-Red and MRT-Blue Loopback Addresses have not been defined. + +8.3. Default MRT profile + + The following set of options defines the default MRT Profile. The + default MRT profile is indicated by the MRT Profile ID value of 0. + + MRT Algorithm: MRT Lowpoint algorithm defined in + [I-D.ietf-rtgwg-mrt-frr-algorithm]. + + MRT-Red MT-ID: TBA-MRT-ARCH-1, final value assigned by IANA + allocated from the LDP MT-ID space (prototype experiments have + used 3997) + + MRT-Blue MT-ID: TBA-MRT-ARCH-2, final value assigned by IANA + allocated from the LDP MT-ID space (prototype experiments have + used 3998) + + GADAG Root Selection Policy: Among the routers in the MRT Island + and with the highest priority advertised, an implementation MUST + pick the router with the highest Router ID to be the GADAG root. + + Forwarding Mechanisms: MRT LDP Labels + + Recalculation: Recalculation of MRTs SHOULD occur as described in + Section 12.2. This allows the MRT forwarding topologies to + support IP/LDP fast-reroute traffic. + + Area/Level Border Behavior: As described in Section 10, ABRs/LBRs + SHOULD ensure that traffic leaving the area also exits the MRT-Red + or MRT-Blue forwarding topology. + +9. LDP signaling extensions and considerations -8. Protocol Extensions and considerations: LDP 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]. + to all the MRT-Blue and MRT-Red MT-IDs in supported MRT profiles. + The FEC-label bindings for the default shortest-path based MT-ID 0 + MUST still be sent (even though it could be inferred from the Rainbow + FEC-label bindings) to ensure continuous operation of normal LDP + forwarding. 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 + an LDP egress router, it is not MRT profile specific. -9. Inter-Area and ABR Forwarding Behavior + [I-D.atlas-mpls-ldp-mrt] contains the IANA request for the Rainbow + MRT MT-ID. + +10. Inter-area Forwarding Behavior + + Unless otherwise specified, in this section we will use the terms + area and ABR to indicate either an OSPF area and OSPF ABR or ISIS + level and ISIS LBR. An ABR/LBR has two forwarding roles. First, it forwards traffic - inside its area. Second, it forwards traffic from one area into + 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 back to the - shortest-path forwarding. + 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 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 + An ABR/LBR 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 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. 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 +10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A + + For LDP forwarding where a single 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_red. To those routers in the same area as the best route to the + destination, the ABR/LBR 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, 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 + L_red for the MRT-Red MT-ID, as expected. However, to routers in + other areas, the ABR/LBR 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/LBR installs all next-hops from the best area: primary next- + The ABR/LBR 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/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 +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/LBR 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 implmentations and + doesn't have other obvious uses. + + Many existing LDP implmentations do however have the ability to + filter FEC-label binding advertisements on a per-neighbor basis. The + Rainbow-FEC allows us to re-use 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. + + The use of the Rainbow-FEC by the ABR for non-best-area + advertisements is RECOMMENDED. An ABR MAY advertise the label for + the default topology in separate MRT-Blue and MRT-Red advertisements. + However, a router that supports the LDP Label MRT 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/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. + the associated MRT loopback address of the selected ABR/LBR + advertising the lowest cost 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. + 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 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/ + 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 route 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]. +----[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 @@ -763,131 +1178,109 @@ / | | \ [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]. - -10. Prefixes Multiply Attached to the MRT Island +11. 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 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. + FRR using LFA has the useful property that it is able 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 be accomplished + with MRT FRR as well. - If ASBR protection is desired, this has additonal complexities if the - ASBRs are in different areas. Similarly, protecting labeled BGP + 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 multi-homed prefix could be: o An out-of-area prefix announced by more than one ABR, o An AS-External route announced by 2 or more ASBRs, o A prefix with iBGP multipath to different ASBRs, o etc. 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. + 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, 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 IP traffic, multi-homed 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 + 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, 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. + Island. 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 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. -10.1. Endpoint Selection +11.1. Protecting Multi-Homed Prefixes using Tunnel Endpoint Selection - 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. + 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 endpoint selection are those to which the + 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]. @@ -916,105 +1309,108 @@ while avoiding both F and S? If so, select B as the end-point. 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 that for reaching B from S - avoiding the link (S,F). + the endpoint and the MRT alternate for reaching B from S that + avoid the link (S,F). - 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. + 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 will not work for LDP traffic without targeted LDP + sesssions for learning the FEC-label binding at the tunnel endpoint. -10.2. Named Proxy-Nodes +11.2. Protecting Multi-Homed Prefixes using Named Proxy-Nodes - 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. + 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 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 + node F is 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 + [I-D.ietf-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). + 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 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 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 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). + 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 discussed below. + 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 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. + 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 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. + 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. -10.2.1. Computing if an Island Neighbor (IN) is loop-free +11.2.1. Computing if an Island Neighbor (IN) is loop-free 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) | \ | | | @@ -1043,70 +1439,70 @@ 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. -10.3. MRT Alternates for Destinations Outside the MRT Island +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 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. -11. Network Convergence and Preparing for the Next Failure +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 re-calculation. -11.1. Micro-forwarding loop prevention and MRTs +12.1. Micro-forwarding loop prevention and MRTs 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. -11.2. MRT Recalculation +12.2. MRT Recalculation 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: @@ -1121,118 +1517,217 @@ their SPTs and traffic to drain from the MRTs. 5. Recompute MRTs 6. Install new MRTs. 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. -12. Acknowledgements +13. Implementation Status + + [RFC Editor: please remove this section prior to publication.] + + This section records the status of known implementations of the + protocol defined by this specification at the time of posting of this + Internet-Draft, and is based on a proposal described in [RFC6982]. + The description of implementations in this section is intended to + assist the IETF in its decision processes in progressing drafts to + RFCs. Please note that the listing of any individual implementation + here does not imply endorsement by the IETF. Furthermore, no effort + has been spent to verify the information presented here that was + supplied by IETF contributors. This is not intended as, and must not + be construed to be, a catalog of available implementations or their + features. Readers are advised to note that other implementations may + exist. + + According to [RFC6982], "this will allow reviewers and working groups + to assign due consideration to documents that have the benefit of + running code, which may serve as evidence of valuable experimentation + and feedback that have made the implemented protocols more mature. + It is up to the individual working groups to use this information as + they see fit". + + Juniper Networks Implementation + + o Organization responsible for the implementation: Juniper Networks + + o Implementation name: MRT-FRR algorithm + o Implementation description: The MRT-FRR algorithm using OSPF as + the IGP has been implemented and verified. + + o The implementation's level of maturity: prototype + + o Protocol coverage: This implementation of the MRT algorithm + includes Island identification, GADAG root selection, Lowpoint + algorithm, augmentation of GADAG with additional links, and + calculation of MRT transit next-hops alternate next-hops based on + draft "draft-ietf-rtgwg-mrt-frr-algorithm-00". This + implementation also includes the M-bit in OSPF based on "draft- + atlas-ospf-mrt-01" as well as LDP MRT Capability based on "draft- + atlas-mpls-ldp-mrt-00". + + o Licensing: proprietary + + o Implementation experience: Implementation was useful for verifying + functionality and lack of gaps. It has also been useful for + improving aspects of the algorithm. + + o Contact information: akatlas@juniper.net, shraddha@juniper.net, + kishoret@juniper.net + + Huawei Technology Implementation + + o Organization responsible for the implementation: Huawei Technology + Co., Ltd. + + o Implementation name: MRT-FRR algorithm and IS-IS extensions for + MRT. + + o Implementation description: The MRT-FRR algorithm, IS-IS + extensions for MRT and LDP multi-topology have been implemented + and verified. + + o The implementation's level of maturity: prototype + + o Protocol coverage: This implementation of the MRT algorithm + includes Island identification, GADAG root selection, Lowpoint + algorithm, augmentation of GADAG with additional links, and + calculation of MRT transit next-hops alternate next-hops based on + draft "draft-enyedi-rtgwg-mrt-frr-algorithm-03". This + implementation also includes IS-IS extension for MRT based on + "draft-li-mrt-00". + + o Licensing: proprietary + o Implementation experience: It is important produce a second + implementation to verify the algorithm is implemented correctly + without looping. It is important to verify the ISIS extensions + work for MRT-FRR. + + o Contact information: lizhenbin@huawei.com, eric.wu@huawei.com + +14. 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 and Chris Bowers for their - suggestions and review. + Bahadur, Harish Sitaraman, and Raveendra Torvi for their suggestions + and review. -13. IANA Considerations +15. IANA Considerations - This doument includes no request to IANA. + Please create an MRT Profile registry for the MRT Profile TLV. The + range is 0 to 255. The default MRT Profile has value 0. Values + 1-200 are by Standards Action. Values 201-220 are for + experimentation. Values 221-255 are for vendor private use. -14. Security Considerations + Please allocate values from the LDP Multi-Topology (MT) ID Name Space + [I-D.ietf-mpls-ldp-multi-topology] for the following: Default Profile + MRT-Red MT-ID (TBA-MRT-ARCH-1) and Default Profile MRT-Blue MT-ID + (TBA-MRT-ARCH-2). Please allocate MT-ID values less than 4096 so + that they can also be signalled in PIM. + +16. Security Considerations This architecture is not currently believed to introduce new security concerns. -15. References +17. References -15.1. Normative References +17.1. Normative References - [I-D.enyedi-rtgwg-mrt-frr-algorithm] - Atlas, A., Envedi, G., Csaszar, A., Gopalan, A., and C. - Bowers, "Algorithms for computing Maximally Redundant - Trees for IP/LDP Fast- Reroute", draft-enyedi-rtgwg-mrt- - frr-algorithm-03 (work in progress), July 2013. + [I-D.ietf-rtgwg-mrt-frr-algorithm] + Enyedi, G., Csaszar, A., Atlas, A., Bowers, C., and A. + Gopalan, "Algorithms for computing Maximally Redundant + Trees for IP/LDP Fast-Reroute", draft-rtgwg-mrt-frr- + algorithm-01 (work in progress), July 2014. [RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, September 2008. [RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC 5714, January 2010. -15.2. Informative References +17.2. 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, . [I-D.atlas-mpls-ldp-mrt] Atlas, A., Tiruveedhula, K., Tantsura, J., and IJ. Wijnands, "LDP Extensions to Support Maximally Redundant - Trees", draft-atlas-mpls-ldp-mrt-00 (work in progress), - July 2013. + Trees", draft-atlas-mpls-ldp-mrt-01 (work in progress), + July 2014. [I-D.atlas-ospf-mrt] - Atlas, A., Hegde, S., Chris, C., and J. Tantsura, "OSPF + Atlas, A., Hegde, S., Bowers, C., and J. Tantsura, "OSPF Extensions to Support Maximally Redundant Trees", draft- - atlas-ospf-mrt-00 (work in progress), July 2013. + atlas-ospf-mrt-02 (work in progress), July 2014. [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", draft-atlas-rtgwg-mrt-mc- arch-02 (work in progress), July 2013. [I-D.bryant-ipfrr-tunnels] Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP Fast Reroute using tunnels", draft-bryant-ipfrr-tunnels-03 (work in progress), November 2007. [I-D.ietf-mpls-ldp-multi-topology] - Zhao, Q., Fang, L., Zhou, C., Li, L., and K. Raza, "LDP - Extensions for Multi Topology Routing", draft-ietf-mpls- - ldp-multi-topology-08 (work in progress), May 2013. + Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D. + King, "LDP Extensions for Multi Topology", draft-ietf- + mpls-ldp-multi-topology-12 (work in progress), April 2014. [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", draft- ietf-rtgwg-ipfrr-notvia-addresses-11 (work in progress), May 2013. [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", draft-ietf-rtgwg-ordered-fib-12 (work in progress), May 2013. [I-D.ietf-rtgwg-remote-lfa] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and S. - Ning, "Remote LFA FRR", draft-ietf-rtgwg-remote-lfa-02 - (work in progress), May 2013. + Ning, "Remote LFA FRR", draft-ietf-rtgwg-remote-lfa-06 + (work in progress), May 2014. - [I-D.litkowski-rtgwg-node-protect-remote-lfa] - Litkowski, S., "Node protecting remote LFA", draft- - litkowski-rtgwg-node-protect-remote-lfa-00 (work in - progress), April 2013. + [I-D.li-isis-mrt] + Li, Z., Wu, N., Zhao, Q., Atlas, A., Bowers, C., and J. + Tantsura, "Intermediate System to Intermediate System (IS- + IS) Extensions for Maximally Redundant Trees(MRT)", draft- + li-isis-mrt-01 (work in progress), July 2014. + + [I-D.psarkar-rtgwg-rlfa-node-protection] + psarkar@juniper.net, p., Gredler, H., Hegde, S., + Raghuveer, H., Bowers, C., and S. Litkowski, "Remote-LFA + Node Protection and Manageability", draft-psarkar-rtgwg- + rlfa-node-protection-04 (work in progress), April 2014. [LFARevisited] Retvari, G., Tapolcai, J., Enyedi, G., and A. Csaszar, "IP Fast ReRoute: Loop Free Alternates Revisited", Proceedings - of IEEE INFOCOM , 2011, . + of IEEE INFOCOM , 2011, + . [LightweightNotVia] Enyedi, G., Retvari, G., Szilagyi, P., and A. Csaszar, "IP Fast ReRoute: Lightweight Not-Via without Additional Addresses", Proceedings of IEEE INFOCOM , 2009, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. @@ -1246,20 +1741,24 @@ 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. + [RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running + Code: The Implementation Status Section", RFC 6982, July + 2013. + Appendix A. General Issues with Area Abstraction 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 | | @@ -1296,20 +1795,29 @@ Email: akatlas@juniper.net Robert Kebler Juniper Networks 10 Technology Park Drive Westford, MA 01886 USA Email: rkebler@juniper.net + + Chris Bowers + Juniper Networks + 1194 N. Mathilda Ave. + Sunnyvale, CA 94089 + USA + + Email: cbowers@juniper.net + Gabor Sandor Enyedi Ericsson Konyves Kalman krt 11. Budapest 1097 Hungary Email: Gabor.Sandor.Enyedi@ericsson.com Andras Csaszar Ericsson