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Internet Engineering Task Force (IETF)                         S. AldrinRequest for Comments: 7882                                  Google, Inc.Category: Informational                                     C. PignataroISSN: 2070-1721                                                    Cisco                                                               G. Mirsky                                                                Ericsson                                                                N. Kumar                                                                   Cisco                                                               July 2016Seamless Bidirectional Forwarding Detection (S-BFD) Use CasesAbstract   This document describes various use cases for Seamless Bidirectional   Forwarding Detection (S-BFD) and provides requirements such that   protocol mechanisms allow for simplified detection of forwarding   failures.   These use cases support S-BFD, which is a simplified mechanism for   using BFD with a large proportion of negotiation aspects eliminated,   accelerating the establishment of a BFD session.  The benefits of   S-BFD include quick provisioning, as well as improved control and   flexibility for network nodes initiating path monitoring.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7882.Aldrin, et al.                Informational                     [Page 1]

RFC 7882                     S-BFD Use Cases                   July 2016Copyright Notice   Copyright (c) 2016 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1. Introduction ....................................................31.1. Terminology ................................................32. Introduction to Seamless BFD ....................................43. Use Cases .......................................................53.1. Unidirectional Forwarding Path Validation ..................5      3.2. Validation of the Forwarding Path prior to           Switching Traffic ..........................................63.3. Centralized Traffic Engineering ............................73.4. BFD in Centralized Segment Routing .........................83.5. Efficient BFD Operation under Resource Constraints .........83.6. BFD for Anycast Addresses ..................................83.7. BFD Fault Isolation ........................................93.8. Multiple BFD Sessions to the Same Target Node ..............93.9. An MPLS BFD Session per ECMP Path .........................104. Detailed Requirements for Seamless BFD .........................115. Security Considerations ........................................126. References .....................................................126.1. Normative References ......................................126.2. Informative References ....................................13   Acknowledgements ..................................................15   Contributors ......................................................15   Authors' Addresses ................................................15Aldrin, et al.                Informational                     [Page 2]

RFC 7882                     S-BFD Use Cases                   July 20161.  Introduction   Bidirectional Forwarding Detection (BFD), as defined in [RFC5880], is   a lightweight protocol used to detect forwarding failures.  Various   protocols, applications, and clients rely on BFD for failure   detection.  Even though the protocol is lightweight and simple, there   are certain use cases where faster setup of sessions and faster   continuity checks of the data-forwarding paths are necessary.  This   document identifies these use cases and consequent requirements, such   that enhancements and extensions result in a Seamless BFD (S-BFD)   protocol.   BFD is a simple and lightweight "Hello" protocol to detect data-plane   failures.  With dynamic provisioning of forwarding paths on a large   scale, establishing BFD sessions for each of those paths not only   creates operational complexity but also causes undesirable delay in   establishing or deleting sessions.  The existing session   establishment mechanism of the BFD protocol has to be enhanced in   order to minimize the time for the session to come up to validate the   forwarding path.   This document specifically identifies various use cases and   corresponding requirements in order to enhance BFD and other   supporting protocols.  Specifically, one key goal is removing the   time delay (i.e., the "seam") between when a network node wants to   perform a continuity test and when the node completes that continuity   test.  Consequently, "Seamless BFD" (S-BFD) has been chosen as the   name for this mechanism.   While the identified requirements could meet various use cases, it is   outside the scope of this document to identify all of the possible   and necessary requirements.  Solutions related to the identified use   cases and protocol-specific enhancements or proposals are outside the   scope of this document as well.  Protocol definitions to support   these use cases can be found in [RFC7880] and [RFC7881].1.1.  Terminology   The reader is expected to be familiar with the BFD [RFC5880], IP   [RFC791] [RFC2460], MPLS [RFC3031], and Segment Routing [SR-ARCH]   terms and protocol constructs.   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and   "OPTIONAL" in this document are to be interpreted as described in   [RFC2119].Aldrin, et al.                Informational                     [Page 3]

RFC 7882                     S-BFD Use Cases                   July 20162.  Introduction to Seamless BFD   BFD, as defined in [RFC5880], requires two network nodes to exchange   locally allocated discriminators.  These discriminators enable the   identification of the sender and the receiver of BFD packets over the   particular session.  Subsequently, BFD performs proactive continuity   monitoring of the forwarding path between the two.  Several   specifications describe BFD's multiple deployment uses:   o  [RFC5881] defines BFD over IPv4 and IPv6 for single IP hops.   o  [RFC5883] defines BFD over multi-hop paths.   o  [RFC5884] defines BFD for MPLS Label Switched Paths (LSPs).   o  [RFC5885] defines BFD for MPLS Pseudowires (PWs).   Currently, BFD is best suited for verifying that two endpoints are   mutually reachable or that an existing connection continues to be up   and alive.  In order for BFD to be able to initially verify that a   connection is valid and that it connects the expected set of   endpoints, it is necessary to provide each endpoint with the   discriminators associated with the connection at each endpoint prior   to initiating BFD sessions.  The discriminators are used to verify   that the connection is up and valid.  Currently, the exchange of   discriminators and the demultiplexing of the initial BFD packets are   application dependent.   If this information is already known to the endpoints of a potential   BFD session, the initial handshake including an exchange of   discriminators is unnecessary, and it is possible for the endpoints   to begin BFD messaging seamlessly.  A key objective of the S-BFD use   cases described in this document is to avoid needing to exchange the   initial packets before the BFD session can be established, with the   goal of getting to the established state more quickly; in other   words, the initial exchange of discriminator information is an   unnecessary extra step that may be avoided for these cases.   In a given scenario, an entity (such as an operator or a centralized   controller) determines a set of network entities to which BFD   sessions might need to be established.  In traditional BFD, each of   those network entities chooses a BFD Discriminator for each BFD   session that the entity will participate in (seeSection 6.3 of   [RFC5880]).  However, a key goal of S-BFD is to provide operational   simplification.  In this context, for S-BFD, each of those network   entities is assigned one or more BFD Discriminators, and those   network entities are allowed to use one Discriminator value for   multiple sessions.  Therefore, there may be only one or a fewAldrin, et al.                Informational                     [Page 4]

RFC 7882                     S-BFD Use Cases                   July 2016   discriminators assigned to a node.  These network entities will   create an S-BFD listener session instance that listens for incoming   BFD Control packets.  When the mappings between specific network   entities and their corresponding BFD Discriminators are known to   other network nodes belonging to the same administrative domain,   then, without having received any BFD packets from a particular   target, a network entity in this network is able to send a BFD   Control packet to the target's assigned discriminator in the   Your Discriminator field.  The target network node, upon reception of   such a BFD Control packet, will transmit a response BFD Control   packet back to the sender.3.  Use Cases   As per the BFD protocol [RFC5880], BFD sessions are established using   a handshake mechanism prior to validating the forwarding path.  This   section outlines some use cases where the existing mechanism may not   be able to satisfy the requirements identified.  In addition, some of   the use cases also stress the need for expedited BFD session   establishment while preserving the benefits of forwarding failure   detection using existing BFD mechanisms.  Both of these high-level   goals result in the S-BFD use cases outlined in this document.3.1.  Unidirectional Forwarding Path Validation   Even though bidirectional verification of forwarding paths is useful,   there are scenarios where verification is only required in one   direction between a pair of nodes.  One such case is when a static   route uses BFD to validate reachability to the next-hop IP router.   In this case, the static route is established from one network entity   to another.  The requirement in this case is only to validate the   forwarding path for that statically established unidirectional path.   Validation of the forwarding path in the direction of the target   entity to the originating entity is not required in this scenario.   Many LSPs have the same unidirectional characteristics and   unidirectional validation requirements.  Such LSPs are common in   Segment Routing and LDP-based MPLS networks.  A final example is when   a unidirectional tunnel uses BFD to validate the reachability of an   egress node.   Additionally, validation of the unidirectional path has operational   implications.  If traditional BFD is to be used, the target network   entity, as well as an initiator, has to be provisioned, even though   reverse-path validation with the BFD session is not required.   However, in the case of unidirectional BFD, there is no need for   provisioning on the target network entity -- only on the source   entity.Aldrin, et al.                Informational                     [Page 5]

RFC 7882                     S-BFD Use Cases                   July 2016   In this use case, a BFD session could be established in a single   direction.  When the target network entity receives the packet, it   identifies the packet as BFD in an application-specific manner (for   example, a destination UDP port number).  Subsequently, the BFD   module processes the packet, using the Your Discriminator value as   context.  Then, the network entity sends a response to the   originator.  This does not necessitate the requirement for   establishment of a bidirectional session; hence, the two-way   handshake to exchange discriminators is not needed.  The target node   does not need to know the My Discriminator value of the source node.   Thus, in this use case a requirement for BFD is to enable session   establishment from the source network entity to the target network   entity without the need to have a session (and state) for the reverse   direction.  Further, another requirement is that the BFD response   from the target back to the sender can take any (in-band or   out-of-band) path.  The BFD module in the target network entity (for   the BFD session), upon receipt of a BFD packet, starts processing the   BFD packet based on the discriminator received.  The source network   entity can therefore establish a unidirectional BFD session without   the bidirectional handshake and exchange of discriminators for   session establishment.3.2.  Validation of the Forwarding Path prior to Switching Traffic   In this use case, BFD is used to verify reachability before sending   traffic via a path/LSP.  This comes at a cost: traffic is prevented   from using the path/LSP until BFD is able to validate reachability;   this could take seconds due to BFD session bring-up sequences   [RFC5880], LSP Ping bootstrapping [RFC5884], etc.  This use case   would be better supported by eliminating the need for the initial BFD   session negotiation.   All it takes to be able to send BFD packets to a target and for the   target to properly demultiplex these packets is for the source   network entities to know what Discriminator values will be used for   the session.  This is also the case for S-BFD: the three-way   handshake mechanism is eliminated during the bootstrapping of BFD   sessions.  However, this information is required at each entity to   verify that BFD messages are being received from the expected   endpoints; hence, the handshake mechanism serves no purpose.   Elimination of the unnecessary handshake mechanism allows for faster   reachability validation of BFD provisioned paths/LSPs.Aldrin, et al.                Informational                     [Page 6]

RFC 7882                     S-BFD Use Cases                   July 2016   In addition, it is expected that some MPLS technologies will require   traffic-engineered LSPs to be created dynamically, perhaps driven by   external applications, as, for example, in Software-Defined   Networking (SDN).  It will be desirable to perform BFD validation as   soon as the LSPs are created, so as to use them.   In order to support this use case, an S-BFD session is established   without the need for session negotiation and exchange of   discriminators.3.3.  Centralized Traffic Engineering   Various technologies in the SDN domain that involve controller-based   networks have evolved such that the intelligence, traditionally   placed in a distributed and dynamic control plane, is separated from   the networking entities themselves; instead, it resides in a   (logically) centralized place.  There are various controllers that   perform the function of establishing forwarding paths for the data   flow.  Traffic engineering is one important function, where the path   of the traffic flow is engineered, depending upon various attributes   and constraints of the traffic paths as well as the network state.   When the intelligence of the network resides in a centralized entity,   the ability to manage and maintain the dynamic network, and its   multiple data paths and node reachability, becomes a challenge.  One   way to ensure that the forwarding paths are valid and working is done   by validation using BFD.  When traffic-engineered tunnels are   created, it is operationally critical to ensure that the forwarding   paths are working, prior to switching the traffic onto the engineered   tunnels.  In the absence of distributed control-plane protocols, it   may be desirable to verify any arbitrary forwarding path in the   network.  With tunnels being engineered by a centralized entity, when   the network state changes, traffic has to be switched with minimum   latency and without black-holing of the data.   It is highly desirable in this centralized traffic-engineering use   case that the traditional BFD session establishment and validation of   the forwarding path do not become a bottleneck.  If the controller or   other centralized entity is able to very rapidly verify the   forwarding path of a traffic-engineered tunnel, it could steer the   traffic onto the traffic-engineered tunnel very quickly, thus   minimizing adverse effects on a service.  This is even more useful   and necessary when the scale of the network and the number of   traffic-engineered tunnels grow.   The cost associated with the time required for BFD session   negotiation and establishment of BFD sessions to identify valid paths   is very high when providing network redundancy is a critical issue.Aldrin, et al.                Informational                     [Page 7]

RFC 7882                     S-BFD Use Cases                   July 20163.4.  BFD in Centralized Segment Routing   A monitoring technique for a Segment Routing network based on a   centralized controller is described in [SR-MPLS].  Specific   Operations, Administration, and Maintenance (OAM) requirements for   Segment Routing are captured in [SR-OAM-REQS].  In validating this   use case, one of the requirements is to ensure that the BFD packet's   behavior is according to the monitoring specified for the segment and   that the packet is U-turned at the expected node.  This criterion   ensures the continuity check to the adjacent Segment Identifier.   To support this use case, the operational requirement is for BFD,   initiated from a centralized controller, to perform liveness   detection for any given segment in its domain.3.5.  Efficient BFD Operation under Resource Constraints   When BFD sessions are being set up, torn down, or modified (i.e.,   when parameters such as intervals and multipliers are being   modified), BFD requires additional packets, other than scheduled   packet transmissions, to complete the negotiation procedures (i.e.,   Poll (P) bits and Final (F) bits; seeSection 4.1 of [RFC5880]).   There are scenarios where network resources are constrained: a node   may require BFD to monitor a very large number of paths, or BFD may   need to operate in low-powered and traffic-sensitive networks; these   include microwave systems, low-powered nanocells, and others.  In   these scenarios, it is desirable for BFD to slow down, speed up,   stop, or resume at will and with a minimal number of additional BFD   packets exchanged to modify the session or establish a new one.   The established BFD session parameters, and such attributes as   transmission interval and receiver interval, need to be modifiable   without changing the state of the session.3.6.  BFD for Anycast Addresses   The BFD protocol requires two endpoints to host BFD sessions, both   sending packets to each other.  This BFD model does not fit well with   anycast address monitoring, as BFD packets transmitted from a network   node to an anycast address will reach only one of potentially many   network nodes hosting the anycast address.   This use case verifies that a source node can send a packet to an   anycast address and that the target node to which the packet is   delivered can send a response packet to the source node.  Traditional   BFD cannot fulfill this requirement, since it does not provide for aAldrin, et al.                Informational                     [Page 8]

RFC 7882                     S-BFD Use Cases                   July 2016   set of BFD agents to collectively form one endpoint of a BFD session.   The concept of a "target listener" in S-BFD fulfills this   requirement.   To support this use case, the BFD sender transmits BFD packets, which   are received by any of the nodes hosting the anycast address to which   the BFD packets are being sent.  The anycast target that receives the   BFD packet responds.  This use case does not imply BFD session   establishment with every node hosting the anycast address.   Consequently, in this anycast use case, target nodes that do not   happen to receive any of the BFD packets do not need to maintain any   state, and the source node does not need to maintain separate state   for each target node.3.7.  BFD Fault Isolation   BFD for multi-hop paths [RFC5883] and BFD for MPLS LSPs [RFC5884]   perform end-to-end validation, traversing multiple network nodes.   BFD has been designed to declare a failure to receive some number of   consecutive packets.  This failure can be caused by a fault anywhere   along these paths.  Fast failure detection allows for rapid fault   detection and consequent rapid path recovery procedures.  However,   operators often have to follow up, manually or automatically, to   attempt to identify and localize the fault that caused BFD sessions   to fail (i.e., fault isolation).  If Equal-Cost Multipath (ECMP) is   used, the usage of other tools to isolate the fault (e.g.,   traceroute) may cause the packets to traverse a different path   through the network.  In addition, the longer it takes from the time   of BFD session failure to the time that fault isolation begins, the   more likely the fault will not be isolated (e.g., a fault may be   corrected via rerouting or some other means during that time).  If   BFD had built-in fault-isolation capability, fault isolation would be   triggered when the fault was first detected.  This embedded fault   isolation would be more effective (i.e., faults would be detected   sooner) if those BFD fault-isolation packets were load-balanced in   the same way as the BFD packets that were dropped.   This use case describes S-BFD fault-isolation capabilities, utilizing   a TTL field (e.g., as described inSection 5.1.1 of [RFC7881]) or   using fields that indicate status.3.8.  Multiple BFD Sessions to the Same Target Node   BFD is capable of providing very fast failure detection, as relevant   network nodes continuously transmit BFD packets at the negotiated   rate.  If BFD packet transmission is interrupted, even for a very   short period of time, BFD can declare a failure irrespective of path   liveness.  On a system where BFD is running, it is possible forAldrin, et al.                Informational                     [Page 9]

RFC 7882                     S-BFD Use Cases                   July 2016   certain events to (intentionally or unintentionally) cause a brief   interruption of BFD packet transmissions.  With distributed   architectures of BFD implementations, this case can be prevented.   This use case is for an S-BFD node running multiple BFD sessions to   the same target node, with those sessions hosted on different system   modules (e.g., in different CPU instances).  This can reduce false   failures, resulting in a more stable network.   To support this use case, a mapping between the multiple   discriminators on a single system and the specific entity within that   system is required.3.9.  An MPLS BFD Session per ECMP Path   BFD for MPLS LSPs, defined in [RFC5884], describes procedures for   running BFD as an LSP in-band continuity check mechanism by using   MPLS Echo Request messages [RFC4379] to bootstrap the BFD session on   the target (i.e., egress) node.Section 4 of [RFC5884] also   describes the possibility of running multiple BFD sessions per   alternative of LSPs.  [RFC7726] further clarifies the procedures, for   both ingress and egress nodes, regarding how to bootstrap, maintain,   and remove multiple BFD sessions for the same <MPLS LSP, FEC> tuple   ("FEC" means Forwarding Equivalence Class).  However, this mechanism   still requires the use of MPLS LSP Ping for bootstrapping,   round trips for initialization, and keeping state at the receiver.   In the presence of ECMP within an MPLS LSP, it may be desirable to   run in-band monitoring that exercises every path of this ECMP.   Otherwise, there will be scenarios where an in-band BFD session   remains up through one path but traffic is black-holing over another   path.  A BFD session per ECMP path of an LSP requires the definition   of procedures that update [RFC5884] in terms of how to bootstrap and   maintain the correct set of BFD sessions on the egress node.   However, for traditional BFD, that requires the constant use of MPLS   Echo Request messages to create and delete BFD sessions on the egress   node when ECMP paths and/or corresponding load-balance hash keys   change.  If a BFD session over any paths of the LSP can be   instantiated, stopped, and resumed without requiring additional   procedures for bootstrapping via an MPLS Echo Request message, it   would greatly simplify both implementations and operations and   would benefit network devices, as less processing would be required   by them.   To support this requirement, multiple S-BFD sessions need to be   established over different ECMP paths between the same pair of source   and target nodes.Aldrin, et al.                Informational                    [Page 10]

RFC 7882                     S-BFD Use Cases                   July 20164.  Detailed Requirements for Seamless BFD   REQ 1:   Upon receipt of an S-BFD packet, a target network entity            (for the S-BFD session) MUST process the packet based on the            discriminator received in the BFD packet.  If the S-BFD            context is found, the target network entity MUST be able to            send a response.   REQ 2:   The source network entity MUST be able to establish a            unidirectional S-BFD session without the bidirectional            handshake of discriminators for session establishment.   REQ 3:   The S-BFD session MUST be able to be established without the            need for the exchange of discriminators during session            negotiation.   REQ 4:   In a Segment Routed network, S-BFD MUST be able to perform            liveness detection initiated from a centralized controller            for any given segment in its domain.   REQ 5:   The established S-BFD session parameters and attributes,            such as transmission interval and reception interval, MUST            be modifiable without changing the state of the session.   REQ 6:   An S-BFD source network entity MUST be able to send Control            packets to an anycast address.  These packets are received            and processed by any node hosting the anycast address.  The            S-BFD entity MUST be able to receive responses to S-BFD            Control packets from any of these anycast nodes, without            establishing a separate S-BFD session with every node            hosting the anycast address.   REQ 7:   S-BFD SHOULD support fault-isolation capability, which MAY            be triggered when a fault is encountered.   REQ 8:   S-BFD SHOULD be able to establish multiple sessions between            the same pair of source and target nodes.  This requirement            enables but does not guarantee the ability to monitor            divergent paths in ECMP environments.  It also provides            resiliency in distributed router architectures.  The mapping            between BFD Discriminators and particular entities (e.g.,            ECMP paths, line cards) is out of scope for the S-BFD            protocol.Aldrin, et al.                Informational                    [Page 11]

RFC 7882                     S-BFD Use Cases                   July 2016   REQ 9:   The S-BFD protocol MUST provide mechanisms for loop            detection and prevention, protecting against malicious            attacks attempting to create packet loops.   REQ 10:  S-BFD MUST incorporate robust security protections against            impersonators, malicious actors, and various active and            passive attacks.  The simple and accelerated establishment            of an S-BFD session should not negatively affect security.5.  Security Considerations   This document details use cases for S-BFD and identifies various   associated requirements.  Some of these requirements are security   related.  The use cases described herein do not expose a system to   abuse or additional security risks.  Since some negotiation aspects   are eliminated, a misconfiguration can result in S-BFD packets being   sent to an incorrect node.  If this receiving node runs S-BFD, the   packet will be discarded due to discriminator mismatch.  If the node   does not run S-BFD, the packet will be naturally discarded.   The proposed new protocols, extensions, and enhancements for S-BFD   supporting these use cases and realizing these requirements will   address associated security considerations.  S-BFD should not have   reduced security capabilities as compared to traditional BFD.6.  References6.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <http://www.rfc-editor.org/info/rfc2119>.   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection              (BFD)",RFC 5880, DOI 10.17487/RFC5880, June 2010,              <http://www.rfc-editor.org/info/rfc5880>.   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection              (BFD) for IPv4 and IPv6 (Single Hop)",RFC 5881,              DOI 10.17487/RFC5881, June 2010,              <http://www.rfc-editor.org/info/rfc5881>.   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection              (BFD) for Multihop Paths",RFC 5883, DOI 10.17487/RFC5883,              June 2010, <http://www.rfc-editor.org/info/rfc5883>.Aldrin, et al.                Informational                    [Page 12]

RFC 7882                     S-BFD Use Cases                   July 2016   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,              "Bidirectional Forwarding Detection (BFD) for MPLS Label              Switched Paths (LSPs)",RFC 5884, DOI 10.17487/RFC5884,              June 2010, <http://www.rfc-editor.org/info/rfc5884>.   [RFC5885]  Nadeau, T., Ed., and C. Pignataro, Ed., "Bidirectional              Forwarding Detection (BFD) for the Pseudowire Virtual              Circuit Connectivity Verification (VCCV)",RFC 5885,              DOI 10.17487/RFC5885, June 2010,              <http://www.rfc-editor.org/info/rfc5885>.6.2.  Informative References   [RFC791]   Postel, J., "Internet Protocol", STD 5,RFC 791,              DOI 10.17487/RFC791, September 1981,              <http://www.rfc-editor.org/info/rfc791>.   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6              (IPv6) Specification",RFC 2460, DOI 10.17487/RFC2460,              December 1998, <http://www.rfc-editor.org/info/rfc2460>.   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol              Label Switching Architecture",RFC 3031,              DOI 10.17487/RFC3031, January 2001,              <http://www.rfc-editor.org/info/rfc3031>.   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol              Label Switched (MPLS) Data Plane Failures",RFC 4379,              DOI 10.17487/RFC4379, February 2006,              <http://www.rfc-editor.org/info/rfc4379>.   [RFC7726]  Govindan, V., Rajaraman, K., Mirsky, G., Akiya, N., and S.              Aldrin, "Clarifying Procedures for Establishing BFD              Sessions for MPLS Label Switched Paths (LSPs)",RFC 7726,              DOI 10.17487/RFC7726, January 2016,              <http://www.rfc-editor.org/info/rfc7726>.   [RFC7880]  Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.              Pallagatti, "Seamless Bidirectional Forwarding Detection              (S-BFD)",RFC 7880, DOI 10.17487/RFC7880, July 2016,              <http://www.rfc-editor.org/info/rfc7880>.   [RFC7881]  Pignataro, C., Ward, D., and N. Akiya, "Seamless              Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6,              and MPLS",RFC 7881, DOI 10.17487/RFC7881, July 2016,              <http://www.rfc-editor.org/info/rfc7881>.Aldrin, et al.                Informational                    [Page 13]

RFC 7882                     S-BFD Use Cases                   July 2016   [SR-ARCH]  Filsfils, C., Ed., Previdi, S., Ed., Decraene, B.,              Litkowski, S., and R. Shakir, "Segment Routing              Architecture", Work in Progress,draft-ietf-spring-segment-routing-09, July 2016.   [SR-MPLS]  Geib, R., Ed., Filsfils, C., Pignataro, C., Ed., and N.              Kumar, "A Scalable and Topology-Aware MPLS Dataplane              Monitoring System", Work in Progress,draft-ietf-spring-oam-usecase-03, April 2016.   [SR-OAM-REQS]              Kumar, N., Pignataro, C., Akiya, N., Geib, R., Mirsky, G.,              and S. Litkowski, "OAM Requirements for Segment Routing              Network", Work in Progress,draft-ietf-spring-sr-oam-requirement-02, July 2016.Aldrin, et al.                Informational                    [Page 14]

RFC 7882                     S-BFD Use Cases                   July 2016Acknowledgements   The authors would like to thank Tobias Gondrom and Eric Gray for   their insightful and useful comments.  The authors appreciate the   thorough review and comments provided by Dale R. Worley.Contributors   The following are key contributors to this document:      Manav Bhatia, Ionos Networks      Satoru Matsushima, Softbank      Glenn Hayden, ATT      Santosh P K      Mach Chen, Huawei      Nobo Akiya, Big Switch NetworksAuthors' Addresses   Sam Aldrin   Google, Inc.   Email: aldrin.ietf@gmail.com   Carlos Pignataro   Cisco Systems, Inc.   Email: cpignata@cisco.com   Greg Mirsky   Ericsson   Email: gregory.mirsky@ericsson.com   Nagendra Kumar   Cisco Systems, Inc.   Email: naikumar@cisco.comAldrin, et al.                Informational                    [Page 15]