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Versions: (draft-aldrin-bfd-seamless-use-case) 00 01 02 03 04 05 06 07 08 RFC 7882

Network Working Group                                          S. Aldrin
Internet-Draft                                               Google, Inc
Intended status: Informational                              C. Pignataro
Expires: November 7, 2016                                          Cisco
                                                               G. Mirsky
                                                                Ericsson
                                                                N. Kumar
                                                                   Cisco
                                                             May 6, 2016


     Seamless Bidirectional Forwarding Detection (S-BFD) Use Cases
                  draft-ietf-bfd-seamless-use-case-08

Abstract

   This document describes various use cases for a Seamless
   Bidirectional Forwarding Detection (S-BFD), and provides requirements
   such that protocol mechanisms allow for a simplified detection of
   forwarding failures.

   These use cases support S-BFD, as a simplified mechanism to use
   Bidirectional Forwarding Detection (BFD) with large portions of
   negotiation aspects eliminated, accelerating the establishment of a
   BFD session.  S-BFD benefits include quick provisioning as well as
   improved control and flexibility to network nodes initiating the path
   monitoring.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   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 November 7, 2016.







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Copyright Notice

   Copyright (c) 2016 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
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Introduction to Seamless BFD  . . . . . . . . . . . . . . . .   4
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Unidirectional Forwarding Path Validation . . . . . . . .   5
     3.2.  Validation of the Forwarding Path Prior to Switching
           Traffic . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Centralized Traffic Engineering . . . . . . . . . . . . .   7
     3.4.  BFD in Centralized Segment Routing  . . . . . . . . . . .   8
     3.5.  Efficient BFD Operation under Resource Constraints  . . .   8
     3.6.  BFD for Anycast Addresses . . . . . . . . . . . . . . . .   8
     3.7.  BFD Fault Isolation . . . . . . . . . . . . . . . . . . .   9
     3.8.  Multiple BFD Sessions to the Same Target Node . . . . . .   9
     3.9.  An MPLS BFD Session Per ECMP Path . . . . . . . . . . . .  10
   4.  Detailed Requirements for a Seamless BFD  . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Bidirectional Forwarding Detection (BFD) is a lightweight protocol,
   as defined in [RFC5880], used to detect forwarding failures.  Various
   protocols and applications rely on BFD as its clients for failure
   detection.  Even though the protocol is lightweight and simple, there



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   are certain use cases where faster setting up of sessions and faster
   continuity check of the data forwarding paths is 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 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 a network node wants to perform
   a continuity test and 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 to the identified uses 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 at [I-D.ietf-bfd-seamless-base] and
   [I-D.ietf-bfd-seamless-ip].

1.1.  Terminology

   The reader is expected to be familiar with the BFD [RFC5880], IP
   [RFC0791] [RFC2460], MPLS [RFC3031], and Segment Routing (SR)
   [I-D.ietf-spring-segment-routing] terminologies and protocol
   constructs.

1.2.  Requirements Language

   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].







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2.  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:

      [RFC5881] defines BFD over IPv4 and IPv6 for single IP hops

      [RFC5883]  defines BFD over multihop paths

      [RFC5884] defines BFD for MPLS Label Switched Paths (LSPs)

      [RFC5885] defines BFD for MPLS Pseudowires (PWs)

   Currently, BFD is best suited to verify 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 verifiable.  Currently, the exchange of
   discriminators and the demultiplexing of the initial BFD packets is
   application dependent.

   If this information is already known to the end-points 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 (see Section 6.3 of
   [RFC5880]).  However, a key goal of a Seamless 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 allowing those network entities to use one discriminator value
   for multiple sessions.  Therefore, there may be only one or a few



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   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 packet 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
   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
   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 benefits of forwarding failure
   detection using existing BFD mechanics.  Both these high-level goals
   result in the S-BFD use cases.

3.1.  Unidirectional Forwarding Path Validation

   Even though bidirectional verification of forwarding path 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 reachability of an
   egress node.

   Additionally, there are operational implications to the
   unidirectional path validation.  If the traditional BFD is to be
   used, the target network entity has to be provisioned as well as an
   initiator, even though the 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
   the source one.




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   In this use case, a BFD session could be established in a single
   direction.  When the targeted network entity receives the packet, it
   identities 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 bi-directional session, hence the two way
   handshake to exchange discriminators is not needed.  The target node
   does not need to know the My Discriminator of the source node.

   Thus, a requirement for BFD for this use case is to enable session
   establishment from source network entity to 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 target back to 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 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

   This use case is when BFD is used to verify reachability before
   sending traffic via a path/LSP.  This comes with a cost, which is
   that traffic is prevented to use the path/LSP until BFD is able to
   validate the reachability, which 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 the
   target properly demultiplexing these, is for the source network
   entities to know what the discriminator values to be used for the
   session.  The same is the case for S-BFD: the three-way handshake
   mechanism is eliminated during the bootstrap of BFD sessions.
   However, this information is required at each entity to verify that
   BFD messages are being received from the expected end-points, hence
   the handshake mechanism serves no purpose.  Elimination of the
   unnecessary handshake mechanism allows for faster reachability
   validation of BFD provisioned paths/LSPs.

   In addition, it is expected that some MPLS technologies will require
   traffic engineered LSPs to be created dynamically, perhaps driven by
   external applications, as e.g. in Software Defined Networks (SDN).




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   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 in establishment of forwarding paths for the
   data flow.  Traffic engineering (TE) 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 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 does 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 effect on a service.  This is even more useful and
   necessary when the scale of the network and number of traffic
   engineered tunnels grows.

   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.





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3.4.  BFD in Centralized Segment Routing

   A monitoring technique of a Segment Routing network based on a
   centralized controller is described in [I-D.ietf-spring-oam-usecase].
   Specific OAM requirements for Segment Routing are captured in
   [I-D.ietf-spring-sr-oam-requirement].  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 criteria ensures
   the continuity check to the adjacent segment-id.

   To support this use case, the operational requirement is for BFD,
   initiated from a centralized controller, to perform liveness
   detection for any given segment under its domain.

3.5.  Efficient BFD Operation under Resource Constraints

   When BFD sessions are being setup, torn down or modified (i.e., when
   parameters such as interval and multiplier are being modified), BFD
   requires additional packets other than scheduled packet transmissions
   to complete the negotiation procedures (i.e., P/F bits).  There are
   scenarios where network resources are constrained: a node may require
   BFD to monitor very large number of paths, or BFD may need to operate
   in low powered and traffic sensitive networks; these include
   microwave, low powered nano-cells, and others.  In these scenarios,
   it is desirable for BFD to slow down, speed up, stop, or resume at-
   will and with minimal number of additional BFD packets exchanged to
   modify the session or establish a new one.

   The established BFD session parameters and attributes like
   transmission interval, receiver interval, etc., 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 a
   set of BFD agents to collectively form one endpoint of a BFD session.
   The concept of a Target Listener in S-BFD solves this requirement.




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   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 being sent.  The anycast target that receives the BFD
   packet, responds.  This use case does not imply the BFD session
   establishment with every node hosting the anycast address.
   Consequently, in this any cast 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 multihop paths [RFC5883] and BFD for MPLS LSPs [RFC5884]
   perform end-to-end validation, traversing multiple network nodes.
   BFD has been designed to declare failure upon lack of consecutive
   packet reception, which can be caused by a fault anywhere along these
   path.  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).  The usage of other tools to isolate the fault (e.g.,
   traceroute) may cause the packets to traverse a different path
   through the network, if Equal-Cost Multipath (ECMP) is used.  In
   addition, the longer it takes from BFD session failure to starting
   fault isolation, the more likely that the fault will not be able to
   be isolated (e.g., a fault can get corrected or routed around).  If
   BFD had built-in fault isolation capability, fault isolation can get
   triggered at the earliest sign of fault detection.  This embedded
   fault isolation will be more effective when those BFD fault isolation
   packets are load balanced in the same way as the BFD packets that
   were dropped, detecting the fault.

   This use case describes S-BFD fault isolation capabilities, utilizing
   a TTL field (e.g., as in Section 5.1.1 of [I-D.ietf-bfd-seamless-ip])
   or using status indicating fields.

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
   liveliness.  It is possible, on a system where BFD is running, for
   certain events (intentionally or unintentionally) to cause a short
   interruption of BFD packet transmissions.  With distributed
   architectures of BFD implementations, this case can be protected.  In
   this case, the use case of an S-BFD node running multiple BFD
   sessions to a targets, with those sessions hosted on different system



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   modules (e.g., in different CPU instances).  This can reduce BFD
   false failures, resulting in more stable network.

   To support this use case, a mapping between the multiple
   discriminators on a single system, and the specific entity within the
   system is required.

3.9.  An MPLS BFD Session Per ECMP Path

   BFD for MPLS LSPs, defined in [RFC5884], describes procedures to run
   BFD as LSP in-band continuity check mechanism, through usage of MPLS
   echo request [RFC4379] to bootstrap the BFD session on the target
   (i.e., egress) node.  Section 4 of [RFC5884] also describes a
   possibility of running multiple BFD sessions per alternative paths of
   LSP.  [RFC7726] further clarified the procedures, both for ingress
   and egress nodes, of how to bootstrap, maintain, and remove multiple
   BFD sessions for the same <MPLS LSP, FEC> tuple.  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 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 of bootstrapping via an MPLS echo request message, it
   would greatly simplify both implementations and operations, and
   benefits network devices as less processing are required by them.

   To support this requirement, multiple S-BFD sessions need to be
   established over different ECMP paths from the same source to target
   node.

4.  Detailed Requirements for a Seamless BFD

   REQ#1:   A target network entity (for the S-BFD session), upon
            receipt of the S-BFD packet, 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.




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   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 exchange of discriminators in 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 under its domain.

   REQ#5:   The established S-BFD session parameters and attributes,
            such as transmission interval, reception interval, etc.,
            MUST be modifiable without changing the state of the
            session.

   REQ#6:   An S-BFD source network entity MUST be able to send S-BFD
            control packets to an anycast address which are received by
            any node hosting that address, and must be able to receive
            responses from any of these anycast nodes, without
            establishing a separate BFD session with every node hosing
            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
            diverge paths in ECMP environments.  It also provides
            resiliency in distributed router architectures.  The mapping
            between BFD discriminators and particular entities (e.g.,
            ECMP paths, or Line Cards) is out the scope of the S-BFD
            specification.

   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, malicions actors, and various active and
            passive attacks.  The simple and accelerated establishment
            of an S-BFD session should not negatively affect security.








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5.  Security Considerations

   This document details the use cases and identifies various associated
   requirements.  Some of these requirements are security related.  The
   use cases herein described do not expose a system to abuse or to
   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 discarted because of the discriminator mismatch.  If the node
   does not run S-BFD, the packet will be naturally discarded.

   The proposed new protocols, extensions, and enhancements for a
   Seamless BFD supporting these use cases and realizing these
   requirements will address the associated security considerations.  A
   Seamless BFD should not have reduced security capabilities as
   compared to traditional BFD.

6.  IANA Considerations

   There are no IANA considerations introduced by this document.

7.  Acknowledgements

   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.

8.  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 Networks

9.  References

9.1.  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>.





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   [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>.

   [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>.

9.2.  Informative References

   [I-D.ietf-bfd-seamless-base]
              Akiya, N., Pignataro, C., Ward, D., Bhatia, M., and J.
              Networks, "Seamless Bidirectional Forwarding Detection
              (S-BFD)", draft-ietf-bfd-seamless-base-09 (work in
              progress), April 2016.

   [I-D.ietf-bfd-seamless-ip]
              Akiya, N., Pignataro, C., and D. Ward, "Seamless
              Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6
              and MPLS", draft-ietf-bfd-seamless-ip-04 (work in
              progress), April 2016.

   [I-D.ietf-spring-oam-usecase]
              Geib, R., Filsfils, C., Pignataro, C., and N. Kumar, "A
              Scalable and Topology-Aware MPLS Dataplane Monitoring
              System", draft-ietf-spring-oam-usecase-03 (work in
              progress), April 2016.








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   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
              and R. Shakir, "Segment Routing Architecture", draft-ietf-
              spring-segment-routing-07 (work in progress), December
              2015.

   [I-D.ietf-spring-sr-oam-requirement]
              Kumar, N., Pignataro, C., Akiya, N., Geib, R., Mirsky, G.,
              and S. Litkowski, "OAM Requirements for Segment Routing
              Network", draft-ietf-spring-sr-oam-requirement-01 (work in
              progress), December 2015.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, 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>.

Authors' Addresses

   Sam Aldrin
   Google, Inc

   Email: aldrin.ietf@gmail.com


   Carlos Pignataro
   Cisco Systems, Inc.

   Email: cpignata@cisco.com



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   Greg Mirsky
   Ericsson

   Email: gregory.mirsky@ericsson.com


   Nagendra Kumar
   Cisco Systems, Inc.

   Email: naikumar@cisco.com









































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