Network Working Group                                          S. Aldrin
Internet-Draft                                               Google, Inc
Intended status: Informational                                 M. Bhatia
Expires: February 1, September 22, 2016                               Ionos Networks
                                                           S. Matsushima
                                                               G. Mirsky
                                                                N. Kumar
                                                           July 31, 2015
                                                          March 21, 2016

       Seamless Bidirectional Forwarding Detection (BFD) Use Case


   This document provides various use cases for Bidirectional Forwarding
   Detection (BFD) and various requirements such that extensions could
   be developed to allow for simplified detection of forwarding

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Introduction to Seamless BFD  . . . . . . . . . . . . . . . .   3
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Unidirectional Forwarding Path Validation . . . . . . . .   4
     3.2.  Validation of forwarding path prior to traffic switching    5
     3.3.  Centralized Traffic Engineering . . . . . . . . . . . . .   5   6
     3.4.  BFD in Centralized Segment Routing  . . . . . . . . . . .   6
     3.5.  BFD  Efficient BFD Operation Under Resource Constraints  . . .   6   7
     3.6.  BFD for Anycast Address . . . . . . . . . . . . . . . . .   7
     3.7.  BFD Fault Isolation . . . . . . . . . . . . . . . . . . .   7
     3.8.  Multiple BFD Sessions to Same Target  . . . . . . . . . .   7   8
     3.9.  MPLS BFD Session Per ECMP Path  . . . . . . . . . . . . .   7   8
   4.  Detailed Requirements . . . . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  10
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  10
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10  12

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 for failure detection.  Even
   though the protocol is simple and lightweight, simple, there are certain use cases, where
   faster setting up of sessions and continuity check of the data
   forwarding paths is necessary.  This document identifies various use
   cases and requirements related to those, such that necessary
   enhancements could be made to BFD protocol
   to meet those requirements. protocol.

   BFD was designed to be 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 creates
   complexity, not only from an operations point of view, but also in
   terms of the speed at which these sessions could be established or
   deleted.  The existing session establishment mechanism of the BFD
   protocol need has to be enhanced in order to minimize the time for the
   session to come up and to validate the forwarding path.

   This document specifically identifies those various use cases where certain and
   corresponding requirements could be derived to be used as reference, so that,
   protocol enhancements could be developed in order to address them. enhance BFD and other
   supporting protocols.  While the identified requirements could meet
   various use cases could be used as reference for certain requirements, , it is outside the scope of this document to
   identify all of the
   requirements for all possible enhancements.  Specific solutions and
   enhancement necessary requirements.  Solutions
   to the identified uses cases and protocol specific enhancements or
   proposals are outside the scope of this document as well.

1.1.  Terminology

   The reader is expected to be familiar with the BFD, IP, MPLS and
   Segment Routing (SR) [I-D.ietf-spring-segment-routing] terminology
   and protocol constructs.  This section identifies only the new
   terminology introduced.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

2.  Introduction to Seamless BFD

   BFD, as defined in [RFC5880], requires two network nodes, to exchange
   locally allocated discriminators.  The discriminator enables
   identification of the sender and receiver of BFD packets of the
   particular session and perform proactive continuity monitoring of the
   forwarding path between the two.  [RFC5881] defines single hop BFD
   whereas [RFC5883]  defines multi-hop BFD, [RFC5884] BFD for MPLS
   LSPs, and [RFC5885] - BFD for PWs.

   Currently, BFD is best suited to verify that two end points are
   reachable or that an existing connection continues to be valid. 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 end
   points, it is necessary to provide the node information associated
   with the connection at each end point prior to initiating BFD
   sessions, such that this information can be used to verify that the
   connection is
   valid. up and verifiable.

   If this information is already known to the end-points of a potential
   BFD session, the initial handshake including an exchange of this
   node-specific information is unnecessary and it is possible for the
   end points to begin BFD messaging seamlessly.  In fact, the initial
   exchange of discriminator information is an unnecessary extra step
   that may be avoided for these cases.

   As an example of how Seamless BFD (S-BFD) might work,

   In a given scenario, where an entity (such as an operator, or
   centralized controller) determines a set of network entities to which
   BFD sessions might need to be established.  Each of those network
   entities is assigned a BFD discriminator, to establish a BFD session.
   These network entities will create a BFD session instance that
   listens for incoming BFD control packets.  Mappings between selected
   network entities and corresponding BFD discriminators are known to
   other network nodes belonging in the same network by some means.  A
   network entity in this network is then able to send a BFD control
   packet to a particular target with the corresponding BFD
   discriminator.  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. requirements identified.  In addition, some of
   the use cases also be identify stress the need for expedited BFD session
   establishment while preserving benefits of forwarding failure
   detection using existing BFD specifications.

3.1.  Unidirectional Forwarding Path Validation

   Even though bidirectional verification of forwarding path is useful,
   there are scenarios when where verification is only required in one
   direction between a pair of nodes.  One such case is 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 path, and validation
   by path.  Validation of
   the forwarding path in the direction of the target entity to the
   originating entity is not required. 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
   networks.  Another example is when a unidirectional tunnel uses BFD
   to validate reachability of an egress node.

   If the traditional BFD is to be used, the target network entity has
   to be provisioned as well, even though the reverse path validation
   with BFD session is not required.  But with  However, in the case of
   unidirectional BFD, the there is no need to provision for provisioning on the target
   network entity is not needed. .  Once the mechanism within the BFD protocol is in
   place, where the source
   network entity knows the target network entity's discriminator, it
   starts the session right away. could be established in a single direction.  When the
   targeted network entity receives the packet, it knows that BFD
   packet, based on the discriminator and processes it.  That  This does not require
   necessitates the requirement for establishment of a bi-directional
   session, hence the two way handshake to exchange discriminators is
   not needed as well.

   The primary needed.

   Thus the requirement in for BFD for this use case is to enable session
   establishment from source network entity to target network entity.

   This translates to a entity
   without the need for to have a session in the reverse direction.  This
   requires to ensure that the target network entity (for the BFD
   session), should upon receipt of BFD packet, MUST start processing for the
   discriminator received in the BFD packet.  This will enable the  The source network entity
   MUST be able to establish a unidirectional BFD session without the
   bidirectional handshake of discriminators for session establishment.

3.2.  Validation of forwarding path prior to traffic switching

   BFD provides data delivery confidence when reachability validation is
   performed prior to traffic utilizing specific paths/LSPs.  However
   this comes with a cost, where, traffic is prevented to use such
   paths/LSPs 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 does not
   require to have sequences could be well
   supported by eliminating the need for session negotiation and
   discriminator exchanges in order to establish the BFD session.

   When these sequences

   All it takes is for handshake are eliminated, the network entities need to know what the
   discriminator values to be used for the session.  The same is the
   case for S-BFD, i.e., when the three-
   way three-way handshake mechanism is eliminated
   during 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, 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, e.g. in Software Defined Networks (SDN).  It
   will be desirable to perform BFD validation very quickly as soon as the LSP?s are
   created, in order to allow
   applications use them.

   In order to utilize dynamically created LSPs in a timely manner. support this use case, the BFD session MUST be able to be
   established without the need for session negotiation and exchange of

3.3.  Centralized Traffic Engineering

   Various technologies in the SDN domain that involve controller based
   networks have evolved where intelligence, traditionally placed in a
   distributed and dynamic control plane, is separated from the
   networking entities along the data
   plane and path, instead resides in a
   logically centralized place.  There are various controllers that
   perform this exact function in establishing establishment of forwarding paths for
   the data flow.  Traffic engineering is one important function, where
   the traffic flow is engineered 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,
   ability to manage and maintain the dynamic network becomes a
   challenge.  One way to ensure the forwarding paths are valid, and
   working, is to establish BFD sessions within done by validation of the network. network using BFD.  When
   traffic engineered tunnels are created, it is operationally critical
   to ensure that the forwarding paths are working working, prior to switching
   the traffic onto the engineered tunnels.  In the absence of control
   plane protocols, it may be desirable to verify verify, not only the
   forwarding path but also of any arbitrary 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.

   Traditional BFD session establishment and validation of the
   forwarding path must not become a bottleneck in the case of
   centralized traffic engineering.  If the controller or other
   centralized entity is able to instantly verify a forwarding path of
   the TE tunnel , it could steer the traffic onto the traffic
   engineered tunnel very quickly thus minimizing adverse effect on a
   service.  This is especially useful and needed when the scale of the
   network and number of TE tunnels is very high.

   The cost associated with BFD session negotiation and establishment of
   BFD sessions to identify valid paths is very high and providing
   network redundancy becomes a critical issue.

3.4.  BFD in Centralized Segment Routing

   A centralized controller based monitoring technique of a Segment Routing network monitoring
   technique based on a
   centralized controller is described in [I-D.geib-spring-oam-usecase]. [I-D.ietf-spring-oam-usecase].
   Various OAM requirements for Segment Routing were captured in
   [I-D.ietf-spring-sr-oam-requirement].  In validating this use case,
   one of the requirements is to ensure the BFD packet's behavior is
   according to the requirement and monitoring of the segment, where the
   packet is U-turned at the expected node.  One of the criterion is to
   ensure the continuity check to the adjacent segment-id.


   To support this use case, BFD MUST be able to perform liveness
   detection initated from centralized controller 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.
   parameters ? such as interval, multiplier, etc 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,
   i.e. microwave, low powered nano-cells, etc.  In these scenarios, it
   is desirable for BFD to slow down, speed up, stop or resume at will
   witho minimal additional BFD packets exchanged to establish a new or
   modified session.

   The established BFD session parameters and attributes like
   transmission interval, receiver interval, etc., MUST be modifiable
   without changing the state of the session.

3.6.  BFD for Anycast Address

   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.

   To support this use case, the BFD MUST be able to send packets in
   order to be received by any of nodes hosting anycast address to which
   the BFD packets being sent and to respond.  This requirement does not
   require BFD session establishment with every node hosting the anycast

3.7.  BFD Fault Isolation

   BFD multi-hop and [RFC5883]and BFD MPLS [RFC5884] traverse 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 the path.  Fast failure detection allows for 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.  Usage of other tools to
   isolate the fault may cause the packets to traverse a different path
   through the network (e.g. if ECMP is used).  In addition, the longer
   it takes from BFD session failure to fault isolation attempt, more
   likely that the fault cannot 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 and such packets will get load balanced in very similar way, if
   not the same, as BFD packets that went missing.

   To support this requirement, BFD SHOULD support fault isolation
   capability using status indicating fields, when encountered.

3.8.  Multiple BFD Sessions to Same Target

   BFD is capable of providing very fast failure detection, as relevant
   network nodes continuously transmitting transmit BFD packets at negotiated rate.
   If BFD packet transmission is interrupted, even for a very short
   period of time, that can result in BFD to declare 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 can be
   protected, if a node was to run multiple BFD sessions to targets,
   hosted on different parts of the system (ex: different CPU
   instances).  This can reduce BFD false failures, resulting in more
   stable network.

3.9.  MPLS BFD Session Per ECMP Path

   BFD for MPLS, 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 egress node.
   Section 4 of [RFC5884] also describes a possibility of running
   multiple BFD sessions per alternative paths of LSP.  However, details
   on how to bootstrap and maintain correct set of BFD sessions on the
   egress node is absent.

   When an LSP has ECMP segment, it may be desirable to run in-band
   monitoring that exercises every path of ECMP.  Otherwise there will
   be scenarios where in-band BFD session remains up through one path
   but traffic is black-holing over another path.  One way to achieve  BFD session per ECMP
   path of LSP is to define requires definition of procedures that update [RFC5884]
   in terms of how to bootstrap and maintain correct set of BFD sessions
   on the egress node.  However, that may require 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 MPLS echo request, it would simplify
   implementations and operations, and benefits network devices as less
   processing are required by them.

4.  Security Considerations

   There are no new security considerations associated with

   To support this draft.

5.  IANA Considerations

   There are no requirement, multiple BFD sessions MUST be able to be
   established over different ECMP paths from the same source to target

4.  Detailed Requirements

   REQ#1- A target network entity (for the BFD session), upon receipt of
   BFD packet, MUST start processing for the discriminator received in
   the BFD packet.

   REQ#2- The source network entity MUST be able to establish a
   unidirectional BFD session without the bidirectional handshake of
   discriminators for session establishment.

   REQ#3 - The BFD session MUST be able to be established without the
   need for session negotiation and exchange of discriminators.

   REQ#4 - BFD MUST be able to perform liveness detection initated from
   centralized controller for any given segment under its domain.

   REQ#5 - The established BFD session parameters and attributes like
   transmission interval, receiver interval, etc., MUST be modifiable
   without changing the state of the session.

   REQ#6 - The BFD MUST be able to send and receive response to control
   packets addressed to an anycast address to be received by any of
   nodes hosting that address.  This requirement does not require BFD
   session establishment with every node hosting the anycast address.

   REQ#7 - BFD SHOULD support fault isolation capability and to indicate
   the same, when fault is encountered.

   REQ#8 - BFD MUST be able to establish multiple sessions between the
   same pair of source and target nodes.  This requirement enables but
   does not guarantee ability to monitor diverge paths in ECMP
   environment.  The mapping between BFD session and particular ECMP
   path is out the scope of BFD specification.

5.  Security Considerations

   This document details the use cases and identifies various
   requirements for the same.  As this document do not propose any new
   protocol or changes to the existing ones, no new security
   considerations have been identified with this draft.

6.  IANA Considerations

   There are no IANA considerations introduced by this draft


7.  Contributors

   Carlos Pignataro

   Cisco Systems


   Glenn Hayden



   Santosh P K



   Mach Chen



   Nobo Akiya

   Cisco Systems



8.  Acknowledgements

   The authors would like to thank Eric Gray for his useful comments.


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,

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              DOI 10.17487/RFC4379, February 2006,

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
              DOI 10.17487/RFC5881, June 2010,

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
              June 2010, <>.

   [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, <>.

   [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,


9.2.  Informative References


              Geib, R., Filsfils, C., Pignataro, C., and N. Kumar, "Use
              Case for a scalable Scalable and topology aware Topology-Aware Segment Routing
              MPLS data plane
              monitoring system", draft-geib-spring-oam-usecase-06 Data Plane Monitoring System", draft-ietf-spring-oam-
              usecase-01 (work in progress), October 2015.

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

              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), July December 2015.

Authors' Addresses

   Sam Aldrin
   Google, Inc
   1600 Amphitheatre Parkway
   Mountain View, CA


   Manav Bhatia
   Ionos Networks


   Satoru Matsushima


   Greg Mirsky


   Nagendra Kumar