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

       Seamless Bidirectional Forwarding Detection (BFD) Use Case


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

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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 February 1, 2016.

Copyright Notice

   Copyright (c) 2015 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
   ( in effect on the date of
   publication of this document.  Please review these documents

Aldrin, et al.          Expires February 1, 2016                [Page 1]

Internet-Draft               S-BFD Use Case                    July 2015

   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   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
     3.4.  BFD in Centralized Segment Routing  . . . . . . . . . . .   6
     3.5.  BFD Efficient Operation Under Resource Constraints  . . .   6
     3.6.  BFD for Anycast Address . . . . . . . . . . . . . . . . .   7
     3.7.  BFD Fault Isolation . . . . . . . . . . . . . . . . . . .   7
     3.8.  Multiple BFD Sessions to Same Target  . . . . . . . . . .   7
     3.9.  MPLS BFD Session Per ECMP Path  . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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

   BFD was designed to be a 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

Aldrin, et al.          Expires February 1, 2016                [Page 2]

Internet-Draft               S-BFD Use Case                    July 2015

   of the BFD protocol need to be enhanced in order to minimize the time
   for the session to come up and validate the forwarding path.

   This document specifically identifies those cases where certain
   requirements could be derived to be used as reference, so that,
   protocol enhancements could be developed to address them.  While the
   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 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) terminology and protocol constructs.  This
   section identifies only the new terminology introduced.

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 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.  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

   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, 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

Aldrin, et al.          Expires February 1, 2016                [Page 3]

Internet-Draft               S-BFD Use Case                    July 2015

   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.  In addition, some of the use
   cases also be identify 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 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 path, and validation
   by the target entity to the originating entity is not required.  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 unidirectional BFD, the
   need to provision 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.  When the targeted network entity
   receives the packet, it knows that BFD packet, based on the
   discriminator and processes it.  That does not require establishment
   of a bi-directional session, hence the two way handshake to exchange
   discriminators is not needed as well.

   The primary requirement in this use case is to enable session
   establishment from source network entity to target network entity.

Aldrin, et al.          Expires February 1, 2016                [Page 4]

Internet-Draft               S-BFD Use Case                    July 2015

   This translates to a need for the target network entity (for the BFD
   session), should start processing for the discriminator received in
   the BFD packet.  This will enable the source network entity 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 for session negotiation and discriminator
   exchanges in order to establish the BFD session.

   When these sequences 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 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, 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 to allow
   applications to utilize dynamically created LSPs in a timely manner.

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 data
   plane and resides in a logically centralized place.  There are
   various controllers that perform this exact function in establishing
   forwarding paths for the data flow.  Traffic engineering is one
   important function, where the traffic flow is engineered depending
   upon various attributes of the traffic 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 the network.  When

Aldrin, et al.          Expires February 1, 2016                [Page 5]

Internet-Draft               S-BFD Use Case                    July 2015

   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 control
   plane protocols, it may be desirable to verify 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 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 Segment Routing network monitoring
   technique is described in [I-D.geib-spring-oam-usecase].  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.

3.5.  BFD Efficient 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.

Aldrin, et al.          Expires February 1, 2016                [Page 6]

Internet-Draft               S-BFD Use Case                    July 2015

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.

3.7.  BFD Fault Isolation

   BFD multi-hop and BFD MPLS 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.

3.8.  Multiple BFD Sessions to Same Target

   BFD is capable of providing very fast failure detection, as relevant
   network nodes continuously transmitting 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

Aldrin, et al.          Expires February 1, 2016                [Page 7]

Internet-Draft               S-BFD Use Case                    July 2015

   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 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 this draft.

5.  IANA Considerations

   There are no IANA considerations introduced by this draft

6.  Contributors

   Carlos Pignataro

   Cisco Systems


   Glenn Hayden



   Santosh P K



   Mach Chen

Aldrin, et al.          Expires February 1, 2016                [Page 8]

Internet-Draft               S-BFD Use Case                    July 2015



   Nobo Akiya

   Cisco Systems


7.  Acknowledgements

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

8.  References

8.1.  Normative References

   [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,

Aldrin, et al.          Expires February 1, 2016                [Page 9]

Internet-Draft               S-BFD Use Case                    July 2015

8.2.  Informative References

              Geib, R., Filsfils, C., Pignataro, C., and N. Kumar, "Use
              case for a scalable and topology aware MPLS data plane
              monitoring system", draft-geib-spring-oam-usecase-06 (work
              in progress), July 2015.

Authors' Addresses

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


   Manav Bhatia
   Ionos Networks


   Satoru Matsushima


   Greg Mirsky


   Nagendra Kumar


Aldrin, et al.          Expires February 1, 2016               [Page 10]