INTERNET-DRAFT                                                Sam Aldrin
Intended Status: Informational                                  (Huawei)
Expires: December 13, 2014                                  Manav Bhatia
                                                             Greg Mirsky
                                                          Nagendra Kumar
                                                       Satoru Matsushima

                                                           June 11, 2014

      Seamless Bidirectional Forwarding Detection (BFD) Use Case


   This document provides various use cases for Bidirectional Forwarding
   Detection (BFD) such that simplified solution and extensions could be
   developed for detecting forwarding failures.

Status of this Memo

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Copyright and License Notice

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   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   ( in effect on the date of
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   described in the Simplified BSD License.

Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     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 to Efficiently Operate 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  . . . . . . . . . . . . . .  8
   4. Security Considerations . . . . . . . . . . . . . . . . . . . .  9
   5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . .  9
   6. References  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1. Normative References  . . . . . . . . . . . . . . . . . . .  9
   7. Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . .  9
   8. Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . 10

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

   There are various ways to detecting faults and BFD protocol was
   designed to be a lightweight "Hello" protocol to detect data plane
   failures. With dynamic provisioning of forwarding paths at a large
   scale, establishing BFD sessions for each of those paths creates
   complexity, not only from operations point of view, but also the
   speed at which these sessions could be established or deleted. The
   existing session establishment mechanism 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. Whilst 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 SR
   terminology and protocol constructs.  This section identifies only
   the new terminology introduced.

2. Introduction to Seamless BFD

   BFD as defined in standard [RFC5880] requires two network nodes, as
   part of handshake, exchange discriminators. This will enable the
   sender and receiver of BFD packets of a session to be identified and
   check the continuity of the forwarding path. [RFC5881] defines single
   hop BFD whereas [RFC5883] and [RFC5884] defines multi-hop BFD.

   In order to establish BFD sessions between network entities and
   seamlessly be able to have the session up and running, BFD protocol
   should be capable of doing that. These sessions have to be
   established a priori to traffic flow and ensure the forwarding path
   is available and connectivity is present. With handshake mechanism

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   within BFD protocol, establishing sessions at a rapid rate and
   ensuring the validity or existence of working forwarding path, prior
   to the session being up and running, becomes complex and time
   consuming. In order to achieve seamless BFD sessions, it requires a
   mechanism where the ability to specify the discriminators and the
   ability to respond to the BFD control packets by the network node,
   should already be negotiated ahead of the session becoming active.
   Seamless BFD by definition will be able to provide those mechanisms
   within the BFD protocol in order to meet the requirements and
   establish BFD sessions seamlessly, with minimal overhead, in order to
   detect forwarding failures.

   As an example of how Seamless BFD (S-BFD) works, a set of network
   entities are first identified, to which BFD sessions have to be
   established. Each of those network nodes,  will be assigned a special
   BFD discriminator, to establish a BFD session. These network nodes
   will also create a BFD session instance that listens for incoming BFD
   control packets. Mappings between selected network entities and
   corresponding special BFD discriminators are known to other network
   nodes belonging in the same network. A network node in such network
   is then able to send a BFD control packet to a particular target with
   corresponding special 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 of the use cases where the existing mechanism
   may not be able to satisfy the requirements. In addition, some of the
   use cases will also be identifying the need for expedited BFD session
   establishment with 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 only one side of the BFD, not both, is
   interested in verifying continuity of the data plane 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. Validating the reverse direction is
   not required in this case. Many of these network scenarios are being
   proposed as part of segment routing [TBD]. Another example is when a
   unidirectional tunnel uses BFD to validate reachability to the egress

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   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 do not require to have a bi-
   directional session establishment, 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.
   This translates to, the target network entity for the BFD session,
   upon receiving the BFD packet, should start processing for the
   discriminator received. This will enable the source network entity to
   establish a unidirectional BFD session without 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.
   Due to this faster reachability validation of BFD provisioned
   paths/LSPs could be achieved. In addition, it is expected that some
   MPLS technologies will require traffic engineered LSPs to get created
   dynamically, driven by external applications, e.g. in Software
   Defined Networks (SDN).  It would be desirable to perform BFD
   validation very quickly to allow applications to utilize dynamically
   created LSPs in timely manner.

3.3. Centralized Traffic Engineering

   Various technologies in the SDN domain have evolved which involves

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   controller based networks, where the intelligence, traditionally
   placed in the distributed and dynamic control plane, is separated
   from the data plane and resides in a logically centralized place.
   There are various controllers which perform this exact function in
   establishing forwarding paths for the data flow. Traffic engineering
   is one important function, where the traffic is engineered depending
   upon various attributes of the traffic as well as the network state.

   When the intelligence of the network resides in the 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 traffic
   engineering 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 is not only the desire to verify the forwarding path
   but also an arbitrary path in the network. With tunnels being
   engineered from the centralized entity, when the network state
   changes, traffic has to be switched without much latency and black
   holing of the data.

   Traditional BFD session establishment and validation of the
   forwarding path must not become 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 too high. Session negotiation and establishment of
   BFD sessions to identify valid paths is way to high in terms of time
   and providing network redundancy becomes a critical issue.

3.4. BFD in Centralized Segment Routing

   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 to Efficiently Operate under Resource Constraints

   When BFD sessions are being setup, torn down or parameters (i.e.
   interval, multiplier, etc) are being modified, BFD protocol requires
   additional packets outside of scheduled packet transmissions to
   complete the negotiation procedures (i.e. P/F bits). There are

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   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 without requiring
   additional BFD packets to be exchanged.

3.6. BFD for Anycast Address

   BFD protocol requires the 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 any fault anywhere along the path.
   Fast failure detection provides great benefits, as it can trigger
   recovery procedures rapidly. However, operators often have to follow
   up, manually or automatically, to attempt to identify and localize
   the fault which caused the BFD sessions to fail. Usage of other tools
   to isolate the fault may cause the packets to traverse differently
   throughout the network (i.e. ECMP). In addition, longer it takes from
   BFD session failure to fault isolation attempt, more likely that
   fault cannot be isolated, i.e. 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 which 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.

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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 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.

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4. Security Considerations

   There are no new security considerations introduced by this draft.

5. IANA Considerations

   There are no new IANA considerations introduced by this draft

6. References

6.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC5880, June 2010.

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC5881, June 2010.

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC5883, June 2010.

   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC5884, June 2010.

7. Authors' Addresses

   Sam Aldrin
   Huawei Technologies
   2330 Central Expressway
   Santa Clara, CA 95051


   Manav Bhatia
   Ionos Networks


   Satoru Matsushima

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   Greg Mirsky


   Nagendra Kumar


8. Contributors

   Carlos Pignataro
   Cisco Systems


   Glenn Hayden


   Santosh P K


   Mach Chen


   Nobo Akiya
   Cisco Systems


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