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    Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net 1

    Bidirectional Forwarding

    Detection (BFD)

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    S Goals

    S Protocol Overview

    S Operation

    SStatus

    S Implementation Details

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    3Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    Detecting Forwarding Failures (I)

    S For IP, classically a function of the routing protocol

    Because formerly, routing = forwarding

    Fault resolution in perhaps tens of seconds

    Perhaps too slow for anything but best-effort IP

    Sometimes there is no routing protocol!

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    4Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    Detecting Forwarding Failures (II)

    S Interface state can be used, but:

    Transient failures are problematic

    So are switched environments

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    5Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    Detecting Forwarding Failures (III)

    S In modern architectures: routing forwarding

    S Would be nice to have a way to detect forwardingwithout relying on interface state

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    6Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    Goals for BFD

    S Detection of forwarding plane-to-forwarding plane

    connectivity (including links, interfaces, etc.)S Faster convergence of routing protocols, particularly on shared

    media like Ethernet

    S

    Semantic separation of forwarding plane connectivity vs.control plane connectivity (works well in concert with

    Graceful Restart)

    S A single mechanism that is independent of media, routing

    protocol, and data protocol

    S No changes to existing protocols

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    7Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    BFD Protocol Overview

    S At its heart, Yet Another Hello Protocol

    S Packets sent at regular intervals; neighbor failure detectedwhen packets stop showing up

    S Intended to be implemented in the forwarding plane to the

    extent possible (avoiding fate sharing with the control plane)S Context defined by encapsulating protocol (sending inside

    IPv4 packets signals IPv4 connectivity; also could be sent

    over IPv6, directly over the datalink, or whatever)

    S Always unicast, even on shared media

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    8Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    BFD Protocol Overview

    S Not just for direct links; can also be used over MPLS LSPs,other tunnels

    SSufficient context in the protocol to keep track of multipleparallel paths between systems

    S Timing is adjustable on-the-fly, allowing for adaptivity toavoid catastrophic collapse due to false failure detection

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    9Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    BFD Operation

    S BFD Control Packets sent in both directions, providing basic

    connectivity check and continuous parameter negotiationS Optional Echo Mode can be negotiated

    BFD Echo Packets transmitted addressed to originating

    system; other system forwards them back through regularforwarding path

    Exercises entire forwarding path in destination system

    May not always be possible or desirable, thus it isnegotiated

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    10Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    BFD Async Mode

    S Control packets flow in each direction

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    11Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    BFD Echo Mode

    S Control packets flow in each direction

    S Echo packets loop through remote system

    S Control packet flow is slower

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    12Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    BFD Operation

    S Combination of continuous negotiation and unicast allowshigh adaptivity

    Less capable systems can negotiate slower detectionwithout penalizing faster systems

    Transmission rate and detection time can be changed on-

    the-fly, allowing for automatic mechanisms for recoveryfrom overly aggressive timers or unusual network events

    Echo mode can be enabled or disabled without taking downBFD session

    S Three-way handshake on both up and down transitions

    S Each system in full control of its own fate

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    13Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    BFD OperationS No discovery protocol, handled by control protocol

    S

    Semantics of BFD session failure are contextual BFD-over-IP implies neighbor failure; IGP neighborshould be torn down

    BFD-over-Ethernet implies switch failure; subnet should be

    withdrawn from routing protocols

    S Works over a wide range of time constants

    Timers specified in microseconds, allowing very fast or

    very slow detection

    Systems specify how quickly they can receive BFD packetsso that boxes of differing abilities can interoperate

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    14Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    BFD Operation

    S Discriminator field used to demultiplex incoming packets oncesession is up

    Each system chooses 32 bit value that is unique among allsessions, puts it in My Discr field

    Remote systems discriminator is looped back in Your

    Discr field

    Your Discr used to choose session for incoming packets

    S Initialization is slightly tricky

    First packet has to bedemuxed on other info--sourceaddress and interface index, for example

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    15Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    BFD Control Message

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    | My Discr |

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    | Your Discr |

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    | Desired Min TX Interval |

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    | Required Min RX Interval |

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    | Required Min Echo RX Interval |

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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    16Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    BFD State Machine

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    17Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    BFD Applications

    S Routing protocol liveness detection (obviously)

    S Switch-to-routerliveness detection

    S Router-to-hostlivenessdetection (a real answer forthe web farms)

    S Virtual link liveness (MPLS, GRE tunnels, etc.--noneed to keep reinventing these)

    S Alarm passthrough (receive alarm from concatenated

    link; take session admin-down with diagnosticconcatenated link down)

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    18Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net

    BFD Status

    S Protocol jointly developed by Juniper and Cisco

    S Internet draft published (draft-katz-ward-bfd-0x.txt)

    S Published as an Informational document (so that

    progress can be made in a reasonable amount of time

    S Rumors of fast track standardization being pursued

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    Copyright 2003 Juniper Networks, Inc. Proprietary and Confidential www.juniper.net 19

    http://www.juniper.net

    Thank You!

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