Simplified BFD Procedures
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Simplified BFD Procedures for Bi-Directional LSPs for MPLS OAM (particularly MPLS-TP OAM) George Swallow cisco. MPLS-TP Assumptions. Bidirectional co-routed LSPs OAM carried in an associated channel ITU standard for detection time is 10 ms Send BFD at 3.3 ms

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MPLS-TP Assumptions

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Mpls tp assumptions

Simplified BFD Proceduresfor Bi-Directional LSPsfor MPLS OAM(particularly MPLS-TP OAM)George Swallowcisco


Mpls tp assumptions

MPLS-TP Assumptions

  • Bidirectional co-routed LSPs

  • OAM carried in an associated channel

  • ITU standard for detection time is 10 ms

    • Send BFD at 3.3 ms

  • Needs to be simple enough for cheap silicon

  • Support many forms of protection switching

    • One difficult case !+! Bidirw/ independent switching

  • Consistent configuration is “guaranteed” by NMS or signaling (GMPLS)


Goals

Goals

  • Maintain Slow Start, but greatly simplified

    • Elimination of Poll and Final sequences

  • Initial checking for agreement on timer values

    • Once session is up, timer values and detect mult can be ignored

  • Reduce the amount of session state that needs to be kept for checking validity in UP state (CC Mode)

    • Comparison based only on the two discriminators plus at most 2 of the first 4 bytes of the BFD packet*

      *Assuming Auth is not in use


Summary of changes

Summary of Changes

  • Poll and Final bits are not used

  • Timers for each direction are fixed

  • Deviations from these timers and the implied detect time are tied to the BFD State

  • Discriminator value considerations


Further changes being considered

Further changes being considered

  • Fixed preamble for connectivity verification

  • Independent Mode with a single BFD session

  • Limited set of required timer values


Timer values

Timer Values

  • Each end is configured with

    Min Required RX

    Min Desired TX

    By default these will be equal, but need not be

    Detect Multiplier

  • These parameters MUST agree for session to come up


Timer value control

Timer ValueControl

  • Up State: TX = configured Min Tx

  • Entering down state from up state

    Normal BFD rules apply

  • All other down state

    TX = 1 sec; detect time = 3.5 sec

  • Init State: TX = 1 second

  • Detect time in Init and Up states

    If no packet has been received with Up state; Detect timer = 3.5 seconds

    • (these procedures assume Tx and Rx < 1 second)

      Detect time changes on receipt of an UP state from neighbor toDetect_Mult*Min Rx


Discriminators

Discriminators

“My Discriminator” is locally configured

Optionally “your discriminator” value MAY be configured with the following procedures

During session initialization

  • If the received “my discriminator” does not match the configured “your discriminator”, the session remains in the down state

    In Up and Init States

  • If the received “My Discriminator” changes the session MAY be declared Down

  • Latter case is what is expected in a configured environment


  • Example of initialization

    Example of Initialization

    B (DOWN)

    A(DOWN)

    TX @ 1 second

    MyDisc=B32; YourDisc=0

    TX @ 1 second

    Down 10 msec/10 msec

    MyDisc=A26; YourDisc=0

    Down 10 msec/10 msec

    B Sta (INIT)

    TX @ 1 second

    A Sta (UP)

    TX @10 msec

    MyDisc=B32; YourDisc=A26

    INIT 10/10 msec

    MyDisc=A26; YourDisc=B32

    UP 10/10 msec

    A Sta (UP)

    TX @10 msec

    Change Detect Time

    A Sta (UP)

    TX @10 msec

    Change Detect Time

    MyDisc=B32; YourDisc=A26

    UP 10/10 msec


    Fixed preamble

    Fixed Preamble

    • Used to hold

      • ASN & IPv4 source and destination

      • various forms of OAM identifiers (MEG-IDs)

    • Many possibilities for formats and identification of formats

      • Nothing decided at this point

      • Size in packet is not a great issue; size of state per session is a big issue


    Independent mode single session

    Independent Mode Single Session

    • Session Initiation same as bi-directional mode

    • On

      • Control Detection Time Expired (1)

      • Path Down (5)

    • Send diag (both imply RDI), but stay in UP state

    • Only enter down state on

      • Returning from a complete reset of LSP

      • Admin Down

      • Neighbor signals Down


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