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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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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)
  • 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


“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



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