Simplified BFD Proceduresfor Bi-Directional LSPs

Goals• Simple static configuration of Active State• Maintain Slow Start, but greatly simplified

– Fixed values for timers, detect multiplier during session (re-)initialization– 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*Could be reduced further pending discussion of control bits

Summary of Changessince IETF 77

• 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

Timer Values

• Each end is configured with Min Required RXMin 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• Up State

On entering the 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 to

Detect_Mult*Min Rx

Discriminators

“My Discriminator” is locally configuredOptionally “your discriminator” value MAY be configured with

the following proceduresDuring session initialization– If the received “my discriminator” does not match the configured

“your discriminator”, the session remains in the down stateIn 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

MyDisc=B32; YourDisc=0 TX @ 1 secondA (DOWN) B (DOWN)

A Sta (UP)TX @ 10 msec

Example of Initialization

MyDisc=B32; YourDisc=A26

MyDisc=A26; YourDisc=B32

MyDisc=A26; YourDisc=0

TX @ 1 second

TX @ 1 secondB Sta (INIT)

Down 10 msec/10 msec

Down 10 msec/10 msec

INIT 10/10 msec

UP 10/10 msec

UP 10/10 msecMyDisc=B32; YourDisc=A26A Sta (UP)

TX @ 10 msecChange Detect Time

A Sta (UP)TX @ 10 msecChange Detect Time

Further Simplifications

• Eliminate ACH TLVs• Modify procedures for Independent Mode

such that there is only one BFD session needed

• Limited set of timer values that are required to be supported

Eliminate ACH TLVs

• Replace with a fixed preamble which can contain– various forms of MEG-IDs – other identifiers

• 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

• 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