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6.2. BFD State Machine

The BFD state machine is quite straightforward. There are three states through which a session normally proceeds: two for establishing a session (Init and Up) and one for tearing down a session (Down). This allows a three-way handshake for both session establishment and session teardown (assuring that both systems are aware of all session state changes). A fourth state (AdminDown) exists so that a session can be administratively put down indefinitely.

Each system communicates its session state in the State (Sta) field in the BFD Control packet, and that received state, in combination with the local session state, drives the state machine.

Down state means that the session is down (or has just been created). A session remains in Down state until the remote system indicates that it agrees that the session is down by sending a BFD Control packet with the State field set to anything other than Up. If that packet signals Down state, the session advances to Init state; if that packet signals Init state, the session advances to Up state. Semantically, Down state indicates that the forwarding path is unavailable, and that appropriate actions should be taken by the applications monitoring the state of the BFD session. A system MAY hold a session in Down state indefinitely (by simply refusing to advance the session state). This may be done for operational or administrative reasons, among others.

Init state means that the remote system is communicating, and the local system desires to bring the session up, but the remote system does not yet realize it. A session will remain in Init state until either a BFD Control Packet is received that is signaling Init or Up state (in which case the session advances to Up state) or the Detection Time expires, meaning that communication with the remote system has been lost (in which case the session advances to Down state).

Up state means that the BFD session has successfully been established, and implies that connectivity between the systems is working. The session will remain in the Up state until either connectivity fails or the session is taken down administratively. If either the remote system signals Down state or the Detection Time expires, the session advances to Down state.

AdminDown state means that the session is being held administratively down. This causes the remote system to enter Down state, and remain there until the local system exits AdminDown state. AdminDown state has no semantic implications for the availability of the forwarding path.

The following diagram provides an overview of the state machine. Transitions involving AdminDown state are deleted for clarity (but are fully specified in sections 6.8.6 and 6.8.16). The notation on each arc represents the state of the remote system (as received in the State field in the BFD Control packet) or indicates the expiration of the Detection Timer.

                        +--+
                        |  | UP, ADMIN DOWN, TIMER
                        |  V
                DOWN  +------+  INIT
         +------------|      |------------+
         |            | DOWN |            |
         |  +-------->|      |<--------+  |
         |  |         +------+         |  |
         |  |                          |  |
         |  |               ADMIN DOWN,|  |
         |  |ADMIN DOWN,          DOWN,|  |
         |  |TIMER                TIMER|  |
         V  |                          |  V
       +------+                      +------+
  +----|      |                      |      |----+
DOWN|    | INIT |--------------------->|  UP  |    |INIT, UP
  +--->|      | INIT, UP             |      |<---+
       +------+                      +------+
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