6.1. Overview

A system may take either an Active role or a Passive role in session initialization. A system taking the Active role MUST send BFD Control packets for a particular session, regardless of whether it has received any BFD packets for that session. A system taking the Passive role MUST NOT begin sending BFD packets for a particular session until it has received a BFD packet for that session, and thus has learned the remote system's discriminator value. At least one system MUST take the Active role (possibly both). The role that a system takes is specific to the application of BFD, and is outside the scope of this specification.

A session begins with the periodic, slow transmission of BFD Control packets. When bidirectional communication is achieved, the BFD session becomes Up.

Once the BFD session is Up, a system can choose to start the Echo function if it desires and the other system signals that it will allow it. The rate of transmission of Control packets is typically kept low when the Echo function is active.

If the Echo function is not active, the transmission rate of Control packets may be increased to a level necessary to achieve the Detection Time requirements for the session.

Once the session is Up, a system may signal that it has entered Demand mode, and the transmission of BFD Control packets by the remote system ceases. Other means of implying connectivity are used to keep the session alive. If either system wishes to verify bidirectional connectivity, it can initiate a short exchange of BFD Control packets (a "Poll Sequence"; see section 6.5) to do so.

If Demand mode is not active, and no Control packets are received in the calculated Detection Time (see section 6.8.4), the session is declared Down. This is signaled to the remote end via the State (Sta) field in outgoing packets.

If sufficient Echo packets are lost, the session is declared Down in the same manner. See section 6.8.5.

If Demand mode is active and no appropriate Control packets are received in response to a Poll Sequence, the session is declared Down in the same manner. See section 6.6.

If the session goes Down, the transmission of Echo packets (if any) ceases, and the transmission of Control packets goes back to the slow rate.

Once a session has been declared Down, it cannot come back up until the remote end first signals that it is down (by leaving the Up state), thus implementing a three-way handshake.

A session MAY be kept administratively down by entering the AdminDown state and sending an explanatory diagnostic code in the Diagnostic field.