3.2. Operating Modes

BFD has two operating modes that may be selected, as well as an additional function that can be used in combination with the two modes.

The primary mode is known as Asynchronous mode. In this mode, the systems periodically send BFD Control packets to one another, and if a number of those packets in a row are not received by the other system, the session is declared to be down.

The second mode is known as Demand mode. In this mode, it is assumed that a system has an independent way of verifying that it has connectivity to the other system. Once a BFD session is established, such a system may ask the other system to stop sending BFD Control packets, except when the system feels the need to verify connectivity explicitly, in which case a short sequence of BFD Control packets is exchanged, and then the far system quiesces. Demand mode may operate independently in each direction, or simultaneously.

An adjunct to both modes is the Echo function. When the Echo function is active, a stream of BFD Echo packets is transmitted in such a way as to have the other system loop them back through its forwarding path. If a number of packets of the echoed data stream are not received, the session is declared to be down. The Echo function may be used with either Asynchronous or Demand mode. Since the Echo function is handling the task of detection, the rate of periodic transmission of Control packets may be reduced (in the case of Asynchronous mode) or eliminated completely (in the case of Demand mode).

Pure Asynchronous mode is advantageous in that it requires half as many packets to achieve a particular Detection Time as does the Echo function. It is also used when the Echo function cannot be supported for some reason.

The Echo function has the advantage of truly testing only the forwarding path on the remote system. This may reduce round-trip jitter and thus allow more aggressive Detection Times, as well as potentially detecting some classes of failure that might not otherwise be detected.

The Echo function may be enabled individually in each direction. It is enabled in a particular direction only when the system that loops the Echo packets back signals that it will allow it, and when the system that sends the Echo packets decides it wishes to.

Demand mode is useful in situations where the overhead of a periodic protocol might prove onerous, such as a system with a very large number of BFD sessions. It is also useful when the Echo function is being used symmetrically. Demand mode has the disadvantage that Detection Times are essentially driven by the heuristics of the system implementation and are not known to the BFD protocol. Demand mode may not be used when the path round-trip time is greater than the desired Detection Time, or the protocol will fail to work properly. See section 6.6 for more details.