7. Operational Considerations

BFD is likely to be deployed as a critical part of network infrastructure. As such, care should be taken to avoid disruption.

Obviously, any mechanism that blocks BFD packets, such as firewalls or other policy processes, will cause BFD to fail.

Mechanisms that control packet scheduling, such as policers, traffic shapers, priority queueing, etc., have the potential of impacting BFD operations if the Detection Time is similar in scale to the scheduled packet transmit or receive rate. The delivery of BFD packets is time-critical, relative to the magnitude of the Detection Time, so this may need to be taken into account in implementation and deployment, particularly when very short Detection Times are to be used.

When BFD is used across multiple hops, a congestion control mechanism MUST be implemented, and when congestion is detected, the BFD implementation MUST reduce the amount of traffic it generates. The exact mechanism used is outside the scope of this specification, and the requirements of this mechanism may differ depending on how BFD is deployed, and how it interacts with other parts of the system (for example, exponential backoff may not be appropriate in cases where routing protocols are interacting closely with BFD).

Note that "congestion" is not only a traffic phenomenon, but also a computational one. It is possible for systems with a large number of BFD sessions and/or very short packet intervals to become CPU-bound. As such, a congestion control algorithm SHOULD be used even across single hops in order to avoid the possibility of catastrophic system collapse, as such failures have been seen repeatedly in other periodic Hello-based protocols.

The mechanisms for detecting congestion are outside the scope of this specification, but may include the detection of lost BFD Control packets (by virtue of holes in the authentication sequence number space, or by BFD session failure) or other means.

The mechanisms for reducing BFD's traffic load are the control of the local and remote packet transmission rate via the Min RX Interval and Min TX Interval fields.

Note that any mechanism that increases the transmit or receive intervals will increase the Detection Time for the session.

It is worth noting that a single BFD session does not consume a large amount of bandwidth. An aggressive session that achieves a detection time of 50 milliseconds, by using a transmit interval of 16.7 milliseconds and a detect multiplier of 3, will generate 60 packets per second. The maximum length of each packet on the wire is on the order of 100 bytes, for a total of around 48 kilobits per second of bandwidth consumption in each direction.