Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

Control packets are encapsulated in user datagram protocol (UDP) packets. The following illustration shows the complete encapsulation of a BFD control packet inside an IPv4 packet.

Figure 1. BFD in IPv4 Packet Format

Field	Description
Diagnostic Code	The reason that the last session failed.
State	The current local session state. Refer to BFD Sessions .
Flag	A bit that indicates packet function. If the poll bit is set, the receiving system must respond as soon as possible, without regard to its transmit interval. The responding system clears the poll bit and sets the final bit in its response. The poll and final bits are used during the handshake and in Demand mode (refer to BFD Sessions). NOTE: The Dell Networking OS does not currently support multi-point sessions, Demand mode, authentication, or control plane independence; these bits are always clear.
Detection Multiplier	The number of packets that must be missed in order to declare a session down.
Length	The entire length of the BFD packet.
My Discriminator	A random number generated by the local system to identify the session.
Your Discriminator	A random number generated by the remote system to identify the session. Discriminator values are necessary to identify the session to which a control packet belongs because there can be many sessions running on a single interface.
Desired Min TX Interval	The minimum rate at which the local system would like to send control packets to the remote system.
Required Min RX Interval	The minimum rate at which the local system would like to receive control packets from the remote system.
Required Min Echo RX	The minimum rate at which the local system would like to receive echo packets. NOTE: The Dell Networking OS does not currently support the echo function.
Authentication Type, Authentication Length, Authentication Data	An optional method for authenticating control packets. NOTE: The Dell Networking OS does not currently support the BFD authentication function.

Two important parameters are calculated using the values contained in the control packet.

Transmit interval

Transmit interval is the agreed-upon rate at which a system sends control packets. Each system has its own transmit interval, which is the greater of the last received remote Desired TX Interval and the local Required Min RX Interval.

Detection time

Detection time is the amount of time that a system does not receive a control packet, after which the system determines that the session has failed. Each system has its own detection time. In Asynchronous mode: Detection time is the remote Detection Multiplier multiplied by greater of the remote Desired TX Interval and the local Required Min RX Interval. In Demand mode: Detection time is the local Detection Multiplier multiplied by the greater of the local Desired Min TX and the remote Required Min RX Interval.