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4.1. Generic BFD Control Packet Format

The Mandatory Section of a BFD Control packet has the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Vers |  Diag   |Sta|P|F|C|A|D|M|  Detect Mult  |    Length     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       My Discriminator                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Your Discriminator                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Desired Min TX Interval                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Required Min RX Interval                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Required Min Echo RX Interval                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

An optional Authentication Section MAY be present:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Auth Type   |   Auth Len    |    Authentication Data...     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Field Definitions​

Version (Vers)

The version number of the protocol. This document defines protocol version 1.

Diagnostic (Diag)

A diagnostic code specifying the local system's reason for the last change in session state. Values are:

  • 0 -- No Diagnostic
  • 1 -- Control Detection Time Expired
  • 2 -- Echo Function Failed
  • 3 -- Neighbor Signaled Session Down
  • 4 -- Forwarding Plane Reset
  • 5 -- Path Down
  • 6 -- Concatenated Path Down
  • 7 -- Administratively Down
  • 8 -- Reverse Concatenated Path Down
  • 9-31 -- Reserved for future use

This field allows remote systems to determine the reason that the previous session failed, for example.

State (Sta)

The current BFD session state as seen by the transmitting system. Values are:

  • 0 -- AdminDown
  • 1 -- Down
  • 2 -- Init
  • 3 -- Up

Poll (P)

If set, the transmitting system is requesting verification of connectivity, or of a parameter change, and is expecting a packet with the Final (F) bit in reply. If clear, the transmitting system is not requesting verification.

Final (F)

If set, the transmitting system is responding to a received BFD Control packet that had the Poll (P) bit set. If clear, the transmitting system is not responding to a Poll.

Control Plane Independent (C)

If set, the transmitting system's BFD implementation does not share fate with its control plane (in other words, BFD is implemented in the forwarding plane and can continue to function through disruptions in the control plane). If clear, the transmitting system's BFD implementation shares fate with its control plane.

The use of this bit is application dependent and is outside the scope of this specification. See specific application specifications for details.

Authentication Present (A)

If set, the Authentication Section is present and the session is to be authenticated (see section 6.7 for details).

Demand (D)

If set, Demand mode is active in the transmitting system (the system wishes to operate in Demand mode, knows that the session is Up in both directions, and is directing the remote system to cease the periodic transmission of BFD Control packets). If clear, Demand mode is not active in the transmitting system.

Multipoint (M)

This bit is reserved for future point-to-multipoint extensions to BFD. It MUST be zero on both transmit and receipt.

Detect Mult

Detection time multiplier. The negotiated transmit interval, multiplied by this value, provides the Detection Time for the receiving system in Asynchronous mode.

Length

Length of the BFD Control packet, in bytes.

My Discriminator

A unique, nonzero discriminator value generated by the transmitting system, used to demultiplex multiple BFD sessions between the same pair of systems.

Your Discriminator

The discriminator received from the corresponding remote system. This field reflects back the received value of My Discriminator, or is zero if that value is unknown.

Desired Min TX Interval

This is the minimum interval, in microseconds, that the local system would like to use when transmitting BFD Control packets, less any jitter applied (see section 6.8.2). The value zero is reserved.

Required Min RX Interval

This is the minimum interval, in microseconds, between received BFD Control packets that this system is capable of supporting, less any jitter applied by the sender (see section 6.8.2). If this value is zero, the transmitting system does not want the remote system to send any periodic BFD Control packets.

Required Min Echo RX Interval

This is the minimum interval, in microseconds, between received BFD Echo packets that this system is capable of supporting, less any jitter applied by the sender (see section 6.8.9). If this value is zero, the transmitting system does not support the receipt of BFD Echo packets.

Auth Type

The authentication type in use, if the Authentication Present (A) bit is set.

  • 0 - Reserved
  • 1 - Simple Password
  • 2 - Keyed MD5
  • 3 - Meticulous Keyed MD5
  • 4 - Keyed SHA1
  • 5 - Meticulous Keyed SHA1
  • 6-255 - Reserved for future use

Auth Len

The length, in bytes, of the authentication section, including the Auth Type and Auth Len fields.

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