6.3. Demultiplexing and the Discriminator Fields

Since multiple BFD sessions may be running between two systems, there needs to be a mechanism for demultiplexing received BFD packets to the proper session.

Each system MUST choose an opaque discriminator value that identifies each session, and which MUST be unique among all BFD sessions on the system. The local discriminator is sent in the My Discriminator field in the BFD Control packet, and is echoed back in the Your Discriminator field of packets sent from the remote end.

Once the remote end echoes back the local discriminator, all further received packets are demultiplexed based on the Your Discriminator field only (which means that, among other things, the source address field can change or the interface over which the packets are received can change, but the packets will still be associated with the proper session).

The method of demultiplexing the initial packets (in which Your Discriminator is zero) is application dependent, and is thus outside the scope of this specification.

Note that it is permissible for a system to change its discriminator during a session without affecting the session state, since only that system uses its discriminator for demultiplexing purposes (by having the other system reflect it back). The implications on an implementation for changing the discriminator value is outside the scope of this specification.