The base BFD specification does not specify the encapsulation type to be used for sending BFD control packets. Choice of the appropriate encapsulation-type to be implemented is based on the network and medium. The encapsulation for BFD over IPv4 networks is specified in draft-ietf-bfd-v4v6-1hop-04.txt, BFD for IPv4 (Single Hop). This specification requires that BFD control packets be sent over UDP with a destination port number of 3784 and the source port number must be within the range 49152 to 65535.
The TTL of all transmitted BFD packets must have an IP TTL of 255.
If authentication is not enabled, all BFD packets received must have an IP TTL of 255.
If authentication is enabled, the IP TTL should be 255. In case the IP TTL is not 255 the BFD packets are still processed, if packet passes the enabled authentication mechanism.
If multiple BFD sessions exist between two nodes, the BFD discriminator is used to demultiplex the BFD control packet to the appropriate BFD session.