Path Telemetry feature provides the ability to determine WAN path state and measure its characteristics including latency (one way delay), jitter, packet loss rate and throughput.
The outer IP header uses the WAN IP addresses on local and peer WAN interfaces of the path. IP header is followed by a UDP header where the destination port is set to be 4793 by default or to be the port number configured by user in CLI. When IPsec is enabled, destination port is set to be 4500. A path telemetry header is inserted in between of UDP/ESP header and the inner IP packet for path characteristics measurement purpose.
Path State Determination
Path telemetry uses keepalive and feedback packets to determine path state. It sends out keepalive periodically (once per second) and if it receives peer’s feedback packet, the path is considered as active and its characteristics is measured. Accordingly, if feedback packet is not received within a certain period of time (for 5 keepalive we sent), the path is considered as inactive and is not used for path selection.