Techniques are provided for detecting network characteristics. At a first endpoint device configured to communicate with a second endpoint device in a network, a request is made to a relay server in a network for an address and port assignment on the relay server. The address and port assignment are obtained from the relay server, and a probing packet is sent at a first time instance to the relay server at an address and port corresponding to the address and port assignment. The first time instance is stored in a timestamp field of the probing packet. A response packet responsive to the probing packet is then received from the address and port on the relay server at a second time instance. The second time instance is stored in a timestamp field of the response packet.