Control/Data                                           Control/Data
  Relay Client (Initiator)                   Relay Server (Responder)
  |   1. UDP(I1)                                                    |
  +---------------------------------------------------------------->|
  |                                                                 |
  |   2. UDP(R1(REG_INFO(RELAY_UDP_HIP,[RELAY_UDP_ESP])))           |
  |<----------------------------------------------------------------+
  |                                                                 |
  |   3. UDP(I2(REG_REQ(RELAY_UDP_HIP),[RELAY_UDP_ESP]))            |
  +---------------------------------------------------------------->|
  |                                                                 |
  |   4. UDP(R2(REG_RES(RELAY_UDP_HIP,[RELAY_UDP_ESP]), REG_FROM,   |
  |          [RELAYED_ADDRESS]))                                    |
  |<----------------------------------------------------------------+
  |                                                                 |