Initiator  NAT1                                 NAT2        Responder
|             | 1. UDP(UPDATE(SEQ, CAND_PRIO,      |                |
|             |        ECHO_REQ_SIGN))             |                |
|             X<-----------------------------------+----------------+
|             |                                    |                |
| 2. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO))    |                |
+-------------+------------------------------------+--------------->|
|             |                                    |                |
| 3. UDP(UPDATE(ACK, ECHO_RESP_SIGN, MAPPED_ADDR)) |                |
|<------------+------------------------------------+----------------+
|             |                                    |                |
| 4. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO))    |                |
|<------------+------------------------------------+----------------+
|             |                                    |                |
| 5. UDP(UPDATE(ACK, ECHO_RESP_SIGN, MAPPED_ADDR)) |                |
+-------------+------------------------------------+--------------->|
|             |                                    |                |
| 6. Other connectivity checks using UPDATE over UDP                |
|<------------+------------------------------------+---------------->
|             |                                    |                |
| 7. UDP(UPDATE(SEQ, ECHO_REQ_SIGN, CAND_PRIO, NOMINATE))           |
+-------------+------------------------------------+--------------->|
|             |                                    |                |
| 8. UDP(UPDATE(SEQ, ACK, ECHO_REQ_SIGN, ECHO_RESP_SIGN,            |
|           NOMINATE))                             |                |
|<------------+------------------------------------+----------------+
|             |                                    |                |
| 9. UDP(UPDATE(ACK, ECHO_RESP_SIGN))              |                |
+-------------+------------------------------------+--------------->+
|             |                                    |                |
| 10. ESP data traffic over UDP                     |               |
+<------------+------------------------------------+--------------->+
|             |                                    |                |