Client   Server
   |       |
   +------>|        Code: 0.03 (PUT)
   |  PUT  |       Token: 0x41
   |       |    Uri-Path: lock
   |       |     Payload: 0 (Unlock)
   |       |
   |<------+        Code: 4.01 (Unauthorized)
   |  4.01 |       Token: 0x41
   |       |        Echo: 0x00000009437468756c687521 (t0 = 9, +MAC)
   |       |
   | ...   | The round trips take 1 second, time is now t1 = 10.
   |       |
   +------>|        Code: 0.03 (PUT)
   |  PUT  |       Token: 0x42
   |       |    Uri-Path: lock
   |       |        Echo: 0x00000009437468756c687521 (t0 = 9, +MAC)
   |       |     Payload: 0 (Unlock)
   |       |
   |       | Verify MAC, compare t1 - t0 = 1 < T => permitted.
   |       |
   |<------+        Code: 2.04 (Changed)
   |  2.04 |       Token: 0x42
   |       |