Client   Server
   |       |
   +------>|        Code: 0.03 (PUT)
   |  PUT  |       Token: 0x41
   |       |    Uri-Path: lock
   |       |     Payload: 0 (Unlock)
   |       |
   |<------+        Code: 4.01 (Unauthorized)
   |  4.01 |       Token: 0x41
   |       |        Echo: 0x05 (e0 = 5, number of total lock
   |       |                            operations performed)
   |       |
   | ...   | No alterations happen to the lock state, e1 has the
   |       | same value e1 = 5.
   |       |
   +------>|        Code: 0.03 (PUT)
   |  PUT  |       Token: 0x42
   |       |    Uri-Path: lock
   |       |        Echo: 0x05
   |       |     Payload: 0 (Unlock)
   |       |
   |       | Compare e1 = e0 => permitted.
   |       |
   |<------+        Code: 2.04 (Changed)
   |  2.04 |       Token: 0x42
   |       |        Echo: 0x06 (e2 = 6, to allow later locking
   |       |                            without more round trips)
   |       |