Timestamping is another way of preventing a replay attack.[3] Synchronization should be achieved using a secure protocol. For example, Bob periodically broadcasts the time on his clock together with a MAC. When Alice wants to send Bob a message, she includes her best estimate of the time on his clock in her message, which is also authenticated. Bob only accepts messages for which the timestamp is within a reasonable tolerance. Timestamps are also implemented during mutual authentication, when both Bob and Alice authenticate each other with unique session IDs, in order to prevent the replay attacks.[4] The advantages of this scheme are that Bob does not need to generate (pseudo-) random numbers and that Alice doesn't need to ask Bob for a random number. In networks that are unidirectional or near unidirectional, it can be an advantage. The trade-off being that replay attacks, if they are performed quickly enough i.e. within that 'reasonable' limit, could succeed.