Time Synchronization Issues

 

Loose time synchronization is an important component in TESLA. Although sophisticated time synchronization protocols exist, they usually require considerable management overhead. Furthermore, they generally have a high complexity and achieve properties that TESLA does not require. An example is the network time protocol (NTP) by Mills [21]. Bishop performs a detailed security analysis of NTP [7]. For these reasons, we outline a simple and secure time synchronization protocol that suffices the humble requirements of TESLA.

The time synchronization requirement that secures TESLA against an active attacker is that the receiver knows an upper bound of the difference between the sender's local time and the receiver's local time, $\Delta$. For simplicity, we assume the clock drift of both sender and receiver are negligible, otherwise they will simply resynchronize periodically. We denote the real difference between the sender and the receiver's time with $\delta$. Hence for loose synchronization, the receiver does not need know $\delta$ but only some $\Delta$ that is guaranteed to be greater or equal to $\delta$. To compute $\Delta$, we can use either a direct or an indirect time synchronization method. In the following, we first discuss a simple protocol for direct time synchronization, and next we discuss how to do indirect time synchronization.