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SECURE TIME SYNCHRONIZATION
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Figure 17.5. Two-way message exchange between pair of nodes. T2 and T3 are measured in node B s clock while T1 and T4 are measured in node A s clock. (Adapted from reference 12.)
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that a node at level n can communicate with a node or a set of nodes at level n 1. In the synchronization phase, the child nodes are synchronized to the parent. The synchronization is initiated by the child node, which sends a synchronization packet at time t1 . This is received by the parent at t2 , and an acknowledgment packet is sent in response at time t3 . The values of t2 and t3 will be included in the acknowledgment packet. This packet is received by the child node at time t4 (see Figure 17.5). Knowing these four time values, the child node can calculate its clock offset relative to the parent node as well as propagation delay as follows: (t2 t1 ) (t4 t3 ) 2 (t2 t1 ) + (t4 t3 ) 2
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The decomposition of the packet delay is depicted in Figure 17.6 similar to references 5 and 6 as shown in Figure 17.2. The transmission and reception time are more detailed here. The transmission time is the time it takes to transmit a packet on a bit-by-bit basis at the physical layer via the wireless link [12]. The reception
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Figure 17.6. Decomposition of packet delay over a wireless link in TPSN. (Adapted from reference 12).
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A TAXONOMY OF SECURE TIME SYNCHRONIZATION ALGORITHMS
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time is the time required to receive and forward all the bits to the link layer. Both transmission time and reception time are generally considered deterministic; however, variations can occur, depending on the underlying hardware structure. The nodes are assumed to have unique ids, and each node has knowledge of its neighboring nodes. TPSN takes advantage of only the symmetric links for pairwise synchronization between nodes even though the network may also have asymmetric links. This can be considered one of the drawbacks of the protocols. Other drawbacks may include its limited suitability for applications serving highly mobile networks and its lack of support for multihop communication. On the other hand, TPSN is scalable and its computation overhead is less than some other protocols such as NTP [9]. The root selection and the tree construction mechanisms need to be reinvoked when topology changes occur due to node failures or other factors.
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Possible Attacks on TPSN. A compromised node cannot cause any problem by requesting a time synchronization message because the message will reach to the parent node only. On the other hand, the compromised node can send erroneous time information to its children. Naturally, the child node is relying on the parent for its clock synchronization; by providing incorrect values for t2 and t3 , the parent can set an arbitrary offset on its child node. What is more, this incorrect offset will then be propagated down the tree. Therefore, the number of nodes whose synchronization can be affected by the compromised node depends on the location of the compromised node on the tree. One way for a malicious attacker to compromise a larger number of nodes is to reposition itself in a higher location on the tree or to answer queries instead of the proper parent. This is surprisingly easy to do in the original algorithm. Yet another type of attack can surface when the compromised node misinforms its level in the tree, basically announcing a lower level than its current level. The compromised node can also attempt to trick other nodes at its level in requesting synchronization updates from itself. Furthermore, the compromised node can disconnect a number of nodes from being included in the tree by simply not participating in the level discovery phase.
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17.3.3 Flooding Time Synchronization Protocol (FTSP) In FTSP [13], the nodes are participating in a process in which a root node is elected. The root is the origin of the time synchronization messages. If a node does not hear a time synchronization message for a while, it declares itself the new root. The protocol requires that if at a later time the node receives a time synchronization message from a node with a lower id than itself, it gives up its root status. When a node receives a time synchronization message from the root, it adjusts its clock and broadcasts its own time to its neighbors. In the message broadcast, the preamble bytes are transmitted rst, followed by sync bytes, message descriptor, the actual message, and nally crc bytes (see Figure 17.7).