Effective Cache based Policies in Wireless Sensor Networks: A Survey <s> Figure 5. Distributed TCP Caching <s> Previous research on transport layer protocols for wireless sensor ::: networks (WSNs) has focused on designing protocols specifically ::: targeted for sensor networks. Most sensor networks applications, ::: however, are only useful when connected to an external network. The ::: deployment of TCP/IP in WSNs would enable direct connection between ::: the WSN and external TCP/IP networks. However, TCP performs badly in ::: wireless environments both in terms of throughput and energy ::: efficiency. To overcome these problems in WSNs we have designed ::: Distributed TCP Caching. This mechanism greatly enhances TCP ::: performance by caching TCP segments on the nodes in the sensor network ::: and locally retransmitting lost segments. Our simulations ::: demonstrate that with these enhancements TCP performs well enough to ::: be useful in wireless sensor networks. <s> BIB001 </s> Effective Cache based Policies in Wireless Sensor Networks: A Survey <s> Figure 5. Distributed TCP Caching <s> The behavior of sensor networks is highly unpredictable because of randomness in individual node state and network structure. When a node fails, the packets transmitted to this faulty node is dropped if it is not cached by an operational node. In this paper some analytical models based on fault tolerance mechanisms have been proposed to be used in different aspects of caching a packet regarding this situation. The ratio of the time to be allocated to cache the packet by an operational node and the expected repair time of the faulty node represents a threshold that decides whether this operational node should cache the packet or it will pass the packet to a selected neighbor node. Analytical models developed in this paper have made detail and clear inspections in these respects. Distributed caching mechanism has been described in this paper, which makes a way to use the energy of the nodes in a distributed fashion to cache a packet. The models and mechanisms, described in this paper are distributed and highly scalable, which ensure energy efficient caching of a packet and thus highly applicable to the sensor networks. <s> BIB002 </s> Effective Cache based Policies in Wireless Sensor Networks: A Survey <s> Figure 5. Distributed TCP Caching <s> Reliable transport protocols like TCP are in principle not well suited for wireless sensor networks (WSNs), due to both the characteristics of the network nodes (low computing power, strong energy constraints) and those of the typical applications running on such nodes (low data rates). Indeed, TCP has not been designed with energy efficiency in mind, and the end-to-end recovery of TCP makes it too energy-expensive for common WSN applications. Distributed TCP Caching (DTC), originally proposed by Dunkels et al., is a mechanism aimed at adapting TCP to wireless sensor networks. DTC improves TCP performance by caching TCP segments in intermediate nodes. However, DTC was designed for, and tested with, a TDMA-based MAC layer without automatic repeat request (ARQ). On the other hand, current WSN link-layer protocols like 802.15.4 are based on Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA), and they readily support ARQ. In this paper, we propose and evaluate several modifications to DTC that result in an enhanced, more energy-efficient transport layer for IP-based WSNs. We explicitly consider the CSMA-CA nature of the link (i.e., the presence of frame collisions), as well as the use of link-layer reliability. Using the INETMANET framework of the OMNET++ simulator, we assess how simple modifications to the TCP caching mechanism and round-trip time estimator can have a significant impact on DTC performance, in terms of energy efficiency and file transfer delay. <s> BIB003
In the above scenario TCP sender sends three segments and the lost nodes are cached by the intermediate nodes. Thus, node 5 caches the lost segment 1 and node 7 caches the lost segment 2. This is, as described, done by Packet Loss Detection. Now in receiving segment 3 by the receiver, it replies with the ACK1. On receiving ACK1 by node 7, it doesn't send its cached segment 2 rather it waits. When ACK1 is received by the node 5, it sends cached copy of segment 1 to the receiver. Node 5 will not send ACK1 to the sender to avoid unnecessary retransmission. On receiving segment 1 by receiver it replies with ACK2. The ACK2 is received by node 7 and it retransmits the cached copy of segment 2. The receiver on receiving this segment ask sender for further segments with ACK4. This way the intermediate nodes, here node 5 and node 7, clears its cached segments and are ready to accept new segments. Thus, DTC helps in reducing the number of packets in the network, avoids end-to-end retransmissions and does local retransmissions utilizing the intermediate nodes cache. The TCP support in Wireless Sensor Network BIB001 does not considers the local (data-link) transmission delay and overhead. The scheme is given only for single TCP data flow. Multiple data flows in TCP supported Wireless Sensor Network may not perform well and needed to be considered. Some of the other areas where caching can be utilized is proposed in BIB002 , BIB003 . In BIB002 , scheme exploits caching for faulty nodes in WSNs and proposes a mechanism to handle the packets when node gets failed. The operational node handles the data during the node failure and recovers it when the faulty node again gets workable. An improvement in DTC BIB003 has been proposed with implementation of Automatic Repeat ReQuest (ARQ). The scheme 