Improving distributed TCP caching for wireless sensor networks

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.