3 Chowkidar: Reliable and Scalable Health Monitoring for Wireless Sensor Network Testbeds S. BAPAT, W. LEAL, T. KWON, P. WEI, and A. ARORA The Ohio State University Wireless sensor network (WSN) testbeds are useful because they provide a way to test applica- tions in an environment that makes it easy to deploy experiments, conﬁgure them statically or dynamically, and gather performance information. However, WSNs are typically composed of low- cost devices and tend to be unreliable, with failures a common phenomenon. Accurate knowledge of network health status, including nodes and links of each type, is critical for correctly conﬁguring applications on WSN testbeds and for interpreting the data collected from them. In this article we present a stabilizing protocol, Chowkidar, that provides accurate and efﬁcient network health monitoring in WSNs. Our approach adapts the well-known problem of message- passing rooted spanning tree construction and its use in propagation of information with feedback (PIF) for the case of a WSN. The Chowkidar protocol is initiated upon demand; that is, it does not involve ongoing maintenance, and it terminates with accurate results, including detection of failure and restart during the monitoring process. Chowkidar is distinguished from others in two important ways. Given the resource constraints of WSNs, it is message-efﬁcient in that it uses only a few messages per node. Also, it tolerates ongoing node and link failure and node restart, in contrast to requiring that faults stop during convergence. We have implemented the Chowkidar protocol as part of enabling a network health status ser- vice that is tightly integrated with a remotely accessible wireless sensor network testbed, Kansei, at The Ohio State University. We present experimental results from this testbed that validate the correctness and performance of Chowkidar. We also report on initial experiences and lessons learnt from the integration of Chowkidar with Kansei, including feedback from both testbed users and administrators who have found Chowkidar to be a useful tool for improving the accuracy and efﬁciency of testbed experimentation and maintenance, and the need for well-deﬁned policies to address issues such as minimizing interference with concurrently running experiments. Finally, we discuss extensions that enhance the functionality and usability of Chowkidar. Categories and Subject Descriptors: C.2.2 [Computer Communication Networks]—Network protocols General Terms: Algorithms, Experimentation, Reliability, Management Additional Key Words and Phrases: Wireless sensor networks, stabilization, tree protocols, PIF, health monitoring, protocol architecture This work was sponsored in part by NSF grants NSF-NETS/NOSS-0520222 and NSF-HDCCSR- 0341703. Author’s address: S. Bapat: email: Bapat.2@osu.edu. Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for proﬁt or direct commercial advantage and that copies show this notice on the ﬁrst page or initial screen of a display along with the full citation. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, to republish, to post on servers, to redistribute to lists, or to use any component of this work in other works requires prior speciﬁc permission and/or a fee. Permissions may be requested from Publications Dept., ACM, Inc., 2 Penn Plaza, Suite 701, New York, NY 10121-0701 USA, fax +1 (212) 869-0481, or permissions@acm.org. C  2009 ACM 1556-4665/2009/01-ART3 $5.00 DOI 10.1145/1462187.1462190 http://doi.acm.org/ 10.1145/1462187.1462190 ACM Transactions on Autonomous and Adaptive Systems, Vol. 4, No. 1, Article 3, Publication date: January 2009.