International Journal of Distributed Systems and Technologies, 2(3), 37-48, July-September 2011 37 Copyright © 2011, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. Keywords: Causally Delivery, Group Communication, Heterogeneous, Hybrid Clock, Liner Clock, Physical Clock, Unnecessarily Ordered Messages Design and Implementation of Hybrid Time (HT) Group Communication Protocol for Homogeneous Broadcast Groups Isamu Tsuneizumi, Seikei University, Japan Ailixier Aikebaier, Seikei University, Japan Makoto Ikeda, Seikei University, Japan Tomoya Enokido, Risho University, Japan Makoto Takizawa, Seikei University, Japan ABSTRACT To realize the cooperation of a group of multiple peer processes (peers), messages sent by peers must be causally delivered to every peer. In a scalable group, it is necessary to reduce the communication overhead to causally deliver messages. In this paper, the authors take advantage of the linear time (LT) and physical time (PT) protocols, as the message length is O(n) for the number n of peers. However, some pairs are un- necessarily ordered, that is, even if a pair of messages is ordered in the protocols, the messages may not be causally ordered. The greater the number of messages that are unnecessarily ordered, the larger the overhead is implied since the messages must be kept in a receipt queue if a message is lost or delayed. This paper dis- cusses a hybrid time group communication (HT) protocol that reduces the number of messages unnecessarily ordered. The HTprotocol is evaluated in terms of the number of unnecessarily ordered messages compared with the PT and LT protocols. It is demonstrated that the number of unnecessarily ordered messages can be reduced in the HT protocol compared with the LT and PT protocols. INTRODUCTION In peer-to-peer (P2P) information systems (Schollmeier, 2001), a group of multiple peer processes (peers) are cooperating to achieve some objectives by exchanging messages with each other. A P2P system is in nature fully distributed with no centralized coordinator and is scalable. Here, messages are required to be causally delivered to peers (Nakamura DOI: 10.4018/jdst.2011070103