Dr. Dobb's Journal - November 2008 - (Page 33)

by Jeremy Blackburn and Ken Christensen Core Technology Green Telnet Modifying a client-server application to save energy Reducing the energy consumption of IT equipment is of growing interest for operational, economic, and environmental reasons. The existing client/server model that uses TCP connections tightly couples application state to connection state. Thus, when a client machine transitions to a sleep state to reduce its energy footprint, the TCP connection drops and the application state is lost on the server. Because of this tight coupling, an idle client machine must remain fully awake to avoid losing its TCP connection to the server and any application state associated with that connection. That is, the client machine must remain in this energy inefficient state or lose any outstanding work (state) in the server. As a result, users often permanently disable power management in client machines. To address this, we have created a “green telnet” that lets clients transition to a low-power, sleep state to save energy and not lose their telnet session (and state) in the server. We have modified telnetd to recognize when a client machine goes to sleep, gracefully handling the loss of the TCP connection, and then buffering data for the sleeping client. When the telnet client wakes up, a new TCP connection is established and the data buffered in the server is delivered to the client. Our new “gtelnetd” daemon is a modification of the Minix 3 telnetd daemon (www.minix3.org) and is as distribution agnostic as possible by relying on fork() and minimal IPC primitives. To enable the client machine to go to sleep without losing its session, we wedge three processes between the network connection and the telnetd application. These three processes coordinate data transfer, data queuing, and sleep/wake state transitions in a manner that is transparent to the telnet session. We also make some changes to the telnet client and introduce several sleep-related control messages. client and server communicate information regarding power state change: • gtWSLE, which the client sends to the server to indicate that it wants to go to sleep. The payload for this control message is a time indicating how long the client intends to be asleep (0 indicates an unknown sleep period). • gtWWAK is issued by the client when it wants to wake up. The payload for this message is a byte count used to recover from any data loss that might have occurred. • gtRECO is sent by the server to let the client know which port number it should reconnect to for the next sleep/wake cycle. Figure 1 shows the flow of control messages when a client goes to sleep (TCP ACKs are not shown). Upon receiving notification from the host powermanagement system, the client sends a gtWSLE message to the server. While the client is asleep the server uses an intermediate queue to buffer data destined for the client. When the client wakes back up, it reconnects to the server and sends a gtWWAK message. The server responds by sending the client the next reconnect port via a gtRECO message and any data necessary for synchronization. To delineate control messages from normal data messages, a 2-byte flag field is inserted into data packets sent between a green telnet client and server as in Figure 2. While the client sleeps, any running applications in the server continue to write data as normal; however, November 2008 l www.ddj.com l Dr. Dobb’s Journal Jeremy is a student in the Department of Computer Science and Engineering at the University of South Florida. Ken is a Professor in the Department of Computer Science and Engineering at the University of South Florida. Making the Client/Server Model Green To make the client/server model green, we must allow for the client to go to sleep and thus lose its TCP connection to the server, but still maintain application state in the server. The first step in this process is to define the protocol with which the client and server will communicate power state changes. We have defined three control messages through which the 33 http://www.minix3.org http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - November 2008

Dr. Dobb's Journal - November 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Developer's Notebook Saving Open Source Conversations iPhone Building Your Own Web Server Green Telnet What's New In Boost Threads? Testing Service Oriented Architectures Test Case Generation, UML, and Eclipse Unit Testing Web Services C3 Programming The Agile Edge Swaine's Flames Effective Concurrency

Dr. Dobb's Journal - November 2008

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