Energy Efficient Client-centric Shaping of Multi-flow TCP Traffic

Transcription

1 Energy Efficient Client-centric Shaping of Multi-flow TCP Traffic Ahmad Nazir Raja Kongens Lyngby 2010 IMM-M.Sc

2 Technical University of Denmark Informatics and Mathematical Modelling Building 321, DK-2800 Kongens Lyngby, Denmark Phone , Fax IMM-M.Sc.: ISSN xxxx-xxxx

3 Abstract With the explosion of mobile devices, there is a pressing need to explore optimal ways of battery consumption. One of the widely used mechanisms for energy conservation is the IEEE b Power Save Mode (PSM). The mechanism suggests that the wireless interface should be transitioned to a low power, sleep state when idle. PSM needs infrastructure support and studies have shown that it adversely affects the throughput in some cases. Other than using PSM, there are various power saving mechanisms that either suggest a trade-off between performance and energy consumption, propose usage of power aware protocols or require additional infrastructure support. We have implemented a modified version of the client-centric PSMT protocol which is capable of minimizing energy consumption while maintaining the throughput. The protocol works by exploiting the TCP flow-control mechanism and our implementation can be deployed without changing the existing TCP functionality. Furthermore, the client-centric nature of the protocol enables easy deployment of the solution. The protocol is application independent but is targeted for bulky TCP transfers. Our solution can be used on Linux based systems and is kept portable as it can be deployed with different wireless devices without major modifications. We have tested our solution with real web-servers and our results show that the protocol can achieve upto 80%, 65% and 45% savings for one, two and four active connections respectively.

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5 Acknowledgments The thesis was conducted in Data Communications Software Lab at Department of Computer Science and Engineering, Aalto University. I owe my deepest gratitude to Associate Professor Christian W. Probst (DTU) and Associate Professor Peter Sjödin (KTH) for supervising the thesis work. I especially want to thank my instructor, Zhihua Jin, for his extended support and active collaboration during all stages of the thesis. His constructive criticism, encouragement and excellent advice has kept me focused and has led me to the completion of the project. I would like to thank Matti Siekkinen for the stimulating discussions that I ve had with him and his feedback on numerous topics has certainly been valuable. I am grateful to Mohammad Ashraful Hoque, who was involved in the initial phases of the project and contributed with his interesting ideas and solutions. I wish to thank my parents who have always been a source of inspiration and motivation for me. Nothing would have been possible without their blessings and constant guidance. I am thankful to my accommodating brother and my loving sister for their support and encouragement. Espoo, 30th June 2010 Ahmad Nazir Raja

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7 Abbreviations and Acronyms AP Access Point APSD Automatic Power Save Delivery BSS Basic Service Set CAM Constantly Awake Mode CSMA Carrier Sense Multiple Access CSMA/CA Carrier Sense Multiple Access/Collision Avoidance CSMA/CD Carrier Sense Multiple Access/Collision Detection CTS Clear to Send DCF Distribution Coordination Function DNS Domain Name Server DTIM Delivery Traffic Indication Map EBSS Extended BSS IBSS Independent BSS IEEE Institute of Electric and Electronic Engineers IOCTL Input Output Control I-TCP Indirect TCP

8 vi LWE Linux Wireless Extensions MIMO Multiple Input Multiple Output MSR Mobility Support Router MSS Maximum Segment Size PCF Point Coordination Function PSM Power Save Mode PSMP Power Save Multi-Poll PSNA Power Saving Network Protocol PSP Power Save Protocol PS-Poll Power Save-Poll PSM-T PSM-Throttling RTS Request to Send RTT Round Trip Time S-APSD Scheduled Automatic Power Save Delivery STA Station STPM Self-Tuning wireless network Power Management TCP Transmission Control Protocol TIM Traffic Indication Map U-APSD Unscheduled Automatic Power Save Delivery UDP User Datagram Protocol VoIP Voice over Internet Protocol WE Wireless Extensions WLAN Wireless Local Area Network WMM-PS Wifi Multimedia-Power Save WNI Wireless Network Interface WNIC Wireless Network Interface Card

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11 Contents Abstract Acknowledgments Abbreviations and Acronyms i iii v 1 Introduction Motivation Related Work Scope Methodology Outline IEEE Power Management IEEE WLAN Power Save Mode Performance of PSM Extensions and Other Techniques Related Work 13 4 Client-centric Traffic Shaping - PSM Throttling PSM Throttling Modifications to PSM Throttling Design Design Objectives The Components Design Overview

12 x CONTENTS 5.4 Debugging Implementation Traffic Shaping Scheduler WNI Control Debugging Protocol Tuning Parameters Other Considerations Experimentation and Results Setup Protocol Parameter Values Experiment Design Single Connections Multiple Connections Discussion and Future Work Conclusion 65

13 Chapter 1 Introduction 1.1 Motivation Mobile computing is currently one of the most popular means for accessing the Internet. Mobile devices with better computation power and greater storage capacity are being produced at a very fast pace. As these capabilities continue to grow, so does the need for better batteries and their optimal usage. Our research is motivated on the same lines i.e. optimal usage of energy in mobile devices without having any affect on the performance as perceived by the user. Wireless cards use considerable amount of energy when idle. To overcome this problem, IEEE b power saving mechanism is most widely used and is known as the Power Save Mode (PSM). PSM suggests that the wireless interface should be switched to a low power, standby state when idle. However, the implementation of such a technique comes with its own complexities since determining the exact time to switch to and from the low power mode is difficult. Staying awake wastes energy while switching to the low power mode early enough might lead to loss of incoming traffic. Further more, the technique can have an adverse affect on the throughput of connections that undergo bulky data transfers. There are a number of strategies that try to fix this issue, some suggest a trade-

14 2 Introduction off between performance and energy consumption, some propose usage of power aware protocols where some suggest the addition of a third entity or proxy between the server and the client responsible for traffic management. Ideally, a solution is required that can be implemented with the least effort and can effectively minimize the time that the wireless device needs to stay in the awake mode without degrading throughput. Hence, the motivation of our research is implementation of a client-centric technique that can be deployed easily on the wireless device, maintains throughput and yields better or at least similar results than existing power saving mechanisms. We have proposed a modified version of the client-centric protocol known as PSM-Throttling (PSMT), for energy efficient usage of wireless network interfaces compliant with IEEE b standard. Apart from being client-centric, the main advantage of the protocol is that it minimizes energy consumption without having an affect on the throughput. Furthermore, we show that our modifications help in increasing the performance of the protocol. 1.2 Related Work There has been some research on client-centric power conservation strategies that focus on yielding better results with a focus on connection throughput. Yan et al. proposed a client-centric mechanism to maximize sleep intervals by convincing the server to send data in bursts [25, 26]. However, their solution increases the data transmission time as a trade-off. They worked on both type of connections i.e. the web access and bulk data transfers while testing their technique with single connections at a time. They acquired their results by simulation the wireless device and the access point. Tan et al. proposed the PSM-Throttling protocol based on a similar client-centric mechanism. PSM- T is capable of minimizing power consumption on TCP-based bulk traffic by effectively utilizing available Internet bandwidth without degrading the application s performance perceived by the user. According to their results, PSM-T is capable of effectively improving energy savings and/or QoS for bulk transfers [22]. They tested their solution on a single connection at a time. Also, their implementation was hardware specific. The mentioned research studies have formed the basis of our work. 1.3 Scope The scope of our work is limited to mobile devices that are equipped with the b wireless cards. We have tested our solution in an infrastructure wireless

15 1.4 Methodology 3 network whereas it should also work fine in an ad-hoc configuration because of the client-centric nature of the protocol. We tackle only TCP traffic which is the most common transport layer protocol used for both web browsing and bulk data transfers (i.e. file downloads as well as multimedia streaming). The protocol requires a stable and constant traffic flow to function and therefore it is most suitable for applications that require bulky data transfers. Our technique is based on exploiting the TCP s congestion control mechanism and hence does not support any UDP traffic such as VoIP, DNS queries etc. Furthermore, the protocol is not suitable for applications with sporadic traffic patterns e.g. gaming applications etc. Lastly, the protocol focuses on scenarios in which the mobile client is the data receiver. Even though, energy savings while sending data is a much simpler case, it is not a very common one in mobile devices and is not accounted for in our solution. 1.4 Methodology We have designed and implemented our solution to be portable which enables deployment on any machine that runs the Linux Kernel. We have tested our solution on various machines including Nokia s N900 that runs the Linux based Maemo OS. However, the results discussed in this thesis are acquired from running our solution on an IBM z61m Thinkpad running Ubuntu 9.10 (Karmic Koala) and using a LinkSys Wireless Router as the access point. We measure the total time saved instead of the actual energy consumption of the device which gives us a good idea of the overall energy savings. We believe that the method yields significantly accurate results as their is a linear relationship between the time for which the wireless network interface is in the idle state and the actual energy savings. This approach for approximation of energy savings is discussed in detail in section 7.3. Our research is constructive and empirical in nature as we implement our solution on a real device and conduct our experiments get results using real web-servers. 1.5 Outline Chapter 2 provides a short overview of IEEE b networks and IEEE PSM functionality. It also describes performance inefficiencies of PSM and lists some industrial variants that are currently being used to solve these issues. Chapter 3 gives an insight into the related research works. Chapter 4 discusses the PSM Throttling protocol proposed by Tan et al. in [22]. The chapter

16 4 Introduction also includes our suggestions for modifications to the protocol which help in improving the performance of the protocol. Chapter 5 deals with the design of our solution by discussing the core components and their interaction with each other. Implementation related details are provided in chapter 6. Chapter 7 contains the experiments and their outcomes along with our analysis of the results and possible extensions to our work. We conclude the thesis in chapter 8.

17 Chapter 2 IEEE Power Management The most basic form of power management in wireless devices is specified in the IEEE standard, referred to as the Power Save Mode (PSM) [19]. In order to understand PSM functionality, it is important to know how different components within a wireless network interact as specified by the standard. 2.1 IEEE WLAN There are four basic components of a wireless network [14]: the Station (STA), the Access Point (AP), the Wireless Medium and the Distributed System (DS). The Stations are the entities that carry wireless network interfaces. These are either laptops, mobile phones etc. Wireless networks are built so that data can be transferred between these mobile devices. The Access Points perform the function of wireless-wired bridging and are responsible for relaying traffic to and from the Stations. The Distributed System is the logical entity responsible for relaying traffic from one wireless network to another with the help of Access Points. These individual networks when connected together are collectively known as a Distributed System whereas the most basic wireless network can also be referred to as a Distributed System.

18 6 IEEE Power Management The Basic Service Set (BSS) is the basic building block of the network. It comprises of a group of stations (with or without an access point) that communicate with each other. BSSs can be chained together with some backbone technology into a larger network called the Extended BSS (EBSS). The BSS can be of two types: 1. Independent BSS: The Independent BSS or IBSS is a group of stations that communicate with each other in an ad-hoc manner. Each station within the network must communicate with another station directly. Therefore, all target stations must be in range of the sender since there is no traffic relaying entity. Also, because of the absence of a central entity, power management within an IBSS is very complex. 2. Infrastructure BSS: Within an infrastructure BSS, all stations communicate with each other with the help of access points. The stations don t need to be in range of each other to communicate, they just need to be able to communicate with the access point. One of the main advantages of infrastructure networks is that since the access points are the central entity, they are aware of the conditions of every station in the BSS. This makes power management easier as access points can temporarily buffer data for stations that are trying to conserve energy. There are two coordination functions specified in standard, which are used to control access to the wireless medium: the Distribution Coordination Function (DCF) and the Point Coordination Function (PCF). The DCF uses a carrier sense multiple access (CSMA) scheme to control access to the underlying transmission medium. Unlike the Ethernet or wired-lan, DCF uses a collision avoidance scheme (CSMA/CA) instead of collision detection (CSMA/CD) which reduces traffic on the medium. The concept is that whenever a station needs to transmit data, it sends a Request-to-Send (RTS) signal which is responded by a Clear-to-Send (CTS) signal by the receiver. The RTS signal helps in silencing all stations which receive the signal. Similarly, the CTS signal also silences stations in the immediate vicinity and also reserves the medium for some time period. The data exchanges one frame at a time and every frame is acknowledged. Keeping in mind the power consumption of the stations, the RTS and CTS signals constitute of a large volume of traffic and also incur additional latency before the transmission starts. Hence, the mechanism consumes significant amount of energy. A threshold can be set on the size of the frame called the RTS-threshold which determines whether RTS/CTS signals should be exchanged before data transmission or not. PCF is built on top of DCF and is used when contention-free service is required. The PCF is not widely implemented [14].

19 2.2 Power Save Mode Power Save Mode According to [19], the wireless network interface card (WNIC) can be in either the Awake or in the Doze state. When operational, the wireless device can be either Transmitting, Receiving, Waiting or Idle. Since maximum energy is required to keep the WNIC in the awake state, PSM focuses in transition the WNIC to the Doze state when idle, without sacrificing connectivity. The standard specifies power management strategies for both the infrastructure and independent wireless networks. In infrastructure networks, the task is relatively simpler as all traffic is routed through the access points. The access points are in a position where they can buffer incoming data for dozing stations and send it to them when requested. On the other hand, power conservation in ad-hoc networks is complex and not as efficient. The sender is relied upon to ensure that the receiver is active. Receivers need to be more awake and can not sleep for same periods of time as in infrastructure BSSs. Even though the standard defines strategies for conserving power within adhoc networks, most of the wireless devices do not support the functionality. The reason is that ad-hoc networks are usually created for very limited time e.g. Data transfer, conference call etc. The vendors don t invest their time in implementing something which is not used by the end user or rarely required. The focus of our work is also limited to infrastructure BSS networks, which we use for conducting our experiments. However, our solution should work fine in an ad-hoc configuration as well PSM in Infrastructure BSSs The access point is the logical center of an infrastructure BSS and is assumed to have access to continuous power unlike the mobile stations. Therefore, it is an ideal candidate that can be held responsible for power management for all associated stations. The main idea behind PSM is that the access point maintains a record of the stations currently operating in PSM, and buffers the packets addressed to these stations until either the stations specifically require to get the packets by sending a polling request, or until they change their operation mode. The stations need to inform the access point when and for how long they intend to sleep so that data can be buffered for them until they wake up or get out of power save mode. Synchronization is the key for smooth functioning of PSM. The access point is responsible for sending signals called the beacons at regular intervals of time, usually 100ms. The beacons contain the timestamp and all listening stations have to synchronize themselves accordingly.

20 8 IEEE Power Management Beacon Incoming frame buffe red for station Beacon AP TIM (No data buffe red for station) TIM (Data buffe red for station) PS-P oll F rame A CK A ctive Station Doze Figure 2.1: Basic PSM Operation in an Infrastructure BSS The beacons also contain the traffic indication map (TIM) which is a virtual bitmap indicating that unicast data is buffered for specific stations. It is the station s responsibility to respond to the TIM to fetch its data from the access point. The station does this by sending a PS-Poll control frame to the access point requesting for the buffered data. The station in PSM can resume sleeping in the following two conditions: if the incoming data frame doesn t carry any indication that more data is buffered for it at the access point or it receives a TIM saying that there is no data buffered for that station. In case of multicast traffic, a special type of traffic indication map is used called the Delivery Traffic Indication Map (DTIM). The multicast data is delivered to the stations immediately after the beacon containing DTIM is sent. The access point is configured to send DTIM containing beacons after certain number of beacons. Multicast traffic is not buffered for the stations and therefore every station must wake up for the beacon containing the DTIM if it wants to receive multicast data. All stations need to listen to the beacons even if they are in the power saving mode. However, stations can be configured to sleep for longer periods of time by setting a listen interval parameter. This parameter determines the number

21 2.3 Performance of PSM 9 Incoming frame buffe red for station AP F rame PS-P oll A CK A ctive Station Doze W ait T ime Figure 2.2: Increase in wait time of beacons that the station can ignore before waking up again. The parameter is also a contract between the station and the access point since the access point needs to agree if it can buffer data for the station for that period of time. By configuring the listen interval with a high value, the station is able to save more energy at a cost of higher latency. 2.3 Performance of PSM The main strategy behind PSM is to switch the mobile device s WNIC to a low power state whenever it is idle. Unfortunately this technique does not account for the time when the device is waiting for data to arrive. This situation can easily occur when the access point is handling multiple clients. The standard does not specify any limit on the waiting time for the client after sending a PS-Poll control frame. There is a high probability that the PS-Poll frames are not responded to immediately, forcing the client to stay active till it receives its data or gets an indication that no more data is buffered for it. Chandra et al. [10] identified a few limiting factors of the power saving mode arguing that the mechanism is only suitable for data traffic that is sporadic whereas

22 10 IEEE Power Management Ongoing data transmission delays beacon AP Beacon P eriod Beacon Interval A ctive Station Doze Figure 2.3: Beacon Transmission gets delayed by ongoing data transfer multimedia streams tend to be isochronous. In high contention networks, PSM fails to save significant amount of energy when the beacon delivery is delayed due to an ongoing data transfer. In such a case, the beacon transmission has to be delayed till after the ongoing data transfer is complete or till the next beacon transmission time arrives. The stations need to wake up as scheduled to listen for the beacons since it is not possible for them to predict this scenario. This adds to the total time a station spends waiting and end up wasting energy and degrading throughput. In general, non-deterministic TIM interval and the associated Wait interval can have a significant impact on power savings of the device [10]. PSM works well for normal web access since the traffic is discontinuous and bursty allowing the WNIC to go into the sleep mode. On the other hand, PSM works poorly during bulk data transmissions since the WNIC needs to stay active continuously. Krashinsky et al. showed in [17] that PSM had an adverse impact on short TCP connections whose performance is dominated by the round trip time (RTT) i.e. PSM can cause an increase in RTT from %. They also showed that the use of PSM could lead to performance inversion where TCP may achieve higher throughput over a lower bandwidth link. Hence, there are two main problems

23 2.4 Extensions and Other Techniques 11 associated with b PSM i.e. it is not application-aware and it has poor support for multiple clients. 2.4 Extensions and Other Techniques There are a number of techniques that focus on improving the battery life of wireless devices. However, in this section we discuss on some of the commercial solutions that concentrate on power saving by minimizing the time that a client spends in waiting or the idle state. When a wireless device is operating without any power saving technique, it is said to be in the Constantly Awake Mode (CAM). Wireless devices usually operate in the CAM mode due to fear of throughput degradation and because power-saving features are disabled by default. PSM is the most basic form of power management in the standard, however there are other power conservation technologies being used commercially. Most of the techniques are based on the PSM and focus on improving its efficiency. Some of them are listed as below: 1. Power Save Mode - Adaptive (PSM-A): As mentioned in 2.3, PSM has a number of performance issues. In some cases, PSM has proved to degrade the throughput [10, 17]. PSM-A is a concept which adapts to the traffic trends deciding whether to use PSM or not. The usual implementation of PSM-A in wireless cards is to stay in CAM and go in PSM only if no traffic arrives e.g. for 75 ms [22]. The bounded slowdown protocol is an example which dynamically adapts to network activity and uses PSM accordingly [17]. Intel came up with their own implementation for adaptive PSM [7]. Cisco products refer to PSM-A as Fast PSP [1]. 2. Automatic Power Save Delivery (APSD): The APSD mechanism is an extension to the standard PSM and was introduced in the 8th amendment to the standard i.e e. The technique is used for data with specific QoS levels and has two operating modes i.e. unscheduled and scheduled. With S-APSD, the clients are scheduled to wake up in a scheduled manner to receive their data frames. In U-APSD, the mobile stations can poll the access point for data frames any time they want, provided that no other data transmission is taking place. The techniques are used in case of real-time transmissions and U-APSD is specially used in cases of applications transferring full-duplex data over WLAN e.g. VoIP. The technique also enables polling with the help of outgoing frames. This minimizes the delay caused by separate polling.

24 12 IEEE Power Management 3. Wifi Multimedia-Power Save (WMM-PS): The technique was introduced with the development of e and the Wifi Multimedia specification [24]. This power saving mechanism is based on U-APSD and is often implemented in handsets. As the name suggests, the main focus here is power management for multimedia applications. 4. Power Save Multi-Poll (PSMP): PSMP is part of the n standard for wireless devices with multiple transceivers. The technique concerns MIMO-based products since they have the capacity to send out and receive multiple streams of data at the same time. The technique uses the U-APSD and S-APSD mechanisms mentioned above. PSMP provides the same delivery-enable and trigger concepts and it extends the client s ability to schedule the frames that it transmits which trigger the delivering the down-link frames [11]. The mechanism reduces contention between the client and the access point and is considered to be appropriate for heavy traffic loads. 5. Spatial Multiplexing Power Save: Spatial Multiplexing Power Save Mechanism is a concept that was also introduced in the n standard with the PSMP technique [11]. It has two operational modes: static and dynamic. In static operational mode, the MIMO-based radios can be switched down to less aggressive radio configurations e.g. 1x1 from 3x3 which means only one transmitter and one receiver will be working. In the dynamic mode, the wireless device can wake up other radios when required. 6. Wake-on-Wireless: The Wake-on-Wireless is a strategy similar to the wake-on-lan technique that is frequently seen in the Ethernet adapters. To reduce the idle power, the device is essentially shut-down and its wireless network card when the device is not being used and can be powered on with an incoming frame [21]. Most of the solutions being used in the industry use a trade-off between energy savings and throughput. In the following chapter, we discuss PSM-Throttling or PSM-T protocol and propose our solution for energy conservation based on it. We have selected the PSM-T protocol as the basis of our work because of its capacity to minimize energy consumption while maintaining the throughput. Other than this unique characteristic of the protocol, it is client-centric which enables relatively easy deployment. However, our solution can also be used adaptively by determining when it s use will prove to be more beneficial.

25 Chapter 3 Related Work The use of PSM is the most basic approach for power conservation in case of mobile devices carrying b wifi cards. However, A number of performance related inefficiencies are attributed to the well known b power saving mechanism [10, 17]. In an attempt to decrease the affect of PSM on throughput, PSM-A or adaptive PSM is most widely used in the industry. The concept is to keep the WNIC in CAM when there is a possibility of arrival of data and only switch to PSM if no more incoming traffic is predicted. This technique performs much better in terms of throughput than the regular PSM but doesn t save as much energy in some cases e.g. Web Access. The mechanism has a number of commercial variants [7, 1]. There has been significant amount of research based on just minimizing the active time of the wireless device so that the WNIC can be switched to the sleep mode as often as possible. In an attempt to improve standard PSM in a similar manner, Krashinsky et al. came up with their own variant called the Bounded Slowdown (BSD) protocol. The BSD protocol dynamically adapts to network activity while guaranteeing that a connection s RTT does not increase by more than a factor p over the base RTT. They showed that PSM has an adverse affect on RTTs since it rounds them to the beacon interval time e.g. 20 ms RTT will be rounded to 100ms where beacon interval is 100ms. They also showed that

26 14 Related Work PSM can incur a performance inversion affect where TCP may achieve higher throughput over lower bandwidth. Even though the BSD protocol is effective in saving energy for web sessions as compared to the PSM and also avoids significant delays, the goal of BSD protocol is to reduce energy consumption during periods of user think times and not during transmission times. A similar technique that follows the adaptive concept is Self-tuning wireless network power management (STPM). STPM adapts to application access patterns, network interface characteristics, and the system on which it is running [4]. Another approach of reducing the idle power to a minimum is Wake on WLAN [21]. The solution suggests that the client should just go to sleep whenever it is not receiving packets and the access point should be responsible for waking up the client. Even though this solution has high potential for energy saving, it requires modification within the hardware of the mobile client. Also, the functionality at the access point needs to be implemented so that it can signal the client to wakeup or sleep. Some studies suggest that the transport layer should me modified for power awareness. Bakre et al. came up with Indirect-TCP (I-TCP) by arguing that TCP, as a transport layer protocol, is not suitable for mobile devices and they compared their solution with BSD protocol [6]. I-TCP splits the standard TCP connection into two connections: one between the access point and the mobile client and the other between the access point and the server. The access point is responsible for bridging the connections and hence is called the Mobility Support Router (MSR). I-TCP avoids the throughput degradation caused by the combined effect of the packet loss in the wireless network and the congestion control mechanism. Based on this work, there has been some research on power conservation of the whole machine rather than just the wireless device [18]. To achieve both the TCP reliability and better power consumption, Anastasi et al. came up with Power Saving Network Architecture (PSNA), an extension of I-TCP [12]. Another area of research is the use of proxies for better energy consumption. Chandra et al. identified that PSM doesn t work well in case of multimedia streams and came up with a server-side proxy solution for shaping multimedia streams [10, 9]. They showed that server-side application specific traffic shaping along with client-side history based prediction could save upto 80% energy required for network reception. However, their focus was on only popular multimedia formats. Shenoy et al. also came up with a proxy based solution [20]. The idea was to use a proxy that resides at the access point and schedules the traffic in such a way that the mobile client s power saving mechanism, being PSM, is used optimally. The main problem with proxy based solutions is that they require a change at the access point. Shaping data for specific multimedia formats is a separate challenge making the proxy solution

27 15 more complex. Most of the above mentioned techniques require a change in the access point, the client s hardware or the transport protocol being used. Client-centric solutions however focus on just modifying the client to get better energy conservation. BSD and STPM are client based solutions but they don t have to capability to control the sender s behavior for better power consumption at the client s end. Based on this concept, Chan et Al. proposed the Ack Regulator to improve TCP performance on a 3G wireless link by regulating the flow of acknowledgments back to the sender [8]. Yan et al. proposed a client-centric scheme for conserving energy by reshaping the TCP traffic into bursts and switching the WNIC into sleep mode between the bursts [25]. The technique lacks specific consideration for streaming traffic and trades-off between the data transmission time and energy consumption. The trade-off works fine for normal web access but can have an adverse affect for some bulk transmissions such as TCP based video streaming. Yan et al. compared their strategies with b PSM and the bounded slowdown protocol using their former work [26]. They use the technique for two basic applications: Web browsing and Large File Downloads or Bulk Transfers. Their results showed that for web browsing, the technique saved 21% energy compared to PSM and incurred less than a 1% increase in transmission time compared to regular TCP. For large file downloads, the scheme saved 27% energy on average with a transmission time increase in 20%. They tested with multiple connections while dealing with the first application i.e web browsing. However, for bulk transfers they performed their tests using a single connection. Wireless device was simulated in case of both [25, 26] which is a separate challenge and requires the solution to be hardware dependent. Using the techniques in [25], Tan et al. came up with a similar client centric mechanism which they called PSM Throttling (PSM-T) which is capable of minimizing power consumption on TCP-based bulk traffic by effectively utilizing available Internet bandwidth without degrading the application s performance perceived by the user. The main objective was to maintain the throughput of the TCP connection, specifically a video stream. The technique was compared with the following power modes: awake mode or the CAM, b PSM, PSM- A and the client-centric approach developed by Yan et al. [25]. Their results show that PSM-T is capable of effectively improving energy savings (upto 75%) and/or QoS for bulk transfers. However, this work also deals with a single connection. There are other solutions as well that give the receiver or the mobile client in this case the capability to control the sender s behavior such as Reception Control Protocol (RCP) [16] or TCP-Real [23]. However, these are new protocols and require development on both connection ends. Our work is based on the client-centric strategies proposed in [22, 26], specifically the PSM-T pro-

28 16 Related Work tocol. The protocol requires modification at the client end only. We focus on using the mentioned client-centric techniques to handle multiple connections by holistically shaping TCP traffic.

29 Chapter 4 Client-centric Traffic Shaping - PSM Throttling As already discussed in earlier sections, one of the most efficient ways to save power is by turning the WNI to a standby state when no packets are arriving or being sent. In case of outgoing traffic, the state of the WNI can be controlled but it is very difficult to predict the arrival times of incoming traffic with a high degree of probability. Due to this fact, the WNI has to be kept in the awake state which wastes energy. Even though, with PSM, the access point synchronizes sending of packets at regular intervals allowing the WNI at the mobile client to sleep in between, it can affect the throughput of the TCP connection and is not an optimum solution for bulk data transfers. Also, the PSM implementation in different access points differ and don t always perform as they are supposed to and can further degrade the throughput. This chapter describes a client centric technique for traffic shaping called PSM Throttling proposed by Tan et al. in [22], along with a few modifications. We implement the modified version for enhanced performance and test it for the thesis.

30 18 Client-centric Traffic Shaping - PSM Throttling 4.1 PSM Throttling PSM Throttling is a technique that detects unused bandwidth between the server and the client and uses it to shape traffic and increase predictability of packet arrival. High predictability of incoming traffic can be used to switch the state of the WNI for energy efficient communication. PSM Throttling is client centric and unlike PSM, it does not require additional infrastructure support. The technique is application independent and can minimize power consumption for TCP-based bulk data transfers. The technique effectively utilizes available Internet bandwidth without degrading the application s performance as perceived by the user [22]. In order to understand the working of the technique, it is important to understand the TCP Flow Control Mechanism which is discussed in the following section TCP Flow Control Server Client A CK r eceived W indow Size = 3KB Send 3KB Data W ait for A CK A CK r eceived Send data and wait.. Figure 4.1: TCP Windowing Flow Control TCP uses a windowing mechanism to control the flow-rate between the sender and the receiver. The sender sends a predetermined number of bytes to the client and waits for acknowledgement before sending more data. The maximum number of bytes that the sender is allowed to send to the receiver is determined with the help of Receiver Window Size that the receiver sets in every acknowl-

31 4.1 PSM Throttling 19 Send Data Server Client Start P ersist T imer and W ait for A CK T imeout! Send A CK (T CP K eepalive) Figure 4.2: TCP Windowing Flow Control with Persist Timer edgement. The sender can not send more data until the previously sent data is acknowledged by the receiver. A receiver may also set the window size with a zero value which means that it s buffer is full and can not accept any data. In such a case, the sender stops sending data and waits for a window update from the receiver. However, in case the window update gets lost, the sender is required to probe the receiver for the window update after a certain amount of time. TCP uses persist timers which determine the wait time before probing the receiver. The probe is sent in the form of a TCP Keep Alive packet which is simply a request for the receiver to send back an acknowledgement, which by default also contains the window size. As the name suggests, TCP Keep Alive packet helps in determining whether the TCP connection has terminated or not. The TCP Keep Alive is simply a TCP ACK with zero payload. The mechanism helps in protecting the TCP from a deadlock situation where the sender and client stop communicating, mainly due to network problems Protocol Design PSM Throttling relies on the fact that there can be unused bandwidth between the server and the client and uses it to minimize energy consumption without sacrificing throughput. Therefore, the technique consists of two parts: Bandwidth Throttle Detection and Traffic Burst Generation. The first part determines whether the protocol should enter the burst generation mode or not.

32 20 Client-centric Traffic Shaping - PSM Throttling Bandwidth Throttle Detection Multimedia servers such as Youtube and Google Video use a pseudo-streaming technique to deliver content [15], which means that the data is sent in small bursts. In such a case, it is easy to identify whether there is unused bandwidth or not. However, in case of other multimedia servers that provide a steady stream of data, it is not as evident to figure out whether they throttle bandwidth or not. Unused bandwidth can be detection with a quick bandwidth throttling detection algorithm with a low overhead. The basic concept behind throttle detection is to exploit the TCP flow control mechanism where the client chokes and un-chokes the connection to determine whether there is un-utilized bandwidth or not. The client chokes the connection by sending an ACK with window size set to zero. On reception of the ACK, TCP at the server-side stops sending any data to the client but the application layer of the server keeps on sending data to it s TCP. The data keeps on accumulating in the TCP s buffer. The client, after some time, sends an acknowledgement with the proper window size signaling the server that it is ready to receive data. Once, this packet is received by the server, the data stored in the buffer of TCP is sent in a burst. On reception of the burst, the client can determine whether the throughput of the connection is controlled by the server or not, with the help of the total bytes in the burst and time taken for the transfer. For efficient execution of the detection algorithm, RTT for the TCP connection is required. TCP time-stamp option is normally enabled for most TCP based services and the same can be used to calculate the RTT. The algorithm begins with the calculation of the flow-rate r, which takes place after some time T (where T >5 secs). Next, the client sends a choke ACK to the server i.e. by setting the receiver window size in the ACK to zero. This restricts the server from sending more packets, however the client might keep getting data for a RTT as packets take half RTT to reach the server. In case of throttle detection, the server s transport layer buffers the data sent by the application layer until it gets a window update. The connection is unchoked by the client by restoring the original window size after 2RTT and the new flow-rate r is calculated for 2RTT * r number of bytes. If r >r, it means that more than half of the bandwidth between the server and the client is not being utilized and can be exploited to save energy at the client side.

33 4.2 Modifications to PSM Throttling Traffic Burst Generation A concept similar to the one used during throttle detection is used to generate bursty traffic. The client sends ACKS with specific window sizes and then waits for the amount of data to arrive. On the arrival of the first packet, the client acknowledges it by sending a choke ACK preventing the server to send in more data. The client keeps on receiving data for 1 RTT after having sent the choke ACK. The client counts the number of bytes it receives and sends an open ACK once all the expected data has arrived. If the time taken to receive the burst is T recv and the interval between is called T idle so that total burst time T burst is equal to the sum T burst = (T recv + T idle ), then it makes sense to shift the WNI to the sleep state for time period T idle as long as it is non-trivial (i.e. T idle RTT). It is very important to set the value of the receiver window size small enough that that all the data can arrive within an RTT. If the number of bytes, as specified in the window size, takes more than 1 RTT to reach the client, then the algorithm will fail as the subsequent choke ACK will prevent the server from sending more data and the client will keep on waiting for the data to arrive. Therefore, an appropriate sized window should be set to avoid any deadlock. On the other hand, a very small sized window will also lead to a lot of overhead and protocol might consume more energy than normal. 4.2 Modifications to PSM Throttling PSM Throttling protocol is the basic technique for energy savings that we used for our work. However, we have proposed a few changes to the protocol design: 1. Use of Timers to transition between states: During the T recv periods of bursty traffic generation, the client waits for all the expected data to arrive. If for some reason, there is a loss of data, the client will keep on waiting. This is because the client chokes the connection on the first acknowledgement and the server is stopped from sending in more data. This causes a temporary deadlock until the server sends a TCP Keep Alive packet to check if the connection is alive or not. We propose that timers should be used, and if no packets arrive after the client having sent a choke ACK, the connection will stop waiting for data and send an open ACK to resume TCP communication. This reduces the time period T recv to a minimum i.e. maximum value can be 1 RTT. This slight modification can help reduce the overall idle time.

34 22 Client-centric Traffic Shaping - PSM Throttling Server Client Open A CK sleep W ait for W indow Update Chok e A CK Open A CK r ecv sleep Send data burst Chok e A CK r ecv sleep Figure 4.3: TCP Windowing Flow Control with Persist Timer 2. Use of TCP Keep Alives: As explained above, there can be scenarios where the protocol might get stuck in a deadlock. The use of TCP Keep Alives can be very useful to quickly resume data transfer without degrading throughput of the connection. 3. Keeping the TCP functionality intact: The PSM Throttling protocol proposes that the TCP functionality should be slightly modified to generate bursty traffic. According to the protocol description in [22], during bursty traffic generation, only the first packet arrived after sending an open ACK is acknowledged by sending back a choke ACK. The authors don t mention whether other packets should be acknowledged or not. Also, retransmission is a complex scenario which should be handled by TCP itself. TCP might acknowledge all the packets coming in or might acknowledge certain packets in a burst depending on it s implementation. We are of the opinion that the new protocol should be implemented as a supporting mechanism rather than a modification to the TCP stack. Modification of TCP leads to numerous problems which should be avoided. 4. Determining the Window Size: The authors of the protocol haven t specific how to chose the appropriate size of the receiver window size. Making the right choice is very important as a higher value might lead to

35 4.2 Modifications to PSM Throttling 23 a temporary deadlock, as explained above, and a very low value can cause extra overhead. 5. Multiple Connections: The original protocol was implemented and tested with only a single connection. In order to make the protocol usable in real life scenarios, the states of multiple connections using the protocol need to be monitored so that the state of the WNI can be controlled accordingly.

36 24 Client-centric Traffic Shaping - PSM Throttling

37 Chapter 5 Design 5.1 Design Objectives Designing the overall solution requires us to make some important decisions. Following are the most important factors that we kept in mind while designing the solution: 1. Least Hardware Dependency and Portability: Even though, control of the wifi device might need some sort of modification of the device driver, the implementation of the psm protocol should be independent of the hardware and it should be possible to implement the protocol with other wireless devices. 2. Existing Protocol Stack Behavior: There should be no (or least) modification of existing functionality of the protocol stack. The end solution should supplement the protocol stack, not modify their behavior. This will also help in reducing the communication overhead. 3. Quick Execution: Since, the overall energy saved depends on how often the WNI is transitioned to the sleep state, it is important to make these decisions quickly. Hence, the execution time of the protocol should be sufficiently fast.

38 26 Design 4. Low Computation Overhead: It is important to keep a balance between the energy saved and extra energy consumed due to communication and computation overhead. It should be kept in mind that computation overhead should be kept to a minimum as it can adversely effect the overall energy consumed. 5. Ease of Deployment: A solution that requires an infrastructural change is difficult to deploy. Since PSM Throttling is a client centric protocol, it is easy to deploy. However, it should be kept in mind that the solution should not be dependent on some other software dependencies which might make it difficult to deploy. 5.2 The Components The solution can be mainly divided into three components, the Traffic Shaping Protocol, the Scheduler and WNI control. The traffic shaping protocol is solely responsible for increasing the predictability of incoming packets and informing the scheduler about the idle times. The scheduler s job is to keep track of the idle times of all the TCP connections and invoking the WNI control component accordingly. The WNI control component is responsible for transitioning the state of the wireless network interface card based on the individual TCP connection states. In implementation, the traffic shaper and the scheduler can be grouped together but the WNI controller has to be kept separate. This separation is necessary for satisfying one of the important design goals i.e. Least Hardware Dependency and Portability. It is very difficult to come up with one single solution which takes care of all of the functionalities, since the WNI control part will most probably be hardware dependent. The components are discussed individually in the following sections: Traffic Shaping Protocol The function of the traffic shaping protocol is to increase the predictability of incoming packets so that the underlying device can be put to sleep as often as possible. As already discussed, the technique used to perform this task in our case is PSM Throttling. The protocol is client centric, application independent and focuses on energy savings for TCP-based bulk transmission without significantly affecting the throughput. The protocol is designed to work with the latest Linux kernel (ver at the time of implementation). This means that the protocol shapes TCP-based traffic on any device which runs a Linux