Cloud gaming is a promising application of the rapidly expanding cloud
Existing cloud gaming systems, however, are closed-source with
proprietary protocols, which raises the bars to setting up testbeds for
experiencing cloud games. In this paper, we present a complete cloud
gaming system, called GamingAnywhere, which is to the best of our knowledge the
first open cloud gaming system. In addition to its openness, we design
GamingAnywhere for high extensibility, portability, and reconfigurability. We
implement GamingAnywhere on Windows, Linux, and OS X, while its client can be
readily ported to other OS's, including iOS and Android. We conduct
extensive experiments to evaluate the performance of GamingAnywhere, and compare
it against two well-known cloud gaming systems: OnLive and StreamMyGame.
Our experimental results indicate that GamingAnywhere is efficient and provides
high responsiveness and video quality. For example, GamingAnywhere yields a
per-frame processing delay of 34 ms, which is 3+ and 10+ times shorter
than OnLive and StreamMyGame, respectively.
Our experiments also reveal that all these performance gains are
achieved without the expense of higher network loads; in fact, GamingAnywhere
incurs less network traffic.
The proposed GamingAnywhere can be employed by the
researchers, game developers, service providers, and end users for
setting up cloud gaming testbeds, which, we believe, will stimulate more
research innovations on cloud gaming systems.
Categories and Subject Descriptors: H.5[Information Systems Applications]: Multimedia Information Systems
General Terms: Design, Measurement
The video game market has been an important sector of both the
software and entertainment industries, e.g., the global market
of video games is expected to grow from 66 billion US dollars
in 2010 to 81 billion in 2016 . Another market
research  further breaks down the market growth into
three categories: boxed-games, online-sold games, and
cloud games. Cloud gaming systems render the game scenes on
cloud servers and stream the encoded game scenes to thin
clients over broadband networks. The control events, from mice,
keyboards, joysticks, and touchscreens are transmitted from the
thin clients back to the cloud servers. Among the three
categories, it is the cloud games market that is expected to
expand the most: nine times over the period of 2011 to 2017, at
which time it is forecast to reach 8 billion US
Cloud gaming systems attract both users and game developers for many
reasons. In particular, cloud gaming systems: (i) free users from
upgrading their hardware for the latest games, (ii) allow users to play
the same games on different platforms, including PCs, laptops, tablets,
and smartphones, and (iii) enable users to play more games by reducing
the hardware/software costs. Cloud gaming systems also allow game
developers to: (i) support more platforms, (ii) ease hardware/software
incompatibility issues, (iii) reduce the production costs, and (iv)
increase the net revenues. In fact, the market potential of cloud gaming
is tremendous and well recognized, as evidenced by Sony's recent
acquisition  of Gaikai , which is a cloud gaming
However, providing a good user experience in cloud gaming
systems is not an easy task, because users expect both
high-quality videos and low response delays. The response
delay refers to the time difference between a user input
received by the thin client and the resulting in-game action
appearing on the thin client's screen. Higher-quality videos,
such as 720p (1280x720) at 50 fps (frame-per-second),
inherently result in higher bit rates, which render the cloud
gaming systems vulnerable to higher network latency, and thus
longer response delay. Since a long response delay results in
degraded user experience, it may turn the users away from the
cloud gaming systems. User studies, for example, show that
networked games require short response delay, even as low as
100 ms, e.g., for first-person shooter games . Given
that each game scene has to go through the real-time video
streaming pipeline of rendering, encoding, transmission,
decoding, and displaying, designing a cloud gaming system to
meet the stringent response delay requirements, while still
achieving high video quality is very challenging.
One simple approach to support cloud gaming is to employ generic desktop
streaming thin clients, such as LogMeIn ,
TeamViewer , and UltraVNC . However, a
measurement study  reveals that generic thin clients
achieve low frame rates, on average 13.5 fps, which clearly lead to
sluggish game plays. A better user experience is possible with thin
clients specifically designed for cloud gaming, e.g.,
Gaikai , OnLive , and StreamMyGame .
Nevertheless, another measurement study  demonstrates that
these cloud gaming systems also suffer from non-trivial response time.
Even under a strong assumption of zero network latency, at least
134 ms and 375 ms average response delays are observed in OnLive and
StreamMyGame, respectively. The measurement
results [4,5] indicate that the problem of designing
cloud gaming systems for high video quality and fast responsiveness
In this work, we design, implement, and evaluate a cloud gaming
system called GamingAnywhere. GamingAnywhere follows three design goals.
First, in contrast to OnLive and StreamMyGame, GamingAnywhere is an
open system, in the sense that a component of the video
streaming pipeline can be easily replaced by another component
implementing a different algorithm, standard, or protocol. For
example, GamingAnywhere by default employs x264 , a
highly-optimized H.264/AVC encoder, to encode captured raw
videos. To expand GamingAnywhere for stereoscopic games, an H.264/MVC
encoder may be plugged into it without significant changes.
Since GamingAnywhere is open, various algorithms, standards, protocols,
and system parameters can be rigorously evaluated using real
experiments, which is impossible on proprietary cloud gaming
systems. Second, GamingAnywhere is cross-platform, and is
currently available on Windows, Linux, and OS X. This is made
possible largely due to the modularized design of GamingAnywhere. Third,
GamingAnywhere has been designed to be efficient, as can be seen,
for example, in its minimizing of time and space overhead by
using shared circular buffers to reduce the number of memory
copy operations. These optimizations allow GamingAnywhere to provide a
high-quality gaming experience with short response delay. In
particular, on a commodity Intel i7 server, GamingAnywhere delivers
real-time 720p videos at ≥ 35 fps, which is equivalent to
less than 28.6 ms of processing time for each video frame,
with a video quality significantly higher than that of existing
cloud gaming systems. In particular, GamingAnywhere achieves a video
quality 3 dB and 19 dB higher than that of OnLive and
StreamMyGame, in terms of average Peak Signal-to-Noise Ratio
(PSNR). PSNR is popular video quality metric, which is
inversely related to mean-squared error [40,p. 29].
This paper makes two main contributions.
We propose an open cloud gaming system, GamingAnywhere, which can be used by
cloud gaming developers, cloud service providers, and system researchers
for setting up a complete cloud gaming testbed. To the best of our
knowledge, this is the first open cloud gaming testbed in the
We conduct extensive experiments using GamingAnywhere to quantify its
performance and overhead. We also derive the optimal setups of system
parameters, which in turn allow users to install and try out GamingAnywhere on
their own servers.
The rest of this paper is organized as follows.
Section 2 surveys related work in the
literature. Section 3 presents the design
goals and Section 4 depicts the system
architecture. This is followed by the detailed implementations
in Section 5.
Section 6 gives the performance evaluation
results. We conclude the paper in Section 7.
2 Related Work
In this section, we survey the existing cloud gaming systems and
the proposals for measuring their performance.
2.1 Cloud Gaming Systems
Cloud gaming systems, or more generally real-time remote
rendering systems, have been studied in the literature. We
classify these systems into three categories: (i) 3D
graphics streaming [12,19], (ii) video
streaming [42,18], and (iii) video streaming
with post-rendering operations [33,16]. These
three approaches differ from one another in how they divide
the workload between the cloud servers and clients.
With the 3D graphics streaming approach [12,19],
the cloud servers intercept the graphics commands, compress the
commands, and stream them to the clients. The clients then
render the game scenes using its graphics chips based on
graphics command sets such as OpenGL and Direct3D. The clients'
graphics chips must be not only compatible with the streamed
graphics commands but also powerful enough to render the game
scenes in high quality and real time. 3D graphics streaming
approach does not use the graphics chips on the cloud servers,
thereby allowing each cloud server to concurrently support
multiple clients. However, as this approach imposes more
workload on the clients, it is less suitable for
resource-constrained devices, such as mobile devices and
In contrast, with the video streaming approach [42,18] the
cloud servers render the 3D graphics commands into 2D videos, compress
the videos, and stream them to the clients. The clients then decode and
display the video streams. The decoding can be done using low-cost video
decoder chips massively produced for consumer electronics. This
approach relieves the clients from computationally-intensive 3D graphics
rendering and is ideal for thin clients on resource-constrained devices.
Since the video streaming approach does not rely on specific 3D chips,
the same thin clients can be readily ported to different platforms,
which are potentially GPU-less.
The approach of video streaming with post-rendering
operations [33,16] is somewhere between the 3D graphics
streaming and video streaming. While the 3D graphics rendering is
performed at the cloud servers, some post-rendering operations are
optionally done on the thin clients for augmenting motions, lighting,
and textures . These post-rendering
operations have low computational complexity and run in real time
Similar to the proprietary cloud gaming
systems [15,29,34], the proposed GamingAnywhere employs the
video streaming approach for lower loads on the thin clients.
Differing from other systems [42,18] in the
literature, GamingAnywhere is open, modularized, cross-platform, and
efficient. To the best of our knowledge, GamingAnywhere is the first
complete system of its kind, and is of interests for
researchers, cloud gaming service providers, game developers,
and end users. Last, GamingAnywhere is flexible and can be extended to
evaluate the potentials and performance impact of
post-rendering operations. This is one of our future tasks.
2.2 Measuring the Performance of Cloud Gaming Systems
Measuring the performance of general-purpose thin client
systems has been considered in the
literature [20,26,43,30,37]. The slow-motion
benchmarking [20,26] runs a slow-motion version of an
application on the server, and collects network packet traces
between the server and thin client. It then analyzes the traces
for the performance of the thin client system. However,
slow-motion benchmarking augments the execution speed of
applications, and is thus less suitable to real-time
applications, including cloud games. The performances of
different thin clients are investigated, including X
Window , Windows NT Terminal Service ,
and VNC (Virtual Network Computing) . Packard and
Gettys  analyze the network traces between the X
Window server and client, under diverse network conditions. The
traces are used to compare the compression ratios of different
compression mechanisms, and to quantify the effects of network
impairments. Wong and Seltzer  measure the
performance of the Windows NT Terminal Service, in terms of
process, memory, and network bandwidth. The Windows NT Terminal
Service is found to be generally efficient with multi-user
access, but the response delay is degraded when the system load
is high. Tolia et al.  quantify the performance of
several applications running on a VNC server, which is
connected to a VNC thin client via a network with diverse
round-trip times (RTT). It is determined that the response
delay of these applications highly depends on the degree of the
application's interactivity and network RTT. However, because
the aforementioned techniques [20,26,43,30,37]
are designed for general-purpose thin clients, the performance
metrics they consider are not applicable to cloud gaming
systems, which impose stringent time constraints.
Recently, the performance and potentials of thin client
has been the subject of research. Chang et al.'s 
methodology to study the performance of games on
general-purpose thin clients has been employed to evaluate
several popular thin clients, including LogMeIn ,
TeamViewer , and UltraVNC .
Chang et al. establish that player performance and
Quality-of-Experience (QoE) depend on video quality and frame
rates. It is observed that the general-purpose thin clients
cannot support cloud games given that the achieved frame rate
is as low as 9.7 fps . Chen et al. 
propose another methodology to quantify the response delay,
which is even more critical to cloud
games [8,17,46]. Two proprietary cloud
gaming systems, OnLive  and
StreamMyGame , are evaluated using this methodology.
Their evaluation results reveal that StreamMyGame suffers from
a high response delay, while OnLive achieves reasonable
response delay. Chen et al. partially attribute the
performance edge of OnLive to its customized hardware platform,
which however entails a high infrastructure
cost . In addition, Lee et
al.  evaluate whether computer games
are equally suitable to the cloud gaming setting and find that
some games are more "compatible" with cloud gaming than
others. Meanwhile, Choy et al. 
evaluate whether a wide-scale cloud gaming infrastructure is
feasible on the current Internet and propose a smart-edge
solution to mitigate user-perceived delays when playing on the
In light of the literature review, the current paper tackles
the following question: Can we do better than OnLive using
commodity desktops? We employs the measurement methodologies
proposed in  to compare the proposed GamingAnywhere against
the well-known cloud gaming systems of OnLive  and
3 Design Objectives
GamingAnywhere aims to provide an open platform for researchers to
develop and study real-time multimedia streaming applications
in the cloud. The design objectives of GamingAnywhere include:
GamingAnywhere adopts a modularized design. Both platform-dependent components
such as audio and video capturing and platform-independent
components such as codecs and network protocols can be easily
modified or replaced. Developers should be able to follow the
programming interfaces of modules in GamingAnywhere to extend the
capabilities of the system. It is not limited only to games, and any
real-time multimedia streaming application such as live casting can
be done using the same system architecture.
In addition to desktops, mobile devices are now becoming one of the
most potential clients of cloud services as wireless networks are
getting increasingly more popular. For this reason, we maintain the
principle of portability when designing and implementing GamingAnywhere. Currently
the server supports Windows and Linux, while the client supports
Windows, Linux, and OS X. New platforms can be easily included by
replacing platform-dependent components in GamingAnywhere. Besides the easily
replaceable modules, the external components leveraged by GamingAnywhere are
highly portable as well. This also makes GamingAnywhere easier to be ported to
mobile devices. For these details please refer to
A system researcher may conduct experiments for real-time multimedia
streaming applications with diverse system parameters. A large
number of built-in audio and video codecs are supported by GamingAnywhere.
In addition, GamingAnywhere exports all available configurations to users
so that it is possible to try out the best combinations of
parameters by simply editing a text-based configuration file and
fitting the system into a customized usage scenario.
GamingAnywhere is publicly available at http://gaminganywhere.org/.
Use of GamingAnywhere in academic research is free of charge but researchers
and developers should follow the license terms claimed in the binary
and source packages.
Figure 1: The deployment scenario of GamingAnywhere.
4 System Architecture
The deployment scenario of GamingAnywhere is shown in Figure 1. A
user first logs into the system via a portal server, which provides a
list of available games to the user. The user then selects a preferred
game and requests to play the game. Upon receipt of the request, the
portal server finds an available game server, launches the selected game
on the server, and returns the game server's URL to the user. Finally,
the user connects to the game server and starts to play.
There is not too much to discuss for the portal server, which
is just like most Web-based services and provides only a simple
login and game selection user interface. If login and game
selection requests are sent from a customized client, it does
not even need a user interface. Actions can be sent as
REST-like [14,10] requests via standard
HTTP or HTTPS protocols.
Therefore, in this section we only focus on the game server and the game
client of GamingAnywhere.
Figure 2 shows the architecture of the game server and the
game client of GamingAnywhere. We define two types of network flows in the
architecture, the data flow and the control flow.
Whereas the data flow is used to stream audio and video (A/V) frames
from the server to the client, the control flow runs in a reverse
direction, being used to send the user's actions from the client to the
The system architecture of GamingAnywhere allows it to support any types of games,
including PC-based and Web-based games.
The game selected by a user runs on a game server. There is an agent
running along with the selected game on the same server. The agent can
be a stand-alone process or a thread injected into the selected game.
The choice depends on the type of the game and how the game is
implemented. The agent has two major tasks. The first task is to capture
the A/V frames of the game, encode the frames using the chosen codecs,
and then deliver the encoded frames to the client via the data flow.
The second task of the agent is to interact with the game. On receipt of
the user's actions from the client, it must behave as the user and play
with the game by re-playing the received keyboard, mouse, joysticks, and
even gesture events.
However, as there exist no standard protocols for delivering
users' actions, we chose to design and implement the transport
protocol for user actions by ourselves.
The client is basically a customized game console implemented by
combining an RTSP/RTP multimedia player and a keyboard/mouse logger.
The system architecture of GamingAnywhere allows observers1 by
nature because the server delivers encoded A/V frames using the standard
RTSP and RTP protocols. In this way, an observer can watch a game play
by simply accessing the corresponding game URL with full-featured
multimedia players, such as the VLC media player , which are
available on almost all OS's and platforms.
Figure 2: A modular view of GamingAnywhere server and client.
Presently, the implementation of GamingAnywhere includes the server and the
client, each of which contains a number of modules whose details of each
module are elaborated in this section. The implementation of GamingAnywhere
depends on several external libraries including
live555 , and SDL library .
The libavcodec/libavformat library is part of the
ffmpeg project, which is a package to record,
convert, and stream audio and video. We use this library to
encode and decode the A/V frames on both the server and the
client. In addition, it is also used to handle the RTP
protocol at the server.
The live555 library is a set of C++ libraries for
multimedia streaming using open standard protocols (RTSP, RTP,
RTCP, and SIP). We use this library to handle RTSP/RTP
protocols [32,31] at the client.
The Simple DirectMedia Layer (SDL) library is a
cross-platform library designed to provide low-level access to
audio, keyboard, mouse, joystick, 3D hardware via OpenGL and a
2D video frame buffer. We use this library to render audio and
video at the client.
All the above libraries have been ported to a number of platforms,
including Windows, Linux, OS X, iOS, and Android.
Figure 3: The relationships among server modules, shared buffers, and
5.1 GamingAnywhere Server
The relationships among server modules are shown in
Figure 3. Some of the modules are
implemented in separate threads. When an agent is launched,
its four modules, i.e., the RTSP server, audio source, video
source, and input replayer are launched as well. The RTSP
server and the input replayer modules are immediately started
to wait for incoming clients (starting from the path 1n and
1i in the figure). The audio source and the video source
modules are kept idle after initialization. When a client is
connected to the RTSP server, the encoder threads are launched
and an encoder must notify the corresponding source module that
it is ready to encode the captured frames. The source modules
then start to capture audio and video frames when one or more
encoders are ready to work.
Encoded audio and video frames are generated concurrently in real time.
The data flows of audio and video frame generations are depicted as the
paths from 1a to 5a and from 1v to 5v, respectively. The details
of each module are explained respectively in the following subsections.
5.1.1 RTSP, RTP, and RTCP Server
The RTSP server thread is the first thread launched in the agent. It
accepts RTSP commands from a client, launches encoders, and setups data
flows for delivering encoded frames. The data flows can be conveyed by a
single network connection or multiple network connections depending on
the preferred transport layer protocol, i.e., TCP or UDP.
In the case of TCP, encoded frames are delivered as interleaved binary
data in RTSP , hence necessitating only one data flow
network connection. Both RTSP commands and RTP/RTCP packets are sent via
the RTSP over TCP connection established with a client.
In the case of UDP, encoded frames are delivered based on the RTP over
UDP protocol. Three network flows are thus required to accomplish the
same task: In addition to the RTSP over TCP connection, two RTP over UDP
flows are used to deliver encoded audio and video frames, respectively.
We implement the mechanisms for handling RTSP commands and delivering
interleaved binary data by ourselves, while using the
libavformat library to do the packetization of RTP and RTCP
packets. If encoded frames are delivered as interleaved binary data, a
raw RTP/RTCP packet can be obtained by allocating a dynamic packet
buffer and then be sent as interleaved binary data. On the other hand,
if encoded frames are delivered via RTP over UDP, they are sent directly
to the client using libavformat.
The RTSP server thread exports a programming interface for encoders to
send encoded frames. When an encoder generates an encoded frame, it can
send out the frame to the client via the interface without knowing the
details about the underlying network connections.
5.1.2 Video Source
Capturing of game screens (frames) is platform-dependent. We
currently provide two implementations of the video source
module to capture the game screens in real time. One
implementation is called the desktop capture module,
which captures the entire desktop screen at a specified rate,
and extracts the desired region when necessary. Another
implementation is called the API intercept module, which
intercepts a game's graphics drawing function calls and
captures the screen directly from the game's back
buffer  immediately whenever the rendering
of a new game screen is completed.
Given a desired frame rate (commonly expressed in frame-per-second,
fps), the two implementations of the video source module work in
different ways. The desktop capture module is triggered in a polling
manner; that is, it actively takes a screenshot of the desktop at a
specified frequency. For example, if the desired frame rate is 24 fps,
the capture interval will be 1/24 sec ( ≈ 41.7 ms). By using a
high-resolution timer, we can keep the rate of screen captures
approximately equal to the desired frame rate.
On the other hand, the API intercept module works in an
event-driven manner. Whenever a game completes the
rendering of an updated screen in the back buffer, the API
intercept module will have an opportunity to capture the screen
for streaming. Because this module captures screens in an
opportunistic manner, we use a token bucket rate
controller  to decide whether the module should
capture a screen in order to achieve the desired streaming
frame rate. For example, assuming a game updates its screen 100
times per second and the desired frame rate is 50 fps, the API
intercept module will only capture one game screen for every
two screen updates. In contrast, if the game's frame rate is
lower than the desired rate, the module will re-use the
last-captured game screens to meet the desired streaming frame
Each captured frame is associated with a timestamp, which
is a zero-based sequence number.
Captured frames along with their timestamps are stored in a
shared buffer owned by the video source module and shared
with video encoders. The video source module serves as the
only buffer writer, while the video encoders are all buffer
readers. Therefore, a reader-writer lock must be acquired
every time before accessing the shared buffer.
Note that although only one video encoder is illustrated in
Figure 3, it is possible to run
multiple video encoders simultaneously depending on the
usage scenario. We discuss this design choice between a
single encoder and multiple encoders in
At present, the desktop capture module is implemented in Linux
and Windows. We use the MIT-SHM extension for the X Window
system to capture the desktop on Linux and use GDI to capture
the desktop graphics on Windows. As for the API intercept
module, it currently supports DirectDraw and Direct3D games by
hooking DirectX APIs on Windows. Both modules support captured
frames of pixel formats in RGBA, BGRA, and YUV420P, with a high
extensibility to incorporate other pixel formats for future
5.1.3 Audio Source
Capturing of audio frames is platform-dependent as well. In our
implementation, we use the ALSA library and Windows audio
session API (WASAPI) to capture sound on Linux and Windows,
The audio source module regularly captures audio frames
(also called audio packets) from an audio device
(normally the default waveform output device).
The captured frames are copied by the audio source module to a
buffer shared with the encoder. The encoder will be awakened
each time an audio frame is generated to encode the new frame.
To simplify the programming interface of GamingAnywhere, we require each
sample of audio frames to be stored as a 32-bit signed integer.
One issue that an audio source module must handle is the frame
discontinuity problem. When there is no application generating any
sound, the audio read function may return either 1) an audio frame with
all zeros, or 2) an error code indicating that no frames are currently
If the second case, an audio source module needs to still emit
silence audio frames to the encoder because encoders normally
expect continuous audio frames no matter whether audible sound
is present or not. Therefore, an audio source module must emit
silence audio frames in the second case to resolve the frame
discontinuity problem. We observed that modern Windows games
often play audio using WASAPI, which suffers from the frame
discontinuity problem. Our WASAPI-based audio source module has
overcome the problem by carefully estimating the duration of
silence periods and generating silence frames accordingly, as
illustrated in Figure 4. From the figure, the
length of the silence frame should ideally be t1−t0;
however, the estimated silence duration may be slightly longer
or shorter if the timer accuracy is not sufficiently high.
Figure 4: Sample audio signals that may cause the frame
5.1.4 Frame Encoding
Audio and video frames are encoded by two different encoder modules,
which are launched when there is at least one client connected to the
GamingAnywhere currently supports two encoding modes: 1)
one-encoder-for-all and 2)
one-encoder-each-client to support different usage
In the one-encoder-for-all mode, the frames
generated by a frame source are only read and encoded by one
encoder regardless of the number of
observers2. Therefore, a total of two
encoders, one for video frames and another for audio frames, are in
charge of encoding tasks.
The benefit of this mode is better efficiency as the CPU usage does not
increase when there are more observers. All the video and audio frames
are encoded only once and the encoded frames are delivered to the
corresponding clients in a unicast manner.
On the other hand, the one-encoder-each-client mode
allocates a dedicated encoder for each client, either a
player or an observer. The benefit is that it is
therefore possible to use different encoding
configurations, such as bit rate, resolution, and quality
parameters, for different clients.
However, the consumed CPU resources would increase
proportionally with the number of encoders. For example,
in our study, each x264 encoder with 1280x720
resolution and 24 fps increases the CPU utilization by
nearly 10% on an Intel 2.66 GHz i5 CPU. In this way, a game
server can only tolerate 10 observers at most when only one
game instance is running. Therefore, the tradeoff between
the one-encoder-for-all mode3 and
one-encoder-each-client mode needs to be seriously
considered as it may have large performance impacts on the
Presently, both the video and audio encoder modules are
implemented using the libavcodec library, which is
part of the ffmpeg project. The libavcodec library
supports various audio and video codecs and is completely
written in the C language. Therefore, GamingAnywhere can use
any codec supported by libavcodec.
In addition, since the libavcodec library is
highly extensible, researchers can easily integrate their
own code into GamingAnywhere to evaluate its performance in cloud
5.1.5 Input Handling
The input handling module is implemented as a separate thread.
This module has two major tasks: 1) to capture input events on
the client, and 2) to replay the events occurring at the client
on the game server.
Unlike audio and video frames, input events are delivered via a
separated connection, which can be TCP or UDP.
Although it is possible to reuse the RTSP connection for sending input
events from the client to the server, we decided not to adopt this
strategy for three reasons: 1) The delivery of input events may be
delayed due to other messages, such as RTCP packets, sent via the same
RTSP connection. 2) Data delivery via RTSP connections incurs slightly
longer delays because RTSP is text-based and parsing text is relatively
time-consuming. 3) There is no such standard of embedding input events
in an RTSP connection. This means that we will need to modify the RTSP
library and inevitably make the system more difficult to maintain.
The implementation of the input handling module is
intrinsically platform-dependent because the input event
structure is OS- and library-dependent. Currently GamingAnywhere
supports the three input formats of Windows, X Window, and SDL.
Upon the receipt of an input event4, the input handling module
first converts the received event into the format required by
the server and sends the event structure to the server. GamingAnywhere
replays input events using the SendInput function on
Windows and the XTEST extension on Linux. While the above
replay functions work quite well for most desktop and game
applications, some games adopt different approaches for
capturing user inputs. For example, the SendInput
function on Windows does not work for Batman and Limbo, which
are two popular action adventure games. In this case, GamingAnywhere can
be configured to use other input replay methods, such as
hooking the GetRawInputData function on Windows to
"feed" input events whenever the function is called by the
Figure 5: The relationships among client modules, shared buffers, and
5.2 GamingAnywhere Client
The client is basically a remote desktop client that displays
real-time game screens which are captured at the server and
delivered in the form of encoded audio and video
The relationships among client modules are shown in
The GamingAnywhere client contains two worker threads: one is used to handle user
inputs (starting from path 1i) and the other is used to render audio
and video frames (starting from path 1r).
In this section, we divide the discussion on the client
design into three parts, i.e., the network protocols, the
decoders, and input handling.
5.2.1 RTSP, RTP, and RTCP Clients
In the GamingAnywhere client, we use the live555 library to handle the
The live555 library is entirely written in C++ with an
event-driven design. We take advantage of the class framework of
live555 and derive from the RTSPClient and
MediaSink classes to register callback functions that handle
Once the RTSP client has successfully set up audio and
video sessions, we create two sink classes to respectively handle the
encoded audio and video frames that are
received from the server.
Both sink classes are inherited from the MediaSink class and
the implemented continuePlaying virtual function is called when
the RTSP client issues the PLAY command.
The continuePlaying function attempts to receive an encoded
frame from the server. When a frame is received successfully, the
function triggers a callback function that puts the frame in a buffer
and decodes the video frame if possible. The continuePlaying
function will then be called again to receive the next frame.
5.2.2 Frame Buffering and Decoding
To provide better gaming experience in terms of latency, the video
decoder currently does not buffer video frames at all. In other
words, the video buffer component in Figure 5 is
simply used to buffer packets that are associated with the latest video
Because live555 provides us with packet payloads
without an RTP header, we detect whether consecutive packets
correspond to the same video frame based on the marker
bit  in each packet.
That is, if a newly received packet has a zero marker bit
(indicating that it is not the last packet associative
with a video frame), it will be appended into the buffer;
otherwise, the decoder will decode a video frame based on all
the packets currently in the buffer, empty the buffer, and
place the newly arrived packet in the buffer.
Although this zero-buffering strategy may lead to inconsistency
in video playback rate when network delays are
unstable , it reduces the
input-response latency due to video playout to a minimum level.
We believe that this design tradeoff can yield a overall better
cloud gaming experience.
The way GamingAnywhere handles audio frames is different from its handling of
video frames. Upon the receipt of audio frames, the RTSP client thread
does not decode the frames, but instead simply places all the received
frames in a shared buffer (implemented as a FIFO queue).
This is because the audio rendering of SDL is implemented using
an on-demand approach. That is, to play audio in SDL, a
callback function needs to be registered and it is called whenever
SDL requires audio frames for playback.
The memory address m to fill audio frames and the number of required
audio frames n are passed as arguments to the callback function. The
callback function retrieves the audio packets from the shared buffer,
decodes the packets, and fills the decoded audio frames into the
designated memory address m.
Note that the callback function must fill exactly n audio
frames into the specified memory address as requested. This
should not be a problem if the number of decoded frames is
more than requested. If not, the function must wait
until there are sufficient frames. We implement the waiting
mechanism for sufficient frames using a mutual exclusive
lock (mutex). If the RTSP client thread has received new
audio frames, it will append the frames to the buffer and
also trigger the callback function to read more frames.
5.2.3 Input Handling
The input handling module on the client has two major tasks.
One is to capture input events made by game players, and the
other is to send captured events to the server.
When an input event is captured, the event structure is
sent to the server directly. Nevertheless, the client still has
to tell the server the format and the length of a captured
At present, GamingAnywhere supports the mechanism for cross-platform SDL event
capturing. In addition, on certain platforms, such as Windows, we
provide more sophisticated input capture mechanisms to cover games with
special input mechanisms and devices. Specifically, we use the
SetWindowsHookEx function with WH_KEYBOARD_LL and
WH_MOUSE_LL hooks to intercept low-level keyboard and mouse
events. By so doing we can perfectly mimic every move of the players'
inputs on the game server.
Figure 6: The network topology of our experiments.
6 Performance Evaluation
In this section, we evaluate GamingAnywhere via extensive experiments, and compare
its performance against two well-known cloud gaming systems.
We have set up a GamingAnywhere testbed in our lab. We conduct the experiments
using Windows 7 desktops with Intel 2.67 GHz i7 processors if not
otherwise specified. For evaluation purposes, we compare the performance
of GamingAnywhere against OnLive  and StreamMyGame (SMG) .
Figure 6 illustrates the experimental setup, which
consists of a server, a client, and a router. The OnLive server resides
in OnLive's data centers, while the GamingAnywhere and SMG servers are installed
on our own PCs. More specifically, the OnLive client connects to the
OnLive server over the Internet, while the GamingAnywhere and SMG clients connect
to their servers via a LAN. To evaluate the performance of the cloud
gaming systems under diverse network conditions, we add a FreeBSD router
between the client and server, and run dummynet on it to inject
constraints of delays, packet losses, and network bandwidths.
Because the OnLive server is outside our LAN, the quality of the network
path between our OnLive client and the server might affect our
evaluations. However, according to our observations, the quality of the
path was consistently good throughout the experiments. The network delay
of the path was around 130 ms with few fluctuations. Furthermore, the
packet loss rates were measured to be less than 10−6 when receiving
OnLive streams at the recommended 5 Mbps. Therefore, the path between
the OnLive server and our client can be considered as a communication
channel with sufficient bandwidth, zero packet loss rate, and a constant
130 ms latency.
Since the performance of cloud gaming systems may be game-dependent, we
consider games from three popular categories: action adventure,
first-person shooter, and real-time strategy. We pick a representative
game from each category, and briefly introduce them in the following.
LEGO Batman: The Videogame (Batman)  is
an action-adventure game, created by Traveller's Tales in 2008. All the
interactive objects in this game are made of Lego bricks.
In this game, players control the characters to fight enemies
and solve puzzles from a third-person perspective.
F.E.A.R. 2: Project Origin (FEAR)  is a
first-person shooter game, developed by Monolith Productions in 2009.
The combat scenes are designed to be as close to those in real life as
possible. In this game, players have great freedom to interact with the
environments, e.g., they can flip over a desk to take cover.
Warhammer 40,000: Dawn of War II (DOW)  is a
real-time strategy game developed by Relic Entertainment in 2009. In the
campaign mode, players control squads to fight against enemies and
destroy the buildings. In the multiplayer mode, up to 8 players play
matches on the same map to complete a mission, such as holding specific
Modern video encoders strive to achieve the highest video
quality with the smallest bit rate by applying complex coding
techniques. However, overly-complex coding techniques are not
feasible for real-time videos given their lengthy encoding
time. As such, we empirically study the tradeoff among the bit
rate, video quality, and frame complexity using x264.
More specifically, we apply the real-time encoding parameters
summarized in Appendix , and exercise a wide
spectrum of other encoding parameters. We then analyze the
resulting video quality and encoding time. Based on our
analysis, we recommend the following x264 encoding
where $r is the encoding rate.
We configure the GamingAnywhere server to use the above-mentioned encoding
and we set the encoding bit rate to be 3 Mbps. For a fair comparison,
all games are streamed at a resolution of 720p. Whereas we configure
GamingAnywhere and OnLive to stream at 50 fps, StreamMyGame only supports
streaming at 25 fps. We design the experiments to evaluate the three
gaming systems from two critical aspects: responsiveness and
video quality. We also conduct experiments to quantify the
network loads incurred by different cloud gaming systems. The details of
the experimental designs and results are given in the rest of this
We define response delay (RD) to be the time difference between a user
submitting a command and the corresponding in-game action appearing on
the screen. Studies [8,17,46] report that players of
various game categories can tolerate different degrees of RD; for
example, it was observed that first-person shooter game players demand
for less than 100 ms RD . However, since measuring RD in
cloud gaming systems is not an easy task (as discussed in
Section 2.2), we adopt the RD measurement procedure
proposed in , in which the RD is divided into three
Processing delay (PD) is the time required for the server to
receive and process a player's command, and to encode and transmit the
corresponding frame to that client.
Playout delay (OD) is the time required for the client to receive,
decode, and render a frame on the display.
Network delay (ND) is the time required for a round
of data exchange between the server and client. ND is also
known as round-trip time (RTT).
Therefore, we have RD=PD+OD+ND.
Figure 7: Response delays of GamingAnywhere, OnLive, and StreamMyGame.
ND can be measured using probing packets, e.g., in ICMP
protocol, and is not controllable by cloud gaming systems.
Moreover, ND in a LAN is much smaller than that in the
Internet. Therefore, for a fair comparison among the cloud
gaming systems, we exclude ND from RD measurements in the rest
of this paper. Measuring PD (at the server) and OD (at the
client) is much more challenging, because they occur internally
in the cloud gaming systems, which may be closed and
proprietary. The procedure detailed in  measures
the PD and OD using external probes only, and thus works for
all the considered cloud gaming systems.
For GamingAnywhere, we further divide the PD and OD into subcomponents by
instrumenting the server and client. More specifically, PD is divided
into: (i) memory copy, which is the time for copying a raw image
out of the games, (ii) format conversion, which is the time for
color-space conversion, (iii) video encoding, which is the time
for video compression, and (iv) packetization, which is the time
for segmenting each frame into one or multiple packets. OD is divided
into: (i) frame buffering, which is the time for receiving all the
packets belonging to the current frame (ii) video decoding, which
is the time for video decompression, and (iii) screen rendering,
which is the time for displaying the decoded frame.
Results. Figure 7 reports the average PD (server)
and OD (client) achieved by the considered cloud gaming systems. From
this figure, we make several observations. First, the OD is small, ≤ 31 ms, for all cloud gaming systems and considered games. This reveals
that all the decoders are efficient, and the decoding time of different
games does not fluctuate too much. Second, GamingAnywhere achieves a much smaller
PD, at most 34 ms, than OnLive and SMG, which are observed to be as
high as 191 and 365 ms, respectively. This demonstrates the
efficiency of the proposed GamingAnywhere: the PDs of OnLive and SMG are 3+ and
10+ times longer than that of GamingAnywhere. Last, among the three systems,
only GamingAnywhere achieves sub-100 ms RD, and may satisfy the stringent delay
requirements of networked games .
Figure 8: Delay decomposition of GamingAnywhere.
Figure 8 presents the decomposed delay
subcomponents of PD and OD. This figure reveals that the GamingAnywhere
server and client are well-tuned, in the sense that all the
steps in the pipeline are fairly efficient. Even for the most
time-consuming video encoding (at the server) and video
rendering (at the client), each frame is finished in at most
16 and 7 ms on average. Such a low delay contributes to the
superior RD of GamingAnywhere, compared to the other well-known cloud
Figure 9: Network loads incurred by the considered cloud gaming systems.
6.3 Network Loads
We next quantify the network loads incurred by GamingAnywhere. In particular, we
recruit an experienced gamer, and ask him to play each game using
different cloud gaming systems. Every game session lasts for 10 minutes,
and the network packets are captured by Wireshark. For a fair
comparison, the player is asked to follow two guidelines. First, he
shall visit as many areas as possible and fight the opponents as in
normal game plays. Second, he shall repeat his actions and trajectories
as much as possible.
Results. Figure 9 plots the uplink and
downlink traffic characteristics, including bit rate, packet
rate, and payload length. The bar charts show the average
values with 95% confidence intervals.
Figures 9(a)-9(c) reveal
that the proposed GamingAnywhere incurs a much lower uplink traffic
loads, compared to OnLive and SMG. The only exception is that,
with Batman, SMG incurs lower uplink packet rate
(Figure 9(b)). However, SMG also produces a
larger uplink payload size (Figure 9(c)),
which leads to a higher uplink bit rate than that of GamingAnywhere
Figures 9(d)-9(f) reveal
that the downlink bit rates of OnLive are between 3-5 Mbps,
while those of SMG are between 10-13 Mbps. This finding
indicates that the compression algorithm employed by OnLive
achieves up to a 4.33 times higher compression rate, compared
to that of SMG.
Figure 10: Achieved video quality in PSNR under different network conditions.
Figure 11: Achieved video quality in SSIM under different network conditions.
We can make another observation on
Figure 9(d): GamingAnywhere incurs a download bit rate
≤ 3 Mbps, which is also much lower than that of SMG.
However, given that we set the encoding bit rate at 3 Mbps,
the download bit rate should never be smaller than that.
We took a closer look and found that, with GamingAnywhere, only Batman
achieves 50 fps; FEAR and DOW only achieve 35-42 fps, which
leads to lower download bit rate and may result in irregular
playouts. Our in-depth analysis shows that, unlike Batman, both
FEAR and DOW use multisampling surfaces, which cannot be
locked for memory copy operations. More specifically, an
additional non-multisampling surface and an extra copy
operation are required for FEAR and DOW, which in turn hurts
the achieved frame rates. As one of our future tasks, we will
optimize the multi-threaded design of the GamingAnywhere server, so as to
minimize the synchronization overhead.
In summary, we have shown that GamingAnywhere incurs much lower network traffic
loads. Even though the current GamingAnywhere implementation only achieves 35-42
fps for games using multisampling surfaces, such a frame rate is still
much higher than the 25 fps of SMG. On the other hand the slightly lower
achieved frame rate may affect the fairness of video quality comparisons
between GamingAnywhere and OnLive. Therefore, in the rest of this section, we only
report results from Batman.
6.4 Video Quality
Video streaming quality directly affects gaming experience, and network
conditions are the keys for high-quality streaming. In this light, we
use dummynet to control three network condition metrics: network
delay (ND), packet loss rate, and network bandwidth. We vary ND between
0-600 ms, packet loss rate between 0-10%, and bandwidth 1-6 Mbps in
our experiments. We also include experiments with unlimited
bandwidth. For OnLive, the ND in the Internet is already 130 ms and thus
we cannot report the results from zero ND. Two video quality metrics,
PSNR [40,p. 29] and Structural Similarity (SSIM) ,
are adopted. We report the average PSNR and SSIM values of the
Results. Figures 10 and 11 present
the PSNR and SSIM values, respectively. We make four observations on
these two figures. First, ND does not affect the video quality too much
(Figures 10(a) and 11(a)). Second, GamingAnywhere
achieves much higher video quality than OnLive and SMG: up to 3 dB and
0.03, and 19 dB and 0.15 gaps are observed, respectively. Third, GamingAnywhere
suffers from quality drops when packet loss rate is nontrivial, as
illustrated in Figures 10(b) and 11(b).
This can be attributed to the missing error resilience mechanism in
GamingAnywhere. Nevertheless, high packet loss rates are less common in modern
networks. Last, Figures 10(c) and 11(c)
show that the video quality of GamingAnywhere suddenly drops when the bandwidth is
smaller than the encoding bit rate of 3 Mbps. A potential future work
to address this is to add a rate adaptation heuristic to
dynamically adjust the encoding bit rate, in order to utilize all the
available bandwidth without overloading the networks.
7 Conclusions and Future Work
In this paper, we presented GamingAnywhere, which is the first open cloud gaming
system designed to be open, extensible, portable, and fully
configurable. Through extensive experiments, we have shown that GamingAnywhere
significantly outperforms two well-known, commercial, cloud gaming
systems: OnLive and StreamMyGame. Compared to GamingAnywhere, for example, OnLive
and StreamMyGame suffer from up to 3 and 10 times higher processing
delays, as well as 3 dB and 19 dB lower video quality, respectively.
GamingAnywhere is also efficient: it incurs lower network loads in both uplink and
downlink directions. Given that GamingAnywhere is open, cloud game developers,
cloud service providers, system researchers, and individual users may
use it to set up a complete cloud gaming testbed. GamingAnywhere in publicly
available at http://gaminganywhere.org. We hope that the
release of GamingAnywhere will stimulate more research innovations on cloud gaming
systems, or multimedia streaming applications in general.
We are actively enhancing GamingAnywhere in several directions. First, we
strive to further reduce the delay at the GamingAnywhere server by
minimizing the synchronization overhead. This will allow us to
increase the achieved frame rate. Second, we are designing a
practical rate control algorithm for GamingAnywhere, which may not be
very useful in resourceful LANs, but is critical for remote
players. Third, we are considering adding error resilience
mechanisms to GamingAnywhere, in order to cope with packet loss due to,
e.g., network congestion, hardware failures, and misconfigured
In addition to playing a game themselves, hobbyists may also like to
watch how other gamers play the same game. An observer can only
watch how a game is played but cannot be involved in the game.
In the current design, there can be one player and unlimited
observers simultaneously in a game session.
3. It is also
possible to provide differential streaming quality for
different clients in the one-encoder-for-all mode by
adopting scalable video codecs such as H.264/SVC.
4. The capturing of
input events on clients will be elaborated in
Sheng-Wei Chen (also known as Kuan-Ta Chen) http://www.iis.sinica.edu.tw/~swc
Last Update September 19, 2017