Practicalizing Delay-Tolerant Mobile Apps with Cedos - NDSL!

Practicalizing Delay-Tolerant Mobile Apps with Cedos - NDSL!
Practicalizing Delay-Tolerant Mobile Apps with Cedos
YoungGyoun Moon, Donghwi Kim, Younghwan Go, Yeongjin Kim,
Yung Yi, Song Chong, and KyoungSoo Park
Department of Electrical Engineering, KAIST
Daejeon, Republic of Korea
{ygmoon, dhkim, yhwan}@ndsl.kaist.edu, [email protected],
{yiyung, songchong}@kaist.edu, [email protected]
ABSTRACT
1.
Delay-tolerant Wi-Fi offloading is known to improve overall mobile network bandwidth at low delay and low cost. Yet, in reality,
we rarely find mobile apps that fully support opportunistic Wi-Fi
access. This is mainly because it is still challenging to develop
delay-tolerant mobile apps due to the complexity of handling network disruptions and delays.
In this work, we present Cedos, a practical delay-tolerant mobile
network access architecture in which one can easily build a mobile app. Cedos consists of three components. First, it provides a
familiar socket API whose semantics conforms to TCP while the
underlying protocol, D2 TP, transparently handles network disruptions and delays in mobility. Second, Cedos allows the developers to explicitly exploit delays in mobile apps. App developers
can express maximum user-specified delays in content download
or use the API for real-time buffer management at opportunistic
Wi-Fi usage. Third, for backward compatibility to existing TCPbased servers, Cedos provides D2 Prox, a protocol-translation Web
proxy. D2 Prox allows intermittent connections on the mobile device side, but correctly translates Web transactions with traditional
TCP servers. We demonstrate the practicality of Cedos by porting
mobile Firefox and VLC video streaming client to using the API.
We also implement delay/disruption-tolerant podcast client and run
a field study with 50 people for eight weeks. We find that up to
92.4% of the podcast traffic is offloaded to Wi-Fi, and one can
watch a streaming video in a moving train while offloading 48%
of the content to Wi-Fi without a single pause.
Wi-Fi has become the most popular secondary network interface
for high-speed mobile Internet access on mobile devices. Many
mobile apps support the “Wi-Fi only” mode that allows the users to
shun expensive cellular communication while enjoying high bandwidth and low delay. In addition, cellular ISPs are actively deploying Wi-Fi access points (APs) to further increase the mobile
Internet access coverage [1, 2, 3].
However, current Wi-Fi usage is often statically bound to the location of mobile devices. While this “on-the-spot” Wi-Fi offloading
is still effective, recent studies suggest that one can further extend
the benefit of Wi-Fi access if we allow delay tolerance between network connections [4, 5]. The basic idea is to hold off using cellular
networks in the absence of Wi-Fi service and to resume the data
transfer when the device meets the next Wi-Fi AP. For example, [5]
reveals that Wi-Fi offloading ratio, the fraction of data offloaded to
Wi-Fi from cellular networks, could be increased to 93.7% if we
allow only one hour of delay between Wi-Fi connections.
Despite the great potential of delay-tolerant Wi-Fi offloading,
existing mobile apps rarely support disrupted network operations.
This is mainly because the burden of handling network disruption or delay is placed solely on app developers. In reality, it is
challenging to build delay-tolerant mobile apps. Existing network
stacks are clumsy at addressing network disruptions/delays since
they are designed to work in real-time environments. Moreover,
popular servers are unfriendly to delay-tolerant apps that intermittently download the content in multiple networks. We find that
many popular mobile apps do not properly address network switchings and few apps correctly handle a few minutes of delay between
network connections.
Delay tolerance in mobile Internet access requires addressing
two mobility events: network disruption and delay between network connections. We refer to network disruption as an event that
a mobile device switches from one network to another due to user
mobility. Network disruption often results in IP address change,
which forces the termination of on-going TCP connections. To recover from a network disruption event, mobile apps should be programmed to resume from the last downloaded offset or to download
the content from the start in a separate connection. Unfortunately,
download resumption cannot be a general option since the content
could be dynamically generated or the application layer protocol
may not support it. Re-downloading from the start is also undesirable since it wastes network bandwidth and power consumption.
Exploiting a long delay (e.g., a few hours) between preferred network connections is another major issue. To maximize the Wi-Fi
usage opportunity, a mobile app may want to resume network transfer only when the mobile device is within the Wi-Fi coverage. But
at the same time, it may want to avoid an indefinite delay but to
Categories and Subject Descriptors
C.2.1 [Computer-Communication Networks]: Network Architecture and Design—Wireless communication
General Terms
Algorithms, Design, Experimentation, Performance
Keywords
Delay-Tolerant Networking; Mobility; Wi-Fi Offloading
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MobiSys’15, May 18–22, 2015, Florence, Italy.
Copyright 2015 ACM 978-1-4503-3494-5/15/05 ...$15.00.
http://dx.doi.org/10.1145/2742647.2742664.
INTRODUCTION
Type
SNS
Podcast
Media
streaming
App
Twitter [6]
Facebook [7]
Podcast Addict [8]
BeyondPod [9]
Podcast Republic [10]
YouTube [11]
TuneIn Radio [12]
Web
browsing
MXPlayer [13]
Google Play Movie [14]
VLC [15]
Chrome [16]
OperaMini [17]
Online
shopping
eBay [18]
Amazon [19]
Google Play Book [20]
Disruption Test
Task
Result
Upload a photo
3 Resume
Download 5MB or
larger podcast file
3 Resume
Stream video
Post comment
Stream audio
Tune into channel
Stream video
3 Resume
7 Fail
3 Resume
7 Fail
3 Resume
7 Fail
Download 100MB file
Load static web page
Download 100MB file
Load static web page
Load page
Load page
Play demo song
Load ebook
7 Fail
7 Fail
3 Resume
7 Fail
7 Fail
7 Fail
7 Fail
7 Fail
Task
Upload a photo
Delay Test
Result
3 Resume
7 Fail if D > 5 min
Download 5MB or 3 Resume
larger podcast file
7 Fail if D > 2 min
7 Fail if D > 1 min
Stream video
7 Playback interrupts
Opportunistic
Wi-Fi Offloading
7 No consideration
Stream audio
7 Playback interrupts
7 “Wi-Fi only” (W1, W3)
7 “Wi-Fi only” (W2)
7 “Wi-Fi only” (W3)
7 No consideration
for non-HD movies
7 No consideration
Stream video
7 Playback interrupts
Download 100MB
file
7 Fail if D > 5 min
7 No consideration
7 Fail if D > 1 min
Load page
Load page
Play demo song
Load ebook
7 Fail if D > 1 min
7 Fail if D > 30 sec
7 Fail
7 Fail if D > 1 min
7 No consideration
7 “Wi-Fi only” (W1, W3)
* ‘Fail’ means no download resumption nor re-downloading. D refers to delay.
Table 1: Behavior of popular mobile apps at mobility events
continue the transfer with a cellular network if the delay exceeds
a certain threshold. To the best of our knowledge, no existing solutions support a long delay of network unavailability in the same
TCP connection.
In this work, we bridge the gap between theory and practice in
delay-tolerant Wi-Fi offloading. We propose Cedos, a practical
delay-tolerant mobile network architecture for mobile apps. Cedos
eases the development of a mobile Internet app with three salient
features. First, it provides a familiar socket-like API whose semantics conforms to TCP in stationary environments. Developers
write the code just like they do with the Berkeley socket API, but
the underlying protocol below the API, D2 TP, transparently handles mobility events such as network disruptions or delays. Second, Cedos network API explicitly exposes how to handle delays
between network connections. Developers may express a deadline
and a content size with the API, and D2 TP completes the content
download in time while exploiting the Wi-Fi network as much as
possible. Also, the API provides real-time buffer management such
that the transport layer automatically switches between cellular and
Wi-Fi networks depending on the buffer availability. This allows to
build a streaming application that maximizes opportunistic Wi-Fi
usage without risk of being interrupted in the middle of streaming. Third, for backward compatibility, Cedos provides a protocol
translation Web proxy, D2 Prox. D2 Prox manages the delay-tolerant
HTTP connections with mobile clients while it downloads the content from the origin Web server using TCP. It also caches popular
content to reduce the bandwidth consumption on the server side.
This paper makes two key contributions. First, while many previous works address challenges in delay-tolerant networking and
Wi-Fi offloading, Cedos provides the first comprehensive solution
to delay-tolerant mobile app development and deployment. There
are many works that separate the host identity from its location [21,
22, 23, 24, 25, 26], but they do not provide network stack abstraction. TCP migrate [27] and Serval [28] support host mobility in
a TCP connection, but they neither allow long delays nor expose
an API to maximize opportunistic network access. Wiffler [4] exploits the delay in Wi-Fi offloading, but their network API does not
conform to the TCP semantics. The goal of Cedos is to popularize delay-tolerant Wi-Fi offloading while significantly reducing the
complexity of app development and deployment.
Second, we demonstrate the practicality of Cedos by porting existing mobile apps, VLC streaming client [29] and Mobile Firefox [30], with different network usage patterns. Not surprisingly,
we find that large, non-interactive file downloads benefit the most
from Cedos, but our real-time buffer management helps maximize
Wi-Fi offloading opportunities even for real-time video streaming.
In our evaluation with Cedos-vlc, we find that almost half (48%)
of the streaming video is delivered via opportunistic Wi-Fi access
without a single playback pause on a moving train. The benefit for
short, interactive Web flows with Cedos-firefox is modest due to
noticeable delay in network switching (e.g., seconds), but we find
that porting process is straightforward since the Cedos network API
is similar to Berkeley socket API.
Along with porting existing apps, we build our first delay-tolerant
podcast app, ReadyCast [31], to demonstrate the utility of delaytolerance in real world. Unlike real-time podcast apps, ReadyCast
allows “download reservation” with a user-specified deadline, and
the content is made ready by the time the user wants. We have conducted a field study with 50 ReadyCast users for 8 weeks, and we
find that Cedos does help download most of the podcast contents
(92.4%) via opportunistic Wi-Fi access. Moreover, D2 TP successfully saves 38% of the data that would have to be re-downloaded
due to natural network disruptions from user mobility.
2.
MOBILE APP BEHAVIOR AT MOBILITY
In this section, we examine how existing mobile apps react to
mobility events. We select 15 popular mobile apps 1 from Google
Play [32] that might benefit from delay-tolerant Wi-Fi access. We
report their behaviors on network disruption and during the delay
at network unavailability.
Methodology: We install each app on a Nexus 5 smartphone [33],
running Android v4.4. The device is connected to the Internet either via a Wi-Fi AP in our lab or through an LTE connection to a
local cellular ISP. To simulate a network disruption event, we had
the device use the Wi-Fi AP initially, but forced it to turn off the WiFi interface to use the LTE interface. This event simulates moving
the device from a Wi-Fi network to a cellular network, which results in IP address change. For a delay test, we had the device use
1
We use the latest version of each app as of November 2014.
Server
Wired Network
Mobile Apps (e.g., podcast downloader, streaming player)
Cellular Network
d2tp_socket();
d2tp_connect();
Estimated Transfer
Duration via 3G/4G
≤ 10min
D2Prox
Wi-Fi
PAUSE
Disruption
Downloaded: 080%
Remaining: 6hr
Flow ID
IPend
A
X.X.X.X
B
Y.Y.Y.Y
Flow
State
RESUME
①
①
②
Downloaded: 80%
Remaining: 3hr
③
Handle IP migration
B
Downloaded: 80100%
Remaining: 10min
d2tp_setsockopt();
D2TP
Wi-Fi / Cellular
Link
Z.Z.Z.Z
d2tp_read();
D2Sched/D2BufMgr
Rem. deadline Rem. cont. size
② 3hr
③ 10min
Pause flow
20 MB
20 MB
Resume flow
(delayed offload) (deadline guarantee)
②
③
Figure 1: Cedos delayed data offloading architecture. D2 TP handles both disruption and long delay. D2 Sched schedules the flows for delayed Wi-Fi
offloading (e.g., downloads a file as much as possible via a Wi-Fi connection, and the remaining 20% via LTE to finish within the deadline).
the Wi-Fi AP, but unplugged the cable of the AP to simulate network unavailability. We replugged the cable after 10, 30, 60, 120,
300, 3600 seconds of delay. This process does not change the IP
address of the device, but injects a period of Wi-Fi network unavailability. Finally, we examine the apps that support Wi-Fi offloading
via “Wi-Fi only” mode by toggling the option during data transfer.
Since we do not have access to source code for majority of the mobile apps, we report only qualitative behavior of each app. During
each test, we had the device download or upload data whose size is
large enough to be interrupted by the mobility event.
Findings: Table 1 summarizes the results of our experiments. We
find that many mobile apps do not handle network disruptions properly while notable exceptions are social networking service (SNS)
apps, some media streaming apps, and podcast apps. All apps use
standard TCP/IP protocols, so we notice that their TCP connection
terminates when the host IP address changes. For graceful operation at network disruption, the developers have to write code to
re-initiate a TCP connection to resume the data transfer, but that
seems to be often neglected in mobile apps. What is surprising
is that some mobile Web browser does not support download resumption even if HTTP supports range queries. In media streaming, app behavior varies - MXPlayer [13] and YouTube apps [11]
resume video streaming even after IP address change, but VLC [15]
and Google Play Movie [14] get stuck. Interestingly, posting comments in the YouTube app is not disruption-tolerant while its video
streaming is. The disparity comes from the fact that the developers have to handle each case differently without the support from
the transport layer. In contrast, all three podcast apps handle short
network disruptions well. We think that podcast app developers are
better-prepared for frequent network disruptions at large file download, and handle this case well.
The behavior for the delay test, however, is more disappointing.
Except Twitter [6] and Podcast Addict [8], all apps fail to resume
data transfer in a few minutes of network unavailability. We also
find that all media streaming apps pause the playback during the
delay. Since none of them support the “Wi-Fi only” mode, a desirable action would be to automatically switch to the cellular interface and to resume streaming if Wi-Fi access becomes unavailable. Twitter and Podcast Addict handle both network disruptions
and delays well, but they do not support download deadline as discussed shortly.
Lastly, we have investigated if the apps support opportunistic WiFi offloading. We find that some apps allow the “Wi-Fi only” mode,
but none of them support any notion of download deadline. Without a deadline, a download in the “Wi-fi only” mode would wait
indefinitely when there is no Wi-Fi connectivity. We further ana-
lyze how the apps behave with the “Wi-Fi only” option. We try
turning on the option while downloading through an LTE interface
and try turning it off while downloading via LTE network. No apps
that support the “Wi-Fi only” mode handle these two cases correctly. They either continue with the cellular interface (W1) or end
up with a download failure in the former case (W2). Or they do
not resume the transfer by switching to the cellular interface in the
latter case (W3).
In summary, we find that many popular mobile apps neither properly handle mobility events nor their behavior is consistent across
app types. This is because the current network API assumes realtime operation and app developers have to write the action on their
own. The goal of our work is to ease their burden by transparently
handling the mobility events in the transport layer while providing
the flexible knob to control the behavior with a familiar API.
3.
DESIGN
In this section, we present the design of Cedos, a practical delaytolerant networking architecture for mobile apps. Cedos transparently addresses mobility events in a new transport layer while it allows flexible control between opportunistic Wi-Fi and cellular network usage.
3.1
Overall Architecture
Figure 1 shows the overall architecture of Cedos. Cedos-based
mobile app initiates a D2 TP connection to D2 Prox which relays
downloading content from the TCP server to the app. D2 TP is a
TCP-like transport layer protocol, but it transparently masks network disruptions and delays. D2 Prox works as a protocol translator,
allowing client-side mobility while it supports backward compatibility to TCP-based servers. We assume that a mobile device can
move around, and transfer data intermittently in multiple networks
while the server is at a fixed location. Each D2 TP connection is
characterized by a maximum-allowed delay, deadline, and a preferred network interface (e.g., Wi-Fi). Until the deadline, D2 TP
strives to transfer as much data as possible through the preferred
network, even by intentionally delaying the transmission if the preferred network is unavailable.
For connections without a deadline or near deadline expiration,
D2 TP uses any available interface to complete the data transfer.
D2 Sched analyzes the current network status (available bandwidth
and interfaces) together with a deadline and a content size, and decides if a flow should transfer data, wait for the preferred interface,
or switch to a faster network. For dynamic content whose size is
unknown (e.g., video stream), D2 BufMgr calculates an appropriate
deadline and size by analyzing the minimum throughput required
C
SYN
(Flow ID = X, Host ID, Deadline)
(1) Flow ID Conflict
RST
SYN
(Flow ID = Y, Host ID, Deadline)
(2) Valid Flow ID
SYN+ACK (Flow ID, Host ID)
ACK
(a) Flow creation
S
C
S
C
S
SYN + Public-Key (n, e)
Encrypt per-conn.
secret key
SYN+ACK + ENC(key)
Data Transfer
Disruption
Retransmit 15 times
(1) Teardown (no deadline)
Extract key via
private key decryption
Deadline Timeout
ACK
(2) Connection Teardown
(b) Secret key sharing
(a) Teardown at disruption
C
S
Reconnect
DATA or ACK
(receive-window > 0)
Detect Address Change
CHG (nonce)
RSP (HMAC-SHA1_key{nonce})
Valid Flow ID Ownership
AUTH
(b) Challenge-and-response
Figure 2: Initialization of D2 TP connection
Figure 3: Flow handling in mobile environments
to maintain the app’s QoE (e.g., bitrate), and chooses how a flow
should transfer data similar to D2 Sched.
SYN packet may include a deadline specified in seconds, as a 4byte option field. When it is missing, the connection falls back to a
normal TCP connection.
If the server already holds a D2 TP connection with the same flow
id, it responds with a RST to ask for retrying with a new flow id. Or,
the server replies with a SYN+ACK packet containing the serverside flow and host ids, and the client establishes the connection with
an ACK. Besides a regular three-way handshake, the D2 TP server
and client need to agree on a per-connection secret key to authenticate each other when the connection is disrupted and resumed later.
Figure 2(b) shows the key exchange process. We employ an asymmetric key cryptographic algorithm (e.g., RSA) and have the client
send its public key (n, e) in the payload of the SYN packet. The
server then generates a secret key, encrypts it with the client’s public key, and sends it in the payload of the SYN+ACK packet. The
client finally decrypts the message with its private key and stores
the extracted secret key into the flow’s state table for future use.
Connection teardown: A D2 TP connection is closed in two ways.
First, when both hosts complete the data transfer, they explicitly
close the connection by exchanging a FIN. Second, if an application closes the connection during network unavailability, the D2 TP
layer waits until the network becomes available again, and does a
normal closure. If the deadline expires before the device meets an
available network, the D2 TP connection terminates itself without
notifying the other. This should not be a problem since the other
party would close the connection as well. The deadline can be updated by the application at any time during the connection to reflect
the effective network bandwidth and actual transferred data size.
3.2
D2 TP: Transport Layer for Mobile Apps
D2 TP is a transport layer protocol for mobile apps, providing
TCP-like, reliable data transfer in stationary environments but it
hides network disruptions and allows delays when a mobile device
is on the move. We choose to take the transport layer approach,
hiding the mobility events from the application layer. This decision
presents two advantages. First, it frees the mobile app developers from directly dealing with network disruptions or delays while
providing a flexible knob to control the deadline. The rationale is
that the developers need to focus on the core program logic rather
than to keep track of the latest offset of an interrupted content or
whether a network interface is on or off. Second, having this mechanism inside the transport layer allows providing more information
as to delay-induced transmission with D2 Sched and D2 BufMgr, explained later. Maintaining the amount of data that has been transferred through Wi-Fi and cellular interfaces allows a more informed
decision whether to transmit or not at any given time.
The key enabler for D2 TP disruption tolerance is in the separation of a connection from its network attachment as in [21, 23,
26]. Our contribution here is to implement the mechanism in a
purely end-to-end fashion and to make it easy to use in real mobile networks. Specifically, each D2 TP connection is identified by
a persistent flow id and a host id that do not change in the course
of mobility. When a mobile device moves to another network, it
resumes the connection with the flow and host ids from where it
left off. One downside may be the overhead of keeping track of
many idle connections at the D2 TP server, but we keep the connection metadata small enough to maintain as many as one million
concurrent flows for less than 1 GB memory.
3.2.1
Connection setup and teardown
D2 TP is similar to TCP except it requires an explicit connection
resumption process when the network is available again after disruption. Connection teardown may occur explicitly or implicitly
depending on the user-defined deadline.
Connection setup: Figure 2(a) shows how a mobile client establishes a D2 TP connection with a server. The client first sends a
SYN packet, but unlike TCP, the SYN packet must include a flow
id and a client-side host id as its option fields. The host id is defined as a 20-byte SHA-1 hash of the MAC address of the cellular
interface. The flow id is the least significant 4 bytes of the SHA1 hash of the host id and a microsecond-granularity timestamp at
connection setup. Since the purpose of the id pair is to identify the
connection between the client and the server, the pair needs to be
unique only on the two machines during connection lifetime. The
3.2.2
Data transfer in mobile environments
Once a connection is established, the client and the server exchange data as in a normal TCP connection. On network disruption, connection is resumed with a new IP address after verifying
the ownership of the connection.
Connection resumption: If a client-side IP address changes due
to network switching, the client can resume the connection by authenticating itself to the server to prevent connection hijacking. It
first piggybacks flow and host ids as options to a normal data or
ACK packet. Then, the server detects the client-side IP address
change, and verifies the connection ownership through a challengeand-response protocol shown in Figure 3(b). For this protocol,
we define three special bit flags (CHG, RSP, and AUTH) in D2 TP
packet header. The server first sends a challenge packet by setting
the CHG bit flag on with an 8-byte random nonce in the payload.
Then, the client calculates HM AC − SHA1key (nonce), using
the shared key at connection setup, and sends it back with the RSP
bit flag on. Once the hash is verified, the server responds with an
authentication packet with the AUTH flag on, which gives an OK
At each scheduling epoch,
sign that the client can resume the data transfer. On verification
failure, the server sends an RST packet to notify the client to start
a new connection.
3.3
D2 TP Flow Management
Cedos achieves efficient mobile network usage by avoiding the
cellular data transfer whenever an app allows a delay. We describe
how a D2 TP flow is managed to determine the scheduling action:
wait, transfer data, or switch interface for minimum QoE guarantee.
3.3.1
Deadline-based flow management
The core enabler for managing an efficient D2 TP flow data transfer is the deadline, set explicitly by Cedos socket API. Basically,
the deadline is examined to decide whether the D2 TP flow should
transfer data through the attached network for efficiency (Wi-Fi offloading), or should change the interface for reliability (deadline
guarantee). For this, we collect information of currently attached
interface, bandwidth for both networks (last measured for unconnected network), flow deadline and content size. When the mobile
client moves out of Wi-Fi, the remaining deadline and time duration
to transfer remaining content through a cellular network are calculated for flow scheduling. Additionally, the remaining deadline is
periodically monitored to determine whether the current network
throughput is large enough to transfer the content in time. Detailed
algorithm on how a D2 TP flow is scheduled will be explained in
Section 3.4.
3.3.2
Exploiting buffer for flow management
Unfortunately, it is difficult to directly apply deadline-based flow
management for dynamic content apps (e.g., streaming) as there
is no deterministic way to identify when the data transfer would
end. A straightforward solution would be to periodically adjust the
deadline and content size to match the minimal bitrate required.
However, this can frequently interrupt streaming as the flow will
not begin data transfer until the deadline is close. Reversely, setting too small deadlines would result in frequent CPU wakeups for
deadline calculation, consuming much of mobile device’s limited
battery resources. Therefore, we propose an alternate method to
calculate an appropriate deadline and content size, to be later used
for flow scheduling.
D2 BufMgr is a flow manager for dynamic content that exploits a
popular design semantics that many streaming apps follow for efficient playback: buffer that maintains to-be-played content. The
goal of D2 BufMgr is to delay data transfer in a cellular network
by consuming already buffered data only until the point that would
result in interruption. For this, we introduce a low threshold, which
represents the remaining buffer point where D2 TP flow should begin transferring data through a cellular network, assuming that either Wi-Fi is unavailable or its signal strength is too weak for continuous streaming. The deadline is thus determined by the estimated time that it takes to actually download the data via cellular
network (= hardware interface switching time + D2 TP’s challengeand-response message exchange time + first data packet arrival time)
while the content size is minimum data required to be transferred
during that time according to its bitrate.
To minimize cellular traffic usage for efficiency, the device must
return to a Wi-Fi network when it has buffered enough data with a
cellular network. To this end, D2 BufMgr introduces a high threshold, that determines when the device stops using a cellular network
by either switching to Wi-Fi if it is available or simply pausing the
data transfer. The high threshold can be calculated similarly, which
should be set large enough to begin downloading the data via WiFi before reaching the low threshold. During cellular traffic usage,
Input: (fi , Ti : i = 1, . . . , m), rc , and rw
Output: I (the interface to use) and i? (the flow to schedule)
P
1. For each flow i, let τi = ij=1 dfj /rc e.
2. if τi ≤ Ti , for all flows i = 1, . . . , m,
3.
if Wi-Fi is unavailable
4.
I = none, i? = 0,
5.
else I = Wi-Fi, i? = 1.
6.
end if
7. else
8.
I = arg maxn={c,w} rn , i? = 1.
9. end if
Figure 4: D2 Sched algorithm
D2 BufMgr resets the deadline and content size to match the high
threshold. Reversely, when the buffer reaches the high threshold,
D2 BufMgr swaps the values back to guarantee the low threshold.
3.4
D2 TP Flow Scheduling
When a mobile device opens a D2 TP flow, a flow scheduler,
called D2 Sched, uses the values set from flow management to schedule the flow such that the flow should be idle or active in transmission and which interface it should use if it is active. In case of
multiple flows, D2 Sched schedules flows by considering the contention between shared mobile resources (e.g., bandwidth).
Scheduling algorithm: The scheduling algorithm of D2 Sched is
described in Figure 4. Before explanation, we introduce several
notations for ease of exposition. At each scheduling epoch with
m D2 TP flows, each D2 TP flow has remaining deadline of Ti , and
content size of fi where i ∈ {1, . . . , m} respectively. We assume
that Ti is indexed in the increasing order of flow index i, i.e., T1 ≤
T2 ≤ ... ≤ Tm . We also let rc and rw be the estimated throughput
(at this scheduling epoch) of cellular and Wi-Fi, respectively (rw =
0 if the device has no Wi-Fi connection), for which we will explain
how to measure them shortly.
In line 1, we first compute τi for each flow i, which corresponds
to the time to finish flow i’s data delivery, if the flows were served
in the increasing order of their remaining deadlines. We call τi
as flow i’s potential completion time. Then, we check whether all
flows’ potential completion times are smaller than their deadlines
(line 2), which means that the system is not in the “urgent” situation, i.e., all flows have sufficient time until their deadlines expire.
In this case, if the device has no Wi-Fi connection, it just waits and
no flow is scheduled (line 4), but schedules the flow with the shortest remaining deadline to use Wi-Fi (line 5) otherwise. However,
if we are in the urgent situation, where there exists a flow whose
remaining deadline is less than its estimated completion time, we
perform the Earliest Deadline First (EDF) scheduling by serving
it via the network interface that would result in a larger expected
throughput (line 8).
Scheduling epochs and throughput estimation: Scheduling in
Figure 4 is periodically carried out every N seconds with no environmental change (e.g., Wi-Fi availability, flow configuration). In
each scheduling epoch, rc or rw is updated by the average throughput over the past N seconds. Clearly, N should be appropriately
chosen, such that it reacts to time-varying network conditions fast
as well as it avoids too frequent switchings between Wi-Fi and cellular networks. In our real-world experiment, we choose it to be 1
second, considering both interface switching time and energy consumption. A smarter alternative is to keep a history of Wi-Fi access
and to adjust N according to the access pattern [34, 35], but we
choose simplicity and fast adaptation at the cost of a slight over-
head. Scheduling is also launched in an asynchronous manner to
the change of flow configuration and Wi-Fi availability. In particular, in case a mobile device acquires a new Wi-Fi connection, it
first spends N seconds without active transmission to estimate rw .
Design rationales: We design D2 Sched by the following rationales. First, D2 Sched naturally enables a single D2 TP flow to be
assigned a low priority over other real-time TCP flows. This is because a sequential scheduling of serving only a single D2 TP flow
inside D2 Sched leads our estimated data rate rc or rw , which approximately corresponds to the throughput of a normal TCP flow, to
be divided over m D2 TP flows. This design comes from the rationale that D2 TP flows are less interactive large flows (e.g., backup of
data in smartphones or buffering of streaming movie) which should
be served with lower priority than other real-time TCP flows. Second, a sequential scheduling of a single flow in an EDF manner is
motivated by the following rationales that (i) an urgent flow has less
opportunities of data transfer than others, and (ii) serving multiple flows concurrently generates frequent switching between flows
with active transmission, causing overheads such as switching delay and small data rates due to the need of TCP’s ramping-up time
(i.e., slow-start). We note that the sequential flow scheduling does
not hurt the performance of offloading efficiency when the scheduling interval is short enough and the achievable sum throughput is
independent of the number of activated flows; D2 Sched is an optimal scheduling policy in terms of offloading ratio for a fixed Wi-Fi
and cellular throughputs [36]. In spite of time-varying nature of
wireless connections, D2 Sched is expected to work in an highly
efficient way, because Cedos’s efficient Wi-Fi offloading targets
at long flows, and mobile users are typically in indoor Wi-Fi networks, where data rate variations are reasonably small [37]. Third,
D2 Sched tries to increase the offloading ratio by delaying flows as
much as possible, i.e., data transfer is even paused when Wi-Fi is
unavailable and remaining deadlines are large.
3.5
Power Efficient CPU/Wi-Fi Wakeup
One practical concern when enabling flow scheduling in mobile
devices is detecting environmental changes at idle device states.
Current mobile OS platforms such as Android minimize battery
consumption when a mobile device becomes idle by entering the
CPU sleep or Wi-Fi “no-attach” mode. In this mode, the CPU is
completely switched off until any external input event arrives, and
all D2 TP flows in the background would stop even if Wi-Fi networks are available for offloading. What is worse is that the device
will not retry scanning Wi-Fi APs during the sleep mode, and it
would miss potential Wi-Fi opportunities. We thus require a mechanism to transparently wake up CPU and scan for Wi-Fi without
user intervention.
One naïve workaround is to prevent the CPU and Wi-Fi interface
from turning off by acquiring locks on them in the sleep mode, but
it is impractical since it would quickly drain the battery. Instead, we
decide to implement a D2 TP flow aware lock, called D-Lock that
periodically locks CPU and Wi-Fi resources. If a preferred network
is available, and there are D2 TP flows that want to use the network,
D-Lock keeps the CPU on even if the device becomes idle. When
the Wi-Fi network becomes unavailable, it allows the device to go
to the sleep mode, but it periodically wakes up the CPU to probe
the availability of Wi-Fi APs. The developers only need to hold a
D-Lock during the D2 TP connection, and it automatically handles
periodic scanning of APs at minimal power usage. Through realworld experiments, we find that locking with a period of 2 minutes is enough to detect most environmental changes with minimal
power, shown in Section 5.2.2.
Server
① Download
the entire file
Server
③ Close when
download (①)
completes
TCP
② Wait until
upload (①)
completes
D2Prox
D2TP
② Opportunistic
transfer to client
(via Wi-Fi/LTE)
③ Forward
the entire file
TCP
D2Prox
① Opportunistic
transfer from client
(via Wi-Fi/LTE)
Client
(a) D2Prox on download case
D2TP
Client
(b) D2Prox on upload case
Figure 5: Architecture of D2 Prox
3.6
D2 Prox: Protocol Translation Proxy
Upgrading the existing infrastructure (e.g., servers, middleboxes)
to support D2 TP is costly and time-consuming. Instead, we propose placing a network-embedded Web caching proxy, D2 Prox,
which enables D2 TP data transfers between a mobile client and
an unmodified server. It basically hides the client-side mobility
from the server by communicating with the client in D2 TP, but
connects to the server using TCP. Note that D2 Prox operates as a
non-transparent proxy that app developers or users can choose, depending on the characteristics of the apps. Although D2 Prox can
sit anywhere in the Internet, we envision that it is located near a
cellular ISP’s core network to curb the latency stretch between an
origin server and a client on a cellular interface.
Figure 5 shows the overall architecture of D2 Prox. When a new
2
D TP connection is requested, D2 Prox creates an entry in the flow
metadata table mapped by the flow id. Each entry consists of client/server
IP addresses, a requested URL, and a connection deadline. Since
TCP does not allow delay or network disruptions, D2 Prox only connects to the server when a full data transfer is possible in a single
TCP connection. For downlink, D2 Prox simply downloads the entire content from the server, caches it in memory or disk, and forwards it to the client if the client is attached to a network. For
uplink transfer (e.g., HTTP POST), D2 Prox merges all of client’s
partial data from multiple physical connections, and forwards it to
the server. D2 Prox removes the flow entry when the connection is
explicitly closed or when the deadline expires but the client is out
of reach.
Since D2 Prox maintains the flow metadata table in memory, a
large number of connections with long deadlines could lead to memory pressure. We mitigate this problem by periodically flushing
idle flow entries with long deadlines to disk. We believe this approach is reasonable in normal situations since such idle connections will be likely to stay paused to minimize the cellular traffic
usage. However, we understand this is not a solution to a Denial-ofService (DoS) attack on D2 Prox, which would require more careful
thoughts. One mitigation approach would be to limit the number of
concurrent D2 Prox flows per mobile device. If a cellular ISP employs D2 Prox, it could further require client authentication before
using D2 Prox.
4.
IMPLEMENTATION
We describe the implementation of Cedos components. We show
the practicality of Cedos by porting two existing mobile apps (Mobile Firefox, VLC) to run on Cedos, and implement a delay-tolerant
podcast client, ReadyCast, for real-world deployment.
D2 TP-based client
1
fd = d2tp_socket();
2
if (non-interactive app) {
3
// set deadline (10min) and flow size (1MB)
4
int deadline = 10*60;
5
int flowSize = 1000*1024;
6
d2tp_setsockopt(fd, D2TP_SO_DEADLINE,
7
&deadline, sizeof(deadline));
8
d2tp_setsockopt(fd, D2TP_SO_ESIZE,
9
&flowSize, sizeof(flowSize));
10 } else if (streaming app) {
11
// set low/high buffer thresholds (20KB/1MB)
12
struct d2tp_rcvbufth thr;
13
thr.low = 20*1024;
14
thr.high = 1000*1024;
15
d2tp_setsockopt(fd, D2TP_SO_RCVBUFTH, &thr,
16
sizeof(thr));
17 }
18 d2tp_connect(fd, &sAddr, sizeof(sAddr));
19 d2tp_write(fd, buf, BUFSIZE);
20 d2tp_read(fd, buf, BUFSIZE);
21 d2tp_close(fd);
D2 Prox as D2 TP-TCP protocol translation Web proxy
1
sfd = d2tp_socket();
2
d2tp_bind(sfd, &sAddr, sizeof(sAddr));
3
d2tp_listen(sfd, 10);
4
cfd = d2tp_accept(sfd, &cAddr, &cSize);
5
d2tp_fcntl(cfd, D2TP_F_SETFL, D2TP_O_NONBLOCK);
6
FD_SET(cfd, rSet);
2
Figure 6: D TP socket API
7
while (1) {
8
d2tp_select(fdmax, &rSet, &wSet, NULL, NULL);
9
// event notification from D2 TP client
4.1 D2 TP
10
if (FD_ISSET(cfd, &rSet) {
11
d2tp_read(cfd, buf, BUFSIZE);
We implement D2 TP in the user level on top of UDP for easy
12
// create tcp connection to server
portability and programming convenience. We implement all TCP
13
tcpfd = d2tp_socket();
core features such as slow start, flow and congestion control, fast re14
connect(tcpfd, &addr, sizeof(addr));
transmit and recovery, timeout and retransmission, delayed ACKs,
15
// forward client message to server
16
d2tp_write(tcpfd, message, BUFSIZE);
selective ACKs and so on. These are implemented as a separate
17
FD_SET(tcpfd, rSet);
thread for each D2 TP application.
18
}
19
// event notification from TCP server
4.1.1 D2 TP Socket API
20
if (FD_ISSET(tcpfd, &rSet) {
2
D TP supports ease of portability by providing a socket API
21
read(tcpfd, buf, BUFSIZE);
where each function maps to a BSD socket function by attaching
22
// forward server data to client
2
the D TP prefix, d2tp_ (see Figure 6). For example, d2tp_socket() 23
d2tp_write(cfd, buf, BUFSIZE);
24
}
returns a UDP socket while its associated context information is
25 }
maintained in the D2 TP thread. Applications use this socket to
Socket operations (Server/Client)
int d2tp_socket(void);
int d2tp_bind(int fd,
const struct sockaddr *addr,
socklen_t addrlen);
int d2tp_connect(int fd,
const struct sockaddr *addr,
socklen_t addrlen);
int d2tp_listen(int fd, int backlog);
int d2tp_accept(int fd, struct sockaddr *addr,
socklen_t *addrlen);
int d2tp_close(int fd);
Data transfer and event notification
ssize_t d2tp_read(int fd, void *buf, size_t count);
ssize_t d2tp_write(int fd, const void *buf,
size_t count);
int d2tp_select(int nfds, fd_set *readfds,
fd_set *writefds, fd_set *exceptfds,
struct timeval *timeout);
Special socket options
int d2tp_fcntl(int fd, int cmd, ... /*arg*/);
int d2tp_getsockopt(int fd, int level,
int optname, void *optval,
socklen_t *optlen);
int d2tp_setsockopt(int sockfd, int level,
int optname, void *optval,
socklen_t *optlen);
bind, connect, listen on a port, and send or receive data with D2 Prox
or any D2 TP-based applications. For event-driven programming,
we support d2tp_select() and have it handle events for both
D2 TP and TCP sockets. Since D2 TP socket is implemented as a
UDP socket, d2tp_select() can simply use select() for
event notification. Besides regular socket functions, we have special socket option support with d2tp_setsockopt() that allows setting a flow deadline (D2TP_SO_DEADLINE), a flow size
(D2TP_SO_ESIZE), and high and low buffer thresholds for flow
scheduling (D2TP_SO_RCVBUFTH). We show example code snippet of D2 TP-based client and D2 Prox in Figure 7.
4.1.2
Detecting Network Interface Availability
To pause or resume the data transfer, the D2 TP layer must detect
the change in the network availability. At start, the D2 TP layer
reads the current status of network interfaces in the routing table
at /proc/net/route. Then, it monitors a netlink socket to be notified
of any status change events of the network interfaces. If D2 Sched
detects any D2 TP flows whose deadlines are about to expire but
the quality of a Wi-Fi network is suspected too poor to meet their
deadlines, the D2 TP layer switches to using the cellular network
interface by modifying the routing table entry.
Figure 7: Sample code with D2 TP API (error processing is omitted)
4.1.3
D-Lock
We implement D-Lock by extending Android WakeLock [38]
using AlarmClock [39]. If there is any active flow, D-Lock acquires
WakeLock to prevent CPU from entering sleep mode. If there is
no active flow, D-lock releases WakeLock and sets AlarmClock to
wake up CPU for periodic Wi-Fi scanning. For every scanning
period, D-Lock checks if D2 TP has any flow that needs to start
before the next scan. If so, D-Lock sets another AlarmClock to
trigger the D2 TP layer to continue the data transfer to meet the
deadline.
4.2
D2 Prox
We base our D2 Prox implementation on a popular open-source
Web server, nginx (v1.2.6), by porting it to use D2 TP socket functions. D2 Prox supports memory and disk caching, and transfers
data to/from a server with normal TCP socket functions when it is
able to send/receive a full content in a single connection. Porting is
Implementation
D2 TP thread and API
D2 Prox (nginx)
Mobile Firefox
VLC
ReadyCast
Total (LoC)
11,166
129,340
8,851,611
3,733,651
9,868
Modified (LoC)
all
55
123
61
all
Table 2: Total implemented/modified lines of code
mostly straightforward by replacing existing socket functions with
D2 TP API, and the total number of modified lines is only 55 out of
129K lines of code as shown in Table 2. We have stress tested it to
confirm correct delay-tolerant operation as well as Web caching.
4.3
Real-world Applications
To show the applicability of Cedos to real-world applications, we
port two existing applications and implement a delay-tolerant podcast downloader. The number of modified lines of code is shown in
Table 2.
Mobile Firefox: Since a Web browser is one of the most popular
applications, we port mobile Firefox (Fennec [30]) to use D2 TP
and retrieve web pages via D2 Prox. Original Fennec is built on
top of NetScape Portable Runtime (NSPR) [40], which provides a
platform-independent abstraction through a unified library API. We
thus identify all network-related functions in Fennec and NSPR,
and change them to use D2 TP API. With only 123 lines of code
modification, Mobile Firefox enjoys seamless web browsing even
after network disruptions. While it works well with large file download, we notice a few seconds of network interface switching delay
at network disruptions when we browse regular Web sites; due to
this reason, we do not evaluate it further in the next section.
VLC: To demonstrate that streaming media players could also benefit from Cedos, we port VLC [29] (v0.2.0), a popular open-source
video player. We modify only 61 lines out of 3.7M lines of code to
download streaming video via D2 Prox. VLC exploits D2 BufMgr
in D2 TP by setting an appropriate deadline, and delays cellular data
transfer if possible to offload to Wi-Fi. We find that by maintaining D2 BufMgr’s buffer thresholds large enough to buffer drainage
during network switching, VLC enjoys a constant video streaming
without a single interruption (or buffer underrun) even in a moving
train, further explained in next section.
Podcast client: Since there exists no deadline-based mobile app
in the market, we have built our own delay-tolerant publish-andsubscribe podcast downloader, ReadyCast [31], and have registered
it with Google Play [32]. ReadyCast supports subscription to any
podcast feed published in the Internet. It allows the user to set the
deadline of a podcast content download, benefiting from the D2 TP
layer for transparent delay management. ReadyCast also uses DLock to minimize the power consumption at idle states while maximizing the offloading opportunities.
5.
EVALUATION
We evaluate Cedos in two ways. First, we evaluate the basic
functionalities of Cedos by measuring the data transfer throughput and bandwidth fairness, network switching delay, effect of flow
scheduling, power consumption at idle states, and D2 Prox scalability. Second, we test if Cedos-based systems effectively offload
cellular traffic to Wi-Fi networks in real-world applications.
5.1
Microbenchmark
We first measure the basic performance of Cedos components.
We use Galaxy S3 (Android 4.1.2 and Linux kernel 3.0.31) with an
Device
Protocol
D2 TP
UDT
TCP
IBR-DTN
Galaxy S3
Throughput CPU
89.1 Mbps
16%
89.3 Mbps
25%
49.2 Mbps
7%
45.1 Mbps
15%
Nexus 5
Throughput
90.6 Mbps
90.5 Mbps
91.3 Mbps
87.0 Mbps
CPU
11%
14%
8%
18%
Table 3: Throughput/CPU usage during wireless data transfer
Protocol (# Connections)
TCP (100)
D2 TP (100)
TCP (50) + D2 TP (50)
Aggregate Throughput
95.1 Mbps
96.3 Mbps
95.0 Mbps
JFI
0.99
0.99
0.98
Table 4: Concurrent connection performance via Wi-Fi
1.4 GHz quadcore CPU and 2 GB RAM or Nexus 5 (Android 4.4.2
with Linux kernel 3.4.0) with a 2.3 GHz quadcore CPU and 2 GB
RAM as a client, and use a machine with an Intel Xeon E5-2650v2
octacore CPU with 32 GB RAM for D2 Prox and an origin server.
For LTE access, we use SKT as our cellular ISP.
Throughput and CPU usage: Table 3 compares the performances
of downloading a 250 MB file using D2 TP, TCP, UDT (UDP-based
reliable data transfer protocol [41]) and IBR-DTN (one of the delaytolerant network group’s disruption-tolerant data transfer implementations [42]) employing TCP as the underlying transport protocol. Both the client and the server run the same transport-layer protocol and a single connection is made between them via an 802.11n
Wi-Fi AP. Overall, we find that D2 TP’s performance is comparable
to those of other protocols except for a slight increase in the CPU
cycles due to user-level protocol implementation atop UDP. Interestingly, we observe much lower TCP performance on Galaxy S3,
which we suspect is due to a kernel implementation bug (after code
review) since we do not see the same problem on Nexus 5.
Fairness: We check if D2 TP provides a fair bandwidth share among
competing flows by measuring Jain’s Fairness Index (JFI) [43]. We
compare following three cases: (i) 100 TCP connections, (ii) 100
D2 TP connections, (iii) 50 TCP and 50 D2 TP connections. We
have each connection download a 1 GB file on a Nexus 5 device,
and show the aggregate throughputs and JFIs in Table 4. We observe that D2 TP provides comparable per-flow bandwidth fairness
to that of TCP both for D2 TP-only flows and for D2 TP flows mixed
with TCP flows.
Network switching delay: We next measure the network switching delays to migrate a D2 TP flow between Wi-Fi and cellular networks. We measure the time between the last data packet from the
previous network and the first data packet in the new network on a
Galaxy S3 device. We measure it for 100 times and show the averages. The switching time includes the CHG-RSP-AUTH process
for source authentication besides hardware interface switching.
We find that it takes 27 to 141 ms to switch from an LTE to a WiFi network with an average of 45 ms. Switching from LTE to Wi-Fi
is fast since the device can use LTE until the Wi-Fi access is fully
ready. So, the network switching delay is the same as the D2 TP
source authentication latency. However, in the reverse direction
from Wi-Fi to LTE, we see 440 to 856 ms of delay with an average
of 581 ms. This is because Android enforces a higher priority level
on the Wi-Fi access, which requires the Wi-Fi interface to be turned
off before the LTE interface is initialized. We note that it takes
88 ms on average for D2 TP source authentication in Wi-Fi-to-LTE
switching.
Deadline (failed to meet)
(i)
(ii)
(i)
(b)
1
X
Wi-Fi LTE
Wi-Fi
10
O
10
20
30
0
10
20 Flow 1
30
LTE
10
X
9X
8X
7X
6X
5X
4X
3X
2X
1X
10
(ii)
0
10
(i)
O
10
O
9
8
O
7
O
6
O
5
O
4
O
3
O
2
O
1X
10
O0
9
O
8
O
7
O
6
O
5
O
4
O
3
O
2
O
1
O
10
0
10
(ii)
LTE
20
150
100
50
0
30
231.9
200
Flow 10
O0
10
O
9
8
O
7
O
6
O
5
O
4
O
3
O
2
O
1
O
(c)
Flow 2
………
(a)
Wi-Fi
250
Deadline (succeeded to meet)
Power (mW)
Data Transfer
Completed
within deadline
43.0
Sleep
67.5
WakeLock D-Lock
(2min)
(1min)
55.1
45.2
D-Lock
(2min)
D-Lock
(10min)
Figure 9: Idle power usage for WakeLock and D-Lock at various scanning periods
Wi-Fi
Wi-Fi
Elapsed Time (sec)
20
30
20
30
20
30
Figure 8: Timecharts for flow scheduling experiments. Deadlines are
shown as vertical bars.
Flow scheduling with D2 Sched: We now evaluate whether D2 Sched
can guarantee complete data transfer within user’s delay tolerance
when experiencing poor Wi-Fi connections (Figure 8(a)) or resource
contention between multiple flows (Figure 8(b, c)). For these experiments, we use Galaxy S3 and have each flow download a 100 MB
file from our server. For each graph, (i) represents a strawman
scheduling method (e.g., stick with Wi-Fi if available, schedule
each flow independently) while (ii) represents D2 Sched’s result.
For poor Wi-Fi connection experiment, we run one flow with a
short deadline (5 min) in a Wi-Fi network whose bandwidth happens to be lower (0.8 Mbps) than that of a cellular network (4.7 Mbps).
This is a valid scenario as it is often the case that a Wi-Fi bandwidth
is lower than that of cellular access if the APs are congested or the
signal strength is weak [4]. As shown in Figure 8(a)(i), blindly
transferring data through Wi-Fi only fails to meet the deadline. On
the other hand in (ii), we see that D2 Sched automatically switches
to LTE to finish the download within flow’s deadline. We note that
D2 Sched still tries to maximize the Wi-Fi offloading ratio, receiving 16% of the data via Wi-Fi, which is close to optimal (17%).
For multiple flow experiment, we first run 10 flows with the same
deadline (30 min) in a cellular network (LTE) without any Wi-Fi
availability. The measured bandwidth of the cellular network is
4.7 Mbps on our client. Figure 8(b) compares the behavior of (i)
scheduling each flow independently vs. (ii) D2 Sched. Under (i),
we find that all flows miss their deadlines since they end up waiting
for Wi-Fi too long. When they start using LTE, the bandwidth is
divided into 10 flows, which delays the flow beyond its independently estimated finish time. In contrast, in (ii), all flows under
D2 Sched finish in time since it schedules some flows earlier, operating with a minimum required time for meeting the deadlines of
all flows. Next, we run 9 flows with a long deadline (30 min) along
with one urgent flow with a much shorter deadline (5 min) in a WiFi network. The measured bandwidth of the network is 6.3 Mbps.
Figure 8(c) shows that the urgent flow misses its deadline under
(i) since the other 9 flows compete for the bandwidth at the same
time even if they have longer deadlines. However, in (ii), D2 Sched
schedules the urgent flow first while holding off remaining 9 flows,
eventually meeting the deadlines of all flows.
Power usage at idle states: We show the effectiveness of our DLock in reducing power consumption at idle states. Specifically, we
compare the power usage of a Galaxy S3 device when we use (i)
Android WakeLock (with WifiLock) vs. (ii) D-Lock. We set the device to use an LTE network during the experiment, and turn on the
Wi-Fi module for periodic scanning. For accurate measurement,
we turn off all the other user apps but the built-in apps, which cannot be killed by a user. We use Power Monitor by Monsoon [44] for
power usage measurement, and show the results for various scanning periods in Figure 9.
We first see that at least 43 mW is needed to keep the device in a
sleep state. If Android WakeLock is acquired, the CPU stays awake
for the entire duration of the idle states while the device would scan
for a Wi-Fi AP every 2 minute (default scanning policy in Galaxy
S3). As a result, the power usage surges up to 231.9 mW, 5.4 times
that of sleep state. Given that the battery capacity of Galaxy S3 is
7,980 mWh (3.7V with 2,100 mAh [45]), almost 70% of its total
battery would be consumed per day even if no other activity is going on. In contrast, D-Lock with 2 minutes as the scanning period
would curb the average power consumption at 55.1 mW, which outperforms WakeLock by a factor of 4.2. This is because the CPU is
mostly turned off with D-Lock except when scanning a Wi-Fi AP.
Even with a 1-minute period, the power usage reduction is significant (3.4x better than WakeLock), and increasing it to 10 minutes
produces the power consumption close to that of sleep states.
Power usage at transfer states: We next examine the power usage of D2 TP during data transfer in comparison to that of TCP. We
measure the power consumption on a Nexus 5 device while downloading a 1 GB file through Wi-Fi using (i) TCP vs. (ii) D2 TP.
We find that for 825 seconds of the data transfer (bandwidth =
9.7 Mbps), TCP uses 1043 mW while D2 TP spends 1096 mW. This
extra 5% of power consumption comes from increased CPU usage
from user-level D2 TP implementation as is also shown in Table 3.
However, we believe D2 TP will be more power-efficient in the mobility events as it avoids re-downloading the content from the start
at network disruptions, also shown in our previous work [46]. Besides, it strives to use power-efficient Wi-Fi as much as possible
over power-hungry cellular radio.
D2 Prox scalability: Finally, we evaluate the performance scalability of D2 Prox by having it handle a large number of concur-
off
idle
transfer
transfer
Cellular
transfer
transfer
off
idle
Power (mW)
1 Mbps 5 Mbps
1,282
1,410
1,301
1,420
568
686
576
689
Table 5: Power consumption with average bandwidth of 1 & 5 Mbps
30
Buffered Data (MB)
Wi-Fi
(i) (iii)
(ii)
(iii)
(i) (iii) (ii)
25
20
15
10
5
0
0
rent flows. For this experiment, we employ a client machine of the
same specification as the server since it is difficult to create tens of
thousands of connections from a single smartphone. We have the
client initiate 64K concurrent connections to a D2 Prox server, and
have each connection download a 1 GB file via a 10 Gbps interface
(through a NetGear XSM7224S switch). We make 10% of the active connections go idle to simulate a network disruption for each
minute, and when all connections become idle, we make 10% of
the idle connections go active again for each minute. We observe
that D2 Prox achieves almost 10 Gbps throughout the experiment
while the bandwidth is evenly distributed among the active flows,
producing a JFI above 0.9. Lastly, D2 Prox scales linearly as we
introduce more D2 Prox nodes to the network.
5.2
Experience with Real-world Applications
We gauge the practicality of Cedos in real-world mobile applications. First, we evaluate if Cedos ensures automatic switching between Wi-Fi and cellular networks even for real-time video streaming with VLC. Second, we carry out a user study with ReadyCast
for 8 weeks. We measure Wi-Fi offloading opportunities with delays in real environments, and observe how the users react to delayed data transfers.
5.2.1
Opportunistic data offloading with VLC
Experiment setup: We watch a long streaming video of constant
bit rate of 640 Kbps with Cedos-enabled VLC on a Nexus 5 device while riding a subway train in Daejeon, South Korea. The
subway line consists of 22 stations, and we take a round trip to go
from one end to the other end, and then come back to the starting
station. Each subway station is equipped with publicly-accessible
Wi-Fi APs but the train itself does not provide Wi-Fi access. So,
the Wi-Fi service is available only when the train is staying at a
station. Our goal is to opportunistically use the Wi-Fi access to
buffer the streaming video but to automatically switch to the cellular network (LTE) if Wi-Fi access is unavailable and the buffered
data is too small to be played back. We place D2 Prox and an origin
server in our lab, which is 2 to 10 Km away from the client with
an average RTT of 42.9 ms via LTE and 13.7 ms via Wi-Fi. We
set D2 BufMgr’s low and high receive socket buffer thresholds to
200 KB and 3 MB, respectively. We set the low threshold large
enough to cover 0.3 seconds of LTE-to-Wi-Fi interface switching
time during video playback. We set the high threshold conservatively large since we observe that many public Wi-Fi APs that advertise themselves as accessible actually do not function properly.
Wi-Fi offloading ratio: The experiment result shows that as much
as 48.7% of the video data is being fetched through public Wi-Fi
APs at the stations. For the entire period of video watching, the WiFi data transfer is possible only for 310 seconds with an average
of 6.29 Mbps while the device uses LTE for much longer, 2,447
seconds. However, D2 BufMgr’s buffering mechanism ensures to
limit the LTE usage by curbing the content download rate to the
video bit rate (640 Kbps). Overall, the result is inspiring since it
shows that almost a half of the large streaming data is delivered
100
200
300
400
500
Time (sec)
Figure 10: Buffered data size over time for subway experiment. (i)
Wi-Fi data transfer, (ii) Wi-Fi no data transfer, (iii) no Wi-Fi
through Wi-Fi even if Wi-Fi access is available intermittently at a
fraction of the stations. We confirm that Cedos effectively offloads
the data to Wi-Fi without a single pause in the playback.
Buffer size at network disruptions: We next analyze how a D2 TP
connection reacts to network disruptions during device mobility.
Figure 10 shows the trace of the buffered data size during the time
the train passes four subway stations. The shaded portions represent the availability of APs where (i) denotes that the APs are working properly while (ii) marks the period of malfunctioning APs that
disallow data transfer. At the 20th second, the client starts buffering the video content through Wi-Fi as much as 26 MB. The device
moves out of the Wi-Fi network at the 60th second, but it uses the
buffered data for playback until the remaining size hits the high
threshold at the 300th second. Then, it downloads the video content through LTE, but it keeps the buffer size at the high threshold
(=3 MB) until it meets a Wi-Fi AP at the next station at the 360th
second. The device attempts to buffer the data through the Wi-Fi
AP, but the AP seems to be unstable and transfers the data for only
about 5 seconds. At the 410th second, the device uses LTE again
and when the device meets a malfunctioning AP at the 450th second, it tries to use Wi-Fi only to switch back to LTE at 500th second
since the remaining buffer size hits the low threshold.
Optimizing Wi-Fi offloading: We now share our experience in optimizing the Wi-Fi offloading process. In our earlier experiments,
we found that only 14.9% of the data was offloaded to Wi-Fi, which
appeared lower than expected given the average Wi-Fi attachment
time per station is 56.4 seconds while the period of Wi-Fi unavailability between two stations is 60.3 seconds on average. We later
found out that once a device is attached to a Wi-Fi AP, it would not
rescan for a better AP even if the available bandwidth of the AP is
very low or even if it is not working. Actually, Wi-Fi data transfer
worked at only 3 out of 44 train stops (round trip of 22-station line)
in our earlier experiments.
We fix this problem by rescanning for a better AP every five
seconds when a streaming application is running in the foreground.
This allows the device to download 3.3 times more data through
Wi-Fi at the cost of 8.5% of extra power consumption. With this
fix, the number of stations that do transfer the data through Wi-Fi
increases to 9 from 3, and the transfer time per station lasts for 34.4
seconds on average. We find that each Wi-Fi transfer at a station
downloads 27.1 MB of the data, which is enough for watching the
video for more than 5 minutes. Unfortunately, while the other 35
train stops report Wi-Fi availability, we could not transfer any data
through their APs. We plan to investigate the reasons in the future.
Another problem we found is that it often takes as much as 3 seconds to turn on the 3G cellular interface when we switch from the
Wi-Fi Offloading Ratio (%)
Fraction of Data Saved (%)
100
80
60
40
20
0
1
51
101
151
201
251
301
Individual
flows
experiencing
network
disruptions
Flows that
experience
network
disruptions
Figure 11: Data portion saved by D2 TP connection resumption
Wi-Fi interface. Switching to LTE is not a big problem, but if LTE
is not available, the device can fall back to the 3G interface. This
implies that we need to have a larger low buffer threshold to reliably switch from Wi-Fi to cellular interfaces. But having a large
low buffer threshold could lead to unnecessarily frequent switchings to cellular networks even if Wi-Fi is available (but its available
bandwidth is temporarily fluctuating), which would reduce the WiFi offloading opportunities.
One workaround is to keep the cellular interface on even if the
device is attached to a Wi-Fi network so that the device could
switch to the cellular interface at any time. One concern with the
approach is that it would consume more power, which is obviously
undesirable. Fortunately, we find that the extra power consumption
is reasonable, ranging from only 0.7 to 1.5% as shown in Table 5
when it is compared with having only the Wi-Fi interface on. So,
we decide to keep the cellular interface on even if the device is
attached to a Wi-Fi AP.
5.2.2
Delayed data offloading with ReadyCast
Experiment setup: We conduct a user study with ReadyCast by
having 50 KAIST students use it for 8 weeks 2 . The students subscribe to any of 3,853 Podcast channels populated in ReadyCast
links, and download the contents by setting an allowed delay between 0 to 6 hours. To simulate ISP-embedded D2 Prox, we place
two D2 Prox servers in Korea Advanced Research Network (KOREN) PoPs in Seoul and in Daejeon, respectively. KOREN is a
government-funded research network testbed in South Korea, offering high-speed linkage to Japan, China, U.S., and Europe as
well [47]. We have ReadyCast choose the nearest D2 Prox server
by resolving a D2 Prox domain name (e.g., dprox.net) with our
custom DNS server. For each download, the client leaves a per-flow
log, which includes a deadline, play timestamps, network disruption/switching records, and data volume transferred via Wi-Fi and
cellular connections. Overall, we have collected a total of 2,834
unique connections that complete the downloads, which is 71.2 GB
in volume. While the users are mostly staying on campus during
weekdays, many of them tend to travel outside the campus during weekends and holidays. So, the dataset covers remote mobility
events beyond the campus networks to some extent.
Network characteristics: We describe the overall mobile network
characteristics logged by ReadyCast. First, we observe that the
users are well-exposed to Wi-Fi networks. We find that the user
devices are attached to Wi-Fi for 62.9% of the download time. Data
transfers via Wi-Fi show a higher average throughput (2.68 Mbps)
than that of cellular access (0.82 Mbps), which matches the trend
in previous experiments [5].
2
We anonymize all usage records for privacy.
100
93.4%
96.2%
99.5%
92.4%
90
80
79.5%
70
60
On-the-spot
Allowed
Delay
Delayed
(Short)
Delayed
(Long)
Delayed
(Long)
0
0 – 1 hr
1 – 3 hr
3 – 6 hr
-
# Downloads
974
362
330
1,168
2,834
Total
Figure 12: Offloading ratios by user-configured delays
User Group with
Monthly Limit (x)
0 < x ≤ 1 GB
1 GB < x ≤ 5 GB
x ≥ 5 GB
Delayed
Download
62.7%
52.2%
47.6%
Average
Allowed Delay
186.1 min
101.7 min
100.9 min
Offloading
Ratio
92.6%
90.7%
88.5%
Table 6: User study statistics based on monthly data cap
336 downloads (11.9%) experience network switchings between
Wi-Fi and cellular networks during the download. The average
number of network switchings is 2.8 times per download. This
means that a network switching is bimodal. The majority of downloads finish within a network, and if there is a network switching
(user mobility), the users switch the networks almost 3 times per
download before completion. Figure 11 shows the fraction of the
data that are transferred before network disruptions, which amount
to 38% of the entire downloaded data. This is the savings by D2 TP
while it would have been wasted on TCP without application-level
resumption. We find that 80% of the downloads have non-zero
user-allowed delays, which implies that the users are willing to benefit from Wi-Fi access even with delays when they expect network
switchings while downloading the content.
Wi-Fi offloading ratio: We next look at how much data is offloaded to Wi-Fi by ReadyCast. Figure 12 shows the breakdown
of Wi-Fi offloading ratios by the delays allowed by the users. Interestingly, even the downloads without any delay (“on-the-spot”)
deliver 79.5% of the data through Wi-Fi. We suspect that this is because the users choose “no delay” when they already know that they
are in the Wi-Fi service area. We find that 70% of the on-the-spot
flows download the data completely without cellular access, and
only 24% of them receive the content solely from 3G/LTE access.
In case a user allows some delays, the offloading ratio increases
rapidly. We observe that almost 93% of the data is offloaded to WiFi even for a few tens of minutes of delays. This implies that one
does not need to wait very long to meet available Wi-Fi APs, and
delayed offloading is a practical solution that could significantly reduce the traffic to cellular networks. If the allowed delay is larger
than three hours, almost all traffic (99.5%) is offloaded to Wi-Fi.
Overall, 92.4% of the total data is delivered through Wi-Fi without
any wasted data transfer with ReadyCast.
User reaction to delays: We analyze how users react to delayed
data transfers by looking at their deadline selection behavior. We
first surveyed users’ cellular data plans, and found the tendency
that the users allow more delays as the monthly data cap is smaller,
as shown in Table 6. The actual offloading ratios are in line with
the deadline selection where more data is offloaded to Wi-Fi with
longer delays.
Fraction of Data Transmitted (%)
100%
100
80%
80
60%
60
29%
13%
39%
8%
28%
58%
0%0
Total
Delayed (Long)
53%
13%
21%
40%
40
20%
20
16%
51%
71%
Week 1
Week 2 Week 3 - 8
Delayed (Short)
On-the-spot
Figure 13: Delay selection by users (short: < 1hr, long: ≥ 1hr)
We next analyze how users’ delay selection changes during the
experiment period. Figure 13 compares the fractions of user-selected
delays for the first two weeks and the remaining six weeks. We see
that almost 40% of the bytes are downloaded with no delay for the
first week. However, once the users get used to delayed downloads,
their delay tolerance grows accordingly, showing as much as 84%
of the downloaded volume is tagged with a positive delay and more
than 70% of the total bytes allow a delay of one hour or more. This
suggests that the users are willing to allow delays as long as the
application supports delayed downloads.
User experience: We conducted a user survey before and after
the field study. Before the experiment, almost half of the users
responded that they usually keep the Wi-Fi interface off to avoid
abrupt connection closures during network disruptions. After the
experiment, 94% of the users responded that ReadyCast was very
useful since it automatically completed the downloads regardless of
network disruptions, and 85% have felt almost no increase in battery consumption, implying that D-Lock with the 2-minute polling
period works well in practice. Many wanted to extend the service
to movies or TV shows, so delay-tolerant Netflix-like service could
be promising.
6.
DISCUSSION
In this section, we discuss other issues in developing and deploying delay-tolerant mobile applications.
Leveraging delay tolerance: Our work reconfirms that allowing
even a slight delay in download offloads a significant fraction of
cellular data to Wi-Fi. We believe there are many ways to encourage delayed downloads different from what we have presented in
this paper. One simple example is automatic synchronization of
periodically-generated contents (e.g., newspapers, TV shows, or
even movies) by a daily deadline. Another way is to develop an
app that automatically learns the user’s acceptable delay and suggests the user a default value per each download. The app can analyze the content types or the download history and can calculate
the time between the download and playback (or usage). Also, if an
app can predict the next content to download (e.g., if it can access
a playlist in on-line music streaming), it can opportunistically fetch
the content through Wi-Fi as we have showed with VLC streaming.
Concurrent multiple network interfaces: In this work, we have
not explored using both Wi-Fi and cellular interfaces at the same
time, but it could be beneficial if the Wi-Fi bandwidth happens to
be too small to finish downloading within a deadline. Allowing to
use both networks is in line with adopting multipath TCP in Apple
iOS 7 [48] and using a download booster [49] for fast download. To
support concurrent multiple network interfaces, we should adjust
D2 TP to conform to multipath TCP, and fix our flow scheduling
algorithm to reflect concurrent data transfers.
Operator-scale deployment cost: We discuss the applicability of
D2 Prox on a cellular ISP scale. A cellular ISP can decide to deploy D2 Prox in a number of vantage points close to the subscribers
and allow mobile apps to use D2 Prox after authentication. This
would efficiently distribute delay-tolerant contents to Wi-Fi with
Cedos apps while the remaining interactive, real-time contents are
delivered through cellular networks in case Wi-Fi is unavailable.
We note the cost of the increasing end-to-end latency due to routing through D2 Prox, but we believe most of delay-tolerant flows
that benefit from Cedos will be long-lived, large-data transfers that
are less affected by it. Moreover, D2 Prox could reduce the latency
by servicing popular content from its cache without contacting the
origin server. By veering a large volume of cellular data traffic
to cheap, widely-deployed Wi-Fi APs, Cedos would allow cellular
ISPs to reduce the cost of expanding and maintaining the expensive
cellular infrastructure.
Handling hard failures: We have focused mainly on how to transparently handle soft failures such as network disruptions and delay.
However, maintaining delay-tolerant, long-lived flows in the face of
hard failures such as app crash or device power outage is another
challenge that needs to be addressed. For example, our ReadyCast
could resume the data transfer after a hard failure by keeping all
flow metadata such as URL, content length, file path and deadline
persistent in its flash storage. We plan to support reliable crash
recovery in D2 TP by using persistently storing flow metadata at
minimum overhead [50].
Security: We discuss a few security issues in the operation of Cedos applications. First, D2 TP servers (or proxies) could be more
vulnerable to state explosion attacks. For example, attackers may
issue a large number of D2 TP connections with D2 Prox, but never
download the content from it. Since D2 Prox keeps track of many
concurrent D2 TP sessions, too many idle connections could exhaust the computing resources of D2 Prox. Second, attackers may
intentionally issue repeated connection or connection resumption
requests. Since the connection or connection resumption procedure requires more CPU cycles than regular TCP connection setup,
attackers may attempt to use up CPU cycles to deny other requests.
However, these attacks are not fundamentally different from other
resource exhaustion attacks on regular TCP-based (or TLS-based)
servers. One approach to mitigating these problems is to authenticate each app user, and to limit the number of concurrent requests
or to enforce a rate on bandwidth consumption per client [51].
7.
RELATED WORKS
There are many related works that address network disruptions
or exploit the delay in Wi-Fi offloading. Due to the sheer number,
comprehensive treatment is hard here, so we highlight the difference of our approach from other works.
7.1
Handling network disruptions/delays
There are various ways to handle network disruptions at content download. A popular approach is to resume the download using an application layer protocol (e.g., HTTP range queries). One
practical downside, however, is that it requires the modification of
each application, which we find is often neglected in mobile apps.
Mosh [52] and ATOM [53] provide seamless data transfer at network disruption by synchronizing to the latest state or by querying
for the latest download offset. However, their applicability is limited as they base on UDP, or it requires mobile apps to support
resumption. 3GPP’s I-WLAN [54] specifies a way to integrate the
cellular operator’s LTE and user-owned Wi-Fi APs. More recently,
IFOM [55] was proposed to support seamless flow-level offloading within I-WLAN architecture. However, implementations of IWLAN and IFOM require tight integration of the two networks,
and may result in heavy burden on the backhaul network (e.g., anchoring on P-GW).
Another workaround is to eliminate the network address binding altogether and to use a unique identifier that persists between
network disruptions. Locator/ID Separation Protocol (LISP) [23],
i3 [26], and HIP [21] bring in a separate id for the location and
use the IP address as an endpoint identifier. However, these protocols require upgrading the existing infrastructure (e.g., routers) or
adding a separate entity (e.g., DNS support), which may be challenging for immediate deployment. Mobile IP [56] uses a fixed
home address to hide a moving device, which is similar to D2 Prox
in relaying the packets. But Cedos supports delayed offloading with
a flexible network API that directly controls the mobility events.
TCP Migrate [27] and MPTCP [57, 58] remove separate entity
requirement by extending TCP to include transport-level flow identifying token. Serval [28] introduces a new service access layer,
which demultiplexes packets based on flow identifiers and migrates
a flow from one address to another. Auspice [59] is a massively
scalable global name service which resolves network location identities under high mobility. msocket [60] is a BSD socket-like API
which provides mid-connection mobility for mobile-to-mobile and
multipath cases by relying on Auspice. However, they all assume a
short reconnection and are unable to handle hours of delay as they
are based on default TCP teardown after 15 retransmissions [61].
Bundle protocol (BP) [62] is the standard communication protocol for Delay Tolerant Networks (DTN), which enables end-to-end
reliable delivery through intermittent connectivity in the challenged
networks (e.g., interplanetary [63] or rural Internet access [64]).
Notable BP implementations include DTN2 [65], IBR-DTN [66],
and ION [67]. Since these protocols are originally designed as
transport protocols in multi-hop opportunistic networks, we find
that they do not fit well into our system environments. First, BP
requires bundling the data before sending it, which makes it unsuitable to deliver dynamically generated data (e.g., media streaming).
Second, their programming API is largely different from the BSD
socket API (e.g., requiring endpoint ID starting with "dtn://"), so
that it would be difficult to port the existing mobile applications.
Cedos adopts D2 TP, a transport layer protocol tailored to Wi-Fi offloading in mobile apps. D2 TP extends our early work, DTP [46],
a disruption-tolerant transport protocol, by completing the API that
allows delay and buffer management with multi-flow scheduling
and by supporting D2 Prox for easy deployment with realistic mobile apps.
7.2
Wi-Fi offloading with delay
A large number of works have shown the potential of Wi-Fi offloading through mobility studies. A recent study on mobility estimates that over 60% of the cellular traffic can be offloaded to WiFi [5, 4]. Lee et. al. [5] report that the offloading ratio would further
improve up to 93.7% if the data transfer could be delayed by one
hour. User surveys [68, 69], incentive mechanism [70], and economic analysis [71] also support the feasibility of Wi-Fi offloading
with delay. These results may change depending on the experimental conditions, but they show that a slight delay helps a lot in
increasing Wi-Fi contact chances for most mobile data transfers.
Wiffler [4] implements a Wi-Fi offloading system that leverages
user delay tolerance to reduce cellular data traffic. However, Wiffler focuses on exploiting delay only in fixed-sized file transfers,
and shifts the burden of handling mobility events to mobile apps.
In contrast, Cedos provides transport-layer handling of the mobility events, which allows easy development of mobile apps. Also,
since Wiffler does not provide multi-flow scheduling, it could make
some flows miss the deadlines when there is resource contention.
TUBE [69] builds an app that allows a user to set a delay on mobile apps. However, its application-level implementation requires
to jailbreak the device (for autopilot) and to set a deadline per app
(not per each flow), which limits flexibility. Like Wiffler, it does
not provide transport-layer API nor network disruption handling.
In Cedos, we aim to provide flexibility to app developers with
minimal migration effort by providing the BSD socket-like API
that supports a broad spectrum of apps, ranging from interactive,
streaming to non-interactive file transfer apps.
We note that recent smartphones introduce advanced Wi-Fi network switching algorithms (e.g., "Smart Network Switch" from
Samsung Android phones) to improve the QoE by automatically
switching the Wi-Fi interface when the signal drops too low or
the network is too slow. These features would help D2 TP to improve the QoE of the users, but blind switchings agnostic to the
need of application workloads (e.g., delay tolerance) could result
in increased cellular data usage. In contrast, our solution derives
network switching decisions based on the deadline or the amount
of data in the buffer to exploit the Wi-Fi network.
8.
CONCLUSION
While delay-tolerant Wi-Fi offloading is known to be a great
idea, there has been little systems support for it. In this paper, we
have presented Cedos, which enables easy development of delaytolerant mobile apps by hiding the complexity of handling mobility
events in a new transport layer, D2 TP. D2 TP is designed to transparently handle network disruptions and delays, while it schedules multiple flows to meet deadlines for maximal Wi-Fi usage.
D2 TP also allows real-time buffer management for video streaming through opportunistic Wi-Fi connections. Finally, we provide
D2 Prox, which enables Cedos apps to coexist with TCP-based servers.
We have demonstrated that it is easy to port existing apps to Cedos,
and confirmed that the benefit is realized in the real-world usage
scenarios. We hope that Cedos further encourages delay-tolerant
app development and practicalizes the concept of delay-tolerant
Wi-Fi offloading.
9.
ACKNOWLEDGMENTS
We would like to thank the anonymous reviewers and our shepherd, Margaret Martonosi, for their insightful comments and suggestions to improve the paper. This work was supported in part by
the ICT R&D program of MSIP/IITP, Republic of Korea [14-91105-001, Development of an NFV-inspired networked switch and an
operating system for multi-middlebox services] and [14-000-04001, Development of 5G Mobile Communication Technologies for
Hyper-connected smart devices], National Research Foundation of
Korea (NRF) grant funded by the Korea government (MSIP) (NRF2013R1A2A2A01067633), and KCA-2011-11913-05004.
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