Engineering, 2013, 5, 919-927
Published Online December 2013 (http://www.scirp.org/journal/eng)
Open Access ENG
Trust Management for Mobile Media Distribution
Raimund K. Ege
Department of Computer Science, Northern Illinois University, DeKalb, USA
Received May 9, 2013; revised June 9, 2013; accepted June 17, 2013
Copyright © 2013 Raimund K. Ege. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Multimedia content delivery to capable smart phones with high-sp eed next-generation Internet connectivity is becoming
commonplace. However, the openness of delivery demands adaptive and robust management of intellectual property
rights. The purpose of this article is to describe a framework to address the central issues in content delivery: a scalable
peer-to-peer-based content delivery model. Our method pairs the delivery with a secure access control model that en-
ables data providers to secure a return from making their original content available. Our work resulted in a prototype
implementation written in Java that includes a client for the And roid mobile platform. Adding robust trust management
to scalable peer-to-peer conten t delivery is the major significance of our work.
Keywords: Multimedia Sharing; Peer-to-Peer Content Delivery; Access Control Management
It is amazing at what rate that multimedia data are intro-
duced to the Internet and consumed. High bandwidth
Internet connectivity is no longer limited to reaching PCs
and laptops: a new generation of devices, such as net-
books and smart phones, is within reach of 3G/4G tele-
communication networks. Smart phones have ushered in
a n ew e ra in omni-present broadband media consumpti on .
Services such as YouTube and FaceBook are populariz-
ing deliverie s of a u dio and video c ontent to anyb ody with
a broadband Internet connection. Almost any kind of
multimedia data has value to somebody. Releasing it to
the Internet carries potential for capturing some of the
value, but also carries the risk that the data will be
consumed without rewarding the original source.
In this article, we describe a framework for multimedia
content delivery that is based on peer-to-peer file sharing.
Peers communicate to discover each other, to establish
trust, and to exchange data. We describe the implemen-
tation of a video player application for the Java and
Android platform that delivers video in a secure and
Delivering multimedia services has many challenges:
the ever increasing size of the data requires elaborate
delivery networks to handle peek network traffic. A
common approach to a large-scale distribution is a peer-
to-peer model, where clients that download data imme-
diately become intermediates in a delivery chain to
further clients. The BitTorrent protocol is an example of
such peer-based dat a delivery.
Another challenge is to secure and protect the property
rights of the media owners. The dynamism of peer-to-
peer communities means that principals who offer
services will meet requests from unrelated or unknown
peers. Peers need to collaborate and obtain services
within an environment that is unfamiliar or even hostile.
Therefore, peers have to manage the risks involved in the
collaboration when prior experience and knowledge
about each other are incomplete. On e way to address this
uncertainty is to develop and establish trust amon g peers.
Trust can be built by either a trusted third party  or by
community-based feedback from past experiences  in
a self-regulating system. Other approaches reported in
the literature use different access control models [3,4]
that qualify and determine authorization based on permi-
ssions defined for peers. In such a complex and collabor-
ative world, a peer can benefit and protect itself only if it
can respond to new peers and enforce access control by
assigning proper privileges to new peers.
The broader goal of our work is to address the trust in
peers who are allowed to participate in the content
delivery process, to minimize the risk and to maximize
the reward garnered from releasing data into the network.
In our prior work [5,6], we focused on modeling the
nature of risk and reward when releasing content to the
Internet. We integrated trust evaluation for usage control
R. K. EGE
with an analysis of risk and reward. Underlying our
framework is a formal computational model of trust and
access control. In the work reported here, we focus on
the implementation aspects of the framework to establish
trust among peers.
Our article is organized as follows: the next section
will elaborate on how the data provid er and its peers can
quantify gain from participating in the content delivery.
It also explains our risk/reward model that enables a data
source to initially decide on whether to share the content
and keep some leverages after its release. Section 3
describes our prototype architecture with its components
to identify and authenticate peers, to maintain trust
information, and to track swarms of peers as they con-
sume multi-media data. Section 4 introduces prototypes
for these components, including a client for the Android
platform and its implementation in Java. The article
concludes with our assessment of how peer-to-peer
systems can shed their freewheeling image via sensible
access control additions. The research reported here is an
extension of a work that appeared in the Proceedings of
the First International Conference on Mobile Services,
Resources, and Users (MOBILITY 2011) .
2. Qualifying the Value of Multimedia
It is amazing at what rate multimedia data is introduced
to the Internet and consumed. Almost any kind of multi-
media data has value to somebody. Releasing it to the
Internet carries potential for reaping some of the value,
but also carries the risk that the data will be consumed
without rewarding the original source. In addition to the
cost of creating the original multimedia data, there is also
a cost associated with releasing the data, i.e., storag e and
For example, consider the life of a typical “viral”
video found on a popular social media site: the video is
captured via a smart phone camera (maybe even acci-
dentally), then is uploaded to the social media site, dis-
cussed (i.e., “liked” and “friended”), and viewed by a
large audience (measured in millions of hits). The video
taker is rewarded with fame, rarely gets a monetary re-
ward, the entity that is getting rewarded is the social me-
dia site, which will accompany the video presentation
with paid advertising.
Let us first recap our model to asses risk and reward,
by quantizing aspects of the information interchange
between the origin al source, the tran smitting mediu m and
the final consumer of the data. Our emphasis here is on
the reward quantity, rather than on how trust in peers
affects the outcome.
In a traditional fee for service model the reward “R” to
the source is the fee “F” paid by the consumer minus the
cost “D” of delive ry :
The cost of delivery “D” consist of the storage cost at
the server, and the cost of feeding it into the Internet. In
the case of a social media site, considerable cost is in-
curred for providing the necessary network of servers
and their bandwidth to the Internet. The social media site
recovers that cost by adding paid advertising on the
source web page as well as adding paid advertising onto
the video stream. The site’s business model recognizes
that these paid advertisings represent significant added
value. As soon as we recognize that the value gained is
not an in-significant amount, the focus of the formula
shifts from providing value to the original data source to
the reward that can be gained by the transmitter. If we
quantify the advertising reward as “A” the formula now
Even in this simplest form, we recognize that “A” has
the potential to outweigh “D” and therefore reduce the
need for “F”. As the social media site recognizes, the
reward lies in “A”, i.e., paid ads that accompany the
Mediation frameworks can capture the mutative nature
of data delivery on the Internet (see also our prior work
). As data travels from a source to a client on a lengthy
path, each node in the path may act as mediator. A me-
diator transforms data from an input perspective to an
output perspective. In the simplest scenario, the data that
is fed into the delivery network by the source and is re-
ceived by the ultimate client unchanged: i.e., each me-
diator just passes its input data along as output data.
However, that is not the necessary scenario anymore: the
great variety of client devices already necessitate that the
data is transformed to enhance the client’s viewing ex-
perience. We apply this mediation approach to each peer
on the path from source to client. Each peer may serve as
a mediator that transforms the content stream in some
fashion. Our implementation employs the stream control
transmission protocol (SCTP) which allows multi-media
to be delivered in multiple concurren t streams. All a peer
needs to do is add an additional stream for a video over-
lay message to the content as it passes through.
The formula for reward can now be extended into the
P2P content delivery domain, where a large number of
peers serve as the transmission/storage medium. Assum-
ing “n” number of peers that participate and potentially
add value the formula for the reward per peer is now:
Di and Ai are now the delivery cost and value incurred
at each peer that participates in the P2P content delivery.
Fi is the fee potentially paid by each peer. Fp is the fee
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paid to the data source provider. Whether or not the data
originator will gain any reward depends on whether the
client and the peers are willing to share their gain from
the added value. In a scenario where clients and peers are
authenticated and the release of the data is predicated by
a contractual agreement, the source will reap the com-
In our model, we quantify the certainty of whether the
client and peers will remit their gain to the source with a
value of trust. Trust is evaluated based on both actual
observations and recommendations from referees. Ob-
servations are based on previous interactions with the
peer. Recommendations may include signed trust-asser-
tions from other principals, or a list of referees that can
be contacted for recommendations. Our model enables an
informed decision on whether to accept a new peer based
on the potential additional reward gained correlated to
the risk/trust encumbered by the new peer.
3. Peer-to-Peer Architecture
The architecture of our peer-to-peer multimedia delivery
framework encompasses components that aim to deliver
multi-media content from a source to a large number of
clients. We assume that the content comes into existence
at a source. A simple example of creating such multime-
dia might be a video clip taken with a camera and a mi-
crophone, or more likely video captured via a smartphone
camera, and then uploaded to the source. We assume that
clients consume the content, e.g., by displaying it on a
computing device monitor, which again might be a
smartphone screen watching an Internet video. We further
assume that there is just one original source, but that there
are many clients that want to receive the data. The clients
value their viewing experience, and our goal is to reward
the source for m aki ng t he vi d eo avai l a bl e.
In a peer-to-peer (P2P) delivery approach, each client
participates in the further delivery of the content. Each
client makes part o r all of th e origin al content av ailable to
further clients. The clients become peers in a peer-to-peer
delivery model. Such an approach is specifically geared
towards being able to scale effortlessly to support millions
of clients without prior notice, i.e., be able to handle a
“mob-like” beh avi or o f t he cli e nt s.
The nature of the source data will dictate the exact de-
tails of delivery: for example, video data is made avail-
able at a few preset quality levels using variable-rate
video encoding. The typical video source data stream is a
series of sequential frames: each frame is identified by its
frame number. Clients request frames in sequence, re-
ceive the frame and reassemble the video stream which is
then displayed using a suitable video decoder and display
utility. The video strea m is encoded in such a fashion that
missing frames don’t prevent a resulting video from being
shown, but rather a video of lesser bit-rate encoding, i.e.,
quality, will result . We explicitly allow the video
stream to be quite malleable, i.e., the quality of delivery
need not be constant and there is no harm if extra frames
find their way into the stream. It is actually a key element
of our approach that the stream can be enriched as part of
the delivery process.
In addition, all frames are encrypted to ensure confi-
dentiality, integrity and authenticated access [10,11].
The central element of our architecture is the central
trust management server. Peer clients need to register
with this server: all information that establishes the trust
in them is maintained here. Other components of the ar-
chitecture are the provider of the orig inal source data, and
peers that consume the multimedia stream. Peers can also
serve as further sources in a peer-to-peer download
The information maintained for each peer is the peer
identification, location, trust value and history of partici-
pation in data stream delivery. When a new source regis-
ters its data stream with the central trust server, th e source
also provides minimum trust criteria, which guide the
trust server when granting peer access to the stream. Only
peers that match the trust requirements with their trust
value and history are given access to the data stream. The
source also sets a bonus trust value, which is used to ad-
just peer’s trust value to reflect their participation in the
content delivery net work .
Figure 1 shows the system schematic of our frame-
work for a content delivery network. The elements are
“source”, “trust server”, “relay peer” and “edge peer”.
The connections between them represent communica-
tion among them as they establish the network and pro-
ceed with data exchange. The content delivery network
shown is populated with one source, the trust server, and
Figure 1. System schematic.
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3 peers: 2 relay peers and one edge pee r.
The source is where the video data is produced, en-
coded, encrypted and made available. The source submits
the stream info to the trust server. Peers connect to the
trust server for authentication and to receive the
download credentials. A peer that only downloads is
called an “edge peer”. Once the peer starts serving the
stream to other peers, it becomes a “relay peer”. All
peers together maintain a peer group, i.e. information on
which peers are actively part of the content delivery
network. The trust server initially informs the peers in
the peer group which source to download from: peer 1 is
fed directly from the source; peer 2 joined somewhat
later and is now being served from the source and peer 1;
the edge peer joined last and is being served from peer 1
and peer 2. In this example, peer 1 and 2 started out as
edge peer, but became relay peers once they had enough
data to start serving as intermediaries on the delivery
path from original source to ultimate consumer.
Peers stay in contact with the trust server while media
data is exchanged. The peer needs authorization to be
able to consume, i.e. decrypt and display, the data. Au-
thorization is granted via an authorization token that is
used for decryption. The token has a limited life span; it
can be revoked by the trust server based on the trust be-
havior of the peer. Once all streaming for a specific data
source has completed, the trust server adjusts the peer’s
trust value in the trust server’s database by a bonus value,
reflecting the peer’s positive cooperation. Conversely, if
the peer violated the trust placed in it, the bonus value is
deducted from the peer’s trust value.
4. Java Implementation
Our implementation has 4 major components: 1) the trust
server; 2) an application that allows a source to submit
information about a data stream; 3) a relay peer that
consumes data, i.e. shows the video, and makes it avail-
able to other peers; and 4) an edge peer to run on a mo-
bile device. All 4 components are implemented in Java.
We chose the Android platform to implement a proof-
of-concept client for a mobile device. Android is part of
the Open Handset Alliance . Android is implemented
in Java and therefore offers a flexible and standard set of
communication and security features.
The communication among the peers within their peer
group uses session initiation protocol (SIP) messaging
based on the Sip2Peer library . The actual media
exchange uses the Java implementation  of the SCTP
 transport layer protocol. For authentication we use
the OpenID  which provides a convenient and flexi-
ble way to establish the iden tity of peers, and OAuth 
which is an open protocol to allow secure authorization
in a simple and standard method from desktop and web
Peer communication is achieved via session SIP mes-
sages. Each message has a message type and carries a
payload. The initial message is of type “p eer_join” that a
new client peer sends to the trust server. Another impor-
tant type of message is “query_media”, which in-quires
about which media is available and maintained by the
peer group. The answer to this message is a list of which
peers are able to serve which parts of the available media.
The answer also provides communication details such as
the IP and port number at which a peer will serve up
frames of the media. Every peer constantly monitors the
rate of response it gets from the other peers and adjusts
its connections to the peers from which the highest
throughput rate can be achieved.
In the following, we will showcase four prototypes of
these components and discuss details of their implemen-
4.1. Trust Server
The central component of our architecture is the Trust
Server. It maintains a database of all peers and a tracks
the collection of data streams that are made available by
sources. The information maintained for each peer is its
name, its OpenID and associated identity and security
information, the exact network location, the current trust
value, and the history of current and past streams that it
Our Trust Server prototype presents a display of all
peers. Figure 2 shows a sn apshot of the P eer Trus t Man-
All peers are maintained centrally by the trust server.
When a peer connects with the trust server, authentica-
tion follows a multi-factor process: 1) the peer’s openId
is validated via the openID provider; 2) the peer’s public
key is retrieved from the trust server’s database; and 3)
the openID is used to request an OAuth authentication
token. In our current implementation, we retrieve the
OAuth token from the same provider that maintains the
peer’s OpenID. The OAuth token is specific to the data
Figure 2. Trust manager peer list.
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stream that the peer has requested. The token is the key
used to decrypt the data stream.
Figure 3 shows the security information for a specific
client, in this case the Diffie-Hellman public key  of
the peer. It is assumed that the peer stores the matching
private key. All communication past authentication is
encrypted using the se key s.
The Trust Server also maintains the list of currently
available data streams. For each stream it captures the
stream’s name, its uniform record locator (URL), the
trust value set by the source to qualif y peers, and the bo-
nus value used to reward/penalize peers for participating.
Figure 4 shows a snapshot of the Active Stream Man-
For example, consider the stream named “Blizzard of
2011”: it can be streamed from the URL “oghma://wel-
come.today.ege.com:5012”. It requires peers to have a
minimum trust value of 65, and if the peer successfully
completes participation, the peer’s trust value will be
incremented by a bonus of 10.
The sn apshot in th e figure a lso shows the log window,
which displays the current status of the trust server: all
Figure 3. Peer security information.
Figure 4. Active stream list.
activities of the content delivery network are lo gged here.
4.2. Submit Source Dialog
Arguably, the source is the most important component of
our framework. Without interesting and compelling
video content, no content delivery network will be suc-
cessful. Figure 5 shows a screen capture of the Secure
Media Submission dialog.
The source provides the stream information such as its
name and exact stream in location, i.e. uniform record
locator (URL). Peers will use this address to retrieve the
stream. The source also set the minimum required trust
level: each peer must have at least this value to partici-
pate in the stream delivery. And finally, the source sets
the trust bonus that successful participation will earn the
peer. The bonus is also used as penalty to a misbehaving
peer’s trust value.
Not shown here are the other parts of our prototype
that allow a peer to adjust stream properties, and also
revoke as stream completely, i.e. remove it from the
content delivery network.
4.3. Relay Peer
The current implementation of our prototype relay peer is
as a Java application. Nothing would prevent us from
providing the same capability as a web application.
The Java application is used to allow a peer to authen-
ticate with the trust server, get a listing of available
streams, make a selection, display the stream and finally
also make the stream available to other peers down-
stream. Figure 6 shows a screen capture of the Java Peer
Once the peer is authenticated with the trust server, it
can request a list of available streams. Figure 6 shows all
streams that are currently available with their name, re-
quired trust value, and potential bonus. The peer can
make any selection. The reasons why all stream are dis-
Figure 5. Secure me dia submission.
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played, even the ones which require a higher trust value
than what the peer currently has, is to give the peer an
incentive to first participate in another stream to add the
bonus to its trust value. However, only streams can actu-
ally be selected for which the peer is currently qualified.
Figure 7 shows such a case:
Once the peer has selected a stream for viewing, the
trust server will transmit the necessary information to
enable the peer to start download. It will get the set of all
locations at which the media stream is available, plus the
necessary access token to enable stream decryption. The
peer then contacts the source locations at their streams’
URLs and starts downloading data, i.e. the sequential
frames of the video stream.
In general, peers can do 3 things: 1) they continuously
request frames from other peers (the original source is
viewed as just another peer) and store them; 2) they may
display the frames as video to the user of the peer device;
3) and they make the stored frames available to other
Figure 8 shows our prototype Java implementation of
Figure 6. Peer stream selection.
Figure 7. Peer stream selection failure.
Figure 8. Peer streaming video.
our Peer Client while it displays the requested video:
Peers don’t need to provide all 3 services. A peer that
provides only service (1) and (2) is an “edge” peer, i.e.,
an end user consumer. A peer that provides service (1)
and (3) is a “relay” peer. Relay peers are specifically
important for peers that have limited access to the public
Internet, i.e., peers behind network boundaries, such as a
NAT firewall. In addition, peers stay in contact with each
other to continuously update the peer group and source
A peer client consists of three processes:
1) A process to communicate with the trust server. The
client initiates the negotiation with the server to get
admitted into the content delivery network. Upon
success, the server informs the client which sources
the client should use accompanied by their access to-
kens. The client will update the tracker on its success
in down-loading th e source data;
2) A process to request data from the given sources.
Frames may be requested from multiple sources.
Frames that are received are decrypted using access
token for the given data source.
3) A process to sequence the frames received from
sources and to assemble them into a usable media
Our prototype uses the SCTP  transport layer pro-
tocol. SCTP is serving in a similar role as the popular
TCP and UDP protocols. It provides some of the same
service features of both, ensuring reliable, in-sequence
transport of messages with congestion con trol. We chose
SCTP because of its ability to deliver multimedia in mul-
tiple streams. Once a client has established a SCTP asso-
ciation with a server, packages can be exchanged with
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high speed and low laten cy. Each association can su pport
multiple streams, where the packages that are sent within
one stream are guaranteed to arrive in sequence. Each
source can divide the original video stream into set of
streams meant to be displayed in an overlay fashion.
Streams can be arranged in a way that the more streams
are fully received by a client, the better the viewing qual-
ity will be. The first stream is used to deliver a basic low
quality version of the video stream. The second and con-
secutive streams carry frames that are overlaid onto the
primary stream for the purpose of increasing the quality.
In our framework we also use the additional streams to
carry content that is “added value”, such as advertising
messages or identifying logos. The ultimate client that
displays the content to a user will combine all streams
into one viewing experience.
4.4. Android Edge Peer
The final component of our prototype framework is our
proof-of-concept edge peer implementation for the An-
droid platform. In the following we will show three
screenshots: Figure 9 shows the login screen, Figure 10
shows the stream selection screen, and finally Figure 11
shows the video being streamed onto the mobile device.
First, Figure 9 shows the login screen. Like in the
Java application, each peer is authenticated with its
OpenID credentials. The user enters userid and password,
Figure 9. Oghmasip login screen.
Figure 10. OghmaSip available video screen.
Figure 11. OghmaSip video delivery screen.
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plus the URL of the central trust server. If the peer is new
to the content delivery network, it will also generate a
public/private pair of Diffie-Hellman keys , keep one
private and submit the pub lic one to the trust server.
Once authentication is achieved, i.e. the OpenID pro-
vider has sent the authorization token, the user is show
which streams are currently available on the next screen.
Figure 10 presents a drop-down list of streams that are
available to be served to this mobile device. The screen
also offers a menu to allow the peer to modify configure-
tion data, see information about other peers in the peer
group, and to manage its own security information.
Once the “play selected video stream” button is
pressed, the user is shown the video display screen, as
shown in Figure 10. Once a sufficient read-ahead buffer
has been accumulated, the video stream starts playing on
the Android device (Figure 11).
Our Android prototype implementation uses a second
feature of the SCTP protocol: we use is its new class
SctpMultiChannel which can establish a one-to-many
association for a single server to multiple clients. The
SctpMultiChannel is able to recognize which client is
sending a request and en ables that the response is sent to
that exact same client. This is much more efficient than a
traditional “server socket” which for each incoming re-
quest spawns a sub-process with its own socket to serve
the client. Each packet that is received on the channel
carries a MessageInfo object which contains information
on the actual client that is the actual other end point of
this association. The code to receive SctpMultiChannel
packets is logically similar to any UPD or TCP style of
socket receive data packets programming.
In this article, we described a framework for new content
delivery networks that almost implements access control
for its participating peers. We have described a prototype
implementation written in Java to establish a P2P net-
work, where a group of peers disseminate information on
which sources are available to download, and it includes
a Java-based client for the Android platform for smart-
phones. Such P2P content delivery has a great potential
to enable large scale d elivery of multimedia content. Our
framework is designed to enable content originators to
assess the potential reward from distributing the content
to the Internet. The reward is quantified as the value
added at each peer in the content delivery network and
gauged relative to the actual cost incurred in data deliv-
ery but also correlated to the risk that su ch open delivery
The scenario of a typical “viral” video found on a so-
cial networking site is considered. The video is captured
via a cell phone camera by a user, and the user then up-
loads the video onto the site. The social networking site
stores the video and makes it available to other users for
free. The video becomes popular and is viewed by a larg e
audience driving traffic to the social networking site. The
only entity that is getting a reward is social media site,
which accompanies the video presentation with paid ad-
vertising. The on ly benefit that the original source of th e
video gets is notoriety. Using our model, the original data
owner can select other venues to make the video avail-
able via a peer-to-peer approach. The selection on who
will participate can be based on how much each peer
contributes in terms of reward but also risk. Peers will
have an interest in being part of the delivery network,
such as Facebook and YouTube which have recogniz ed its
value. P eers migh t even ad d their own va lue to the d eliv-
ery and share the proceeds with the original source.
Whereas in the social media approach, the reward is only
reaped by one, and the original source has shouldered all
the risk, i.e., lost all reward from the content, and our
model will enable a more equitable mechanism for shar-
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