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global consumer trac by 2018, including video on-demand (VoD), live stream-
ing, and peer-to-peer (P2P) file sharing. In fact, as the Internet access has become
ubiquitous, continuously faster, and cheaper, streaming video has become main-
stream. Users are progressively moving from the old-fashioned scheduled televi-
sion to VoD services. This contributes to increase the expectations of consumers
on Internet video delivery.
Since broadcasting future seems to be online, customers have become more
sensitive to VoD quality, expecting ever-higher bitrates and lower rebuffering.
Contrary to many traditional workloads, e.g. social network messaging or search
engines, specifying just latency as quality of service (QoS) metric does not suf-
fice. Instead, streaming trac requires proper average bitrate to avoid rebuffer-
ing and improve user experience. For example, Dobrain
et al.
[
13
] found that
a 1 % increase in buffering ratio can reduce the consumer's expected viewing
time by more than three minutes. Balachandran
et al.
, observe that increased
average bitrate in Internet video delivery leads to a better user experience for
viewers with mobile devices [
3
]. This suggests that service-level agreement (SLA)
contracts must include average bitrate as a key QoS metric.
Yet, current Content Delivery Networks (CDN) platforms are not ready to
fulfil the requirements of the increasing demand for VoD services and meet con-
sumers' expectations. Through fine-grained client-side measurements from over
200 million client viewing sessions, Liu
et al.
[
21
] showed that 20 % of these ses-
sions experience a rebuffering ratio of at least 10 %, 14 % of users have to wait
more than 10 s for video to start up, more than 28 % of sessions have an average
bitrate less than 500 Kbps, and 10 % of users fail to see any video at all.
To deal with these issues, CDN providers have started to combine datacen-
ters and edge network resources in hybrid designs
2
. This includes
peer-assisted
VoD systems
[
17
] whose deployment requires hybrid CDN platforms. The aim
of peer-assisted VoD systems is to take advantage of both infrastructure-based
resources and P2P communication facilities. Huang
et al.
[
17
] suggest the use
of peer-assisted VoD systems to improve resource allocation for Internet video
delivery. They argue that devices on edge networks, e.g. set-top-boxes, contribute
with storage and bandwidth to video delivery, reducing dramatically the burden
on infrastructure-based servers, and cutting operations costs. Many recent stud-
ies [
10
,
18
,
25
] confirm that exploring peer-assisted VoD system permits enhanc-
ing resource allocation for streaming videos, but none has properly evaluated
the performance of video delivery regarding SLA enforcement.
In fact, there exists an increasing need for more research in easy-to-deploy,
self-adapting techniques for ensuring tough QoS guarantees brought by the cloud
paradigm. However, ecient resource allocation on hybrid CDNs to meet user
expectations imposes big challenges, particularly for resource-hungry services as
VoD. This paper identifies
adaptive content replication
as one of such challenges.
Adaptive replication plays an important role on the content availability of dis-
tributed systems, contributing directly to both storage and bandwidth provision.
2
Akamai acquires Red Swoosh.
http://www.akamai.com/html/about/press/releases/
2007/press 041207.html
,
April 2007.
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