to client-server based end-to-end services. This ap-
proach has been thoroughly analyzed and studied in
the literature. For instance, in (ASTRALS, 2010;
Schierl et al., 2007; Renzi et al., 2008) N RTP (Real
Time Protocol) sessions transmitted by the server are
fused by the MANE into a single RTP flow for each
client according to network conditions. Still, other
contributions (Liebl et al., 2006; Tizon and Pesquet-
Popescu, 2008) propose to use MANEs to perform
an optimized packet scheduling and radio resource
sharing over the last wireless hop of a network by
mapping scalable content layer dependencies to flow
priorities. Unfortunately, the aforementioned use of
MANEs presents several disadvantages: 1) the inser-
tion of an intermediate media-aware device into the
streaming scenario, and 2) the need for modifying
both RTP and RTCP (Real Time Control Protocol)
packets to adapt them to the customized content. De-
ploying MANEs into the streaming system requires to
know beforehand where the final clients are located.
Since the success of streaming services is only
achievable if respecting the self-regulatory nature of
transmissions within the Internet, it is mandatory to
avoid either overloading or under-utilizing network
resources. This justifies the need for providing con-
gestion control techniques. Several congestion con-
trol mechanisms have been presented for streaming
applications in the literature, e.g. see (Feamster et al.,
2001; Ma and Ooi, 2007; Mujica-V. et al., 2004;
Papadimitriou and Tsaoussidis, 2007) and references
therein. However, most of them are source-based
(i.e. the transmitting node is in charge for implement-
ing congestion-aware techniques), which requires ac-
tive probing, information piggybacking or acknowl-
edgement mechanisms. Several drawbacks can be in-
ferred from the application of source-based conges-
tion protocols to heterogeneous IP networks. On one
hand, demanding feedback from the client implies
an overhead in terms of both processing complexity
at the client and bandwidth over-utilization. On the
other hand, in networks where both wired and wire-
less technologies coexist links with highly asymmet-
ric characteristics are likely to appear. Therefore, tak-
ing the two-way path into account is not desirable
in heterogeneous networks, since asymmetric charac-
teristics of paths cannot be reliably estimated at the
server side. Thus, worst conditions prevail in two-
way path congestion control, as it is not possible to
distinguish whether the problem arises in the uplink
or the downlink. The heterogeneous nature of future
networks implies the need for new receiver-driven
congestion control mechanisms. We here propose to
discard considering two-way paths and, in-contrast, to
only account for the down-link state in our receiver-
driven network congestion.
This position paper outlines the key design prin-
ciples of a receiver-driven streaming system based on
scalable multimedia content. Both the management
of the multimedia content and the congestion and flow
control logic are placed on the client, hence minimiz-
ing the computational complexity of the server. In
our approach, not only streaming standards are kept
unmodified, but we also profit from the information
already embedded by such protocols. Furthermore,
each constituent component of our proposed architec-
ture is independent from each other. Another novel
contribution of our work hinges on the metrics uti-
lized for the congestion and flow control mechanisms,
for which we introduce a novel LER (Loss Event
Rate) metric which is proven to offer enhanced stabil-
ity to bursty losses with respect to conventional packet
loss rate metrics.
The remainder of this manuscript is organized
as follows: first Section 2 introduces the reader to
the fundamentals of scalable multimedia streaming
1
,
whereas Section 3 presents our novel receiver-driven
end-to-end streaming system proposal for distributing
scalable media content. Finally, concluding remarks
and future research lines are drawn in Section 4.
2 SCALABLE STREAMING
Due to the heterogeneity of the actual networks and
the proliferation of a wide range of final devices, it
is essential to adapt the streaming content for each
specific context. Early approaches have been based
on storing a number of replicas of the same origi-
nal content or, alternately, on transcoding the original
content in a case-by-case basis. Recently, research
efforts have been conducted towards the generation
of inherently scalable multimedia content as a means
to provide different versions of the same multime-
dia content, without resorting to multiple successive
transcoding tasks. Consequently, processing redun-
dancy and storage occupancy of the encoded multi-
media content are minimized.
This growing interest in scalable codification has
led to several research lines: the SVC (H.264/SVC,
2009) and MVC (H.264/MVC, 2009) extensions of
the so-called Advanced Video Codec (H.264/AVC).
The H.264/SVC standard attains high compression
rates while simultaneously combining three scalabil-
ity levels into a single encoded bitstream, namely spa-
tial (resolution), temporal (frame rate) and signal-to-
noise ratio (SNR) scalability (fidelity). H.264/SVC
1
The authors recommend the reader to skip Section 2 if
familiar with the concepts tackled therein.
SIGMAP 2010 - International Conference on Signal Processing and Multimedia Applications
40