2. STREAMING OVERLAY
Modern streaming services have to handle high scalability
demands. In most cases, a simple C/S-based system offers the
resources required by streaming users but is not scalable to
thousands of parallel views. The P2P dissemination paradigm
on the other hand offers promising scalability properties,
as every client contributes resources to keep the system
working. However, P2P systems are expensive in terms of
overhead and control based on the number of concurrent
accesses. In contrast to existing hybrid P2P-based video
dissemination services such as ToMo [1] or mTreebone [2]
this demonstration shows a transition-enabled framework for
networking mechanisms on the basis of a live video streaming
service. Transition-enabled means in this context that C/S as
well as different P2P-based video dissemination protocols can
coexist and are used when environmental conditions favor
their selection. The framework is not limited to an exchange
of topologies such as it is demonstrated in this work, but
allows to switch arbitrary networking mechanisms if they
comply to specified interfaces.
This demonstration implements an adaptation mechanism
to seamlessly switch between two content dissemination mechanisms: a central C/S and a decentralized P2P-based dissemination scheme. With this it circumvents issues resulting from
significantly varying environmental conditions. Streaming
should be continued in a consistent quality for all users even
though the operations on the network change. The used P2P
system is based on Wichtlhuber et al.’s TRANSIT [3]. The
C/S system follows a classical star topology, where joining
nodes contact a server and are provided with small video
segments, so called chunks by only this central source. The
P2P-based mechanism allows clients to retrieve video chunks
from the server but additionally from each available other
node. Therefore, a tracking functionality is hosted on the
central server to provide node contacts used for the initial neighborhoods in the P2P topology, while the exchange of
media chunks is then done completely decentralized. As [3]
has shown in live streaming scenarios: a tree topology is
beneficial to reduce delay between streaming nodes due to
the push-based delivery of chunks. Thus, the system aims to
arrange clients in a tree, but allows an hybrid video chunk
exchange in cases of high churn or if chunks are dropped due
to unreliable UDP transmissions. In such cases, the client
can request chunks in a pull-based, mesh-like manner from
each other node currently in its neighborhood. Transitions
and mechanism reconfigurations are shown between C/S and
P2P, but the P2P-based system allows further replacements
of tree and mesh-topologies.