  In this paper, we attempt to revisit the problem of multi-party conferencing
from a practical perspective, and to rethink the design space involved in this
problem. We believe that an emphasis on low end-to-end delays between any two
parties in the conference is a must, and the source sending rate in a session
should adapt to bandwidth availability and congestion. We present Celerity, a
multi-party conferencing solution specifically designed to achieve our
objectives. It is entirely Peer-to-Peer (P2P), and as such eliminating the cost
of maintaining centrally administered servers. It is designed to deliver video
with low end-to-end delays, at quality levels commensurate with available
network resources over arbitrary network topologies where bottlenecks can be
anywhere in the network. This is in contrast to commonly assumed P2P scenarios
where bandwidth bottlenecks reside only at the edge of the network. The
highlight in our design is a distributed and adaptive rate control protocol,
that can discover and adapt to arbitrary topologies and network conditions
quickly, converging to efficient link rate allocations allowed by the
underlying network. In accordance with adaptive link rate control, source video
encoding rates are also dynamically controlled to optimize video quality in
arbitrary and unpredictable network conditions. We have implemented Celerity in
a prototype system, and demonstrate its superior performance over existing
solutions in a local experimental testbed and over the Internet.
