  A protocol named Threshold Bipolar (TB) is proposed as a fetching strategy at
the startup stage of p2p live streaming systems. In this protocol, chunks are
fetched consecutively from buffer head at the beginning. After the buffer is
filled into a threshold, chunks at the buffer tail will be fetched first while
keeping the contiguously filled part in the buffer above the threshold even
when the buffer is drained at a playback rate. High download rate, small
startup latency and natural strategy handover can be reached at the same time
by this protocol. Important parameters in this protocol are identified. The
buffer progress under this protocol is then expressed as piecewise lines
specified by those parameters. Startup traces of peers measured from PPLive are
studied to show the real performance of TB protocol in a real system. A simple
design model of TB protocol is proposed to reveal important considerations in a
practical design.
