Streaming Media Supplement sa2150 and sa2250

3
Chapter 1 About Streaming Media
Sometimes network conditions or design can make this default streaming strategy unworkable. The players for
all three formats have the ability to fall back to or be configured to use alternative strategies. The three main
fallback strategies are:
• using TCP, rather than UDP, on the data channel, so that both control and data channels use TCP as the
underlying transport protocol
• interleaving control commands and data over a single channel, using the different control and data transfer
protocols over a single underlying transport protocol, namely TCP
• streaming over HTTP, with TCP as the underlying transport protocol
A player falls back to an alternative strategy according to a built-in algorithm, and it’s not possible to stop the
process. The player falls back to one strategy, and if that strategy fails, falls back to yet another. Which
strategies are used, and in which order, varies from player to player.
The most common reason for configuring players to use an alternative strategy is the presence of a firewall that
drops UDP packets.
Origin servers can stream media data at different bitrates chosen according to the needs of the client player.
When the player requests a particular bit rate, that is called stream selection. The server, meanwhile, has
strategies for consuming the least bandwidth possible, referred to as stream thinning. Media-IXT supports
multi-bitrate streams from RealNetworks and WMT origin servers. The term multi-bitrate clip refers to the
technique where an origin server stores several versions of the same content within one file, encoded for
streaming at different bitrates.
Streaming over HTTP is normally the last-resort fallback strategy for streaming, but sometimes is chosen as the
primary streaming strategy for a deployment. That choice raises issues which are addressed at the end of this
section.
Media streaming formats and protocols
The three streaming formats use different control and data transfer protocols.
Some protocols are proprietary, while others are based on public Internet RFCs: RTSP (Real Time Streaming
Protocol) is based on RFC 2326, and RTP (Real Time Protocol) is based on RFCs 2326 and 1889.
Windows Media servers stream WMT content to Windows Media Players.
RealNetworks servers stream RealNetworks content to RealPlayers, and QuickTime content to QuickTime
players.
QuickTime servers stream QuickTime content to QuickTime players.
In each case, Media-IXT proxies and caches this content.
This table summarizes protocol use by format:
Streaming format
Control
protocol
Data transfer
protocol Protocol ownership
Underlying
transport
protocol
RealNetworks RTSP RDT (formerly
known as RDP)
RTSP is Internet RFC-based
RDT is proprietary
UDP or TCP
WMT (current) MMS MMST proprietary TCP
WMT (current) MMS MMSU proprietary UDP
WMT (planned) RTSP RTP Internet RFC-based UDP or TCP
QuickTime RTSP RTP Internet RFC-based UDP or TCP