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9.2.4
RTSP
The Real Time Streaming Protocol (RTSP) is used to control the continuous transmission
of multimedia content via IP based networks (media streams).
Thereby, RTSP uses a direct (bidirectional) communication with the RTSP streaming
server of the camera; on the one hand in order to determine the applicable transmission
protocol for the RTP data transfer (UDP or TCP), on the other hand to transmit control ac
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tions of IP based RTSP-capable applications (players) such as the starting and stopping
of video transmission.
The encoding, packet assembly and transport of the data streams from server to client is
thereby carried out directly (unidirectionally) via the Real-Time Transport Protocol (RTP).
Usually, the RTP transmissions of the streaming content are realised via UDP (User
Datagram Protocol), whereas the RTSP transmissions are realised via a TCP connection
(TCP = Transmission Control Protocol).
The following points need to be considered for RTP transmissions via UDP:
•
UDP is a so-called “unreliable” and connectionless protocol.
No connection is established to the receiver/client prior to the data transmission.
The receiver/client does not send a confirmation of the receipt of the data.
During the transmission via UDP packet losses (e.g. lack of individual images) can oc
-
cur.
Lost packets will not be sent again.
• Usually, UDP packets that come from outside (internet) into the local area network are
blocked by internet routers/firewalls in general.
•
UDP allows for smooth and fast data transmission with relatively low delays, i.e. without
a time offset of the IP packets (jitter).
• Each RTSP/RTP transmission via UDP requires three ports to be open: a static port for
the RTSP control commands (standard port number 554) and two dynamic ports for the
RTP data stream.
The following points need to be considered for RTP/RTSP transmissions via TCP:
•
TCP is a so-called “reliable” and connection-oriented protocol.
A connection to the receiver/client is established prior to the data transmission.
The receiver/client sends a confirmation of the receipt of each IP packet.
During the data transmission via TCP there are usually no packet losses (unless in the
case of a buffer overload in the camera due to permanent network overload). However,
the transmission may be slower than with UDP.
• Usually, only the RTSP port at the internet router/firewall needs to be open in order to
allow for data transmissions of RTP/RTSP/TCP packets from the internet to the local
area network.
•
With RTSP the transmission of RTP streams can be embedded in the existing RTSP/
TCP connection; a separate UDP transmission or an additional port for the RTP data
stream is not necessary.
