doc/protocols: add description for the RTP protocol

With some edits by Stefano.
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/protocols.texi

@@ -596,6 +596,66 @@ ffplay "rtmp://myserver/live/mystream live=1"
 
 Real-Time Protocol.
 
+The required syntax for an RTP URL is:
+rtp://@var{hostname}[:@var{port}][?@var{option}=@var{val}...]
+
+@var{port} specifies the RTP port to use.
+
+The following URL options are supported:
+
+@table @option
+
+@item ttl=@var{n}
+Set the TTL (Time-To-Leave) value (for multicast only).
+
+@item rtcpport=@var{n}
+Set the remote RTCP port to @var{n}.
+
+@item localrtpport=@var{n}
+Set the local RTP port to @var{n}.
+
+@item localrtcpport=@var{n}'
+Set the local RTCP port to @var{n}.
+
+@item pkt_size=@var{n}
+Set max packet size (in bytes) to @var{n}.
+
+@item connect=0|1
+Do a @code{connect()} on the UDP socket (if set to 1) or not (if set
+to 0).
+
+@item sources=@var{ip}[,@var{ip}]
+List allowed source IP addresses.
+
+@item block=@var{ip}[,@var{ip}]
+List disallowed (blocked) source IP addresses.
+
+@item write_to_source=0|1
+Send packets to the source address of the latest received packet (if
+set to 1) or to a default remote address (if set to 0).
+
+@item localport=@var{n} (DEPRECATED)
+Set the local port to @var{n}.
+
+@end table
+
+Important notes:
+
+@enumerate
+
+@item
+if @option{rtcpport} is not set the RTCP port will be set to the RTP
+port value plus 1.
+
+@item
+If @option{localport} (the local RTP port) is not set any available
+port will be used for the local RTP and RTCP ports.
+
+@item
+If @option{localrtcpport} (the local RTCP port) is not set it will be
+set to the the local RTP port value plus 1.
+@end enumerate
+
 @section rtsp
 
 RTSP is not technically a protocol handler in libavformat, it is a demuxer