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RTSP Version
RTSP URL
The "rtsp" and "rtspu" schemes are used to refer to network resources via the RTSP protocol. This section defines the scheme-specific syntax and semantics for RTSP URLs.
rtsp_URL = (“rtsp:" | "rtspu :”)
"//" host [“:" port] [abs_path]
host = A legal Internet host domain name of IP address
(in dotted decimal form), as defined by
of RFC 1123 \cite{rfc1123}> port = *DIGIT .
The scheme rtsp requires that commands are issued via a reliable protocol (within the Internet, TCP), while the scheme rtspu identifies an unreliable protocol (within the Internet, UDP).
If the port is empty or not given, port 554 is assumed. The semantics are that the identified resource can be controlled by RTSP at the server listening for TCP (scheme "rtsp") connections or UDP (scheme "rtspu") packets on that port of host, and the Request-URI for the resource is rtsp_URL.
A presentation or a stream is identified by a textual media identifier, using the character set and escape conventions of URLs. URLs may refer to a stream or an aggregate of streams, i.e., a presentation.
For example, the RTSP URL:
rtsp://media.example.com:554/twister/audiotrack
identifies the audio stream within the presentation "twister", which can be controlled via RTSP requests issued over a TCP connection to port 554 of host media.example.com.
Also, the RTSP URL:
rtsp://media.example.com:554/twister
identifies the presentation "twister", which may be composed of audio and video streams.
This does not imply a standard way to reference streams in URLs. The presentation description defines the hierarchical relationships in the presentation and the URLs for the individual streams. A presentation description may name a stream "a.mov" and the whole presentation "b.mov".

The path components of the RTSP URL are opaque to the client and do not imply any particular file system structure for the server.
This decoupling also allows presentation descriptions to be used with non-RTSP media control protocols simply by replacing the scheme in the URL.
Conference Identifiers

Conference identifiers are opaque to RTSP and are encoded using standard URL encoding methods (i.e., LWS is escaped with %). They can contain any octet value. The conference identifier MUST be globally unique

Conference identifiers are used to allow RTSP sessions to obtain parameters from multimedia conferences the media server is participating in. These conferences are created by protocols outside the scope of this specification, e.g., H.323 or SIP. Instead of the RTSP client explicitly providing transport information, for example, it asks the media server to use the values in the conference description instead.

Session Identifiers
Session identifiers are opaque strings of arbitrary length. Linear white space must be URL-escaped. A session identifier MUST be chosen randomly and MUST be at least eight octets long to make guessing it more difficult.
SMPTE Relative Timestamps
A SMPTE relative timestamp expresses time relative to the start of the clip. Relative timestamps are expressed as SMPTE time codes for frame-level access accuracy. The time code has the format hours:minutes:seconds:frames.subframes, with the origin at the start of the clip. The default smpte format is "SMPTE 30 drop" format, with frame rate is 29.97 frames per second. Other SMPTE codes MAY be supported through the use of alternative use of "smpte time". For the "frames" field in the time value can assume the values 0 through 29. The difference between 30 and 29.97 frames per second is handled by dropping the first two frame indices (values 00 and 01) of every minute, except every tenth minute. If the frame value is zero, it may be omitted. Sub frames are measured in one-hundredth of a frame.
Examples:
smpte=10:12:33:20-


Normal Play Time:
Normal play time (NPT) indicates the stream absolute position relative to the beginning of the presentation. The timestamp consists of a decimal fraction. The part left of the decimal may be expressed in either seconds or hours, minutes, and seconds. The part right of the decimal point measures fractions of a second.
The beginning of a presentation corresponds to 0.0 seconds. Negative values are not defined. The special constant now is defined as the current instant of a live event. It may be used only for live events.
It is often digitally displayed on a VCR. NPT advances normally when in normal play mode (scale = 1), advances at a faster rate when in fast scan forward (high positive scale ratio), decrements when in scan reverse (high negative scale ratio) and is fixed in pause mode. NPT is (logically) equivalent to SMPTE time codes."
Absolute Time
Absolute time is expressed as ISO 8601 timestamps, using UTC (GMT). Fractions of a second may be indicated.
Example for November 8, 1996 at 14h37 and 20 and a quarter seconds UTC: 19961108T143720.25Z
Option Tags
Option tags are unique identifiers used to designate new options in RTSP. These tags are used in require and Proxy- Require header fields.
Syntax: option-tag = 1*xchar
The creator of a new RTSP option should either prefix the option with a reverse domain name (e.g., "com.foo.mynewfeature" is an apt name for a feature whose inventor can be reached at "foo.com"), or register the new option with the Internet Assigned Numbers Authority (IANA).