Section 3. Document Conventions

3.1.  Notational Conventions
   All the mechanisms specified in this document are described in both
   prose and the Augmented Backus-Naur form (ABNF) described in detail
   in [RFC5234].
   Indented paragraphs are used to provide informative background and
   motivation.  This is intended to give readers who were not involved
   with the formulation of the specification an understanding of why
   things are the way they are in RTSP.
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].
   The word, "unspecified" is used to indicate functionality or features
   that are not defined in this specification.  Such functionality
   cannot be used in a standardized manner without further definition in
   an extension specification to RTSP.
3.2.  Terminology
   Aggregate control:  The concept of controlling multiple streams using
      a single timeline, generally one maintained by the server.  A
      client, for example, uses aggregate control when it issues a
      single play or pause message to simultaneously control both the
      audio and video in a movie.  A session that is under aggregate
      control is referred to as an "aggregated session".
   Aggregate control URI:  The URI used in an RTSP request to refer to
      and control an aggregated session.  It normally, but not always,
      corresponds to the presentation URI specified in the session
      description.  See Section 13.3 for more information.
   Client:  The client is the requester of media service from the media
      server.
   Connection:  A transport-layer virtual circuit established between
      two programs for the purpose of communication.
   Container file:  A file that may contain multiple media streams that
      often constitute a presentation when played together.  The concept
      of a container file is not embedded in the protocol.  However,
      RTSP servers may offer aggregate control on the media streams
      within these files.
   Continuous media:  Data where there is a timing relationship between
      source and sink; that is, the sink needs to reproduce the timing
      relationship that existed at the source.  The most common examples
      of continuous media are audio and motion video.  Continuous media
      can be real time (interactive or conversational), where there is a
      "tight" timing relationship between source and sink or it can be
      streaming where the relationship is less strict.
   Feature tag:  A tag representing a certain set of functionality,
      i.e., a feature.
   IRI:  An Internationalized Resource Identifier is similar to a URI
      but allows characters from the whole Universal Character Set
      (Unicode/ISO 10646), rather than the US-ASCII only.  See [RFC3987]
      for more information.
   Live:  A live presentation or session originates media from an event
      taking place at the same time as the media delivery.  Live
      sessions often have an unbound or only loosely defined duration
      and seek operations may not be possible.
   Media initialization:  The datatype- or codec-specific
      initialization.  This includes such things as clock rates, color
      tables, etc.  Any transport-independent information that is
      required by a client for playback of a media stream occurs in the
      media initialization phase of stream setup.
   Media parameter:  A parameter specific to a media type that may be
      changed before or during stream delivery.
   Media server:  The server providing media-delivery services for one
      or more media streams.  Different media streams within a
      presentation may originate from different media servers.  A media
      server may reside on the same host or on a different host from
      which the presentation is invoked.
   (Media) Stream:  A single media instance, e.g., an audio stream or a
      video stream as well as a single whiteboard or shared application
      group.  When using RTP, a stream consists of all RTP and RTCP
      packets created by a media source within an RTP session.
   Message:  The basic unit of RTSP communication, consisting of a
      structured sequence of octets matching the syntax defined in
      Section 20 and transmitted over a transport between RTSP agents.
      A message is either a request or a response.
   Message body:  The information transferred as the payload of a
      message (request or response).  A message body consists of meta-
      information in the form of message body headers and content in the
      form of an arbitrary number of data octets, as described in
      Section 9.
   Non-aggregated control:  Control of a single media stream.
   Presentation:  A set of one or more streams presented to the client
      as a complete media feed and described by a presentation
      description as defined below.  Presentations with more than one
      media stream are often handled in RTSP under aggregate control.
   Presentation description:  A presentation description contains
      information about one or more media streams within a presentation,
      such as the set of encodings, network addresses, and information
      about the content.  Other IETF protocols, such as SDP ([RFC4566]),
      use the term "session" for a presentation.  The presentation
      description may take several different formats, including but not
      limited to SDP format.
   Response:  An RTSP response to a request.  One type of RTSP message.
      If an HTTP response is meant, it is indicated explicitly.
   Request:  An RTSP request.  One type of RTSP message.  If an HTTP
      request is meant, it is indicated explicitly.
   Request-URI:  The URI used in a request to indicate the resource on
      which the request is to be performed.
   RTSP agent:  Either an RTSP client, an RTSP server, or an RTSP proxy.
      In this specification, there are many capabilities that are common
      to these three entities such as the capability to send requests or
      receive responses.  This term will be used when describing
      functionality that is applicable to all three of these entities.
   RTSP session:  A stateful abstraction upon which the main control
      methods of RTSP operate.  An RTSP session is a common context; it
      is created and maintained on a client's request and can be
      destroyed by either the client or server.  It is established by an
      RTSP server upon the completion of a successful SETUP request
      (when a 200 OK response is sent) and is labeled with a session
      identifier at that time.  The session exists until timed out by
      the server or explicitly removed by a TEARDOWN request.  An RTSP
      session is a stateful entity; an RTSP server maintains an explicit
      session state machine (see Appendix B) where most state
      transitions are triggered by client requests.  The existence of a
      session implies the existence of state about the session's media
      streams and their respective transport mechanisms.  A given
      session can have one or more media streams associated with it.  An
      RTSP server uses the session to aggregate control over multiple
      media streams.
   Origin server:  The server on which a given resource resides.
   Seeking:  Requesting playback from a particular point in the content
      time line.
   Transport initialization:  The negotiation of transport information
      (e.g., port numbers, transport protocols) between the client and
      the server.
   URI:  A Universal Resource Identifier; see [RFC3986].  The URIs used
      in RTSP are generally URLs as they give a location for the
      resource.  As URLs are a subset of URIs, they will be referred to
      as URIs to cover also the cases when an RTSP URI would not be a
      URL.
   URL:  A Universal Resource Locator is a URI that identifies the
      resource through its primary access mechanism rather than
      identifying the resource by name or by some other attribute(s) of
      that resource.