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RFC 7826


Real-Time Streaming Protocol Version 2.0

Part 8 of 13, p. 159 to 185
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18.34.  Proxy-Authenticate

   The Proxy-Authenticate response-header field MUST be included as part
   of a 407 (Proxy Authentication Required) response.  The field-value
   consists of a challenge that indicates the authentication scheme and
   parameters applicable to the proxy for this Request-URI.  The
   definition of the header is in [RFC7235], and any applicable HTTP
   authentication schemes appear in other RFCs, such as Digest [RFC7616]
   and Basic [RFC7617].

   The HTTP access authentication process is described in [RFC7235].
   This header MUST only be used in response messages related to client-
   to-server requests.

18.35.  Proxy-Authentication-Info

   The Proxy-Authentication-Info response-header is used by the proxy to
   communicate some information regarding the successful authentication
   to the proxy in the message response in some authentication schemes,
   such as the Digest scheme [RFC7616].  The definition of the header is
   in [RFC7615], and any applicable HTTP authentication schemes appear
   in other RFCs.  This header MUST only be used in response messages
   related to client-to-server requests.  This header has hop-by-hop

18.36.  Proxy-Authorization

   The Proxy-Authorization request-header field allows the client to
   identify itself (or its user) to a proxy that requires
   authentication.  The Proxy-Authorization field-value consists of
   credentials containing the authentication information of the user
   agent for the proxy or realm of the resource being requested.  The
   definition of the header is in [RFC7235], and any applicable HTTP
   authentication schemes appear in other RFCs, such as Digest [RFC7616]
   and Basic [RFC7617].

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   The HTTP access authentication process is described in [RFC7235].
   Unlike Authorization, the Proxy-Authorization header field applies
   only to the next-hop proxy.  This header MUST only be used in client-
   to-server requests.

18.37.  Proxy-Require

   The Proxy-Require request-header field is used to indicate proxy-
   sensitive features that MUST be supported by the proxy.  Any Proxy-
   Require header features that are not supported by the proxy MUST be
   negatively acknowledged by the proxy to the client using the
   Unsupported header.  The proxy MUST use the 551 (Option Not
   Supported) status code in the response.  Any feature tag included in
   the Proxy-Require does not apply to the endpoint (server or client).
   To ensure that a feature is supported by both proxies and servers,
   the tag needs to be included in also a Require header.

   See Section 18.43 for more details on the mechanics of this message
   and a usage example.  See discussion in the proxies section
   (Section 15.1) about when to consider that a feature requires proxy

   Example of use:

      Proxy-Require: play.basic

18.38.  Proxy-Supported

   The Proxy-Supported general-header field enumerates all the
   extensions supported by the proxy using feature tags.  The header
   carries the intersection of extensions supported by the forwarding
   proxies.  The Proxy-Supported header MAY be included in any request
   by a proxy.  It MUST be added by any proxy if the Supported header is
   present in a request.  When present in a request, the receiver MUST
   copy the received Proxy-Supported header in the response.

   The Proxy-Supported header field contains a list of feature tags
   applicable to proxies, as described in Section 4.5.  The list is the
   intersection of all feature tags understood by the proxies.  To
   achieve an intersection, the proxy adding the Proxy-Supported header
   includes all proxy feature tags it understands.  Any proxy receiving
   a request with the header MUST check the list and remove any feature
   tag(s) it does not support.  A Proxy-Supported header present in the
   response MUST NOT be modified by the proxies.  These feature tags are
   the ones the proxy chains support in general and are not specific to
   the request resource.

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     C->P1: OPTIONS rtsp:// RTSP/2.0
            Supported: foo, bar, blech
            User-Agent: PhonyClient/1.2

    P1->P2: OPTIONS rtsp:// RTSP/2.0
            Supported: foo, bar, blech
            Proxy-Supported: proxy-foo, proxy-bar, proxy-blech
            Via: 2.0

    P2->S:  OPTIONS rtsp:// RTSP/2.0
            Supported: foo, bar, blech
            Proxy-Supported: proxy-foo, proxy-blech
            Via: 2.0, 2.0

     S->C:  RTSP/2.0 200 OK
            Supported: foo, bar, baz
            Proxy-Supported: proxy-foo, proxy-blech
            Via: 2.0, 2.0

18.39.  Public

   The Public response-header field lists the set of methods supported
   by the response sender.  This header applies to the general
   capabilities of the sender, and its only purpose is to indicate the
   sender's capabilities to the recipient.  The methods listed may or
   may not be applicable to the Request-URI; the Allow header field
   (Section 18.6) MAY be used to indicate methods allowed for a
   particular URI.

   Example of use:


   In the event that there are proxies between the sender and the
   recipient of a response, each intervening proxy MUST modify the
   Public header field to remove any methods that are not supported via
   that proxy.  The resulting Public header field will contain an
   intersection of the sender's methods and the methods allowed through
   by the intervening proxies.

      In general, proxies should allow all methods to transparently pass
      through from the sending RTSP agent to the receiving RTSP agent,
      but there may be cases where this is not desirable for a given
      proxy.  Modification of the Public response-header field by the

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      intervening proxies ensures that the request sender gets an
      accurate response indicating the methods that can be used on the
      target agent via the proxy chain.

18.40.  Range

   The Range general-header specifies a time range in PLAY
   (Section 13.4), PAUSE (Section 13.6), SETUP (Section 13.3), and
   PLAY_NOTIFY (Section 13.5) requests and responses.  It MAY be
   included in GET_PARAMETER requests from the client to the server with
   only a Range format and no value to request the current media
   position, whether the session is in Play or Ready state in the
   included format.  The server SHALL, if supporting the range format,
   respond with the current playing point or pause point as the start of
   the range.  If an explicit stop point was used in the previous PLAY
   request, then that value shall be included as stop point.  Note that
   if the server is currently under any type of media playback
   manipulation affecting the interpretation of the Range header, like
   scale value other than 1, that fact is also required to be included
   in any GET_PARAMETER response by including the Scale header to
   provide complete information.

   The range can be specified in a number of units.  This specification
   defines smpte (Section 4.4.1), npt (Section 4.4.2), and clock
   (Section 4.4.3) range units.  While octet ranges (Byte Ranges) (see
   Section 2.1 of [RFC7233]) and other extended units MAY be used, their
   behavior is unspecified since they are not normally meaningful in
   RTSP.  Servers supporting the Range header MUST understand the NPT
   range format and SHOULD understand the SMPTE range format.  If the
   Range header is sent in a time format that is not understood, the
   recipient SHOULD return 456 (Header Field Not Valid for Resource) and
   include an Accept-Ranges header indicating the supported time formats
   for the given resource.


     Range: clock=19960213T143205Z-

   The Range header contains a range of one single range format.  A
   range is a half-open interval with a start and an end point,
   including the start point but excluding the end point.  A range may
   either be fully specified with explicit values for start point and
   end point or have either the start or end point be implicit.  An
   implicit start point indicates the session's pause point, and if no
   pause point is set, the start of the content.  An implicit end point
   indicates the end of the content.  The usage of both implicit start

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   and end points is not allowed in the same Range header; however, the
   omission of the Range header has that meaning, i.e., from pause point
   (or start) until end of content.

      As noted, Range headers define half-open intervals.  A range of
      A-B starts exactly at time A, but ends just before B.  Only the
      start time of a media unit such as a video or audio frame is
      relevant.  For example, assume that video frames are generated
      every 40 ms.  A range of 10.0-10.1 would include a video frame
      starting at 10.0 or later time and would include a video frame
      starting at 10.08, even though it lasted beyond the interval.  A
      range of 10.0-10.08, on the other hand, would exclude the frame at

      Please note the difference between NPT timescales' "now" and an
      implicit start value.  Implicit values reference the current
      pause-point, while "now" is the current time.  In a time-
      progressing session with recording (retention for some or full
      time), the pause point may be 2 min into the session while now
      could be 1 hour into the session.

   By default, range intervals increase, where the second point is
   larger than the first point.


       Range: npt=10-15

   However, range intervals can also decrease if the Scale header (see
   Section 18.46) indicates a negative scale value.  For example, this
   would be the case when a playback in reverse is desired.


       Scale: -1
       Range: npt=15-10

   Decreasing ranges are still half-open intervals as described above.
   Thus, for range A-B, A is closed and B is open.  In the above
   example, 15 is closed and 10 is open.  An exception to this rule is
   the case when B=0 is in a decreasing range.  In this case, the range
   is closed on both ends, as otherwise there would be no way to reach 0
   on a reverse playback for formats that have such a notion, like NPT
   and SMPTE.

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       Scale: -1
       Range: npt=15-0

   In this range, both 15 and 0 are closed.

   A decreasing range interval without a corresponding negative value in
   the Scale header is not valid.

18.41.  Referrer

   The Referrer request-header field allows the client to specify, for
   the server's benefit, the address (URI) of the resource from which
   the Request-URI was obtained.  The URI refers to that of the
   presentation description, typically retrieved via HTTP.  The Referrer
   request-header allows a server to generate lists of back-links to
   resources for interest, logging, optimized caching, etc.  It also
   allows obsolete or mistyped links to be traced for maintenance.  The
   Referrer field MUST NOT be sent if the Request-URI was obtained from
   a source that does not have its own URI, such as input from the user

   If the field-value is a relative URI, it SHOULD be interpreted
   relative to the Request-URI.  The URI MUST NOT include a fragment

   Because the source of a link might be private information or might
   reveal an otherwise private information source, it is strongly
   recommended that the user be able to select whether or not the
   Referrer field is sent.  For example, a streaming client could have a
   toggle switch for openly/anonymously, which would respectively
   enable/disable the sending of Referrer and From information.

   Clients SHOULD NOT include a Referrer header field in an (non-secure)
   RTSP request if the referring page was transferred with a secure

18.42.  Request-Status

   This request-header is used to indicate the end result for requests
   that take time to complete, such as PLAY (Section 13.4).  It is sent
   in PLAY_NOTIFY (Section 13.5) with the end-of-stream reason to report
   how the PLAY request concluded, either in success or in failure.  The
   header carries a reference to the request it reports on using the
   CSeq number and the Session ID used in the request reported on.  This
   is not ensured to be unambiguous due to the fact that the CSeq number
   is scoped by the transport connection.  Agents originating requests

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   can reduce the issue by using a monotonically increasing counter
   across all sequential transports used.  The header provides both a
   numerical status code (according to Section 8.1.1) and a human-
   readable reason phrase.

   Request-Status: cseq=63 status=500 reason="Media data unavailable"

   Proxies that renumber the CSeq header need to perform corresponding
   remapping of the cseq parameter in this header when forwarding the
   request to the next-hop agent.

18.43.  Require

   The Require request-header field is used by agents to ensure that the
   other endpoint supports features that are required in respect to this
   request.  It can also be used to query if the other endpoint supports
   certain features; however, the use of the Supported general-header
   (Section 18.51) is much more effective in this purpose.  In case any
   of the feature tags listed by the Require header are not supported by
   the server or client receiving the request, it MUST respond to the
   request using the error code 551 (Option Not Supported) and include
   the Unsupported header listing those feature tags that are NOT
   supported.  This header does not apply to proxies; for the same
   functionality with respect to proxies, see the Proxy-Require header
   (Section 18.37) with the exception of media-modifying proxies.
   Media-modifying proxies, due to their nature of handling media in a
   way that is very similar to a server, do need to understand also the
   server's features to correctly serve the client.

      This is to make sure that the client-server interaction will
      proceed without delay when all features are understood by both
      sides and only slow down if features are not understood (as in the
      example below).  For a well-matched client-server pair, the
      interaction proceeds quickly, saving a round trip often required
      by negotiation mechanisms.  In addition, it also removes state
      ambiguity when the client requires features that the server does
      not understand.

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   Example (Not complete):

   C->S:   SETUP rtsp:// RTSP/2.0
           CSeq: 302
           Require: funky-feature
           Funky-Parameter: funkystuff

   S->C:   RTSP/2.0 551 Option not supported
           CSeq: 302
           Unsupported: funky-feature

   In this example, "funky-feature" is the feature tag that indicates to
   the client that the fictional Funky-Parameter field is required.  The
   relationship between "funky-feature" and Funky-Parameter is not
   communicated via the RTSP exchange, since that relationship is an
   immutable property of "funky-feature" and thus should not be
   transmitted with every exchange.

   Proxies and other intermediary devices MUST ignore this header.  If a
   particular extension requires that intermediate devices support it,
   the extension should be tagged in the Proxy-Require field instead
   (see Section 18.37).  See discussion in the proxies section
   (Section 15.1) about when to consider that a feature requires proxy

18.44.  Retry-After

   The Retry-After response-header field can be used with a 503 (Service
   Unavailable) or 553 (Proxy Unavailable) response to indicate how long
   the service is expected to be unavailable to the requesting client.
   This field MAY also be used with any 3rr (Redirection) response to
   indicate the minimum time the user agent is asked to wait before
   issuing the redirected request.  A response using 413 (Request
   Message Body Too Large) when the restriction is temporary MAY also
   include the Retry-After header.  The value of this field can be
   either an RTSP-date or an integer number of seconds (in decimal)
   after the time of the response.


   Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
   Retry-After: 120

   In the latter example, the delay is 2 minutes.

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18.45.  RTP-Info

   The RTP-Info general-header field is used to set RTP-specific
   parameters in the PLAY and GET_PARAMETER responses or PLAY_NOTIFY and
   GET_PARAMETER requests.  For streams using RTP as transport protocol,
   the RTP-Info header SHOULD be part of a 200 response to PLAY.

      The exclusion of the RTP-Info in a PLAY response for RTP-
      transported media will result in a client needing to synchronize
      the media streams using RTCP.  This may have negative impact as
      the RTCP can be lost and does not need to be particularly timely
      in its arrival.  Also, functionality that informs the client from
      which packet a seek has occurred is affected.

   The RTP-Info MAY be included in SETUP responses to provide
   synchronization information when changing transport parameters, see
   Section 13.3.  The RTP-Info header and the Range header MAY be
   included in a GET_PARAMETER request from client to server without any
   values to request the current playback point and corresponding RTP
   synchronization information.  When the RTP-Info header is included in
   a Request, the Range header MUST also be included.  The server
   response SHALL include both the Range header and the RTP-Info header.
   If the session is in Play state, then the value of the Range header
   SHALL be filled in with the current playback point and with the
   corresponding RTP-Info values.  If the server is in another state, no
   values are included in the RTP-Info header.  The header is included
   in PLAY_NOTIFY requests with the Notify-Reason of the end of stream
   to provide RTP information about the end of the stream.

   The header can carry the following parameters:

   url:  Indicates the stream URI for which the following RTP parameters
         correspond; this URI MUST be the same as used in the SETUP
         request for this media stream.  Any relative URI MUST use the
         Request-URI as base URI.  This parameter MUST be present.

   ssrc: The SSRC to which the RTP timestamp and sequence number
         provided applies.  This parameter MUST be present.

   seq:  Indicates the sequence number of the first packet of the stream
         that is direct result of the request.  This allows clients to
         gracefully deal with packets when seeking.  The client uses
         this value to differentiate packets that originated before the
         seek from packets that originated after the seek.  Note that a
         client may not receive the packet with the expressed sequence
         number and instead may receive packets with a higher sequence
         number due to packet loss or reordering.  This parameter is
         RECOMMENDED to be present.

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   rtptime:  MUST indicate the RTP timestamp value corresponding to the
         start time value in the Range response-header or, if not
         explicitly given, the implied start point.  The client uses
         this value to calculate the mapping of RTP time to NPT or other
         media timescale.  This parameter SHOULD be present to ensure
         inter-media synchronization is achieved.  There exists no
         requirement that any received RTP packet will have the same RTP
         timestamp value as the one in the parameter used to establish

      A mapping from RTP timestamps to NTP format timestamps (wallclock)
      is available via RTCP.  However, this information is not
      sufficient to generate a mapping from RTP timestamps to media
      clock time (NPT, etc.).  Furthermore, in order to ensure that this
      information is available at the necessary time (immediately at
      startup or after a seek), and that it is delivered reliably, this
      mapping is placed in the RTSP control channel.

      In order to compensate for drift for long, uninterrupted
      presentations, RTSP clients should additionally map NPT to NTP,
      using initial RTCP sender reports to do the mapping, and later
      reports to check drift against the mapping.


   RTP-Info:url="rtsp://" ssrc=0A13C760:seq=45102;

   Lets assume that Audio uses a 16 kHz RTP timestamp clock and Video
   a 90 kHz RTP timestamp clock.  Then, the media synchronization is
   depicted in the following way.

   NPT    3.0---3.1---3.2-X-3.3---3.4---3.5---3.6
   Audio               PA A
   Video                  V    PV

   X: NPT time value = 3.25, from Range header.
   A: RTP timestamp value for Audio from RTP-Info header (12345678).
   V: RTP timestamp value for Video from RTP-Info header (29567112).
   PA: RTP audio packet carrying an RTP timestamp of 12344878, which
       corresponds to NPT = (12344878 - A) / 16000 + 3.25 = 3.2
   PV: RTP video packet carrying an RTP timestamp of 29573412, which
       corresponds to NPT = (29573412 - V) / 90000 + 3.25 = 3.32

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18.46.  Scale

   The Scale general-header indicates the requested or used view rate
   for the media resource being played back.  A scale value of 1
   indicates normal play at the normal forward viewing rate.  If not 1,
   the value corresponds to the rate with respect to normal viewing
   rate.  For example, a value of 2 indicates twice the normal viewing
   rate ("fast forward") and a value of 0.5 indicates half the normal
   viewing rate.  In other words, a value of 2 has content time increase
   at twice the playback time.  For every second of elapsed (wallclock)
   time, 2 seconds of content time will be delivered.  A negative value
   indicates reverse direction.  For certain media transports, this may
   require certain considerations to work consistently; see Appendix C.1
   for description on how RTP handles this.

   The transmitted-data rate SHOULD NOT be changed by selection of a
   different scale value.  The resulting bitrate should be reasonably
   close to the nominal bitrate of the content for scale = 1.  The
   server has to actively manipulate the data when needed to meet the
   bitrate constraints.  Implementation of scale changes depends on the
   server and media type.  For video, a server may, for example, deliver
   only key frames or selected frames.  For audio, it may time-scale the
   audio while preserving pitch or, less desirably, deliver fragments of
   audio, or completely mute the audio.

   The server and content may restrict the range of scale values that it
   supports.  The supported values are indicated by the Media-Properties
   header (Section 18.29).  The client SHOULD only indicate request
   values to be supported.  However, as the values may change as the
   content progresses, a requested value may no longer be valid when the
   request arrives.  Thus, a non-supported value in a request does not
   generate an error, it only forces the server to choose the closest
   value.  The response MUST always contain the actual scale value
   chosen by the server.

   If the server does not implement the possibility to scale, it will
   not return a Scale header.  A server supporting scale operations for
   PLAY MUST indicate this with the use of the "play.scale" feature tag.

   When indicating a negative scale for a reverse playback, the Range
   header MUST indicate a decreasing range as described in
   Section 18.40.

   Example of playing in reverse at 3.5 times normal rate:

     Scale: -3.5
     Range: npt=15-10

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18.47.  Seek-Style

   When a client sends a PLAY request with a Range header to perform a
   random access to the media, the client does not know if the server
   will pick the first media samples or the first random access point
   prior to the request range.  Depending on the use case, the client
   may have a strong preference.  To express this preference and provide
   the client with information on how the server actually acted on that
   preference, the Seek-Style general-header is defined.

   Seek-Style is a general-header that MAY be included in any PLAY
   request to indicate the client's preference for any media stream that
   has the random access properties.  The server MUST always include the
   header in any PLAY response for media with random access properties
   to indicate what policy was applied.  A server that receives an
   unknown Seek-Style policy MUST ignore it and select the server
   default policy.  A client receiving an unknown policy MUST ignore it
   and use the Range header and any media synchronization information as
   basis to determine what the server did.

   This specification defines the following seek policies that may be
   requested (see also Section 4.7.1):

   RAP:  Random Access Point (RAP) is the behavior of requesting the
      server to locate the closest previous random access point that
      exists in the media aggregate and deliver from that.  By
      requesting a RAP, media quality will be the best possible as all
      media will be delivered from a point where full media state can be
      established in the media decoder.

   CoRAP:  Conditional Random Access Point (CoRAP) is a variant of the
      above RAP behavior.  This policy is primarily intended for cases
      where there is larger distance between the random access points in
      the media.  CoRAP uses the RAP policy if the condition that there
      is a Random Access Point closer to the requested start point than
      to the current pause point is fulfilled.  Otherwise, no seeking is
      performed and playback will continue from the current pause point.
      This policy assumes that the media state existing prior to the
      pause is usable if delivery is continued.  If the client or server
      knows that this is not the fact, the RAP policy should be used.
      In other words, in most cases when the client requests a start
      point prior to the current pause point, a valid decoding
      dependency chain from the media delivered prior to the pause and
      to the requested media unit will not exist.  If the server
      searched to a random access point, the server MUST return the
      CoRAP policy in the Seek-Style header and adjust the Range header
      to reflect the position of the selected RAP.  In case the random
      access point is farther away and the server chooses to continue

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      from the current pause point, it MUST include the "Next" policy in
      the Seek-Style header and adjust the Range header start point to
      the current pause point.

   First-Prior:  The first-prior policy will start delivery with the
      media unit that has a playout time first prior to the requested
      time.  For discrete media, that would only include media units
      that would still be rendered at the request time.  For continuous
      media, that is media that will be rendered during the requested
      start time of the range.

   Next:  The next media units after the provided start time of the
      range: for continuous framed media, that would mean the first next
      frame after the provided time and for discrete media, the first
      unit that is to be rendered after the provided time.  The main
      usage for this case is when the client knows it has all media up
      to a certain point and would like to continue delivery so that a
      complete uninterrupted media playback can be achieved.  An example
      of such a scenario would be switching from a broadcast/multicast
      delivery to a unicast-based delivery.  This policy MUST only be
      used on the client's explicit request.

   Please note that these expressed preferences exist for optimizing the
   startup time or the media quality.  The "Next" policy breaks the
   normal definition of the Range header to enable a client to request
   media with minimal overlap, although some may still occur for
   aggregated sessions.  RAP and First-Prior both fulfill the
   requirement of providing media from the requested range and forward.
   However, unless RAP is used, the media quality for many media codecs
   using predictive methods can be severely degraded unless additional
   data is available as, for example, already buffered, or through other
   side channels.

18.48.  Server

   The Server general-header field contains information about the
   software used by the origin server to create or handle the request.
   This field can contain multiple product tokens and comments
   identifying the server and any significant subproducts.  The product
   tokens are listed in order of their significance for identifying the


   Server: PhonyServer/1.0

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   If the response is being forwarded through a proxy, the proxy
   application MUST NOT modify the Server response-header.  Instead, it
   SHOULD include a Via field (Section 18.57).  If the response is
   generated by the proxy, the proxy application MUST return the Server
   response-header as previously returned by the server.

18.49.  Session

   The Session general-header field identifies an RTSP session.  An RTSP
   session is created by the server as a result of a successful SETUP
   request, and in the response, the session identifier is given to the
   client.  The RTSP session exists until destroyed by a TEARDOWN or a
   REDIRECT or is timed out by the server.

   The session identifier is chosen by the server (see Section 4.3) and
   MUST be returned in the SETUP response.  Once a client receives a
   session identifier, it MUST be included in any request related to
   that session.  This means that the Session header MUST be included in
   a request, using the following methods: PLAY, PAUSE, PLAY_NOTIFY and
   The Session header MUST NOT be included in the following methods, if
   these requests are pipelined and if the session identifier is not yet

   In an RTSP response, the session header MUST be included in methods,
   SETUP, PLAY, PAUSE, and PLAY_NOTIFY, and it MAY be included in
   methods TEARDOWN and REDIRECT.  If included in the request of the
   following methods it MUST also be included in the response: OPTIONS,
   GET_PARAMETER, and SET_PARAMETER.  It MUST NOT be included in
   DESCRIBE responses.

   Note that a session identifier identifies an RTSP session across
   transport sessions or connections.  RTSP requests for a given session
   can use different URIs (Presentation and media URIs).  Note, that
   there are restrictions depending on the session as to which URIs are
   acceptable for a given method.  However, multiple "user" sessions for
   the same URI from the same client will require use of different
   session identifiers.

      The session identifier is needed to distinguish several delivery
      requests for the same URI coming from the same client.

   The response 454 (Session Not Found) MUST be returned if the session
   identifier is invalid.

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   The header MAY include a parameter for session timeout period.  If
   not explicitly provided, this value is set to 60 seconds.  As this
   affects how often session keep-alives are needed, values smaller than
   30 seconds are not recommended.  However, larger-than-default values
   can be useful in applications of RTSP that have inactive but
   established sessions for longer time periods.

      The 60-second value was chosen as the session timeout value as it
      results in keep-alive messages that are not too frequent and low
      sensitivity to variations in request/response timing.  If one
      reduces the timeout value to below 30 seconds, the corresponding
      request/response timeout becomes a significant part of the session
      timeout.  The 60-second value also allows for reasonably rapid
      recovery of committed server resources in case of client failure.

18.50.  Speed

   The Speed general-header field requests the server to deliver
   specific amounts of nominal media time per unit of delivery time,
   contingent on the server's ability and desire to serve the media
   stream at the given speed.  The client requests the delivery speed to
   be within a given range with a lower and upper bound.  The server
   SHALL deliver at the highest possible speed within the range, but not
   faster than the upper bound, for which the underlying network path
   can support the resulting transport data rates.  As long as any speed
   value within the given range can be provided, the server SHALL NOT
   modify the media quality.  Only if the server is unable to deliver
   media at the speed value provided by the lower bound shall it reduce
   the media quality.

   Implementation of the Speed functionality by the server is OPTIONAL.
   The server can indicate its support through a feature tag,
   play.speed.  The lack of a Speed header in the response is an
   indication of lack of support of this functionality.

   The speed parameter values are expressed as a positive decimal value,
   e.g., a value of 2.0 indicates that data is to be delivered twice as
   fast as normal.  A speed value of zero is invalid.  The range is
   specified in the form "lower bound - upper bound".  The lower-bound
   value may be smaller or equal to the upper bound.  All speeds may not
   be possible to support.  Therefore, the server MAY modify the
   requested values to the closest supported.  The actual supported
   speed MUST be included in the response.  However, note that the use
   cases may vary and that Speed value ranges such as 0.7-0.8, 0.3-2.0,
   1.0-2.5, and 2.5-2.5 all have their usages.

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     Speed: 1.0-2.5

   Use of this header changes the bandwidth used for data delivery.  It
   is meant for use in specific circumstances where delivery of the
   presentation at a higher or lower rate is desired.  The main use
   cases are buffer operations or local scale operations.  Implementers
   should keep in mind that bandwidth for the session may be negotiated
   beforehand (by means other than RTSP) and, therefore, renegotiation
   may be necessary.  To perform Speed operations, the server needs to
   ensure that the network path can support the resulting bitrate.
   Thus, the media transport needs to support feedback so that the
   server can react and adapt to the available bitrate.

18.51.  Supported

   The Supported general-header enumerates all the extensions supported
   by the client or server using feature tags.  The header carries the
   extensions supported by the message-sending client or server.  The
   Supported header MAY be included in any request.  When present in a
   request, the receiver MUST respond with its corresponding Supported
   header.  Note that the Supported header is also included in 4xx and
   5xx responses.

   The Supported header contains a list of feature tags, described in
   Section 4.5, that are understood by the client or server.  These
   feature tags are the ones the server or client supports in general
   and are not specific to the request resource.


     C->S:  OPTIONS rtsp:// RTSP/2.0
            Supported: foo, bar, blech
            User-Agent: PhonyClient/1.2

     S->C:  RTSP/2.0 200 OK
            Supported: bar, blech, baz

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18.52.  Terminate-Reason

   The Terminate-Reason request-header allows the server, when sending a
   REDIRECT or TEARDOWN request, to provide a reason for the session
   termination and any additional information.  This specification
   identifies three reasons for Redirections and may be extended in the

   Server-Admin:  The server needs to be shut down for some
      administrative reason.

   Session-Timeout:  A client's session has been kept alive for extended
      periods of time and the server has determined that it needs to
      reclaim the resources associated with this session.

   Internal-Error  An internal error that is impossible to recover from
      has occurred, forcing the server to terminate the session.

   The Server may provide additional parameters containing information
   around the redirect.  This specification defines the following ones.

   time:  Provides a wallclock time when the server will stop providing
      any service.

   user-msg:  A UTF-8 text string with a message from the server to the
      user.  This message SHOULD be displayed to the user.

18.53.  Timestamp

   The Timestamp general-header describes when the agent sent the
   request.  The value of the timestamp is of significance only to the
   agent and may use any timescale.  The responding agent MUST echo the
   exact same value and MAY, if it has accurate information about this,
   add a floating-point number indicating the number of seconds that has
   elapsed since it has received the request.  The timestamp can be used
   by the agent to compute the round-trip time to the responding agent
   so that it can adjust the timeout value for retransmissions when
   running over an unreliable protocol.  It also resolves retransmission
   ambiguities for unreliable transport of RTSP.

   Note that the present specification provides only for reliable
   transport of RTSP messages.  The Timestamp general-header is
   specified in case the protocol is extended in the future to use
   unreliable transport.

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18.54.  Transport

   The Transport general-header indicates which transport protocol is to
   be used and configures its parameters such as destination address,
   compression, multicast time-to-live and destination port for a single
   stream.  It sets those values not already determined by a
   presentation description.

   A Transport request-header MAY contain a list of transport options
   acceptable to the client, in the form of multiple transport
   specification entries.  Transport specifications are comma separated
   and listed in decreasing order of preference.  Each transport
   specification consists of a transport protocol identifier, followed
   by any number of parameters separated by semicolons.  A Transport
   request-header MAY contain multiple transport specifications using
   the same transport protocol identifier.  The server MUST return a
   Transport response-header in the response to indicate the values
   actually chosen, if any.  If no transport specification is supported,
   no transport header is returned and the response MUST use the status
   code 461 (Unsupported Transport) (Section 17.4.25).  In case more
   than one transport specification was present in the request, the
   server MUST return the single transport specification (transport-
   spec) that was actually chosen, if any.  The number of transport-spec
   entries is expected to be limited as the client will receive guidance
   on what configurations are possible from the presentation

   The Transport header MAY also be used in subsequent SETUP requests to
   change transport parameters.  A server MAY refuse to change
   parameters of an existing stream.

   The transport protocol identifier defines, for each transport
   specification, which transport protocol to use and any related rules.
   Each transport protocol identifier defines the parameters that are
   required to occur; additional optional parameters MAY occur.  This
   flexibility is provided as parameters may be different and provide
   different options to the RTSP agent.  A transport specification may
   only contain one of any given parameter within it.  A parameter
   consists of a name and optionally a value string.  Parameters MAY be
   given in any order.  Additionally, a transport specification may only
   contain either the unicast or the multicast transport type parameter.
   The transport protocol identifier, and all parameters, need to be
   understood in a transport specification; if not, the transport
   specification MUST be ignored.  An RTSP proxy of any type that uses
   or modifies the transport specification, e.g., access proxy or
   security proxy, MUST remove specifications with unknown parameters

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   before forwarding the RTSP message.  If that results in no remaining
   transport specification, the proxy SHALL send a 461 (Unsupported
   Transport) (Section 17.4.25) response without any Transport header.

      The Transport header is restricted to describing a single media
      stream.  (RTSP can also control multiple streams as a single
      entity.)  Making it part of RTSP rather than relying on a
      multitude of session description formats greatly simplifies
      designs of firewalls.

   The general syntax for the transport protocol identifier is a list of
   slash-separated tokens:


   Which, for RTP transports, takes the form:


   The default value for the "lower-transport" parameters is specific to
   the profile.  For RTP/AVP, the default is UDP.

   There are two different methods for how to specify where the media
   should be delivered for unicast transport:

   dest_addr:  The presence of this parameter and its values indicates
         the destination address or addresses (host address and port
         pairs for IP flows) necessary for the media transport.

   No dest_addr:  The lack of the dest_addr parameter indicates that the
         server MUST send media to the same address from which the RTSP
         messages originates.

   The choice of method for indicating where the media is to be
   delivered depends on the use case.  In some cases, the only allowed
   method will be to use no explicit address indication and have the
   server deliver media to the source of the RTSP messages.

   For multicast, there are several methods for specifying addresses,
   but they are different in how they work compared with unicast:

   dest_addr with client picked address:  The address and relevant
         parameters, like TTL (scope), for the actual multicast group to
         deliver the media to.  There are security implications
         (Section 21) with this method that need to be addressed because
         an RTSP server can be used as a DoS attacker on an existing
         multicast group.

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   dest_addr using Session Description Information:  The information
         included in the transport header can all be coming from the
         session description, e.g., the SDP "c=" and "m=" lines.  This
         mitigates some of the security issues of the previous methods
         as it is the session provider that picks the multicast group
         and scope.  The client MUST include the information if it is
         available in the session description.

   No dest_addr:  The behavior when no explicit multicast group is
         present in a request is not defined.

   An RTSP proxy will need to take care.  If the media is not desired to
   be routed through the proxy, the proxy will need to introduce the
   destination indication.

   Below are the configuration parameters associated with transport:

   General parameters:

   unicast / multicast:  This parameter is a mutually exclusive
         indication of whether unicast or multicast delivery will be
         attempted.  One of the two values MUST be specified.  Clients
         that are capable of handling both unicast and multicast
         transmission need to indicate such capability by including two
         full transport-specs with separate parameters for each.

   layers:  The number of multicast layers to be used for this media
         stream.  The layers are sent to consecutive addresses starting
         at the dest_addr address.  If the parameter is not included, it
         defaults to a single layer.

   dest_addr:  A general destination address parameter that can contain
         one or more address specifications.  Each combination of
         protocol/profile/lower transport needs to have the format and
         interpretation of its address specification defined.  For
         RTP/AVP/UDP and RTP/AVP/TCP, the address specification is a
         tuple containing a host address and port.  Note, only a single
         destination parameter per transport spec is intended.  The
         usage of multiple destinations to distribute a single media to
         multiple entities is unspecified.

         The client originating the RTSP request MAY specify the
         destination address of the stream recipient with the host
         address as part of the tuple.  When the destination address is
         specified, the recipient may be a different party than the
         originator of the request.  To avoid becoming the unwitting
         perpetrator of a remote-controlled DoS attack, a server MUST
         perform security checks (see Section 21.2.1) and SHOULD log

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         such attempts before allowing the client to direct a media
         stream to a recipient address not chosen by the server.
         Implementations cannot rely on TCP as a reliable means of
         client identification.  If the server does not allow the host
         address part of the tuple to be set, it MUST return 463
         (Destination Prohibited).

         The host address part of the tuple MAY be empty, for example
         ":58044", in cases when it is desired to specify only the
         destination port.  Responses to requests including the
         Transport header with a dest_addr parameter SHOULD include the
         full destination address that is actually used by the server.
         The server MUST NOT remove address information that is already
         present in the request when responding, unless the protocol
         requires it.

   src_addr:  A general source address parameter that can contain one or
         more address specifications.  Each combination of
         protocol/profile/lower transport needs to have the format and
         interpretation of its address specification defined.  For
         RTP/AVP/UDP and RTP/AVP/TCP, the address specification is a
         tuple containing a host address and port.

         This parameter MUST be specified by the server if it transmits
         media packets from an address other than the one RTSP messages
         are sent to.  This will allow the client to verify the source
         address and give it a destination address for its RTCP feedback
         packets, if RTP is used.  The address or addresses indicated in
         the src_addr parameter SHOULD be used both for the sending and
         receiving of the media stream's data packets.  The main reasons
         are threefold: First, indicating the port and source address(s)
         lets the receiver know where from the packets is expected to
         originate.  Second, traversal of NATs is greatly simplified
         when traffic is flowing symmetrically over a NAT binding.
         Third, certain NAT traversal mechanisms need to know to which
         address and port to send so-called "binding packets" from the
         receiver to the sender, thus creating an address binding in the
         NAT that the sender-to-receiver packet flow can use.

            This information may also be available through SDP.
            However, since this is more a feature of transport than
            media initialization, the authoritative source for this
            information should be in the SETUP response.

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   mode: The mode parameter indicates the methods to be supported for
         this session.  The currently defined valid value is "PLAY".  If
         not provided, the default is "PLAY".  The "RECORD" value was
         defined in RFC 2326; in this specification, it is unspecified
         but reserved.  RECORD and other values may be specified in the

   interleaved:  The interleaved parameter implies mixing the media
         stream with the control stream in whatever protocol is being
         used by the control stream, using the mechanism defined in
         Section 14.  The argument provides the channel number to be
         used in the $ block (see Section 14) and MUST be present.  This
         parameter MAY be specified as an interval, e.g.,
         interleaved=4-5 in cases where the transport choice for the
         media stream requires it, e.g., for RTP with RTCP.  The channel
         number given in the request is only a guidance from the client
         to the server on what channel number(s) to use.  The server MAY
         set any valid channel number in the response.  The declared
         channels are bidirectional, so both end parties MAY send data
         on the given channel.  One example of such usage is the second
         channel used for RTCP, where both server and client send RTCP
         packets on the same channel.

            This allows RTP/RTCP to be handled similarly to the way that
            it is done with UDP, i.e., one channel for RTP and the other
            for RTCP.

   MIKEY:  This parameter is used in conjunction with transport
         specifications that can utilize MIKEY [RFC3830] for security
         context establishment.  So far, only the SRTP-based RTP
         profiles SAVP and SAVPF can utilize MIKEY, and this is defined
         in Appendix C.1.4.1.  This parameter can be included both in
         request and response messages.  The binary MIKEY message SHALL
         be Base64-encoded [RFC4648] before being included in the value
         part of the parameter, where the encoding adheres to the
         definition in Section 4 of RFC 4648 and where the padding bits
         are set to zero.


   ttl:  multicast time-to-live for IPv4.  When included in requests,
         the value indicates the TTL value that the client requests the
         server to use.  In a response, the value actually being used by
         the server is returned.  A server will need to consider what
         values that are reasonable and also the authority of the user
         to set this value.  Corresponding functions are not needed for
         IPv6 as the scoping is part of the IPv6 multicast address

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   These parameters MAY only be used if the media-transport protocol is

   ssrc: The ssrc parameter, if included in a SETUP response, indicates
         the RTP SSRC [RFC3550] value(s) that will be used by the media
         server for RTP packets within the stream.  The values are
         expressed as a slash-separated sequence of SSRC values, each
         SSRC expressed as an eight-digit hexadecimal value.

         The ssrc parameter MUST NOT be specified in requests.  The
         functionality of specifying the ssrc parameter in a SETUP
         request is deprecated as it is incompatible with the
         specification of RTP [RFC3550].  If the parameter is included
         in the Transport header of a SETUP request, the server SHOULD
         ignore it, and choose appropriate SSRCs for the stream.  The
         server SHOULD set the ssrc parameter in the Transport header of
         the response.

   RTCP-mux:  Used to negotiate the usage of RTP and RTCP multiplexing
         [RFC5761] on a single underlying transport stream/flow.  The
         presence of this parameter in a SETUP request indicates the
         client's support and requires the server to use RTP and RTCP
         multiplexing.  The client SHALL only include one transport
         stream in the Transport header specification.  To provide the
         server with a choice between using RTP/RTCP multiplexing or
         not, two different transport header specifications must be

   The parameter setup and connection defined below MAY only be used if
   the media-transport protocol of the lower-level transport is
   connection oriented (such as TCP).  However, these parameters MUST
   NOT be used when interleaving data over the RTSP connection.

   setup:  Clients use the setup parameter on the Transport line in a
         SETUP request to indicate the roles it wishes to play in a TCP
         connection.  This parameter is adapted from [RFC4145].  The use
         of this parameter in RTP/AVP/TCP non-interleaved transport is
         discussed in Appendix C.2.2; the discussion below is limited to
         syntactic issues.  Clients may specify the following values for
         the setup parameter:

         active:  The client will initiate an outgoing connection.

         passive:  The client will accept an incoming connection.

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         actpass:  The client is willing to accept an incoming
            connection or to initiate an outgoing connection.

         If a client does not specify a setup value, the "active" value
         is assumed.

         In response to a client SETUP request where the setup parameter
         is set to "active", a server's 2xx reply MUST assign the setup
         parameter to "passive" on the Transport header line.

         In response to a client SETUP request where the setup parameter
         is set to "passive", a server's 2xx reply MUST assign the setup
         parameter to "active" on the Transport header line.

         In response to a client SETUP request where the setup parameter
         is set to "actpass", a server's 2xx reply MUST assign the setup
         parameter to "active" or "passive" on the Transport header

         Note that the "holdconn" value for setup is not defined for
         RTSP use, and MUST NOT appear on a Transport line.

   connection:  Clients use the connection parameter in a transport
         specification part of the Transport header in a SETUP request
         to indicate the client's preference for either reusing an
         existing connection between client and server (in which case
         the client sets the "connection" parameter to "existing") or
         requesting the creation of a new connection between client and
         server (in which cast the client sets the "connection"
         parameter to "new").  Typically, clients use the "new" value
         for the first SETUP request for a URL, and "existing" for
         subsequent SETUP requests for a URL.

         If a client SETUP request assigns the "new" value to
         "connection", the server response MUST also assign the "new"
         value to "connection" on the Transport line.

         If a client SETUP request assigns the "existing" value to
         "connection", the server response MUST assign a value of
         "existing" or "new" to "connection" on the Transport line, at
         its discretion.

         The default value of "connection" is "existing", for all SETUP
         requests (initial and subsequent).

   The combination of transport protocol, profile and lower transport
   needs to be defined.  A number of combinations are defined in the
   Appendix C.

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   Below is a usage example, showing a client advertising the capability
   to handle multicast or unicast, preferring multicast.  Since this is
   a unicast-only stream, the server responds with the proper transport
   parameters for unicast.

     C->S: SETUP rtsp:// RTSP/2.0
           CSeq: 302
           Transport: RTP/AVP;multicast;mode="PLAY",
           Accept-Ranges: npt, smpte, clock
           User-Agent: PhonyClient/1.2

     S->C: RTSP/2.0 200 OK
           CSeq: 302
           Date: Fri, 20 Dec 2013 10:20:32 +0000
           Session: rQi1hBrGlFdiYld241FxUO
           Transport: RTP/AVP;unicast;dest_addr=""/
           Accept-Ranges: npt
           Media-Properties: Random-Access=0.6, Dynamic,

18.55.  Unsupported

   The Unsupported response-header lists the features not supported by
   the responding RTSP agent.  In the case where the feature was
   specified via the Proxy-Require field (Section 18.37), if there is a
   proxy on the path between the client and the server, the proxy MUST
   send a response message with a status code of 551 (Option Not
   Supported).  The request MUST NOT be forwarded.

   See Section 18.43 for a usage example.

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18.56.  User-Agent

   The User-Agent general-header field contains information about the
   user agent originating the request or producing a response.  This is
   for statistical purposes, the tracing of protocol violations, and
   automated recognition of user agents for the sake of tailoring
   responses to avoid particular user agent limitations.  User agents
   SHOULD include this field with requests.  The field can contain
   multiple product tokens and comments identifying the agent and any
   subproducts which form a significant part of the user agent.  By
   convention, the product tokens are listed in order of their
   significance for identifying the application.


   User-Agent: PhonyClient/1.2

18.57.  Via

   The Via general-header field MUST be used by proxies to indicate the
   intermediate protocols and recipients between the user agent and the
   server on requests and between the origin server and the client on
   responses.  The field is intended to be used for tracking message
   forwards, avoiding request loops, and identifying the protocol
   capabilities of all senders along the request/response chain.

   Each of multiple values in the Via field represents each proxy that
   has forwarded the message.  Each recipient MUST append its
   information such that the end result is ordered according to the
   sequence of forwarding applications.  So messages originating with
   the client or server do not include the Via header.  The first proxy
   or other intermediate adds the header and its information into the
   field.  Any additional intermediate adds additional field-values.
   Resulting in the server seeing the chains of intermediates in a
   client-to-server request and the client seeing the full chain in the
   response message.

   Proxies (e.g., Access Proxy or Translator Proxy) SHOULD NOT, by
   default, forward the names and ports of hosts within the private/
   protected region.  This information SHOULD only be propagated if
   explicitly enabled.  If not enabled, the via-received of any host
   behind the firewall/NAT SHOULD be replaced by an appropriate
   pseudonym for that host.

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   For organizations that have strong privacy requirements for hiding
   internal structures, a proxy MAY combine an ordered subsequence of
   Via header field entries with identical sent-protocol values into a
   single such entry.  Applications MUST NOT combine entries that have
   different received-protocol values.

18.58.  WWW-Authenticate

   The WWW-Authenticate header is specified in [RFC7235]; its usage
   depends on the used authentication schemes, such as Digest [RFC7616]
   and Basic [RFC7617].  The WWW-Authenticate response-header field MUST
   be included in 401 (Unauthorized) response messages.  The field-value
   consists of at least one challenge that indicates the authentication
   scheme(s) and parameters applicable to the Request-URI.  This header
   MUST only be used in response messages related to client to server

   The HTTP access authentication process is described in [RFC7235] with
   some clarification in Section 19.1.  User agents are advised to take
   special care in parsing the WWW-Authenticate field-value as it might
   contain more than one challenge, or if more than one WWW-Authenticate
   header field is provided, the contents of a challenge itself can
   contain a comma-separated list of authentication parameters.

(page 185 continued on part 9)

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