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rfc2326.txt：源码内容

RFC 2326 Real Time Streaming Protocol April 1998
Bandwidth = "Bandwidth" ":" 1*DIGIT
Example:
Bandwidth: 4000
12.7 Blocksize
This request header field is sent from the client to the media server
asking the server for a particular media packet size. This packet
size does not include lower-layer headers such as IP, UDP, or RTP.
The server is free to use a blocksize which is lower than the one
requested. The server MAY truncate this packet size to the closest
multiple of the minimum, media-specific block size, or override it
with the media-specific size if necessary. The block size MUST be a
positive decimal number, measured in octets. The server only returns
an error (416) if the value is syntactically invalid.
12.8 Cache-Control
The Cache-Control general header field is used to specify directives
that MUST be obeyed by all caching mechanisms along the
request/response chain.
Cache directives must be passed through by a proxy or gateway
application, regardless of their significance to that application,
since the directives may be applicable to all recipients along the
request/response chain. It is not possible to specify a cache-
directive for a specific cache.
Cache-Control should only be specified in a SETUP request and its
response. Note: Cache-Control does not govern the caching of
responses as for HTTP, but rather of the stream identified by the
SETUP request. Responses to RTSP requests are not cacheable, except
for responses to DESCRIBE.
Cache-Control = "Cache-Control" ":" 1#cache-directive
cache-directive = cache-request-directive
| cache-response-directive
cache-request-directive = "no-cache"
| "max-stale"
| "min-fresh"
| "only-if-cached"
| cache-extension
cache-response-directive = "public"
| "private"
| "no-cache"
| "no-transform"
| "must-revalidate"
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| "proxy-revalidate"
| "max-age" "=" delta-seconds
| cache-extension
cache-extension = token [ "=" ( token | quoted-string ) ]
no-cache:
Indicates that the media stream MUST NOT be cached anywhere.
This allows an origin server to prevent caching even by caches
that have been configured to return stale responses to client
requests.
public:
Indicates that the media stream is cacheable by any cache.
private:
Indicates that the media stream is intended for a single user
and MUST NOT be cached by a shared cache. A private (non-
shared) cache may cache the media stream.
no-transform:
An intermediate cache (proxy) may find it useful to convert
the media type of a certain stream. A proxy might, for
example, convert between video formats to save cache space or
to reduce the amount of traffic on a slow link. Serious
operational problems may occur, however, when these
transformations have been applied to streams intended for
certain kinds of applications. For example, applications for
medical imaging, scientific data analysis and those using
end-to-end authentication all depend on receiving a stream
that is bit-for-bit identical to the original entity-body.
Therefore, if a response includes the no-transform directive,
an intermediate cache or proxy MUST NOT change the encoding of
the stream. Unlike HTTP, RTSP does not provide for partial
transformation at this point, e.g., allowing translation into
a different language.
only-if-cached:
In some cases, such as times of extremely poor network
connectivity, a client may want a cache to return only those
media streams that it currently has stored, and not to receive
these from the origin server. To do this, the client may
include the only-if-cached directive in a request. If it
receives this directive, a cache SHOULD either respond using a
cached media stream that is consistent with the other
constraints of the request, or respond with a 504 (Gateway
Timeout) status. However, if a group of caches is being
operated as a unified system with good internal connectivity,
such a request MAY be forwarded within that group of caches.
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max-stale:
Indicates that the client is willing to accept a media stream
that has exceeded its expiration time. If max-stale is
assigned a value, then the client is willing to accept a
response that has exceeded its expiration time by no more than
the specified number of seconds. If no value is assigned to
max-stale, then the client is willing to accept a stale
response of any age.
min-fresh:
Indicates that the client is willing to accept a media stream
whose freshness lifetime is no less than its current age plus
the specified time in seconds. That is, the client wants a
response that will still be fresh for at least the specified
number of seconds.
must-revalidate:
When the must-revalidate directive is present in a SETUP
response received by a cache, that cache MUST NOT use the
entry after it becomes stale to respond to a subsequent
request without first revalidating it with the origin server.
That is, the cache must do an end-to-end revalidation every
time, if, based solely on the origin server's Expires, the
cached response is stale.)
12.9 Conference
This request header field establishes a logical connection between a
pre-established conference and an RTSP stream. The conference-id must
not be changed for the same RTSP session.
Conference = "Conference" ":" conference-id Example:
Conference: 199702170042.SAA08642@obiwan.arl.wustl.edu%20Starr
A response code of 452 (452 Conference Not Found) is returned if the
conference-id is not valid.
12.10 Connection
See [H14.10]
12.11 Content-Base
See [H14.11]
12.12 Content-Encoding
See [H14.12]
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12.13 Content-Language
See [H14.13]
12.14 Content-Length
This field contains the length of the content of the method (i.e.
after the double CRLF following the last header). Unlike HTTP, it
MUST be included in all messages that carry content beyond the header
portion of the message. If it is missing, a default value of zero is
assumed. It is interpreted according to [H14.14].
12.15 Content-Location
See [H14.15]
12.16 Content-Type
See [H14.18]. Note that the content types suitable for RTSP are
likely to be restricted in practice to presentation descriptions and
parameter-value types.
12.17 CSeq
The CSeq field specifies the sequence number for an RTSP request-
response pair. This field MUST be present in all requests and
responses. For every RTSP request containing the given sequence
number, there will be a corresponding response having the same
number. Any retransmitted request must contain the same sequence
number as the original (i.e. the sequence number is not incremented
for retransmissions of the same request).
12.18 Date
See [H14.19].
12.19 Expires
The Expires entity-header field gives a date and time after which the
description or media-stream should be considered stale. The
interpretation depends on the method:
DESCRIBE response:
The Expires header indicates a date and time after which the
description should be considered stale.
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A stale cache entry may not normally be returned by a cache (either a
proxy cache or an user agent cache) unless it is first validated with
the origin server (or with an intermediate cache that has a fresh
copy of the entity). See section 13 for further discussion of the
expiration model.
The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that
time.
The format is an absolute date and time as defined by HTTP-date in
[H3.3]; it MUST be in RFC1123-date format:
Expires = "Expires" ":" HTTP-date
An example of its use is
Expires: Thu, 01 Dec 1994 16:00:00 GMT
RTSP/1.0 clients and caches MUST treat other invalid date formats,
especially including the value "0", as having occurred in the past
(i.e., "already expired").
To mark a response as "already expired," an origin server should use
an Expires date that is equal to the Date header value. To mark a
response as "never expires," an origin server should use an Expires
date approximately one year from the time the response is sent.
RTSP/1.0 servers should not send Expires dates more than one year in
the future.
The presence of an Expires header field with a date value of some
time in the future on a media stream that otherwise would by default
be non-cacheable indicates that the media stream is cacheable, unless
indicated otherwise by a Cache-Control header field (Section 12.8).
12.20 From
See [H14.22].
12.21 Host
This HTTP request header field is not needed for RTSP. It should be
silently ignored if sent.
12.22 If-Match
See [H14.25].
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This field is especially useful for ensuring the integrity of the
presentation description, in both the case where it is fetched via
means external to RTSP (such as HTTP), or in the case where the
server implementation is guaranteeing the integrity of the
description between the time of the DESCRIBE message and the SETUP
message.
The identifier is an opaque identifier, and thus is not specific to
any particular session description language.
12.23 If-Modified-Since
The If-Modified-Since request-header field is used with the DESCRIBE
and SETUP methods to make them conditional. If the requested variant
has not been modified since the time specified in this field, a
description will not be returned from the server (DESCRIBE) or a
stream will not be set up (SETUP). Instead, a 304 (not modified)
response will be returned without any message-body.
If-Modified-Since = "If-Modified-Since" ":" HTTP-date
An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
12.24 Last-Modified
The Last-Modified entity-header field indicates the date and time at
which the origin server believes the presentation description or
media stream was last modified. See [H14.29]. For the methods
DESCRIBE or ANNOUNCE, the header field indicates the last
modification date and time of the description, for SETUP that of the
media stream.
12.25 Location
See [H14.30].
12.26 Proxy-Authenticate
See [H14.33].
12.27 Proxy-Require
The Proxy-Require header 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 if not supported. Servers
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RFC 2326 Real Time Streaming Protocol April 1998
should treat this field identically to the Require field.
See Section 12.32 for more details on the mechanics of this message
and a usage example.
12.28 Public
See [H14.35].
12.29 Range
This request and response header field specifies a range of time.
The range can be specified in a number of units. This specification
defines the smpte (Section 3.5), npt (Section 3.6), and clock
(Section 3.7) range units. Within RTSP, byte ranges [H14.36.1] are
not meaningful and MUST NOT be used. The header may also contain a
time parameter in UTC, specifying the time at which the operation is
to be made effective. Servers supporting the Range header MUST
understand the NPT range format and SHOULD understand the SMPTE range
format. The Range response header indicates what range of time is
actually being played or recorded. If the Range header is given in a
time format that is not understood, the recipient should return "501
Not Implemented".
Ranges are half-open intervals, including the lower point, but
excluding the upper point. In other words, a range of a-b starts
exactly at time a, but stops just before b. Only the start time of a
media unit such as a video or audio frame is relevant. As an 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 10.08.
Range = "Range" ":" 1#ranges-specifier
[ ";" "time" "=" utc-time ]
ranges-specifier = npt-range | utc-range | smpte-range
Example:
Range: clock=19960213T143205Z-;time=19970123T143720Z
The notation is similar to that used for the HTTP/1.1 [2] byte-
range header. It allows clients to select an excerpt from the media
object, and to play from a given point to the end as well as from
the current location to a given point. The start of playback can be
scheduled for any time in the future, although a server may refuse
to keep server resources for extended idle periods.
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12.30 Referer
See [H14.37]. The URL refers to that of the presentation description,
typically retrieved via HTTP.
12.31 Retry-After
See [H14.38].
12.32 Require
The Require header is used by clients to query the server about
options that it may or may not support. The server MUST respond to
this header by using the Unsupported header to negatively acknowledge
those options which are NOT supported.
This is to make sure that the client-server interaction will
proceed without delay when all options are understood by both
sides, and only slow down if options are not understood (as in the
case above). 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.
Require = "Require" ":" 1#option-tag
Example:
C->S: SETUP rtsp://server.com/foo/bar/baz.rm RTSP/1.0
CSeq: 302
Require: funky-feature
Funky-Parameter: funkystuff
S->C: RTSP/1.0 551 Option not supported
CSeq: 302
Unsupported: funky-feature
C->S: SETUP rtsp://server.com/foo/bar/baz.rm RTSP/1.0
CSeq: 303
S->C: RTSP/1.0 200 OK
CSeq: 303
In this example, "funky-feature" is the feature tag which 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.
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Proxies and other intermediary devices SHOULD ignore features that
are not understood in this field. If a particular extension requires
that intermediate devices support it, the extension should be tagged
in the Proxy-Require field instead (see Section 12.27).
12.33 RTP-Info
This field is used to set RTP-specific parameters in the PLAY
response.
url:
Indicates the stream URL which for which the following RTP
parameters correspond.
seq:
Indicates the sequence number of the first packet of the
stream. 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.
rtptime:
Indicates the RTP timestamp corresponding to the time value in
the Range response header. (Note: For aggregate control, a
particular stream may not actually generate a packet for the
Range time value returned or implied. Thus, there is no
guarantee that the packet with the sequence number indicated
by seq actually has the timestamp indicated by rtptime.) The
client uses this value to calculate the mapping of RTP time to
NPT.
A mapping from RTP timestamps to NTP timestamps (wall clock) is
available via RTCP. However, this information is not sufficient to
generate a mapping from RTP timestamps to NPT. 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.
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Syntax:
RTP-Info = "RTP-Info" ":" 1#stream-url 1*parameter
stream-url = "url" "=" url
parameter = ";" "seq" "=" 1*DIGIT
| ";" "rtptime" "=" 1*DIGIT
Example:
RTP-Info: url=rtsp://foo.com/bar.avi/streamid=0;seq=45102,
url=rtsp://foo.com/bar.avi/streamid=1;seq=30211
12.34 Scale
A scale value of 1 indicates normal play or record at the normal
forward viewing rate. If not 1, the value corresponds to the rate
with respect to normal viewing rate. For example, a ratio of 2
indicates twice the normal viewing rate ("fast forward") and a ratio
of 0.5 indicates half the normal viewing rate. In other words, a
ratio of 2 has normal play time increase at twice the wallclock rate.
For every second of elapsed (wallclock) time, 2 seconds of content
will be delivered. A negative value indicates reverse direction.
Unless requested otherwise by the Speed parameter, the data rate
SHOULD not be changed. Implementation of scale changes depends on the
server and media type. For video, a server may, for example, deliver
only key frames or selected key frames. For audio, it may time-scale
the audio while preserving pitch or, less desirably, deliver
fragments of audio.
The server should try to approximate the viewing rate, but may
restrict the range of scale values that it supports. The response
MUST contain the actual scale value chosen by the server.
If the request contains a Range parameter, the new scale value will
take effect at that time.
Scale = "Scale" ":" [ "-" ] 1*DIGIT [ "." *DIGIT ]
Example of playing in reverse at 3.5 times normal rate:
Scale: -3.5
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12.35 Speed
This request header fields parameter requests the server to deliver
data to the client at a particular speed, contingent on the server's
ability and desire to serve the media stream at the given speed.
Implementation by the server is OPTIONAL. The default is the bit rate
of the stream.
The parameter value is expressed as a decimal ratio, e.g., a value of
2.0 indicates that data is to be delivered twice as fast as normal. A
speed of zero is invalid. If the request contains a Range parameter,
the new speed value will take effect at that time.
Speed = "Speed" ":" 1*DIGIT [ "." *DIGIT ]
Example:
Speed: 2.5
Use of this field changes the bandwidth used for data delivery. It is
meant for use in specific circumstances where preview of the
presentation at a higher or lower rate is necessary. Implementors
should keep in mind that bandwidth for the session may be negotiated
beforehand (by means other than RTSP), and therefore re-negotiation
may be necessary. When data is delivered over UDP, it is highly
recommended that means such as RTCP be used to track packet loss
rates.
12.36 Server
See [H14.39]
12.37 Session
This request and response header field identifies an RTSP session
started by the media server in a SETUP response and concluded by
TEARDOWN on the presentation URL. The session identifier is chosen by
the media server (see Section 3.4). Once a client receives a Session
identifier, it MUST return it for any request related to that
session. A server does not have to set up a session identifier if it
has other means of identifying a session, such as dynamically
generated URLs.
Session = "Session" ":" session-id [ ";" "timeout" "=" delta-seconds ]
The timeout parameter is only allowed in a response header. The
server uses it to indicate to the client how long the server is
prepared to wait between RTSP commands before closing the session due
to lack of activity (see Section A). The timeout is measured in
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seconds, with a default of 60 seconds (1 minute).
Note that a session identifier identifies a RTSP session across
transport sessions or connections. Control messages for more than one
RTSP URL may be sent within a single RTSP session. Hence, it is
possible that clients use the same session for controlling many
streams constituting a presentation, as long as all the streams come
from the same server. (See example in Section 14). However, multiple
"user" sessions for the same URL from the same client MUST use
different session identifiers.
The session identifier is needed to distinguish several delivery
requests for the same URL coming from the same client.
The response 454 (Session Not Found) is returned if the session
identifier is invalid.
12.38 Timestamp
The timestamp general header describes when the client sent the
request to the server. The value of the timestamp is of significance
only to the client and may use any timescale. The server 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 is
used by the client to compute the round-trip time to the server so
that it can adjust the timeout value for retransmissions.
Timestamp = "Timestamp" ":" *(DIGIT) [ "." *(DIGIT) ] [ delay ]
delay = *(DIGIT) [ "." *(DIGIT) ]
12.39 Transport
This request 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.
Transports are comma separated, listed in order of preference.
Parameters may be added to each transport, separated by a semicolon.
The Transport header MAY also be used to change certain transport
parameters. A server MAY refuse to change parameters of an existing
stream.
The server MAY return a Transport response header in the response to
indicate the values actually chosen.
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A Transport request header field may contain a list of transport
options acceptable to the client. In that case, the server MUST
return a single option which was actually chosen.
The syntax for the transport specifier is
transport/profile/lower-transport.
The default value for the "lower-transport" parameters is specific to
the profile. For RTP/AVP, the default is UDP.
Below are the configuration parameters associated with transport:
General parameters:
unicast | multicast:
mutually exclusive indication of whether unicast or multicast
delivery will be attempted. Default value is multicast.
Clients that are capable of handling both unicast and
multicast transmission MUST indicate such capability by
including two full transport-specs with separate parameters
for each.
destination:
The address to which a stream will be sent. The client may
specify the multicast address with the destination parameter.
To avoid becoming the unwitting perpetrator of a remote-
controlled denial-of-service attack, a server SHOULD
authenticate the client and SHOULD log such attempts before
allowing the client to direct a media stream to an address not
chosen by the server. This is particularly important if RTSP
commands are issued via UDP, but implementations cannot rely
on TCP as reliable means of client identification by itself. A
server SHOULD not allow a client to direct media streams to an
address that differs from the address commands are coming
from.
source:
If the source address for the stream is different than can be
derived from the RTSP endpoint address (the server in playback
or the client in recording), the source MAY be specified.
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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layers:
The number of multicast layers to be used for this media
stream. The layers are sent to consecutive addresses starting
at the destination address.
mode:
The mode parameter indicates the methods to be supported for
this session. Valid values are PLAY and RECORD. If not
provided, the default is PLAY.
append:
If the mode parameter includes RECORD, the append parameter
indicates that the media data should append to the existing
resource rather than overwrite it. If appending is requested
and the server does not support this, it MUST refuse the
request rather than overwrite the resource identified by the
URI. The append parameter is ignored if the mode parameter
does not contain RECORD.
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 10.12.
The argument provides the channel number to be used in the $
statement. This parameter may be specified as a range, e.g.,
interleaved=4-5 in cases where the transport choice for the
media stream requires it.
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.
Multicast specific:
ttl:
multicast time-to-live
RTP Specific:
port:
This parameter provides the RTP/RTCP port pair for a multicast
session. It is specified as a range, e.g., port=3456-3457.
client_port:
This parameter provides the unicast RTP/RTCP port pair on
which the client has chosen to receive media data and control
information. It is specified as a range, e.g.,
client_port=3456-3457.
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server_port:
This parameter provides the unicast RTP/RTCP port pair on
which the server has chosen to receive media data and control
information. It is specified as a range, e.g.,
server_port=3456-3457.
ssrc:
The ssrc parameter indicates the RTP SSRC [24, Sec. 3] value
that should be (request) or will be (response) used by the
media server. This parameter is only valid for unicast
transmission. It identifies the synchronization source to be
associated with the media stream.
Transport = "Transport" ":"
1#transport-spec
transport-spec = transport-protocol/profile[/lower-transport]
*parameter
transport-protocol = "RTP"
profile = "AVP"
lower-transport = "TCP" | "UDP"
parameter = ( "unicast" | "multicast" )
| ";" "destination" [ "=" address ]
| ";" "interleaved" "=" channel [ "-" channel ]
| ";" "append"
| ";" "ttl" "=" ttl
| ";" "layers" "=" 1*DIGIT
| ";" "port" "=" port [ "-" port ]
| ";" "client_port" "=" port [ "-" port ]
| ";" "server_port" "=" port [ "-" port ]
| ";" "ssrc" "=" ssrc
| ";" "mode" = <"> 1#mode <">
ttl = 1*3(DIGIT)
port = 1*5(DIGIT)
ssrc = 8*8(HEX)
channel = 1*3(DIGIT)
address = host
mode = <"> *Method <"> | Method
Example:
Transport: RTP/AVP;multicast;ttl=127;mode="PLAY",
RTP/AVP;unicast;client_port=3456-3457;mode="PLAY"
The Transport header is restricted to describing a single RTP
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.
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12.40 Unsupported
The Unsupported response header lists the features not supported by
the server. In the case where the feature was specified via the
Proxy-Require field (Section 12.32), if there is a proxy on the path
between the client and the server, the proxy MUST insert a message
reply with an error message "551 Option Not Supported".
See Section 12.32 for a usage example.
12.41 User-Agent
See [H14.42]
12.42 Vary
See [H14.43]
12.43 Via
See [H14.44].
12.44 WWW-Authentica
See [H14.46].
13 Caching
In HTTP, response-request pairs are cached. RTSP differs
significantly in that respect. Responses are not cacheable, with the
exception of the presentation description returned by DESCRIBE or
included with ANNOUNCE. (Since the responses for anything but
DESCRIBE and GET_PARAMETER do not return any data, caching is not
really an issue for these requests.) However, it is desirable for the
continuous media data, typically delivered out-of-band with respect
to RTSP, to be cached, as well as the session description.
On receiving a SETUP or PLAY request, a proxy ascertains whether it
has an up-to-date copy of the continuous media content and its
description. It can determine whether the copy is up-to-date by
issuing a SETUP or DESCRIBE request, respectively, and comparing the
Last-Modified header with that of the cached copy. If the copy is not
up-to-date, it modifies the SETUP transport parameters as appropriate
and forwards the request to the origin server. Subsequent control
commands such as PLAY or PAUSE then pass the proxy unmodified. The
proxy delivers the continuous media data to the client, while
possibly making a local copy for later reuse. The exact behavior
allowed to the cache is given by the cache-response directives
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described in Section 12.8. A cache MUST answer any DESCRIBE requests
if it is currently serving the stream to the requestor, as it is
possible that low-level details of the stream description may have
changed on the origin-server.
Note that an RTSP cache, unlike the HTTP cache, is of the "cut-
through" variety. Rather than retrieving the whole resource from the
origin server, the cache simply copies the streaming data as it
passes by on its way to the client. Thus, it does not introduce
additional latency.
To the client, an RTSP proxy cache appears like a regular media
server, to the media origin server like a client. Just as an HTTP
cache has to store the content type, content language, and so on for
the objects it caches, a media cache has to store the presentation
description. Typically, a cache eliminates all transport-references
(that is, multicast information) from the presentation description,
since these are independent of the data delivery from the cache to
the client. Information on the encodings remains the same. If the
cache is able to translate the cached media data, it would create a
new presentation description with all the encoding possibilities it
can offer.
14 Examples
The following examples refer to stream description formats that are
not standards, such as RTSL. The following examples are not to be
used as a reference for those formats.
14.1 Media on Demand (Unicast)
Client C requests a movie from media servers A ( audio.example.com)
and V (video.example.com). The media description is stored on a web
server W . The media description contains descriptions of the
presentation and all its streams, including the codecs that are
available, dynamic RTP payload types, the protocol stack, and content
information such as language or copyright restrictions. It may also
give an indication about the timeline of the movie.
In this example, the client is only interested in the last part of
the movie.
C->W: GET /twister.sdp HTTP/1.1
Host: www.example.com
Accept: application/sdp
W->C: HTTP/1.0 200 OK
Content-Type: application/sdp
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v=0
o=- 2890844526 2890842807 IN IP4 192.16.24.202
s=RTSP Session
m=audio 0 RTP/AVP 0
a=control:rtsp://audio.example.com/twister/audio.en
m=video 0 RTP/AVP 31
a=control:rtsp://video.example.com/twister/video
C->A: SETUP rtsp://audio.example.com/twister/audio.en RTSP/1.0
CSeq: 1
Transport: RTP/AVP/UDP;unicast;client_port=3056-3057
A->C: RTSP/1.0 200 OK
CSeq: 1
Session: 12345678
Transport: RTP/AVP/UDP;unicast;client_port=3056-3057;
server_port=5000-5001
C->V: SETUP rtsp://video.example.com/twister/video RTSP/1.0
CSeq: 1
Transport: RTP/AVP/UDP;unicast;client_port=3058-3059
V->C: RTSP/1.0 200 OK
CSeq: 1
Session: 23456789
Transport: RTP/AVP/UDP;unicast;client_port=3058-3059;
server_port=5002-5003
C->V: PLAY rtsp://video.example.com/twister/video RTSP/1.0
CSeq: 2
Session: 23456789
Range: smpte=0:10:00-
V->C: RTSP/1.0 200 OK
CSeq: 2
Session: 23456789
Range: smpte=0:10:00-0:20:00
RTP-Info: url=rtsp://video.example.com/twister/video;
seq=12312232;rtptime=78712811
C->A: PLAY rtsp://audio.example.com/twister/audio.en RTSP/1.0
CSeq: 2
Session: 12345678
Range: smpte=0:10:00-
A->C: RTSP/1.0 200 OK
CSeq: 2
Session: 12345678
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Range: smpte=0:10:00-0:20:00
RTP-Info: url=rtsp://audio.example.com/twister/audio.en;
seq=876655;rtptime=1032181
C->A: TEARDOWN rtsp://audio.example.com/twister/audio.en RTSP/1.0
CSeq: 3
Session: 12345678
A->C: RTSP/1.0 200 OK
CSeq: 3
C->V: TEARDOWN rtsp://video.example.com/twister/video RTSP/1.0
CSeq: 3
Session: 23456789
V->C: RTSP/1.0 200 OK
CSeq: 3
Even though the audio and video track are on two different servers,
and may start at slightly different times and may drift with respect
to each other, the client can synchronize the two using standard RTP
methods, in particular the time scale contained in the RTCP sender
reports.
14.2 Streaming of a Container file
For purposes of this example, a container file is a storage entity in
which multiple continuous media types pertaining to the same end-user
presentation are present. In effect, the container file represents an
RTSP presentation, with each of its components being RTSP streams.
Container files are a widely used means to store such presentations.
While the components are transported as independent streams, it is
desirable to maintain a common context for those streams at the
server end.
This enables the server to keep a single storage handle open
easily. It also allows treating all the streams equally in case of
any prioritization of streams by the server.
It is also possible that the presentation author may wish to prevent
selective retrieval of the streams by the client in order to preserve
the artistic effect of the combined media presentation. Similarly, in
such a tightly bound presentation, it is desirable to be able to
control all the streams via a single control message using an
aggregate URL.
The following is an example of using a single RTSP session to control
multiple streams. It also illustrates the use of aggregate URLs.
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Client C requests a presentation from media server M . The movie is
stored in a container file. The client has obtained an RTSP URL to
the container file.
C->M: DESCRIBE rtsp://foo/twister RTSP/1.0
CSeq: 1
M->C: RTSP/1.0 200 OK
CSeq: 1
Content-Type: application/sdp
Content-Length: 164
v=0
o=- 2890844256 2890842807 IN IP4 172.16.2.93
s=RTSP Session
i=An Example of RTSP Session Usage
a=control:rtsp://foo/twister
t=0 0
m=audio 0 RTP/AVP 0
a=control:rtsp://foo/twister/audio
m=video 0 RTP/AVP 26
a=control:rtsp://foo/twister/video
C->M: SETUP rtsp://foo/twister/audio RTSP/1.0
CSeq: 2
Transport: RTP/AVP;unicast;client_port=8000-8001
M->C: RTSP/1.0 200 OK
CSeq: 2
Transport: RTP/AVP;unicast;client_port=8000-8001;
server_port=9000-9001
Session: 12345678
C->M: SETUP rtsp://foo/twister/video RTSP/1.0
CSeq: 3
Transport: RTP/AVP;unicast;client_port=8002-8003
Session: 12345678
M->C: RTSP/1.0 200 OK
CSeq: 3
Transport: RTP/AVP;unicast;client_port=8002-8003;
server_port=9004-9005
Session: 12345678
C->M: PLAY rtsp://foo/twister RTSP/1.0
CSeq: 4
Range: npt=0-
Session: 12345678
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M->C: RTSP/1.0 200 OK
CSeq: 4
Session: 12345678
RTP-Info: url=rtsp://foo/twister/video;
seq=9810092;rtptime=3450012
C->M: PAUSE rtsp://foo/twister/video RTSP/1.0
CSeq: 5
Session: 12345678
M->C: RTSP/1.0 460 Only aggregate operation allowed
CSeq: 5
C->M: PAUSE rtsp://foo/twister RTSP/1.0
CSeq: 6
Session: 12345678
M->C: RTSP/1.0 200 OK
CSeq: 6
Session: 12345678
C->M: SETUP rtsp://foo/twister RTSP/1.0
CSeq: 7
Transport: RTP/AVP;unicast;client_port=10000
M->C: RTSP/1.0 459 Aggregate operation not allowed
CSeq: 7
In the first instance of failure, the client tries to pause one
stream (in this case video) of the presentation. This is disallowed
for that presentation by the server. In the second instance, the
aggregate URL may not be used for SETUP and one control message is
required per stream to set up transport parameters.
This keeps the syntax of the Transport header simple and allows
easy parsing of transport information by firewalls.
14.3 Single Stream Container Files
Some RTSP servers may treat all files as though they are "container
files", yet other servers may not support such a concept. Because of
this, clients SHOULD use the rules set forth in the session
description for request URLs, rather than assuming that a consistent
URL may always be used throughout. Here's an example of how a multi-
stream server might expect a single-stream file to be served:
Accept: application/x-rtsp-mh, application/sdp
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CSeq: 1
S->C RTSP/1.0 200 OK
CSeq: 1
Content-base: rtsp://foo.com/test.wav/
Content-type: application/sdp
Content-length: 48
v=0
o=- 872653257 872653257 IN IP4 172.16.2.187
s=mu-law wave file
i=audio test
t=0 0
m=audio 0 RTP/AVP 0
a=control:streamid=0
C->S SETUP rtsp://foo.com/test.wav/streamid=0 RTSP/1.0
Transport: RTP/AVP/UDP;unicast;
client_port=6970-6971;mode=play
CSeq: 2
S->C RTSP/1.0 200 OK
Transport: RTP/AVP/UDP;unicast;client_port=6970-6971;
server_port=6970-6971;mode=play
CSeq: 2
Session: 2034820394
C->S PLAY rtsp://foo.com/test.wav RTSP/1.0
CSeq: 3
Session: 2034820394
S->C RTSP/1.0 200 OK
CSeq: 3
Session: 2034820394
RTP-Info: url=rtsp://foo.com/test.wav/streamid=0;
seq=981888;rtptime=3781123
Note the different URL in the SETUP command, and then the switch back
to the aggregate URL in the PLAY command. This makes complete sense
when there are multiple streams with aggregate control, but is less
than intuitive in the special case where the number of streams is
one.
In this special case, it is recommended that servers be forgiving of
implementations that send:
C->S PLAY rtsp://foo.com/test.wav/streamid=0 RTSP/1.0
CSeq: 3
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In the worst case, servers should send back:
S->C RTSP/1.0 460 Only aggregate operation allowed
CSeq: 3
One would also hope that server implementations are also forgiving of
the following:
C->S SETUP rtsp://foo.com/test.wav RTSP/1.0
Transport: rtp/avp/udp;client_port=6970-6971;mode=play
CSeq: 2
Since there is only a single stream in this file, it's not ambiguous
what this means.
14.4 Live Media Presentation Using Multicast
The media server M chooses the multicast address and port. Here, we
assume that the web server only contains a pointer to the full
description, while the media server M maintains the full description.
C->W: GET /concert.sdp HTTP/1.1
Host: www.example.com
W->C: HTTP/1.1 200 OK
Content-Type: application/x-rtsl
<session>
<track src="rtsp://live.example.com/concert/audio">
</session>
C->M: DESCRIBE rtsp://live.example.com/concert/audio RTSP/1.0
CSeq: 1
M->C: RTSP/1.0 200 OK
CSeq: 1
Content-Type: application/sdp
Content-Length: 44
v=0
o=- 2890844526 2890842807 IN IP4 192.16.24.202
s=RTSP Session
m=audio 3456 RTP/AVP 0
a=control:rtsp://live.example.com/concert/audio
c=IN IP4 224.2.0.1/16
C->M: SETUP rtsp://live.example.com/concert/audio RTSP/1.0
CSeq: 2
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Transport: RTP/AVP;multicast
M->C: RTSP/1.0 200 OK
CSeq: 2
Transport: RTP/AVP;multicast;destination=224.2.0.1;
port=3456-3457;ttl=16
Session: 0456804596
C->M: PLAY rtsp://live.example.com/concert/audio RTSP/1.0
CSeq: 3
Session: 0456804596
M->C: RTSP/1.0 200 OK
CSeq: 3
Session: 0456804596
14.5 Playing media into an existing session
A conference participant C wants to have the media server M play back
a demo tape into an existing conference. C indicates to the media
server that the network addresses and encryption keys are already
given by the conference, so they should not be chosen by the server.
The example omits the simple ACK responses.
C->M: DESCRIBE rtsp://server.example.com/demo/548/sound RTSP/1.0
CSeq: 1
Accept: application/sdp
M->C: RTSP/1.0 200 1 OK
Content-type: application/sdp
Content-Length: 44
v=0
o=- 2890844526 2890842807 IN IP4 192.16.24.202
s=RTSP Session
i=See above
t=0 0
m=audio 0 RTP/AVP 0
C->M: SETUP rtsp://server.example.com/demo/548/sound RTSP/1.0
CSeq: 2
Transport: RTP/AVP;multicast;destination=225.219.201.15;
port=7000-7001;ttl=127
Conference: 199702170042.SAA08642@obiwan.arl.wustl.edu%20Starr
M->C: RTSP/1.0 200 OK
CSeq: 2
Transport: RTP/AVP;multicast;destination=225.219.201.15;
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port=7000-7001;ttl=127
Session: 91389234234
Conference: 199702170042.SAA08642@obiwan.arl.wustl.edu%20Starr
C->M: PLAY rtsp://server.example.com/demo/548/sound RTSP/1.0
CSeq: 3
Session: 91389234234
M->C: RTSP/1.0 200 OK
CSeq: 3
14.6 Recording
The conference participant client C asks the media server M to record
the audio and video portions of a meeting. The client uses the
ANNOUNCE method to provide meta-information about the recorded
session to the server.
C->M: ANNOUNCE rtsp://server.example.com/meeting RTSP/1.0
CSeq: 90
Content-Type: application/sdp
Content-Length: 121
v=0
o=camera1 3080117314 3080118787 IN IP4 195.27.192.36
s=IETF Meeting, Munich - 1
i=The thirty-ninth IETF meeting will be held in Munich, Germany
u=http://www.ietf.org/meetings/Munich.html
e=IETF Channel 1 <ietf39-mbone@uni-koeln.de>
p=IETF Channel 1 +49-172-2312 451
c=IN IP4 224.0.1.11/127
t=3080271600 3080703600
a=tool:sdr v2.4a6
a=type:test
m=audio 21010 RTP/AVP 5
c=IN IP4 224.0.1.11/127
a=ptime:40
m=video 61010 RTP/AVP 31
c=IN IP4 224.0.1.12/127
M->C: RTSP/1.0 200 OK
CSeq: 90
C->M: SETUP rtsp://server.example.com/meeting/audiotrack RTSP/1.0
CSeq: 91
Transport: RTP/AVP;multicast;destination=224.0.1.11;
port=21010-21011;mode=record;ttl=127
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M->C: RTSP/1.0 200 OK
CSeq: 91
Session: 50887676
Transport: RTP/AVP;multicast;destination=224.0.1.11;
port=21010-21011;mode=record;ttl=127
C->M: SETUP rtsp://server.example.com/meeting/videotrack RTSP/1.0
CSeq: 92
Session: 50887676
Transport: RTP/AVP;multicast;destination=224.0.1.12;
port=61010-61011;mode=record;ttl=127
M->C: RTSP/1.0 200 OK
CSeq: 92
Transport: RTP/AVP;multicast;destination=224.0.1.12;
port=61010-61011;mode=record;ttl=127
C->M: RECORD rtsp://server.example.com/meeting RTSP/1.0
CSeq: 93
Session: 50887676
Range: clock=19961110T1925-19961110T2015
M->C: RTSP/1.0 200 OK
CSeq: 93
15 Syntax
The RTSP syntax is described in an augmented Backus-Naur form (BNF)
as used in RFC 2068 [2].
15.1 Base Syntax
OCTET = <any 8-bit sequence of data>
CHAR = <any US-ASCII character (octets 0 - 127)>
UPALPHA = <any US-ASCII uppercase letter "A".."Z">
LOALPHA = <any US-ASCII lowercase letter "a".."z">
ALPHA = UPALPHA | LOALPHA
DIGIT = <any US-ASCII digit "0".."9">
CTL = <any US-ASCII control character
(octets 0 - 31) and DEL (127)>
CR = <US-ASCII CR, carriage return (13)>
LF = <US-ASCII LF, linefeed (10)>
SP = <US-ASCII SP, space (32)>
HT = <US-ASCII HT, horizontal-tab (9)>
<"> = <US-ASCII double-quote mark (34)>
CRLF = CR LF
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LWS = [CRLF] 1*( SP | HT )
TEXT = <any OCTET except CTLs>
tspecials = "(" | ")" | "<" | ">" | "@"
| "," | ";" | ":" | "" | <">
| "/" | "[" | "]" | "?" | "="
| "{" | "}" | SP | HT
token = 1*<any CHAR except CTLs or tspecials>
quoted-string = ( <"> *(qdtext) <"> )
qdtext = <any TEXT except <">>
quoted-pair = "" CHAR
message-header = field-name ":" [ field-value ] CRLF
field-name = token
field-value = *( field-content | LWS )
field-content = <the OCTETs making up the field-value and
consisting of either *TEXT or
combinations of token, tspecials, and
quoted-string>
safe = "$" | "-" | "_" | "." | "+"
extra = "!" | "*" | "$'$" | "(" | ")" | ","
hex = DIGIT | "A" | "B" | "C" | "D" | "E" | "F" |
"a" | "b" | "c" | "d" | "e" | "f"
escape = "%" hex hex
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "="
unreserved = alpha | digit | safe | extra
xchar = unreserved | reserved | escape
16 Security Considerations
Because of the similarity in syntax and usage between RTSP servers
and HTTP servers, the security considerations outlined in [H15]
apply. Specifically, please note the following:
Authentication Mechanisms:
RTSP and HTTP share common authentication schemes, and thus
should follow the same prescriptions with regards to
authentication. See [H15.1] for client authentication issues,
and [H15.2] for issues regarding support for multiple
authentication mechanisms.
Abuse of Server Log Information:
RTSP and HTTP servers will presumably have similar logging
mechanisms, and thus should be equally guarded in protecting
the contents of those logs, thus protecting the privacy of the
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users of the servers. See [H15.3] for HTTP server
recommendations regarding server logs.
Transfer of Sensitive Information:
There is no reason to believe that information transferred via
RTSP may be any less sensitive than that normally transmitted
via HTTP. Therefore, all of the precautions regarding the
protection of data privacy and user privacy apply to
implementors of RTSP clients, servers, and proxies. See
[H15.4] for further details.
Attacks Based On File and Path Names:
Though RTSP URLs are opaque handles that do not necessarily
have file system semantics, it is anticipated that many
implementations will translate portions of the request URLs
directly to file system calls. In such cases, file systems
SHOULD follow the precautions outlined in [H15.5], such as
checking for ".." in path components.
Personal Information:
RTSP clients are often privy to the same information that HTTP
clients are (user name, location, etc.) and thus should be
equally. See [H15.6] for further recommendations.
Privacy Issues Connected to Accept Headers:
Since may of the same "Accept" headers exist in RTSP as in
HTTP, the same caveats outlined in [H15.7] with regards to
their use should be followed.
DNS Spoofing:
Presumably, given the longer connection times typically
associated to RTSP sessions relative to HTTP sessions, RTSP
client DNS optimizations should be less prevalent.
Nonetheless, the recommendations provided in [H15.8] are still
relevant to any implementation which attempts to rely on a
DNS-to-IP mapping to hold beyond a single use of the mapping.
Location Headers and Spoofing:
If a single server supports multiple organizations that do not
trust one another, then it must check the values of Location
and Content-Location headers in responses that are generated
under control of said organizations to make sure that they do
not attempt to invalidate resources over which they have no
authority. ([H15.9])
In addition to the recommendations in the current HTTP specification
(RFC 2068 [2], as of this writing), future HTTP specifications may
provide additional guidance on security issues.
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The following are added considerations for RTSP implementations.
Concentrated denial-of-service attack:
The protocol offers the opportunity for a remote-controlled
denial-of-service attack. The attacker may initiate traffic
flows to one or more IP addresses by specifying them as the
destination in SETUP requests. While the attacker's IP address
may be known in this case, this is not always useful in
prevention of more attacks or ascertaining the attackers
identity. Thus, an RTSP server SHOULD only allow client-
specified destinations for RTSP-initiated traffic flows if the
server has verified the client's identity, either against a
database of known users using RTSP authentication mechanisms
(preferably digest authentication or stronger), or other
secure means.
Session hijacking:
Since there is no relation between a transport layer
connection and an RTSP session, it is possible for a malicious
client to issue requests with random session identifiers which
would affect unsuspecting clients. The server SHOULD use a
large, random and non-sequential session identifier to
minimize the possibility of this kind of attack.
Authentication:
Servers SHOULD implement both basic and digest [8]
authentication. In environments requiring tighter security for
the control messages, the RTSP control stream may be
encrypted.
Stream issues:
RTSP only provides for stream control. Stream delivery issues
are not covered in this section, nor in the rest of this memo.
RTSP implementations will most likely rely on other protocols
such as RTP, IP multicast, RSVP and IGMP, and should address
security considerations brought up in those and other
applicable specifications.
Persistently suspicious behavior:
RTSP servers SHOULD return error code 403 (Forbidden) upon
receiving a single instance of behavior which is deemed a
security risk. RTSP servers SHOULD also be aware of attempts
to probe the server for weaknesses and entry points and MAY
arbitrarily disconnect and ignore further requests clients
which are deemed to be in violation of local security policy.
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Appendix A: RTSP Protocol State Machines
The RTSP client and server state machines describe the behavior of
the protocol from RTSP session initialization through RTSP session
termination.
State is defined on a per object basis. An object is uniquely
identified by the stream URL and the RTSP session identifier. Any
request/reply using aggregate URLs denoting RTSP presentations
composed of multiple streams will have an effect on the individual
states of all the streams. For example, if the presentation /movie
contains two streams, /movie/audio and /movie/video, then the
following command:
PLAY rtsp://foo.com/movie RTSP/1.0
CSeq: 559
Session: 12345678
will have an effect on the states of movie/audio and movie/video.
This example does not imply a standard way to represent streams in
URLs or a relation to the filesystem. See Section 3.2.
The requests OPTIONS, ANNOUNCE, DESCRIBE, GET_PARAMETER,
SET_PARAMETER do not have any effect on client or server state and
are therefore not listed in the state tables.
A.1 Client State Machine
The client can assume the following states:
Init:
SETUP has been sent, waiting for reply.
Ready:
SETUP reply received or PAUSE reply received while in Playing
state.
Playing:
PLAY reply received
Recording:
RECORD reply received
In general, the client changes state on receipt of replies to
requests. Note that some requests are effective at a future time or
position (such as a PAUSE), and state also changes accordingly. If no
explicit SETUP is required for the object (for example, it is
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available via a multicast group), state begins at Ready. In this
case, there are only two states, Ready and Playing. The client also
changes state from Playing/Recording to Ready when the end of the
requested range is reached.
The "next state" column indicates the state assumed after receiving a
success response (2xx). If a request yields a status code of 3xx, the
state becomes Init, and a status code of 4xx yields no change in
state. Messages not listed for each state MUST NOT be issued by the
client in that state, with the exception of messages not affecting
state, as listed above. Receiving a REDIRECT from the server is
equivalent to receiving a 3xx redirect status from the server.
state message sent next state after response
Init SETUP Ready
TEARDOWN Init
Ready PLAY Playing
RECORD Recording
TEARDOWN Init
SETUP Ready
Playing PAUSE Ready
TEARDOWN Init
PLAY Playing
SETUP Playing (changed transport)
Recording PAUSE Ready
TEARDOWN Init
RECORD Recording
SETUP Recording (changed transport)
A.2 Server State Machine
The server can assume the following states:
Init:
The initial state, no valid SETUP has been received yet.
Ready:
Last SETUP received was successful, reply sent or after
playing, last PAUSE received was successful, reply sent.
Playing:
Last PLAY received was successful, reply sent. Data is being
sent.
Recording:
The server is recording media data.
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In general, the server changes state on receiving requests. If the
server is in state Playing or Recording and in unicast mode, it MAY
revert to Init and tear down the RTSP session if it has not received
"wellness" information, such as RTCP reports or RTSP commands, from
the client for a defined interval, with a default of one minute. The
server can declare another timeout value in the Session response
header (Section 12.37). If the server is in state Ready, it MAY
revert to Init if it does not receive an RTSP request for an interval
of more than one minute. Note that some requests (such as PAUSE) may
be effective at a future time or position, and server state changes
at the appropriate time. The server reverts from state Playing or
Recording to state Ready at the end of the range requested by the
client.
The REDIRECT message, when sent, is effective immediately unless it
has a Range header specifying when the redirect is effective. In such
a case, server state will also change at the appropriate time.
If no explicit SETUP is required for the object, the state starts at
Ready and there are only two states, Ready and Playing.
The "next state" column indicates the state assumed after sending a
success response (2xx). If a request results in a status code of 3xx,
the state becomes Init. A status code of 4xx results in no change.
state message received next state
Init SETUP Ready
TEARDOWN Init
Ready PLAY Playing
SETUP Ready
TEARDOWN Init
RECORD Recording
Playing PLAY Playing
PAUSE Ready
TEARDOWN Init
SETUP Playing
Recording RECORD Recording
PAUSE Ready
TEARDOWN Init
SETUP Recording
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Appendix B: Interaction with RTP
RTSP allows media clients to control selected, non-contiguous
sections of media presentations, rendering those streams with an RTP
media layer[24]. The media layer rendering the RTP stream should not
be affected by jumps in NPT. Thus, both RTP sequence numbers and RTP
timestamps MUST be continuous and monotonic across jumps of NPT.
As an example, assume a clock frequency of 8000 Hz, a packetization
interval of 100 ms and an initial sequence number and timestamp of
zero. First we play NPT 10 through 15, then skip ahead and play NPT
18 through 20. The first segment is presented as RTP packets with
sequence numbers 0 through 49 and timestamp 0 through 39,200. The
second segment consists of RTP packets with sequence number 50
through 69, with timestamps 40,000 through 55,200.
We cannot assume that the RTSP client can communicate with the RTP
media agent, as the two may be independent processes. If the RTP
timestamp shows the same gap as the NPT, the media agent will
assume that there is a pause in the presentation. If the jump in
NPT is large enough, the RTP timestamp may roll over and the media
agent may believe later packets to be duplicates of packets just
played out.
For certain datatypes, tight integration between the RTSP layer and
the RTP layer will be necessary. This by no means precludes the
above restriction. Combined RTSP/RTP media clients should use the
RTP-Info field to determine whether incoming RTP packets were sent
before or after a seek.
For continuous audio, the server SHOULD set the RTP marker bit at the
beginning of serving a new PLAY request. This allows the client to
perform playout delay adaptation.
For scaling (see Section 12.34), RTP timestamps should correspond to
the playback timing. For example, when playing video recorded at 30
frames/second at a scale of two and speed (Section 12.35) of one, the
server would drop every second frame to maintain and deliver video
packets with the normal timestamp spacing of 3,000 per frame, but NPT
would increase by 1/15 second for each video frame.
The client can maintain a correct display of NPT by noting the RTP
timestamp value of the first packet arriving after repositioning. The
sequence parameter of the RTP-Info (Section 12.33) header provides
the first sequence number of the next segment.
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Appendix C: Use of SDP for RTSP Session Descriptions
The Session Description Protocol (SDP, RFC 2327 [6]) may be used to
describe streams or presentations in RTSP. Such usage is limited to
specifying means of access and encoding(s) for:
aggregate control:
A presentation composed of streams from one or more servers
that are not available for aggregate control. Such a
description is typically retrieved by HTTP or other non-RTSP
means. However, they may be received with ANNOUNCE methods.
non-aggregate control:
A presentation composed of multiple streams from a single
server that are available for aggregate control. Such a
description is typically returned in reply to a DESCRIBE
request on a URL, or received in an ANNOUNCE method.
This appendix describes how an SDP file, retrieved, for example,
through HTTP, determines the operation of an RTSP session. It also
describes how a client should interpret SDP content returned in reply
to a DESCRIBE request. SDP provides no mechanism by which a client
can distinguish, without human guidance, between several media
streams to be rendered simultaneously and a set of alternatives
(e.g., two audio streams spoken in different languages).
C.1 Definitions
The terms "session-level", "media-level" and other key/attribute
names and values used in this appendix are to be used as defined in
SDP (RFC 2327 [6]):
C.1.1 Control URL
The "a=control:" attribute is used to convey the control URL. This
attribute is used both for the session and media descriptions. If
used for individual media, it indicates the URL to be used for
controlling that particular media stream. If found at the session
level, the attribute indicates the URL for aggregate control.
Example:
a=control:rtsp://example.com/foo
This attribute may contain either relative and absolute URLs,
following the rules and conventions set out in RFC 1808 [25].
Implementations should look for a base URL in the following order:
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1. The RTSP Content-Base field
2. The RTSP Content-Location field
3. The RTSP request URL
If this attribute contains only an asterisk (*), then the URL is
treated as if it were an empty embedded URL, and thus inherits the
entire base URL.
C.1.2 Media streams
The "m=" field is used to enumerate the streams. It is expected that
all the specified streams will be rendered with appropriate
synchronization. If the session is unicast, the port number serves as
a recommendation from the server to the client; the client still has
to include it in its SETUP request and may ignore this
recommendation. If the server has no preference, it SHOULD set the
port number value to zero.
Example:
m=audio 0 RTP/AVP 31
C.1.3 Payload type(s)
The payload type(s) are specified in the "m=" field. In case the
payload type is a static payload type from RFC 1890 [1], no other
information is required. In case it is a dynamic payload type, the
media attribute "rtpmap" is used to specify what the media is. The
"encoding name" within the "rtpmap" attribute may be one of those
specified in RFC 1890 (Sections 5 and 6), or an experimental encoding
with a "X-" prefix as specified in SDP (RFC 2327 [6]). Codec-
specific parameters are not specified in this field, but rather in
the "fmtp" attribute described below. Implementors seeking to
register new encodings should follow the procedure in RFC 1890 [1].
If the media type is not suited to the RTP AV profile, then it is
recommended that a new profile be created and the appropriate profile
name be used in lieu of "RTP/AVP" in the "m=" field.
C.1.4 Format-specific parameters
Format-specific parameters are conveyed using the "fmtp" media
attribute. The syntax of the "fmtp" attribute is specific to the
encoding(s) that the attribute refers to. Note that the packetization
interval is conveyed using the "ptime" attribute.
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C.1.5 Range of presentation
The "a=range" attribute defines the total time range of the stored
session. (The length of live sessions can be deduced from the "t" and
"r" parameters.) Unless the presentation contains media streams of
different durations, the range attribute is a session-level
attribute. The unit is specified first, followed by the value range.
The units and their values are as defined in Section 3.5, 3.6 and
3.7.
Examples:
a=range:npt=0-34.4368
a=range:clock=19971113T2115-19971113T2203
C.1.6 Time of availability
The "t=" field MUST contain suitable values for the start and stop
times for both aggregate and non-aggregate stream control. With
aggregate control, the server SHOULD indicate a stop time value for
which it guarantees the description to be valid, and a start time
that is equal to or before the time at which the DESCRIBE request was
received. It MAY also indicate start and stop times of 0, meaning
that the session is always available. With non-aggregate control, the
values should reflect the actual period for which the session is
available in keeping with SDP semantics, and not depend on other
means (such as the life of the web page containing the description)
for this purpose.
C.1.7 Connection Information
In SDP, the "c=" field contains the destination address for the media
stream. However, for on-demand unicast streams and some multicast
streams, the destination address is specified by the client via the
SETUP request. Unless the media content has a fixed destination
address, the "c=" field is to be set to a suitable null value. For
addresses of type "IP4", this value is "0.0.0.0".
C.1.8 Entity Tag
The optional "a=etag" attribute identifies a version of the session
description. It is opaque to the client. SETUP requests may include
this identifier in the If-Match field (see section 12.22) to only
allow session establishment if this attribute value still corresponds
to that of the current description. The attribute value is opaque and
may contain any character allowed within SDP attribute values.
Example:
a=etag:158bb3e7c7fd62ce67f12b533f06b83a
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One could argue that the "o=" field provides identical
functionality. However, it does so in a manner that would put
constraints on servers that need to support multiple session
description types other than SDP for the same piece of media
content.
C.2 Aggregate Control Not Available
If a presentation does not support aggregate control and multiple
media sections are specified, each section MUST have the control URL
specified via the "a=control:" attribute.
Example:
v=0
o=- 2890844256 2890842807 IN IP4 204.34.34.32
s=I came from a web page
t=0 0
c=IN IP4 0.0.0.0
m=video 8002 RTP/AVP 31
a=control:rtsp://audio.com/movie.aud
m=audio 8004 RTP/AVP 3
a=control:rtsp://video.com/movie.vid
Note that the position of the control URL in the description implies
that the client establishes separate RTSP control sessions to the
servers audio.com and video.com.
It is recommended that an SDP file contains the complete media
initialization information even if it is delivered to the media
client through non-RTSP means. This is necessary as there is no
mechanism to indicate that the client should request more detailed
media stream information via DESCRIBE.
C.3 Aggregate Control Available
In this scenario, the server has multiple streams that can be
controlled as a whole. In this case, there are both media-level
"a=control:" attributes, which are used to specify the stream URLs,
and a session-level "a=control:" attribute which is used as the
request URL for aggregate control. If the media-level URL is
relative, it is resolved to absolute URLs according to Section C.1.1
above.
If the presentation comprises only a single stream, the media-level
"a=control:" attribute may be omitted altogether. However, if the
presentation contains more than one stream, each media stream section
MUST contain its own "a=control" attribute.
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Example:
v=0
o=- 2890844256 2890842807 IN IP4 204.34.34.32
s=I contain
i=<more info>
t=0 0
c=IN IP4 0.0.0.0
a=control:rtsp://example.com/movie/
m=video 8002 RTP/AVP 31
a=control:trackID=1
m=audio 8004 RTP/AVP 3
a=control:trackID=2
In this example, the client is required to establish a single RTSP
session to the server, and uses the URLs
rtsp://example.com/movie/trackID=1 and
rtsp://example.com/movie/trackID=2 to set up the video and audio
streams, respectively. The URL rtsp://example.com/movie/ controls the
whole movie.
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Appendix D: Minimal RTSP implementation
D.1 Client
A client implementation MUST be able to do the following :
* Generate the following requests: SETUP, TEARDOWN, and one of PLAY
(i.e., a minimal playback client) or RECORD (i.e., a minimal
recording client). If RECORD is implemented, ANNOUNCE must be
implemented as well.
* Include the following headers in requests: CSeq, Connection,
Session, Transport. If ANNOUNCE is implemented, the capability to
include headers Content-Language, Content-Encoding, Content-
Length, and Content-Type should be as well.
* Parse and understand the following headers in responses: CSeq,
Connection, Session, Transport, Content-Language, Content-
Encoding, Content-Length, Content-Type. If RECORD is implemented,
the Location header must be understood as well. RTP-compliant
implementations should also implement RTP-Info.
* Understand the class of each error code received and notify the
end-user, if one is present, of error codes in classes 4xx and
5xx. The notification requirement may be relaxed if the end-user
explicitly does not want it for one or all status codes.
* Expect and respond to asynchronous requests from the server, such
as ANNOUNCE. This does not necessarily mean that it should
implement the ANNOUNCE method, merely that it MUST respond
positively or negatively to any request received from the server.
Though not required, the following are highly recommended at the time
of publication for practical interoperability with initial
implementations and/or to be a "good citizen".
* Implement RTP/AVP/UDP as a valid transport.
* Inclusion of the User-Agent header.
* Understand SDP session descriptions as defined in Appendix C
* Accept media initialization formats (such as SDP) from standard
input, command line, or other means appropriate to the operating
environment to act as a "helper application" for other
applications (such as web browsers).
There may be RTSP applications different from those initially
envisioned by the contributors to the RTSP specification for which
the requirements above do not make sense. Therefore, the
recommendations above serve only as guidelines instead of strict
requirements.
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D.1.1 Basic Playback
To support on-demand playback of media streams, the client MUST
additionally be able to do the following:
* generate the PAUSE request;
* implement the REDIRECT method, and the Location header.
D.1.2 Authentication-enabled
In order to access media presentations from RTSP servers that require
authentication, the client MUST additionally be able to do the
following:
* recognize the 401 status code;
* parse and include the WWW-Authenticate header;
* implement Basic Authentication and Digest Authentication.
D.2 Server
A minimal server implementation MUST be able to do the following:
* Implement the following methods: SETUP, TEARDOWN, OPTIONS and
either PLAY (for a minimal playback server) or RECORD (for a
minimal recording server). If RECORD is implemented, ANNOUNCE
should be implemented as well.
* Include the following headers in responses: Connection,
Content-Length, Content-Type, Content-Language, Content-Encoding,
Transport, Public. The capability to include the Location header
should be implemented if the RECORD method is. RTP-compliant
implementations should also implement the RTP-Info field.
* Parse and respond appropriately to the following headers in
requests: Connection, Session, Transport, Require.
Though not required, the following are highly recommended at the time
of publication for practical interoperability with initial
implementations and/or to be a "good citizen".
* Implement RTP/AVP/UDP as a valid transport.
* Inclusion of the Server header.
* Implement the DESCRIBE method.
* Generate SDP session descriptions as defined in Appendix C
There may be RTSP applications different from those initially
envisioned by the contributors to the RTSP specification for which
the requirements above do not make sense. Therefore, the
recommendations above serve only as guidelines instead of strict
requirements.
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D.2.1 Basic Playback
To support on-demand playback of media streams, the server MUST
additionally be able to do the following:
* Recognize the Range header, and return an error if seeking is not
supported.
* Implement the PAUSE method.
In addition, in order to support commonly-accepted user interface
features, the following are highly recommended for on-demand media
servers:
* Include and parse the Range header, with NPT units.
Implementation of SMPTE units is recommended.
* Include the length of the media presentation in the media
initialization information.
* Include mappings from data-specific timestamps to NPT. When RTP
is used, the rtptime portion of the RTP-Info field may be used to
map RTP timestamps to NPT.
Client implementations may use the presence of length information
to determine if the clip is seekable, and visibly disable seeking
features for clips for which the length information is unavailable.
A common use of the presentation length is to implement a "slider
bar" which serves as both a progress indicator and a timeline
positioning tool.
Mappings from RTP timestamps to NPT are necessary to ensure correct
positioning of the slider bar.
D.2.2 Authentication-enabled
In order to correctly handle client authentication, the server MUST
additionally be able to do the following:
* Generate the 401 status code when authentication is required for
the resource.
* Parse and include the WWW-Authenticate header
* Implement Basic Authentication and Digest Authentication
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Appendix E: Authors' Addresses
Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA
EMail: schulzrinne@cs.columbia.edu
Anup Rao
Netscape Communications Corp.
501 E. Middlefield Road
Mountain View, CA 94043
USA
EMail: anup@netscape.com
Robert Lanphier
RealNetworks
1111 Third Avenue Suite 2900
Seattle, WA 98101
USA
EMail: robla@real.com
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Appendix F: Acknowledgements
This memo is based on the functionality of the original RTSP document
submitted in October 96. It also borrows format and descriptions from
HTTP/1.1.
This document has benefited greatly from the comments of all those
participating in the MMUSIC-WG. In addition to those already
mentioned, the following individuals have contributed to this
specification:
Rahul Agarwal, Torsten Braun, Brent Browning, Bruce Butterfield,
Steve Casner, Francisco Cortes, Kelly Djahandari, Martin Dunsmuir,
Eric Fleischman, Jay Geagan, Andy Grignon, V. Guruprasad, Peter
Haight, Mark Handley, Brad Hefta-Gaub, John K. Ho, Philipp Hoschka,
Anne Jones, Anders Klemets, Ruth Lang, Stephanie Leif, Jonathan
Lennox, Eduardo F. Llach, Rob McCool, David Oran, Maria Papadopouli,
Sujal Patel, Ema Patki, Alagu Periyannan, Igor Plotnikov, Pinaki
Shah, David Singer, Jeff Smith, Alexander Sokolsky, Dale Stammen, and
John Francis Stracke.
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References
1 Schulzrinne, H., "RTP profile for audio and video conferences
with minimal control", RFC 1890, January 1996.
2 Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
Berners-Lee, "Hypertext transfer protocol - HTTP/1.1", RFC
2068, January 1997.
3 Yergeau, F., Nicol, G., Adams, G., and M. Duerst,
"Internationalization of the hypertext markup language", RFC
2070, January 1997.
4 Bradner, S., "Key words for use in RFCs to indicate
requirement levels", BCP 14, RFC 2119, March 1997.
5 ISO/IEC, "Information technology - generic coding of moving
pictures and associated audio information - part 6: extension
for digital storage media and control," Draft International
Standard ISO 13818-6, International Organization for
Standardization ISO/IEC JTC1/SC29/WG11, Geneva, Switzerland,
Nov. 1995.
6 Handley, M., and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
7 Franks, J., Hallam-Baker, P., and J. Hostetler, "An extension to
HTTP: digest access authentication", RFC 2069, January 1997.
8 Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
1980.
9 Hinden, B. and C. Partridge, "Version 2 of the reliable data
protocol (RDP)", RFC 1151, April 1990.
10 Postel, J., "Transmission control protocol", STD 7, RFC 793,
September 1981.
11 H. Schulzrinne, "A comprehensive multimedia control
architecture for the Internet," in Proc. International
Workshop on Network and Operating System Support for Digital
Audio and Video (NOSSDAV), (St. Louis, Missouri), May 1997.
12 International Telecommunication Union, "Visual telephone
systems and equipment for local area networks which provide a
non-guaranteed quality of service," Recommendation H.323,
Telecommunication Standardization Sector of ITU, Geneva,
Switzerland, May 1996.
Schulzrinne, et. al. Standards Track [Page 90]
RFC 2326 Real Time Streaming Protocol April 1998
13 McMahon, P., "GSS-API authentication method for SOCKS version
5", RFC 1961, June 1996.
14 J. Miller, P. Resnick, and D. Singer, "Rating services and
rating systems (and their machine readable descriptions),"
Recommendation REC-PICS-services-961031, W3C (World Wide Web
Consortium), Boston, Massachusetts, Oct. 1996.
15 J. Miller, T. Krauskopf, P. Resnick, and W. Treese, "PICS
label distribution label syntax and communication protocols,"
Recommendation REC-PICS-labels-961031, W3C (World Wide Web
Consortium), Boston, Massachusetts, Oct. 1996.
16 Crocker, D. and P. Overell, "Augmented BNF for syntax
specifications: ABNF", RFC 2234, November 1997.
17 Braden, B., "Requirements for internet hosts - application and
support", STD 3, RFC 1123, October 1989.
18 Elz, R., "A compact representation of IPv6 addresses", RFC
1924, April 1996.
19 Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform
resource locators (URL)", RFC 1738, December 1994.
20 Yergeau, F., "UTF-8, a transformation format of ISO 10646",
RFC 2279, January 1998.
22 Braden, B., "T/TCP - TCP extensions for transactions
functional specification", RFC 1644, July 1994.
22 W. R. Stevens, TCP/IP illustrated: the implementation, vol. 2.
Reading, Massachusetts: Addison-Wesley, 1994.
23 Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: a transport protocol for real-time applications", RFC
1889, January 1996.
24 Fielding, R., "Relative uniform resource locators", RFC 1808,
June 1995.
Schulzrinne, et. al. Standards Track [Page 91]
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Full Copyright Statement
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BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Schulzrinne, et. al. Standards Track [Page 92]