Internet Engineering Task Force MMUSIC WG Internet Draft H. Schulzrinne draft-ietf-mmusic-rfc2326bis-07.txt Columbia U. July 19, 2004 A. Rao Expires: January, 2005 Cisco R. Lanphier RealNetworks M. Westerlund Ericsson A. Narasimhan Princeton Real Time Streaming Protocol (RTSP) STATUS OF THIS MEMO By submitting this Internet-Draft, I (we) certify that any applicable patent or other IPR claims of which I am (we are) aware have been disclosed, and any of which I (we) become aware will be disclosed, in accordance with RFC 3668 (BCP 79). Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. Abstract This memorandum is a revision of RFC 2326, which is currently a Proposed Standard. The Real Time Streaming Protocol, or RTSP, is an application-level protocol for control over the delivery of data with real-time properties. RTSP provides an extensible framework to enable controlled, on-demand delivery of real-time data, such as audio and video. Sources of data can include both live data feeds and stored clips. This protocol is intended to control multiple data delivery H. Schulzrinne et. al. [Page 1] Internet Draft RTSP July 19, 2004 sessions, provide a means for choosing delivery channels such as UDP, multicast UDP and TCP, and provide a means for choosing delivery mechanisms based upon RTP (RFC 3550). H. Schulzrinne et. al. [Page 2] Internet Draft RTSP July 19, 2004 Table of Contents 1 Introduction ........................................ 9 1.1 RTSP Specification Update ........................... 9 1.2 Purpose ............................................. 10 1.3 Notational Conventions .............................. 11 1.4 Terminology ......................................... 12 1.5 Protocol Properties ................................. 15 1.6 Extending RTSP ...................................... 17 1.7 Overall Operation ................................... 18 1.8 RTSP States ......................................... 19 1.9 Relationship with Other Protocols ................... 20 2 RTSP Use Cases ...................................... 20 2.1 On-demand Playback of Stored Content ................ 20 2.2 Unicast distribution of Live Content ................ 22 2.3 On-demand Playback using Multicast .................. 22 2.4 Inviting a RTSP server into a conference ............ 23 2.5 Live Content using Multicast ........................ 24 3 Protocol Parameters ................................. 25 3.1 RTSP Version ........................................ 25 3.2 RTSP URL ............................................ 25 3.3 Session Identifiers ................................. 26 3.4 SMPTE Relative Timestamps ........................... 27 3.5 Normal Play Time .................................... 27 3.6 Absolute Time ....................................... 28 3.7 Feature-tags ........................................ 28 3.8 Entity Tags ......................................... 29 4 RTSP Message ........................................ 29 4.1 Message Types ....................................... 29 4.2 Message Headers ..................................... 30 4.3 Message Body ........................................ 30 4.4 Message Length ...................................... 30 5 General Header Fields ............................... 30 6 Request ............................................. 30 6.1 Request Line ........................................ 31 6.2 Request Header Fields ............................... 32 7 Response ............................................ 32 7.1 Status-Line ......................................... 33 7.1.1 Status Code and Reason Phrase ....................... 33 7.1.2 Response Header Fields .............................. 35 8 Entity .............................................. 35 8.1 Entity Header Fields ................................ 35 8.2 Entity Body ......................................... 35 H. 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[Page 3] Internet Draft RTSP July 19, 2004 9 Connections ......................................... 36 9.1 Pipelining .......................................... 36 9.2 Reliability and Acknowledgements .................... 36 9.3 Unreliable Transport ................................ 39 9.4 The usage of connections ............................ 39 9.5 Timing Out RTSP messages ............................ 41 9.6 Use of IPv6 ......................................... 41 10 Capability Handling ................................. 42 11 Method Definitions .................................. 43 11.1 OPTIONS ............................................. 44 11.2 DESCRIBE ............................................ 45 11.3 SETUP ............................................... 47 11.4 PLAY ................................................ 50 11.5 PAUSE ............................................... 54 11.6 TEARDOWN ............................................ 58 11.7 GET_PARAMETER ....................................... 58 11.8 SET_PARAMETER ....................................... 59 11.9 REDIRECT ............................................ 61 11.10 PING ................................................ 63 12 Embedded (Interleaved) Binary Data .................. 64 13 Status Code Definitions ............................. 65 13.1 Success 1xx ......................................... 66 13.1.1 100 Continue ........................................ 66 13.2 Success 2xx ......................................... 66 13.3 Redirection 3xx ..................................... 66 13.3.1 TBW ................................................. 66 13.3.2 301 Moved Permanently ............................... 66 13.3.3 302 Found ........................................... 66 13.3.4 303 See Other ....................................... 67 13.3.5 304 Not Modified .................................... 67 13.3.6 305 Use Proxy ....................................... 67 13.4 Client Error 4xx .................................... 67 13.4.1 400 Bad Request ..................................... 67 13.4.2 405 Method Not Allowed .............................. 68 13.4.3 451 Parameter Not Understood ........................ 68 13.4.4 452 reserved ........................................ 68 13.4.5 453 Not Enough Bandwidth ............................ 68 13.4.6 454 Session Not Found ............................... 68 13.4.7 455 Method Not Valid in This State .................. 68 13.4.8 456 Header Field Not Valid for Resource ............. 68 13.4.9 457 Invalid Range ................................... 69 13.4.10 458 Parameter Is Read-Only .......................... 69 13.4.11 459 Aggregate Operation Not Allowed ................. 69 13.4.12 460 Only Aggregate Operation Allowed ................ 69 13.4.13 461 Unsupported Transport ........................... 69 13.4.14 462 Destination Unreachable ......................... 69 13.4.15 470 Connection Authorization Required ............... 69 13.4.16 471 Connection Credentials not accepted ............. 69 H. 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[Page 4] Internet Draft RTSP July 19, 2004 13.5 Server Error 5xx .................................... 69 13.5.1 551 Option not supported ............................ 70 14 Header Field Definitions ............................ 70 14.1 Accept .............................................. 72 14.2 Accept-Credentials .................................. 72 14.3 Accept-Encoding ..................................... 77 14.4 Accept-Language ..................................... 77 14.5 Accept-Ranges ....................................... 77 14.6 Allow ............................................... 77 14.7 Authorization ....................................... 77 14.8 Bandwidth ........................................... 78 14.9 Blocksize ........................................... 78 14.10 Cache-Control ....................................... 78 14.11 Connection .......................................... 81 14.12 Content-Base ........................................ 81 14.13 Content-Encoding .................................... 81 14.14 Content-Language .................................... 81 14.15 Content-Length ...................................... 81 14.16 Content-Location .................................... 81 14.17 Content-Type ........................................ 81 14.18 CSeq ................................................ 82 14.19 Date ................................................ 82 14.20 ETag ................................................ 82 14.21 Expires ............................................. 83 14.22 From ................................................ 83 14.23 Host ................................................ 84 14.24 If-Match ............................................ 84 14.25 If-Modified-Since ................................... 84 14.26 If-None-Match ....................................... 84 14.27 Last-Modified ....................................... 85 14.28 Location ............................................ 85 14.29 Proxy-Authenticate .................................. 85 14.30 Proxy-Require ....................................... 85 14.31 Public .............................................. 85 14.32 Range ............................................... 86 14.33 Referer ............................................. 88 14.34 Retry-After ......................................... 88 14.35 Require ............................................. 88 14.36 RTP-Info ............................................ 89 14.37 Scale ............................................... 91 14.38 Speed ............................................... 92 14.39 Server .............................................. 92 14.40 Session ............................................. 92 14.41 Supported ........................................... 94 14.42 Timestamp ........................................... 95 14.43 Transport ........................................... 95 14.44 Unsupported ......................................... 101 14.45 User-Agent .......................................... 102 H. 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[Page 5] Internet Draft RTSP July 19, 2004 14.46 Vary ................................................ 102 14.47 Via ................................................. 102 14.48 WWW-Authenticate .................................... 102 15 Caching ............................................. 102 16 Examples ............................................ 103 16.1 Media on Demand (Unicast) ........................... 103 16.2 Streaming of a Container file ....................... 106 16.3 Single Stream Container Files ....................... 109 16.4 Live Media Presentation Using Multicast ............. 111 16.5 Capability Negotiation .............................. 112 17 Security Framework .................................. 113 17.1 RTSP and HTTP Authentication ........................ 113 17.2 RTSP over TLS ....................................... 114 17.3 Security and Proxies ................................ 114 17.3.1 Accept-Credentials .................................. 115 17.3.2 User approved TLS procedure ......................... 116 18 Syntax .............................................. 118 18.1 Base Syntax ......................................... 118 18.2 RTSP Protocol Definition ............................ 119 18.2.1 Generic Protocol elements ........................... 119 18.2.2 Message Syntax ...................................... 120 18.2.3 Header Syntax ....................................... 124 19 Security Considerations ............................. 126 20 IANA Considerations ................................. 128 20.1 Feature-tags ........................................ 129 20.1.1 Description ......................................... 129 20.1.2 Registering New Feature-tags with IANA .............. 129 20.1.3 Registered entries .................................. 130 20.2 RTSP Methods ........................................ 130 20.2.1 Description ......................................... 130 20.2.2 Registering New Methods with IANA ................... 130 20.2.3 Registered Entries .................................. 131 20.3 RTSP Status Codes ................................... 131 20.3.1 Description ......................................... 131 20.3.2 Registering New Status Codes with IANA .............. 131 20.3.3 Registered Entries .................................. 131 20.4 RTSP Headers ........................................ 131 20.4.1 Description ......................................... 131 20.4.2 Registering New Headers with IANA ................... 131 20.4.3 Registered entries .................................. 132 20.5 Transport Header registries ......................... 132 20.5.1 Transport Protocols ................................. 132 20.5.2 Profile ............................................. 133 20.5.3 Lower Transport ..................................... 133 20.5.4 Transport modes ..................................... 134 20.6 Cache Directive Extensions .......................... 134 20.7 Accept-Credentials policies ......................... 135 20.8 SDP attributes ...................................... 135 H. 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[Page 6] Internet Draft RTSP July 19, 2004 A RTSP Protocol State Machine ......................... 136 A.1 States .............................................. 137 A.2 State variables ..................................... 137 A.3 Abbreviations ....................................... 137 A.4 State Tables ........................................ 137 B Media Transport Alternatives ........................ 141 B.1 RTP ................................................. 141 B.1.1 AVP ................................................. 141 B.1.2 AVP/UDP ............................................. 141 B.1.3 AVP/TCP ............................................. 143 B.1.4 Handling NPT Jumps in the RTP Media Layer ........... 143 B.1.5 Handling RTP Timestamps after PAUSE ................. 145 B.1.6 RTSP / RTP Integration .............................. 148 B.1.7 Scaling with RTP .................................... 148 B.1.8 Maintaining NPT synchronization with RTP timestamps .......................................... 148 B.1.9 Continuous Audio .................................... 148 B.1.10 Multiple Sources in an RTP Session .................. 148 B.1.11 Usage of SSRCs and the RTCP BYE Message During a RTSP Session ........................................ 148 B.2 Future Additions .................................... 149 C Use of SDP for RTSP Session Descriptions ............ 150 C.1 Definitions ......................................... 150 C.1.1 Control URL ......................................... 150 C.1.2 Media Streams ....................................... 151 C.1.3 Payload Type(s) ..................................... 152 C.1.4 Format-Specific Parameters .......................... 152 C.1.5 Range of Presentation ............................... 152 C.1.6 Time of Availability ................................ 153 C.1.7 Connection Information .............................. 153 C.1.8 Entity Tag .......................................... 153 C.2 Aggregate Control Not Available ..................... 154 C.3 Aggregate Control Available ......................... 155 C.4 RTSP external SDP delivery .......................... 156 D Minimal RTSP implementation ......................... 156 D.1 Client .............................................. 156 D.1.1 Basic Playback ...................................... 157 D.1.2 Authentication-enabled .............................. 157 D.2 Server .............................................. 157 D.2.1 Basic Playback ...................................... 158 D.2.2 Authentication-enabled .............................. 159 E Open Issues ......................................... 159 F Changes ............................................. 160 G Author Addresses .................................... 166 H Contributors ........................................ 167 I Acknowledgements .................................... 167 J Normative References ................................ 168 K Informative References .............................. 169 H. Schulzrinne et. al. [Page 7] Internet Draft RTSP July 19, 2004 H. Schulzrinne et. al. [Page 8] Internet Draft RTSP July 19, 2004 1 Introduction 1.1 RTSP Specification Update This document is a draft to an update of RTSP, a proposed standard | defined in RFC 2326 [1]. The goal the update is to progress RTSP to | draft standard status. Many flaws have been identified in RTSP since | its publication. While this draft tries to address these flaws, not | all known issues have been resolved. Appendix F catalogs the issues | that have already been addressed. Known open issues are listed in | appendix E. | The possibility of progressing RTSP to draft standard without | republishing RTSP as a proposed standard depends on the changes | necessary to make the protocol work. | A list of bugs against the specification is available at | "http://rtspspec.sourceforge.net". These bugs should be taken into | account when reading this specification. Input on the unresolved bugs | and other issues can be sent via e-mail to the MMUSIC WG's mailing | list mmusic@ietf.org and the authors. Not all of the contents of RFC 2326 are part of this draft. In an | attempt to prevent bloat, the specification has been reduced and | split. The content of this draft is the core specification of the protocol. It contains the general idea behind RTSP and the basic functionality necessary to establish an on-demand play-back session. It also contains the mechanisms for extending the protocol. Any other functionality will be published as extension documents. The Working | group is currently working on: o NAT and FW traversal mechanisms for RTSP are described in a document called "How to make Real-Time Streaming Protocol (RTSP) traverse Network Address Translators (NAT) and interact with Firewalls." [24]. There have also been discussion or proposals about the following extensions to RTSP: o Mute and Unmute Extension [25]. o RTSP Stream Switching [26]. o Live Streaming Relays [27]. o Unreliable transport of RTSP messages (rtspu). o The Record functionality. H. Schulzrinne et. al. [Page 9] Internet Draft RTSP July 19, 2004 o A text body type with suitable syntax for basic parameters to be used in SET_PARAMETER, and GET_PARAMETER. Including IANA registry within the defined name space. o A RTSP MIB. 1.2 Purpose The Real-Time Streaming Protocol (RTSP) establishes and controls single or several time-synchronized streams of continuous media such as audio and video. Put simply, RTSP acts as a "network remote control" for multimedia servers. There is no notion of a RTSP connection in the protocol. Instead, a RTSP server maintains a session labelled by an identifier to associate groups of media streams and their states. A RTSP session is not tied to a transport-level connection such as a TCP connection. During a session, a client may open and close many reliable transport connections to the server to issue RTSP requests for that session. This memorandum describes the use of RTSP over a reliable connection based transport level protocol such as TCP. RTSP may be implemented over an unreliable connectionless transport protocol such as UDP. While nothing in RTSP precludes this, additional definition of this problem area must be handled as an extension to the core specification. The mechanisms of RTSP's operation over UDP were left out of this spec. because they were poorly defined in RFC 2326 [1] and the tradeoff in size and complexity of this spec. for a small gain in a targeted problem space was not deemed justifiable. The set of streams to be controlled in a RTSP session is defined by a | presentation description. This memorandum does not define a format for the presentation description. However appendix C defines how SDP [2] is used for this purpose. The streams controlled by RTSP may use RTP [3] for their data transport, but the operation of RTSP does not depend on the transport mechanism used to carry continuous media. | RTSP is intentionally similar in syntax and operation to HTTP/1.1 [4] so that extension mechanisms to HTTP can in most cases also be added to RTSP. However, RTSP differs in a number of important aspects from HTTP: o RTSP introduces a number of new methods and has a different protocol identifier. H. Schulzrinne et. al. [Page 10] Internet Draft RTSP July 19, 2004 o RTSP has the notion of a session built into the protocol. o A RTSP server needs to maintain state by default in almost all cases, as opposed to the stateless nature of HTTP. o Both a RTSP server and client can issue requests. o Data is usually carried out-of-band by a different protocol. Session descriptions returned in a DESCRIBE response (see Section 11.2) and interleaving of RTP with RTSP over TCP are exceptions to this rule (see Section 12). o RTSP is defined to use ISO 10646 (UTF-8) rather than ISO 8859-1, consistent with HTML internationalization efforts [28]. o The Request-URL always contains the absolute URL. Because of backward compatibility with a historical blunder, HTTP/1.1 [4] carries only the absolute path in the request and puts the host name in a separate header field. This makes "virtual hosting" easier, where a single host with one IP address hosts several document trees. The protocol supports the following operations: Retrieval of media from media server: The client can either request a presentation description via RTSP DESCRIBE, HTTP or some other method. If the presentation is being multicast, the presentation description contains the multicast addresses and ports to be used for the continuous media. If the presentation is to be sent only to the client via unicast, the client provides the destination for security reasons. Invitation of a media server to a conference: A media server can be "invited" to join an existing conference to play back media into the presentation. This mode is useful for distributed teaching applications. Several parties in the conference may take turns "pushing the remote control buttons". RTSP requests may be handled by proxies, tunnels and caches as in HTTP/1.1 [4]. 1.3 Notational Conventions H. Schulzrinne et. al. [Page 11] Internet Draft RTSP July 19, 2004 Since many of the definitions and syntax are identical to HTTP/1.1, this specification only points to the section where they are defined rather than copying it. For brevity, [HX.Y] is to be taken to refer to Section X.Y of the current HTTP/1.1 specification (RFC 2616 [4]). All the mechanisms specified in this document are described in both prose and the augmented Backus-Naur form (BNF) described in detail in RFC 2234 [5]. Indented and smaller-type paragraphs are used to provide background | and motivation. This is intended to give readers who were not | involved with the formulation of the specification an understanding | of why things are the way they are in RTSP. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [6]. The word, unspecified, is used to indicate functionality or features | that are not defined in this specification. Such functionality cannot | be used in a standardized manner without further definition and | review in an extension specification to RTSP. 1.4 Terminology Some of the terminology has been adopted from HTTP/1.1 [4]. Terms not listed here are defined as in HTTP/1.1. Aggregate control: The concept of controlling multiple streams using a single timeline, generally maintained by the server. A client, for example, uses aggregate control when it issues a single play or pause message to simultaneously control both the audio and video in a movie. Aggregate control URL: The URL used in a RTSP request to refer to and control an aggregated session. It normally, but not always, corresponds to the presentation URL specified in the session description. See Section 11.3 for more information. Conference: a multiparty, multimedia presentation, where "multi" implies greater than or equal to one. Client: The client requests media service from the media server. Connection: A transport layer virtual circuit established between two programs for the purpose of communication. H. Schulzrinne et. al. [Page 12] Internet Draft RTSP July 19, 2004 Container file: A file which may contain multiple media streams | which often constitutes a presentation when played | together. The concept of a container file is not embedded | in the protocol. However, RTSP servers may offer aggregate | control on the media streams within these files. Continuous media: Data where there is a timing relationship between source and sink; that is, the sink must reproduce the timing relationship that existed at the source. The most common examples of continuous media are audio and motion video. Continuous media can be real-time (interactive), where there is a "tight" timing relationship between source and sink, or streaming (playback), where the relationship is less strict. Entity: The information transferred as the payload of a request or response. An entity consists of meta-information in the form of entity-header fields and content in the form of an entity-body, as described in Section 8. Feature-tag: A tag representing a certain set of functionality, i.e. a feature. Live: Normally used to describe a presentation or session with media coming from ongoing event. This generally results in that the session has a unbound or only loosely defined duration, and that no seek operations are possible. Media initialization: Datatype/codec specific initialization. This includes such things as clock rates, color tables, etc. Any transport-independent information which is required by a client for playback of a media stream occurs in the media initialization phase of stream setup. Media parameter: Parameter specific to a media type that may be changed before or during stream playback. Media server: The server providing playback services for one or more media streams. Different media streams within a presentation may originate from different media servers. A | media server may reside on the same host or on a different | host from which the presentation is invoked. Media server indirection: Redirection of a media client to a different media server. (Media) stream: A single media instance, e.g., an audio stream or a video stream as well as a single whiteboard or shared H. Schulzrinne et. al. [Page 13] Internet Draft RTSP July 19, 2004 application group. When using RTP, a stream consists of all RTP and RTCP packets created by a source within an RTP session. This is equivalent to the definition of a DSM-CC stream([29]). Message: The basic unit of RTSP communication, consisting of a structured sequence of octets matching the syntax defined in Section 18 and transmitted over a connection or a | connectionless transport. Non-Aggregated Control: Control of a single media stream. Only possible in RTSP sessions with a single media. Participant: Member of a conference. A participant may be a machine, e.g., a playback server. Presentation: A set of one or more streams presented to the client as a complete media feed and described by a | presentation description as defined below. In the RTSP | context, this generally implies aggregate control over the | streams, but does not necessarily have to. Presentation description: A presentation description contains information about one or more media streams within a presentation, such as the set of encodings, network addresses and information about the content. Other IETF protocols such as SDP (RFC 2327 [2]) use the term "session" for a presentation. The presentation description may take several different formats, including but not limited to the session description protocol format, SDP. | Response: A RTSP response. If an HTTP response is meant, that is indicated explicitly. Request: A RTSP request. If an HTTP request is meant, that is indicated explicitly. Request URL: The URL used in a request to indicate the resource | on which the request shall be performed. RTSP agent: Refers to either a RTSP client, a RTSP server, or a | RTSP Proxy. In this specification, there are many | capabilities that are common to these three entities such | as the capability to send requests or receive responses. | This term will be used when describing functionality that | is applicable to all three of these entities. H. Schulzrinne et. al. [Page 14] Internet Draft RTSP July 19, 2004 RTSP session: A stateful abstraction upon which the main control methods of RTSP operate. A RTSP session is a server entity; it is created, maintained and destroyed by the server. It is established by a RTSP server upon the completion of a successful SETUP request (when 200 OK response is sent) and is labelled by a session identifier at that time. The session exists until timed out by the server or explicitly removed by a TEARDOWN request. A RTSP session is a stateful | entity; a RTSP server maintains an explicit session state machine (see Appendix A) where most state transitions are triggered by client requests. The existence of a session implies the existence of state about the session's media streams and their respective transport mechanisms. A given session can have zero or more media streams associated with it. A RTSP server uses the session to aggregate control over multiple media streams. Transport initialization: The negotiation of transport information (e.g., port numbers, transport protocols) between the client and the server. URI: Universal Resource Identifier, see RFC 2396 [13]. In RTSP the used URIs are as general rule in fact URL's as they gives an location for the resource. Therefore although RTSP URLs are a subset of URIs, they will be refered as URLs. URL: Universal Resource Locator, is an URI which identifies the resource through its primary access mechanism, rather than identifying the resource by name or by some other attribute(s) of that resource. 1.5 Protocol Properties RTSP has the following properties: Extendable: New methods and parameters can be easily added to RTSP. Easy to parse: RTSP can be parsed by standard HTTP or MIME parsers. Secure: RTSP re-uses web security mechanisms, either at the transport level (TLS, RFC 2246 [7]) or within the protocol itself. All HTTP authentication mechanisms such as basic (RFC 2616 [4]) and digest authentication (RFC 2617 [8]) are directly applicable. Transport-independent: RTSP does not preclude the use of an H. Schulzrinne et. al. [Page 15] Internet Draft RTSP July 19, 2004 unreliable datagram protocol (UDP) (RFC 768 [9]), a reliable datagram protocol (RDP, RFC 1151, not widely used [30]) as it would be possible to implement application- level reliability. The use of a connectionless datagram protocol such as UDP or RDP requires additional definition that may be provided as extensions to the core RTSP specification. The usage of the reliable stream protocol TCP (RFC 793 [10]) is what is currently defined as transport protocol of RTSP messages. Multi-server capable: Each media stream within a presentation can reside on a different server. The client automatically establishes several concurrent control sessions with the different media servers. Media synchronization is performed at the transport level. Separation of stream control and conference initiation: Stream control is divorced from inviting a media server to a conference. In particular, SIP [31] or H.323 [32] may be used to invite a server to a conference. Suitable for professional applications: RTSP supports frame- level accuracy through SMPTE time stamps to allow remote digital editing. Presentation description neutral: The protocol does not impose a particular presentation description or metafile format and can convey the type of format to be used. However, the presentation description must contain at least one RTSP URL. Proxy and firewall friendly: The protocol should be readily handled by both application and transport-layer (SOCKS [33]) firewalls. A firewall may need to understand the SETUP method to open a "hole" for the media stream. HTTP-friendly: Where sensible, RTSP reuses HTTP concepts, so that the existing infrastructure can be reused. This infrastructure includes PICS (Platform for Internet Content Selection [34,35]) for associating labels with content. However, RTSP does not just add methods to HTTP since the controlling continuous media requires server state in most cases. Appropriate server control: If a client can start a stream, it must be able to stop a stream. Servers should not start streaming to clients in such a way that clients cannot stop the stream. H. Schulzrinne et. al. [Page 16] Internet Draft RTSP July 19, 2004 Transport negotiation: The client can negotiate the transport method prior to actually needing to process a continuous media stream. An earlier requirement in RTSP was multi-client capability. However, it was determined that a better approach was to make sure that the protocol is easily extensible to the multi-client scenario. Stream identifiers can be used by several control streams, so that "passing the remote" would be possible. The protocol would not address how several clients negotiate access; this is left to either a "social protocol" or some other floor control mechanism. 1.6 Extending RTSP Since not all media servers have the same functionality, media servers by necessity will support different sets of requests. For example: o A server may not be capable of seeking (absolute positioning) if it is to support live events only. o Some servers may not support setting stream parameters and thus not support GET_PARAMETER and SET_PARAMETER. A server SHOULD implement all header fields described in Section 14. It is up to the creators of presentation descriptions not to ask the impossible of a server. This situation is similar in HTTP/1.1 [4], where the methods described in [H19.5] are not likely to be supported across all servers. RTSP can be extended in three ways, listed here in order of the magnitude of changes supported: o Existing methods can be extended with new parameters, e.g. headers, as long as these parameters can be safely ignored by the recipient. If the client needs negative acknowledgement | when a method extension is not supported, a tag corresponding to the extension may be added in the Require: field (see Section 14.35). o New methods can be added. If the recipient of the message does not understand the request, it responds with error code 501 (Not Implemented) and the sender should not attempt to use this method again. A client may also use the OPTIONS method to inquire about methods supported by the server. The server MUST H. Schulzrinne et. al. [Page 17] Internet Draft RTSP July 19, 2004 list the methods it supports using the Public response header. o A new version of the protocol can be defined, allowing almost all aspects (except the position of the protocol version number) to change. The basic capability discovery mechanism can be used to both discover support for a certain feature and to ensure that a feature is available when performing a request. For detailed explanation of this see chapter 10. 1.7 Overall Operation Each presentation and media stream is identified by an RTSP URL. The overall presentation and the properties of the media the presentation is made up of are defined by a presentation description file, the format of which is outside the scope of this specification. The presentation description file may be obtained by the client using HTTP or other means such as email and may not necessarily be stored on the media server. For the purposes of this specification, a presentation description is assumed to describe one or more presentations, each of which maintains a common time axis. For simplicity of exposition and without loss of generality, it is assumed that the presentation description contains exactly one such presentation. A presentation may contain several media streams. The presentation description file contains a description of the media streams making up the presentation, including their encodings, language, and other parameters that enable the client to choose the most appropriate combination of media. In this presentation description, each media stream that is individually controllable by RTSP is identified by a RTSP URL, which points to the media server handling that particular media stream and names the stream stored on that server. Several media streams can be located on different servers; for example, audio and video streams can be split across servers for load sharing. The description also enumerates which transport methods the server is capable of. Besides the media parameters, the network destination address and port need to be determined. Several modes of operation can be distinguished: Unicast: The media is transmitted to the source of the RTSP request, with the port number chosen by the client. Alternatively, the media is transmitted on the same reliable stream as RTSP. H. Schulzrinne et. al. [Page 18] Internet Draft RTSP July 19, 2004 Multicast, server chooses address: The media server picks the multicast address and port. This is the typical case for a live or near-media-on-demand transmission. Multicast, client chooses address: If the server is to participate in an existing multicast conference, the multicast address, port and encryption key are given by the conference description, established by means outside the scope of this specification. 1.8 RTSP States RTSP controls a stream which may be sent via a separate protocol, independent of the control channel. For example, RTSP control may occur on a TCP connection while the data flows via UDP. Thus, data delivery continues even if no RTSP requests are received by the media server. Also, during its lifetime, a single media stream may be controlled by RTSP requests issued sequentially on different TCP connections. Therefore, the server needs to maintain "session state" to be able to correlate RTSP requests with a stream. The state transitions are described in Appendix A. Many methods in RTSP do not contribute to state. However, the following play a central role in defining the allocation and usage of stream resources on the server: SETUP, PLAY, PAUSE, REDIRECT, PING and TEARDOWN. SETUP: Causes the server to allocate resources for a stream and create a RTSP session. PLAY: Starts data transmission on a stream allocated via SETUP. PAUSE: Temporarily halts a stream without freeing server resources. REDIRECT: Indicates that the session should be moved to new server / location PING: Prevents the identified session from being timed out. TEARDOWN: Frees resources associated with the stream. The RTSP session ceases to exist on the server. RTSP methods that contribute to state use the Session header field (Section 14.40) to identify the RTSP session whose state is being manipulated. The server generates session identifiers in response to SETUP requests (Section 11.3). H. Schulzrinne et. al. [Page 19] Internet Draft RTSP July 19, 2004 1.9 Relationship with Other Protocols RTSP has some overlap in functionality with HTTP. It also may interact with HTTP in that the initial contact with streaming content is often to be made through a web page. The current protocol specification aims to allow different hand-off points between a web server and the media server implementing RTSP. For example, the presentation description can be retrieved using HTTP or RTSP, which reduces round trips in web-browser-based scenarios, yet also allows for stand alone RTSP servers and clients which do not rely on HTTP at all. However, RTSP differs fundamentally from HTTP in that most data delivery takes place out-of-band in a different protocol. HTTP is an asymmetric protocol where the client issues requests and the server responds. In RTSP, both the media client and media server can issue requests. RTSP requests are also stateful; they may set parameters and continue to control a media stream long after the request has been acknowledged. Re-using HTTP functionality has advantages in at least two areas, namely security and proxies. The requirements are very similar, so having the ability to adopt HTTP work on caches, proxies and authentication is valuable. RTSP assumes the existence of a presentation description format that can express both static and temporal properties of a presentation containing several media streams. Session Description Protocol (SDP) [2] is generally the format of choice; however, RTSP is not bound to it. For data delivery, most real-time media will use RTP as a transport protocol. While RTSP works well with RTP, it is not tied to RTP. 2 RTSP Use Cases This section describes some of the use cases for RTSP. They are | listed in descending order of importance in regards to ensuring that | all necessary functionality is present. This specification does only | fully support usage of the two first. Also in these first two cases | are there special cases that will not be supported without | extensions, e.g. the redirection of media to another address than the | controlling entity. | 2.1 On-demand Playback of Stored Content | An RTSP capable server stores content suitable for being streamed to | a client. A client desiring playback of any of the stored content | uses RTSP to set up the media transport required for the desired | H. Schulzrinne et. al. [Page 20] Internet Draft RTSP July 19, 2004 content. Then RTSP is used to initiate, halt and manipulate the | transmission of the content. There are also requirement on being able | to use RTSP to carry necessary description and synchronization | information for the content. | The above high level description can be broken down into a number of | functionalities that RTSP needs to be capable of. | Presentation Description: The possibility to carry | initialization information about the presentation | (content), for example, which media codec(s) that are | needed for the content. Other information that are | important; how many media stream that the presentation | contains; what transport protocols to use for the media | streams; and identifiers for these media streams. This | information is required before setup of the content is | possible. The information is also needed by the client to | determine if it is capable at all to support the content. | This information is not required to be sent using RTSP, | instead other external protocols can be utilized to | transport presentation descriptions. Two good examples are | the use of HTTP [4] or email to fetch or receive | presentation descriptions like SDP [2]. | Setup: Performing setup of some or all of the media streams in a | presentation. The setup itself consist of determining which | protocols for media transport to use; the necessary | parameters for the protocol, like addresses and ports. | Control of Transmission: After the necessary media streams has | been established the client can request the server to start | transmitting the content. There is need to allow the client | to arbitrary times start or stop the transmission of the | content. There are also exist need to be able to start the | transmission at an any point in the timeline of the | presentation. | Synchronization: For media transport protocols like RTP [18] it | might be beneficial to carry synchronization information | within RTSP. Either due to the lack of inter media | synchronization within the protocol itself, or the | potential delay before the synchronization is established | (which is the case for RTP when using RTCP). | Termination There is also need to be able to terminate the | established contexts. | H. Schulzrinne et. al. [Page 21] Internet Draft RTSP July 19, 2004 For this use cases there is a number of assumption about how it | works. These are listed below: | On-Demand content: The content available is stored at the server | and can be accessed at any time during a time period when | it is intended to be available. | Independent sessions: A server is capable of serving a number of | clients simultaneously, including from the same piece of | content at different points in that presentations time- | line. | Unicast Transport: Content for each individual client is | transmitted to them using unicast traffic. | It is also possible to redirect the media traffic to another | destination than where the entity controlling traffic uses. However | allowing this without appropriate mechanisms for checking that the | destination approves of this is a denial of service threat. | 2.2 Unicast distribution of Live Content | This use cases is not that different from the above on-demand content | case (see section 2.1. The difference is really the restriction the | content itself establish. Live content is continuously distributed | as it becomes available from a source, i.e. the main difference to | on-demand is that one starts distributing content before the end of | it has become available to the server. | In many cases the consumer of live content is only interested in | consuming what is actually happens "now", i.e. very similar to | broadcast TV. However in this case it is assumed that there exist no | broadcast or multicast channel to the users, and instead the server | functions as a distribution node, sending the same content to | multiple receivers, using unicast traffic between server and client. | This unicast traffic and the transport parameters are individually | negotiated for each receiving client. | Another aspect of live content is that it has often very limited time | of availability, as it is only is available for the duration of the | event the content covers. A example of such a live content could for | example be a music concert, which lasts 2 hour and starts at a | predetermined time. Thus there is need to announce when and for how | long the live content is available. | 2.3 On-demand Playback using Multicast | It is possible to use RTSP to request that media is delivered to a | H. Schulzrinne et. al. [Page 22] Internet Draft RTSP July 19, 2004 multicast group. The entity setting up the session (the controller) | will then control when and what media that is delivered to the group. | Also this use case has some potential for denial of service attacks, | in this case flooding any multicast group. Therefore there is need | for a mechanism indicating that the group actually accepts the | traffic from the RTSP server. | An open issue in this use case is how one ensures that all receivers | listening to the multicast or broadcast receives the session | presentation configuring the receivers. | 2.4 Inviting a RTSP server into a conference | If one has an established conference or group session, it is possible | to have a RTSP server distribute media to the whole group. The | transmission to the group is simplest controlled by a single | participant or leader of the conference. Shared control might be | possible, but would require further investigation and possibly | extensions. There are some protocol mechanisms missing for this | scenario. | For reasonable complexity in the media transmission stage, this use | case assumes that there exist either multicast or a conference focus | that redistribute media to all participants. | In some more detail, this use case is intended to be able to handle | the following scenario: A conference leader or participant (from here | called the controller) has some pre-stored content on a RTSP server | that he likes to share with the group. The controller sets up a RTSP | session at the streaming server for the content the controller likes | to share. The session description for the content is retrieved to the | controller. The media destination for the media content is set to the | shared multicast group or conference focus. When desired by the | controller, he/she can start and stop the transmission of the media | to the conference group. | There are several issues with this use case that is not solved by | this core specification for RTSP: | o Denial of service threat, to avoid a RTSP server from being a | unknowing participant of a denial of service attack the server | must be able to verify the destinations acceptance for the | media. Such a mechanism does not yet exist that can be used to | verify the approval to received media, instead only policies | can be used, which can be made to work in controlled | environments. | o The problem of distributing the presentation description to | H. Schulzrinne et. al. [Page 23] Internet Draft RTSP July 19, 2004 all participants in the group. To enable a media receiver to | decode the content correctly the media configuration | information will need to be distributed reliable to all | participants. This will most likely require support from an | external protocol. | o Passing the control. If it is desired to be able to pass the | control of the RTSP session between the participants some | support will be required by an external protocol for the | necessary exchange of state information and possibly floor | control of who is controlling the RTSP session. | So if there interest in this use case further work on the necessary | extensions has to be performed. | 2.5 Live Content using Multicast | This use case does in its simplest form not require any use of RTSP | at all. This is what multicast conferences being announce with SAP | and SDP are intended to handle. However in use cases where more | advance features like access control to the multicast session is | desired, RTSP could be used for session establishment. | A client desiring to join a live multicasted media session with | cryptographic (encryption) access control could use RTSP in the | following way. The source of the session, announces the session and | gives all interested to join, a RTSP URI. The client connects to the | server and requests the presentation description allowing for | configuration the reception. In this step it is possible to use | secured transport for the client, and also desired levels of | authentication, for example for charging purposes or simply access | control. An RTSP link also allows for load balancing between multiple | servers. However if this the only thing that occurs it can probably | be solved as simple using HTTP. | However for session where the sender likes to keep track of each | individual receiver during the session, and possibly use this side | channel for pushing out key-updates or other side information that is | desirable to be done on a per receiver basis, and the receivers are | not know prior to the session start, the state establishment that | RTSP provides can be beneficial. In this case a client would | establish a RTSP session to the multicast group. The RTSP server will | not transmit any media, instead it will simply point to the multicast | group. However the client and server will be able to keep the session | alive for as long as the receiver participates in the session. Thus | enabling for example server to client pushes of updates. | This use cases will most likely not be able to actually implement | H. Schulzrinne et. al. [Page 24] Internet Draft RTSP July 19, 2004 some extensions in relation to the server to client push mechanism. | Here a method like ANNOUNCE might be suitable, however it will | require a RTSP extension to revive the method. 3 Protocol Parameters 3.1 RTSP Version HTTP Specification Section [H3.1] applies, with HTTP replaced by RTSP. This specification defines version 1.0 of RTSP. 3.2 RTSP URL The "rtsp", "rtsps" and "rtspu" schemes are used to refer to network resources via the RTSP protocol. This section defines the scheme- specific syntax and semantics for RTSP URLs. The RTSP URL is case sensitive. Informative RTSP URL syntax: rtsp[u|s]://host[:port]/abspath[?query]#fragment See section 18.2.1 for the formal definition of the RTSP URL syntax. Note that fragment and query identifiers do not have a well-defined meaning at this time, i.e. their usage is unspecified, with the interpretation left to the RTSP server. The URL scheme rtsp requires that commands are issued via a reliable | protocol (within the Internet, TCP), while the scheme rtspu is | intended to identify RTSP over an unreliable protocol (within the | Internet, UDP). The scheme rtsps identifies a reliable transport | using secure transport (TLS [7]). The rtspu is not defined in this | specification, and are for future extensions of the protocol to | define how to use. If the port is empty or not given, port 554 SHALL be assumed. The semantics are that the identified resource can be controlled by RTSP at the server listening for TCP (scheme "rtsp") connections or UDP (scheme "rtspu") packets on that port of host, and the Request-URL for the resource is rtsp_URL. For the scheme rtsps the TCP and UDP port 322 is registered and SHALL be assumed. The use of IP addresses in URLs SHOULD be avoided whenever possible (see RFC 1924 [11]). Note: Using qualified domain names in any URL is H. Schulzrinne et. al. [Page 25] Internet Draft RTSP July 19, 2004 one requirement for making it possible for RFC 2326 implementations of RTSP to use IPv6. This specification is updated to allow for literal IPv6 addresses in RTSP URLs using the host specification in RFC 2732 [12]. A presentation or a stream is identified by a textual media identifier, using the character set and escape conventions [H3.2] of URLs (RFC 2396 [13]). URLs may refer to a stream or an aggregate of streams, i.e., a presentation. Accordingly, requests described in Section 11 can apply to either the whole presentation or an individual stream within the presentation. Note that some request methods can only be applied to streams, not presentations and vice versa. For example, the RTSP URL: rtsp://media.example.com:554/twister/audiotrack identifies the audio stream within the presentation "twister", which can be controlled via RTSP requests issued over a TCP connection to port 554 of host media.example.com Also, the RTSP URL: rtsp://media.example.com:554/twister identifies the presentation "twister", which may be composed of audio and video streams. This does not imply a standard way to reference streams in URLs. The presentation description defines the hierarchical relationships in the presentation and the URLs for the individual streams. A presentation description may name a stream "a.mov" and the whole presentation "b.mov". The path components of the RTSP URL are opaque to the client and do not imply any particular file system structure for the server. This decoupling also allows presentation descriptions to be used with non-RTSP media control protocols simply by replacing the scheme in the URL. 3.3 Session Identifiers H. Schulzrinne et. al. [Page 26] Internet Draft RTSP July 19, 2004 Session identifiers are strings of any arbitrary length. A session identifier MUST be chosen randomly and MUST be at least eight characters long to make guessing it more difficult. (See Section 19.) 3.4 SMPTE Relative Timestamps A SMPTE relative timestamp expresses time relative to the start of the clip. Relative timestamps are expressed as SMPTE time codes for frame-level access accuracy. The time code has the format hours:minutes:seconds:frames.subframes, with the origin at the start of the clip. The default smpte format is"SMPTE 30 drop" format, with frame rate is 29.97 frames per second. Other SMPTE codes MAY be supported (such as "SMPTE 25") through the use of alternative use of "smpte time". For the "frames" field in the time value can assume the values 0 through 29. The difference between 30 and 29.97 frames per second is handled by dropping the first two frame indices (values 00 and 01) of every minute, except every tenth minute. If the frame value is zero, it may be omitted. Subframes are measured in one-hundredth of a frame. Examples: smpte=10:12:33:20- smpte=10:07:33- smpte=10:07:00-10:07:33:05.01 smpte-25=10:07:00-10:07:33:05.01 3.5 Normal Play Time Normal play time (NPT) indicates the stream absolute position relative to the beginning of the presentation, not to be confused with the Network Time Protocol (NTP). The timestamp consists of a decimal fraction. The part left of the decimal may be expressed in either seconds or hours, minutes, and seconds. The part right of the decimal point measures fractions of a second. The beginning of a presentation corresponds to 0.0 seconds. Negative values are not defined. The special constant now is defined as the current instant of a live type event. It MAY only be used for live type events, and SHALL NOT be used for on-demand content. NPT is defined as in DSM-CC: "Intuitively, NPT is the clock the viewer associates with a program. It is often digitally displayed on a VCR. NPT advances normally when in normal play mode (scale = 1), advances at a faster rate when in fast scan forward (high positive scale ratio), decrements when in scan reverse (high negative scale H. Schulzrinne et. al. [Page 27] Internet Draft RTSP July 19, 2004 ratio) and is fixed in pause mode. NPT is (logically) equivalent to SMPTE time codes." [29] Examples: npt=123.45-125 npt=12:05:35.3- npt=now- The syntax conforms to ISO 8601. The npt-sec notation is optimized for automatic generation, the ntp-hhmmss notation for consumption by human readers. The "now" constant allows clients to request to receive the live feed rather than the stored or time-delayed version. This is needed since neither absolute time nor zero time are appropriate for this case. 3.6 Absolute Time Absolute time is expressed as ISO 8601 timestamps, using UTC (GMT). Fractions of a second may be indicated. Example for November 8, 1996 at 14h37 and 20 and a quarter seconds UTC: 19961108T143720.25Z 3.7 Feature-tags Feature-tags are unique identifiers used to designate features in RTSP. These tags are used in Require (Section 14.35), Proxy-Require (Section 14.30), Unsupported (Section 14.44), and Supported (Section 14.41) header fields. Feature tag needs to indicate if they apply to servers only, proxies only, or both server and proxies. The creator of a new RTSP feature-tag should either prefix the feature-tag with a reverse domain name (e.g., "com.example.mynewfeature" is an apt name for a feature whose inventor can be reached at "example.com"), or register the new feature-tag with the Internet Assigned Numbers Authority (IANA), see IANA Section 20. H. Schulzrinne et. al. [Page 28] Internet Draft RTSP July 19, 2004 The usage of feature tags are further described in section 10 that | deals with capability handling. | 3.8 Entity Tags | Entity tags opaque strings that are used to compare two entities from | the same resource, for example in caches or to optimize setup after a | redirect. Further explanation is present in [H3.11]. For explanation | on how to compare Entity tags see [H13.3]. Entity tags can be carried | in the ETag header (see section 14.20) or in SDP (see section C.1.8). | Entity tags are used in RTSP to make some methods conditional. The | methods are made conditional through the inclusion of headers, see | 14.24 and 14.26. 4 RTSP Message RTSP is a text-based protocol and uses the ISO 10646 character set in UTF-8 encoding (RFC 2279 [14]). Lines are terminated by CRLF, but receivers should be prepared to also interpret CR and LF by themselves as line terminators. Text-based protocols make it easier to add optional parameters in a self-describing manner. Since the number of parameters and the frequency of commands is low, processing efficiency is not a concern. Text-based protocols, if done carefully, also allow easy implementation of research prototypes in scripting languages such as Tcl, Visual Basic and Perl. The 10646 character set avoids tricky character set switching, but is invisible to the application as long as US-ASCII is being used. This is also the encoding used for RTCP. ISO 8859-1 translates directly into Unicode with a high-order octet of zero. ISO 8859-1 characters with the most-significant bit set are represented as 1100001x 10xxxxxx. (See RFC 2279 [14]) RTSP messages can be carried over any lower-layer transport protocol that is 8-bit clean. RTSP messages are vulnerable to bit errors and SHOULD NOT be subjected to them. Requests contain methods, the object the method is operating upon and parameters to further describe the method. Methods are idempotent, unless otherwise noted. Methods are also designed to require little or no state maintenance at the media server. 4.1 Message Types H. Schulzrinne et. al. [Page 29] Internet Draft RTSP July 19, 2004 See [H4.1]. 4.2 Message Headers See [H4.2]. 4.3 Message Body See [H4.3] 4.4 Message Length When a message body is included with a message, the length of that body is determined by one of the following (in order of precedence): 1. Any response message which MUST NOT include a message body (such as the 1xx, 204, and 304 responses) is always terminated by the first empty line after the header fields, regardless of the entity-header fields present in the message. (Note: An empty line consists of only CRLF.) 2. If a Content-Length header field (section 14.15) is present, its value in bytes represents the length of the message-body. If this header field is not present, a value of zero is assumed. Note that RTSP does not (at present) support the HTTP/1.1 "chunked" transfer coding(see [H3.6.1]) and requires the presence of the Content-Length header field. Given the moderate length of presentation descriptions returned, the server should always be able to determine its length, even if it is generated dynamically, making the chunked transfer encoding unnecessary. 5 General Header Fields See [H4.5], except that Pragma, Trailer, Transfer-Encoding, Upgrade, and Warning headers are not defined. RTSP further defines the CSeq, and Timestamp. The general headers are listed in table 1: 6 Request A request message from a client to a server or vice versa includes, | within the first line (Request Line) of that message, the method to | be applied to the resource, the identifier of the resource, and the | H. Schulzrinne et. al. [Page 30] Internet Draft RTSP July 19, 2004 Header Name Comment _________________________________ Cache-Control See section 14.10 Connection See section 14.11 CSeq See section 14.18 Date See section 14.19 Supported See section 14.41 Timestamp See section 14.42 Via See section 14.47 Table 1: The General headers used in RTSP. protocol version in use. Then follows zero or more headers that can | be of general (Section 5), request (Section 6.2), or entity (Section | 8.1) type. Then an empty line, i.e. a line with only the two | characters Carriage Return (CR) and Line Feed (LF), indicates the end | of the header part. Optionally a message body (entity) follows to the | end of the message. The length of the message body is indicated | through the Content-Length entity header. 6.1 Request Line The request line, provides the most important things about the request: What method, on what resources and using which RTSP version. The methods that is defined by this specification can be seen in Table 6.1. The resource is identified through an absolute RTSP URL (see section 3.2. SP SP CRLF Please note: The request line's syntax can't be freely changed in | future versions of RTSP, as this line indicates the version of the | messages and need to be parsable also by older versions. Note that in contrast to HTTP/1.1 [4], RTSP requests always contain the absolute URL (that is, including the scheme, host and port) rather than just the absolute path. HTTP/1.1 requires servers to understand the absolute URL, but clients are supposed to use the Host request header. This is purely needed for backward-compatibility with HTTP/1.0 servers, a consideration that does not apply to RTSP. H. Schulzrinne et. al. [Page 31] Internet Draft RTSP July 19, 2004 Method Defined In Section _________________________________ DESCRIBE Section 11.2 GET_PARAMETER Section 11.7 OPTIONS Section 11.1 PAUSE Section 11.5 PLAY Section 11.4 PING Section 11.10 REDIRECT Section 11.9 SETUP Section 11.3 SET_PARAMETER Section 11.8 TEARDOWN Section 11.6 Table 2: The RTSP Methods The asterisk "*" in the Request-URL means that the request does not apply to a particular resource, but to the server or proxy itself, and is only allowed when the method used does not necessarily apply to a resource. One example would be as follows: OPTIONS * RTSP/1.0 An OPTIONS in this form will determine the capabilities of the server or the proxy that first receives the request. If one needs to address the server explicitly, then one should use an absolute URL with the server's address. OPTIONS rtsp://example.com RTSP/1.0 6.2 Request Header Fields The RTSP headers in Table 3 can be included in a request with the purpose to give further define how the request should be fulfilled. A request header MAY also be response header, see section 7.1.2. 7 Response H. Schulzrinne et. al. [Page 32] Internet Draft RTSP July 19, 2004 Header Defined in Section _____________________________________ Accept Section 14.1 Accept-Encoding Section 14.3 Accept-Language Section 14.4 Authorization Section 14.7 Bandwidth Section 14.8 Blocksize Section 14.9 From Section 14.22 If-Match Section 14.24 If-Modified-Since Section 14.25 If-None-Match Section 14.26 Proxy-Require Section 14.30 Range Section 14.32 Referer Section 14.33 Require Section 14.35 Scale Section 14.37 Session Section 14.40 Speed Section 14.38 Supported Section 14.41 Transport Section 14.43 User-Agent Section 14.45 Table 3: The RTSP request headers [H6] applies except that HTTP-Version is replaced by RTSP-Version. Also, RTSP defines additional status codes and does not define some HTTP codes. The valid response codes and the methods they can be used with are defined in Table 4. After receiving and interpreting a request message, the recipient responds with an RTSP response message. 7.1 Status-Line The first line of a Response message is the Status-Line, consisting of the protocol version followed by a numeric status code, and the textual phrase associated with the status code, with each element separated by SP characters. No CR or LF is allowed except in the final CRLF sequence. SP SP CRLF 7.1.1 Status Code and Reason Phrase H. Schulzrinne et. al. [Page 33] Internet Draft RTSP July 19, 2004 The Status-Code element is a 3-digit integer result code of the attempt to understand and satisfy the request. These codes are fully defined in Section 13. The Reason-Phrase is intended to give a short textual description of the Status-Code. The Status-Code is intended for use by automata and the Reason-Phrase is intended for the human user. The client is not required to examine or display the Reason- Phrase. The first digit of the Status-Code defines the class of response. The last two digits do not have any categorization role. There are 5 values for the first digit: o 1xx: Informational - Request received, continuing process o 2xx: Success - The action was successfully received, understood, and accepted o 3rr: Redirection - Further action must be taken in order to complete the request o 4xx: Client Error - The request contains bad syntax or cannot be fulfilled o 5xx: Server Error - The server failed to fulfill an apparently valid request The individual values of the numeric status codes defined for RTSP/1.0, and an example set of corresponding Reason-Phrase's, are presented in table 4. The reason phrases listed here are only recommended they may be replaced by local equivalents without affecting the protocol. Note that RTSP adopts most HTTP/1.1 [4] status codes and adds RTSP-specific status codes starting at x50 to avoid conflicts with newly defined HTTP status codes. RTSP status codes are extensible. RTSP applications are not required to understand the meaning of all registered status codes, though such understanding is obviously desirable. However, applications MUST understand the class of any status code, as indicated by the first digit, and treat any unrecognized response as being equivalent to the x00 status code of that class, with the exception that an unrecognized response MUST NOT be cached. For example, if an unrecognized status code of 431 is received by the client, it can safely assume that there was something wrong with its request and treat the response as if it had received a 400 status code. In such cases, user agents SHOULD present to the user the entity returned with the response, since that entity is likely to include human- readable information which will explain the unusual status. H. Schulzrinne et. al. [Page 34] Internet Draft RTSP July 19, 2004 7.1.2 Response Header Fields The response-header fields allow the request recipient to pass additional information about the response which cannot be placed in the Status-Line. These header fields give information about the server and about further access to the resource identified by the Request-URL. All headers currently being classified as response headers are listed in table 5. Response-header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields MAY be given the semantics of response- header fields if all parties in the communication recognize them to be response-header fields. Unrecognized header fields are treated as entity-header fields. 8 Entity Request and Response messages MAY transfer an entity if not otherwise restricted by the request method or response status code. An entity consists of entity-header fields and an entity-body, although some responses will only include the entity-headers. The SET_PARAMETER, and GET_PARAMETER request and response, and DESCRIBE response MAY have an entity. All 4xx and 5xx responses MAY also have an entity. In this section, both sender and recipient refer to either the client or the server, depending on who sends and who receives the entity. 8.1 Entity Header Fields Entity-header fields define optional meta-information about the entity-body or, if no body is present, about the resource identified by the request. The entity header fields are listed in table 8.1. The extension-header mechanism allows additional entity-header fields to be defined without changing the protocol, but these fields cannot be assumed to be recognizable by the recipient. Unrecognized header fields SHOULD be ignored by the recipient and forwarded by proxies. 8.2 Entity Body See [H7.2] with the addition that a RTSP message with an entity body MUST include the Content-Type and Content-Length headers. H. Schulzrinne et. al. [Page 35] Internet Draft RTSP July 19, 2004 9 Connections RTSP requests can be transmitted in several different ways: o persistent transport connections used for several request- response transactions; o one connection per request/response transaction; o connectionless mode. The type of transport is defined by the RTSP URL (Section 3.2). For the scheme "rtsp", a connection is assumed, while the scheme "rtspu" calls for RTSP requests to be sent without setting up a connection. Unlike HTTP, RTSP allows the media server to send requests to the media client. However, this is only supported for persistent connections, as the media server otherwise has no reliable way of reaching the client. Also, this is the only way that requests from media server to client are likely to traverse firewalls. 9.1 Pipelining A client that supports persistent connections or connectionless mode MAY "pipeline" its requests (i.e., send multiple requests without waiting for each response). A server MUST send its responses to those requests in the same order that the requests were received. 9.2 Reliability and Acknowledgements The transmission of RTSP over UDP was optionally to implement and specified in RFC 2326. However that definition was not satisfactory for interoperable implementations. Due to lack of interest, this specification does not specify how RTSP over UDP shall be implemented. However to maintain backwards compatibility in the message format certain RTSP headers must be maintained. These mechanism are described below. The next section Unreliable Transport (section 9.3) provides documentation of certain features that are necessary for transport protocols like UDP. Any RTSP request according to this specification SHALL NOT be sent to a multicast address. Any RTSP request SHALL be acknowledged. If a reliable transport protocol is used to carry RTSP, requests MUST NOT be retransmitted; the RTSP application MUST instead rely on the underlying transport to provide reliability. If both the underlying reliable transport such as TCP and H. Schulzrinne et. al. [Page 36] Internet Draft RTSP July 19, 2004 Code Reason Method __________________________________________________________ 100 Continue all __________________________________________________________ 200 OK all 201 Created RECORD 250 Low on Storage Space RECORD __________________________________________________________ 300 Multiple Choices all 301 Moved Permanently all 302 Found all 303 See Other all 305 Use Proxy all __________________________________________________________ 400 Bad Request all 401 Unauthorized all 402 Payment Required all 403 Forbidden all 404 Not Found all 405 Method Not Allowed all 406 Not Acceptable all 407 Proxy Authentication Required all 408 Request Timeout all 410 Gone all 411 Length Required all 412 Precondition Failed DESCRIBE, SETUP 413 Request Entity Too Large all 414 Request-URL Too Long all 415 Unsupported Media Type all 451 Parameter Not Understood SET_PARAMETER 452 reserved n/a 453 Not Enough Bandwidth SETUP 454 Session Not Found all 455 Method Not Valid In This State all 456 Header Field Not Valid all 457 Invalid Range PLAY, PAUSE 458 Parameter Is Read-Only SET_PARAMETER 459 Aggregate Operation Not Allowed all 460 Only Aggregate Operation Allowed all 461 Unsupported Transport all 462 Destination Unreachable all 470 Connection Authorization Required all 471 Connection Credentials not accepted all __________________________________________________________ 500 Internal Server Error all 501 Not Implemented all 502 Bad Gateway all 503 Service Unavailable all 504 Gateway Timeout all 505 RTSP Version Not Supported all H. Schulzrinne et. al. [Page 37] Internet Draft RTSP July 19, 2004 Table 4: Status codes and their usage with RTSP methods Header Defined in Section ______________________________________ Accept-Ranges Section 14.5 ETag Section 14.20 Location Section 14.28 Proxy-Authenticate Section 14.29 Public Section 14.31 Range Section 14.32 Retry-After Section 14.34 RTP-Info Section 14.36 Scale Section 14.37 Session Section 14.40 Server Section 14.39 Speed Section 14.38 Transport Section 14.43 Unsupported Section 14.44 Vary Section 14.46 WWW-Authenticate Section 14.48 Table 5: The RTSP response headers Header Defined in Section ____________________________________ Allow Section 14.6 Content-Base Section 14.12 Content-Encoding Section 14.13 Content-Language Section 14.14 Content-Length Section 14.15 Content-Location Section 14.16 Content-Type Section 14.17 Expires Section 14.21 Last-Modified Section 14.27 Table 6: The RTSP entity headers the RTSP application retransmit requests, it is possible that each packet loss results in two retransmissions. The receiver cannot typically take advantage of the application-layer retransmission since the transport stack will not deliver the application-layer retransmission before the first attempt has reached the receiver. If the packet loss is caused by congestion, multiple retransmissions at different layers will exacerbate the H. Schulzrinne et. al. [Page 38] Internet Draft RTSP July 19, 2004 congestion. Each request carries a sequence number in the CSeq header (Section 14.18), which MUST be incremented by one for each distinct request transmitted to the destination end-point. The initial sequence number MAY be chosen arbitrary, but is RECOMMENDED to begin with 0. If a request is repeated because of lack of acknowledgement, the request MUST carry the original sequence number (i.e., the sequence number is not incremented). 9.3 Unreliable Transport This section provides some information to future specification of RTSP over unreliable transport. Requests shall be acknowledged by the receiver. If there is no acknowledgement, the sender may resend the same message after a timeout of one round-trip time (RTT). The round-trip time is estimated as in TCP (RFC 1123) [15], with an initial round-trip value of 500 ms. An implementation MAY cache the last RTT measurement as the initial value for future connections. If RTSP is used over a small-RTT LAN, standard procedures for optimizing initial TCP round trip estimates, such as those used in T/TCP (RFC 1644) [36], can be beneficial. The Timestamp header (Section 14.42) is used to avoid the retransmission ambiguity problem [37] and obviates the need for Karn's algorithm. If a request is repeated because of lack of acknowledgement, the request must carry the original sequence number (i.e., the sequence number is not incremented). A number of RTSP messages destined for the same control end point may be packed into a single lower-layer PDU. The default port for the RTSP server is 554 for UDP. 9.4 The usage of connections Systems implementing RTSP MUST support carrying RTSP over TCP. The default port for the RTSP server is 554 for TCP. A number of RTSP packets destined for the same control end point may be encapsulated into a TCP stream. RTSP data MAY be interleaved with RTP and RTCP packets, see section 12. Unlike HTTP, an RTSP message MUST contain a Content-Length header field whenever that message contains a payload (entity). Otherwise, an RTSP packet is terminated with an empty line H. Schulzrinne et. al. [Page 39] Internet Draft RTSP July 19, 2004 immediately following the last message header. TCP can be used for both persistent connections and for one message exchange per connection, as presented above. This section gives further rules and recommendations on how to handle these connections so maximum interoperability and flexibility can be achieved. A server SHALL handle both persistent connections and one request/response transaction per connection. A persistent connection MAY be used for all transactions between the server and client, including messages to multiple RTSP sessions. However the persistent connection MAY also be closed after a few message exchanges, e.g. the initial setup and play command in a session. Later when the client wishes to send a new request, e.g. pause, to the session a new connection is opened. This connection may either be for a single message exchange or can be kept open for several messages, i.e. persistent. A major motivation for allowing non-persistent connections are that they ensure fault tolerance. A second one is to allow for application layer mobility. A server and client supporting non-persistent connection can survive a loss of a TCP connection, e.g. due to a NAT timeout. When the client has discovered that the TCP connection has been lost, it can set up a new one when there is need to communicate. The client MAY close the connection at any time when no outstanding request/response transactions exist. The server SHOULD NOT close the connection unless at least one RTSP session timeout period has passed without data traffic. A server SHOULD NOT initiate a close of a connection directly after responding to a TEARDOWN request for the whole session. A server SHOULD NOT close the connection as a result of responding to a request with an error code. Doing this would prevent or result in extra overhead for the client when testing advanced or special types of requests. The client SHOULD NOT have more than one connection to the server at any given point. If a client or proxy handles multiple RTSP sessions on the same server, it is RECOMMENDED to use only a single connection. Older services which was implemented according to RFC 2326 sometimes requires the client to use persistent connection. The client closing the connection may result in that the server removes the session. To achieve interoperability with old servers any client is strongly RECOMMENDED to use persistent connections. A Client is also strongly RECOMMENDED to use persistent connections as it allows the server to send request to the client. In cases H. Schulzrinne et. al. [Page 40] Internet Draft RTSP July 19, 2004 where no connection exist between the server and the client, this may cause the server to be forced to drop the RTSP session without notifying the client why, due to the lack of signalling channel. An example of such a case is when the server desires to send a REDIRECT request for a RTSP session to the client. A server implemented according to this specification MUST respond that it supports the "play.basic" feature-tag above. A client MAY send a request including the Supported header in a request to determine support of non-persistent connections. A server supporting non-persistent connections will return the "play.basic" feature-tag in its response. If the client receives the feature-tag in the response, it can be certain that the server handles non-persistent connections. 9.5 Timing Out RTSP messages Receivers of a request (responder) SHOULD respond to requests in a timely manner even when a reliable transport such as TCP is used. Similarly, the sender of a request (requestor) SHOULD wait for a sufficient time for a response before concluding that the responder will not be acting upon its request. A responder SHOULD respond to all requests within 5 seconds. If the responder recognizes that processing of a request will take longer than 5 seconds, it SHOULD send a 100 response as soon as possible. It SHOULD continue sending a 100 response every 5 seconds thereafter until it is ready to send the final response to the requestor. After sending a 100 response, the receiver MUST send a final response indicating the success or failure of the request. A requestor SHOULD wait at least 10 seconds for a response before concluding that the responder will not be responding to its request. After receiving a 100 response, the requestor SHOULD continue waiting for further responses. If more than 10 seconds elapses without receiving any response, the requestor MAY assume the responder is unresponsive and abort the connection. A requestor SHOULD wait longer than 10 seconds for a response if it is experiencing significant transport delays on its connection to the responder. The requestor is capable of determining the RTT using the Timestamp header (section 14.42) in any RTSP request. 9.6 Use of IPv6 This specification has been updated so that it supports IPv6. However this support was not present in RFC 2326 therefore some interoperability issues exist. A RFC 2326 implementation can support H. Schulzrinne et. al. [Page 41] Internet Draft RTSP July 19, 2004 IPv6 as long as no explicit IPv6 addresses are used within RTSP messages. This require that any RTSP URL pointing at a IPv6 host must use fully qualified domain name and not a IPv6 address. Further the Transport header must not use the parameters source and destination. Implementations according to this specification MUST understand IPv6 addresses in URLs, and headers. By this requirement the feature-tag "play.basic" can be used to determine that a server or client is capable of handling IPv6 within RTSP. 10 Capability Handling This chapter describes the capability handling mechanism available in RTSP which allows RTSP to be extended. Extensions to this version of the protocol are basically done in two ways. First, new headers can be added. Secondly, new methods can be added. The capability handling mechanism is designed to handle these two cases. When a method is added the involved parties can use the OPTIONS method to discover if it is supported. This is done by issuing a OPTIONS request to the other party. Depending on the URL it will either apply in regards to a certain media resource, the whole server in general, or simply the next hop. The OPTIONS response will contain a Public header which declares all methods supported for the indicated resource. It is not necessary to use OPTIONS to discover support of a method, the client could simply try the method. If the receiver of the request does not support the method it will respond with an error code indicating the the method is either not implemented (501) or does not apply for the resource (405). The choice between the two discovery methods depends on the requirements of the service. To handle functionality additions that are not new methods feature- tags are defined. Each feature-tag represents a certain block of functionality. The amount of functionality that a feature-tag represents can vary significantly. A simple feature-tag can simple represent the functionality a single header gives. Another feature- tag is "play.basic" which represents the minimal playback implementation according to the updated specification. The feature-tags are then used to determine if the client, server or proxy supports the functionality that is necessary to achieve the desired service. To determine support of a feature-tag several different headers can be used, each explained below: Supported: The supported header is used to determine the complete set of functionality that both client and server H. Schulzrinne et. al. [Page 42] Internet Draft RTSP July 19, 2004 has. The intended usage is to determine before one needs to use a functionality that it is supported. It can be used in any method however OPTIONS is the most suitable as one at the same time determines all methods that are implemented. When sending a request the requestor declares all its capabilities by including all supported feature-tags. This results in that the receiver learns the requestors feature support. The receiver then includes its set of features in the response. Require: The Require header can be included in any request where the end point, i.e. the client or server, is required to understand the feature to correctly perform the request. This can for example be a SETUP request where the server must understand a certain parameter to be able to set up the media delivery correctly. Ignoring this parameter would not have the desired effect and is not acceptable. Therefore the end-point receiving a request containing a Require must negatively acknowledge any feature that it does not understand and not perform the request. The response in cases where features are not understood are 551 (Option Not Supported). Also the features that are not understood are given in the Unsupported header in the response. Proxy-Require: This method has the same purpose and workings as Require except that it only applies to proxies and not the end point. Features that needs to be supported by both proxies and end-point needs to be included in both the Require and Proxy-Require header. Unsupported: This header is used in 551 error response to tell which feature(s) that was not supported. Such a response is only the result of the usage of the Require and/or Proxy- Require header where one or more feature where not supported. This information allows the requestor to make the best of situations as it knows which features that was not supported. 11 Method Definitions The method indicates what is to be performed on the resource identified by the Request-URL. The method name is case-sensitive. New methods may be defined in the future. Method names may not start with a $ character (decimal 24) and must be a token as defined by the ABNF. Methods are summarized in Table 11. H. Schulzrinne et. al. [Page 43] Internet Draft RTSP July 19, 2004 method direction object Server req. Client req. ___________________________________________________________________ DESCRIBE C -> S P,S recommended recommended GET_PARAMETER C -> S, S -> C P,S optional optional OPTIONS C -> S, S -> C P,S R=Req, Sd=Opt Sd=Req, R=Opt PAUSE C -> S P,S recommended recommended PING C -> S, S -> C P,S recommended optional PLAY C -> S P,S required required REDIRECT S -> C P,S optional optional SETUP C -> S S required required SET_PARAMETER C -> S, S -> C P,S optional optional TEARDOWN C -> S P,S required required Table 7: Overview of RTSP methods, their direction, and what objects | (P: presentation, S: stream) they operate on. Legend: R=Respond, | Sd=Send, Opt: Optional, Req: Required, Rec: Recommended Notes on Table 11: PAUSE is recommended, but not required. For | example, a fully functional server can be built to deliver live feeds | that does not support this method. If a RTSP agent does not support a | particular method, it MUST return 501 (Not Implemented) and the | requesting RTSP agent, in turn, SHOULD NOT try this method again for | the given agent / resource combination. 11.1 OPTIONS The semantics of the RTSP OPTIONS method is equivalent to that of the | HTTP OPTIONS method described in [H9.2]. In, RTSP, however, OPTIONS | is bi-directional, where a client can request it to a server and vice | versa. A client MUST implement the capability to send an OPTIONS | request and a server or a proxy MUST implement the capability to | respond to an OPTIONS request. The client, server or proxy MAY also | implement the converse of their required capability. | An OPTIONS request may be issued at any time. Such a request does not | modify the session state. However, it may prolong the session | lifespan (see below). The URL in an OPTIONS request determines the | scope of the request and the corresponding response. If the request | URL refers to a specific media resource on a given host, the scope is | limited to the set of methods supported for that media resource by | the indicated RTSP agent. A request URL with only the host address | limits the scope to the specified RTSP agent's general capabilities | without regard to any specific media. If the request URL is an | asterisk ("*"), the scope is limited to the general capabilities of | the next hop (i.e. the RTSP agent in direct communication with the | request sender). | H. Schulzrinne et. al. [Page 44] Internet Draft RTSP July 19, 2004 Regardless of scope of the request, the Public header MUST always be | included in the OPTIONS response listing the methods that are | supported by the responding RTSP agent. In addition, if the scope of | the request is limited to a media resource, the Allow header MAY be | included in the response to enumerate the set of methods that are | allowed for that resource. If the given resource is not available, | the RTSP agent SHOULD return an appropriate response code such as 3rr | or 4xx. The Supported header can be included in the request to query | the set of features that are supported by the responding RTSP agent. | The OPTIONS method can be used to keep an RTSP session alive. | However, it is not the preferred means of session keep-alive | signalling, see section 14.40. An OPTIONS request intended for | keeping alive a RTSP session MUST include the Session header with the | associated session ID. Such a request SHOULD also use the media or | the aggregated control URL as the request URL. Example: C->S: OPTIONS * RTSP/1.0 CSeq: 1 User-Agent: PhonyClient/1.2 Require: Proxy-Require: gzipped-messages Supported: play.basic S->C: RTSP/1.0 200 OK CSeq: 1 Public: DESCRIBE, SETUP, TEARDOWN, PLAY, PAUSE Supported: play.basic, implicit-play, gzipped-messages Server: PhonyServer/1.0 Note that some of the feature-tags in Require and Proxy-Require are necessarily fictional features (one would hope that we would not purposefully overlook a truly useful feature just so that we could have a strong example in this section). 11.2 DESCRIBE The DESCRIBE method is used to retrieve the description of a | presentation or media object from a server. The request URL of the | DESCRIBE request identifies the media resource of interest. The | client MAY include the Accept header in the request to list the | description formats that it understands. The server SHALL respond | with a description of the requested resource and return the | H. Schulzrinne et. al. [Page 45] Internet Draft RTSP July 19, 2004 description in the entity of the response. The DESCRIBE reply- response pair constitutes the media initialization phase of RTSP. Example: C->S: DESCRIBE rtsp://server.example.com/fizzle/foo RTSP/1.0 CSeq: 312 User-Agent: PhonyClient 1.2 Accept: application/sdp, application/rtsl, application/mheg S->C: RTSP/1.0 200 OK CSeq: 312 Date: 23 Jan 1997 15:35:06 GMT Server: PhonyServer 1.0 Content-Type: application/sdp Content-Length: 376 v=0 o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4 s=SDP Seminar i=A Seminar on the session description protocol u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps e=mjh@isi.edu (Mark Handley) c=IN IP4 224.2.17.12/127 t=2873397496 2873404696 a=recvonly m=audio 3456 RTP/AVP 0 m=video 2232 RTP/AVP 31 m=application 32416 UDP WB a=orient:portrait The DESCRIBE response MUST contain all media initialization information for the resource(s) that it describes. Servers SHOULD NOT | use the DESCRIBE response as a means of media indirection. | By forcing a DESCRIBE response to contain all media | initialization for the set of streams that it describes, | and discouraging use of DESCRIBE for media indirection, we | avoid looping problems that might result from other | approaches. | Media initialization is a requirement for any RTSP-based system, but | the RTSP specification does not dictate that this must be done via | the DESCRIBE method. There are four ways that an RTSP client may | H. Schulzrinne et. al. [Page 46] Internet Draft RTSP July 19, 2004 receive initialization information: | o via a RTSP DESCRIBE method | o via some other protocol (HTTP, email attachment, etc.) | o via the command line or standard input | If a client obtains a valid description from an alternate source, the client MAY use this description for initialization purposes without issuing a DESCRIBE request for the same media. It is RECOMMENDED that minimal servers support the DESCRIBE method, and highly recommended that minimal clients support the ability to | act as "helper applications" that accept a media initialization file | from standard input, command line, and/or other means that are | appropriate to the operating environment of the clients. 11.3 SETUP The SETUP request for a URL specifies the transport mechanism to be | used for the streamed media. The SETUP method may be used in three | different cases; Create a RTSP session, add a media to a session, and | change the transport parameters of already set up media stream. Using | SETUP to create or add media to a session when in PLAY state is | unspecified. Otherwise SETUP can be used in all three states; INIT, | and READY, for both purposes and in PLAY to change the transport | parameters. The Transport header, see section 14.43, specifies the transport parameters acceptable to the client for data transmission; the response will contain the transport parameters selected by the server. This allows the client to enumerate in priority order the transport mechanisms and parameters acceptable to it, while the server can select the most appropriate. It is expected that the session description format used will enable the client to select a limited number possible configurations that are offered to the server to choose from. All transport parameters SHOULD be included in the Transport header, the use of other headers for this purpose is discouraged due to middle boxes. For the benefit of any intervening firewalls, a client SHOULD indicate the transport parameters even if it has no influence over these parameters, for example, where the server advertises a fixed multicast address. H. Schulzrinne et. al. [Page 47] Internet Draft RTSP July 19, 2004 Since SETUP includes all transport initialization information, firewalls and other intermediate network devices (which need this information) are spared the more arduous task of parsing the DESCRIBE response, which has been reserved for media initialization. In a SETUP response the server SHOULD include the Accept-Ranges header (see section 14.5 to indicate which time formats that are acceptable to use for this media resource. C->S: SETUP rtsp://example.com/foo/bar/baz.rm RTSP/1.0 CSeq: 302 Transport: RTP/AVP;unicast;client_port=4588-4589, RTP/AVP/TCP;unicast;interleaved=0-1 S->C: RTSP/1.0 200 OK CSeq: 302 Date: 23 Jan 1997 15:35:06 GMT Server: PhonyServer 1.0 Session: 47112344;timeout=60 Transport: RTP/AVP;unicast;client_port=4588-4589; server_port=6256-6257;ssrc=2A3F93ED Accept-Ranges: NPT In the above example the client want to create a RTSP session containing the media resource "rtsp://example.com/foo/bar/baz.rm". The transport parameters acceptable to the client is either RTP/AVP/UDP (UDP per default) to be received on client port 4588 and 4589 or RTP/AVP interleaved on the RTSP control channel. The server selects the RTP/AVP/UDP transport and adds the ports it will send and received RTP and RTCP from, and the RTP SSRC that will be used by the server. The server MUST generate a session identifier in response to a successful SETUP request, unless a SETUP request to a server includes a session identifier, in which case the server MUST bundle this setup request into the existing session (aggregated session) or return error 459 (Aggregate Operation Not Allowed) (see Section 13.4.11). An Aggregate control URL MUST be used to control an aggregated session. This URL MUST be different from the stream control URLs of the individual media streams included in the aggregate. The Aggregate control URL is to be specified by the session description if the server supports aggregated control and aggregated control is desired for the session. However even if aggregated control is offered the client MAY chose to not set up the session in aggregated control. If H. Schulzrinne et. al. [Page 48] Internet Draft RTSP July 19, 2004 an Aggregate control URL is not specified in the session description, it is normally an indication that non-aggregated control should be used. The SETUP of media streams in an aggregate which has not been given an aggregated control URL is unspecified. While the session ID sometimes has enough information for aggregate control of a session, the Aggregate control URL is still important for some methods such as SET_PARAMETER where the control URL enables the resource in question to be easily identified. The Aggregate control URL is also useful for proxies, enabling them to route the request to the appropriate server, and for logging, where it is useful to note the actual resource that a request was operating on. Finally, presence of the Aggregate control URL allows for backwards compatibility with RFC 2326 [1]. A session will exist until it is either removed by a TEARDOWN request or is timed-out by the server. The server MAY remove a session that has not demonstrated liveness signs from the client within a certain timeout period. The default timeout value is 60 seconds; the server MAY set this to a different value and indicate so in the timeout field of the Session header in the SETUP response. For further discussion see chapter 14.40. Signs of liveness for a RTSP session are: o Any RTSP request from a client which includes a Session header with that session's ID. o If RTP is used as a transport for the underlying media streams, an RTCP sender or receiver report from the client for any of the media streams in that RTSP session. If a SETUP request on a session fails for any reason, the session state, as well as transport and other parameters for associated streams SHALL remain unchanged from their values as if the SETUP request had never been received by the server. A client MAY issue a SETUP request for a stream that is already set up or playing in the session to change transport parameters, which a server MAY allow. If it does not allow this, it MUST respond with error 455 (Method Not Valid In This State). Reasons to support changing transport parameters, is to allow for application layer mobility and flexibility to utilize the best available transport as it becomes available. In a SETUP response for a request to change the transport parameters while in Play state, the server SHOULD include the Range to indicate H. Schulzrinne et. al. [Page 49] Internet Draft RTSP July 19, 2004 from what point the new transport parameters are used. Further if RTP is used for delivery the server SHOULD also include the RTP-Info header to indicate from what timestamp and RTP sequence number the change has taken place. If both RTP-Info and Range is included in the response the "rtp_time" parameter and range MUST be for the corresponding time, i.e. be used in the same way as for PLAY to ensure the correct synchronization information is present. If the transport parameter change while in PLAY state results in a change of synchronization related information, for example changing RTP SSRC, the server MUST provide in the SETUP response the necessary synchronization information. However the server is RECOMMENDED to avoid changing the synchronization information if possible. 11.4 PLAY The PLAY method tells the server to start sending data via the mechanism specified in SETUP. A client MUST NOT issue a PLAY request until any outstanding SETUP requests have been acknowledged as successful. PLAY requests are valid when the session is in READY state; the use of PLAY requests when the session is in PLAY state is deprecated. A PLAY request MUST include a Session header to indicate which session the request applies to. In an aggregated session the PLAY request MUST contain an aggregated control URL. A server SHALL responde with error 460 (Only Aggregate Operation Allowed) if the client PLAY request URL is for one of the media. The media in an aggregate SHALL be played in sync. If a client want individual control of the media it must use separate RTSP sessions for each media. The PLAY request SHALL position the normal play time to the beginning of the range specified by the Range header and delivers stream data until the end of the range if given, else to the end of the media is reached. To allow for precise composition multiple ranges MAY be specified in one PLAY Request. The range values are valid if all given ranges are part of any media within the aggregate. If a given range value points outside of the media, the response SHALL be the 457 (Invalid Range) error code. The below example will first play seconds 10 through 15, then, immediately following, seconds 20 to 25, and finally seconds 30 through the end. C->S: PLAY rtsp://audio.example.com/audio RTSP/1.0 CSeq: 835 Session: 12345678 H. Schulzrinne et. al. [Page 50] Internet Draft RTSP July 19, 2004 Range: npt=10-15, npt=20-25, npt=30- See the description of the PAUSE request for further examples. A PLAY request without a Range header is legal. It SHALL start playing a stream from the beginning (npt=0-) unless the stream has been paused. If a stream has been paused via PAUSE, stream delivery resumes at the pause point. The stream SHALL play until the end of the media. The Range header MUST NOT contain a time parameter. The usage of time in PLAY method has been deprecated. If a request with time parameter is received the server SHOULD respond with a 457 (Invalid Range) to indicate that the time parameter is not supported. Server MUST include a "Range" header in any PLAY response. The response MUST use the same format as the request's range header contained. If no Range header was in the request, the NPT time format SHOULD be used unless the client showed support for an other format more appropriate. Also for a session with live media streams the Range header MUST indicate a valid time. It is RECOMMENDED that normal play time is used, either the "now" indicator, for example "npt=now-", or the time since session start as an open interval, e.g. "npt=96.23-". An absolute time value (clock) for the corresponding time MAY be given, i.e. "clock=20030213T143205Z-". The UTC clock format SHOULD only be used if client has shown support for it. A media server only supporting playback MUST support the npt format and MAY support the clock and smpte formats. For a on-demand stream, the server MUST reply with the actual range | that will be played back, i.e. for which duration all media having | content at this time is delivered. This may differ from the requested | range if alignment of the requested range to valid frame boundaries | is required for the media source. Note that some media streams in an | aggregate may need to be delivered from even earlier points. Also | some media format has very long duration per individual data unit, | therefore it might be necessary for the client to parse the data | unit, and select where to start. | Example: Single audio stream (MIDI) | C->S: PLAY rtsp://example.com/audio RTSP/1.0 | CSeq: 836 | Session: 12345678 | H. Schulzrinne et. al. [Page 51] Internet Draft RTSP July 19, 2004 Range: npt=7.05- | S->C: RTSP/1.0 200 OK | CSeq: 836 | Date: 23 Jan 1997 15:35:06 GMT | Server: PhonyServer 1.0 | Range: npt=3.52- | RTP-Info:url=rtsp://example.com/audio; | seq=14783;rtptime=2345962545 | S->C: RTP Packet TS=2345962545 => NPT=3.52 | Duration: 4.15 seconds | In this example the client receives the first media packet that | stretches all the way up and past the requested playtime. Thus it is | a client decision if it desires to render to the user the time | between 3.52 and 7.05 that the user requested. In most cases it is | probably suitable to not render that time period. | For live media sources it might be impossible to specify from which | point in time all media streams that has active content can actually | be delivered. Therefore a server MAY specify a start time (or now-) | in the range header, for which not all media will be available from. If no range is specified in the request, the start position SHALL still be returned in the reply. If the medias that are part of an aggregate has different lengths, the PLAY request SHALL be performed as long as the given range is valid for any media, for example the longest media. Media will be sent whenever it is available for the given play-out point. A PLAY response MAY include a header(s) carrying synchronization | information. As the information necessary is dependent on the media | transport format, further rules specifying the header and its usage | is needed. For RTP the RTP-Info header is specified, see section | 14.36. After playing the desired range, the pres