SIPPING E. Burger Internet-Draft SnowShore Networks, Inc. Expires: December 29, 2003 June 30, 2003 Basic Network Media Services with SIP draft-burger-sipping-netann-06 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 29, 2003. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract In SIP-based networks, there is a need to provide basic network media services. Such services include network announcements, user interaction, and conferencing services. These services are basic building blocks, from which one can construct interesting applications. In order to have interoperability between servers offering these building blocks (also known as Media Servers) and application developers, one needs to be able to locate and invoke such services in a well-defined manner. This document describes a mechanism for providing an interoperable protocol interface between Application Servers, which provide application services to SIP-based networks, and Media Servers, which provide the basic media processing building blocks. Burger Expires December 29, 2003 [Page 1] Internet-Draft SIP Media Services June 2003 Conventions used in this document RFC2119 [1] provides the interpretations for the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" found in this document. Note that the words "must" and "should" appear in the section Early Media Announcement (Section 7). In that section, these key words do NOT carry the strength described in RFC2119. That section is informative only. Table of Contents 1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Announcement Service . . . . . . . . . . . . . . . . . . . . . 6 3.1 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.2 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.3 Protocol Diagram . . . . . . . . . . . . . . . . . . . . . . . 8 3.4 Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 9 4. Prompt and Collect Service . . . . . . . . . . . . . . . . . . 12 4.1 Formal Syntax for Prompt and Collect Service . . . . . . . . . 12 5. Conference Service . . . . . . . . . . . . . . . . . . . . . . 14 5.1 Protocol Diagram . . . . . . . . . . . . . . . . . . . . . . . 15 5.2 Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 16 6. The User Part . . . . . . . . . . . . . . . . . . . . . . . . 17 7. Early Media Announcement . . . . . . . . . . . . . . . . . . . 19 7.1 IMPORTANT NOTICE . . . . . . . . . . . . . . . . . . . . . . . 19 7.2 Description . . . . . . . . . . . . . . . . . . . . . . . . . 19 7.3 Protocol Diagram . . . . . . . . . . . . . . . . . . . . . . . 21 8. Security Considerations . . . . . . . . . . . . . . . . . . . 22 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 23 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24 Normative References . . . . . . . . . . . . . . . . . . . . . 25 Informative References . . . . . . . . . . . . . . . . . . . . 26 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 27 Intellectual Property and Copyright Statements . . . . . . . . 28 Burger Expires December 29, 2003 [Page 2] Internet-Draft SIP Media Services June 2003 1. Overview In SIP-based media networks (RFC3261 [2]), there is a need to provide basic network media services. Such services include playing announcements, initiating a media mixing session (conference), and prompting and collecting information with a user. These services are basic in nature, are few in number, and fundamentally have not changed in 25 years of enhanced telephony services. Moreover, given their elemental nature, one would not expect them to change in the future. Announcements are media played to the user. Announcements can be static media files, media files generated in real-time, media streams generated in real-time, multimedia objects, or combinations of the above. Media mixing is the act of mixing different RTP streams, as described in RFC1889 [9]. Note that the service described here will suffice for simple mixing of media for a basic conferencing service. One can create a complete conferencing service using this basic building block. However, this service does not address the interesting application-level issues such as floor control for conferencing, etc. MSCML [10] addresses this need. However, that is beyond the scope of this document. Interested readers should read conferencing-framework [11] for details on the IETF SIP conferencing framework. Prompt and collect is where the server prompts the user for some information, as in an announcement, and then collects the user's response. This can be a one-step interaction, for example by playing an announcement, "Please enter your pass code", followed by collecting a string of digits. It can also be a more complex interaction, specified, for example, by VoiceXML [12] or MSCML [10]. Burger Expires December 29, 2003 [Page 3] Internet-Draft SIP Media Services June 2003 2. Mechanism In the context of SIP control of media servers, we take advantage of the fact that the standard SIP URI has a user part. Media servers do not have users. Thus we use the user address, or the left-hand-side of the URI, as a service indicator. Note that the set of services is small, well defined, and well contained. The section The User Part (Section 6) discusses the issues with using a fixed set of user-space names. For per-service security, the media server MAY use any of the security protocols described in RFC3261 [2]. The media server MAY issue 401 challenges for authentication. The media server, upon receiving the INVITE, notes the service indicator. Depending on the service indicator, the media server will either honor the request or return a failure response code. The service indicator is the concatenation of the service name and an optional service instance identifier, separated by an equal sign. Per RFC3261 [2], the service indicator is case insensitive. The service name MUST be from the set alphanumeric characters plus dash (US-ASCII %2C). The service name MUST NOT include an equal sign (US-ASCII %3C). The service name MAY have long- and short-forms, as SIP does for headers. A given service indicator MAY have an associated set of parameters. Such parameters MUST follow the convention set out for SIP URI parameters. That is, a semi-colon separated list of keyword=values. Certain services may have an association with a unique service instance on the media server. For example, a given media server can host multiple, separate conference sessions. To identify unique service instances, a unique identifier modifies the service name. The unique identifier MUST meet the rules for a legal user part of a SIP URI. An equal sign, US-ASCII %3D, MUST separate the service indicator from the unique identifier. Note that since the service indicator is case insensitive, the service instance identifier is also case insensitive. The requesting client issues a SIP INVITE to the media server, specifying the requested service and any appropriate parameters. Burger Expires December 29, 2003 [Page 4] Internet-Draft SIP Media Services June 2003 If the media server can perform the requested service, it does so, following the processing steps described in the service definition document. If the media server cannot perform the requested service or does not recognize the service indicator, it MUST respond with the response code 488 NOT ACCEPTABLE HERE. This is appropriate, as 488 refers to a problem with the user part of the URI. Moreover, 606 is not appropriate, as some other media server may be able to satisfy the request. RFC3261 [2] describes the 488 and 606 response codes. Some services require a unique identifier. Most services automatically create a service instance upon the first INVITE with the given identifier. However, if a service requires an existing service instance, and no such service instance exists on the media server, the media server MUST respond with the response code 404 NOT FOUND. This is appropriate as the service itself exists on the media server, but the particular service instance does not. It is as if the user was not home. Burger Expires December 29, 2003 [Page 5] Internet-Draft SIP Media Services June 2003 3. Announcement Service A network announcement is the delivery of an multimedia resource, such as a prompt file, to a terminal device. Note the multimedia resource may be any multimedia object that the media server supports. This service can play a single object with multiple streams, such as a video and audio prompt. However, this service cannot play multiple objects on the same SIP dialog. There are two types of network announcements. The differentiating characteristic between the two types is whether the network fully sets up the SIP dialog before playing the announcement. The analog in the PSTN is whether answer supervision is supplied; i.e. does the announcement server answer the call prior to delivering the announcement. Playing an announcement after call setup is straightforward. First, the requesting device issues an INVITE to the media server requesting the announcement service. The media server negotiates the SDP and responds with a 200 OK. After receiving the ACK from the requesting device, the media server plays the requested object and issues a BYE to the requesting device. The operation for playing an announcement before the SIP dialog is fully established, as early media, is discouraged. The section Early Media Announcement (Section 7) describes a method that works for such operations. Note, however, that section is informative in nature. If the media server supports announcements, but it cannot find the referenced URI, it MUST respond with the 404 NOT FOUND response code. If the media server receives an INVITE for the announcement service without a "play=" parameter, it MUST respond with the 404 NOT FOUND response code, as there is no default value for the announcement service. If there is an error retrieving the announcement, the media server MUST respond with a 404 NOT FOUND response code. In addition, the media server SHOULD include a Warning header with appropriate explanatory text explaining what failed. The Request URI fully describes the announcement service through the use of the user part of the address and additional URI parameters. The user portion of the address, "annc", specifies the announcement service on the media server. The service has several associated URI parameters that control the content and delivery of the announcement. These parameters are described below: Burger Expires December 29, 2003 [Page 6] Internet-Draft SIP Media Services June 2003 play Specifies the resource or announcement sequence to be played. repeat Specifies how many times the media server should repeat the announcement or sequence named by the "play=" parameter. delay Specifies a delay interval between announcement repetitions. The delay is measured in milliseconds. duration Specifies the maximum duration of the announcement. The media server will discontinue the announcement and end the call if the maximum duration has been reached. The duration is measured in milliseconds. locale Specifies the language and country variant of the announcement sequence named in the "play=" parameter. The language is defined as a two-letter code per ISO 639-1 [3]. The country variant is also defined as a two-letter code per ISO 3166-1 [4]. These elements are concatenated with a single underbar (%x5F) character. The mechanics of announcement sequences are beyond the scope of this document. param[n] Provides a mechanism for passing values that are to be substituted into an announcement sequence. Up to 9 parameters ("param1=" through "param9=") may be specified. The mechanics of announcement sequences are beyond the scope of this document. The "play=" parameter is mandatory and MUST be present. All other parameters are OPTIONAL. NOTE: Some encodings are not self-describing. Thus the implementation relies on filename extension conventions for determining the media type. Note that RFC3261 [2] implies that proxies are supposed to pass parameters through unchanged. However, be aware that non-conforming proxies may strip Request-URI parameters. That said, given the likely scenarios for the mechanisms presented in this document, this should not be an issue. Most likely, the proxy inserting the parameters is the last proxy before the media server. If the service provider deploys a proxy for load balancing or service location purposes, the service provider should ensure their choice of proxy will preserve the parameters. The form of the SIP Request URI for announcements is as follows. Note that the backslash, CRLF, and spacing before the "play=" in the example is for readability purposes only. sip:annc@ms2.example.net; \ Burger Expires December 29, 2003 [Page 7] Internet-Draft SIP Media Services June 2003 play="http://audio.example.net/allcircuitsbusy.g711" sip:annc@ms2.example.net; \ play="file://fileserver.example.net/geminii/yourHoroscope.wav" 3.1 Operation The scenarios below assume there is a SIP Proxy, application server, or media gateway controller between the caller and the media server. However, the announcement service works as described below even if the caller invokes the service directly. We chose to discuss the proxy case, as it will be the most common case. 3.2 Operation The caller issues an INVITE to the serving SIP Proxy. The SIP Proxy determines what audio prompt to play to the caller. The proxy responds to the caller with 100 TRYING. The proxy issues an INVITE to the media server, requesting the appropriate prompt to play coded in the play= parameter. The media server responds with 200 OK. The proxy sends a 200 OK to the caller. The caller then issues an ACK. The proxy then issues an ACK to the media server. With the call established, the media server plays the requested prompt. When the media server completes the play of the prompt, it issues a BYE to the proxy. The proxy then issues a BYE to the caller. 3.3 Protocol Diagram Caller Proxy Media Server | INVITE | | |----------------------->| INVITE | | 100 TRYING |----------------------->| |<-----------------------| 200 OK | | 200 OK |<-----------------------| |<-----------------------| | | ACK | | |----------------------->| ACK | | |----------------------->| | | | | Play Announcement (RTP) | |<================================================| | | | | | BYE | Burger Expires December 29, 2003 [Page 8] Internet-Draft SIP Media Services June 2003 | BYE |<-----------------------| |<-----------------------| | | 200 OK | 200 OK | |----------------------->|----------------------->| | | | 3.4 Formal Syntax The following syntax specification uses the augmented Backus-Naur Form (BNF) as described in RFC2234 [5]. ANNC-URL = "sip:" annc-ind "@" hostport annc-parameters annc-ind = "annc" annc-parameters = ";" play-param [ ";" content-param ] [ ";" delay-param] [ ";" duration-param ] [ ";" repeat-param ] [ ";" locale-param ] [ ";" variable-params ] play-param = "play=" prompt-url content-param = "content-type=" MIME-type delay-param = "delay=" delay-value delay-value = 1*DIGIT duration-param = "duration=" duration-value duration-value = 1*DIGIT repeat-param = "repeat=" repeat-value repeat-value = 1*DIGIT locale-param = "locale=" locale-value locale-value = 2ALPHA %x5F 2ALPHA variable-params = param-name "=" variable-value param-name = "param" DIGIT ; e.g., "param1" Burger Expires December 29, 2003 [Page 9] Internet-Draft SIP Media Services June 2003 variable-value = 1*(ALPHA | DIGIT) The MIME-type is the MIME [6] content type for the announcement, such as audio/basic, audio/G729, audio/mpeg, video/mpeg, and so on. To date, none of the IETF audio MIME registrations have parameters. Vendor-specific registrations, such as audio/x-wav, do have parameters. However, they are not strictly needed for prompt fetching. On the other hand, the prevalence of parameters may change in the future. In addition, existing video registrations have parameters, such as video/DV. To accommodate this, and retain compatibility with the SIP URI structure, the MIME-type parameter separator (semicolon, %3b) and value separator (equal, %d3) MUST be escaped. For example: sip:annc@ms.example.net; \ play=file://fs.example.net/clips/my-intro.dvi; \ content-type=video/mpeg%3bencode%d3314M-25/625-50 The locale-value consists of a 2-letter language code as specified in ISO 639-1 [3] and a 2-letter country code specified in ISO 3166-1 [4] separated by a single underbar (%x5Fh) character. The definition of hostport is as specified by RFC3261 [2]. The syntax of prompt-url consists of a URL scheme as specified by RFC2396 [7] or a special token indicating a provisioned announcement sequence. We expect the URL to be one of the following schemes. o http o ftp o file (referencing a local or NFS (RFC3010 [13]) o nfs (RFC2224 [14]) If a provisioned announcement sequence is to be played the value of prompt-url will have the following form: prompt-url = "/provisioned/" announcement-id announcement-id = 1*(ALPHA | DIGIT) Note that the scheme "/provisioned/" was chosen because of a Burger Expires December 29, 2003 [Page 10] Internet-Draft SIP Media Services June 2003 hesitation to register a "provisioned:" URI scheme. This document is strictly focused on the SIP interface for the announcement service and as such does not detail how announcement sequences are provisioned or defined. Note that the media type of the object the prompt-url refers to can be most anything, including audio file formats, text file formats, or URI lists. See the Prompt and Collect Service (Section 4) section for more on this topic. Burger Expires December 29, 2003 [Page 11] Internet-Draft SIP Media Services June 2003 4. Prompt and Collect Service This service is also known as a voice dialog. It establishes an aural dialog with the user. The dialog service follows the model of the announcement service. However, the service indicator is "dialog". The dialog service takes a parameter, voicexml=, indicating the URI of the VoiceXML script to execute. sip:dialog@mediaserver.example.net; \ voicexml=http://vxmlserver.example.net/cgi-bin/script.vxml A Media Server MAY accept additional SIP request URI parameters and deliver them to the VoiceXML interpreter session as session variables. 4.1 Formal Syntax for Prompt and Collect Service The following syntax specification uses the augmented Backus-Naur Form (BNF) as described in RFC2234 [5]. DIALOG-URL = "sip:" dialog-ind "@" hostport dialog-parameters dialog-ind = "dialog" dialog-parameters = ";" dialog-param [ vxml-parameters ] dialog-param = "voicexml=" dialog-url vxml-parameters = vxml-param [ vxml-parameters ] vxml-param = ";" vxml-keyword "=" vxml-value vxml-keyword = token vxml-value = token The dialog-url is the URI of the VoiceXML script. If present, other parameters get passed to the VoiceXML interpreter session with the assigned vxml-keyword vxml-value pairs. Note that all vxml-keywords MUST have values. If there is a vxml-keyword without a corresponding vxml-value, the media server MUST reject the request with a 400 BAD REQUEST response code. In addition, the media server MUST state "Missing VXML Value" in the reason phrase. Burger Expires December 29, 2003 [Page 12] Internet-Draft SIP Media Services June 2003 The media server presents the parameters as environment variables in the connection object. Specifically, the parameter appears in the connection.sip tree. If the Media Server does not support the passing of keyword-value pairs to the VoiceXML interpreter session, it MUST ignore the parameters. Burger Expires December 29, 2003 [Page 13] Internet-Draft SIP Media Services June 2003 5. Conference Service One identifies mixing sessions through their SIP request URIs. To create a mixing session, one sends an INVITE to a request URI that represents the session. If the URI does not already exist on the media server and the requested resources are available, the media server creates a new mixing session. If there is an existing URI for the session, then the media server interprets it as a request for the new session to join the existing session. The form of the SIP request URI for conferencing is: sip:conf=uniqueIdentifier@mediaserver.example.net The left-hand side of the request URI is actually the username of the request in the request URI and the To header. The host portion of the URI identifies a particular media server. The "conf" user name conveys to the media server that this is a request for the mixing service. The uniqueIdentifier can be any value that is compliant with the SIP URI specification. It is the responsibility of the conference control application to ensure the identifier is unique within the scope of any potential conflict. In the terminology of the conferencing framework conferencing-framework [11], this URI convention tells the media server that the application server is requesting it to act as a Focus. The conf-id value identifies the particular focus instance. It is worth noting that the conference URI shared between the application and media provides enhanced security, as the SIP control interface does not have to be exposed to participants. It also allows the assignment of a specific media server to be delayed as long as possible, thereby simplifying resource management. One can add additional legs to the conference by INVITEing them to the above mentioned request URI. Per the matching rules of RFC3261 [2], the conf-id parameter is part of the matching string. Conversely, one can remove legs by issuing a BYE in the corresponding dialog. The mixing session, and thus the conference-specific request URI, remains active so long as there is at least one SIP dialog associated with the given request URI. If the Request-URI has "conf" as the user part, but does not have a conf-id parameter, the media server responds with a 404 NOT FOUND. NOTE: The media server could create a unique conference instance and return the conf-id string to the UAC if there is no conf-id present. However, such an operation may have other operational Burger Expires December 29, 2003 [Page 14] Internet-Draft SIP Media Services June 2003 issues, such as permissions and billing. Thus an application server or proxy is a better place to do such an operation. Moreover, such action would make the media server into a Conference Factory in the terminology of conference-framework [11]. That is not the appropriate behavior for a media server. 5.1 Protocol Diagram This diagram shows the establishment of a three-way conference. This section is informative. P1 P2 P3 Application Server Media Server | | | | | | INVITE sip:public-conf@as.c.net | | |---------------------------------->| INVITE sip:conf=123@ms.c.net | | | |------------------>| | | | | 200 OK | | 200 OK | |<------------------| |<----------------------------------| | | | | RTP w/ P1 | | |<=====================================================>| | | | | | | INVITE sip:public-conf@as.c.net | | | |-------------------------->| INVITE sip:conf=123@ms.c.net | | | |------------------>| | | | | 200 OK | | | 200 OK | |<------------------| | |<--------------------------| | | | | | | | | | RTP w/ P1+P2-P2 | | | |<=============================================>| | | | RTP w/ P1+P2-P1 | | |<=====================================================>| | | | | | | INVITE sip:public-conf@as.c.net | | | | |----------------->| INVITE sip:conf=123@ms.c.net | | | |------------------>| | | | | 200 OK | | | | 200 OK |<------------------| | | |<-----------------| | | | | | | | | | RTP w/ P1+P2+P3-P3 | | | |<====================================>| | | | RTP w/ P1+P2+P3-P2 | | |<=============================================>| | | | RTP w/ P1+P2+P3-P1 | |<=====================================================>| Burger Expires December 29, 2003 [Page 15] Internet-Draft SIP Media Services June 2003 | | | | | | | | | | Using the terminology of conference-framework [11], the Application Server is the Conference Factory and the Media Server is the Conference Focus. Note that the above call flow does not show any 100 TRYING messages that would typically flow from the Application Server to the UAC's, nor does it show the ACK's from the UAC's to the Application Server or from the Application Server to the Media Server. Each leg can drop out either under the supervision of the UAC by the UAC sending a BYE or under the supervision of the Application Server by the Application Server issuing a BYE. In either case, the Application Server will either issue a BYE on behalf of the UAC or issue it directly to the Media Server, corresponding to the respective disconnect case. It is left as a trivial exercise to the reader for how the Application Server can mute legs, create side conferences, and so forth. Note that the Application Server is a server to the participants (UAC's). However, the Application Server is a client for mixing services to the Media Server. 5.2 Formal Syntax The following syntax specification uses the augmented Backus-Naur Form (BNF) as described in RFC2234 [5]. CONF-URL = "sip:" conf-ind "=" instance-id "@" hostport conf-ind = "conf" instance-id = token The SIP-PARAM-LIST is the SIP Request-URI parameter list as described in RFC3261 [2]. Note that all of the parameters in the parameter list are part of the URI matching algorithm. Burger Expires December 29, 2003 [Page 16] Internet-Draft SIP Media Services June 2003 6. The User Part There has been considerable debate about the wisdom of using fixed user parts in a request URI. The most common objection is that the user part should be opaque and a local matter. The other objection is that using a fixed user part removes those specified user addresses from the user address space. We will address the latter issue first. The common example is the Postmaster address defined by RFC2821 [15]. The objection is that by using the Postmaster token for something special, one removes that token for anyone. Thus, the Postmaster General of the United States, for example, cannot have the mail address Postmaster@usps.gov. One may debate whether this is a significant limitation, however. One may point out that "annc", for example, has the potential for more conflict than Postmaster. This is true. However, one cannot confuse the namespace at a Media Server with the namespace for an organization. For example, let us take the case where a network offers services for "Ann Charles". She likes to use the name "annc", and thus she would like to use "sip:annc@provider.net". We offer that there is ABSOLUTELY NO NAME COLLISION WHATSOEVER. Why is this so? This is so because sip:annc@provider.net will resolve to the specific user at a specific device for Ann. As an example, provider.net's SIP Proxy Server can resolve sip:annc@provider.net to annc@anns-phone.provider.net . One directs requests for the media service annc directly to the Media Server, e.g., sip:annc@ms21.ap.provider.net . Moreover, by definition, Ann Charles, or anything other than the announcement service, will NEVER be directly on the Media Server. If that were not true, no phone in the world could use the user part "eburger", as eburger is a reserved user part in the SnowShore domain. The most important thing to note about this convention is that the left-hand side of the request URI is opaque to the network. The only network elements that need to know about the convention are the Media Server and client. Some have proposed that such naming be a pure matter of local convention. For example, the thesis of the informational RFC3087 [16] is that you can address services using a request URI. However, some have taken the examples in the document to an extreme. Namely, that the only way to address services is via arbitrary, opaque, long user parts. It is possible to provision the service names, rather than fixed names. While this can work in a closed network, where the Application Servers and Media Servers are in the same administrative Burger Expires December 29, 2003 [Page 17] Internet-Draft SIP Media Services June 2003 domain, this does not work across domains. This is because the client of the media service has to know the local name for each service / domain pair. This is particularly onerous for situations where there is an ad hoc relationship between the application and the media service. Without a well-known relationship between service and service address, how would the client locate the service? One very important result of using the user part as the service descriptor is that we can use all of the standard SIP machinery, without modification. For example, Media Servers with different capabilities can SIP Register their capabilities as users. For example, a mixing-only device will register the "conf" user, while a multi-purpose Media Server will register all of the users. Note that this is why the URI to play is a parameter. Doing otherwise would overburden a normal SIP proxy or redirect server. Likewise, this scheme lets us leverage the standard SIP proxy behavior of using an intelligent redirect server or proxy server to provide high-available services. For example, two Media Servers can register with a SIP redirect server for the annc user. If one of the Media Servers fails, the registration will expire and all requests for the announcement service ("calls to the annc user") get sent to the surviving Media Server. Burger Expires December 29, 2003 [Page 18] Internet-Draft SIP Media Services June 2003 7. Early Media Announcement 7.1 IMPORTANT NOTICE This section is entirely informative. It does not define any particular standards-track protocol or best practice. This section describes one implementation of an early media announcement service. There are many issues surrounding early media. Thus this document does not advocate the use of such a facility. This document includes this description of an early media announcement service purely to document one use of an early media call flow. All of the MUST and SHOULD language is likewise historical and NOT part of the normative definition described in this document. 7.2 Description In some situations, one would like to play an announcement without providing an answer indication. One need for this is to replicate and expand upon the existing telephone network. Network operators require this capability to provide informational network announcements, such as "The person you are trying to reach is unavailable. Good Bye." or "We are sorry, but all circuits are busy. Please try your call again later. Good Bye." Note that simply redirecting the caller to a media server, with the media server issuing a 200 OK response, may not be appropriate. The call request itself has not completed successfully. Likewise, if a proxy record-routes the request, the proxy will still forward a 200 OK response from the media server. Another alternative would be to use a back-to-back user agent (B2BUA). The B2BUA would perform the early media signaling to the UAC, while doing a normal, complete SIP dialog with the media server. That unquestionably makes the media server signaling easier. However, the signaling between the B2BUA would be identical to that described below. Moreover, many network operators would rather deploy SIP Proxies and not B2BUAs. To support the appropriate paradigm, the media server issues a 100 TRYING response, followed immediately by a 183 SESSION PROGRESS response with SDP. This enables the media server to send early media to the caller. The request to the media server should include the 100rel extension Burger Expires December 29, 2003 [Page 19] Internet-Draft SIP Media Services June 2003 header RFC3262 [8]. This ensures the media server reliably delivers its SDP to the caller. At this point, the media server sends the requested audio. After playing the audio, the media server issues a 487 REQUEST CANCELED response code to the requesting device. NOTE: 487 conforms to RFC2543 [17]. However, it is not strictly the correct response code per RFC3261 [2]. The concept is that the media server is responding to an implicit CANCEL. This works for most devices, as per RFC3261, a 4xx result code means the request was terminated. Unfortunately, there are non-conforming devices that do not terminate a session, even if they receive a 4xx result code. Be aware of that if you wish to implement this, non-normative, section of this document. If the media server does not support announcements, it MUST respond with the 488 NOT ACCEPTABLE HERE response code. The INVITE to the media server contains the parameter "early=yes" to invoke the Early Media Prompting service. The media server responds with 100 TRYING followed by 183 SESSION PROGRESS. Assuming the initial invite included the 100rel extension header RFC3262 [8], which this document RECOMMENDS, the media server waits for the acknowledgement of the 183 (PRACK). At that point, the media server sends the announcement to the caller. The procedures in RFC3262 [8] describe what to do if the PRACK is late or does not arrive. In addition, RFC3262 [8] describes provisional offer/answer SDP negotiation. RFC3261 [2] describes the 183 SESSION PROGRESS result code. As stated above, if the Media Server cannot fetch the URI in the "play=" parameter, the Media Server will reply with a 404 NOT FOUND, possibly with an explanation of the failure in the Warning: header. Otherwise, after the media server completes the streaming of the prompt, it must send a 487 REQUEST TERMINATED to the Proxy. Note: When the early media service is used the requester is implicitly asking the media server to cancel the transaction as soon as the announcement is played. Since 487 is associated with an explicit CANCEL request it is appropriate for this use as well. The proxy sends the appropriate error response to the caller. That could be 487 or any other appropriate code reflective of the failure situation. Burger Expires December 29, 2003 [Page 20] Internet-Draft SIP Media Services June 2003 Note that this document does not define the behavior of an early media put on hold. 7.3 Protocol Diagram Caller Proxy Media Server | INVITE | | |----------------------->| INVITE | | 100 TRYING |----------------------->| |<-----------------------| 100 TRYING | | |<-----------------------| | | 183 SESSION PROGRESS | | 183 SESSION PROGRESS |<-----------------------| |<-----------------------| | | PRACK | | * |----------------------->| PRACK | * | |----------------------->| * | | 200 OK | * | 200 OK |<-----------------------| * |<-----------------------| | * | | | | Play Announcement (RTP) | |<================================================| | | 487 REQUEST TERMINATED | | 487 REQUEST TERMINATED |<-----------------------| |<-----------------------| | | ACK | ACK | |----------------------->|----------------------->| | | | * If the UAC requests or supports 100rel in the initial INVITE. Burger Expires December 29, 2003 [Page 21] Internet-Draft SIP Media Services June 2003 8. Security Considerations Untrusted network elements could use the protocol described here for providing information services. Many extant billing arrangements are for completed calls. Successful call completion occurs with a 2xx result code. This can be an issue for the early media announcement service. This is one of the many reasons why the early media announcement service is depreciated. Exposing network services with well-known addresses may not be desirable. In this case, the Media Server should offer local policy, e.g., only accept requests from authorized clients. Barring that, one can use a SIP Proxy to enforce the local policy. Burger Expires December 29, 2003 [Page 22] Internet-Draft SIP Media Services June 2003 9. Contributors Jeff Van Dyke and Andy Spitzer of SnowShore did just about all of the work developing netann, in conjunction with many application developers, media server manufacturers, and service providers, some of whom are listed in the Acknowledgements section. All I did was do the theory and write it up. That also means all of the mistakes are mine, as well. Burger Expires December 29, 2003 [Page 23] Internet-Draft SIP Media Services June 2003 10. Acknowledgements We would like to thank Kevin Summers and Ravindra Kabre of Sonus Networks for their constructive comments, as well as Jonathan Rosenberg of Dynamicsoft and Tim Melanchuk of Convedia for their encouragement. In addition, the discussion at the Las Vegas Interim Workgroup Meeting in 2002 was invaluable for clearing up the issues surrounding the left-hand-side of the request URI. Christer Holmberg helped tune the language of the multimedia announcement service. Orit Levin from Radvision gave a close read on the most recent version of the draft document. Pete Danielsen from Lucent has consistently provided excellent reviews of the many of the different versions of this document. Pascal Jalet provided the theoretical underpinning and David Rio provided the experimental evidence for why the conference identifer belongs in the user part of the request-URI. I am particularly indebted to Alan Johnston for his review of this document and ensuring its conformance with the SIP conference control work in the IETF. The authors would like to give a special thanks to Walter O'Connor for doing much of the initial implementation. Burger Expires December 29, 2003 [Page 24] Internet-Draft SIP Media Services June 2003 Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [3] ISO, "Codes for the representation of names of languages -- Part 1: Alpha-2 code", ISO 639-1, July 2002. [4] ISO, "Codes for the representation of names of countries and their subdivisions -- Part 1: Country codes", ISO 3166-1, October 1997. [5] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. [6] Borenstein, N. and N. Freed, "MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies", RFC 1521, September 1993. [7] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998. [8] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional Responses in Session Initiation Protocol (SIP)", RFC 3262, June 2002. Burger Expires December 29, 2003 [Page 25] Internet-Draft SIP Media Services June 2003 Informative References [9] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. [10] Burger, E., Van Dyke, J. and A. Spitzer, "SnowShore Media Server Control Markup Language and Protocol", draft-vandyke-mscml-00 (work in progress), November 2002. [11] Rosenberg, J., "A Framework for Conferencing with the Session Initiation Protocol", draft-ietf-sipping-conferencing-framework-00 (work in progress), May 2003. [12] World Wide Web Consortium, "Voice Extensible Markup Language (VoiceXML) Version 2.0", W3C Candidate Recommendation , February 2003, . [13] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., Beame, C., Eisler, M. and D. Noveck, "NFS version 4 Protocol", RFC 3010, December 2000. [14] Callaghan, B., "NFS URL Scheme", RFC 2224, October 1997. [15] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [16] Campbell, B. and R. Sparks, "Control of Service Context using SIP Request-URI", RFC 3087, April 2001. [17] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg, "SIP: Session Initiation Protocol", RFC 2543, March 1999. [18] Charlton, N., Gasson, M., Gybels, G., Spanner, M. and A. van Wijk, "User Requirements for the Session Initiation Protocol (SIP) in Support of Deaf, Hard of Hearing and Speech-impaired Individuals", RFC 3351, August 2002. Burger Expires December 29, 2003 [Page 26] Internet-Draft SIP Media Services June 2003 Author's Address Eric Burger SnowShore Networks, Inc. 285 Billerica Rd. Chelmsford, MA 01824-4120 USA EMail: e.burger@ieee.org Burger Expires December 29, 2003 [Page 27] Internet-Draft SIP Media Services June 2003 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION Burger Expires December 29, 2003 [Page 28] Internet-Draft SIP Media Services June 2003 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Burger Expires December 29, 2003 [Page 29]