Alert-Info URNs for the Session Initiation Protocol (SIP)

The Session Initiation Protocol (SIP) allows for user agent servers (UAS) and proxies to provide the specific ringback or ring tone to the user agent (UA). In RFC 3261 this is done by including a URI reference in the Alert-Info header field, that points to the tone. The URI reference is most commonly the HTTP URI to the audio file. On the receipt of the Alert-Info header the user agent may fetch the referenced ringback or ring tone and play it to the user. This mechanism does not ensure interoperability when there is no common understanding of the referenced content (different countries or vendors, hearing impaired) or when the user wants his own tones configured in the end device. If caller and callee are from the different countries, the understanding of the tones may vary significantly. Hearing impaired users may not sense the specific tone if it is provided as an audio file. The tone per se is also not useful for automata. There are currently interoperability issues around the use of the Alert-Info header field when not-using an external ring file. For example, consider the PBX special ringtone for an external (to the PBX) caller. Different vendors use different approaches such as: Alert-Info: <file://ring.pcm>;alert=normal where ring.pcm is a dummy file or: Alert-Info: <file://normal.ring.pcm> or: Alert-Info: <sip:normal-ringtone@example.com> As a result, Alert-Info currently only works when the same vendor provides proxy and UA, as only then is the same "fake" proprietary URI convention is used. Another limitation of the current solution is that the referenced tones are tied to particular rendering. It is not possible to provide a semantic indication that signals the intent and allows the recipient to decide how to render the received information in an appropriate way. To solve the described issues, this specification defines the new URN namespace 'alert' for the Alert-Info header that can be understood by an automaton, would allow for programmatic handling including user interface adaptation, or conversion to equivalent protocol parameters in the Public Switched Telephone Network (PSTN) when the client is a gateway. The work to standardize an Alert-Info URN will increase SIP interoperability for this header field by replacing proprietary conventions. Using 'alert' namespace provides syntax for several different application spaces: Names for services such as call waiting or automatic callback, not tied to any particular rendering. Names for common ring-tones generated by PBX phone for cases such as an internal enterprise caller, external caller, ringback after a transfer failure or expiration of a hold timer, etc. Names for country-specific tones. Names for things with specific renderings that aren't purely audio. They might be static icons, video sequences, text, etc. Some advantages of a URN rather than a URI reference to a downloadable resource: Do not need to download it or deal with security issues associated with dereferencing. No formatting or compatibility issues. No security risk of rendering something unexpected and undesirable. The tone can be stored locally in whatever format and at whatever quality level is appropriate, because it is specified "by name" rather than "by value". It is easier to make policy decisions about whether to use it or not. It facilitates translation for the hearing impaired. The downside is that if the recipient does not understand the URN then it will only be able to render a default ringback or ring-tone. To provide the general awareness about the Alert-Info URNs this document provides IANA template for registering the URNs and defines several typical identifiers.

This section discusses the requirements for an identifier to transport the semantic of a specific ring or ringback tone. REQ-1: The mechanism will allow user agents (UAs) and proxies to o provide a semantic indication in the Alert-Info SIP-header that signals the intent of the rendering and allows the recipient to decide how to render the received information. REQ-2: The mechanism will allow to ensure interoperability for services as call waiting, forward, call forwarding, transfer-recall, auto-callback, hold-recall, crisis. REQ-3: The mechanism will allow to render common PBX ring tone types. REQ-4: The mechanism will allow to render specific country ring- and busy-tones. REQ-5: The mechanism will allow to render tones for emergency alerts. REQ-6: The mechanism will allow rendering using other means than tones, e.g. text or images. REQ-7: The mechanism will allow rendering to be semantic, not biased towards a a particular representation which might not be suitable for all devices or users. REQ-7: The mechanism will allow to store the actual encoding locally rather than fetching it. REQ-9: The mechanism will allow the identifier to be specified "by name" rather than "by value", to enable local policy decisions whether to use it or not . REQ-10: The mechanism will be flexible and can be used for use cases not described in this specification . REQ-11: The mechanism will allow transmission in SIP requests and responses.

Below is the registration template for the 'alert' URN scheme according to the RFC 3406 alert 1 TBD IETF Laura Liess l.liess@telekom.de The Namespace Specific String (NSS) for the "alert" URNs is called alert-identifier and has a hierarchical structure. The left-most label is called "alert-category" and is separated from right-side of the alert-identifier, the alert-indication, by a semicolon. In this specification, two categories of alert-identifiers are described: the "tone" alert-identifiers and the "service" alert-identifiers. The "tone" alert-identifier has the general form: urn:alert:tone:{tone-indication} where the tone-indication identifies the semantic of a tone. The "service" alert-identifier has the general form: urn:alert:service:{service-indication} where the service-indication is a unique identifier for a service as call-waiting or transfer-recall. The tone-indication and service-indication are hierarchical identifiers, consisting of one label or a sequence of labels separated by periods. The left-most label is the most significant one and is called 'top-level indication' , while names to the right are called 'sub-indication'. The set of allowable characters is the same as that for domain names . Labels are case-insensitive, but MUST be specified in all lower-case. Labels can be removed right-to-left excepting the left-most label; the resulting tone-indication or service-indication is still valid, referring to a more generic tone-indication or service-indication. In other words, if a tone 'x.y.z' exists, the tones 'x' and 'x.y' are also valid tones. Each alert indication identifier SHALL explicitly define it's validity respective the sub-indications. The ABNF for the "tone" and "service" alert URNs is shown below:

None The alert URN is believed to be relevant to a large cross-section of Internet users, including both technical and non-technical users, on a variety of devices and with a variety of perception capabilities. The 'alert' URN will allow Internet users to receive more information and enable them to better make decisions about accepting an offered call, or get better feedback on the progress of a call they have made. User interfaces for the perception impaired users can better render the ringback indication based on the 'alert' URN. The assignment of identifiers is described in . The 'alert' URN does not prescribe a particular resolution mechanism, but it is assumed that a number of different entities could operate and offer such mechanisms. There do not appear to be other URN namespaces that serve the same need of uniquely identifying 'alert' communication and information services. An 'alert' URN identifies a logical service or tone, specified in the 'alert' indication registration (see ). Resolution of the registered URN will return a particular instance of the alert identifier. Alert identifier URNs MUST be unique for each unique indication; this is guaranteed through the registration of each alert indication within this namespace, described in . The 'alert' URN for the same indication is expected to be persistent, as long as it is registered with IANA. The process of identifier assignment is described in . 'alert' URNs are statically resolved according to the IANA registry. 'alert' URNs are compared according to case-insensitive string equality. The BNF in the 'Declaration of syntactic structure' above constrains the syntax for this URN scheme. Validation determines whether a given string is currently a validly-assigned URN . Static validation is performed based on the currently registered 'alert' URNs at IANA. The scope for this URN is public and global.

This section describes the use cases that are supported by the 'alert' URNs.

This section defines some commonly encountered ring-tones on PBX or business phones. They are as follows:

This tone indicates that the default or normal ring-tone should be rendered. This is most useful when Alert-Info header field parameters are being used. For example, in , an Alert-Info header field needs to be present containing the "appearance" parameter, but no special ringtone need be specified. If no special alerting is to be used, urn:alert:tone:normal should be used.

This tone is used to indicate that the caller is external to the enterprise or PBX system. This could be a call from the PSTN or from a SIP trunk. For this case, urn:alert:tone:external should be used.

This tone is used to indicate that the caller is internal to the enterprise or PBX system. The call could have been originated from another user on this PBX or on another PBX within the enterprise. For this case, the urn:alert:tone:internal should be used.

This section defines some commonly encountered tones in the public telephone service, according to the and . They are as follows.

This tone indicates that the default or normal ring-tone should be rendered. For this case, urn:alert:tone:ringing should be used.

This tone indicates that a tone should be rendered that indicates to the caller that the callee is busy. For this case, urn:alert:tone:busy should be used.

This tone indicates that a tone should be rendered that indicates to the participants in a call that the privacy of the conversation has been breached. For this case, urn:alert:tone:intrusion should be used.

This tone indicates that a tone should be rendered which advises the caller when to begin a message which will be recorded. For this case, urn:alert:tone:record should be used.

These tones are used to indicate specific PBX and telephony services.

The call waiting Service permits a callee to be notified of an incoming call whilst the media resources are not available for the incoming call and the callee is engaged in an active or held call. Subsequently, the callee can either accept, reject, or ignore the incoming call. There is an interest on the caller side to be informed about the call waiting situation on the callee side. Having this information the caller can decide whether to continue waiting for callee to pickup or better to call some time later when it is estimated that the callee could have finished the ongoing conversation. To provide this information, the callee's UAS or proxy aware of the call waiting condition can add the call-waiting indication URN to the Alert-Info header. As call-waiting information may be subject to the callee's privacy concerns, the exposure of this information SHALL be done only if explicitly required by the user. The urn:alert:service:call-waiting should be used for this case.

This feature is used in a 180 response when a call forwarding feature has been initiated on an INVITE. Many PBX system implement a forwarding "beep" followed by normal ringing to indicate this. The urn:alert:service:forward should be used for this case. Note that a 181 response can be used in place of this URN.

This feature is used when a blind transfer has been performed by a server on behalf of the transferor and fails. Instead of failing the call, the server calls back the transferor, giving them another chance to transfer or otherwise deal with the call. This service tone is used to distinguish this INVITE from any other normal incoming call. The urn:alert:service:transfer-recall should be used for this case.

This feature is used when a user has utilized a server to implement an automatic callback service. When the user is available, the server calls back the user and utilizes this service tone to distinguish this from any other normal incoming call. The urn:alert:service:auto-callback should be used for this case.

This feature is used when a server implements a call hold timer on behalf of an endpoint. After a certain period of time of being on hold, the user who placed the call on hold is alerted to either retrieve the call or otherwise dispose of the call. This service tone is used to distinguish this case from any other normal incoming call. The urn:alert:service:hold-recall should be used for this case.

This feature is used for emergency announcements sent over PBX systems such as building evacuations, alarms, etc. This service tone is used to distinguish this case from any other normal incoming call. The urn:alert:service:crisis should be used for this case.

This sub-level is used to indicate that a priority level alert should be applied for the type of alerting specified. For example, urn:alert:tone:priority would be used to generate priority alerting.

This sub-level is used to indicate that the alert type specified should be rendered shorter than normal. In contact centers, this is sometimes referred to as "abbreviated ringing" or a "zip tone". For example, urn:alert:tone:short would generate a shorter than normal ring-tone.

This sub-level is used to indicate that the alerting type specified show be rendered after a short delay. In some bridged line/shared line appearance implementations, this is used so that the bridged line does not ring at exactly the same time as the main line, but is delayed a few seconds. For example, urn:alert:tone:delayed would generate an external ring-tone, delayed by a few seconds.

In the PSTN, different tones are used in different countries. End users are accustomed to hear the callee's country ringing or busy tone. The ISO 3166-1 country code is used as a top-level to indicate which country specific tone should be rendered. For example, to indicate tones from South Africa, the following URN would be used: <urn:alert:tone:za> The particular rendering for many country specific ringing and busy tones is described in :

In some cases, more than one URN will need to be specified to fully define a particular tone. This is done by including multiple URNs. For example, an internal, priority call could be indicated by Alert-Info: <urn:alert:tone:internal>, <urn:alert:tone:priority>. A priority call waiting tone could be indicated by Alert-Info: <urn:alert:service:call-waiting>, <urn:alert:tone:priority>. And finally, a short Albanian auto-callback tone could be indicated Alert-Info: <urn:alert:service:auto-callback>, <urn:alert:tone:short>, <urn:alert:tone:al>.

Upon receiving a SIP request or a SIP 180 Ringing response with an Alert-Info header that contains a single or multiple 'alert' URNs, the User Agent (UA) attempts to match the received URNs with the known indications. If no match is found, the User Agent (UA) ignores the received 'alert' URNs and proceeds with the normal operation. If the one or multiple URNs matches a known indication, the User Agent (UA) renders the indication(s) to the user according to the tone or service identifier in the received alert URN. The User Agent (UA) is responsible for the non disturbing rendering if multiple indications and network resources are to be rendered simultaneously.

A SIP proxy MAY add a URN or multiple URNs to the Alert-Info header in a SIP request or a 180 Ringing provisional response when it needs to provide additional information about the call or about the provided service. A SIP Proxy SHOULD NOT add a mixture of the 'alert' URNs and URIs to the Alert-Info header that may cause disturbing rendering interference at the recepient's User Agent (UA). Following example shows both the network audio resource referenced by the HTTP URI and the URN indication for the call-waiting service transported by the Alert-Info header in a 180 Ringing provisional response.

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This section registers a new URN scheme with the registration template provided in section Registration Template. Below, the section 7.1 details how to register new alert-identifiers. Descriptions of alert-indications for the first two alert-identifiers described in this document , service and tone, are given in Section 7.2 and Section 7.3, respectively. Finally, Section 7.4 contains the initial registration table.

Alert URNs are identified by labels managed by IANA, according to the processes outlined in in a new registry called "Alert URN Labels". Thus, creating a new alert-identifier requires IANA action. The policy for adding a new alert-identifier category is 'Standards Action'. (This document defines the alert-identifier categories 'service' and 'tone'.) The policy for assigning labels to alert top-level indications or sub-indications and the rules to combine top-level indications and sub-indications may differ for each alert-identifier category and MUST be defined by the document describing the coresponding alert indications. The entries in the registration table have the following format:

Each alert-category or alert-indication label MUST NOT exceed 27 characters.

This section defines the alert-identifier registration within the IANA registry defined in Section 7.1, using the alert-category labels 'service' and 'tone'. The 'tone' alert-category label provides information about ringing tones generated by a UAS and rendered to the callee in response to an INVITE. The 'service' alert-category label describes tones that should be generated by the UAC or UAS and rendered to the caller or calee upon receipt of a 180 response or a request. The normal rendering is audio, however there can be other renderings applicable if needed by the user interface specifics.

The following table contains the initial IANA registration for "tone" top-level alert URN indications for PBX-tones.

The following table contains the initial IANA registration for "tone" top-level alert URN indications for public telephone network tones.

The following list contains the initial IANA registration for alert URN indications for national variant public telephone network tones.

The following table contains the initial IANA registration for "service" alert-indications.

Editor's Note: RFC XXXX should be replaced with this specification.

Alert URN Indications from the same group should not be combined.

The alert-identifier labels are protocol elements and are not normally seen by users. Thus, the character set for these elements is restricted, as described in Section 6.

As an identifier, the alert URN does not appear to raise any particular security issues. The indications described by the 'alert' URN are meant to be well-known, so privacy considerations do not apply to the URN. Provision of the specific indications from callee to caller may raise privacy issues. Such provision SHALL always be explicitly authorised by the callee.

The draft is based on the ideas expressed by Paul Kyzivat on the BLISS WG mailing list. The authors wish to thank Dean Willis, Adam Roach, Paul Kyzivat, Martin Huelsemann, Shida Schubert, John Elwell and Tom Taylor for their comments and suggestions.