Alert-Info URNs for the Session Initiation Protocol (SIP)
draft-liess-dispatch-alert-info-urns-01

Abstract

The Session Initiation Protocol (SIP) supports the capability to provide a reference to the alternative ringback tone (RBT) for caller, or ring tone (RT) for callee using the Alert-Info header. However, the reference addresses only the network resources with specific rendering properties. There is currently no support for predefined standard identifiers for ringback tones or semantic indications without being tied to a particular rendering. To overcome this limitation and support new applications, a new family of URNs for use in SIP Alert-Info header fields is defined in this specification.

Requirements Language

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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

Status of this Memo

This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and 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.

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Copyright Notice

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Table of Contents

1.  Introduction
2.  Requirements
3.  Registration template
4.  Top Level Alert URN Indication Definitions
    4.1.  PBX Tones
        4.1.1.  normal
        4.1.2.  external
        4.1.3.  internal
    4.2.  Service Tones
        4.2.1.  call-waiting
        4.2.2.  forward
        4.2.3.  transfer-recall
        4.2.4.  auto-callback
        4.2.5.  hold-recall
        4.2.6.  priority
        4.2.7.  zip
        4.2.8.  delayed
    4.3.  Country-specific ringback tone indications for the public telephone network
    4.4.  Combinations of URNs
5.  User Agent Behavior
6.  Proxy Behavior
7.  IANA Considerations
    7.1.  New alert-identifiers
    7.2.  Alert-category
    7.3.  Initial IANA Registration
        7.3.1.  Indications for PBX-tones
        7.3.2.  Additional Indications for PBX-tones
        7.3.3.  Indications for Country-specific Ringback tones in the public telephone network
        7.3.4.  Indications for Services
        7.3.5.  Combination Rules for Alert URN Indications
8.  Internationalization Considerations
9.  Security Considerations
10.  Acknowledgements
11.  References
    11.1.  Normative References
    11.2.  Informative References
§  Authors' Addresses

1.  Introduction

The Session Initiation Protocol (SIP) [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) 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 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 phones 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 ringback 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 specification does not change the usage of the SIP Alert-Info-header defined in the RFC3261. The Alert-Info-header can be used in INVITE requests and 180 Ringing responses.

2.  Requirements

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 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 interoperability for services as call waiting, forward, call forwarding, transfer-recall, auto-callback, hold-recall, crisis.

REQ-3: The mechanism will allow rendering common PBX ring tone types.

REQ-4: The mechanism will allow rendering specific country ringback tones.

REQ-5: The mechanism will allow rendering 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 particular representation which might not be suitable for all devices or users. The Alert-Info URN must be semantic whenever the semantic is known to the sender.

REQ-8: The mechanism will allow non-semantic, rendering-oriented Alert-Info URNs when the semantic of the rendering is not known by the sender and the sender can assume that there is a good chance that for the receiver to know how to render the non-semantic URN. This is the case, e.g. for:

- ringing tones used in closed in enterprise networks

- ringing tones used in some countries

- when TDM gateways must map ring-/ringback-tones from legacy protocols to SIP at the edge of a network .

REQ-9: The mechanism must ensure that an UA receiving Alert-Info URNs or portions of an Alert-Info URN it does not understand, it can ignore them.

REQ-10: The mechanism will allow storage of the actual encoding locally rather than fetching it.

REQ-11: 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-12: The mechanism will allow transmission in the Alert-Info header of SIP INVITE requests and 180 Ringing responses.

REQ-13: The mechanism will be flexible, so new identifiers can be defined in the future, when SIP-applications evolve. E.g. Alert-Info URNs could identify specific media by name, such as "Beethoven's Fifth", and the end device could render some small part of it as a ring tone.

3.  Registration template

Below is the registration template for the 'alert' URN scheme according to the [RFC2141] (Moats, R., “URN Syntax,” May 1997.) and [RFC3406] (Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom, “Uniform Resource Names (URN) Namespace Definition Mechanisms,” October 2002.)

Namespace ID:

alert

Registration Information:

Registration version:

1

Registration date:

TBD

Declared registrant of the namespace:

Registering organization:

IETF

Designated contact:

Laura Liess

Designated contact email:

l.liess@telekom.de

Declaration of syntactic structure:

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, three categories of alert-identifiers are described: the "tone" alert-identifiers, the "ringback" 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 or rendering characteristics of a ring tone. Alert-Info URNs containing the "tone" alert-identifier can be used in INVITE requests, but not in 180 Ringing responses.

The "ringback" alert-identifier has the general form:

urn:alert:ringback:{ringback-indication}

where the ringback-indication is a unique identifier for a country-specific ringback tone. Alert-Info URNs containing the "ringback" alert-identifier can be used in 180 Ringing responses, but not in INVITE requests.

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. Alert-Info URNs containing the "service" alert-identifier can be used in both INVITE requests and 180 Ringing responses.

The tone-indication, the ringback-indication and the 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 [RFC1123] (Braden, R., “Requirements for Internet Hosts - Application and Support,” October 1989.). 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, ringback-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 [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.) for the "tone" and "service" alert URNs is shown below:

     alert-URN       = "URN:alert:" alert-identifier
     alert-identifier= alert-category ":" alert-indication
     alert-category  = "tone"/  "ringback"/  "service"
     alert-indication= top-level *("." sub-indication)
     top-level       = let-dig [ *25let-dig-hyp let-dig ]
     sub-indication  = let-dig [ *let-dig-hyp let-dig ]
     let-dig-hyp     = let-dig / "-"
     let-dig         = ALPHA / DIGIT
     ALPHA           = %x41-5A / %x61-7A   ; A-Z / a-z
     DIGIT           = %x30-39 ; 0-9

Relevant ancillary documentation:

None

Community considerations:

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 Section 7 (IANA Considerations). 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.

Namespace considerations:

There do not appear to be other URN namespaces that serve the same need of uniquely identifying 'alert' communication and information services.

Identifier uniqueness considerations:

An 'alert' URN identifies a logical service or tone, specified in the 'alert' indication registration (see Section 7 (IANA Considerations)). 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 Section 7 (IANA Considerations).

Identifier persistence considerations:

The 'alert' URN for the same indication is expected to be persistent, as long as it is registered with IANA.

Process of identifier assignment:

The process of identifier assignment is described in Section 7 (IANA Considerations).

Process for identifier resolution:

'alert' URNs are statically resolved according to the IANA registry.

Rules for lexical equivalence:

'alert' URNs are compared according to case-insensitive string equality.

Conformance with URN syntax:

The BNF in the 'Declaration of syntactic structure' above constrains the syntax for this URN scheme.

Validation mechanism:

Validation determines whether a given string is currently a validly-assigned URN [RFC3406] (Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom, “Uniform Resource Names (URN) Namespace Definition Mechanisms,” October 2002.). Static validation is performed based on the currently registered 'alert' URNs at IANA.

Scope:

The scope for this URN is public and global.

4.  Top Level Alert URN Indication Definitions

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

4.1.  PBX Tones

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

4.1.1.  normal

This tone indicates that the default or normal ring-tone should be rendered. This is essentially a no-operation Alert-Info URN and should be treated by the UA as if no Alert-Info URN is present. This is most useful when Alert-Info header field parameters are being used. For example, in [I‑D.ietf‑bliss‑shared‑appearances] (Johnston, A., Soroushnejad, M., and V. Venkataramanan, “Shared Appearances of a Session Initiation Protocol (SIP) Address of Record (AOR),” March 2010.), 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.

4.1.2.  external

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.

4.1.3.  internal

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.

4.2.  Service Tones

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

4.2.1.  call-waiting

The call waiting Service [TS24.615] (, “3GPP TS 24.615 Communication Waiting (CW) using IP Multimedia (IM) Core Network (CN) subsystem,” .) 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.

4.2.2.  forward

This feature is used in a 180-Ringing 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.

4.2.3.  transfer-recall

This feature is used when a blind transfer [RFC5589] (Sparks, R., Johnston, A., and D. Petrie, “Session Initiation Protocol (SIP) Call Control - Transfer,” June 2009.) 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.

4.2.4.  auto-callback

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.

4.2.5.  hold-recall

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.

4.2.6.  priority

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.

4.2.7.  zip

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:zip would generate a shorter than normal ring-tone.

4.2.8.  delayed

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 a normal ring-tone, delayed by a few seconds.

4.3.  Country-specific ringback tone indications for the public telephone network

In the PSTN, different tones are used in different countries. End users are accustomed to hear the callee's country ringback tone.

The ISO 3166-1 country code [ISO 3166‑1] (, “ISO 3166-1 English country names and code elements,” .) is used as a top-level indication to infor the UAC about the country-specific tone which should be rendered to the caller. For example, to indicate ringback tones from South Africa, the following URN would be used: <urn:alert:ringback:za>.

4.4.  Combinations of URNs

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>.

5.  User Agent Behavior

Upon receiving a SIP INVITE 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 Alert-Info URNs with the known indications or indication combinations. The User Agent (UA) ignores the Alert-Info URNs for which no match is found and proceeds with the normal operation. If one or multiple URNs match(es) a known indication or a known indication combination, the User Agent (UA) renders the indication or the indication combination to the user accordingly. The User Agent (UA) is responsible for the non disturbing rendering if multiple indications and network resources are to be rendered simultaneously.

6.  Proxy Behavior

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 'alert' URNs and URIs to the Alert-Info header that may cause disturbing rendering interference at the recipient'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.

   Alert-Info: <http://www.example.com/sound/moo.wav>,
    <urn:alert:service:call-waiting>

7.  IANA Considerations

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 three alert-identifiers described in this document , 'service', 'tone' and 'ringback', are given in Section 7.2 and Section 7.3, respectively. Finally, Section 7.4 contains the initial registration table.

7.1.  New alert-identifiers

Alert URNs are identified by labels managed by IANA, according to the processes outlined in [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.) 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' , 'tone' and 'ringback'.) 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 corresponding alert indications. The entries in the registration table have the following format:

   Category/           Reference    Description
   Top-Indication/
   Sub-Indication
   ------------------------------------------------------------
   foo                 RFC XXXX     Description of the 'foo'
                                    alert-category
   bar                 RFC YYYY     Description of the 'bar'
                                    alert-indication of the foo
                                    alert-category

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

7.2.  Alert-category

This section defines the alert-identifier registration within the IANA registry defined in Section 7.1, using the alert-category labels 'service', 'tone' and 'ringback'.

The 'tone' alert-category label provides information about ringing tones rendered by a UAS to the callee. The 'tone' alert-category label may be generated by a UAC or a proxy and transmitted in a INVITE request to the UAS.

The ‘ringback’ alert-category label provides information about ringback tone rendered by a UAC to the caller. The 'ringback' alert-category label may be generated by a UAS or a proxy and transmitted in a 180 Ringing response.

The 'service' alert-category label describes special tones rendered by a UAC or the UAS to the caller or the callee. The 'service' alert-category label may be generated by a UAC, a UAS or a proxy and transmitted in a INVITE request or a 180 Ringing response.

The normal rendering is audio, however there can be other renderings applicable if needed by the user interface specifics.

7.3.  Initial IANA Registration

7.3.1.  Indications for PBX-tones

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

    Top-level     Reference   Description
    Indication
    -----------------------------------------------------------
    normal         RFC XXXX    Normal ring-tone
    internal       RFC XXXX    Internal PBX caller PBX ring-tone
    external       RFC XXXX    External PBX caller PBX ring-tone

7.3.2.  Additional Indications for PBX-tones

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

    Top-level    Reference   Description
    Indication
    ------------------------------------------------
    priority     RFC XXXX    Priority ring-tone
    zip          RFC XXXX    zip ring-tone
    delayed      RFC XXXX    Delayed ring-tone

7.3.3.  Indications for Country-specific Ringback tones in the public telephone network

For the alert-category "ringback", the alert-indication contains the ISO 3166-1 country code [ISO 3166-1]. For example, to indicate the Romanian ringback tone, the country-code “ro” is used and the Alert-Info URN will be <urn:alert:ringback:ro>.

7.3.4.  Indications for Services

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

        Top-level           Reference   Description
        Indication
        --------------------------------------------------------
        call-waiting     RFC XXXX    Call waiting service indication
        forward          RFC XXXX    Call forwarding service
                                     indication
        transfer-recall  RFC XXXX    Transfer Recall service
                                     indication
        auto-callback    RFC XXXX    Auto Callback service indication
        hold-recall      RFC XXXX    Hold Recall service indication
        crisis           RFC XXXX    Crisis alerting indication

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

7.3.5.  Combination Rules for Alert URN Indications

Senders may insert one or more Alert-Info URNs. Following rules apply:

- Senders should not insert two different URNs from one of the groups described above.

- Any indication for PBX-tones can be combined with any additional indication for PBX-tones.

- Indications and additional indications for PBX-tones can not be combined with indication for country specific ringback (they are sent in diferent messages).

-Indications and additional indications for PBX-tones as well as indication for country specific ringback can be combind with indications for services which can be sent in the same SIP message (INVITE or 180 Ringing).

- For stand-alone additional indication for PBX-tones, the receiver should assume the "normal" indication for PBX-tones.

UAs may have rules how to render specific combinations of Alert-Info URNs. An UA receiving a combination of Alert-Info URN which it does not know how to render, it may choose to render a subset of it or to ignore the content of the Alert-Info URN Header.

8.  Internationalization Considerations

The alert-identifier labels are protocol elements [RFC3536] (Hoffman, P., “Terminology Used in Internationalization in the IETF,” May 2003.) and are not normally seen by users. Thus, the character set for these elements is restricted, as described in Section 6.

9.  Security Considerations

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.

10.  Acknowledgements

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

11.  References

11.1. Normative References

11.2. Informative References

Authors' Addresses