Alert-Info URNs for the Session Initiation Protocol (SIP)
draft-alexeitsev-bliss-alert-info-urns-02

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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 tied rendering. To overcome this limitations and support new applications a family of the URNs 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.).

Table of Contents

1.  Introduction
2.  Registration template
3.  Top Level Alert URN Indication Definitions
    3.1.  PBX Tones
        3.1.1.  normal
        3.1.2.  external
        3.1.3.  internal
    3.2.  Service Tones
        3.2.1.  call-waiting
        3.2.2.  forward
        3.2.3.  transfer-recall
        3.2.4.  auto-callback
        3.2.5.  hold-recall
        3.2.6.  crisis
4.  Alert Level URN Sub-Indications Definitions
    4.1.  priority
    4.2.  short
    4.3.  delayed
5.  User Agent Behavior
6.  Proxy Behavior
7.  IANA Considerations
    7.1.  New alert-identifiers
    7.2.  Alert-category
    7.3.  Initial IANA Registration
8.  Internationalization Considerations
9.  Security Considerations
10.  Acknowledgements
11.  References
    11.1.  Normative References
    11.2.  Informative References
Appendix A.  An Appendix
§  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. Current solution is sufficient for human users that share the same understanding of the tones. However 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. 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 and support new applications 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.

Using 'alert' namespace provides syntax for several different application spaces:

• 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 services such as call waiting or automatic callback, not tied to any particular rendering.
• 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.

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.

2.  Registration template

Below is the registration template for the 'alert' URN scheme according to the RFC 3406 (Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom, “Uniform Resource Names (URN) Namespace Definition Mechanisms,” October 2002.) [RFC3406]

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:

Namespace Specific String (NSS) in the 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 [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 or service-indication. In other words, if a tone 'x.y.z' exists, the tone 'x' and 'x.y' are also valid tone. Each alert indication identifier SHALL explicitly define it's validity respective the sub-indications.

The ABNF [RFC4234] (Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” October 2005.) 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"/"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 the IANA Considerations (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 IANA Considerations (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 the IANA Considerations (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.

3.  Top Level Alert URN Indication Definitions

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

3.1.  PBX Tones

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

3.1.1.  normal

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 [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.

3.1.2.  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.

3.1.3.  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.

3.2.  Service Tones

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

3.2.1.  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.

3.2.2.  forward

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.

3.2.3.  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.

3.2.4.  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.

3.2.5.  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.

3.2.6.  crisis

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.

4.  Alert Level URN Sub-Indications Definitions

These sub-indications can be applied to any of the Top Level Alert Indications defined in the previous sections.

4.1.  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:internal.priority would be used to generate alerting for a priority internal PBX call.

4.2.  short

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

4.3.  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:external.delayed would generate an external ring-tone, delayed by a few seconds.

5.  User Agent Behavior

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.

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 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.

   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 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.

7.1.  New alert-identifiers

Alert URNs identifiers and alert-indications are identified by labels managed by IANA, according to the processes outlined in [RFC2434] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” October 1998.) 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-indications may differ for each alert-identifier category and MUST be defined by the document describing the coresponding alert-identifier.

Entries in the registration table have the following format:

   Category/Indication  Reference  Description
   --------------------------------------------------------------------
   foo                  RFC XXXX     Description of the 'foo' alert-category
   bar                  RFC YYYY     Description of the 'bar' alert-indication
                                     of the foo alert-category
   bar.moo              RFC ZZZZ     Description of the 'bar.moo'
                                     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' 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.

7.3.  Initial IANA Registration

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

Alert-Indication       Reference  Description
--------------------------------------------------------
normal                 RFC XXXX    Normal ring-tone
normal.priority        RFC XXXX    Priority normal ring-tone
normal.short           RFC XXXX    Short normal ring-tone
normal.delayed         RFC XXXX    Delayed normal ring-tone
internal               RFC XXXX    Internal PBX caller PBX ring-tone
internal.priority      RFC XXXX    Priority internal PBX caller PBX ring-tone
internal.short         RFC XXXX    Short internal PBX caller PBX ring-tone
internal.delayed       RFC XXXX    Delayed internal PBX caller PBX ring-tone
external               RFC XXXX    External PBX caller PBX ring-tone
external.priority      RFC XXXX    Priority external PBX caller PBX ring-tone
external.short         RFC XXXX    Short external PBX caller PBX ring-tone
external.delayed       RFC XXXX    Delayed external PBX caller PBX ring-tone

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

Alert-Indication             Reference  Description
--------------------------------------------------------
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.

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.

11.  References

11.1. Normative References

11.2. Informative References

Appendix A.  An Appendix

Authors' Addresses