Internet Engineering Task Force (IETF) L. Liess, Ed.
Request for Comments: 7462 R. Jesske
Updates: 3261 Deutsche Telekom AG
Category: Standards Track A. Johnston
ISSN: 2070-1721 Avaya
D. Worley
Ariadne
P. Kyzivat
Huawei
March 2015
URNs for the Alert-Info Header Field of the
Session Initiation Protocol (SIP)
Abstract
The Session Initiation Protocol (SIP) supports the capability to
provide a reference to a specific rendering to be used by the User
Agent (UA) as an alerting signal (e.g., a ring tone or ringback tone)
when the user is alerted. This is done using the Alert-Info header
field. However, the reference (typically a URL) addresses only a
specific network resource with specific rendering properties. There
is currently no support for standard identifiers for describing the
semantics of the alerting situation or the characteristics of the
alerting signal, without being tied to a particular rendering. To
overcome these limitations and support new applications, a new family
of URNs for use in Alert-Info header fields (and situations with
similar requirements) is defined in this specification.
This document normatively updates RFC 3261, which defines the Session
Initiation Protocol (SIP). It changes the usage of the Alert-Info
header field defined in RFC 3261 by additionally allowing its use in
any non-100 provisional response to INVITE. This document also
permits proxies to add or remove an Alert-Info header field and to
add or remove Alert-Info header field values.
Liess, et al. Standards Track [Page 1]RFC 7462 Alert URNs March 2015
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7462.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
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Liess, et al. Standards Track [Page 2]
RFC 7462 Alert URNs March 2015
Table of Contents
1. Introduction ....................................................5
2. Requirements Language ...........................................7
3. Terminology .....................................................7
4. Updates to RFC 3261 .............................................7
4.1. Allow Alert-Info in Provisional Responses ..................7
4.2. Proxies May Alter Alert-Info Header Fields .................8
5. Requirements ....................................................8
6. Use Cases ......................................................10
6.1. PBX Ring Tones ............................................10
6.1.1. Normal .............................................10
6.1.2. External ...........................................10
6.1.3. Internal ...........................................11
6.1.4. Priority ...........................................11
6.1.5. Short ..............................................11
6.1.6. Delayed ............................................11
6.2. Service Tones .............................................11
6.2.1. Call Waiting .......................................11
6.2.2. Forward ............................................12
6.2.3. Transfer Recall ....................................12
6.2.4. Auto Callback ......................................12
6.2.5. Hold Recall ........................................12
6.3. Country-Specific Ringback Tone Indications for the
Public Switched ...........................................12
7. URN Specification for the "alert" Namespace Identifier .........12
8. "alert" URN Values .............................................18
8.1. <alert-category> Values ...................................18
8.2. <alert-indication> Values .................................18
8.2.1. <alert-indication> Values for the
<alert-category> "service" .........................19
8.2.2. <alert-indication> Values for the
<alert-category> "source" ..........................19
8.2.3. <alert-indication> Values for the
<alert-category> "priority" ........................19
8.2.4. <alert-Indication> Values for the
<alert-category> "duration" ........................20
8.2.5. <alert-indication> Values for the
<alert-category> "delay" ...........................20
8.2.6. <alert-indication> Values for the
<alert-category> "locale" ..........................20
9. IANA Considerations ............................................20
9.1. URN Namespace Identifier "alert" ..........................20
9.2. 'Alert URN Identifiers' Registry ..........................20
9.2.1. Initial IANA Registration ..........................21
9.2.1.1. The "service" <alert-category> and
<alert-identifier>s .......................22
Liess, et al. Standards Track [Page 3]RFC 7462 Alert URNs March 2015
9.2.1.2. The "source" <alert-category> and
<alert-identifier>s .......................23
9.2.1.3. The "priority" <alert-category>
and <alert-identifier>s ...................24
9.2.1.4. The "duration" <alert-category>
and <alert-identifier>s ...................24
9.2.1.5. The "delay" <alert-category> and
<alert-identifier>s .......................25
9.2.1.6. The "locale" <alert-category> and
<alert-identifier>s .......................25
9.3. 'Alert URN Providers' Registry ............................26
10. Extension Rules ...............................................26
10.1. General Extension Rules ..................................26
10.2. Private Extension Rules ..................................27
10.3. Examples .................................................28
10.3.1. Subsetting an Existing URN ........................28
10.3.2. A New Value within an <alert-category> ............29
10.3.3. A New <alert-category> ............................29
10.3.4. Subsetting a Private Extension URN ................29
11. Combinations of "alert" URNs ..................................30
11.1. Priority Rules ...........................................30
11.2. Multi-mode Signals .......................................31
12. Non-normative Algorithm for Handling Combinations of URNs .....32
12.1. Algorithm Description ....................................32
12.2. Examples of How the Algorithm Works ......................34
12.2.1. Example 1 .........................................34
12.2.2. Example 2 .........................................35
12.2.3. Example 3 .........................................37
12.2.4. Example 4 .........................................38
12.2.5. Example 5 .........................................39
13. User Agent Behaviour ..........................................40
14. Proxy Behaviour ...............................................41
15. Internationalization Considerations ...........................42
16. Security Considerations .......................................42
17. References ....................................................43
17.1. Normative References .....................................43
17.2. Informative References ...................................44
Acknowledgements ..................................................45
Authors' Addresses ................................................46
Liess, et al. Standards Track [Page 4]RFC 7462 Alert URNs March 2015
1. Introduction
The Session Initiation Protocol (SIP) [RFC3261] includes a means to
suggest to a User Agent (UA) a particular ringback tone or ring tone
to be used during session establishment. In [RFC3261], this is done
by including a URI, in the Alert-Info header field, that specifies a
reference to the tone. The URI is most commonly the HTTP URL to an
audio file. On the receipt of the Alert-Info header field, the UA
may fetch the referenced ringback tone or ring tone and play it to
the user.
This mechanism hinders interoperability when there is no common
understanding of the meaning of the referenced tone, which might be
country- or vendor-specific. It can lead to problems for the user
trying to interpret the tone and for the UA wanting to substitute its
own tone (e.g., in accordance with user preferences) or provide an
alternative alerting mode (e.g., for deaf and hard-of-hearing users).
If the caller and the callee are from different countries, their
understanding of the tones may differ significantly. Deaf or hard-
of-hearing 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 using URLs of audio files is that the
referenced tones are tied to particular renderings. There is no
method to signal the semantic intention of the alert while enabling
the recipient UA to choose the specific alert indication (such as a
particular tone, vibration, or visual display) to use to signal the
intention. Similarly, there is no method to signal particular
rendering features (such as short duration, delay, or country-
specific conventions).
The issues with URLs that reference audio files can be avoided by
using fixed URLs with specific meanings. However, this approach has
its own interoperability issues. For example, consider the Private
Branch Exchange (PBX) special ring tone for an external (to the PBX)
caller. Different vendors use different approaches such as:
Alert-Info: <file://ring.pcm>;alert=external
where ring.pcm is a dummy file name, or:
Alert-Info: <file://external.ring.pcm>
Alert-Info: <sip:external-ringtone@example.com>
As a result, the Alert-Info header field currently only works when
the same vendor provides a PBX and UA, and only then if the same
artificial proprietary URI convention is used.
Liess, et al. Standards Track [Page 5]RFC 7462 Alert URNs March 2015
To solve the described issues, this specification defines the new URN
namespace "alert" for the SIP Alert-Info header field that allows for
programmatic user interface adaptation and for conversion of
equivalent alerting tones in the Public Switched Telephone Network
(PSTN) when the client is a gateway. The work to standardize an
"alert" URN will increase SIP interoperability for this header field
by replacing proprietary conventions used today.
The "alert" namespace provides a syntax for several different
application spaces, for example:
o Names for service indications, such as call waiting or automatic
callback, not tied to any particular rendering.
o Names for common ring tones generated by PBX phones for cases such
as an internal enterprise caller, external caller, ringback tone
after a transfer failure or expiration of a hold timer, etc.
o Names for country-specific ringback tones.
o 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 URL of a downloadable
resource:
o There is no need to download it or deal with security issues
associated with dereferencing.
o There are no formatting or compatibility issues.
o There is no security risk of rendering something unexpected and
undesirable.
o 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".
o It is easier to make policy decisions about whether or not to use
it.
o It facilitates translation for the deaf and hard of hearing.
The downside is that if the recipient does not understand the URN,
then it will only be able to render a default ringback tone or ring
tone.
Liess, et al. Standards Track [Page 6]RFC 7462 Alert URNs March 2015
This document creates a new URN namespace and registry for alert
indications and registers some initial values.
In practice, this specification extends the usage of the Alert-Info
header field in that it will cause the use of a new class of URIs and
the use of multiple URIs. Backward compatibility issues are not
expected, as devices that do not understand an "alert" URN should
ignore it, and devices should not malfunction upon receiving multiple
Alert-Info header field values (<alert-param>s in [RFC3261]) (which
was syntactically permitted before, but rarely used).
2. 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].
3. Terminology
This specification uses a number of terms to refer to the roles
involved in the use of alerting indications in SIP. A "specifier"
sends an "alerting indication" (one or more URNs in an Alert-Info
header field) to a "renderer", which then "renders" a "signal" or
"rendering" based on the indication to a human user. A "category" is
a characteristic whose "values" can be used to classify indications.
This specification uses the terms "ring tone" and "ringback tone". A
"ring tone" or "calling signal" (terminology used in [E182]) is a
signal generated by the callee's end device, advising the callee
about an incoming call. A "ringback tone" or "ringing tone"
(terminology used in [E182]) is a signal advising the caller that a
connection has been made and that a ring tone is being rendered to
the callee.
4. Updates to RFC 3261
4.1. Allow Alert-Info in Provisional Responses
This specification changes the usage of the Alert-Info header field
defined in [RFC3261] by additionally allowing its use in any non-100
provisional response to INVITE.
Previously, the Alert-Info header field was only permitted in 180
(Ringing) responses. But in telephony, other situations indicated by
SIP provisional responses, such as 181 (Call Is Being Forwarded) and
182 (Call Is Being Queued), are often indicated by tones. Extending
the applicability of the Alert-Info header field allows the telephony
practice to be implemented in SIP.
Liess, et al. Standards Track [Page 7]
RFC 7462 Alert URNs March 2015
To support this change, the following paragraph replaces the the
first paragraph of Section 20.4 of [RFC3261]:
When present in an INVITE request, the Alert-Info header field
specifies an alternative ring tone to the User Agent Server (UAS).
When present in a non-100 provisional response, the Alert-Info
header field specifies an alternative ringback tone to the UAC. A
typical usage is for a proxy to insert this header field to
provide a distinctive ring feature.
4.2. Proxies May Alter Alert-Info Header Fields
A SIP proxy MAY add or remove an Alert-Info header field, and it MAY
add or remove Alert-Info header field values, in a SIP request or a
non-100 provisional response.
5. Requirements
This section discusses the requirements for an alerting indication to
transport the semantics of the alerting situation or the
characteristics of the rendering.
REQ-1: The mechanism will allow UAs and proxies to provide in the
Alert-Info header field an alerting indication that describes
the semantics of the signaling situation or the
characteristics of the rendering and allows the recipient to
decide how to render the received information to the user.
REQ-2: The mechanism will allow the alerting indication to be
specified "by name" rather than "by value", to enable local
policy decisions whether or not to use it.
REQ-3: The mechanism will enable alerting indications to represent a
wide variety of signals, which have many largely orthogonal
characteristics.
REQ-4: The mechanism will enable the set of alerting indications to
support extensibility by a wide variety of organizations that
are not coordinated with each other. Extensions will be able
to:
add further values to any existing category
add further categories that are orthogonal to existing
categories
semantically subdivide the meaning provided by any
existing indication
Liess, et al. Standards Track [Page 8]RFC 7462 Alert URNs March 2015
REQ-5: The mechanism will be flexible, so new alerting indications
can be defined in the future, when SIP-applications evolve.
For example, "alert" 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.
REQ-6: The mechanism will provide only an indication capability,
not a negotiation capability.
REQ-7: The mechanism will not require an alerting indication to
depend on context provided by a previous alerting indication
in either direction.
REQ-8: The mechanism will allow transmission in the Alert-Info
header field of SIP INVITE requests and provisional 1xx
responses excepting the 100 responses.
REQ-9: The mechanism will be able to accommodate both renderers
that are customized with a limited or uncommon set of
signals that they can render and renderers that are provided
with a set of signals that have uncommon semantics. (The
canonical example is a UA for the deaf and hard of hearing,
customized with an alternative set of signals, video or text
instead of audio. By REQ-6, the renderer has no way of
transmitting this fact to the specifier.)
REQ-10: The mechanism will allow an alerting indication to reliably
carry all extensions if the specifier and the renderer have
designs that are properly coordinated.
REQ-11: The mechanism will allow a renderer to select a tone that
approximates to that intended by the specifier if the
renderer is unable to provide the precise tone indicated.
REQ-12: The mechanism will support alerting indications relating to
services such as call waiting, call forwarding, transfer
recall, auto callback, and hold recall.
REQ-13: The mechanism will allow rendering common PBX ring tone
types.
REQ-14: The mechanism will allow rendering specific country ringback
tones.
REQ-15: The mechanism will allow rendering tones for emergency
alerts. (Use cases and definitions of URN values for
emergency calls are not a subject of this specification.)
Liess, et al. Standards Track [Page 9]RFC 7462 Alert URNs March 2015
REQ-16: The mechanism will allow rendering using other means than
tones, e.g., text or images.
REQ-17: The mechanism will allow PSTN gateways to map ring/ringback
tones from legacy protocols to SIP at the edge of a network,
e.g., national ring tones as defined in TIA/EIA-41-D and
3GPP2 A.S0014. (Use cases and values definition for this
situation are not a subject of this specification.)
REQ-18: The mechanism will ensure that if an UA receives "alert"
URNs or portions of an "alert" URN it does not understand,
it can ignore them.
REQ-19: The mechanism will allow storage of the actual encoding of
the rendering locally rather than fetching it.
REQ-20: The mechanism must provide a simple way to combine two or
more alerting indications to produce an alerting indication
that requests a combination of the intentions of the two
alerting indications, where any contradictions or conflicts
between the two alerting indications are resolved in favor
of the intention of the first alerting indication.
6. Use Cases
This section describes some use cases for which the "alert" URN
mechanism is needed today.
6.1. PBX Ring Tones
This section defines some commonly encountered ring tones on PBX or
business phones. They are as listed in the following subsections.
6.1.1. Normal
This tone indicates that the default or normal ring tone should be
rendered. This is essentially a no-operation "alert" URN and should
be treated by the UA as if no "alert" URN is present. This is most
useful when Alert-Info header field parameters are being used. For
example, in [RFC7463], an Alert-Info header field needs to be present
containing the "appearance" parameter, but no special ring tone needs
to be specified.
6.1.2. 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.
Liess, et al. Standards Track [Page 10]RFC 7462 Alert URNs March 2015
6.1.3. 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.
6.1.4. Priority
A PBX tone needs to indicate that a priority level alert should be
applied for the type of alerting specified (e.g., internal alerting).
6.1.5. Short
In this case, the alerting type specified (e.g., internal alerting)
should be rendered shorter than normal. In contact centers, this is
sometimes referred to as "abbreviated ringing" or a "zip tone".
6.1.6. Delayed
In this case, the alerting type specified should 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.
6.2. Service Tones
These tones are used to indicate specific PBX and public network
telephony services.
6.2.1. Call Waiting
The call-waiting service [TS24.615] permits a callee to be notified
of an incoming call while 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, a callee's UA (or proxy)
that is aware of the call-waiting condition can add the call-waiting
indication to the Alert-Info header field in the 180 (Ringing)
response.
Liess, et al. Standards Track [Page 11]
RFC 7462 Alert URNs March 2015
6.2.2. 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. Note that a 181 response can be used in place of this URN.
6.2.3. Transfer Recall
This feature is used when a blind transfer [RFC5589] 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 a normal
incoming call.
6.2.4. Auto Callback
This feature is used when a user has utilized a server to implement
an automatic callback service [RFC6910]. When the user is available,
the server calls back the user and utilizes this service tone to
distinguish this INVITE from a normal incoming call.
6.2.5. 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 a normal incoming call.
6.3. Country-Specific Ringback Tone Indications for the Public Switched
Telephone Network
In the PSTN, different tones are used in different countries. End
users are accustomed to hear the callee's country ringback tone and
would like to have this feature for SIP.
7. URN Specification for the "alert" Namespace Identifier
This section provides the registration template for the "alert" URN
namespace identifier (NID) according to [RFC2141] and [RFC3406].
Namespace ID: alert
Registration Information:
Registration version: 1
Registration date: 2014-12-10
Liess, et al. Standards Track [Page 12]RFC 7462 Alert URNs March 2015
Declared registrant of the namespace:
Registering organization: Real-time Applications and
Infrastructure Area, IETF
Designated contact: RAI Area Director
Designated contact email: rai-ads@ietf.org
Declaration of syntactic structure:
The Namespace Specific String (NSS) for the "alert" URNs is called
an <alert-identifier> and has a hierarchical structure. The first
colon-separated part after "alert" is called the <alert-category>;
the parts to the right of that are <alert-ind-part>s, and together
form the <alert-indication>. The general form is
urn:alert:<alert-category>:<alert-indication>.
The following <alert-category> identifiers are defined in this
document: "service" , "priority" , "source" , "duration", "delay",
and "locale". The <alert-category> set can be extended in the
future, either by standardization or by private action. The
<alert-category>s describe distinct features of alerting signals.
Any "alert" URN defined in this specification is syntactically
valid for ring and ringback tones and can be used in SIP INVITE
requests or in provisional 1xx responses excepting the 100
response.
The ABNF [RFC5234] for the "alert" URNs is shown below:
alert-URN = "urn:alert:" alert-identifier
alert-identifier = alert-category ":" alert-indication
alert-category = alert-name
alert-indication = alert-ind-part *(":" alert-ind-part)
alert-ind-part = alert-name
alert-name = alert-label / private-name
private-name = alert-label "@" provider
provider = alert-label
alert-label = 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
<alert-label>s MUST comply with the syntax for Non-Reserved LDH
labels [RFC5890]. Registered URNs and components thereof MUST be
transmitted as registered (including case).
Relevant ancillary documentation: RFC 7462
Liess, et al. Standards Track [Page 13]RFC 7462 Alert URNs March 2015
Namespace considerations: This specification defines a URN namespace
"alert" for URNs representing signals or renderings that are
presented to users to inform them of events and actions. The
initial usage is to specify ring tones and ringback tones when
dialogs are established in SIP, but they can also be used for
other communication-initiation protocols (e.g., H.323), and more
generally, in any situation (e.g., web pages or endpoint device
software configurations) to describe how a user should be
signaled.
An "alert" URN does not describe a complete signal, but rather it
describes a particular characteristic of the event it is signaling
or a feature of the signal to be presented. The complete
specification of the signal is a sequence of "alert" URNs
specifying the desired characteristics/significance of the signal
in priority order, with the most important aspects specified by
the earlier URNs. This allows the sender of a sequence of URNs to
compose very detailed specifications from a restricted set of
URNs, and to clearly specify which aspects of the specification it
considers most important.
The initial scope of usage is in the Alert-Info header field, in
initial INVITE requests (to indicate how the called user should be
alerted regarding the call) and non-100 provisional (1xx)
responses to those INVITE requests (to indicate the ringback, how
the calling user should be alerted regarding the progress of the
call).
In order to ensure widespread adoption of these URNs for
indicating ring tones and ringback tones, the scheme must allow
replication of the current diversity of these tones. Currently,
these tones vary between the PSTNs of different nations and
between equipment supplied by different vendors. Thus, the scheme
must accommodate national variations and proprietary extensions in
a way that minimizes the information that is lost during
interoperation between systems that follow different national
variations or that are supplied by different vendors.
The scheme allows definition of private extension URNs that refine
and extend the information provided by standard URNs. Private
extension URNs can also refine and extend the information provided
by other private extension URNs. Private extensions can also
define entirely new categories of information about calls. We
expect these extensions to be used extensively when existing PBX
products are converted to support SIP operation.
Liess, et al. Standards Track [Page 14]RFC 7462 Alert URNs March 2015
The device that receives an Alert-Info header field containing a
sequence of "alert" URNs provides to the user a rendering that
represents the semantic content of the URNs. The device is given
great leeway in choosing the rendering, but it is constrained by
rules that maximize interoperability between systems that support
different sets of private extensions. In particular, earlier URNs
in the sequence have priority of expression over later URNs in the
sequence, and URNs that are not usable in their entirety (because
they contain unknown extensions or are incompatible with previous
URNs) are successively truncated in attempt to construct a URN
that retains some information and is renderable in the context.
Due to the practical importance of private extensions for the
adoption of URNs for alerting calls and the very specific rules
for private extensions and the corresponding processing rules that
allow quality interoperation in the face of private extensions,
the requirements of the "alert" URN scheme cannot be met by a
fixed enumeration of URNs and corresponding meanings. In
particular, the existing namespace "urn:ietf:params" does not
suffice (unless the private extension apparatus is applied to that
namespace).
There do not appear to be other URN namespaces that uniquely
identify the semantic of a signal or rendering feature. Unlike
most other currently registered URN namespaces, the "alert" URN
does not identify documents and protocol objects (e.g., [RFC3044],
[RFC3120], [RFC3187], [RFC3188], [RFC4179], [RFC4195], [RFC4198]),
types of telecommunications equipment [RFC4152], people, or
organizations [RFC3043].
The <alert-URN>s are hierarchical identifiers. An <alert-URN>
asserts some fact or feature of the offered SIP dialog, or some
fact or feature of how it should be presented to a user, or of how
it is being presented to a user. Removing an <alert-ind-part>
from the end of an <alert-URN> (which has more than one <alert-
ind-part>) creates a shorter <alert-URN> with a less specific
meaning; the set of dialogs to which the longer <alert-URN>
applies is necessarily a subset of the set of dialogs to which the
shorter <alert-URN> applies. (If the starting <alert-URN>
contains only one <alert-ind-part>, and thus the <alert-ind-part>
cannot be removed to make a shorter <alert-URN>, we can consider
the set of dialogs to which the <alert-URN> applies to be a subset
of the set of all dialogs.)
The specific criteria defining the subset to which the longer
<alert-URN> applies, within the larger set of dialogs, is
considered to be the meaning of the final <alert-ind-part>. This
meaning is relative to and depends upon the preceding <alert-
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category> and <alert-ind-part>s (if any). The meanings of two
<alert-ind-part>s that are textually the same but are preceded by
different <alert-category>s or <alert-ind-part>s have no necessary
connection. (An <alert-category> considered alone has no meaning
in this sense.)
The organization owning the <provider> within a <private-name>
specifies the meaning of that <private-name> when it is used as an
<alert-ind-part>. (The organization owning a <provider> is
specified by the registry described in Section 9.3.)
The organization owning the <provider> within a <private-name> (in
either an <alert-category> or an <alert-ind-part>) specifies the
meaning of each <alert-ind-part>, which is an <alert-label> that
follows that <private-name> and that precedes the next <alert-ind-
part> which is a <private-name> (if any).
The meaning of all other <alert-ind-part>s (i.e., those that are
not <private-name>s and do not follow a <private-name>) is defined
by standardization.
Community considerations: The "alert" URNs are relevant to a large
cross-section of Internet users, namely those that initiate and
receive communication connections via the Session Initiation
Protocol. These users include both technical and non-technical
users, on a variety of devices and with a variety of perception
capabilities. The "alert" URNs will allow Internet users to
receive more information about offered calls and enable them to
better make decisions about accepting an offered call, and to get
better feedback on the progress of a call they have made.
User interfaces that utilize alternative sensory modes can better
render the ring and ringback tones based on the "alert" URNs
because the URNs provide more detailed information regarding the
intention of communications than is provided by current SIP
mechanisms.
Process of identifier assignment:
Assignment of standardized "alert" URNs is by insertion into the
IANA registry described in Section 9.2. This process defines the
meanings of <alert-ind-part>s that have standardized meanings, as
described in "Namespace Considerations".
A new URN MUST NOT be registered if it is equal by the comparison
rules to an already registered URN.
Liess, et al. Standards Track [Page 16]RFC 7462 Alert URNs March 2015
Private extensions are "alert" URNs that include <alert-ind-part>s
that are <private-name>s and <alert-label>s that appear after a
<private-name> (either as an <alert-category> or an <alert-
indication>). If such an <alert-ind-part> is a <private-name>,
its meaning is defined by the organization that owns the
<provider> that appears in the <private-name>. If the <alert-ind-
part> is an <alert-label>, its meaning is defined by the
organization that owns the <provider> that appears in the closest
<private-name> preceding the <alert-label>. The organization
owning a <provider> is specified by the registry described in
Section 9.3.
Identifier uniqueness and persistence considerations: An "alert" URN
identifies a semantic feature of a call or a sensory feature of
how the call alerting should be a rendered at the caller's or
callee's end device.
For standardized <alert-ind-part>s in URNs, uniqueness and
persistence of their meanings is guaranteed by the fact that they
are registered with IANA in accordance with the procedures of
Section 9.2; the feature identified by a particular "alert" URN is
distinct from the feature identified by any other standardized
"alert" URN.
Assuring uniqueness and persistence of the meanings of private
extensions is delegated to the organizations that define private
extension <alert-ind-part>s. The organization responsible for a
particular <alert-ind-part> in a particular "alert" URN is the
owner of a syntactically determined <provider> part within the
URN.
An organization SHOULD use only one <provider> value for all of
the <private-name>s it defines.
Process for identifier resolution: The process of identifier
resolution is the process by which a rendering device chooses a
rendering to represent a sequence of "alert" URNs. The device is
allowed great leeway in making this choice, but the process MUST
obey the rules of Section 11.1. The device is expected to provide
renderings that users associate with the meanings assigned to the
URNs within their cultural context. A non-normative example
resolution algorithm is given in Section 12.1.
Rules for lexical equivalence: "alert" URNs are compared according
to case-insensitive string equality.
Liess, et al. Standards Track [Page 17]RFC 7462 Alert URNs March 2015
Conformance with URN syntax: All "alert" URNs must conform to the
ABNF in the "Declaration of Syntactic Structure" in Section 7.
That ABNF is a subset of the generic URN syntax [RFC2141].
<alert-label>s are constrained to be Non-Reserved LDH labels
[RFC5890], that is, "ordinary ASCII labels". Future
standardization may allow <alert-label>s that are A-labels
[RFC5890], and so interpreters of "alert" URNs MUST operate
correctly (per Section 11.1) when given such URNs as input.
Validation mechanism: An "alert" URN containing no private
extensions can be validated based on the IANA registry of
standardized "alert" URNs. Validating an "alert" URN containing
private extensions requires obtaining information regarding the
private extensions defined by the organization that owns the
<provider> in the relevant <private-name>. The identity of the
organization can be determined from the IANA registry described in
Section 9.2. However, if an "alert" URN contains at least one
<alert-identifier> that precedes the first <private-name>, the
portion of the "alert" URN that precedes the first <private-name>
must itself be a valid standardized "alert" URN, which may be
validated as above.
Scope: The scope for this URN is public and global.
8. "alert" URN Values
8.1. <alert-category> Values
The following <alert-category> values are defined in this document:
- service
- source
- priority
- duration
- delay
- locale
8.2. <alert-indication> Values
This section describes the "alert" URN indication values for the
<alert-category>s defined in this document.
For each <alert-category>, a default <alert-indication> is defined,
which is essentially a no-operation "alert" URN and should be treated
by the UA as if no "alert" URN for the respective category is
present. "alert" URN default indications are most useful when Alert-
Info header field parameters are being used. For example, in
Liess, et al. Standards Track [Page 18]RFC 7462 Alert URNs March 2015
[RFC7463], an Alert-Info header field needs to be present containing
the "appearance" parameter, but no special ringtone need be
specified.
The <private-name> syntax is used for extensions defined by
independent organizations, as described in Section 10.2.
8.2.1. <alert-indication> Values for the <alert-category> "service"
- normal (default)
- call-waiting
- forward
- recall:callback
- recall:hold
- recall:transfer
- <private-name>
Examples: <urn:alert:service:call-waiting> or
<urn:alert:service:recall:transfer>.
8.2.2. <alert-indication> Values for the <alert-category> "source"
- unclassified (default)
- internal
- external
- friend
- family
- <private-name>
(These <alert-indication>s will rarely be provided by the sending UA;
rather they will usually be inserted by a proxy acting on behalf of
the recipient UA to inform the recipient UA about the origins of a
call.)
Examples: <urn:alert:source:external>.
8.2.3. <alert-indication> Values for the <alert-category> "priority"
- normal (default)
- low
- high
- <private-name>
Examples: <urn:alert:priority:high>.
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8.2.4. <alert-Indication> Values for the <alert-category> "duration"
- normal (default)
- short
- long
- <private-name>
Examples: <urn:alert:duration:short>.
8.2.5. <alert-indication> Values for the <alert-category> "delay"
- none (default)
- yes
- <private-name>
Examples: <urn:alert:delay:yes>.
8.2.6. <alert-indication> Values for the <alert-category> "locale"
- default (default)
- country:<ISO 3166-1 country code>
- <private-name>
The ISO 3166-1 country code [ISO3166-1] is used to inform the
renderer on the other side of the call that a country-specific
rendering should be used. For example, to indicate ringback tones
from South Africa, the following URN would be used:
<urn:alert:locale:country:za>.
9. IANA Considerations
9.1. URN Namespace Identifier "alert"
This section registers a new URN namespace identifier (NID), "alert",
in accordance with [RFC3406] with the registration template provided
in Section 7.
9.2. 'Alert URN Identifiers' Registry
Standard "alert" URNs are recorded as <alert-identifier>s in a new
registry called "Alert URN Identifiers". Thus, creating a new
standard "alert" URN requires IANA action. IANA manages the "Alert
URN Identifiers" registry under the policy 'Specification Required'
[RFC5226] following the guidelines in Section 10.1.
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The registry contains entries in the following formats:
<alert-category>/ Reference Description
<alert-identifier>
---------------------------------------------------------------
foo [RFCxyz] Description of the 'foo'
<alert-category>;
foo:bar [RFCabc] Description of the 'foo:bar'
<alert-identifier>
foo:<range> [RFCdef] Description of the
'foo:<category>' <alert-identifer>s (which will
reference the <range> value)
The first value in each row is the value that is registered, which is
either: (1) an <alert-category> value, (2) an <alert-identifier>
value, composed of an <alert-category> followed by an <alert-
indication>, in turn composed of one or more <alert-label>s, or (3) a
pattern for <alert-identifier> values (e.g., for the "locale" <alert-
category> in Section 9.2.1.6).
The second value in each row is the reference to the required
specification for the value.
The third value in each row is a short description of the semantics
of the value.
A new URN MUST NOT be registered if it is equal by the comparison
rules (that is, case-insensitive string comparison) to an already
registered URN.
<alert-category> and <alert-identifier> values that contain <private-
name>s are not managed by IANA. The process of assigning these
values is described in Section 10.2.
9.2.1. Initial IANA Registration
This document defines the <alert-category>s 'service', 'source',
'priority', 'duration', 'delay' and 'locale'. The entries to be
added to the 'Alert URN Identifiers' registry table for each <alert-
category> are given in the respective sections below.
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9.2.1.1. The "service" <alert-category> and <alert-identifier>s
The following table contains the initial IANA registration for the
"service" <alert-category> and <alert-identifier>s. The value of
this indicator is set to a value different from "normal" if the
caller or callee is informed that a specific telephony service has
been initiated.
<alert-category>/ Reference Description
<alert-identifier>
-----------------------------------------------------------
service RFC 7462 Specific telephony
service used in this
call
service:normal RFC 7462 Normal ring/ringback
rendering (default value)
service:call-waiting RFC 7462 Call waiting was
initiated at the other side
of the call
service:forward RFC 7462 Call has been forwarded
service:recall:callback RFC 7462 Recall due to callback
service:recall:hold RFC 7462 Recall due to call hold
service:recall:transfer RFC 7462 Recall due to transfer
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9.2.1.2. The "source" <alert-category> and <alert-identifier>s
The following table contains the initial IANA registration for the
"source" <alert-category> and <alert-identifier>. The value of this
indicator provides information about the user at the other side of
the call.
<alert-category>/ Reference Description
<alert-identifier>
-----------------------------------------------------------
source RFC 7462 Classification
of the other party
to the call
source:unclassified RFC 7462 Unclassified ring/ringback
rendering (default value)
source:internal RFC 7462 User at the other side of
the call is internal to the
enterprise or PBX system
source:external RFC 7462 User at the other side of
the call is external to the
enterprise or PBX system
source:friend RFC 7462 User at the other side of
the call is a friend
source:family RFC 7462 User at the other side of
the call is a family member
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9.2.1.3. The "priority" <alert-category> and <alert-identifier>s
The following table contains the initial IANA registration for the
"priority" <alert-category> and <alert-identifier>s. The value of
this indicator provides information about the priority the alerted
user should give to the call.
<alert-category>/ Reference Description
<alert-identifier>
-----------------------------------------------------------
priority RFC 7462 Priority of the
call
priority:normal RFC 7462 Normal ring/ringback
rendering (default value)
priority:low RFC 7462 Low priority call
priority:high RFC 7462 High priority call
9.2.1.4. The "duration" <alert-category> and <alert-identifier>s
The following table contains the initial IANA registration for the
"duration" <alert-category> and <alert-identifier>s. The value of
this indicator provides information about the duration of the
alerting signals compared to the default alerting signals.
<alert-category>/ Reference Description
<alert-identifier>
-----------------------------------------------------------
duration RFC 7462 Duration of alerting signal
duration:normal RFC 7462 Normal ring/ringback
rendering (default value)
duration:short RFC 7462 Shorter than normal
duration:long RFC 7462 Longer than normal
Liess, et al. Standards Track [Page 24]RFC 7462 Alert URNs March 2015
9.2.1.5. The "delay" <alert-category> and <alert-identifier>s
The following table contains the initial IANA registration for the
"delay" <alert-category> and <alert-identifier>s. The value of this
indicator provides information about whether the presentation of the
alerting signal should be delayed compared to the default
presentation process. For more details see Section 6.1.6.
<alert-category>/ Reference Description
<alert-identifier>
-----------------------------------------------------------
delay RFC 7462 Delay of rendering
of alerting signal
delay:none RFC 7462 Immediate alerting
(default value)
delay:yes RFC 7462 Delayed alerting
9.2.1.6. The "locale" <alert-category> and <alert-identifier>s
The following table contains the initial IANA registration for the
"locale" <alert-category> and <alert-identifier>s. The value of this
indicator provides information about whether the alerting signals
characteristic of the specified location should be used.
<alert-category>/ Reference Description
<alert-identifier>
-----------------------------------------------------------
locale RFC 7462 Location-specific
alerting signals
locale:default RFC 7462 Alerting not location
specific
(default value)
locale:country:<ISO 3166-1 country code>
RFC 7462 Alerting according to the
conventions of the specified
country
Liess, et al. Standards Track [Page 25]RFC 7462 Alert URNs March 2015
9.3. 'Alert URN Providers' Registry
Values of <provider>, which are used to create <private-name>s, are
recorded in a new registry called "Alert URN Providers". (Private
extension "alert" URNs that are defined are not recorded by IANA.)
The registry is managed by IANA under the policy 'First Come First
Served' [RFC5226].
The registry contains entries in the following format:
<provider> Registrant Contact URI
---------------------------------------------------------------------
example IETF rai-ads@ietf.org
The first value in each row is the <provider> value that is
registered. This value is case-insensitive and MUST comply with the
syntax for Non-Reserved LDH labels [RFC5890].
The second value in each row is the name of the registrant of the
value.
The third value is a contact URI for the registrant.
The registry initially contains the one entry shown above, which can
be used for constructing examples of private extension URNs.
10. Extension Rules
10.1. General Extension Rules
The set of "alert" URNs is extensible. An extension "at the top
level" creates a new <alert-category> (which represents a new
alerting characteristic), an extension "at the second level" creates
a new <alert-indication> value for an existing <alert-category>, an
extension "at the third level" creates a subdivision of an existing
<alert-indication> (that has one <alert-ind-part>), etc. URNs allow
(in principle) indefinite subdivision of existing <alert-indication>
values, although most of the standard "alert" URNs have only one
level of subdivision and a few have two levels of subdivision.
Extensions, either standard or private, MUST conform to the following
principles:
A new <alert-category> is independent of all previously existing
<alert-category>s: For any combination of one <alert-identifier> in
the new <alert-category> with any one <alert-identifier> in any of
the previously existing <alert-category>s, there are potential calls
to which the combination can be meaningfully applied.
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A new <alert-identifier> that has more than one <alert-ind-part> is a
semantic refinement of a parent <alert-identifier>, the parent being
obtained by deleting the final <alert-ind-part>. The new <alert-
identifier> has as parent the most specific previously existing
<alert-identifier> whose meaning includes all potential calls to
which the new <alert-identifier> could be meaningfully applied.
A new <alert-identifier> has no semantic overlap with any sibling
<alert-identifier> (<alert-identifier>s that differ only in the final
<alert-ind-part>). That is, there could be no call to which both
<alert-identifier>s could be meaningfully applied.
The process for defining new standard "alert" URNs is described in
Section 9.2; all such definitions require registering a publicly
available specification. The process for defining new "alert" URNs
via the private extension mechanism is described in Section 10.2.
10.2. Private Extension Rules
The <private-name> syntax is used to create private extensions,
extensions that are not registered with IANA. The <private-name> has
the form of an <alert-label> followed by "@" and then a <provider>
that designates the organization defining the extension. Both
<alert-label> and <provider> have the same syntax as an ordinary
ASCII DNS label. A private extension URN is created by using a
<private-name> as either an <alert-category> or an <alert-ind-part>.
If the <private-name> is used as an <alert-category>, the
characteristic of the alerting signal that the <alert-category>
describes is defined by the organization. If the <private-name> is
used as the first <alert-ind-part>, the organization defines an
alternative value for the standardized <alert-category> of the URN.
If the <private-name> is used as the second or later <alert-ind-
part>, the organization defines the meaning of the URN as a subset of
the meaning of the shorter URN resulting when the <private-name> (and
any subsequent <alert-ind-part>s) are removed.
Within a URN, all <alert-label> components that follow a <private-
name> but are before any following <private-name>s are additional
private extensions whose meaning is defined by the organization
defining the nearest preceding <private-name>.
A URN that contains a private extension can be further subdivided by
the private extension of a different organization: the second
organization appends an <alert-ind-part> that is a <private-name>
containing a the <provider> value for the second organization.
Liess, et al. Standards Track [Page 27]
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The meaning of a <private-name> or an <alert-label> that is defined
privately (because of a preceding <private-name>) is only fixed
within the context provided by the sequence of preceding
<alert-name>s; these components have no meaning in isolation and
there is no necessary relationship between the meaning of textually
identical <alert-name>s that are preceded by different sequences of
<alert-name>s.
Creating private extension "alert" URNs is not a Standards Action and
they are not registered with IANA.
The organization defining a private extension is responsible for
ensuring persistence of the meaning of the private extension.
Private extensions MUST conform to the principles of Section 10.1,
both in regard to previously existing standard <alert-URN>s and in
regard to any previously existing private extensions using the same
<provider> value, and any other private extensions that the
organization is aware of. In particular, a private extension MUST
NOT duplicate any standard URN or any private extension that the
organization is aware of. (In either of those cases, the
organization MUST use the existing URN for its purposes.)
An organization obtains a <provider> value for constructing <private-
name>s by registering the value with IANA as provided in Section 9.3.
10.3. Examples
10.3.1. Subsetting an Existing URN
The organization registering the <provider> "example" can define
distinctive versions of <urn:alert:service:call-waiting>:
urn:alert:service:call-waiting:abc@example
urn:alert:service:call-waiting:def@example
It can create a more specialized URN that applies to a subset of the
situations to which the first URN above applies:
urn:alert:service:call-waiting:abc@example:xyz
Because "xyz" follows "abc@example" (and there is no intervening
<private-name>), its meaning is defined by the owner of the
<provider> "example".
Liess, et al. Standards Track [Page 28]RFC 7462 Alert URNs March 2015
10.3.2. A New Value within an <alert-category>
The organization registering the <provider> "example" can define URNs
in the "service" category to express a new service that is not
covered by any of the standardized URNs:
urn:alert:service:ghi@example
However, before defining such a URN, the organization should verify
that the set of calls to which the URN applies is not a subset of the
set of calls for some existing URN. If it is a subset, the extension
URN should be a subdivision of the existing URN.
10.3.3. A New <alert-category>
The organization registering the <provider> "example" can define an
extension <alert-category> named "jkl@example" with two <alert-
indication>s "a1" and "a2":
urn:alert:jkl@example:a1
urn:alert:jkl@example:a2
10.3.4. Subsetting a Private Extension URN
The organization registering the <provider> "foo" wants to define a
set of URNs that specify the different ring patterns used by a
"distinctive ring" service to alert for incoming calls that are
directed to different directory numbers. These ring patterns are
composed of groups of ring sounds that have particular patterns of
lengths.
The company can create a private <alert-category> "distinctive@foo",
and within it assign three 'alert' URNs that indicate the three
different ring patterns used by the company's service:
urn:alert:distinctive@foo:long-long
urn:alert:distinctive@foo:short-long-short
urn:alert:distinctive@foo:short-short-long
Later, the company registering the <provider> "bar" wants to define
an additional 'alert' URN for the ring pattern "short short", which
it uses to support a fourth directory number for a phone instrument.
Liess, et al. Standards Track [Page 29]RFC 7462 Alert URNs March 2015
The company can create a <private-name> to be used with the
"distinctive@foo" <alert-category>:
urn:alert:distinctive@foo:short-short@bar
11. Combinations of "alert" URNs
11.1. Priority Rules
This section describes combination rules for the case when all the
Alert-Info header fields only contain "alert" URNs. Other
combinations of URIs in the Alert-Info header fields of the same SIP
message are not defined in this specification.
In many cases, more than one URN will be needed to fully define a
particular tone. This is done by including multiple "alert" URNs, in
one or more Alert-Info header fields in a request or a response. For
example, an internal, priority call could be indicated by Alert-Info:
<urn:alert:source:internal>, <urn:alert:priority:high>. A priority
call-waiting tone could be indicated by Alert-Info:
<urn:alert:service:call-waiting>, <urn:alert:priority:high>.
The sender of the Alert-Info header field may include an arbitrary
list of "alert" URNs, even if they are redundant or contradictory.
An earlier URN has priority over any later contradictory URN. This
allows any element to modify a list of URNs to require a feature
value (by adding a URN at the beginning of the list) or to suggest a
feature value (by adding a URN at the end of the list).
The receiving UA matches the received "alert" URN combination with
the signal(s) it is able to render.
The implementation is free to ignore an "alert" URN if it does not
recognize the URN, or if it is incapable of rendering its effect in
the context. Similarly, it can remove a final series of one or more
<alert-ind-part>s of an "alert" URN to create a "more generic" URN
that it recognizes and whose meaning it can render in the context.
The exact way in which a UA renders a received combination of "alert"
URNs is left as an implementation issue. However, the implementation
MUST comply to following rules:
(a) Each "alert" URN has precedence over all URNs that follow it,
and its interpretation is subordinate to all URNs that precede
it.
Liess, et al. Standards Track [Page 30]RFC 7462 Alert URNs March 2015
(b) If the UA cannot implement the effect of a URN (because it does
not recognize the URN or the URN's effect is precluded by
preceding URNs), the UA repeatedly removes the final <alert-ind-
part> of the URN until either:
(1) the resulting URN is recognized and can be given effect by
some signal (without reducing the degree of expression of
any preceding URN), or
(2) the resulting URN is reduced to having no <alert-ind-part>
in which case, that URN in the series cannot be given
effect, and so is ignored.
(c) In case that after processing all the received URNs, the UA can
generate more than one signal that are equally effective at
expressing the URNs (under the preceding rules), one of those
signals is selected. When selecting from the set of equally
effective signals, the least specific signal in the set should
be chosen: a signal should not be chosen if a less-specific
signal is also in the set. (Specificity is to be judged based
on the defined meanings of the signals to the user.) (For
example, if each signal is considered to express certain <alert-
indication>s of certain <alert-category>s, one signal is less-
specific than a second signal if the first signal's <alert-
indication>s are a subset or are prefixes of the second signal's
<alert-indication>s.) However, a more-specific signal may be
chosen if the choice is based on information derived from the
containing SIP message. For example, a signal implying
<urn:alert:priority:high> may be chosen if the SIP message
contains the header field "Priority: urgent".
In all situations, the set of signals that can be rendered and their
significances may change based on user preferences and local policy.
In addition, the chosen signal may change based on the status of the
UA. For example, if a call is active on the UA, all audible signals
may become unavailable, or audible signals may be available only if
<urn:alert:priority:high> is specified.
11.2. Multi-mode Signals
There are cases when the device can render two signal modes (e.g.,
audio and visual, or video and text) at the same time.
Formally, the device must be considered to be making its choice from
the set of all combined signals that it can render (pairs of one
signal from the first mode and one signal from the second mode), and
that choice must conform to the above rules. However, it can be
proven that if the device makes its rendering choice for each of the
Liess, et al. Standards Track [Page 31]RFC 7462 Alert URNs March 2015
two modes independently, with each choice separately conforming to
the above rules, its combined choice also conforms to the above
rules, when it is regarded as a choice from among all possible
combinations.
In such a situation, it may simplify implementation to make each
choice separately. It is an implementation decision whether to chose
from among combined signals or to combine choices made from each
signal mode.
12. Non-normative Algorithm for Handling Combinations of URNs
The following text is a non-normative example of an algorithm for
handling combinations of URNs that complies with the rules in
Sections 10 and 11. Thus, it demonstrates that the rules are
consistent and implementable. (Of course, a device may use any other
algorithm that complies with Sections 10 and 11.)
12.1. Algorithm Description
For each <alert-category> (feature) known by the implementation,
there is a "feature tree" of the known <alert-indication>s for that
<alert-category>, with the sequence of <alert-ind-part>s in an
<alert-indication> specifying the path in the tree from the root to
the node representing the <alert-indication>. For this description,
we will name each tree and its root node by the <alert-category>
name, and name each non-root node by the <alert-identifier>. Each
URN thus corresponds to one non-root node in one feature tree. For
example, there is a tree named "source", whose root node is also
named "source", and which has the children source:internal,
source:external, source:friend, and source:family. The URN
<urn:alert:source:external> is placed at the node "source:external"
in the "source" tree. If the implementation understands
<urn:alert:source:foo@example>, there is a node source:foo@example
that is a child of node "source". If the implementation understands
<urn:alert:source:external:bar@example>, there is a node
source:external:bar@example that is a child of node source:external.
(Of course, there are an infinite number of potential additional
nodes in the tree for private values, but we don't have to represent
those nodes explicitly unless the device has a signal representing
the private value.)
We assign similar locations to signals, but each signal has a
position in *every* tree, describing the specific combination of
meanings that it carries. If a signal has a simple meaning, such as
"external source", its place in the "source" tree is source:external,
Liess, et al. Standards Track [Page 32]
RFC 7462 Alert URNs March 2015
showing that it carries the "external source" meaning, but its place
in every other feature tree is at the root node, meaning that it has
no particular meaning for those features.
A signal that has a complex meaning may have non-root positions in
more than one feature tree. For example, an "external, high
priority" signal would be placed at source:external and priority:high
in those trees, but be at the root in all other feature trees.
In order to assure that the algorithm always selects at least one
signal, we require that there is a "default" signal, whose position
in every feature tree is at the root. This default signal will never
be excluded from the set of acceptable signals for any set of URNs,
but will be the lowest priority signal for any set of URNs.
The algorithm proceeds by considering each URN in the received Alert-
Info header fields from left to right, while revising a set of
signals. The set of signals starts as the entire set of signals
available to the device. Each URN excludes some signals from the
set, and "sorts" the signals that remain in the set according to how
well they represent the URN. (The details of these operations are
described below.) The first URN is the "major sort", and has the
most influence on the position of a signal in the set. The second
URN is a "minor sort", in that it arranges the orders of the signals
that are tied within the first sort, the third URN arranges the
orders of the signals that are tied within the first two sorts, etc.
At the end of the algorithm, a final, "most minor" sort is done,
which orders the signals that remain tied under all the sorts driven
by the URNs. This final sort places the least specific signals
(within their tied groups) "first". (If one signal's position in
each feature tree is ancestral or the same as a second signal's
position in that tree, the first signal is "less specific" than the
second signal. Other cases are left to the implementation to
decide.)
Once all the URNs are processed and the sorting of the signals that
have not been excluded is done, the device selects the first signal
in the set.
Here is how a single sort step proceeds, examining a single URN to
modify the set of signals (by excluding some signals and further
sorting the signals that remain):
o The URN specifies a specific node in a specific feature tree.
Liess, et al. Standards Track [Page 33]
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o All signals in the set that are, within that feature tree,
positioned at the URN's node, or at an ancestor node of the URN's
node, are kept. All other signals are removed from the set
(because they have meanings that are incompatible with the URN's
meaning).
o Each group of signals that are tied under the previous sorts are
further sorted into groups based on how much of the URN's meaning
they represent: those which are positioned at the node of the URN
are tied for first position, those which are positioned at the
parent node of the URN are tied for second position, etc., and
those which are positioned at the root node of the feature tree
are tied for last position.
12.2. Examples of How the Algorithm Works
The following examples show how the algorithm described in the
previous section works:
12.2.1. Example 1
The device has a set of four alerting signals. We list their primary
meanings, and the locations that they are placed in the feature
trees:
Signal 1
Meaning: external
Locations:
- source:external
- priority (that is, the root node of the priority tree)
Signal 2
Meaning: internal
Locations:
- source:internal
- priority
Signal 3
Meaning: low
Locations:
- source
- priority:low
Liess, et al. Standards Track [Page 34]RFC 7462 Alert URNs March 2015
Signal 4
Meaning: high
Locations:
- source
- priority:high
To which we add:
Signal 5
Meaning: default
Locations:
- source
- priority
If the device receives <urn:alert:source:internal>, then the sort is:
Signals at source:internal: (this is, first place)
Signal 2: internal
Signals at source: (tied for second place)
Signal 3: low
Signal 4: high
Signal 5: default
And these signals are excluded from the set:
Signal 1: external
So, in this example, the sorting algorithm properly gives first place
to Signal 2 "internal".
12.2.2. Example 2
Let us add to the set of signals in Example 1 ones that express
combinations like "internal, high priority", but let us specifically
exclude the combination "internal, low priority" so as to set up some
tricky examples. This enlarges our set of signals:
Signal 1
Meaning: default
Locations:
- source
- priority
Liess, et al. Standards Track [Page 35]RFC 7462 Alert URNs March 2015
Signal 2
Meaning: external
Locations:
- source:external
- priority
Signal 3
Meaning: internal
Locations:
- source:internal
- priority
Signal 4
Meaning: low
Locations:
- source
- priority:low
Signal 5
Meaning: high
Locations:
- source
- priority:high
Signal 6
Meaning: external high
Locations:
- source:external
- priority:high
Signal 7
Meaning: external low
Locations:
- source:external
- priority:low
Signal 8
Meaning: internal high
Locations:
- source:internal
- priority:high
Liess, et al. Standards Track [Page 36]RFC 7462 Alert URNs March 2015
If the device receives <urn:alert:source:internal>, then the sort is:
Signals at source:internal: (that is, tied for first place)
- Signal 3: internal
- Signal 8: internal high
Signals at source: (tied for second place)
- Signal 4: low
- Signal 5: high
- Signal 1: default
Signals excluded from the set:
- Signal 2: external
- Signal 7: external low
- Signal 6: external high
Two signals are tied for the first place, but the final sort orders
them:
- Signal 3: internal
- Signal 8: internal high
because it puts the least-specific signal first. So, the Signal 3
"internal" is chosen.
12.2.3. Example 3
The same device receives <urn:alert:source:external>,
<urn:alert:priority:low>. The first sort (due to
<urn:alert:source:external>) is:
Signals at source:external:
- Signal 2: external
- Signal 7: external low
- Signal 6: external high
Signals at source:
- Signal 4: low
- Signal 5: high
- Signal 1: default
Liess, et al. Standards Track [Page 37]RFC 7462 Alert URNs March 2015
Signals excluded:
- Signal 3: internal
- Signal 8: internal high
The second sort (due to <urn:alert:priority:low>) puts signals at
priority:low before signals at priority, and excludes signal at
priority:high:
- Signal 7: external low
- Signal 2: external
- Signal 4: low
- Signal 1: default
Excluded:
- Signal 6: external high
- Signal 5: high
- Signal 3: internal
- Signal 8: internal high
So, we choose Signal 7 "external low".
12.2.4. Example 4
Suppose the same device receives <urn:alert:source:internal>,
<urn:alert:priority:low>. Note that there is no signal that
corresponds to this combination.
The first sort is based on source:internal, and results in this
order:
- Signal 3: internal
- Signal 8: internal high
- Signal 4: low
- Signal 5: high
- Signal 1: default
Excluded:
- Signal 2: external
- Signal 7: external low
- Signal 6: external high
Liess, et al. Standards Track [Page 38]RFC 7462 Alert URNs March 2015
The second sort is based on priority:low, and results in this order:
- Signal 3: internal
- Signal 4: low
- Signal 1: default
Excluded:
- Signal 8: internal high
- Signal 5: high
- Signal 7: external low
- Signal 2: external
- Signal 6: external high
So, we choose the Signal 3 "internal".
Note that <urn:alert:priority:low> could not be given effect because
it followed <urn:alert:source:internal>. If the two URNs had
appeared in the reverse order, the Signal 2 "external" would have
been chosen, because <urn:alert:priority:low> would have been given
precedence.
12.2.5. Example 5
Let us set up a simple set of signals, with three signals giving
priority:
Signal 1
Meaning: default
Locations:
- priority
Signal 2
Meaning: low
Locations:
- priority:low
Signal 3
Meaning: high
Locations:
- priority:high
Liess, et al. Standards Track [Page 39]RFC 7462 Alert URNs March 2015
Notice that we've used the "default" signal to cover "normal
priority". That is so the signal will cover situations where no
priority URN is present, as well as the ones with
<urn:alert:priority:normal>. So, we're deliberately failing to
distinguish "priority:normal" from the default priority.
If the device receives <urn:alert:priority:low>, the sort is:
- Signal 2: low
- Signal 1: default
Excluded:
- Signal 3: high
and Signal 2 "low" is chosen.
Similarly, if the device receives <urn:alert:priority:high>, Signal 3
is chosen.
If the device receives <urn:alert:priority:normal>, the sort is:
-Signal 1 :default
Excluded:
- Signal 2: low
- Signal 3: high
and Signal 1 "default" is chosen.
If no "priority" URN is received, Signal 1 "default" will be put
before Signal 2 "low" and Signal 3 "high" by the final sort, and so
it will be chosen.
13. User Agent Behaviour
A SIP UA MAY add a URN or multiple URNs to the Alert-Info header
field in a SIP request or a provisional 1xx response (excepting a 100
response) when it needs to provide additional information about the
call or about the provided service.
Upon receiving a SIP INVITE request or a SIP provisional response
with an Alert-Info header field that contains a combination of Alert-
Info URNs, the UA attempts to match the received Alert- Info URNs
combination with a signal it can render. The process the UA uses
MUST conform to the rules described in Section 11. (A non-normative
algorithm example for the process is described in Section 12.)
Liess, et al. Standards Track [Page 40]RFC 7462 Alert URNs March 2015
The UA must produce a reasonable rendering regardless of the
combination of URIs (of any schemes) in the Alert-Info header field:
it MUST produce a rendering based on the URIs that it can understand
and act on (if any), interpreted as prescribed by local policy, and
ignore the other URIs. In particular, unless the containing message
is a request and is immediately rejected, the UA SHOULD provide some
alert unless it is instructed not to (for example, by Alert-Info URIs
that it understands, the presence of a Replaces or Joins header
field, local policy, or direction of the user).
Subsequent provisional responses, even within the same dialog, may
contain different Alert-Info header field values. The Alert-Info
header field values received within different provisional responses
are treated independently. If subsequent provisional responses
containing different Alert-Info header field values were received
within the same dialog, the UA SHOULD render, at any time, the last
received Alert-Info header field value. The handling of provisional
responses containing different Alert-Info header field values that
were not received within the same dialog is left as an implementation
issue.
14. Proxy Behaviour
A SIP proxy MAY add or remove an Alert-Info header field, and MAY add
or remove Alert-Info header field values, in a SIP request or a
non-100 provisional response when it needs to modify the information
about the call or about the provided services.
There are many reasons a proxy may choose do this, for example, (1)
to add indications based on information that the proxy can determine
about the call, such as that it is coming from an external source, or
that the INVITE contains a "Priority: urgent" header field; (2) to
add indication that a particular service is being invoked at this end
of the call; (3) to remove undesirable indications, such as possibly
deceptive indications from untrusted sources; and (4) to remove
indications that contain information that should be suppressed for
privacy reasons.
The following example shows a typical example of a 180 (Ringing)
provisional response that has been modified by a proxy. The response
sent by the UAS to the proxy was very similar, but had no Alert-Info
header field. The proxy has added Alert-Info header field values
specifying both a network audio resource referenced by the HTTP URI
and the URN indication for the call-waiting service. This allows the
UAC to render the network audio resource, to choose a rendering based
on the URN, or to perform some combination of these actions. Due to
Section 10, the UAC must produce some reasonable rendering in this
situation.
Liess, et al. Standards Track [Page 41]
RFC 7462 Alert URNs March 2015
SIP/2.0 180 Ringing
Alert-Info: <http://www.example.com/sound/moo.wav>,
<urn:alert:service:call-waiting>
To: Bob <sip:bob@biloxi.example.com>;tag=a6c85cf
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710
Contact: <sip:bob@192.0.2.4>
CSeq: 314159 INVITE
Via: SIP/2.0/UDP server10.biloxi.example.com;
branch=z9hG4bK4b43c2ff8.1
Content-Length: 0
15. Internationalization Considerations
The <alert-identifier> labels are protocol elements [RFC6365] and are
not normally seen by users. Thus, the character set for these
elements is restricted, as described in Section 7.
Allowance has been made for the possibility of internationalizing
<alert-identifier>s by allowing them to be A-labels: a processor that
does not understand such <alert-identifier>s is required to ignore
them as specified in Sections 7 and 11.1.
The URNs <urn:alert:locale:country:<ISO 3166-1 country code>> select
renderings that are conventional in the specified country.
16. 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.
However, the provision of specific indications may raise privacy
issues by revealing information about the source UA, e.g., its
nature, its dialog state, or services initiated at its end of the
call. For example, call-waiting (Section 6.2.1) and call-forwarding
(Section 6.2.2) services can reveal the dialog state of the UA. Such
a provision SHALL always require authorization on behalf of the user
of the source UA (usually through accessing configured policy).
Authorization SHALL NOT assume that there is any limitation of the
potential recipients of the indications without obtaining specific
information about the SIP transaction.
Based on local policy, a UA MAY choose to ignore undesirable
indications (e.g., possibly deceptive indications from untrusted
sources), and it MAY choose not to send indications that are
Liess, et al. Standards Track [Page 42]RFC 7462 Alert URNs March 2015
otherwise valid in the context (e.g., for privacy reasons). A proxy
acting on behalf of a UA MAY add or delete indications going to or
from the UA for the same reasons.
Since the alert indications can be sensitive, end-to-end SIP
encryption mechanisms using S/MIME MAY be used to protect it. UAs
that implement alert indications SHOULD also implement SIP over TLS
[RFC5246] and the sips: scheme [RFC5630].
17. References
17.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997,
<http://www.rfc-editor.org/info/rfc2141>.
[RFC3261] 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, <http://www.rfc-editor.org/info/rfc3261>.
[RFC3406] Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
"Uniform Resource Names (URN) Namespace Definition
Mechanisms", BCP 66, RFC 3406, October 2002,
<http://www.rfc-editor.org/info/rfc3406>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008, <http://www.rfc-editor.org/info/rfc5226>.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5630] Audet, F., "The Use of the SIPS URI Scheme in the Session
Initiation Protocol (SIP)", RFC 5630, October 2009,
<http://www.rfc-editor.org/info/rfc5630>.
Liess, et al. Standards Track [Page 43]RFC 7462 Alert URNs March 2015
17.2. Informative References
[E182] ITU-T, "Application of tones and recorded announcements in
telephone services", ITU-T Recommendation E.182, 1998,
<http://www.itu.int/rec/T-REC-E.182-199803-I/en>.
[ISO3166-1]
ISO, "English country names and code elements", ISO
3166-1, <http://www.iso.org/iso/
english_country_names_and_code_elements>.
[RFC3043] Mealling, M., "The Network Solutions Personal Internet
Name (PIN): A URN Namespace for People and Organizations",
RFC 3043, January 2001,
<http://www.rfc-editor.org/info/rfc3043>.
[RFC3044] Rozenfeld, S., "Using The ISSN (International Serial
Standard Number) as URN (Uniform Resource Names) within an
ISSN-URN Namespace", RFC 3044, January 2001,
<http://www.rfc-editor.org/info/rfc3044>.
[RFC3120] Best, K. and N. Walsh, "A URN Namespace for XML.org", RFC
3120, June 2001, <http://www.rfc-editor.org/info/rfc3120>.
[RFC3187] Hakala, J. and H. Walravens, "Using International Standard
Book Numbers as Uniform Resource Names", RFC 3187, October
2001, <http://www.rfc-editor.org/info/rfc3187>.
[RFC3188] Hakala, J., "Using National Bibliography Numbers as
Uniform Resource Names", RFC 3188, October 2001,
<http://www.rfc-editor.org/info/rfc3188>.
[RFC4152] Tesink, K. and R. Fox, "A Uniform Resource Name (URN)
Namespace for the Common Language Equipment Identifier
(CLEI) Code", RFC 4152, August 2005,
<http://www.rfc-editor.org/info/rfc4152>.
[RFC4179] Kang, S., "Using Universal Content Identifier (UCI) as
Uniform Resource Names (URN)", RFC 4179, October 2005,
<http://www.rfc-editor.org/info/rfc4179>.
[RFC4195] Kameyama, W., "A Uniform Resource Name (URN) Namespace for
the TV-Anytime Forum", RFC 4195, October 2005,
<http://www.rfc-editor.org/info/rfc4195>.
[RFC4198] Tessman, D., "A Uniform Resource Name (URN) Namespace for
Federated Content", RFC 4198, November 2005,
<http://www.rfc-editor.org/info/rfc4198>.
Liess, et al. Standards Track [Page 44]RFC 7462 Alert URNs March 2015
[RFC5589] Sparks, R., Johnston, A., and D. Petrie, "Session
Initiation Protocol (SIP) Call Control - Transfer", BCP
149, RFC 5589, June 2009,
<http://www.rfc-editor.org/info/rfc5589>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
September 2011, <http://www.rfc-editor.org/info/rfc6365>.
[RFC6910] Worley, D., Huelsemann, M., Jesske, R., and D. Alexeitsev,
"Completion of Calls for the Session Initiation Protocol
(SIP)", RFC 6910, April 2013,
<http://www.rfc-editor.org/info/rfc6910>.
[RFC7463] Johnston, A., Ed., Soroushnejad, M., Ed., and V.
Venkataramanan, "Shared Appearances of a Session
Initiation Protocol (SIP) Address of Record (AOR)", RFC
7463, March 2015,
<http://www.rfc-editor.org/info/rfc7463>.
[TS24.615]
3GPP, "Communication Waiting (CW) using IP Multimedia (IM)
Core Network (CN) subsystem; Protocol Specification", 3GPP
TS 24.615, September 2015.
Acknowledgements
The authors wish to thank Denis Alexeitsev, the editor of the initial
version in BLISS, Anwar Siddiqui for his contributions to the
document, Christer Holmberg for his careful review of the document,
Adam Roach, Dean Willis, Martin Huelsemann, Shida Schubert, John
Elwell, and Tom Taylor for their comments and suggestions and Alfred
Hoenes for his extensive comments and proposals related to new
namespace identifiers for URNs.
Liess, et al. Standards Track [Page 45]RFC 7462 Alert URNs March 2015
Authors' Addresses
Laura Liess (editor)
Deutsche Telekom AG
Heinrich-Hertz Str 3-7
Darmstadt, Hessen 64295
Germany
Phone: +49 6151 5812761
EMail: laura.liess.dt@gmail.com
Roland Jesske
Deutsche Telekom AG
Heinrich-Hertz Str. 3-7
Darmstadt, Hessen 64295
Germany
Phone: +49 6151 5812766
EMail: r.jesske@telekom.de
Alan Johnston
Avaya, Inc.
St. Louis, MO
United States
EMail: alan.b.johnston@gmail.com
Dale R. Worley
Ariadne Internet Services, Inc.
738 Main St.
Waltham, MA 02451
United States
Phone: +1 781 647 9199
EMail: worley@ariadne.com
Paul Kyzivat
Huawei
United States
EMail: pkyzivat@alum.mit.edu
Liess, et al. Standards Track [Page 46]
