💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc8433.txt captured on 2023-04-26 at 13:53:18.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
Independent Submission D. Worley
Request for Comments: 8433 Ariadne
Category: Informational August 2018
ISSN: 2070-1721
A Simpler Method for Resolving Alert-Info URNs
Abstract
The "alert" namespace of Uniform Resource Names (URNs) can be used in
the Alert-Info header field of Session Initiation Protocol (SIP)
requests and responses to inform a voice over IP (VoIP) telephone
(user agent) of the characteristics of the call that the user agent
has originated or terminated. The user agent must resolve the URNs
into a signal; that is, it must select the best available signal to
present to its user to indicate the characteristics of the call.
RFC 7462 describes a non-normative algorithm for signal selection.
This document describes a more efficient alternative algorithm: a
user agent's designer can, based on the user agent's signals and
their meanings, construct a finite state machine (FSM) to process the
URNs to select a signal in a way that obeys the restrictions given in
the definition of the "alert" URN namespace.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8433.
Worley Informational [Page 1]
RFC 8433 Resolving Alert-Info URNs August 2018
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction ....................................................3
1.1. Requirements Governing Resolution Algorithms ...............4
1.2. Summary of the New Resolution Algorithm ....................5
1.3. Conventions Used in This Document ..........................7
2. Selecting the Signals and Their Corresponding "alert" URNs ......7
3. General Considerations for Processing Alert-Info ................9
4. Constructing the Finite State Machine for a Very Simple
Example ........................................................10
4.1. Listing the Expressed URNs ................................11
4.2. Constructing the Alphabet of Symbols ......................11
4.3. Constructing the States and Transitions ...................13
4.4. Summary ...................................................17
4.5. Examples of Processing Alert-Info URNs ....................19
5. Further Examples ...............................................20
5.1. Example with "source" and "priority" URNs .................20
5.2. Example 1 of RFC 7462 .....................................24
5.3. Examples 2, 3, and 4 of RFC 7462 ..........................30
5.4. An Example That Subsets Internal Sources ..................33
5.5. An Example of "alert:service" URNs ........................34
5.6. An Example Using Country Codes ............................34
6. Prioritizing Signals ...........................................40
7. Dynamic Sets of Signals ........................................41
8. Security Considerations ........................................43
9. IANA Considerations ............................................43
10. References ....................................................44
10.1. Normative References .....................................44
10.2. Informative References ...................................44
Acknowledgments ...................................................45
Author's Address ..................................................45
Worley Informational [Page 2]
RFC 8433 Resolving Alert-Info URNs August 2018
1. Introduction
When a SIP user agent (UA) server receives an incoming INVITE
request, it chooses an alerting signal (the ring tone) to present to
its user (the called user) by processing the Alert-Info header
field(s) in the incoming INVITE request [RFC3261]. Similarly, a SIP
UA client determines an alerting signal (the ringback tone) to
present to its user (the calling user) by processing the Alert-Info
header field(s) in the incoming provisional response(s) to its
outgoing INVITE request.
[RFC3261] envisioned that the Alert-Info header field value would be
a URL that the UA could use to retrieve the encoded media of the
signal. This usage has security problems and is inconvenient to
implement in practice.
[RFC7462] introduced an alternative practice: the Alert-Info values
can be URNs in the "alert" URN namespace that specify features of the
call or of the signal that should be signaled to the user. [RFC7462]
defined a large set of "alert" URNs and procedures for extending
the set.
A UA is unlikely to provide more than a small set of alerting
signals, and there are an infinite number of possible combinations of
"alert" URNs. Thus, a UA is often required to select an alerting
signal that renders only a subset of the information in the
Alert-Info header field(s) -- which is the resolution process for
"alert" URNs. The requirements for resolving "alert" URNs are given
in Section 11.1 of [RFC7462].
Section 12 of [RFC7462] gives a (non-normative) resolution algorithm
for selecting a signal that satisfies the requirements of
Section 11.1 of that document. That algorithm can be used regardless
of the set of alerting signals that the UA provides and their
specified meanings. The existence of the algorithm defined in
[RFC7462] demonstrates that the resolution requirements can always be
satisfied. However, the algorithm is complex and slow.
The purpose of this document is to describe an improved
implementation -- a more efficient resolution algorithm for selecting
signals that conforms to the requirements of Section 11.1 of
[RFC7462]. (Of course, like any such algorithm, it is non-normative,
and the implementation is free to use any algorithm that conforms to
the requirements of Section 11.1 of [RFC7462].)
In the algorithm defined in this document, once the UA designer has
chosen the set of signals that the UA produces and the "alert" URNs
that they express, a finite state machine (FSM) is constructed that
Worley Informational [Page 3]
RFC 8433 Resolving Alert-Info URNs August 2018
selects alerting signals based on the URNs in the Alert-Info header
field(s) in a SIP message. The incoming "alert" URNs are
preprocessed in a straightforward manner into a sequence of "symbols"
drawn from a fixed finite set; these symbols are then used as input
to the FSM. After processing the input, the state of the FSM selects
the correct alerting signal to present to the user.
Both the preprocessor and the FSM are determined only by the selected
set of signals and the set of "alert" URNs expressed by the signals,
so the processing machinery can be fixed at the time of designing
the UA.
1.1. Requirements Governing Resolution Algorithms
The requirements for the resolution of "alert" URNs are given in
Section 11.1 of [RFC7462] and can be described as follows:
o The "alert" URNs are processed from left to right. Each "alert"
URN has precedence over all URNs that follow it, and its
interpretation is subordinate to all URNs that precede it.
o As each URN is processed, one of the UA's signals is chosen that
expresses that URN as far as can be done without reducing the
degree to which any of the preceding URNs were expressed by the
signal chosen for the preceding URN. Thus, as processing
proceeds, the chosen signals become increasingly specific and
contain more information, but all of the information about a
particular URN that is expressed by the signal chosen for that URN
is also expressed by the signals chosen for all following URNs.
o If the entirety of the current URN cannot be expressed by any
allowed signal, then each of the trailing alert-ind-parts (the
sections separated by colons) is in turn removed until the reduced
URN can be expressed by some signal that also expresses at least
the same reduced versions of the preceding URNs that were
expressed by the signal chosen for the preceding URN. This can be
described as "a signal that expresses as much of the current URN
as possible while still expressing as much of the previous URNs as
the preceding signal did."
So, for instance, consider processing
Alert-Info: urn:alert:category-a:part-a1:part-a2,
urn:alert:category-b:part-b1:part-b2
If the UA has no signal for urn:alert:category-a:part-a1:part-a2, it
removes part-a2 from the URN and checks whether it has a signal for
the less-specific URN urn:alert:category-a:part-a1. If it has no
Worley Informational [Page 4]
RFC 8433 Resolving Alert-Info URNs August 2018
signal for that URN, it gives up on the URN entirely (since
urn:alert:category-a doesn't exist and can be considered to express
nothing about the call), and the chosen signal is the default signal
of the UA, i.e., the signal that is used when there is no Alert-Info.
But let us suppose the UA has a signal for
urn:alert:category-a:part-a1 and chooses that signal when processing
the first URN. All processing after this point will be restricted to
signals that express urn:alert:category-a:part-a1 or a more specific
URN of the category "category-a".
The UA then goes on to examine the next URN,
urn:alert:category-b:part-b1:part-b2. If there is a signal that
expresses both urn:alert:category-a:part-a1 and
urn:alert:category-b:part-b1:part-b2, then the UA chooses that
signal. If there is no such signal, the second URN is reduced to
urn:alert:category-b:part-b1, and the UA checks for a signal that
expresses that URN along with urn:alert:category-a:part-a1. If there
is no such signal that matches that relaxed requirement, the second
URN is reduced to urn:alert:category-b, which is discarded, and the
chosen signal for the first URN is chosen for the second URN. In any
case, all processing after this point will be restricted to signals
that express urn:alert:category-a:part-a1 or a more specific URN of
the category "category-a" and that also express the chosen part of
urn:alert:category-b:part-b1:part-b2.
This process is continued until the last "alert" URN is processed;
the signal chosen for the last URN is the signal that the UA uses.
1.2. Summary of the New Resolution Algorithm
The purpose of this document is to describe a resolution algorithm
that conforms to Section 11.1 of [RFC7462] but is simpler than the
algorithm described in Section 12 of [RFC7462]: once the UA designer
has chosen a set of signals and the URNs that they express, an FSM is
constructed that selects alerting signals based on the URNs in the
Alert-Info header field(s) in a SIP message.
o The designer selects the set of signals that the UA produces,
matching each signal to a set of "alert" URNs that together
specify the meaning that is carried by the signal. (If the signal
is a "default" signal that has no specific meaning, the set is
empty. If the signal carries the meaning of one "alert" URN, the
set contains that URN. If the signal carries a meaning that is
the logical AND of two or more "alert" URNs, the set contains
those URNs.)
Worley Informational [Page 5]
RFC 8433 Resolving Alert-Info URNs August 2018
o Based on the UA's signals and their meanings, the designer
constructs an "alphabet" containing a finite number of symbols;
each possible "alert" URN is mapped into one particular symbol.
o The designer constructs an FSM whose input is the alphabet of
symbols and whose states describe the information extracted from
the Alert-Info URNs.
o Each state of the FSM has an associated signal. Processing the
Alert-Info URNs will leave the FSM in some particular state; the
UA renders the signal that is attached to that final state.
To select a ring tone or ringback tone based on a SIP message, the UA
processes the "alert" URNs in the Alert-Info header field from left
to right. Initially, the FSM is in a designated initial state. The
UA maps each successive URN into the corresponding symbol and then
executes the state transition of the FSM specified by the symbol.
The state of the FSM after processing the URNs determines which
signal the UA will render to the user.
Note that the UA generally has two FSMs, because a UA usually wants
to signal different information in ring tones than it signals in
ringback tones. One FSM is used to select the ring tone to render
for an incoming INVITE request. The other FSM is used to select the
ringback tone to render based on an incoming provisional response to
an outgoing INVITE request. Both FSMs are constructed in the same
way, but the constructions are based on different lists of signals
and corresponding URNs.
All of the steps of the method after the designer has selected the
signals and their URNs are algorithmic, and the algorithm of those
steps ensures that the operation of the FSM will satisfy the
constraints of Section 11.1 of [RFC7462]. A Python implementation of
the algorithmic steps is provided in [code].
In simple situations, a suitable FSM or equivalent ad hoc code can be
constructed by hand using ad hoc analysis. Generally, this is only
practical in situations where a small number of alert-categories and
alert-indications are signaled and the categories interact in a
simple, uniform way. For example, the examples in Sections 5.1 and
5.2 could be constructed by ad hoc analysis. But automatic
processing is valuable if the situation is too complicated to
construct a correct FSM by ad hoc analysis, or if the set of signals
will change too frequently for human production to be economical.
Worley Informational [Page 6]
RFC 8433 Resolving Alert-Info URNs August 2018
1.3. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Selecting the Signals and Their Corresponding "alert" URNs
The designer must select signals that the UA will generate and define
the meanings that the signals will have to the user. Based on this,
the designer determines for each signal the "alert" URN or
combination of "alert" URNs that (1) indicate that signal's meaning
in SIP messages and (2) consequently should elicit that signal from
the UA.
For example, suppose the UA has a particular ring tone for calls from
an external source. A call from an external source is marked with
the URN urn:alert:source:external (specified in Section 9 of
[RFC7462]). Thus, the table of signals includes:
Signal URN(s)
---------------------------- -------------------------------
external source urn:alert:source:external
Similarly, if the UA has a particular ring tone for calls from an
internal source, the table includes:
Signal URN(s)
---------------------------- -------------------------------
internal source urn:alert:source:internal
If the UA has ring tones for calls that are marked as having higher
or lower priority, then the table includes:
Signal URN(s)
---------------------------- -------------------------------
high priority urn:alert:priority:high
low priority urn:alert:priority:low
Note that the UA must be able to signal for a message that has no
"alert" URNs in the Alert-Info header field, which means that there
must always be a default signal that has zero corresponding URNs:
Signal URN(s)
---------------------------- -------------------------------
default (none)
Worley Informational [Page 7]
RFC 8433 Resolving Alert-Info URNs August 2018
A signal can be defined to indicate a combination of conditions. For
instance, a signal that is used only for high-priority, internal-
source calls expresses two URNs and will only be used when both URNs
are present in Alert-Info:
Signal URN(s)
------------------------------ -------------------------------
high priority, internal source urn:alert:priority:high,
urn:alert:source:internal
A signal can be defined to cover a number of related conditions by
specifying a URN that is the common prefix of the URNs for the
various conditions. For instance, the URNs for "recall due to
callback", "recall due to call hold", and "recall due to transfer"
all start with urn:alert:service:recall, and so one signal can be
provided for all of them by:
Signal URN(s)
---------------------------- -------------------------------
recall urn:alert:service:recall
But if a specific signal is also provided for "recall due to
callback" by this entry:
Signal URN(s)
---------------------------- ---------------------------------
recall generally urn:alert:service:recall
recall due to callback urn:alert:service:recall:callback
then if the message contains urn:alert:service:recall:callback, the
"recall due to callback" signal will be chosen instead of "recall
generally" because the UA chooses the signal that most completely
expresses the information in the Alert-Info header field.
The designer may wish to define extension URNs that provide more
specific information about a call than the standard "alert" URNs do.
One method is to add additional components to standard URNs. For
instance, an extra-high priority could be indicated by the URN
urn:alert:priority:high:extra@example. The final "extra@example" is
an "alert-ind-part" that is a private extension. (See Sections 7 and
10.2 of [RFC7462] for a discussion of private extensions.) In any
case, adding an alert-ind-part to a URN makes its meaning more
specific, in that any call to which the longer URN can be applied can
also have the shorter URN applied. In this case, "extra-high-
priority calls" are considered a subset of "high-priority calls".
Worley Informational [Page 8]
RFC 8433 Resolving Alert-Info URNs August 2018
Signal URN(s)
--------------------- -----------------------------------------
high priority urn:alert:priority:high
extra-high priority urn:alert:priority:high:extra@example.com
Of course, for this extension to be useful, the senders of SIP
messages (e.g., other UAs) must generate the extension URN in
suitable circumstances.
In some circumstances, the designer may want to create an entirely
new category of "alert" URNs to indicate a type of information that
is not indicated by any standard category of URNs. In that case, the
designer uses a private extension as the alert-category (the third
component of the URN), combined with whatever alert-ind-part (fourth
component) values are desired. For example, a simplified version of
the U.S. military security designations could be:
Signal URN(s)
----------------------- ---------------------------------------
unclassified urn:alert:security@example:unclassified
confidential urn:alert:security@example:confidential
secret urn:alert:security@example:secret
top secret urn:alert:security@example:top-secret
The designer should ensure that the new alert-category is orthogonal
to all defined standard alert-categories, in that any combination of
one of the new URNs with one of the standard URNs is meaningful in
that there could be a message carrying both URNs.
In addition, the set of alert-ind-parts for the new alert-category
should be comprehensive and disjoint, in that every message can be
described by exactly one of them.
3. General Considerations for Processing Alert-Info
In this section, we will discuss various considerations that arise
when processing Alert-Info. These have to be taken care of properly
in order to conform to the standards, as well as to ensure a good
user experience. But since they are largely independent of the
generated FSM and its processing, they are gathered here in a
separate section.
The UA may have a number of different FSMs for processing URNs.
Generally, there will be different FSMs for processing Alert-Info in
incoming INVITE requests and for incoming provisional responses to
outgoing INVITE requests. But any situation that changes the set of
signals that the UA is willing to generate specifies a different set
of signals and corresponding URNs and thus generates a different FSM.
Worley Informational [Page 9]
RFC 8433 Resolving Alert-Info URNs August 2018
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.
Similarly, if the set of signals is customized by user action or
local policy, the generated FSM must be updated. This can be done by
(1) regenerating it according to the method described here or
(2) generating a "generic" FSM and instantiating it based on the
available signals. (See Section 7 for a discussion of this.)
Note that the values in an Alert-Info header field are allowed to be
URIs of any scheme and, within the "urn" scheme, are allowed to have
any namespace [RFC3261]. The processing of URIs that are not "alert"
URNs is not considered by this document, nor is that processing
specified by [RFC7462]. But the algorithm designer must consider
what to do with such URIs if they are encountered. The simplest
choice is to ignore them. Alternatively, the algorithm may examine
the URI to determine if it names an alerting signal or describes how
to retrieve an alerting signal, and, if so, choose to render that
signal rather than process the "alert" URNs to select a signal. In
any case, the remainder of this document assumes that (1) the signal
is to be chosen based on the "alert" URNs in Alert-Info and (2) all
Alert-Info URIs that are not "alert" URNs have been removed.
The UA may also receive "alert" URNs that are semantically invalid in
various ways. For example, the URN may have only three components,
despite the fact that all valid "alert" URNs have at least one
alert-ind-part and thus four components. The only useful strategy is
to ignore such URNs (and possibly log them for analysis).
The method described here is robust in its handling of categories and
alert-ind-parts that are unknown to the UA; as a consequence, it is
also robust if they are not valid standardized URNs. Thus, these
error conditions need not be handled specially.
4. Constructing the Finite State Machine for a Very Simple Example
Constructing the FSM involves:
1. Listing the URNs that are expressed by the various signals of
the UA.
2. From the expressed URNs, constructing the finite alphabet of
symbols into which input URNs are mapped and that drive the state
transitions of the FSM.
Worley Informational [Page 10]
RFC 8433 Resolving Alert-Info URNs August 2018
3. Constructing the states of the FSM and the transitions between
them.
4. Selecting a signal to be associated with each FSM state.
We will explain the process using a very simple example in which
there are two signals -- one expressing "internal source" and one
expressing "external source" -- along with a default signal (for when
there is no source information to signal). The "internal source"
signal expresses urn:alert:source:internal, and the "external source"
signal expresses urn:alert:source:external.
4.1. Listing the Expressed URNs
The first step is to establish for each of the UA's signals what call
characteristics it represents, which is to say, the set of "alert"
URNs that are its information content.
Signal URN(s)
---------------------------- -------------------------------
default (none)
internal source urn:alert:source:internal
external source urn:alert:source:external
From the totality of these expressed URNs, the designer can then
determine which sets of URNs must be distinguished from each other.
In our simple example, the expressed URNs are:
urn:alert:source:external
urn:alert:source:internal
4.2. Constructing the Alphabet of Symbols
In order to reduce the infinite set of possible "alert" URNs to a
finite alphabet of input symbols that cause the FSM's transitions,
the designer must partition the "alert" URNs into a finite set of
categories.
Once we've listed all the expressed URNs, we can list all of the
alert-categories that are relevant to the UA's signaling; "alert"
URNs in any other alert-category cannot affect the signaling and can
be ignored. (The easiest way to ignore the non-relevant URNs is to
skip over them during Alert-Info processing. A more formal method is
to map all of them into one "Other" symbol and then, for each state
of the FSM, have the "Other" symbol transition to that same state.)
Worley Informational [Page 11]
RFC 8433 Resolving Alert-Info URNs August 2018
Within each relevant alert-category, we now define a distinct
symbol for every expressed URN and for all of their "ancestor" URNs
(those that can be created by removing one or more trailing
alert-ind-parts). In order to name the symbols in a way that
distinguishes them from the corresponding URNs, we remove the initial
"urn:alert:" and capitalize each alert-ind-part. Thus, in our
example, we get these symbols:
Source
Source:External
Source:Internal
Note that there is a "Source" symbol even though there is no
corresponding URN. (urn:alert:source is not a valid URN -- see
Section 7 of [RFC7462] -- although the processing algorithm must be
prepared to screen out such a purported URN if it appears in the
Alert-Info header field.) However, its existence as a symbol will be
useful later when we construct the FSM.
For each of these symbols, we add a symbol that classifies URNs that
extend the symbol's corresponding URN with alert-ind-parts that
cannot be expressed by signals:
Source:Other
Source:External:Other
Source:Internal:Other
The latter two classify URNs, such as
urn:alert:source:external:foo@example, that extend URNs that we
already have symbols for. The first is for classifying URNs, such as
urn:alert:source:bar@example, that have first alert-ind-parts that
contradict all the "source" URNs that the UA can signal.
These steps give us this set of symbols:
Source
Source:External
Source:External:Other
Source:Internal
Source:Internal:Other
Source:Other
We can then simplify the set of symbols by removing the ones like
Source:External:Other and Source:Internal:Other that consist of
adding "Other" to a symbol that corresponds to an expressed URN that
is not ancestral to any other expressed URNs. This works because
adding further alert-ind-parts to a URN that is a leaf in regard to
Worley Informational [Page 12]
RFC 8433 Resolving Alert-Info URNs August 2018
the set of signals has no additional effect. In this example,
urn:alert:source:external:foo@example has the same effect as
urn:alert:source:external for both (1) causing a signal to be chosen
and (2) suppressing the effect of later URNs.
This leaves the following symbols for the "source" category:
Source
Source:External
Source:Internal
Source:Other
These can be visually summarized by showing the infinite tree of
possible source "alert" URNs and how it is partitioned into subtrees
that map to each of these symbols. We also mark with "*" the
expressed URNs.
urn:alert
|
{ | }
{ source } --> 1
{ | }
|
+--------------------+------------------+
| | |
{ | } { | } { | }
{ external* } --> 2 { internal* } --> 3 { ... } --> 4
{ | } { | } { }
{ ... } { ... }
{ } { }
1 = Source
2 = Source:External
3 = Source:Internal
4 = Source:Other
4.3. Constructing the States and Transitions
The UA processes the Alert-Info URNs from left to right using an FSM,
with each successive URN causing the FSM to transition to a new
state. Each state of the FSM records the information that has so far
been extracted from the URNs. The state of the FSM after processing
all the URNs determines which signal the UA will render to the user.
We label each state with a set of symbols, one from each relevant
category, that describe the information that's been extracted from
all of the URNs that have so far been processed. The initial state
is labeled with the "null" symbols that are just the category names,
Worley Informational [Page 13]
RFC 8433 Resolving Alert-Info URNs August 2018
because no information has yet been recorded. In our simple example,
the initial state is labeled "Source", since that's the only relevant
category.
State: Source (initial state)
Each state has a corresponding alerting signal, which is the signal
that the UA will produce when URN processing leaves the FSM in that
state. The signal is the one that best expresses the information
that has been extracted from the URNs. Usually, the choice of signal
is obvious to the designer, but there are certain constraints that
the choice must satisfy. The main constraint is that the signal's
expressed URNs must be semantic supersets of (i.e., identical to or a
prefix of) the URNs corresponding to the symbols in the state's
label. In particular, if the expressed URN of the signal in a
certain category is shorter than the state's label, we show that in
the state's name by putting parentheses around the trailing part of
the symbol that is not expressed by the signal. For instance, if the
symbol in the label is "Source:External" but the signal only
expresses "Source" (i.e., no "source" URN at all), then the symbol in
the label is modified to be "Source:(External)".
The reason for this nonintuitive construction is that in some states,
the FSM has recorded information that the chosen signal cannot
express.
Note that the parentheses are part of the state name, so in some
circumstances there may be two or more distinct states labeled with
the same symbols but with different placement of parentheses within
the symbols. These similar state names are relevant when the FSM can
record information from multiple "alert" URNs but cannot express all
of them -- depending on the order in which the URNs appear, the UA
may have to render different signals, so it needs states that record
the same information but render different subsets of that
information.
The initial state's label is the string of null symbols for the
relevant categories, so the only allowed signal is the default
signal, which expresses no URNs:
State: Source (initial state)
Signal: default
Worley Informational [Page 14]
RFC 8433 Resolving Alert-Info URNs August 2018
From each state, we must construct the transition for each possible
input symbol. For a particular current state and symbol, we
construct the label of the next state by combining the input symbol
with the symbol in the current state's label for the same category.
If one of the symbols is a prefix of the other, we select the longer
one; if not, we select the symbol in the current state's label.
Thus, in our simple example, the initial state has the following
transitions:
State: Source (initial state)
Signal: default
Transitions:
Source:External -> Source:External
Source:Internal -> Source:Internal
Source:Other -> Source:Other
In all of these transitions, the input symbol is compatible with the
matching label of the current state, "Source", so the next state's
label is the full input symbol.
However, there is a further constraint on the next state: its signal
must express URNs that at least contain the expressed URNs of the
signal of the current state. Within that constraint, and being
compatible with the next state's label, for the category of the input
URN, the next state's signal must express the longest URN that can be
expressed by any signal.
In our example, this means that the next Source:External state has
the "external source" signal, which expresses
urn:alert:source:external. Since that signal expresses all of the
state's label, it is the chosen state. Similarly, the next
Source:Internal state has the "internal source" signal. But for the
transition on input Source:Other, the "Source:Other" state must have
the default signal, as there is no signal that expresses
urn:alert:source:[some-unknown-alert-ind-part]. So the next state is
"Source:(Other)", where the parentheses record that the "Other" part
of the label is not expressed by the state's signal.
Worley Informational [Page 15]
RFC 8433 Resolving Alert-Info URNs August 2018
Thus, the current state and the next states that it can transition
to are:
State: Source (initial state)
Signal: default
Transitions:
Source:External -> Source:External
Source:Internal -> Source:Internal
Source:Other -> Source:(Other)
State: Source:External
Signal: external source (urn:alert:source:external)
State: Source:Internal
Signal: internal source (urn:alert:source:internal)
State: Source:(Other)
Signal: default
Looking at the state Source:External, we see that it is incompatible
with all input symbols other than Source:External, and thus all of
its transitions are to itself:
State: Source:External
Signal: external source (urn:alert:source:external)
Transitions:
Source:External -> Source:External
Source:Internal -> Source:External
Source:Other -> Source:External
and similarly:
State: Source:Internal
Signal: internal source (urn:alert:source:internal)
Transitions:
Source:External -> Source:Internal
Source:Internal -> Source:Internal
Source:Other -> Source:Internal
State: Source:(Other)
Signal: default
Transitions:
Source:External -> Source:(Other)
Source:Internal -> Source:(Other)
Source:Other -> Source:(Other)
Worley Informational [Page 16]
RFC 8433 Resolving Alert-Info URNs August 2018
4.4. Summary
The FSM can be constructed by processing the file "very-simple.txt"
with the program "alert-info-fsm.py" in [code]. The program's output
shows the stages of the construction, which are as follows:
1. The signals have the meanings:
Signal URN(s)
---------------------------- -------------------------------
default (none)
internal source urn:alert:source:internal
external source urn:alert:source:external
2. The expressed URNs are:
urn:alert:source:external
urn:alert:source:internal
3. The relevant categories of "alert" URNs are only:
source
4. Thus, the infinite universe of possible "alert" URNs can be
reduced to these symbols, which are the categories of URNs that
are different in ways that are significant to the resolution
process:
Source
Source:External
Source:Internal
Source:Other
5. The FSM is:
State: Source (initial state)
Signal: default
Transitions:
Source:External -> Source:External
Source:Internal -> Source:Internal
Source:Other -> Source:(Other)
State: Source:External
Signal: external source (urn:alert:source:external)
Transitions:
Source:External -> Source:External
Source:Internal -> Source:External
Source:Other -> Source:External
Worley Informational [Page 17]
RFC 8433 Resolving Alert-Info URNs August 2018
State: Source:Internal
Signal: internal source (urn:alert:source:internal)
Transitions:
Source:External -> Source:Internal
Source:Internal -> Source:Internal
Source:Other -> Source:Internal
State: Source:(Other)
Signal: default
Transitions:
Source:External -> Source:(Other)
Source:Internal -> Source:(Other)
Source:Other -> Source:(Other)
* Each state is labeled by a set of symbols that describe the
information that has been extracted from the URNs so far.
* Each state has a signal that is a semantic superset of the
state's label, i.e., the signal's expressed URNs match the
initial portion of the label symbols. If Alert-Info
processing finishes with the FSM in a state, the UA will
render the state's signal to the user.
* The state's label is marked to show what subset of the symbols
are expressed by the state's signal. Two states can have the
same label but different signals.
* If a transition's input symbol is compatible with (is a
semantic subset of) the current state's label for that
category, the next state's label is updated with the input
symbol. If not, the next state is the current state. This is
how the state's label records what information has been
accumulated while processing the Alert-Info URNs.
* A transition's next state has a signal that semantically
subsets the current state's signal as much as possible in the
category of the input symbol. (In most cases, the choice of
signal is unique. In rare cases, there may be more than one
signal that meets this criterion, so the designer may have
some flexibility.)
Worley Informational [Page 18]
RFC 8433 Resolving Alert-Info URNs August 2018
4.5. Examples of Processing Alert-Info URNs
In the trivial case where the UA receives no Alert-Info URNs,
processing begins and ends with the FSM in the initial state, and the
default signal is selected.
If the UA receives
Alert-Info: <urn:alert:source:internal>
then processing progresses:
State: Source
Process: Source:Internal (urn:alert:source:internal)
State: Source:Internal
Signal: internal source
If the UA receives
Alert-Info: <urn:alert:source:external>,
<urn:alert:source:internal>
then processing progresses:
State: Source
Process: Source:External (urn:alert:source:external)
State: Source:External
Process: Source:Internal (urn:alert:source:internal)
State: Source:External
Signal: external source
If the UA receives
Alert-Info: <urn:alert:source:unclassified>,
<urn:alert:source:internal>
then processing progresses:
State: Source
Process: Source:Other (urn:alert:source:unclassified)
State: Source:(Other)
Process: Source:Internal (urn:alert:source:internal)
State: Source:(Other)
Signal: default
Worley Informational [Page 19]
RFC 8433 Resolving Alert-Info URNs August 2018
If the UA receives
Alert-Info: <urn:alert:priority:high>,
<urn:alert:source:internal>
then processing progresses:
State: Source
Ignore: urn:alert:priority:high
State: Source
Process: Source:Internal (urn:alert:source:internal)
State: Source:Internal
Signal: internal source
5. Further Examples
5.1. Example with "source" and "priority" URNs
Now consider an example where the UA can signal "external source",
"internal source", "low priority", and "high priority" individually
or in any combination of source and priority, along with a default
signal. This example is essentially the Cartesian product of two
copies of the example in Section 4: one dealing with the call's
source and one dealing with the call's priority. So there are a
total of 9 signals:
Signal URN(s)
---------------------------- -------------------------------
default (none)
external source urn:alert:source:external
internal source urn:alert:source:internal
low priority urn:alert:priority:low
low priority/external source urn:alert:priority:low,
urn:alert:source:external
low priority/internal source urn:alert:priority:low,
urn:alert:source:internal
high priority urn:alert:priority:high
high priority/external source urn:alert:priority:high,
urn:alert:source:external
high priority/internal source urn:alert:priority:high,
urn:alert:source:internal
The expressed URNs are:
urn:alert:source:external
urn:alert:source:internal
urn:alert:priority:low
urn:alert:priority:high
Worley Informational [Page 20]
RFC 8433 Resolving Alert-Info URNs August 2018
The relevant categories of "alert" URNs are only:
source
priority
The alphabet of symbols is:
Source
Source:External
Source:Internal
Source:Other
Priority
Priority:Low
Priority:High
Priority:Other
The 16 states are as follows, where 9 states are "sink" states from
which no further information can be recorded, as all transitions from
the state lead to itself.
State: Priority/Source
Signal: default
Transitions:
Priority:Other -> Priority:(Other)/Source
Priority:High -> Priority:High/Source
Priority:Low -> Priority:Low/Source
Source:Other -> Priority/Source:(Other)
Source:External -> Priority/Source:External
Source:Internal -> Priority/Source:Internal
State: Priority:(Other)/Source
Signal: default
Transitions:
Priority:Other -> Priority:(Other)/Source
Priority:High -> Priority:(Other)/Source
Priority:Low -> Priority:(Other)/Source
Source:Other -> Priority:(Other)/Source:(Other)
Source:External -> Priority:(Other)/Source:External
Source:Internal -> Priority:(Other)/Source:Internal
State: Priority:(Other)/Source:(Other)
Signal: default
Transitions:
any -> Priority:(Other)/Source:(Other)
Worley Informational [Page 21]
RFC 8433 Resolving Alert-Info URNs August 2018
State: Priority:(Other)/Source:External
Signal: external source
Transitions:
any -> Priority:(Other)/Source:External
State: Priority:(Other)/Source:Internal
Signal: internal source
Transitions:
any -> Priority:(Other)/Source:Internal
State: Priority:High/Source
Signal: high priority
Transitions:
Priority:Other -> Priority:High/Source
Priority:High -> Priority:High/Source
Priority:Low -> Priority:High/Source
Source:Other -> Priority:High/Source:(Other)
Source:External -> Priority:High/Source:External
Source:Internal -> Priority:High/Source:Internal
State: Priority:High/Source:(Other)
Signal: high priority
Transitions:
any -> Priority:High/Source:(Other)
State: Priority:High/Source:External
Signal: high priority/external source
Transitions:
any -> Priority:High/Source:External
State: Priority:High/Source:Internal
Signal: high priority/internal source
Transitions:
any -> Priority:High/Source:Internal
State: Priority:Low/Source
Signal: low priority
Transitions:
Priority:Other -> Priority:Low/Source
Priority:High -> Priority:Low/Source
Priority:Low -> Priority:Low/Source
Source:Other -> Priority:Low/Source:(Other)
Source:External -> Priority:Low/Source:External
Source:Internal -> Priority:Low/Source:Internal
Worley Informational [Page 22]
RFC 8433 Resolving Alert-Info URNs August 2018
State: Priority:Low/Source:(Other)
Signal: low priority
Transitions:
any -> Priority:Low/Source:(Other)
State: Priority:Low/Source:External
Signal: low priority/external source
Transitions:
any -> Priority:Low/Source:External
State: Priority:Low/Source:Internal
Signal: low priority/internal source
Transitions:
any -> Priority:Low/Source:Internal
State: Priority/Source:(Other)
Signal: default
Transitions:
Priority:Other -> Priority:(Other)/Source:(Other)
Priority:High -> Priority:High/Source:(Other)
Priority:Low -> Priority:Low/Source:(Other)
Source:Other -> Priority/Source:(Other)
Source:External -> Priority/Source:(Other)
Source:Internal -> Priority/Source:(Other)
State: Priority/Source:External
Signal: external source
Transitions:
Priority:Other -> Priority:(Other)/Source:External
Priority:High -> Priority:High/Source:External
Priority:Low -> Priority:Low/Source:External
Source:Other -> Priority/Source:External
Source:External -> Priority/Source:External
Source:Internal -> Priority/Source:External
State: Priority/Source:Internal
Signal: internal source
Transitions:
Priority:Other -> Priority:(Other)/Source:Internal
Priority:High -> Priority:High/Source:Internal
Priority:Low -> Priority:Low/Source:Internal
Source:Other -> Priority/Source:Internal
Source:External -> Priority/Source:Internal
Source:Internal -> Priority/Source:Internal
Worley Informational [Page 23]
RFC 8433 Resolving Alert-Info URNs August 2018
An example of processing that involves multiple "source" URNs and one
"priority" URN:
Alert-Info: <urn:alert:source:internal>,
<urn:alert:source:unclassified>,
<urn:alert:priority:high>
in which case processing progresses:
State: Source/Priority
Process: Source:Internal (urn:alert:source:internal)
State: Source:Internal/Priority
Process: Source:(Other) (urn:alert:source:unclassified)
State: Source:Internal/Priority
Process: Priority:High (urn:alert:priority:high)
State: Source:Internal/Priority:High
Signal: internal source/high priority
5.2. Example 1 of RFC 7462
A more complicated example is provided in Section 12.2.1 of
[RFC7462]. It is like the example in Section 5.1 of this document,
except that the UA can only signal "external source", "internal
source", "low priority", and "high priority" individually but not in
combination, as well as a default signal:
Signal URN(s)
---------------------------- -------------------------------
default (none)
internal source urn:alert:source:external
external source urn:alert:source:internal
low priority urn:alert:priority:low
high priority urn:alert:priority:high
The signals can express the following URNs:
urn:alert:source:external
urn:alert:source:internal
urn:alert:priority:low
urn:alert:priority:high
The relevant categories of "alert" URNs are:
source
priority
Worley Informational [Page 24]
RFC 8433 Resolving Alert-Info URNs August 2018
The alphabet of symbols is:
Source
Source:External
Source:Internal
Source:Other
Priority
Priority:Low
Priority:High
Priority:Other
In this example, the FSM has 20 states because both "source" and
"priority" URNs are recorded, but the order in which the two appear
affects the signal:
State: Priority/Source
Signal: default
Transitions:
Priority:Other -> Priority:(Other)/Source
Priority:High -> Priority:High/Source
Priority:Low -> Priority:Low/Source
Source:Other -> Priority/Source:(Other)
Source:External -> Priority/Source:External
Source:Internal -> Priority/Source:Internal
State Priority:(Other)/Source can transition to states that can
signal the source, because the recorded priority can't be signaled
and thus does not block the signaling of the source:
State: Priority:(Other)/Source
Signal: default
Transitions:
Priority:Other -> Priority:(Other)/Source
Priority:High -> Priority:(Other)/Source
Priority:Low -> Priority:(Other)/Source
Source:Other -> Priority:(Other)/Source:(Other)
Source:External -> Priority:(Other)/Source:External
Source:Internal -> Priority:(Other)/Source:Internal
State: Priority:(Other)/Source:(Other)
Signal: default
Transitions:
any -> Priority:(Other)/Source:(Other)
State: Priority:(Other)/Source:External
Signal: external source
Transitions:
any -> Priority:(Other)/Source:External
Worley Informational [Page 25]
RFC 8433 Resolving Alert-Info URNs August 2018
State: Priority:(Other)/Source:Internal
Signal: internal source
Transitions:
any -> Priority:(Other)/Source:Internal
Because there are no signals for combinations of "source" and
"priority" URNs, processing a "source" URN from the state
Priority:High/Source leads to a state that records the priority
information but does not signal it:
State: Priority:High/Source
Signal: high priority
Transitions:
Priority:Other -> Priority:High/Source
Priority:High -> Priority:High/Source
Priority:Low -> Priority:High/Source
Source:Other -> Priority:High/Source:(Other)
Source:External -> Priority:High/Source:(External)
Source:Internal -> Priority:High/Source:(Internal)
State: Priority:High/Source:(Other)
Signal: high priority
Transitions:
any -> Priority:High/Source:(Other)
From the state Priority:High/Source, "source" URNs transition to
states that record both source and priority but signal only priority,
one of which is Priority:High/Source:(External). But from
Priority/Source:External, the symbol Priority:High transitions to the
state Priority:(High)/Source:External, which records the same
information but signals the source, not the priority. One state is
reached by processing a "priority" URN and then a "source" URN,
whereas the other is reached by processing a "source" URN and then a
"priority" URN.
State: Priority:High/Source:(External)
Signal: high priority
Transitions:
any -> Priority:High/Source:(External)
State: Priority:High/Source:(Internal)
Signal: high priority
Transitions:
any -> Priority:High/Source:(Internal)
Worley Informational [Page 26]
RFC 8433 Resolving Alert-Info URNs August 2018
and similarly for Priority:Low/Source:
State: Priority:Low/Source
Signal: low priority
Transitions:
Priority:Other -> Priority:Low/Source
Priority:High -> Priority:Low/Source
Priority:Low -> Priority:Low/Source
Source:Other -> Priority:Low/Source:(Other)
Source:External -> Priority:Low/Source:(External)
Source:Internal -> Priority:Low/Source:(Internal)
State: Priority:Low/Source:(Other)
Signal: low priority
Transitions:
any -> Priority:Low/Source:(Other)
State: Priority:Low/Source:(External)
Signal: low priority
Transitions:
any -> Priority:Low/Source:(External)
State: Priority:Low/Source:(Internal)
Signal: low priority
Transitions:
any -> Priority:Low/Source:(Internal)
State: Priority/Source:(Other)
Signal: default
Transitions:
Priority:Other -> Priority:(Other)/Source:(Other)
Priority:High -> Priority:High/Source:(Other)
Priority:Low -> Priority:Low/Source:(Other)
Source:Other -> Priority/Source:(Other)
Source:External -> Priority/Source:(Other)
Source:Internal -> Priority/Source:(Other)
State: Priority/Source:External
Signal: external source
Transitions:
Priority:Other -> Priority:(Other)/Source:External
Priority:High -> Priority:(High)/Source:External
Priority:Low -> Priority:(Low)/Source:External
Source:Other -> Priority/Source:External
Source:External -> Priority/Source:External
Source:Internal -> Priority/Source:External
Worley Informational [Page 27]
RFC 8433 Resolving Alert-Info URNs August 2018
State: Priority:(High)/Source:External
Signal: external source
Transitions:
any -> Priority:(High)/Source:External
State: Priority:(Low)/Source:External
Signal: external source
Transitions:
any -> Priority:(Low)/Source:External
State: Priority/Source:Internal
Signal: internal source
Transitions:
Priority:Other -> Priority:(Other)/Source:Internal
Priority:High -> Priority:(High)/Source:Internal
Priority:Low -> Priority:(Low)/Source:Internal
Source:Other -> Priority/Source:Internal
Source:External -> Priority/Source:Internal
Source:Internal -> Priority/Source:Internal
State: Priority:(High)/Source:Internal
Signal: internal source
Transitions:
any -> Priority:(High)/Source:Internal
State: Priority:(Low)/Source:Internal
Signal: internal source
Transitions:
any -> Priority:(Low)/Source:Internal
As an example of processing, if the UA receives
Alert-Info: <urn:alert:source:internal>
then processing progresses:
State: Priority/Source
Process: Source:Internal (urn:alert:source:internal)
State: Priority/Source:Internal
Signal: internal source
Worley Informational [Page 28]
RFC 8433 Resolving Alert-Info URNs August 2018
A more complicated example involves multiple "source" URNs that do
not select a non-default signal and one "priority" URN that can be
signaled:
Alert-Info: <urn:alert:source:unclassified>,
<urn:alert:source:internal>,
<urn:alert:priority:high>
in which case processing progresses:
State: Priority/Source
Process: Source:Other (urn:alert:source:unclassified)
State: Priority/Source:(Other)
Process: Source:Internal (urn:alert:source:internal)
State: Priority/Source:(Other)
Process: Priority:High (urn:alert:priority:high)
State: Priority:High/Source:(Other)
Signal: high priority
The only output of the FSM is the state's signal. Based on this,
several groups of states in this FSM can be merged using standard FSM
optimization algorithms:
states with signal "high priority":
Priority:High/Source
Priority:High/Source:(Other)
Priority:High/Source:(External)
Priority:High/Source:(Internal)
states with signal "low priority":
Priority:Low/Source
Priority:Low/Source:(Other)
Priority:Low/Source:(External)
Priority:Low/Source:(Internal)
states with signal "external source":
Priority/Source:External
Priority:(High)/Source:External
Priority:(Low)/Source:External
Priority:(Other)/Source:External
states with signal "internal source":
Priority/Source:Internal
Priority:(High)/Source:Internal
Priority:(Low)/Source:Internal
Priority:(Other)/Source:Internal
Worley Informational [Page 29]
RFC 8433 Resolving Alert-Info URNs August 2018
This reduces the FSM to eight states:
Priority/Source
Priority:(Other)/Source
Priority:(Other)/Source:(Other)
Priority:High/Source [aggregated]
Priority:Low/Source [aggregated]
Priority/Source:(Other)
Priority/Source:External [aggregated]
Priority/Source:Internal [aggregated]
5.3. Examples 2, 3, and 4 of RFC 7462
Examples 2, 3, and 4 of [RFC7462] are similar to the example in
Section 5.1 of this document, but they do not include a signal for
the combination "internal source, low priority" to make resolution
examples work asymmetrically.
The FSM for this example has the same alphabet as the FSM of
Section 5.1. Most of the states of this FSM are the same as the
states of the FSM of Section 5.1, but the state
Source:Internal/Priority:Low is missing because there is no signal
for that combination. It is replaced by two states:
1. One state is Source:Internal/Priority:(Low); it records that
Source:Internal was specified first (and is to be signaled) and
that Priority:Low was specified later (and cannot be signaled --
but it still prevents any further "priority" URNs from having an
effect).
2. The other state is Source:(Internal)/Priority:Low; it records the
reverse sequence of events.
The changes in the FSM are:
State: Priority:Low/Source
Signal: low priority
Transitions:
Source:Internal -> Priority:Low/Source:(Internal)
(other transitions unchanged)
State: Priority:Low/Source:(Internal)
Signal: low priority
Transitions:
any -> Priority:Low/Source:(Internal)
Worley Informational [Page 30]
RFC 8433 Resolving Alert-Info URNs August 2018
State: Priority/Source:Internal
Signal: internal source
Transitions:
Priority:Low -> Priority:(Low)/Source:Internal
(other transitions unchanged)
State: Priority:(Low)/Source:Internal
Signal: internal source
Transitions:
any -> Priority:(Low)/Source:Internal
An example of processing that involves multiple "source" URNs and one
"priority" URN:
Alert-Info: <urn:alert:source:internal>,
<urn:alert:source:unclassified>,
<urn:alert:priority:high>
then processing progresses:
State: Priority/Source
Process: Source:Internal (urn:alert:source:internal)
State: Priority/Source:Internal
Process: Source:Other (urn:alert:source:unclassified)
State: Priority/Source:Internal
Process: Priority:High (urn:alert:priority:high)
State: Priority:High/Source:Internal
Signal: internal source/high priority
If the UA receives
Alert-Info: <urn:alert:source:internal>
then processing progresses:
State: Priority/Source
Process: Source:Internal (urn:alert:source:internal)
State: Priority/Source:Internal
Signal: internal source
Worley Informational [Page 31]
RFC 8433 Resolving Alert-Info URNs August 2018
If the UA receives
Alert-Info: <urn:alert:source:external>,
<urn:alert:priority:low>
then processing progresses:
State: Priority/Source
Process: Source:External (urn:alert:source:external)
State: Priority/Source:External
Process: Priority:Low (urn:alert:priority:low)
State: Priority:Low/Source:External
Signal: external source/low priority
Suppose the same UA receives
Alert-Info: <urn:alert:source:internal>,
<urn:alert:priority:low>
Note that there is no signal that corresponds to this combination.
In that case, the processing is:
State: Priority/Source
Process: Source:Internal (urn:alert:source:internal)
State: Priority/Source:Internal
Process: Priority:Low (urn:alert:priority:low)
State: Priority:(Low)/Source:Internal
Signal: internal source
If the order of the URNs is reversed, what is signaled is the meaning
of the now-different first URN:
Alert-Info: <urn:alert:priority:low>,
<urn:alert:source:internal>
State: Priority/Source
Process: Priority:Low (urn:alert:priority:low)
State: Priority:Low/Source
Process: Source:Internal (urn:alert:source:internal)
State: Priority:Low/Source:(Internal)
Signal: low priority
Worley Informational [Page 32]
RFC 8433 Resolving Alert-Info URNs August 2018
Notice that the existence of the new states prevents later URNs of a
category from overriding earlier URNs of that category, even if the
earlier one was not itself signalable and the later one would be
signalable in the absence of the earlier one:
Alert-Info: <urn:alert:priority:low>,
<urn:alert:source:internal>,
<urn:alert:source:external>
State: Priority/Source
Process: Priority:Low (urn:alert:priority:low)
State: Priority:Low/Source
Process: Source:Internal (urn:alert:source:internal)
State: Priority:Low/Source:(Internal)
Process: Source:External (urn:alert:source:external)
State: Priority:Low/Source:(Internal)
Signal: low priority
This situation shows the necessity of states whose labels contain
parentheses. If the second transition had been to the state
Priority:Low/Source (on the basis that there is no proper state
Priority:Low/Source:Internal), then the third transition would have
been to the state Priority:Low/Source:External, and the signal would
have been "external source/low priority".
5.4. An Example That Subsets Internal Sources
In the example of Section 4, there are signals for "external source"
and "internal source". Let us add to that example a signal for
"source internal from a VIP (Very Important Person)". That last
signal expresses the private extension URN
urn:alert:source:internal:vip@example, which is a subset of
urn:alert:source:internal, which is expressed by the "source
internal" signal. There are a total of three expressed URNs, one of
which is a subset of another:
urn:alert:source:internal
urn:alert:source:internal:vip@example
urn:alert:source:external
This generates the following alphabet of symbols, which includes two
"Other" symbols for the "source" category:
Source
Source:Internal
Source:Internal:Vip@example
Source:Internal:Other
Source:Other
Worley Informational [Page 33]
RFC 8433 Resolving Alert-Info URNs August 2018
5.5. An Example of "alert:service" URNs
In this example, there are signals for "service forward" (the call
has been forwarded) and "source recall callback" (a recall due to a
callback). This gives two expressed URNs:
urn:alert:service:forward
urn:alert:service:recall:callback
This generates the following alphabet of symbols. Note that there
are two "Other" symbols, because the "alert:service" URNs have an
additional level of qualification.
Service
Service:Forward
Service:Recall
Service:Recall:Callback
Service:Recall:Other
Service:Other
5.6. An Example Using Country Codes
In this example, we consider how a UA generates ringback signals when
the UA wishes to reproduce the traditional behavior where the caller
hears the ringback signals defined by the telephone service in the
callee's country rather than the ringback signals defined by the
service in the caller's country. In the Alert-Info header field of
the 180 (Ringing) provisional response, we assume that the called UA
provides an "alert:country" URN [RFC7462] containing the ISO 3166-1
[ISO-3166-1] alpha-2 country code of the callee's country.
The UA has a default signal and a "non-country" signal for
urn:alert:service:call-waiting. For the example country with code
"XA", the UA has a default signal and signals for
urn:alert:service:call-waiting and urn:alert:service:forward. For
the example country with code "XB", the UA has a default signal and a
signal for urn:alert:service:forward. These inconsistencies between
the non-country signals and the country signals are chosen to
demonstrate the flexibility of the construction method, showing that
three systems of signals can be combined correctly even when the
systems were established without coordination between them.
Worley Informational [Page 34]
RFC 8433 Resolving Alert-Info URNs August 2018
The signals are:
Signal URN(s)
-------------------------- ----------------------------------
default (none)
call-waiting urn:alert:service:call-waiting
XA default urn:alert:country:xa
XA call-waiting urn:alert:country:xa,
urn:alert:service:call-waiting
XA forward urn:alert:country:xa,
urn:alert:service:forward
XB default urn:alert:country:xb
XB forward urn:alert:country:xb,
urn:alert:service:forward
The expressed URNs are:
urn:alert:country:xa
urn:alert:country:xb
urn:alert:service:call-waiting
urn:alert:service:forward
The relevant categories of "alert" URNs are only:
country
service
The alphabet of symbols is:
Country
Country:[other]
Country:Xa
Country:Xb
Service
Service:[other]
Service:Call-waiting
Service:Forward
Worley Informational [Page 35]
RFC 8433 Resolving Alert-Info URNs August 2018
The 17 states are as follows:
State: 0 Country/Service
Signal: default
Transitions:
Country:[other] -> 1 Country:([other])/Service
Country:Xa -> 5 Country:Xa/Service
Country:Xb -> 9 Country:Xb/Service
Service:[other] -> 13 Country/Service:([other])
Service:Call-waiting -> 14 Country/Service:Call-waiting
Service:Forward -> 16 Country/Service:(Forward)
State: 1 Country:([other])/Service
Signal: default
Transitions:
Country:[other] -> 1 Country:([other])/Service
Country:Xa -> 1 Country:([other])/Service
Country:Xb -> 1 Country:([other])/Service
Service:[other] -> 2 Country:([other])/Service:([other])
Service:Call-waiting -> 3 Country:([other])/Service:Call-waiting
Service:Forward -> 4 Country:([other])/Service:(Forward)
State: 2 Country:([other])/Service:([other])
Signal: default
Transitions:
any -> 2 Country:([other])/Service:([other])
State: 3 Country:([other])/Service:Call-waiting
Signal: call-waiting
Transitions:
any -> 3 Country:([other])/Service:Call-waiting
State: 4 Country:([other])/Service:(Forward)
Signal: default
Transitions:
any -> 4 Country:([other])/Service:(Forward)
State: 5 Country:Xa/Service
Signal: XA default
Transitions:
Country:[other] -> 5 Country:Xa/Service
Country:Xa -> 5 Country:Xa/Service
Country:Xb -> 5 Country:Xa/Service
Service:[other] -> 6 Country:Xa/Service:([other])
Service:Call-waiting -> 7 Country:Xa/Service:Call-waiting
Service:Forward -> 8 Country:Xa/Service:Forward
Worley Informational [Page 36]
RFC 8433 Resolving Alert-Info URNs August 2018
State: 6 Country:Xa/Service:([other])
Signal: XA default
Transitions:
any -> 6 Country:Xa/Service:([other])
State: 7 Country:Xa/Service:Call-waiting
Signal: XA call-waiting
Transitions:
any -> 7 Country:Xa/Service:Call-waiting
State: 8 Country:Xa/Service:Forward
Signal: XA forward
Transitions:
any -> 8 Country:Xa/Service:Forward
State: 9 Country:Xb/Service
Signal: XB default
Transitions:
Country:[other] -> 9 Country:Xb/Service
Country:Xa -> 9 Country:Xb/Service
Country:Xb -> 9 Country:Xb/Service
Service:[other] -> 10 Country:Xb/Service:([other])
Service:Call-waiting -> 11 Country:Xb/Service:(Call-waiting)
Service:Forward -> 12 Country:Xb/Service:Forward
State: 10 Country:Xb/Service:([other])
Signal: XB default
Transitions:
any -> 10 Country:Xb/Service:([other])
State: 11 Country:Xb/Service:(Call-waiting)
Signal: XB default
Transitions:
any -> 11 Country:Xb/Service:(Call-waiting)
State: 12 Country:Xb/Service:Forward
Signal: XB forward
Transitions:
any -> 12 Country:Xb/Service:Forward
Worley Informational [Page 37]
RFC 8433 Resolving Alert-Info URNs August 2018
State: 13 Country/Service:([other])
Signal: default
Transitions:
Country:[other] -> 2 Country:([other])/Service:([other])
Country:Xa -> 6 Country:Xa/Service:([other])
Country:Xb -> 10 Country:Xb/Service:([other])
Service:[other] -> 13 Country/Service:([other])
Service:Call-waiting -> 13 Country/Service:([other])
Service:Forward -> 13 Country/Service:([other])
State: 14 Country/Service:Call-waiting
Signal: call-waiting
Transitions:
Country:[other] -> 3 Country:([other])/Service:Call-waiting
Country:Xa -> 7 Country:Xa/Service:Call-waiting
Country:Xb -> 15 Country:(Xb)/Service:Call-waiting
Service:[other] -> 14 Country/Service:Call-waiting
Service:Call-waiting -> 14 Country/Service:Call-waiting
Service:Forward -> 14 Country/Service:Call-waiting
State: 15 Country:(Xb)/Service:Call-waiting
Signal: call-waiting
Transitions:
any -> 15 Country:(Xb)/Service:Call-waiting
State: 16 Country/Service:(Forward)
Signal: default
Transitions:
Country:[other] -> 4 Country:([other])/Service:(Forward)
Country:Xa -> 8 Country:Xa/Service:Forward
Country:Xb -> 12 Country:Xb/Service:Forward
Service:[other] -> 16 Country/Service:(Forward)
Service:Call-waiting -> 16 Country/Service:(Forward)
Service:Forward -> 16 Country/Service:(Forward)
Call-waiting can be signaled in conjunction with country XA but not
in conjunction with country XB, as the UA does not have a signal to
present call-waiting alerts for country XB. Thus, the ordering of
urn:alert:service:call-waiting with urn:alert:country:xa does not
matter, but if urn:alert:country:xb appears before
urn:alert:service:call-waiting, call-waiting cannot be signaled.
Worley Informational [Page 38]
RFC 8433 Resolving Alert-Info URNs August 2018
On the other hand, if urn:alert:service:call-waiting appears before
urn:alert:country:xb, then call-waiting is signaled, but using the
non-country signal.
Alert-Info: urn:alert:country:xa,
urn:alert:service:call-waiting
State: 0 Country/Service
Process: Country:Xa (urn:alert:country:xa)
State: 5 Country:Xa/Service
Process: Service:Call-waiting (urn:alert:service:call-waiting)
State: 7 Country:Xa/Service:Call-waiting
Signal: XA call-waiting
Alert-Info: urn:alert:service:call-waiting,
urn:alert:country:xa
State: 0 Country/Service
Process: Service:Call-waiting (urn:alert:service:call-waiting)
State: 14 Country/Service:Call-waiting
Process: Country:Xa (urn:alert:country:xa)
State: 7 Country:Xa/Service:Call-waiting
Signal: XA call-waiting
Alert-Info: urn:alert:country:xb,
urn:alert:service:call-waiting
State: 0 Country/Service
Process: Country:Xb (urn:alert:country:xb)
State: 9 Country:Xb/Service
Process: Service:Call-waiting (urn:alert:service:call-waiting)
State: 11 Country:Xb/Service:(Call-waiting)
Signal: XB default
Alert-Info: urn:alert:service:call-waiting,
urn:alert:country:xb
State: 0 Country/Service
Process: Service:Call-waiting (urn:alert:service:call-waiting)
State: 14 Country/Service:Call-waiting
Process: Country:Xb (urn:alert:country:xb)
State: 15 Country:(Xb)/Service:Call-waiting
Signal: call-waiting
Worley Informational [Page 39]
RFC 8433 Resolving Alert-Info URNs August 2018
6. Prioritizing Signals
The specifications provided in [RFC7462] are oriented toward giving
the sender of Alert-Info control over which of the "alert" URNs are
most important. But in some situations, the UA may prefer to
prioritize expressing one URN category over another regardless of the
order in which their URNs appear in Alert-Info. This section
describes how that can be accommodated within the framework of
[RFC7462] and presents an example FSM resulting from that approach.
This example uses the signals of Section 5.2, viz., "external
source", "internal source", "low priority", and "high priority", but
this time, we want to signal "high priority" in preference to any
other signal that might be applicable.
We accommodate this within the framework of [RFC7462] by assigning
the signal "high priority" for each of these combinations of URNs:
urn:alert:priority:high
urn:alert:priority:high, urn:alert:source:internal
urn:alert:priority:high, urn:alert:source:external
The result is that the signal "high priority" is the "best" signal
for any combination of urn:alert:priority:high with "source" URNs.
Constructing the symbols produces the same results as before. The
signals can express the following URNs:
urn:alert:source:external
urn:alert:source:internal
urn:alert:priority:low
urn:alert:priority:high
The relevant categories of "alert" URNs are:
source
priority
The alphabet of symbols is:
Source
Source:External
Source:Internal
Source:Other
Priority
Priority:Low
Priority:High
Priority:Other
Worley Informational [Page 40]
RFC 8433 Resolving Alert-Info URNs August 2018
When the FSM is constructed, it is the same as the FSM of
Section 5.2, except that certain states are effectively renamed and
merged, because any "source" is defined to be expressed if high
priority is expressed:
Priority:(High)/Source:External and
Priority:High/Source:(External) become:
State: Priority:High/Source:External
Signal: high priority
Priority:(High)/Source:Internal and
Priority:High/Source:(Internal) become:
State: Priority:High/Source:Internal
Signal: high priority
This reduces the FSM to 18 states. In addition, these two new
states, along with a number of other states, can be merged by FSM
optimization, since all of them have the signal "high priority" and
from them, there are no transitions to states outside this set. The
optimized FSM has 10 states.
7. Dynamic Sets of Signals
This section discusses how to construct FSMs for a UA that allows
variable sets of signals -- for example, if the user can configure
the use of ring tones. Several approaches can be used:
o Whenever the set of ring tones is changed, re-execute the
processes of Section 4.
o Whenever the set of ring tones is changed, rebuild the list of
expressed URNs (Section 4.1) and reconstruct the alphabet of
symbols (Section 4.2). Then, use an algorithm for dynamically
constructing the states of the FSM as needed during Alert-Info
processing.
o If the sets of possible URNs expressed by the ring tones are
sufficiently limited, the steps of Section 4 can be carried out
"generically", and the generic FSM can be specialized for the
current ring tone configuration.
The remainder of this section gives an example of the third approach.
Worley Informational [Page 41]
RFC 8433 Resolving Alert-Info URNs August 2018
For the example, we will use a set of ring tones that express the
identity of the caller. To signal this information, a private
extension "alert" URN category, "caller@example", is used:
urn:alert:caller@example:alice@example.com
urn:alert:caller@example:bob@example.com
etc.
which we can express by the generic pattern
urn:alert:caller@example:IDENTITY
where "IDENTITY" is replaced in succession by the set of caller
identities that have their own ring tones to generate the set of
expressed URNs.
The alphabet is then:
Caller@example
Caller@example:IDENTITY
Caller@example:Other
where "IDENTITY" is replaced in succession by the set of caller
identities. The "Caller@example:Other" symbol includes all URNs of
the category "caller@example" that are not included in any of the
"Caller@example:IDENTITY" symbols, i.e, where the second
alert-ind-part is not one of the known caller identities.
The states and transitions of the FSM are:
State: Caller@example (initial state)
Signal: default
Transitions:
Caller@example:IDENTITY -> Caller@example:IDENTITY
Caller@example:Other -> Caller@example:(Other)
State: Caller@example:IDENTITY
Signal: signal for caller IDENTITY
Transitions:
any -> Caller@example:IDENTITY
State: Caller@example:(Other)
Signal: default
Transitions:
any -> Caller@example:(Other)
Worley Informational [Page 42]
RFC 8433 Resolving Alert-Info URNs August 2018
where again, the second state is replicated once for each caller
identity that has a ring tone, with "IDENTITY" replaced with the
caller identity.
8. Security Considerations
The security considerations discussed in Section 16 of [RFC7462]
regarding the use and processing of "alert" URNs MUST be followed
when the algorithm described in this document is used.
Like any implementation of [RFC7462], implementations of the
algorithm defined in this document MUST take into account that the
value of a received Alert-Info header field may contain URIs of any
scheme, may contain syntactically invalid values, and may be
syntactically invalid overall. The handling of syntactically invalid
values is specified by [RFC3261]. The handling of URIs other than
"alert" URIs is outside the scope of this document (and outside the
scope of [RFC7462]) and MAY be subject to local policy.
Like the algorithm described in Section 12 of [RFC7462], the output
of the algorithm defined in this document is limited to a choice
among the signals that it has been configured for, limiting the
security issues regarding the processing of its output. This
algorithm will use at most linear time and constant space to process
a sequence of "alert" URNs. This is significantly more efficient
than the algorithm of [RFC7462] and minimizes the security
vulnerabilities of this processing step that are due to resource
consumption.
However, the process defined in this document for constructing an FSM
can use more than linear time and constant space -- probably
exponential time and space in the worst case. This SHOULD be taken
into consideration whenever an FSM is constructed using this
algorithm and MUST be taken into consideration when it is done
dynamically by a UA. Whenever an FSM is constructed by a process
that is not under the direct supervision of a human user, procedures
MUST be used to ensure that (1) the processing and memory consumption
are limited to acceptable amounts and (2) if the FSM construction is
aborted due to excessive consumption, the designated consumers of the
FSM MUST have appropriate fallback procedures.
9. IANA Considerations
This document has no IANA actions.
Worley Informational [Page 43]
RFC 8433 Resolving Alert-Info URNs August 2018
10. References
10.1. Normative References
[ISO-3166-1]
International Organization for Standardization, "Codes for
the representation of names of countries and their
subdivisions -- Part 1: Country codes", ISO
Standard 3166-1:2013, November 2013,
<https://www.iso.org/iso-3166-country-codes.html>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC7462] Liess, L., Ed., Jesske, R., Johnston, A., Worley, D., and
P. Kyzivat, "URNs for the Alert-Info Header Field of the
Session Initiation Protocol (SIP)", RFC 7462,
DOI 10.17487/RFC7462, March 2015,
<https://www.rfc-editor.org/info/rfc7462>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in
RFC 2119 Key Words", BCP 14, RFC 8174,
DOI 10.17487/RFC8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>.
10.2. Informative References
[code] Worley, D., "draft-worley-alert-info-fsm.aux",
February 2017, <http://svn.resiprocate.org/rep/
ietf-drafts/worley/draft-worley-alert-info-fsm.aux>.
Worley Informational [Page 44]
RFC 8433 Resolving Alert-Info URNs August 2018
Acknowledgments
Thanks to Paul Kyzivat, whose relentless identification of the
weaknesses of earlier versions made the final document much, much
better than it would have been, by changing it from the exposition of
a concept into a practical tool. Thanks to Rifaat Shekh-Yusef, Eric
Burger, and Gonzalo Camarillo for their thorough reviews. Thanks to
the earlier Independent Submissions Editor, Nevil Brownlee, for his
work obtaining reviewers, and the later Independent Submissions
Editor, Adrian Farrel, for prompting me to write the Security
Considerations section (which I had expected to be trivial but
was not).
Author's Address
Dale R. Worley
Ariadne Internet Services
738 Main St.
Waltham, MA 02451
United States of America
Email: worley@ariadne.com
Worley Informational [Page 45]