RFC 8433
A Simpler Method for Resolving Alert-Info URNs
Pages: 45
Informational
Informational
Part 1 of 2 – Pages 1 to 20
Independent Submission D. Worley Request for Comments: 8433 Ariadne Category: Informational August 2018 ISSN: 2070-1721 A Simpler Method for Resolving Alert-Info URNsAbstract
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.
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
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
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
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.)
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.
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)
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".
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.
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.
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.)
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
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,
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
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.
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)
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
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.)
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
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
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