| SALUD | D. Worley |
| Internet-Draft | Ariadne |
| Intended status: Informational | March 24, 2018 |
| Expires: September 25, 2018 |
A Simpler Method for Resolving Alert-Info URNs
draft-worley-alert-info-fsm-09
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 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.
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When a SIP user agent 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 user agent 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 user agent 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 which 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 user agent 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 user agent is often required to select an alerting signal which 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 which satisfies the requirements of section 11.1. That algorithm can be used regardless of the set of alerting signals that the user agent provides and their specified meanings. The existence of this algorithm 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 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 this algorithm, once the user agent designer has chosen the set of signals that the user agent produces and the "alert" URNs that they express, a finite state machine 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, which are then used as input to the finite state machine. After processing the input, the state of the finite state machine selects the correct alerting signal to present to the user.
Both the preprocessor and the finite state machine 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 user agent.
The requirements for the resolution of "alert" URNs are given in section 11.1 of [RFC7462] and can be described as follows:
Alert-Info: urn:alert:category-a:part-a1:part-a2,
urn:alert:category-b:part-b1:part-b2
So, for instance, consider processing
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 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.
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 user agent designer has chosen a set of signals and the URNs that they express, a finite state machine is constructed that selects alerting signals based on the URNs in the Alert-Info header field(s) in a SIP message.
To select a ring tone or ringback tone based on a SIP message, the user agent processes the "alert" URNs in the Alert-Info header field from left to right. Initially the FSM is in a designated initial state. The user agent 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 user agent will render to the user.
Note that the user agent generally has two FSMs, because a user agent 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 assures 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. E.g., the examples in Section 5 and Section 6 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.
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 indicate that meaning in SIP messages, and consequently should elicit that signal from the UA.
Signal URN(s)
---------------------------- -------------------------------
external source urn:alert:source:external
Signal URN(s)
---------------------------- -------------------------------
internal source urn:alert:source:internal
Signal URN(s)
---------------------------- -------------------------------
high priority urn:alert:priority:high
low priority urn:alert:priority:low
Signal URN(s)
---------------------------- -------------------------------
default (none)
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)
------------------------------ -------------------------------
high priority, internal source urn:alert:priority:high,
urn:alert:source:internal
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)
---------------------------- -------------------------------
recall urn:alert:service:recall
Signal URN(s)
---------------------------- -------------------------------
recall generally urn:alert:service:recall
recall due to callback urn:alert:service:recall:callback
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)
--------------------- -------------------------------
high priority urn:alert:priority:high
extra high priority urn:alert:priority:high:extra@example.com
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-high@example. The final "extra-high@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)
----------------------- -------------------------------
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
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:
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.
In this section, we will discuss various considerations which arise when processing Alert-Info. These have to be taken care of properly in order to conform to the standards, as well as to endure a good user experience. But since they are largely independent of the generated finite state machine and its processing, they are gathered here in a separate section.
The UA may have a number of different finite state machines (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 regenerating it according to the method described here, or by generating a "generic" FSM and instantiating it based on the available signals. (See Section 12 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 processing the "alert" URNs to select a signal. In any case, the remainder of this document assumes that the signal is to be chosen based on the "alert" URNs in Alert-Info, and that 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. E.g., the URN may have only three components, despite 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 which are unknown to the UA, and as a consequence, it is also robust if they are not valid standardized URNs. Thus, these error conditions need not be handled specially.
Constructing the FSM involves:
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.
Signal URN(s)
---------------------------- -------------------------------
default (none)
internal source urn:alert:source:internal
external source urn:alert:source:external
urn:alert:source:external
urn:alert:source:internal
The first step is to establish for each of the user agent's signals what call characteristics it represents, which is to say, the set of "alert" URNs which are its information content.
In order to reduce the infinite set of possible "alert" URNs to a finite alphabet of input symbols which 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 user agent's signaling; "alert" URNs in any other alert-category cannot affect the signaling and can be ignored. (The easiest method 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.)
Source
Source:External
Source:Internal
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: [RFC7462] section 7 -- 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.
Source:Other
Source:External:Other
Source:Internal:Other
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
Source:External
Source:External:Other
Source:Internal
Source:Internal:Other
Source:Other
These steps give us this set of symbols:
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 which corresponds to an expressed URN which is not ancestral to any other expressed URNs. This works because adding further alert-ind-parts to a URN which 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, both for causing a signal to be chosen as well as for suppressing the effect of later URNs.
Source
Source:External
Source:Internal
Source:Other
urn:alert
|
{ | }
{ source } --> 1
{ | }
|
+--------------------+------------------+
| | |
{ | } { | } { | }
{ external* } --> 2 { internal* } --> 3 { ... } --> 4
{ | } { | } { }
{ ... } { ... }
{ } { }
1 = Source
2 = Source:External
3 = Source:Internal
4 = Source:Other
This leaves the following symbols for the "source" category:
The user agent processes the Alert-Info URNs left-to-right using a finite state machine (FSM), with each successive URN causing the FSM to transition to a new state. Each state of the FSM records the information which has so far been extracted from the URNs. The state of the FSM after processing all the URNs determines which signal the user agent will render to the user.
We label each state with a set of symbols, one from each relevant category, which 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 user agent 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 unintuitive 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 state and symbol, we construct the label of the destination state by combining the input symbol with the symbol in the start 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 start state's label.
State: Source (initial state)
Signal: default
Transitions:
Source:External -> Source:External
Source:Internal -> Source:Internal
Source:Other -> Source:Other
Thus, in our simple example, the initial state has the following transitions:
However, there is a further constraint on the destination state: Its signal must express URNs that at least contain the expressed URNs of the signal of the start state. Within that constraint, and being compatible with the destination state's label, for the category of the input URN, the destination state's signal must express the longest URN that can be expressed by any signal.
In our example, this means that the destination 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 destination 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 destination state is "Source:(Other)", where the parentheses record that the "Other" part of the label is not expressed by the state's signal.
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
Thus, the initial state and the states it can transition to are:
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)
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:
Signal URN(s)
---------------------------- -------------------------------
default (none)
internal source urn:alert:source:internal
external source urn:alert:source:external.
urn:alert:source:external
urn:alert:source:internal
source
Source
Source:External
Source:Internal
Source:Other
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)
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:
In the trivial case where the user agent receives no Alert-Info URNs, then processing begins and ends with the FSM in the initial state and selects the default signal.
Alert-Info: <urn:alert:source:internal>
State: Source
Process: Source:Internal (urn:alert:source:internal)
State: Source:Internal
Signal: internal source
If the user agent receives
Alert-Info: <urn:alert:source:external>,
<urn:alert:source:internal>
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 user agent receives
Alert-Info: <urn:alert:source:unclassified>,
<urn:alert:source:internal>
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 user agent receives
Alert-Info: <urn:alert:priority:high>,
<urn:alert:source:internal>
State: Source
Ignore: urn:alert:priority:high
State: Source
Process: Source:Internal (urn:alert:source:internal)
State: Source:Internal
Signal: internal source
If the user agent receives
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
Now consider an example where the user agent 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 is a total of 9 signals:
urn:alert:source:external
urn:alert:source:internal
urn:alert:priority:low
urn:alert:priority:high
The expressed URNs are:
source
priority
The relevant categories of "alert" URNs are only:
Source
Source:External
Source:Internal
Source:Other
Priority
Priority:Low
Priority:High
Priority:Other
The alphabet of symbols is:
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)
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
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
The 16 states are as follows, where 10 states have a simple structure because from them, no further information can be recorded.
Alert-Info: <urn:alert:source:internal>,
<urn:alert:source:unclassified>,
<urn:alert:priority:high>
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
An example of processing that involves multiple "source" URNs and one "priority" URN:
Signal URN(s)
---------------------------- -------------------------------
default (none)
internal source urn:alert:source:external
external source urn:alert:source:internal
high low urn:alert:priority:low
high priority urn:alert:priority:high
A more complicated example is in section 12.2.1 of [RFC7462]. It is like the example in Section 5 of this document, except that the user agent can only signal "external source", "internal source", "low priority", and "high priority" individually but not in combination, as well as a default signal:
urn:alert:source:external
urn:alert:source:internal
urn:alert:priority:low
urn:alert:priority:high
The signals can express the following URNs:
source
priority
The relevant categories of "alert" URNs are:
Source
Source:External
Source:Internal
Source:Other
Priority
Priority:Low
Priority:High
Priority:Other
The alphabet of symbols is:
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)
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
Transitions:
any -> Priority:High/Source:(External)
State: Priority:High/Source:(Internal)
Signal: high priority
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)
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
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
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:
Alert-Info: <urn:alert:source:internal>
State: Priority/Source
Process: Source:Internal (urn:alert:source:internal)
State: Priority/Source:Internal
Signal: internal source
As an example of processing, if the user agent receives
Alert-Info: <urn:alert:source:unclassified>,
<urn:alert:source:internal>,
<urn:alert:priority:high>
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
A more complicated example involves multiple "source" URNs which do not select a non-default signal and one "priority" URN which can be signaled:
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
Since the only characteristic of a state that affects the output of the FSM is the state's signal, several groups of states in this FSM can be merged using standard FSM optimization algorithms:
Examples 2, 3, and 4 of [RFC7462] are similar to the example in Section 5, 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. Most of the states of this FSM are the same as the states of the FSM of Section 5, but the state Source:Internal/Priority:Low is missing because there is no signal for that combination. It is replaced by two states: 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 can not be signaled -- but it still prevents any further "priority" URN from having an effect). The other state is Source:(Internal)/Priority:Low; it records the reverse sequence of events.
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)
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
The changes in the FSM are:
Alert-Info: <urn:alert:source:internal>,
<urn:alert:source:unclassified>,
<urn:alert:priority:high>
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
An example of processing that involves multiple "source" URNs and one "priority" URN:
Alert-Info: <urn:alert:source:internal>
State: Priority/Source
Process: Source:Internal (urn:alert:source:internal)
State: Priority/Source:Internal
Signal: internal source
If the user agent receives
Alert-Info: <urn:alert:source:external>,
<urn:alert:priority:low>
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
If the user agent receives
Alert-Info: <urn:alert:source:internal>,
<urn:alert:priority:low>
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
Suppose the same user agent receives
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
If the order of the URNs is reversed, what is signaled is the meaning of now-different first URN:
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
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:
urn:alert:source:internal
urn:alert:source:internal:vip@example
urn:alert:source:external
Source
Source:Internal
Source:Internal:Vip@example
Source:Internal:Other
Source:Other
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". 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 is a total of 3 expressed URNs, one of which is a subset of another:
urn:alert:service:forward
urn:alert:service:recall:callback
Service
Service:Forward
Service:Recall
Service:Recall:Callback
Service:Recall:Other
Service:Other
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 2 expressed URNs:
In this example, we consider how a UA generates ringback signals when the UA wishes to reproduce the traditional behavior that 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 containing the 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.
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 signals are:
urn:alert:country:xa
urn:alert:country:xb
urn:alert:service:call-waiting
urn:alert:service:forward
The expressed URNs are:
country
service
The relevant categories of "alert" URNs are only:
Country
Country:[other]
Country:Xa
Country:Xb
Service
Service:[other]
Service:Call-waiting
Service:Forward
The alphabet of symbols is:
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
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
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)
The 15 states are as follows:
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
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. 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.
The specifications 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 user agent may prefer to prioritize expressing one URN category over another regardless of the order 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 6, 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.
urn:alert:priority:high
urn:alert:priority:high, urn:alert:source:internal
urn:alert:priority:high, urn:alert:source:external
We accommodate this within the framework of [RFC7462] by assigning the signal "high priority" for each of these combinations of URNs:
urn:alert:source:external
urn:alert:source:internal
urn:alert:priority:low
urn:alert:priority:high
source
priority
Source
Source:External
Source:Internal
Source:Other
Priority
Priority:Low
Priority:High
Priority:Other
The intermediate steps of the method produce the same results as before. The signals can express the following URNs:
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
When the FSM is constructed, it is the same as the FSM for Section 6, except that certain states are effectively renamed and merged, because any "source" is defined to be expressed if high priority is expressed:
This section discusses how to construct FSMs for a user agent that allows variable sets of signals, for example, if the user can configure the use of ringtones. Several approaches can be used:
The remainder of this section gives an example of the third approach.
urn:alert:caller@example:alice@example.com
urn:alert:caller@example:bob@example.com
etc.
urn:alert:caller@example:IDENTITY
For the example, we will use a set of ringtones that express the identify of the caller. To signal this information, a private extension "alert" URN category is used, "caller@example":
Caller@example
Caller@example:IDENTITY
Caller@example:Other
The alphabet is then:
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)
The states and transitions of the FSM are:
[Note to RFC Editor: Please remove this entire section upon publication as an RFC.]
Clarify that this algorithm is non-normative, as is the one in section 12 of RFC 7462, and implementations are free to choose any algorithm.
Correct the discussion in the example of Section 10.
Revamp the introduction.
Use the term "resolve" for processing "alert" URNs to select a signal.
Editorial improvements from independent submission reviewer.
Editorial improvements from independent submission reviewer.
Add note at end of introduction that you can do this by hand in simple cases.
Add the country-code example.
Editorial improvements.
Editorial improvements.
Correct indenting of some lines.
Recast exposition to feature the implementation of the construction algorithm.
Reorganized the text, including describing how the FSM states are constructed.
| [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. |
| [RFC7462] | Liess, L., 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. |
| [code] | Worley, D., "draft-worley-alert-info-fsm.aux", February 2017. |
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 Independent Submissions Editor, Nevil Brownlee, for his work obtaining reviewers.