| SALUD | D. Worley |
| Internet-Draft | Ariadne |
| Intended status: Standards Track | May 12, 2016 |
| Expires: November 13, 2016 |
Simpler Algorithms for Processing Alert-Info URNs
draft-worley-alert-info-fsm-01
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. Based on the URNs in the Alert-Info header field, the user agent must select an the best available signal to present to its user to indicate the characteristics of the call. This document describes a method by which a user agent's designer can, based on the user agent's signals and their meanings, constructing 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. In many situations, the resulting FSM is simpler and faster than the previously described selection algorithm.
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A SIP user agent server determines 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. 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 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 a signal. This usage has security problems and is inconvenient to implement in practice. [RFC7462] introduced an alternative practice: The values could 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.
However, a user agent is unlikely to provide alerting signals that can render more than a small subset of the possible combinations of "alert" URNs, so the user agent is frequently required to select one alerting signal which renders only a subset of the information in the Alert-Info header field(s). The requirements for the process of selecting an alerting signal based on "alert" URNs are given in section 11.1 of [RFC7462].
Section 12 of [RFC7462] gives one possible algorithm for selecting a signal which satisfies section 11.1. This algorithm can be used regardless of the set of alerting signals that the user agent provides and their specified meanings. This demonstrates that the rules can always be satisfied. However, the algorithm is complex and slow.
The purpose of this document is to describe an easier method for user agent designers to construct an algorithm for selecting alerting signals based on the Alert-Info header fields in a SIP message. Based on the user agent's signals and their meanings, the designer constructs:
Note that the user agent has two FSMs. One FSM is used to select the ring tone to present for an incoming INVITE request. The other FSM is used to select the ringback tone to present based on an incoming provisional response to an outgoing INVITE request.
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 present to the user.
If the FSM is correctly constructed by the designer, the constraints of section 11.1 will be satisfied.
Note that the values in an Alert-Info header field are allowed to be URIs of any schema, and within the "urn" schema, 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 all Alert-Info URIs that are not "alert" URNs have been removed.
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.
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. 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:
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 achieve is to skip over them during Alert-Info processing. A more formal method is to map all of these URNs into one "Other" symbol, and then for each state of the FSM, have the Other symbol transition to that 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 existance 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:
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 URN.
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 describes 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 present 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)". 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.
The initial state's label is the 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 (no URNs)
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 (no URNs)
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 (no URNs)
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 (no URNs)
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 (no URNs)
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:
default
internal source
external source
urn:alert:source:external
urn:alert:source:internal
source
Source
Source:External
Source:Internal
Source:Other
State: Source (initial state)
Signal: default (no URNs)
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 (no URNs)
Transitions:
Source:External -> Source:(Other)
Source:Internal -> Source:(Other)
Source:Other -> Source:(Other)
To summarize the steps of the method:
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
Process: skip urn:alert:priority:high
State: Source
Process: Source:Internal (urn:alert:source:internal)
State: Source:Internal
Signal: internal source
If the user agent receives
default
external source
internal source
low priority
low priority/external source
low priority/internal source
high priority
high priority/external source
high priority/internal source
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 3, 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: Source/Priority
Signal: default
Transitions:
Source:External -> Source:External/Priority
Source:Internal -> Source:Internal/Priority
Source:Other -> Source:(Other)/Priority
Priority:High -> Source/Priority:High
Priority:Low -> Source/Priority:Low
Priority:Other -> Source/Priority:(Other)
State: Source:External/Priority
Signal: external source
Transitions:
Priority:High -> Source:External/Priority:High
Priority:Low -> Source:External/Priority:Low
Priority:Other -> Source:External/Priority:(Other)
other -> Source:External/Priority
State: Source:Internal/Priority
Signal: internal source
Transitions:
Priority:High -> Source:Internal/Priority:High
Priority:Low -> Source:Internal/Priority:Low
Priority:Other -> Source:Internal/Priority:(Other)
other -> Source:Internal/Priority
State: Source:(Other)/Priority
Signal: default
Transitions:
Priority:High -> Source:(Other)/Priority:High
Priority:Low -> Source:(Other)/Priority:Low
Priority:Other -> Source:(Other)/Priority:(Other)
other -> Source:(Other)/Priority
State: Source/Priority:High
Signal: high priority
Transitions:
Source:External -> Source:External/Priority:High
Source:Internal -> Source:Internal/Priority:High
Source:Other -> Source:(Other)/Priority:High
other -> Source/Priority:High
State: Source:External/Priority:High
Signal: external source/high priority
Transitions:
any -> Source:External/Priority:High
State: Source:Internal/Priority:High
Signal: internal source/high priority
Transitions:
any -> Source:Internal/Priority:High
State: Source:(Other)/Priority:High
Signal: high priority
Transitions:
any -> Source:(Other)/Priority:High
State: Source/Priority:Low
Signal: low priority
Transitions:
Source:External -> Source:External/Priority:Low
Source:Internal -> Source:Internal/Priority:Low
Source:Other -> Source:(Other)/Priority:Low
other -> Source/Priority:Low
State: Source:External/Priority:Low
Signal: external source/low priority
Transitions:
any -> Source:External/Priority:Low
State: Source:Internal/Priority:Low
Signal: internal source/low priority
Transitions:
any -> Source:Internal/Priority:Low
State: Source:(Other)/Priority:Low
Signal: low priority
Transitions:
any -> Source:(Other)/Priority:Low
State: Source/Priority:(Other)
Signal: default
Transitions:
Source:External -> Source:External/Priority:(Other)
Source:Internal -> Source:Internal/Priority:(Other)
Source:Other -> Source:(Other)/Priority:(Other)
other -> Source/Priority:(Other)
State: Source:External/Priority:(Other)
Signal: external source
Transitions:
any -> Source:External/Priority:(Other)
State: Source:Internal/Priority:(Other)
Signal: internal source
Transitions:
any -> Source:Internal/Priority:(Other)
State: Source:(Other)/Priority:(Other)
Signal: default
Transitions:
any -> Source:(Other)/Priority:(Other)
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:
default
internal source
external source
high low
high priority
A more complicated example is in section 12.2.1 of [RFC7462]. It is like the example in Section 4, 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: Source/Priority
Signal: default
Transitions:
Source:External -> Source:External/Priority
Source:Internal -> Source:Internal/Priority
Source:Other -> Source:(Other)/Priority
Priority:Low -> Source/Priority:Low
Priority:High -> Source/Priority:High
Priority:Other -> Source/Priority:Other
State: Source:External/Priority
Signal: external source
Transitions:
Priority:High -> Source:External/Priority:(High)
Priority:Low -> Source:External/Priority:(Low)
Priority:Other -> Source:External/Priority:(Other)
other -> Source:External/Priority
State: Source:External/Priority:(High)
Signal: external source
Transitions:
any -> Source:External/Priority:(High)
State: Source:External/Priority:(Low)
Signal: external source
Transitions:
any -> Source:External/Priority:(Low)
State: Source:External/Priority:(Other)
Signal: external source
Transitions:
any -> Source:External/Priority:(Other)
State: Source:Internal/Priority
Signal: external source
Transitions:
Priority:High -> Source:Internal/Priority:(High)
Priority:Low -> Source:Internal/Priority:(Low)
Priority:Other -> Source:Internal/Priority:(Other)
other -> Source:Internal/Priority
State: Source:Internal/Priority:(High)
Signal: internal source
Transitions:
any -> Source:Internal/Priority:(High)
State: Source:Internal/Priority:(Low)
Signal: internal source
Transitions:
any -> Source:Internal/Priority:(Low)
State: Source:Internal/Priority:(Other)
Signal: internal source
Transitions:
any -> Source:Internal/Priority:(Other)
State: Source:(Other)/Priority
Signal: default
Transitions:
Priority:High -> Source:(Other)/Priority:High
Priority:Low -> Source:(Other)/Priority:Low
Priority:Other -> Source:(Other)/Priority:(Other)
other -> Source:(Other)/Priority
State: Source:(Other)/Priority:High
Signal: high priority
Transitions:
any -> Source:(Other)/Priority:High
State: Source:(Other)/Priority:Low
Signal: low priority
Transitions:
any -> Source:(Other)/Priority:Low
State: Source:(Other)/Priority:(Other)
Signal: default
Transitions:
any -> Source:(Other)/Priority:(Other)
State: Source/Priority:High
Signal: high priority
Transitions:
Source:External -> Source:(External)/Priority:High
Source:Internal -> Source:(Internal)/Priority:High
Source:Other -> Source:(Other)/Priority:High
other -> Source/Priority:High
State: Source:(External)/Priority:High
Signal: high priority
Transitions:
any -> Source:(External)/Priority:High
State: Source:(Internal)/Priority:High
Signal: high priority
Transitions:
any -> Source:(Internal)/Priority:High
State: Source/Priority:Low
Signal: low priority
Transitions:
Source:External -> Source:(External)/Priority:Low
Source:Internal -> Source:(Internal)/Priority:Low
Source:Other -> Source:(Other)/Priority:Low
other -> Source/Priority:Low
State: Source:(External)/Priority:Low
Signal: low priority
Transitions:
any -> Source:(External)/Priority:Low
State: Source:(Internal)/Priority:Low
Signal: low priority
Transitions:
any -> Source:(Internal)/Priority:Low
State: Source/Priority:(Other)
Signal: default
Transitions:
Source:External -> Source:External/Priority:(Other)
Source:Internal -> Source:Internal/Priority:(Other)
Source:Other -> Source:(Other)/Priority:(Other)
other -> Source/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: Section 4:
Alert-Info: <urn:alert:source:internal>
State: Source/Priority
Process: Source:Internal (urn:alert:source:internal)
State: Source:Internal/Priority
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: Source/Priority
Process: Source:Other (urn:alert:source:unclassified)
State: Source:(Other)/Priority
Process: Source:Internal (urn:alert:source:internal)
State: Source:(Other)/Internal
Process: Priority:High (urn:alert:priority:high)
State: Source:(Other)/Priority:High
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 "external source":
Source:External/Priority:(High)
Source:External/Priority:(Low)
Source:External/Priority:(Other)
states with signal "internal source":
Source:Internal/Priority:(High)
Source:Internal/Priority:(Low)
Source:Internal/Priority:(Other)
states with signal "high priority":
Source:(Other)/Priority:High
Source:(External)/Priority:High
Source:(Internal)/Priority:High
states with signal "low priority":
Source:(Other)/Priority:Low
Source:(External)/Priority:Low
Source:(Internal)/Priority:Low
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:
Example 2 of [RFC7462] is similar to the example in Section 4, but it does not include a signal for the combination "internal source, low priority" to make resolution examples work inconsistently.
The FSM for this example has the same alphabet as the FSM of Section 4. Most of the states of this FSM are the same as the states of the FSM of Section 4, 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: Source:Internal/Priority
Signal: internal source
Transitions:
Priority:Low -> Source:Internal/Priority:(Low)
(other transitions unchanged)
State: Source/Priority:Low
Signal: low priority
Transitions:
Source:Internal -> Source:(Internal)/Priority:Low
(other transitions unchanged)
State: Source:Internal/Priority:(Low)
Signal: internal source
Transitions:
any -> Source:Internal/Priority:(Low)
State: Source:(Internal)/Priority:Low
Signal: low priority
Transitions:
any -> Source:(Internal)/Priority:Low
The changes in the FSM are:
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:
Alert-Info: <urn:alert:source:internal>
State: Source/Priority
Process: Source:Internal (urn:alert:source:internal)
State: Source:Internal/Priority
Signal: internal source
If the user agent receives
Alert-Info: <urn:alert:source:external>,
<urn:alert:priority:low>
State: Source/Priority
Process: Source:External (urn:alert:source:external)
State: Source:External/Priority
Process: Priority:Low (urn:alert:priority:low)
State: Source:External/Priority:Low
Signal: external source/low priority
If the user agent receives
Alert-Info: <urn:alert:source:internal>,
<urn:alert:priority:low>
State: Source/Priority
Process: Source:Internal (urn:alert:source:internal)
State: Source:Internal/Priority
Process: Priority:Low (urn:alert:priority:low)
State: Source:Internal/Priority:(Low)
Signal: internal source
Suppose the same user agent receives
Alert-Info: <urn:alert:priority:low>,
<urn:alert:source:internal>
State: Source/Priority
Process: Priority:Low (urn:alert:priority:low)
State: Source/Priority:Low
Process: Source:Internal (urn:alert:source:internal)
State: Source:(Internal)/Priority:Low
Signal: low priority
If the order of the URNs is reversed, what is signaled is still the the meaning of now different first URN:
Alert-Info: <urn:alert:priority:low>,
<urn:alert:source:internal>,
<urn:alert:source:external>
State: Source/Priority
Process: Priority:Low (urn:alert:priority:low)
State: Source/Priority:Low
Process: Source:Internal (urn:alert:source:internal)
State: Source:(Internal)/Priority:Low
Process: Source:External (urn:alert:source:external)
State: Source:(Internal)/Priority:Low
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:
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 the example of Section 3, 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:source:internal:vip@example, which is a subset of urn: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:
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 may prefer to prioritize expressing one URN category over another regardless of the order the URNs appear in Alert-Info. This section describes how that can be accommodated within the framework of [RFC7462], and presents the FSM that this method generates.
This example uses the signals of Section 5, 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:
Source:External/Priority:(High) and
Source:(External)/Priority:High become:
State: Source:External/Priority:High
Signal: high priority
Source:Internal/Priority:(High) and
Source:(Internal)/Priority:High become:
State: Source:Internal/Priority:High
Signal: high priority
When the FSM is constructed, it is the same as the FSM for Section 5, except that certain states are effectively renamed and merged, because any "source" is defined to be expressed if high priority is expressed:
State: Source/Priority
Signal: default
Transitions:
Source:External -> Source:External/Priority
Source:Internal -> Source:Internal/Priority
Source:Other -> Source:(Other)/Priority
Priority:Low -> Source/Priority:Low
| Priority:High -> Source:*/Priority:High
Priority:Other -> Source/Priority:Other
State: Source:External/Priority
Signal: external source
Transitions:
| Priority:High -> Source:*/Priority:High
Priority:Low -> Source:External/Priority:(Low)
Priority:Other -> Source:External/Priority:(Other)
other -> Source:External/Priority
| State: Source:*/Priority:High
| Signal: high priority
| Transitions:
| any -> Source:*/Priority:High
State: Source:External/Priority:(Low)
Signal: external source
Transitions:
any -> Source:External/Priority:(Low)
State: Source:External/Priority:(Other)
Signal: external source
Transitions:
any -> Source:External/Priority:(Other)
State: Source:Internal/Priority
Signal: external source
Transitions:
| Priority:High -> Source:*/Priority:High
Priority:Low -> Source:Internal/Priority:(Low)
Priority:Other -> Source:Internal/Priority:(Other)
other -> Source:Internal/Priority
State: Source:Internal/Priority:(Low)
Signal: internal source
Transitions:
any -> Source:Internal/Priority:(Low)
State: Source:Internal/Priority:(Other)
Signal: internal source
Transitions:
any -> Source:Internal/Priority:(Other)
State: Source:(Other)/Priority
Signal: default
Transitions:
| Priority:High -> Source:*/Priority:High
Priority:Low -> Source:(Other)/Priority:Low
Priority:Other -> Source:(Other)/Priority:(Other)
other -> Source:(Other)/Priority
State: Source:(Other)/Priority:Low
Signal: low priority
Transitions:
any -> Source:(Other)/Priority:Low
State: Source:(Other)/Priority:(Other)
Signal: default
Transitions:
any -> Source:(Other)/Priority:(Other)
State: Source/Priority:Low
Signal: low priority
Transitions:
Source:External -> Source:(External)/Priority:Low
Source:Internal -> Source:(Internal)/Priority:Low
Source:Other -> Source:(Other)/Priority:Low
other -> Source/Priority:Low
State: Source:(External)/Priority:Low
Signal: low priority
Transitions:
any -> Source:(External)/Priority:Low
State: Source:(Internal)/Priority:Low
Signal: low priority
Transitions:
any -> Source:(Internal)/Priority:Low
State: Source/Priority:(Other)
Signal: default
Transitions:
Source:External -> Source:External/Priority:(Other)
Source:Internal -> Source:Internal/Priority:(Other)
Source:Other -> Source:(Other)/Priority:(Other)
other -> Source/Priority:(Other)
The final FSM has 15 states. The changes from the FSM of Section 5 are marked with change bars.
This section discusses how to construct FSMs for user agent that allows the user to 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 (no URNs)
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 (no URNs)
Transitions:
any -> Caller@example:(Other)
The states and transitions of the FSM are:
| [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. |