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A Simpler Method for Processing Alert-Info URNs
draft-worley-alert-info-fsm-03

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This is an older version of an Internet-Draft that was ultimately published as RFC 8433.
Author Dale R. Worley
Last updated 2016-09-08
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draft-worley-alert-info-fsm-03
SALUD                                                          D. Worley
Internet-Draft                                                   Ariadne
Intended status: Informational                         September 8, 2016
Expires: March 12, 2017

            A Simpler Method for Processing Alert-Info URNs
                     draft-worley-alert-info-fsm-03

Abstract

   The "alert" namespace of uniform resource names (URNs) can be used in
   the Alert-Info header field of Session Initiation Protocol (SIP)
   requests and responses to inform a 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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 12, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Selecting the Signals and Their Corresponding "alert" URNs  .   5
   3.  General Considerations for Processing Alert-Info  . . . . . .   8
   4.  Constructing the Finite State Machine for a Very Simple
       Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Listing the Expressed URNs  . . . . . . . . . . . . . . .   9
     4.2.  Constructing the Alphabet . . . . . . . . . . . . . . . .  10
     4.3.  Constructing the States and Transitions . . . . . . . . .  12
     4.4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  15
     4.5.  Examples of Processing Alert-Info URNs  . . . . . . . . .  17
   5.  Example with "source" and "priority" URNs . . . . . . . . . .  18
   6.  Example 1 of RFC 7462 . . . . . . . . . . . . . . . . . . . .  22
   7.  Examples 2, 3, and 4 of RFC 7462  . . . . . . . . . . . . . .  28
   8.  An Example that Subsets Internal Sources  . . . . . . . . . .  31
   9.  An Example of "service" URNs  . . . . . . . . . . . . . . . .  31
   10. Prioritizing Signals  . . . . . . . . . . . . . . . . . . . .  32
   11. Dynamic Sets of Signals . . . . . . . . . . . . . . . . . . .  33
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  35
   13. Revision History  . . . . . . . . . . . . . . . . . . . . . .  35
     13.1.  Changes from draft-worley-alert-info-fsm-02 to draft-
            worley-alert-info-fsm-03 . . . . . . . . . . . . . . . .  35
     13.2.  Changes from draft-worley-alert-info-fsm-01 to draft-
            worley-alert-info-fsm-02 . . . . . . . . . . . . . . . .  35
     13.3.  Changes from draft-worley-alert-info-fsm-00 to draft-
            worley-alert-info-fsm-01 . . . . . . . . . . . . . . . .  35
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  36
     14.2.  Informative References . . . . . . . . . . . . . . . . .  36
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   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.[RFC3261] Similarly, a
   SIP user agent client determines an alerting signal (the ringback

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   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] and can be described as follows:

   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.

   As each URN is processed, one of the UA's signals is chosen which
   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.

   If the entirety of the current URN cannot be expressed by any allowed
   signal, then in turn, each of the trailing alert-ind-parts (the
   sections separated by colons) is removed until the reduced URN can be
   expressed by some signal, and that signal 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

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   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, 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-a category.

   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-a category, 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 that URN is the signal that the UA uses.

   Section 12 of [RFC7462] gives one possible 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.  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
   selecting signals that conforms to section 11.1.  Once the user agent
   designer has chosen a set of signals, 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.

   o  The designer selects the set of signals that the user agent
      produces, matching each signal to the "alert" URN or the

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      combination of "alert" URNs which are the meaning carried by the
      signal.

   o  Based on the user agent'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 a finite state machine (FSM) whose input
      is the alphabet of symbols and whose states describe information
      extracted from the Alert-Info URNs.

   o  Each state of the FSM has an attached signal.  Processing the
      Alert-Info URNs will leave the FSM in some particular state; the
      UA presents the signal that is attached to that final state.

   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.

   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 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.  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 is algorithmic, and the algorithm assures that
   the operation of the FSM will satisfy the constraints of section 11.1
   of [RFC7462].  An implementation of the algorithmic steps is provided
   in [code]

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 indicate that meaning in SIP
   messages, and consequently should elicit that signal from the UA.

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   For example, suppose the UA has a particular ring tone for calls from
   an external source.  A call from an exteral 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 which 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 a 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:

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       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-high@example.  The final "extra-
   high@example" is an "alert-ind-part" that is a private extension.
   (See section 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-high@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:

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      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 should be
   described by exactly one of them.

3.  General Considerations for Processing Alert-Info

   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 seperate 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 11 for a discussion of this.)

   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

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   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.

   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,
   also 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 which are expressed by the various signals of
       the user agent.

   2.  From the expressed URNs, constructing the finite alphabet of
       symbols into which input URNs are mapped and which 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 user agent's signals
   what call characteristics it represents, which is to say, the set of
   "alert" URNs which are its information content.

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       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

   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 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
   [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.

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   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:

       Source:Other
       Source:External:Other
       Source:Internal:Other

   The latter two classify URNs like
   urn:alert:source:external:foo@example, which extend URNs that we
   already have symbols for.  The first is for classifying URNs, such as
   urn:alert:source:bar@example, which have first alert-ind-parts that
   contradict all the "source" URNs that the user agent can signal.

   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.

   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 symbols
   that correspond to expressed URNs.

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                               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 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 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

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   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

   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.

   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 state, "Source", so the destination state's
   label is the full input symbol.

   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

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   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.

   Thus, the initial state and the states 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

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       State: Source:(Other)
       Signal: default
       Transitions:
           Source:External -> Source:(Other)
           Source:Internal -> Source:(Other)
           Source:Other -> Source:(Other)

4.4.  Summary

   To summarize the steps of the method:

   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 FSM can be constructed by processing the file "very-
       simple.txt" with the program "alert-info-fsm.py" in [code].  The
       stages of the construction are:

   3.  The expressed URNs are

       urn:alert:source:external
       urn:alert:source:internal

   4.  The relevant categories of "alert" URNs are only:

       source

   5.  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

   6.  The FSM is:

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       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 which describe the
          information which has been extracted from the URNs so far.

       *  Each state has a signal which 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 user agent
          will present 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 start state's label, the destination
          state's label is the input symbol.  If not, the destination
          state is the same state.  This is how the state's label
          records what information has been accumulated while processing
          the Alert-Info URNs.

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       *  A transition's destination state has a signal which
          semantically subsets the start 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 user agent receives no Alert-Info URNs,
   then processing begins and ends with the FSM in the initial state and
   selects the default signal.

   If the user agent 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 user agent 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 user agent receives

       Alert-Info: <urn:alert:source:unclassified>,
           <urn:alert:source:internal>

   then processing progresses:

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       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>

   then processing progresses:

       State: Source
           Ignore: urn:alert:priority:high
       State: Source
           Process: Source:Internal (urn:alert:source:internal)
       State: Source:Internal
       Signal: internal source

5.  Example with "source" and "priority" URNs

   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:

       Signal                          URN(s)
       ----------------------------    -------------------------------
       default                         (none)
       external source                 urn:alert:source:external
       internal source                 urn:alert:source:internal
       low priority                    urn:alert:priority:low
       low priority/external source    urn:alert:priority:low,
                                           urn:alert:source:external
       low priority/internal source    urn:alert:priority:low,
                                           urn:alert:source:internal
       high priority                   urn:alert:priority:high
       high priority/external source   urn:alert:priority:high,
                                           urn:alert:source:external
       high priority/internal source   urn:alert:priority:high,
                                           urn:alert:source:internal

   The expressed URNs are:

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       urn:alert:source:external
       urn:alert:source:internal
       urn:alert:priority:low
       urn:alert:priority:high

   The relevant categories of "alert" URNs are only:

       source
       priority

   The alphabet of symbols is:

       Source
       Source:External
       Source:Internal
       Source:Other
       Priority
       Priority:Low
       Priority:High
       Priority:Other

   The 16 states are as follows, where 10 states have a simple structure
   because from them, no further information can be recorded.

   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)

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   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

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   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

   An example of processing that involves multiple "source" URNs and one
   "priority" URN:

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       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

6.  Example 1 of RFC 7462

   A more complicated example is in section 12.2.1 of [RFC7462].  It is
   like the example in Section 5, 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:

       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

   The signals can express the following URNs:

       urn:alert:source:external
       urn:alert:source:internal
       urn:alert:priority:low
       urn:alert:priority:high

   The relevant categories of "alert" URNs are:

       source
       priority

   The alphabet of symbols is:

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       Source
       Source:External
       Source:Internal
       Source:Other
       Priority
       Priority:Low
       Priority:High
       Priority:Other

   In this example, the FSM has 20 states because both "source" and
   "priority" URNs are recorded, but the order in which the two appear
   affects the signal:

       State: Priority/Source
       Signal: default
       Transitions:
           Priority:Other -> Priority:(Other)/Source
           Priority:High -> Priority:High/Source
           Priority:Low -> Priority:Low/Source
           Source:Other -> Priority/Source:(Other)
           Source:External -> Priority/Source:External
           Source:Internal -> Priority/Source:Internal

   State Priority:(Other)/Source can transition to states that can
   signal source, because the recorded priority can't be signaled and
   thus does not block the signalling of the source:

       State: Priority:(Other)/Source
       Signal: default
       Transitions:
           Priority:Other -> Priority:(Other)/Source
           Priority:High -> Priority:(Other)/Source
           Priority:Low -> Priority:(Other)/Source
           Source:Other -> Priority:(Other)/Source:(Other)
           Source:External -> Priority:(Other)/Source:External
           Source:Internal -> Priority:(Other)/Source:Internal

       State: Priority:(Other)/Source:(Other)
       Signal: default
       Transitions:
           any -> Priority:(Other)/Source:(Other)

       State: Priority:(Other)/Source:External
       Signal: external source
       Transitions:
           any -> Priority:(Other)/Source:External

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       State: Priority:(Other)/Source:Internal
       Signal: internal source
       Transitions:
           any -> Priority:(Other)/Source:Internal

   Because there are no signals for combinations of "source" and
   "priority" URNs, processing a "source" URN from the state
   Priority:High/Source leads to a state that records the priority
   information, but does not signal it:

       State: Priority:High/Source
       Signal: high priority
       Transitions:
           Priority:Other -> Priority:High/Source
           Priority:High -> Priority:High/Source
           Priority:Low -> Priority:High/Source
           Source:Other -> Priority:High/Source:(Other)
           Source:External -> Priority:High/Source:(External)
           Source:Internal -> Priority:High/Source:(Internal)

       State: Priority:High/Source:(Other)
       Signal: high priority
       Transitions:
           any -> Priority:High/Source:(Other)

   From the state Priority:High/Source, "source" URNs transition to
   states that record both source and priority but signal only priority,
   one of which is Priority:High/Source:(External).  But from Priority/
   Source:External, the symbol Priority:High transitions to the state
   Priority:(High)/Source:External, which records the same information
   but signals the source, not the priority. -- One state is reached by
   processing a "priority" URN and then a "source" URN, whereas the
   other is reached by processing a "source" URN and then a "priority"
   URN.

       State: Priority:High/Source:(External)
       Signal: high priority
       Transitions:
           any -> Priority:High/Source:(External)

       State: Priority:High/Source:(Internal)
       Signal: high priority
       Transitions:
           any -> Priority:High/Source:(Internal)

   And similarly for Priority:Low/Source:

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       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

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       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

         <t>As an example of processing, if the user agent receives
         <figure><artwork><![CDATA[
       Alert-Info: <urn:alert:source:internal>

   then processing progresses:

       State: Priority/Source
           Process: Source:Internal (urn:alert:source:internal)
       State: Priority/Source:Internal
       Signal: internal source

   A more complicated example involves multiple "source" URNs which do
   not select a non-default signal and one "priority" URN which can be
   signaled:

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       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: Source:(Other)/Internal
           Process: Priority:High (urn:alert:priority:high)
       State: Priority:High/Source:(Other)
       Signal: high priority

         <t>As an example of processing, if the user agent receives
         <figure><artwork><![CDATA[
       Alert-Info: <urn:alert:source:internal>

   then processing progresses:

       State: Priority/Source
           Process: Source:Internal (urn:alert:source:internal)
       State: Priority/Source:Internal
       Signal: internal source

   A more complicated example involves multiple "source" URNs which do
   not select a non-default signal and one "priority" URN which can be
   signaled:

       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

   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:

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       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

   This reduces the FSM to 7 states.

7.  Examples 2, 3, and 4 of RFC 7462

   Example 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.

   The changes in the FSM are:

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       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

   An example of processing that involves multiple "source" URNs and one
   "priority" URN:

       Alert-Info: <urn:alert:source:internal>,
           <urn:alert:source:unclassified>,
           <urn:alert:priority:high>

       State: Priority/Source
           Process: Source:Internal (urn:alert:source:internal)
       State: Priority/Source:Internal
           Process: Source:Other (urn:alert:source:unclassified)
       State: Priority/Source:Internal
           Process: Priority:High (urn:alert:priority:high)
       State: Priority:High/Source:Internal
       Signal: internal source/high priority

   If the user agent receives

       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

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       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

   Suppose the same user agent receives

       Alert-Info: <urn:alert:source:internal>,
           <urn:alert:priority:low>

   Note that there is no signal that corresponds to this combination.
   In that case, the processing is:

       State: Priority/Source
           Process: Source:Internal (urn:alert:source:internal)
       State: Priority/Source:Internal
           Process: Priority:Low (urn:alert:priority:low)
       State: Priority:(Low)/Source:Internal
       Signal: internal source

   If the order of the URNs is reversed, what is signaled is still the
   the meaning of now different first URN:

       Alert-Info: <urn:alert:priority:low>,
           <urn:alert:source:internal>

       State: Priority/Source
           Process: Priority:Low (urn:alert:priority:low)
       State: Priority:Low/Source
           Process: Source:Internal (urn:alert:source:internal)
       State: Priority:Low/Source:(Internal)
       Signal: low priority

   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:

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       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

   whereas if the second transition had been to the state Priority:Low/
   Source (on the basis that there is no proper state Priority:Low/
   Source:Internal), then the third transition would have been to the
   state Priority:Low/Source:External, and the signal would have been
   "external source/low priority".

8.  An Example that Subsets Internal Sources

   In the 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:source:internal
       urn:alert:source:internal:vip@example
       urn:alert:source:external

   This generates the following alphabet of symbols, which includes two
   Other symbols for the category source:

       Source
       Source:Internal
       Source:Internal:Vip@example
       Source:Internal:Other
       Source:Other

9.  An Example of "service" URNs

   In this example there are signals for "service forward" (the call has
   been forwarded) and "source recall callback" (a recall due to a
   callback).  This gives 2 expressed URNs:

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       urn:alert:service:forward
       urn:alert:service:recall:callback

   This generates the following alphabet of symbols:

       Service
       Service:Forward
       Service:Recall
       Service:Recall:Callback
       Service:Recall:Other
       Service:Other

10.  Prioritizing Signals

   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 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.

   We accommodate this within the framework of [RFC7462] by assigning
   the signal "high priority" for each of these combinations of URNs:

       urn:alert:priority:high
       urn:alert:priority:high, urn:alert:source:internal
       urn:alert:priority:high, urn:alert:source:external

   The result is that the "high priority" signal is the "best" signal
   for any combination of urn:alert:priority:high with "source" URNs.

   The intermediate steps of the method produce the same results as
   before.  The signals can express the following URNs:

       urn:alert:source:external
       urn:alert:source:internal
       urn:alert:priority:low
       urn:alert:priority:high

   The relevant categories of "alert" URNs are:

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       source
       priority

   The alphabet of symbols is:

       Source
       Source:External
       Source:Internal
       Source:Other
       Priority
       Priority:Low
       Priority:High
       Priority:Other

   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:

       Priority:(High)/Source:External and
       Priority:High/Source:(External) become:
           State: Priority:High/Source:External
           Signal: high priority

       Priority:(High)/Source:Internal and
       Priority:High/Source:(Internal) become:
           State: Priority:High/Source:Internal
           Signal: high priority

   This reduces the FSM to 18 states.  In addition, these two new
   states, along with a number of other states can be merged by FSM
   optimization, since all of them have the signal "high priority" and
   from them, there are no transitions to states outside this set.  The
   final FSM has 10 states.

11.  Dynamic Sets of Signals

   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:

   o  Whenever the set of ringtones is changed, re-execute the processes
      listed in Section 4.

   o  Whenever the set of ringtones is changed, rebuild the list of
      expressed URNs (Section 4.1) and reconstruct the alphabet of
      symbols (Section 4.2).  Then use an algorithm for dynamically

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      constructing states of the FSM as they are needed during Alert-
      Info processing.

   o  If the set of possible URNs expressed by the ringtones is
      sufficiently limited, the steps of Section 4 can be carried out
      "generically", and the generic FSM can be specialized for the
      current ringtone configuration.

   The remainder of this section gives an example of the third approach.

   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":

       urn:alert:caller@example:alice@example.com
       urn:alert:caller@example:bob@example.com
       etc.

   which we can express by the generic pattern

       urn:alert:caller@example:IDENTITY

   where "IDENTITY" is replaced in succession by the set of caller
   identities that have their own ringtones to generate the set of
   expressed URNs.

   The alphabet is then:

         Caller@example
         Caller@example:IDENTITY
         Caller@example:Other

   where "IDENTITY" is replaced in succession by the set of caller
   identities.  The "Caller@example:Other" symbol includes all URNs of
   the category "caller@example" that are not included in any of the
   other symbols.

   The states and transitions of the FSM are:

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       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)

   where again, the second state is replicated once for each caller
   identity that has a ringtone, with "IDENTITY" replaced with the
   caller identity.

12.  Acknowledgments

   Thanks to Paul Kyzivat, whose relentless identification of the
   weaknesses of earlier versions made the final document much, much
   better than it would have been by changing it from the exposition of
   a concept into a practical tool.

13.  Revision History

   [Note to RFC Editor: Please remove this entire section upon
   publication as an RFC.]

13.1.  Changes from draft-worley-alert-info-fsm-02 to draft-worley-
       alert-info-fsm-03

13.2.  Changes from draft-worley-alert-info-fsm-01 to draft-worley-
       alert-info-fsm-02

   Recast exposition to feature the implementation of the construction
   algorithm.

13.3.  Changes from draft-worley-alert-info-fsm-00 to draft-worley-
       alert-info-fsm-01

   Reorganized the text, including describing how the FSM states are
   constructed.

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14.  References

14.1.  Normative References

   [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,
              <http://www.rfc-editor.org/info/rfc3261>.

   [RFC7462]  Liess, L., Ed., Jesske, R., Johnston, A., Worley, D., and
              P. Kyzivat, "URNs for the Alert-Info Header Field of the
              Session Initiation Protocol (SIP)", RFC 7462,
              DOI 10.17487/RFC7462, March 2015,
              <http://www.rfc-editor.org/info/rfc7462>.

14.2.  Informative References

   [code]     Worley, D., "draft-worley-alert-info-fsm.aux", June 2019,
              <http://svn.resiprocate.org/rep/ietf-drafts/worley/
              draft-worley-alert-info-fsm.aux>.

Author's Address

   Dale R. Worley
   Ariadne Internet Services
   738 Main St.
   Waltham, MA  02451
   US

   Email: worley@ariadne.com

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