A Simpler Method for Resolving Alert-Info URNs
RFC 8433

Document Type RFC - Informational (August 2018; No errata)
Last updated 2018-08-27
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Independent Submission                                         D. Worley
Request for Comments: 8433                                       Ariadne
Category: Informational                                      August 2018
ISSN: 2070-1721

             A Simpler Method for Resolving Alert-Info URNs

Abstract

   The "alert" namespace of Uniform Resource Names (URNs) can be used in
   the Alert-Info header field of Session Initiation Protocol (SIP)
   requests and responses to inform a voice over IP (VoIP) telephone
   (user agent) of the characteristics of the call that the user agent
   has originated or terminated.  The user agent must resolve the URNs
   into a signal; that is, it must select the best available signal to
   present to its user to indicate the characteristics of the call.

   RFC 7462 describes a non-normative algorithm for signal selection.
   This document describes a more efficient alternative algorithm: a
   user agent's designer can, based on the user agent's signals and
   their meanings, construct a finite state machine (FSM) to process the
   URNs to select a signal in a way that obeys the restrictions given in
   the definition of the "alert" URN namespace.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates for any level of Internet Standard;
   see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8433.

Worley                        Informational                     [Page 1]
RFC 8433                Resolving Alert-Info URNs            August 2018

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements Governing Resolution Algorithms ...............4
      1.2. Summary of the New Resolution Algorithm ....................5
      1.3. Conventions Used in This Document ..........................7
   2. Selecting the Signals and Their Corresponding "alert" URNs ......7
   3. General Considerations for Processing Alert-Info ................9
   4. Constructing the Finite State Machine for a Very Simple
      Example ........................................................10
      4.1. Listing the Expressed URNs ................................11
      4.2. Constructing the Alphabet of Symbols ......................11
      4.3. Constructing the States and Transitions ...................13
      4.4. Summary ...................................................17
      4.5. Examples of Processing Alert-Info URNs ....................19
   5. Further Examples ...............................................20
      5.1. Example with "source" and "priority" URNs .................20
      5.2. Example 1 of RFC 7462 .....................................24
      5.3. Examples 2, 3, and 4 of RFC 7462 ..........................30
      5.4. An Example That Subsets Internal Sources ..................33
      5.5. An Example of "alert:service" URNs ........................34
      5.6. An Example Using Country Codes ............................34
   6. Prioritizing Signals ...........................................40
   7. Dynamic Sets of Signals ........................................41
   8. Security Considerations ........................................43
   9. IANA Considerations ............................................43
   10. References ....................................................44
      10.1. Normative References .....................................44
      10.2. Informative References ...................................44
   Acknowledgments ...................................................45
   Author's Address ..................................................45

Worley                        Informational                     [Page 2]
RFC 8433                Resolving Alert-Info URNs            August 2018

1.  Introduction

   When a SIP user agent (UA) server receives an incoming INVITE
   request, it chooses an alerting signal (the ring tone) to present to
   its user (the called user) by processing the Alert-Info header
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