Non-Interactive Emergency Calls
draft-ietf-ecrit-data-only-ea-20

ECRIT                                                           B. Rosen
Internet-Draft
Intended status: Standards Track                          H. Schulzrinne
Expires: August 2, 2020                                      Columbia U.
                                                           H. Tschofenig
                                                             ARM Limited
                                                              R. Gellens
                                              Core Technology Consulting
                                                        January 30, 2020

                    Non-Interactive Emergency Calls
                    draft-ietf-ecrit-data-only-ea-20

Abstract

   RFC 6443 'Framework for Emergency Calling Using Internet Multimedia'
   describes how devices use the Internet to place emergency calls and
   how Public Safety Answering Points (PSAPs) handle Internet multimedia
   emergency calls natively.  The exchange of multimedia traffic for
   emergency services involves a Session Initiation Protocol (SIP)
   session establishment starting with a SIP INVITE that negotiates
   various parameters for that session.  These calls involve a person,
   who uses the interactive media to communicate with the PSAP.

   In some cases, however, the transmission of application data is all
   that is needed, and no interactive media channel is established.
   Examples of such environments include alerts issued by a temperature
   sensor, burglar alarm, or chemical spill sensor.  Often these alerts
   are conveyed as one-shot data transmissions.  These type of
   interactions are called 'non-interactive emergency calls'.  This
   document describes use of a SIP MESSAGE transaction containing a
   container for the data based on the Common Alerting Protocol (CAP).
   MESSAGE does not establish a session, which differentiates this type
   of emergency request from a SIP INVITE, which would.  Any device that
   needs to initiate a request for emergency services where no
   interactive media channel will be established would use the
   mechanisms in this document.

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 https://datatracker.ietf.org/drafts/current/.

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   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 August 2, 2020.

Copyright Notice

   Copyright (c) 2020 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Architectural Overview  . . . . . . . . . . . . . . . . . . .   4
   4.  Protocol Specification  . . . . . . . . . . . . . . . . . . .   6
     4.1.  CAP Transport . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Profiling of the CAP Document Content . . . . . . . . . .   7
     4.3.  Sending a non-interactive Emergency Call  . . . . . . . .   8
   5.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  425 (Bad Alert Message) Response Code . . . . . . . . . .   9
     5.2.  The AlertMsg-Error Header Field . . . . . . . . . . . . .   9
   6.  Call Backs  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Handling Large Amounts of Data  . . . . . . . . . . . . . . .  11
   8.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Registration of the
            'application/EmergencyCallData.cap+xml' MIME type  . . .  17
     10.2.  IANA Registration of 'cap' Additional Data Block . . . .  18
     10.3.  IANA Registration for 425 Response Code  . . . . . . . .  18
     10.4.  IANA Registration of New AlertMsg-Error Header Field . .  19
     10.5.  IANA Registration for the SIP AlertMsg-Error Codes . . .  19
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  20
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  20

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     12.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   [RFC6443] describes how devices use the Internet to place emergency
   calls and how Public Safety Answering Points (PSAPs) handle Internet
   multimedia emergency calls natively.  The exchange of multimedia
   traffic for emergency services involves a SIP session establishment
   starting with a SIP INVITE that negotiates various parameters for
   that session.

   In some cases, however, there is only application data to be conveyed
   from the end devices to a PSAP or an intermediary.  Examples of such
   environments includes sensors issuing alerts, or certain types of
   medical monitors.  These messages may be one-shot alerts to emergency
   authorities and do not require establishment of a session.  These
   type of interactions are called 'non-interactive emergency calls'.
   In this document, we use the term "call" so that similarities between
   non-interactive alerts and sessions with interactive media are more
   obvious.

   Non-Interactive emergency calls are similar to regular emergency
   calls in the sense that they require the emergency indications,
   emergency call routing functionality and may even have the same
   location requirements.  However, the communication interaction will
   not lead to the exchange of interactive media, that is, Real-Time
   Protocol packets, such as voice, video data or real-time text.

   The Common Alerting Protocol (CAP) [cap] is a format for exchanging
   emergency alerts and public warnings.  CAP is mainly used for
   conveying alerts and warnings between authorities and from
   authorities to citizens/individuals.  This document is concerned with
   citizen to authority "alerts", where the alert is a call without any
   interactive media.

   This document describes a method of including a CAP message in a SIP
   transaction by defining it as a block of "additional data" as defined
   in [RFC7852].  The CAP message is included either by value (the CAP
   message is in the body of the message, using a CID) or by reference
   (a URI is included in the message, which when dereferenced returns
   the CAP message).  The additional data mechanism is also used to send
   alert specific data beyond that available in the CAP message.  This
   document also describes how a SIP MESSAGE [RFC3428] transaction can
   be used to send a non-interactive call.

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   SIP is the Session Initiation Protocol [RFC3261]

   PIDF-LO is Presence Information Data Format - Location Object, a data
   structure for carrying location [RFC4119]

   LoST is the Location To Service Translation protocol [RFC5222]

   CID is Content InDirection [RFC2392]

   CAP is the Common Alerting Protocol [cap]

   PSAP is a Public Safety Answering Point, the call center for
   emergency calls.

   ESRP is an Emergency Services Routing Proxy, a type of SIP Proxy
   Server used in some emergency services networks

3.  Architectural Overview

   This section illustrates two envisioned usage modes: targeted and
   location-based emergency alert routing.

   1.  Emergency alerts containing only data are targeted to an
       intermediary recipient responsible for evaluating the next steps.
       These steps could include:

       1.  Sending a non-interactive call containing only data towards a
           Public Safety Answering Point (PSAP);

       2.  Establishing a third-party initiated emergency call towards a
           PSAP that could include audio, video, and data.

   2.  Emergency alerts may be targeted to a Service URN used for IP-
       based emergency calls where the recipient is not known to the
       originator.  In this scenario, the alert may contain only data
       (e.g., a CAP, Geolocation header field and one or more Call-Info
       header fields containing Additional Data [RFC7852] in a SIP
       MESSAGE).

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   Figure 1 shows a deployment variant where a sensor is pre-configured
   (using techniques outside the scope of this document) to issue an
   alert to an aggregator that processes these messages and performs
   whatever steps are necessary to appropriately react to the alert.
   For example, a security firm may use different sensor inputs to
   dispatch their security staff to a building they protect or to
   initiate a third-party emergency call.

    +------------+              +------------+
    | Sensor     |              | Aggregator |
    |            |              |            |
    +---+--------+              +------+-----+
        |                              |
     Sensors                           |
     trigger                           |
     emergency                         |
     alert                             |
        |    SIP MESSAGE with CAP      |
        |----------------------------->|
        |                              |
        |                           Aggregator
        |                           processes
        |                           emergency
        |                           alert
        |      SIP 200 (OK)            |
        |<-----------------------------|
        |                              |
        |                              |

                Figure 1: Targeted Emergency Alert Routing

   In Figure 2 a scenario is shown whereby the alert is routed using
   location information and a Service URN.  An emergency services
   routing proxy (ESRP) may use LoST (a protocol defined by [RFC5222]
   which translates a location to a URI used to route an emergency call)
   to determine the next hop proxy to route the alert message to.  A
   possible receiver is a PSAP and the recipient of the alert may be a
   call taker.  In the generic case, there is very likely no prior
   relationship between the originator and the receiver, e.g., a PSAP.
   A PSAP, for example, is likely to receive and accept alerts from
   entities it has no previous relationship with.  This scenario
   corresponds to the classic emergency services use case and the
   description in [RFC6881] is applicable.  In this use case, the only
   difference between an emergency call and an emergency non-interactive
   call is that the former uses INVITE, creates a session, and
   negotiates one or more media streams, while the latter uses MESSAGE,
   does not create a session, and does not have interactive media.

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      +----------+         +----------+                  +-----------+
      |Sensor or |         |  ESRP    |                  |   PSAP    |
      |Aggregator|         |          |                  |           |
      +----+-----+         +---+------+                  +----+------+
           |                   |                              |
        Sensors                |                              |
        trigger                |                              |
        emergency              |                              |
        alert                  |                              |
           |                   |                              |
           |                   |                              |
           | SIP MESSAGE w/CAP |                              |
           | (including Service URN,                          |
           | such as urn:service:sos)                         |
           |------------------>|                              |
           |                   |                              |
           |              ESRP performs                       |
           |              emergency alert                     |
           |              routing                             |
           |                   |  MESSAGE with CAP            |
           |                   |  (including identity info)   |
           |                   |----------------------------->|
           |                   |                              |
           |                   |                           PSAP
           |                   |                           processes
           |                   |                           emergency
           |                   |                           alert
           |                   |      SIP 200 (OK)            |
           |                   |<-----------------------------|
           |                   |                              |
           |  SIP 200 (OK)     |                              |
           |<------------------|                              |
           |                   |                              |
           |                   |                              |

             Figure 2: Location-Based Emergency Alert Routing

4.  Protocol Specification

4.1.  CAP Transport

   A CAP message may be sent in the initial message of any SIP
   transaction.  However, this document only addresses sending a CAP
   message in a SIP MESSAGE transaction for a one-shot, non-interactive
   emergency call.  Behavior with other transactions is not defined.

   The CAP message is included in a SIP message as an additional-data
   block [RFC7852].  Accordingly, it is introduced to the SIP message

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   with a Call-Info header field with a purpose of
   "EmergencyCallData.cap".  The header field may contain a URI that is
   used by the recipient (or in some cases, an intermediary) to obtain
   the CAP message.  Alternatively, the Call-Info header field may
   contain a Content Indirect url [RFC2392] and the CAP message included
   in the body of the message.  In the latter case, the CAP message is
   located in a MIME block of the type 'application/
   emergencyCallData.cap+xml'.

   If the SIP server does not support the functionality required to
   fulfill the request then a 501 Not Implemented will be returned as
   specified in [RFC3261].  This is the appropriate response when a User
   Agent Server (UAS) does not recognize the request method and is not
   capable of supporting it for any user.

   The 415 Unsupported Media Type error will be returned as specified in
   [RFC3261] if the SIP server is refusing to service the request
   because the message body of the request is in a format not supported
   by the server for the requested method.  The server MUST return a
   list of acceptable formats using the Accept, Accept-Encoding, or
   Accept-Language header fields, depending on the specific problem with
   the content.

4.2.  Profiling of the CAP Document Content

   The usage of CAP MUST conform to the specification provided with
   [cap].  For usage with SIP the following additional requirements are
   imposed:

   sender:  The following restrictions and conditions apply to setting
      the value of the <sender> element:

      *  Originator is a SIP entity, Author indication irrelevant: When
         the alert was created by a SIP-based originator and it is not
         useful to be explicit about the author of the alert, then the
         <sender> element MUST be populated with the SIP URI of the user
         agent.

      *  Originator is a non-SIP entity, Author indication irrelevant:
         When the alert was created by a non-SIP based entity and the
         identity of this original sender is to be preserved, then this
         identity MUST be placed into the <sender> element.  In this
         situation it is not useful to be explicit about the author of
         the alert.  The specific type of identity being used will
         depend on the technology used by the original originator.

      *  Author indication relevant: When the author is different from
         the actual originator of the message and this distinction

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         should be preserved, then the <sender> element MUST NOT contain
         the SIP URI of the user agent.

   incidents:  The <incidents> element MUST be present.  This incident
      identifier MUST be chosen in such a way that it is unique for a
      given <sender, expires, incidents> combination.  Note that the
      <expires> element is optional and may not be present.

   scope:  The value of the <scope> element MAY be set to "Private" if
      the alert is not meant for public consumption.  The <addresses>
      element is, however, not used by this specification since the
      message routing is performed by SIP and the respective address
      information is already available in other SIP header fields.
      Populating information twice into different parts of the message
      may lead to inconsistency.

   parameter:  The <parameter> element MAY contain additional
      information specific to the sender, conforming to the CAP message
      syntax.

   area:  It is RECOMMENDED to omit this element when constructing a
      message.  If the CAP message already contains an <area> element,
      then the specified location information SHOULD be copied into a
      PIDF-LO structure (the data format for location used by emergency
      calls on the Internet) referenced by the SIP 'Geolocation' header
      field.  If there is a need to copy the PIDF-LO structure
      referenced by 'geolocation' to <area>, implementers must be aware
      that <area> is limited to a circle or polygon, and conversion of
      other shapes will be required.  Points SHOULD be converted to a
      circle with a radius equal to the uncertainty of the point.  Arc-
      bands and ellipses SHOULD be converted to an equivalent polygon.
      3D locations SHOULD be converted to their equivalent 2D forms.

4.3.  Sending a non-interactive Emergency Call

   A non-interactive emergency call is sent using a SIP MESSAGE
   transaction with a CAP URI or body part as described above in a
   manner similar to how an emergency call with interactive media is
   sent, as described in [RFC6881].  The MESSAGE transaction does not
   create a session nor establish interactive media streams, but
   otherwise, the header content of the transaction, routing, and
   processing of non-interactive calls are the same as those of other
   emergency calls.

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5.  Error Handling

   This section defines a new error response code and a header field for
   additional information.

5.1.  425 (Bad Alert Message) Response Code

   This SIP extension creates a new location-specific response code,
   defined as follows:

      425 (Bad Alert Message)

   The 425 response code is a rejection of the request due to its
   included alert content, indicating that it was malformed or not
   satisfactory for the recipient's purpose.

   A SIP intermediary can also reject an alert it receives from a User
   Agent (UA) when it detects that the provided alert is malformed.

   Section 5.2 describes an AlertMsg-Error header field with more
   details about what was wrong with the alert message in the request.
   This header field MUST be included in the 425 response.

   It is only appropriate to generate a 425 response when the responding
   entity has no other information in the request that is usable by the
   responder.

   A 425 response code MUST NOT be sent in response to a request that
   lacks an alert message, as the user agent in that case may not
   support this extension.

   A 425 response is a final response within a transaction, and MUST NOT
   terminate an existing dialog.

5.2.  The AlertMsg-Error Header Field

   The AlertMsg-Error header field provides additional information about
   what was wrong with the original request.  In some cases the provided
   information will be used for debugging purposes.

   The AlertMsg-Error header field has the following ABNF [RFC5234]:

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      message-header   /= AlertMsg-Error
                              ; (message-header from 3261)
      AlertMsg-Error   = "AlertMsg-Error" HCOLON
                              ErrorValue
      ErrorValue       =  error-code
                               *(SEMI error-params)
      error-code       = 1*3DIGIT
      error-params     = error-code-text
                               / generic-param ; from RFC3261
      error-code-text  = "code" EQUAL quoted-string ; from RFC3261

   HCOLON, SEMI, and EQUAL are defined in [RFC3261].  DIGIT is defined
   in [RFC5234].

   The AlertMsg-Error header field MUST contain only one ErrorValue to
   indicate what was wrong with the alert payload the recipient
   determined was bad.

   The ErrorValue contains a 3-digit error code indicating what was
   wrong with the alert in the request.  This error code has a
   corresponding quoted error text string that is human readable.  The
   text string is OPTIONAL, but RECOMMENDED for human readability,
   similar to the string phrase used for SIP response codes.  The
   strings in this document are recommendations, and are not
   standardized -- meaning an operator can change the strings -- but
   MUST NOT change the meaning of the error code.  Similar to how RFC
   3261 specifies, there MUST NOT be more than one string per error
   code.

   The AlertMsg-Error header field MAY be included in any response if an
   alert message was in the request part of the same transaction.  For
   example, a UA includes an alert in a MESSAGE to a PSAP.  The PSAP can
   accept this MESSAGE, thus creating a dialog, even though its UA
   determined that the alert message contained in the MESSAGE was bad.
   The PSAP merely includes an AlertMsg-Error header field value in the
   200 OK to the MESSAGE, thus informing the UA that the MESSAGE was
   accepted but the alert provided was bad.

   If, on the other hand, the PSAP cannot accept the transaction without
   a suitable alert message, a 425 response is sent.

   A SIP intermediary that requires the UA's alert message in order to
   properly process the transaction may also sends a 425 with an
   AlertMsg-Error code.

   This document defines an initial list of AlertMsg-Error values for
   any SIP response, including provisional responses (other than 100

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   Trying) and the new 425 response.  There MUST be no more than one
   AlertMsg-Error code in a SIP response.

   AlertMsg-Error: 100 ; code="Cannot Process the Alert Payload"

   AlertMsg-Error: 101 ; code="Alert Payload was not present or could
   not be found"

   AlertMsg-Error: 102 ; code="Not enough information to determine the
   purpose of the alert"

   AlertMsg-Error: 103 ; code="Alert Payload was corrupted"

   Additionally, if an entity cannot or chooses not to process the alert
   message from a SIP request, a 500 (Server Internal Error) SHOULD be
   used with or without a configurable Retry-After header field.

6.  Call Backs

   This document does not describe any method for the recipient to call
   back the sender of a non-interactive call.  Usually, these alerts are
   sent by automata, which do not have a mechanism to receive calls of
   any kind.  The identifier in the 'From' header field may be useful to
   obtain more information, but any such mechanism is not defined in
   this document.  The CAP message may contain related contact
   information for the sender.

7.  Handling Large Amounts of Data

   It is not atypical for sensors to have large quantities of data that
   they may wish to send.  Including large amounts of data (tens of
   kilobytes) in a MESSAGE is not advisable, because SIP entities are
   usually not equipped to handle very large messages.  In such cases,
   the sender SHOULD make use of the by-reference mechanisms defined in
   [RFC7852], which involves making the data available via HTTPS (either
   at the originator or at another entity), placing a URI to the data in
   the 'Call-Info' header field, and the recipient uses HTTPS to
   retrieve the data.  The CAP message itself can be sent by-reference
   using this mechanism, as well as any or all of the Additional Data
   blocks that may contain sensor-specific data.

8.  Example

   The following example shows a CAP document indicating a BURGLARY
   alert issued by a sensor called 'sensor1@example.com'.  The location
   of the sensor can be obtained from the attached location information
   provided via the 'geolocation' header field contained in the SIP
   MESSAGE structure.  Additionally, the sensor provided some data along

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   with the alert message, using proprietary information elements
   intended only to be processed by the receiver, a SIP entity acting as
   an aggregator.

      MESSAGE sip:aggregator@example.com SIP/2.0
      Via: SIP/2.0/TCP sensor1.example.com;branch=z9hG4bK776sgdkse
      Max-Forwards: 70
      From: sip:sensor1@example.com;tag=49583
      To: sip:aggregator@example.com
      Call-ID: asd88asd77a@2001:DB8:0:0FF
      Geolocation: <cid:abcdef@example.com>
        ;routing-allowed=yes
      Supported: geolocation
      Accept: application/pidf+xml,application/EmergencyCallData.cap+xml
      CSeq: 1 MESSAGE
      Call-Info: cid:abcdef2@example.com;purpose=EmergencyCallData.cap
      Content-Type: multipart/mixed; boundary=boundary1
      Content-Length: ...

      --boundary1

      Content-Type: application/EmergencyCallData.cap+xml
      Content-ID: <abcdef2@example.com>
      Content-Disposition: by-reference;handling=optional
     <?xml version="1.0" encoding="UTF-8"?>

     <alert xmlns="urn:oasis:names:tc:emergency:cap:1.1">
       <identifier>S-1</identifier>
       <sender>sip:sensor1@example.com</sender>
       <sent>2008-11-19T14:57:00-07:00</sent>
       <status>Actual</status>
       <msgType>Alert</msgType>
       <scope>Private</scope>
       <incidents>abc1234</incidents>
       <info>
           <category>Security</category>
           <event>BURGLARY</event>
           <urgency>Expected</urgency>
           <certainty>Likely</certainty>
           <severity>Moderate</severity>
           <senderName>SENSOR 1</senderName>
           <parameter>
             <valueName>SENSOR-DATA-NAMESPACE1</valueName>
             <value>123</value>
           </parameter>
           <parameter>
             <valueName>SENSOR-DATA-NAMESPACE2</valueName>

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             <value>TRUE</value>
           </parameter>
       </info>
      </alert>

      --boundary1

      Content-Type: application/pidf+xml
      Content-ID: <abcdef2@example.com>
      Content-Disposition: by-reference;handling=optional
      <?xml version="1.0" encoding="UTF-8"?>
          <presence
             xmlns="urn:ietf:params:xml:ns:pidf"
             xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
             xmlns:gbp=
                    "urn:ietf:params:xml:ns:pidf:geopriv10:basicPolicy"
             xmlns:cl="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr"
             xmlns:gml="http://www.opengis.net/gml"
             xmlns:dm="urn:ietf:params:xml:ns:pidf:data-model"
             entity="pres:alice@atlanta.example.com">
           <dm:device id="sensor">
             <gp:geopriv>
               <gp:location-info>
                 <gml:location>
                   <gml:Point srsName="urn:ogc:def:crs:EPSG::4326">
                     <gml:pos>32.86726 -97.16054</gml:pos>
                   </gml:Point>
                </gml:location>
               </gp:location-info>
               <gp:usage-rules>
                 <gbp:retransmission-allowed>false
                 </gbp:retransmission-allowed>
                 <gbp:retention-expiry>2010-11-14T20:00:00Z
                 </gbp:retention-expiry>
               </gp:usage-rules>
               <gp:method>802.11</gp:method>
             </gp:geopriv>
             <dm:timestamp>2010-11-04T20:57:29Z</dm:timestamp>
           </dm:device>
         </presence>
      --boundary1--

       Figure 3: Example Message conveying an Alert to an aggregator

   The following shows the same CAP document sent as a non-interactive
   emergency call towards a PSAP.

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      MESSAGE urn:service:sos SIP/2.0
      Via: SIP/2.0/TCP sip:aggreg.1.example.com;branch=z9hG4bK776abssa
      Max-Forwards: 70
      From: sip:aggregator@example.com;tag=32336
      To: 112
      Call-ID: asdf33443a@example.com
      Route: sip:psap1.example.gov
      Geolocation: <cid:abcdef@example.com>
        ;routing-allowed=yes
      Supported: geolocation
      Accept: application/pidf+xml,application/EmergencyCallData.cap+xml
      Call-info: cid:abcdef2@example.com;purpose=EmergencyCallData.cap
      CSeq: 1 MESSAGE
      Content-Type: multipart/mixed; boundary=boundary1
      Content-Length: ...

      --boundary1

      Content-Type: application/EmergencyCallData.cap+xml
      Content-ID: <abcdef2@example.com>
     <?xml version="1.0" encoding="UTF-8"?>

     <alert xmlns="urn:oasis:names:tc:emergency:cap:1.1">
       <identifier>S-1</identifier>
       <sender>sip:sensor1@example.com</sender>
       <sent>2008-11-19T14:57:00-07:00</sent>
       <status>Actual</status>
       <msgType>Alert</msgType>
       <scope>Private</scope>
       <incidents>abc1234</incidents>
       <info>
           <category>Security</category>
           <event>BURGLARY</event>
           <urgency>Expected</urgency>
           <certainty>Likely</certainty>
           <severity>Moderate</severity>
           <senderName>SENSOR 1</senderName>
           <parameter>
             <valueName>SENSOR-DATA-NAMESPACE1</valueName>
             <value>123</value>
           </parameter>
           <parameter>
             <valueName>SENSOR-DATA-NAMESPACE2</valueName>
             <value>TRUE</value>
           </parameter>
       </info>
      </alert>

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      --boundary1

      Content-Type: application/pidf+xml
      Content-ID: <abcdef2@example.com>
      <?xml version="1.0" encoding="UTF-8"?>
          <presence
             xmlns="urn:ietf:params:xml:ns:pidf"
             xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
             xmlns:gbp=
                    "urn:ietf:params:xml:ns:pidf:geopriv10:basicPolicy"
             xmlns:cl="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr"
             xmlns:gml="http://www.opengis.net/gml"
             xmlns:dm="urn:ietf:params:xml:ns:pidf:data-model"
             entity="pres:alice@atlanta.example.com">
           <dm:device id="sensor">
             <gp:geopriv>
               <gp:location-info>
                 <gml:location>
                   <gml:Point srsName="urn:ogc:def:crs:EPSG::4326">
                     <gml:pos>32.86726 -97.16054</gml:pos>
                   </gml:Point>
                </gml:location>
               </gp:location-info>
               <gp:usage-rules>
                 <gbp:retransmission-allowed>false
                 </gbp:retransmission-allowed>
                 <gbp:retention-expiry>2010-11-14T20:00:00Z
                 </gbp:retention-expiry>
               </gp:usage-rules>
               <gp:method>802.11</gp:method>
             </gp:geopriv>
             <dm:timestamp>2010-11-04T20:57:29Z</dm:timestamp>
           </dm:device>
         </presence>
      --boundary1--

          Figure 4: Example Message conveying an Alert to a PSAP

9.  Security Considerations

   This section discusses security considerations when SIP user agents
   issue emergency alerts utilizing MESSAGE and CAP.  Location specific
   threats are not unique to this document and are discussed in
   [RFC7378] and [RFC6442].

   The ECRIT emergency services architecture [RFC6443] considers classic
   individual-to-authority emergency calling where the identity of the
   emergency caller does not play a role at the time of the call

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   establishment itself, i.e., a response to the emergency call does not
   depend on the identity of the caller.  In the case of emergency
   alerts generated by devices such as sensors, the processing may be
   different in order to reduce the number of falsely generated
   emergency alerts.  Alerts could get triggered based on certain sensor
   input that might have been caused by factors other than the actual
   occurrence of an alert-relevant event.  For example, a sensor may
   simply be malfunctioning.  For this reason, not all alert messages
   are directly sent to a PSAP, but rather may be pre-processed by a
   separate entity, potentially under supervision by a human, to filter
   alerts and potentially correlate received alerts with others to
   obtain a larger picture of the ongoing situation.

   In any case, for alerts initiated by sensors, the identity could play
   an important role in deciding whether to accept or ignore an incoming
   alert message.  With the scenario shown in Figure 1 it is very likely
   that only authorized sensor input will be processed.  For this
   reason, it needs to be possible to refuse to accept alert messages
   from an unknown origin.  Two types of information elements can be
   used for this purpose:

   1.  SIP itself provides security mechanisms that allow the
       verification of the originator's identity.  These mechanisms can
       be re-used, such as P-Asserted-Identity [RFC3325] or SIP Identity
       [RFC8224].  The latter provides a cryptographic assurance while
       the former relies on a chain of trust model.

   2.  CAP provides additional security mechanisms and the ability to
       carry further information about the sender's identity.
       Section 3.3.4.1 of [cap] specifies the signing algorithms of CAP
       documents.

   In addition to the desire to perform identity-based access control,
   the classic communication security threats need to be considered,
   including integrity protection to prevent forgery or replay of alert
   messages in transit.  To deal with replay of alerts, a CAP document
   contains the mandatory <identifier>, <sender>, <sent> elements and an
   optional <expire> element.  Together, these elements make the CAP
   document unique for a specific sender and provide time restrictions.
   An entity that has already received a CAP message within the
   indicated timeframe is able to detect a replayed message and, if the
   content of that message is unchanged, then no additional security
   vulnerability is created.  Additionally, it is RECOMMENDED to make
   use of SIP security mechanisms, such as SIP Identity [RFC8224], to
   tie the CAP message to the SIP message.  To provide protection of the
   entire SIP message exchange between neighboring SIP entities, the
   usage of TLS is REQUIRED.

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   Note that none of the security mechanism in this document protect
   against a compromised sensor sending crafted alerts.  Privacy
   provided for any emergency calls, including non-interactive messages,
   is subject to local regulations.

10.  IANA Considerations

10.1.  Registration of the 'application/EmergencyCallData.cap+xml' MIME
       type

   To:  ietf-types@iana.org

   Subject:  Registration of MIME media type application/
      EmergencyCallData.cap+xml

   MIME media type name:  application

   MIME subtype name:  cap+xml

   Required parameters:  (none)

   Optional parameters:  charset; Indicates the character encoding of
      enclosed XML.  Default is UTF-8 [RFC3629].

   Encoding considerations:  Uses XML, which can employ 8-bit
      characters, depending on the character encoding used.  See
      [RFC7303], Section 3.2.

   Security considerations:  This content type is designed to carry
      payloads of the Common Alerting Protocol (CAP).  RFC XXX [Replace
      by the RFC number of this specification] discusses security
      considerations for this.

   Interoperability considerations:  This content type provides a way to
      convey CAP payloads.

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   Published specification:  RFC XXX [Replace by the RFC number of this
      specification].

   Applications which use this media type:  Applications that convey
      alerts and warnings according to the CAP standard.

   Additional information:  OASIS has published the Common Alerting
      Protocol at http://www.oasis-open.org/committees/
      documents.php&wg_abbrev=emergency

   Person and email address to contact for further information:  Hannes
      Tschofenig, hannes.tschofenig@gmx.net

   Intended usage:  Limited use

   Author/Change controller:  IETF ECRIT working group

   Other information:  This media type is a specialization of
      application/xml [RFC7303], and many of the considerations
      described there also apply to application/cap+xml.

10.2.  IANA Registration of 'cap' Additional Data Block

   This document registers a new block type in the sub-registry called
   'Emergency Call Data Types' of the Emergency Call Additional Data
   Registry defined in [RFC7852].  The token is "cap", the Data About is
   "The Call" and the reference is this document.

10.3.  IANA Registration for 425 Response Code

   In the SIP Response Codes registry, the following is added

   Reference: RFC-XXXX (i.e., this document)

   Response code: 425 (recommended number to assign)

   Default reason phrase: Bad Alert Message

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      Registry:
        Response Code                               Reference
        ------------------------------------------  ---------
        Request Failure 4xx
          425 Bad Alert Message                   [this doc]

   This SIP Response code is defined in Section 5.

10.4.  IANA Registration of New AlertMsg-Error Header Field

   The SIP AlertMsg-error header field is created by this document, with
   its definition and rules in Section 5, to be added to the IANA
   Session Initiation Protocol (SIP) Parameters registry with two
   actions:

   1.  Update the Header Fields registry with

      Registry:
        Header Name        compact    Reference
        -----------------  -------    ---------
        AlertMsg-Error             [this doc]

   2.  In the portion titled "Header Field Parameters and Parameter
       Values", add

                                               Predefined
      Header Field        Parameter Name       Values      Reference
      -----------------   -------------------  ----------  ---------
      AlertMsg-Error      code                 yes         [this doc]

10.5.  IANA Registration for the SIP AlertMsg-Error Codes

   This document creates a new registry for SIP, called "AlertMsg-Error
   Codes".  AlertMsg-Error codes provide reasons for an error discovered
   by a recipient, categorized by the action to be taken by the error
   recipient.  The initial values for this registry are shown below.

   Registry Name: AlertMsg-Error Codes

   Reference: [this doc]

   Registration Procedures: Specification Required

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   Code Default Reason Phrase                                 Reference
   ---- ---------------------------------------------------   ---------
   100  "Cannot Process the Alert Payload"                    [this doc]

   101  "Alert Payload was not present or could not be found" [this doc]

   102  "Not enough information to determine
         the purpose of the alert"                            [this doc]

   103  "Alert Payload was corrupted"                         [this doc]

   Details of these error codes are in Section 5.

11.  Acknowledgments

   The authors would like to thank the participants of the Early Warning
   adhoc meeting at IETF#69 for their feedback.  Additionally, we would
   like to thank the members of the NENA Long Term Direction Working
   Group for their feedback.

   Additionally, we would like to thank Martin Thomson, James
   Winterbottom, Shida Schubert, Bernard Aboba, Marc Linsner, Christer
   Holmberg and Ivo Sedlacek for their review comments.

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", March 1997.

   [cap]      Jones, E. and A. Botterell, "Common Alerting Protocol v.
              1.2", October 2005, <https://docs.oasis-
              open.org/emergency/cap/v1.2/CAP-v1.2-os.pdf>.

   [RFC2392]  Levinson, E., "Content-ID and Message-ID Uniform Resource
              Locators", RFC 2392, DOI 10.17487/RFC2392, August 1998,
              <https://www.rfc-editor.org/info/rfc2392>.

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

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   [RFC3428]  Campbell, B., Ed., Rosenberg, J., Schulzrinne, H.,
              Huitema, C., and D. Gurle, "Session Initiation Protocol
              (SIP) Extension for Instant Messaging", RFC 3428,
              DOI 10.17487/RFC3428, December 2002,
              <https://www.rfc-editor.org/info/rfc3428>.

   [RFC4119]  Peterson, J., "A Presence-based GEOPRIV Location Object
              Format", RFC 4119, DOI 10.17487/RFC4119, December 2005,
              <https://www.rfc-editor.org/info/rfc4119>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC7303]  Thompson, H. and C. Lilley, "XML Media Types", RFC 7303,
              DOI 10.17487/RFC7303, July 2014,
              <https://www.rfc-editor.org/info/rfc7303>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC6442]  Polk, J., Rosen, B., and J. Peterson, "Location Conveyance
              for the Session Initiation Protocol", RFC 6442,
              DOI 10.17487/RFC6442, December 2011,
              <https://www.rfc-editor.org/info/rfc6442>.

   [RFC6881]  Rosen, B. and J. Polk, "Best Current Practice for
              Communications Services in Support of Emergency Calling",
              BCP 181, RFC 6881, DOI 10.17487/RFC6881, March 2013,
              <https://www.rfc-editor.org/info/rfc6881>.

   [RFC7852]  Gellens, R., Rosen, B., Tschofenig, H., Marshall, R., and
              J. Winterbottom, "Additional Data Related to an Emergency
              Call", RFC 7852, DOI 10.17487/RFC7852, July 2016,
              <https://www.rfc-editor.org/info/rfc7852>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

12.2.  Informative References

   [RFC7378]  Tschofenig, H., Schulzrinne, H., and B. Aboba, Ed.,
              "Trustworthy Location", RFC 7378, DOI 10.17487/RFC7378,
              December 2014, <https://www.rfc-editor.org/info/rfc7378>.

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   [RFC8224]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 8224,
              DOI 10.17487/RFC8224, February 2018,
              <https://www.rfc-editor.org/info/rfc8224>.

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              DOI 10.17487/RFC3325, November 2002,
              <https://www.rfc-editor.org/info/rfc3325>.

   [RFC5222]  Hardie, T., Newton, A., Schulzrinne, H., and H.
              Tschofenig, "LoST: A Location-to-Service Translation
              Protocol", RFC 5222, DOI 10.17487/RFC5222, August 2008,
              <https://www.rfc-editor.org/info/rfc5222>.

   [RFC6443]  Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
              "Framework for Emergency Calling Using Internet
              Multimedia", RFC 6443, DOI 10.17487/RFC6443, December
              2011, <https://www.rfc-editor.org/info/rfc6443>.

Authors' Addresses

   Brian Rosen
   470 Conrad Dr
   Mars,  PA   16046
   US

   Phone:
   Email: br@brianrosen.net

   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027
   US

   Phone: +1 212 939 7004
   Email: hgs+ecrit@cs.columbia.edu
   URI:   http://www.cs.columbia.edu

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   Hannes Tschofenig
   ARM Limited

   Austria

   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at

   Randall Gellens
   Core Technology Consulting

   Email: rg+ietf@coretechnologyconsulting.com
   URI:   http://www.coretechnologyconsulting.com

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