1. RFC 8876
Internet Engineering Task Force (IETF)                          B. Rosen
Request for Comments: 8876                                              
Category: Standards Track                                 H. Schulzrinne
ISSN: 2070-1721                                              Columbia U.
                                                           H. Tschofenig
                                                              R. Gellens
                                              Core Technology Consulting
                                                          September 2020

                    Non-interactive Emergency Calls


   Use of the Internet for emergency calling is described in RFC 6443,
   'Framework for Emergency Calling Using Internet Multimedia'.  In some
   cases of emergency calls, the transmission of application data is all
   that is needed, and no interactive media channel is established: a
   situation referred to as 'non-interactive emergency calls', where,
   unlike most emergency calls, there is no two-way interactive media
   such as voice or video or text.  This document describes use of a SIP
   MESSAGE transaction that includes a container for the data based on
   the Common Alerting Protocol (CAP).  That type of emergency request
   does not establish a session, distinguishing it from SIP INVITE,
   which does.  Any device that needs to initiate a request for
   emergency services without an interactive media channel would use the
   mechanisms in this document.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in 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

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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.  Terminology
   3.  Architectural Overview
   4.  Protocol Specification
     4.1.  CAP Transport
     4.2.  Profiling of the CAP Document Content
     4.3.  Sending a Non-interactive Emergency Call
   5.  Error Handling
     5.1.  425 (Bad Alert Message) Response Code
     5.2.  The AlertMsg-Error Header Field
   6.  Call Backs
   7.  Handling Large Amounts of Data
   8.  Example
   9.  Security Considerations
   10. IANA Considerations
     10.1.  'application/EmergencyCallData.cap+xml' Media Type
     10.2.  'cap' Additional Data Block
     10.3.  425 Response Code
     10.4.  AlertMsg-Error Header Field
     10.5.  SIP AlertMsg-Error Codes
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Authors' Addresses

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 include sensors issuing alerts, and certain types of
   medical monitors.  These messages may be alerts to emergency
   authorities and do not require establishment of a session.  These
   types 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

   Non-interactive emergency calls are similar to regular emergency
   calls in the sense that they require the emergency indications,
   emergency call routing functionality, and location.  However, the
   communication interaction will not lead to the exchange of
   interactive media, that is, Real-Time Transport Protocol [RFC3550]
   packets, such as voice, video, 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 the public.  The scope of this document is conveying
   CAP alerts from private devices to emergency service authorities, as
   a call without any interactive media.

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

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

   Non-interactive emergency call:  An emergency call where there is no
      two-way interactive media

   SIP:  Session Initiation Protocol [RFC3261]

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

   LoST:  Location To Service Translation protocol [RFC5222]

   CID:  Content-ID [RFC2392]

   CAP:  Common Alerting Protocol [CAP]

   PSAP:  Public Safety Answering Point, the call center for emergency

   ESRP:  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:

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

       b.  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 [RFC5031] 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 SIP MESSAGE with CAP content, a Geolocation header
       field, and one or more Call-Info header fields containing
       additional data [RFC7852]).

   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 where 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.
   For example, a PSAP is likely to receive and accept alerts from
   entities it has no previous relationship with.  This scenario is
   similar to a classic voice emergency services call, 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.

      +----------+         +----------+                  +-----------+
      |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

   This document addresses sending a CAP alert in a SIP MESSAGE
   transaction for a non-interactive emergency call.  Behavior with
   other transactions is not defined.

   The CAP alert is included in a SIP message as an additional data
   block [RFC7852].  Accordingly, it is conveyed in the SIP message 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 alert.
   Alternatively, the Call-Info header field may contain a Content-ID
   URL [RFC2392] and the CAP alert included in the body of the message.
   In the latter case, the CAP alert 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 (where "sender" and "author" are as defined in CAP and
   "originator" is the entity sending the CAP alert, which may be
   different from the entity sending the SIP MESSAGE):

   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

      *  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 originator.

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

   area:  It is RECOMMENDED to omit this element when constructing a
      message.  If the CAP alert is given to the SIP entity to transport
      and it 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
      the CAP alert is being created by the SIP entity using a PIDF-LO
      structure referenced by 'geolocation' to construct <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 polygons with similar coverage, and 3D locations
      SHOULD be converted to 2D forms with similar coverage.

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.

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 response code defined as follows:

      425 (Bad Alert Message)

   The 425 response code is a rejection of the request, indicating that
   it was malformed enough that no reasonable emergency response to the
   alert can be determined.

   A SIP intermediary can also use this code to 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 usually the case that emergency calls are not rejected if there
   is any useful information that can be acted upon.  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 (i.e., CAP data), 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]:

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

   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.  The code space for ErrorValue is separate
   from SIP Status Codes.

   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, suppose a UA includes an alert in a MESSAGE to a PSAP.  The
   PSAP can accept this MESSAGE, 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 send a 425 response 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
   Trying) and the new 425 response.  There MUST NOT be more than one
   AlertMsg-Error code in a SIP response.  AlertMsg-Error values sent in
   provisional responses MUST be sent using the mechanism defined in
   [RFC3262]; or, if that mechanism is not negotiated, they MUST be
   repeated in the final response to the transaction.

   AlertMsg-Error: 100 ; message="Cannot process the alert payload"

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

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

   AlertMsg-Error: 103 ; message="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 alert may contain related contact information
   for the sender.

7.  Handling Large Amounts of Data

   Sensors may 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 [RFC2818] (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 alert itself can be sent by reference using this
   mechanism, as can any or all of the additional data blocks that may
   contain sensor-specific data.

   There are no rate-limiting mechanisms for any SIP transactions that
   are standardized, although implementations often include such
   functions.  Non-interactive emergency calls are typically handled the
   same as any emergency call, which means a human call-taker is
   involved.  Implementations should take note of this limitation,
   especially when calls are placed automatically without human

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
   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::ff
      Geolocation: <cid:abcdef@example.com>
      Supported: geolocation
      CSeq: 1 MESSAGE
      Call-Info: cid:abcdef2@example.com;purpose=EmergencyCallData.cap
      Content-Type: multipart/mixed; boundary=boundary1
      Content-Length: ...

      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">
           <senderName>SENSOR 1</senderName>

      Content-Type: application/pidf+xml
      Content-ID: <abcdef2@example.com>

      <?xml version="1.0" encoding="UTF-8"?>
           <dm:device id="sensor">
                   <gml:Point srsName="urn:ogc:def:crs:EPSG::4326">
                     <gml:pos>44.85249659 -93.238665712</gml:pos>

       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.

      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>
      Supported: geolocation
      Call-info: cid:abcdef2@example.com;purpose=EmergencyCallData.cap
      CSeq: 1 MESSAGE
      Content-Type: multipart/mixed; boundary=boundary1
      Content-Length: ...


      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">
           <senderName>SENSOR 1</senderName>


      Content-Type: application/pidf+xml
      Content-ID: <abcdef2@example.com>
      <?xml version="1.0" encoding="UTF-8"?>
           <dm:device id="sensor">
                   <gml:Point srsName="urn:ogc:def:crs:EPSG::4326">
                     <gml:pos>44.85249659 -93.2386657124</gml:pos>

           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 Emergency Context Resolution with Internet Technologies (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
   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 authenticated sensor input will be processed.  For
   this reason, it needs to be possible to refuse to accept alert
   messages from unknown origins.  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, 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.  These mechanisms can be reused.

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

   The specific policy and mechanisms used in a given deployment are out
   of scope for this document.

   There is no rate limiting mechanisms in SIP, and all kinds of
   emergency calls, including those defined in this document, could be
   used by malicious actors or misbehaving devices to effect a denial-
   of-service attack on the emergency services.  The mechanism defined
   in this document does not introduce any new considerations, although
   it may be more likely that devices that place non-interactive
   emergency calls without a human initiating them may be more likely
   than those that require a user to initiate them.

   Implementors should note that automated emergency calls may be
   prohibited or regulated in some jurisdictions, and there may be
   penalties for "false positive" calls.

   This document describes potential retrieval of information by
   dereferencing URIs found in a Call Info header of a SIP MESSAGE.
   These may include a CAP alert as well as other additional data
   [RFC7852] blocks.  The domain of the device sending the SIP MESSAGE;
   the domain of the server holding the CAP alert, if sent by reference;
   and the domain of other additional data blocks, if sent by reference,
   may all be different.  No assumptions can be made that there are
   trust relationships between these entities.  Recipients MUST take
   precautions in retrieving any additional data blocks passed by
   reference, including the CAP alert, because the URI may point to a
   malicious actor or entity not expecting to be referred to for this
   purpose.  The considerations in handling URIs in [RFC3986] apply.

   Use of timestamps to prevent replay is subject to the availability of
   accurate time at all participants.  Because emergency event
   notification via this mechanism is relatively low frequency and
   generally involves human interaction, implementations may wish to
   consider messages with times within a small number of seconds of each
   other to be effectively simultaneous for the purposes of detecting
   replay.  Implementations may also wish to consider that most deployed
   time distribution protocols likely to be used by these systems are
   not presently secure.

   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>, and <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 alert 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 the SIP Identity PASSporT
   [RFC8225], to tie the CAP alert to the SIP message.  To provide
   protection of the entire SIP message exchange between neighboring SIP
   entities, the usage of TLS is RECOMMENDED.  [RFC6443] discusses the
   issues of using TLS with emergency calls, which are equally
   applicable to non-interactive emergency calls.

   Note that none of the security mechanisms in this document protect
   against a compromised sensor sending crafted alerts.  Confidentiality
   provided for any emergency calls, including non-interactive messages,
   is subject to local regulations.  Privacy issues are discussed in
   [RFC7852] and are applicable here.

10.  IANA Considerations

10.1.  'application/EmergencyCallData.cap+xml' Media Type

   Type name:  application

   Subtype name:  EmergencyCallData.cap+xml

   Required parameters:  N/A

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

   Encoding considerations:  7bit, 8bit, or binary.  See Section 3.2 of

   Security considerations:  This content type is designed to carry
      payloads of the Common Alerting Protocol (CAP).  RFC 8876
      discusses security considerations for this.

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

   Published specification:  RFC 8876

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

   Fragment identifier considerations: N/A

   Additional information:  OASIS has published the Common Alerting
      Protocol at <https://docs.oasis-open.org/emergency/cap/v1.2/CAP-

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

   Intended usage:  Limited use

   Author/Change controller:  The IESG

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

10.2.  'cap' Additional Data Block

   Per this document, IANA has registered a new block type in the
   "Emergency Call Data Types" subregistry 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.  425 Response Code

   In the SIP "Response Codes" registry, the following has been added
   under Request Failure 4xx.

             | Response Code | Description       | Reference |
             | 425           | Bad Alert Message | RFC 8876  |

                 Table 1: Response Codes Registry Addition

   This SIP Response code is defined in Section 5.

10.4.  AlertMsg-Error Header Field

   The SIP AlertMsg-Error header field is created by this document, with
   its definition and rules in Section 5.  The IANA "Session Initiation
   Protocol (SIP) Parameters" registry has been updated as follows.

   1.  In the "Header Fields" subregistry, the following has been added:

                   | Head Name      | compact | Reference |
                   | AlertMsg-Error |         | RFC 8876  |

                   Table 2: Header Fields Registry Addition

   2.  In the "Header Field Parameters and Parameter Values"
       subregistry, the following has been added:

         | Header Field   | Parameter Name | Predefined | Reference |
         |                |                | Values     |           |
         | AlertMsg-Error | code           | no         | RFC 8876  |

            Table 3: Header Field Parameters and Parameter Values
                              Registry Addition

10.5.  SIP AlertMsg-Error Codes

   This document creates a new registry called "SIP 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.
   The registration procedure is Specification Required [RFC8126].

        | Code | Default Reason Phrase               | Reference |
        | 100  | "Cannot process the alert payload"  | RFC 8876  |
        | 101  | "Alert payload was not present or   | RFC 8876  |
        |      | could not be found"                 |           |
        | 102  | "Not enough information to          | RFC 8876  |
        |      | determine the purpose of the alert" |           |
        | 103  | "Alert payload was corrupted"       | RFC 8876  |

           Table 4: SIP AlertMsg-Error Codes Registry Creation

   Details of these error codes are in Section 5.

11.  References

11.1.  Normative References

   [CAP]      Jones, E. and A. Botterell, "Common Alerting Protocol
              Version 1.2", OASIS Standard CAP-V1.2, July 2010,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2392]  Levinson, E., "Content-ID and Message-ID Uniform Resource
              Locators", RFC 2392, DOI 10.17487/RFC2392, August 1998,

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,

   [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,

   [RFC3262]  Rosenberg, J. and H. Schulzrinne, "Reliability of
              Provisional Responses in Session Initiation Protocol
              (SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002,

   [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,

   [RFC4119]  Peterson, J., "A Presence-based GEOPRIV Location Object
              Format", RFC 4119, DOI 10.17487/RFC4119, December 2005,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [RFC7303]  Thompson, H. and C. Lilley, "XML Media Types", RFC 7303,
              DOI 10.17487/RFC7303, July 2014,

   [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>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC6442]  Polk, J., Rosen, B., and J. Peterson, "Location Conveyance
              for the Session Initiation Protocol", RFC 6442,
              DOI 10.17487/RFC6442, December 2011,

   [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,

   [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,

   [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>.

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,

11.2.  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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [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,

   [RFC5031]  Schulzrinne, H., "A Uniform Resource Name (URN) for
              Emergency and Other Well-Known Services", RFC 5031,
              DOI 10.17487/RFC5031, January 2008,

   [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,

   [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,

   [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>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.


   The authors would like to thank the participants of the Early Warning
   ad hoc 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.

Authors' Addresses

   Brian Rosen
   470 Conrad Dr
   Mars, PA 16046
   United States of America

   Email: br@brianrosen.net

   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY 10027
   United States of America

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

   Hannes Tschofenig

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

   Randall Gellens
   Core Technology Consulting

   Email: rg+ietf@coretechnologyconsulting.com
   URI:   http://www.coretechnologyconsulting.com
  1. RFC 8876