Non-Interactive Emergency Calls
draft-ietf-ecrit-data-only-ea-22

Abstract

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 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/.

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 September 10, 2020.

Copyright Notice

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

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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. Registration of the 'application/EmergencyCallData.cap+xml' media type
• 10.2. IANA Registration of 'cap' Additional Data Block
• 10.3. IANA Registration for 425 Response Code
• 10.4. IANA Registration of New AlertMsg-Error Header Field
• 10.5. IANA Registration for the SIP AlertMsg-Error Codes
• 11. Acknowledgments
• 12. References
• 12.1. Normative References
• 12.2. 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 one-shot 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 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 location. 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 (the message includes a URI that, 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.

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.

A non-interactive emergency call is an emergency call where there is no two-way interactive media.

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-ID [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 [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 CAP, Geolocation header field and one or more Call-Info header fields containing Additional Data [RFC7852] in a SIP MESSAGE).

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. 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

A CAP message is 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 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-ID 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 (where "sender" and "author" are as defined in CAP and "Originator" is the entity sending the alert):

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 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 message syntax.

area:

It is RECOMMENDED to omit this element when constructing a message. If the CAP message 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 message 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 location-specific 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 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, 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 RFC3261)
   AlertMsg-Error   = "AlertMsg-Error" HCOLON
                           ErrorValue
   ErrorValue       =  error-code
                            *(SEMI error-params)
   error-code       = 3DIGIT
   error-params     = error-code-text
                            / generic-param ; from RFC3261
   error-code-text  = "message" 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. 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 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, 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 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 [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 message 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 initiation.

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>
     ;routing-allowed=yes
   Supported: geolocation
   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>2020-01-04T20:57:35Z</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>

   --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>44.85249659 -93.238665712</gml:pos>
                </gml:Point>
             </gml:location>
            </gp:location-info>
            <gp:usage-rules>
              <gbp:retransmission-allowed>false
              </gbp:retransmission-allowed>
              <gbp:retention-expiry>2020-02-04T20:57:29Z
              </gbp:retention-expiry>
            </gp:usage-rules>
            <gp:method>802.11</gp:method>
          </gp:geopriv>
          <dm:timestamp>2020-01-04T20:57:29Z</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.

                  
   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
   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>2020-01-04T20:57:35Z</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>

   --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>44.85249659 -93.2386657124</gml:pos>
                </gml:Point>
             </gml:location>
            </gp:location-info>
            <gp:usage-rules>
              <gbp:retransmission-allowed>false
              </gbp:retransmission-allowed>
              <gbp:retention-expiry>2020-02-04T20:57:25Z
              </gbp:retention-expiry>
            </gp:usage-rules>
            <gp:method>802.11</gp:method>
          </gp:geopriv>
          <dm:timestamp>2020-01-04T20:57:25Z</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 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 3.3.4.1 of [cap] specifies the signing algorithms of CAP documents.

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 message 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 message, 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 message, 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 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>, <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 the SIP Identity PASSporT [RFC8225], 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 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. Registration of the 'application/EmergencyCallData.cap+xml' media type

To:

ietf-types@iana.org

Subject:

Registration of media type application/ EmergencyCallData.cap+xml

Type name:

application

Subtype name:

cap+xml

Required parameters:

(none)

Optional parameters:

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

Encoding considerations:

7bit, 8bit or binary. 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.

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.

Fragment Identifier Considerations: N/A

.

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:

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/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

            
   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

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

            
                                            Predefined
   Header Field        Parameter Name       Values      Reference
   -----------------   -------------------  ----------  ---------
   AlertMsg-Error      code                 no          [this doc]
          

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

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

            
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

12.2. Informative References

Authors' Addresses