| ECRIT | B. Rosen |
| Internet-Draft | |
| Intended status: Standards Track | H. Schulzrinne |
| Expires: July 31, 2020 | Columbia U. |
| H. Tschofenig | |
| ARM Limited | |
| R. Gellens | |
| Core Technology Consulting | |
| January 28, 2020 |
Non-Interactive Emergency Calls
draft-ietf-ecrit-data-only-ea-19
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.
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Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.
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[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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
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
This section illustrates two envisioned usage modes: targeted and location-based emergency alert routing.
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.
+----------+ +----------+ +-----------+
|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
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 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. Alternative, 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 MUST 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 MUST 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.
The usage of CAP MUST conform to the specification provided with [cap]. For usage with SIP the following additional requirements are imposed:
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.
This section defines a new error response code and a header field for additional information.
This SIP extension creates a new location-specific response code, defined as follows:
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.
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 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. That said, the strings are complete enough for rendering to the user, if so desired. 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 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.
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.
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.
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:DB8:0:0FF
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>
<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.
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>
--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
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 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:
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.
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.
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.
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.
Registry:
Header Name compact Reference
----------------- ------- ---------
AlertMsg-Error [this doc]
Predefined
Header Field Parameter Name Values Reference
----------------- ------------------- ---------- ---------
AlertMsg-Error code yes [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:
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.
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.
| [RFC7378] | Tschofenig, H., Schulzrinne, H. and B. Aboba, "Trustworthy Location", RFC 7378, DOI 10.17487/RFC7378, December 2014. |
| [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. |
| [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. |
| [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. |