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
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 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
https://www.rfc-editor.org/info/rfc8876.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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
Acknowledgments
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
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 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",
"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.
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
calls
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
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 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
syntax.
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
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
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 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 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 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
[RFC7303].
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-
v1.2-os.pdf>
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/
EmergencyCallData.cap+xml.
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,
<https://docs.oasis-open.org/emergency/cap/v1.2/CAP-
v1.2-os.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2392] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, DOI 10.17487/RFC2392, August 1998,
<https://www.rfc-editor.org/info/rfc2392>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002,
<https://www.rfc-editor.org/info/rfc3262>.
[RFC3428] Campbell, B., Ed., Rosenberg, J., Schulzrinne, H.,
Huitema, C., and D. Gurle, "Session Initiation Protocol
(SIP) Extension for Instant Messaging", RFC 3428,
DOI 10.17487/RFC3428, December 2002,
<https://www.rfc-editor.org/info/rfc3428>.
[RFC4119] Peterson, J., "A Presence-based GEOPRIV Location Object
Format", RFC 4119, DOI 10.17487/RFC4119, December 2005,
<https://www.rfc-editor.org/info/rfc4119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7303] Thompson, H. and C. Lilley, "XML Media Types", RFC 7303,
DOI 10.17487/RFC7303, July 2014,
<https://www.rfc-editor.org/info/rfc7303>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[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,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC6442] Polk, J., Rosen, B., and J. Peterson, "Location Conveyance
for the Session Initiation Protocol", RFC 6442,
DOI 10.17487/RFC6442, December 2011,
<https://www.rfc-editor.org/info/rfc6442>.
[RFC6881] Rosen, B. and J. Polk, "Best Current Practice for
Communications Services in Support of Emergency Calling",
BCP 181, RFC 6881, DOI 10.17487/RFC6881, March 2013,
<https://www.rfc-editor.org/info/rfc6881>.
[RFC7852] Gellens, R., Rosen, B., Tschofenig, H., Marshall, R., and
J. Winterbottom, "Additional Data Related to an Emergency
Call", RFC 7852, DOI 10.17487/RFC7852, July 2016,
<https://www.rfc-editor.org/info/rfc7852>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8225] Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
<https://www.rfc-editor.org/info/rfc8225>.
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,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8224] Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 8224,
DOI 10.17487/RFC8224, February 2018,
<https://www.rfc-editor.org/info/rfc8224>.
[RFC5031] Schulzrinne, H., "A Uniform Resource Name (URN) for
Emergency and Other Well-Known Services", RFC 5031,
DOI 10.17487/RFC5031, January 2008,
<https://www.rfc-editor.org/info/rfc5031>.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
DOI 10.17487/RFC3325, November 2002,
<https://www.rfc-editor.org/info/rfc3325>.
[RFC5222] Hardie, T., Newton, A., Schulzrinne, H., and H.
Tschofenig, "LoST: A Location-to-Service Translation
Protocol", RFC 5222, DOI 10.17487/RFC5222, August 2008,
<https://www.rfc-editor.org/info/rfc5222>.
[RFC6443] Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
"Framework for Emergency Calling Using Internet
Multimedia", RFC 6443, DOI 10.17487/RFC6443, December
2011, <https://www.rfc-editor.org/info/rfc6443>.
[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>.
Acknowledgments
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
Austria
Email: Hannes.Tschofenig@gmx.net
URI: https://www.tschofenig.priv.at
Randall Gellens
Core Technology Consulting