< draft-ietf-ecrit-data-only-ea-01.txt   draft-ietf-ecrit-data-only-ea-02.txt >
ECRIT B. Rosen ECRIT B. Rosen
Internet-Draft NeuStar, Inc. Internet-Draft NeuStar, Inc.
Intended status: Experimental H. Schulzrinne Intended status: Experimental H. Schulzrinne
Expires: April 28, 2011 Columbia U. Expires: January 11, 2012 Columbia U.
H. Tschofenig H. Tschofenig
Nokia Siemens Networks Nokia Siemens Networks
October 25, 2010 July 10, 2011
Common Alerting Protocol (CAP) based Data-Only Emergency Alerts using Common Alerting Protocol (CAP) based Emergency Alerts using the Session
the Session Initiation Protocol (SIP) Initiation Protocol (SIP)
draft-ietf-ecrit-data-only-ea-01.txt draft-ietf-ecrit-data-only-ea-02.txt
Abstract Abstract
The Common Alerting Protocol (CAP) is a document format for The Common Alerting Protocol (CAP) is a document format for
exchanging emergency alerts and public warnings. CAP is mainly used exchanging emergency alerts and public warnings. CAP is mainly used
for conveying alerts and warnings between authorities and from for conveying alerts and warnings between authorities and from
authorities to citizen/individuals. This document describes how authorities to citizen/individuals. This document describes how
data-only emergency alerts allow devices to issue alerts using the devices use CAP to issue emergency alerts.
CAP document format.
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Architectural Overview . . . . . . . . . . . . . . . . . . . . 5 3. Architectural Overview . . . . . . . . . . . . . . . . . . . . 5
4. Protocol Specification . . . . . . . . . . . . . . . . . . . . 7 4. Protocol Specification . . . . . . . . . . . . . . . . . . . . 7
4.1. CAP Transport . . . . . . . . . . . . . . . . . . . . . . 7 4.1. CAP Transport . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Profiling of the CAP Document Content . . . . . . . . . . 7 4.2. Profiling of the CAP Document Content . . . . . . . . . . 7
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5. Error Handling . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 5.1. 425 (Bad Alert Message) Response Code . . . . . . . . . . 9
6.1. Forgery . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.2. The AlertMsg-Error Header Field . . . . . . . . . . . . . 9
6.2. Replay Attack . . . . . . . . . . . . . . . . . . . . . . 10 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.3. Injecting False Alerts . . . . . . . . . . . . . . . . . . 11 7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7.1. Registration of the 8.1. Registration of the 'application/cap+xml' MIME type . . . 16
'application/common-alerting-protocol+xml' MIME type . . . 12 8.2. IANA Registration for 425 Response Code . . . . . . . . . 17
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14 8.3. IANA Registration of New AlertMsg-Error Header Field . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 8.4. IANA Registration for the SIP AlertMsg-Error Codes . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . . 15 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . . 15 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17 10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
10.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction 1. Introduction
The Common Alerting Protocol (CAP) [cap] is an XML document format The Common Alerting Protocol (CAP) [cap] is an XML document format
for exchanging emergency alerts and public warnings. CAP is mainly for exchanging emergency alerts and public warnings. CAP is mainly
used for conveying alerts and warnings between authorities and from used for conveying alerts and warnings between authorities and from
authorities to citizen/individuals. This document describes how authorities to citizen/individuals. This document describes how
data-only emergency calls are able to utilize the same CAP document data-only emergency calls are able to utilize the same CAP document
format. format.
Emergency alerts containing data are similar to regular emergency
calls in the sense that they require emergency call routing
functionality and may even have the same location requirements. On
the other hand, the communication interaction may occur without
establishment of a voice or video channel.
Data-only emergency alerts are similar to regular emergency calls in Data-only emergency alerts are similar to regular emergency calls in
the sense that they require emergency call routing functionality and the sense that they require emergency call routing functionality and
may even have the same location requirements. On the other hand, the may even have the same location requirements. On the other hand, the
initial communication interaction will not lead to the establishment initial communication interaction will not lead to the establishment
of a voice or video channel. of a voice or video channel.
Based on the deployment experience with non-IP based systems we Based on the deployment experience with non-IP based systems, two
distinguish between two types of environments, namely (1) data-only major deployment scenarios are envisaged:
emergency alerts that are targeted directly to a recipient
responsible for evaluating the alerts and for taking the necessary 1. Emergency alerts containing only data are targeted to a recipient
steps, including triggering an emergency call towards a Public Safety responsible for evaluating the next steps, which could include:
Answering Point (PSAP) and (2) alerts that are targeted to a Service
URN as used for regular IP-based emergency calls where the recipient 1. Sending an alert containing only data toward a Public Safety
is not known to the originator. We describe these two cases in more Answering Point (PSAP);
detail in Section 3.
2. Establishing an emergency call with a PSAP that could include
audio/video as well as data
2. Emergency alerts targeted to a Service URN used for IP-based
emergency calls where the recipient is not known to the
originator. In this scenario, the alert may contain only data
(e.g. a CAP and a PIDF-LO payload in a SIP MESSAGE) or could be
included along with establishment of an audio/video channel (e.g.
SIP INVITE)
We describe these two cases in more detail in Section 3.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
This document utilizes terminology introduced in This document utilizes terminology introduced in
[I-D.ietf-atoca-requirements]. [I-D.ietf-atoca-requirements]. In particular, the terms for author,
originator, receiver and recipient, are relevant for this document.
The originator and the receiver are SIP-based entities while the
author and the recipient are entities that relate to the alert
message delivery, when this is relevant for the communication.
3. Architectural Overview 3. Architectural Overview
This section illustrates two envisioned usage modes; targeted and This section illustrates two envisioned usage modes; targeted and
location-based emergency alert routing. Figure 1 shows a deployment location-based emergency alert routing. Figure 1 shows a deployment
variant where a sensor, as the author and originator of the alert, is variant where a sensor, as the author and originator of the alert, is
pre-configured (using techniques outside the scope of this document) pre-configured (using techniques outside the scope of this document)
to issue an alert to a receiver or an aggregator, a special form of to issue an alert to a receiver or an aggregator, a special form of
mediator, that processes these messages and performs whatever steps mediator, that processes these messages and performs whatever steps
are necessary to appropriately react on the alert. For example, a are necessary to appropriately react on the alert. For example, a
security firm may use different sensor inputs to dispatch their security firm may use different sensor inputs to dispatch their
security staff to a building they protect. security staff to a building they protect or to initiate a third
party emergency call.
+------------+ +------------+ +------------+ +------------+
| Sensor | | Aggregator | | Sensor | | Aggregator |
| | | | | | | |
+---+--------+ +------+-----+ +---+--------+ +------+-----+
| | | |
Sensors | Sensors |
trigger | trigger |
emergency | emergency |
alert | alert |
skipping to change at page 5, line 41 skipping to change at page 5, line 42
| emergency | emergency
| alert | alert
| 200 (OK) | | 200 (OK) |
|<-----------------------------| |<-----------------------------|
| | | |
| | | |
Figure 1: Targeted Emergency Alert Routing Figure 1: Targeted Emergency Alert Routing
In Figure 2 a scenario is shown whereby the alert is routed using In Figure 2 a scenario is shown whereby the alert is routed using
location information and the Service URN. In case the LoST location information and the Service URN. An emergency services
resolution is done at an emergency services routing proxy rather than routing proxy (ESRP) may use LoST to determine the next hop proxy to
at the entity issuing the alert since it may not know the address of route the alert message to. A possible receiver is a PSAP and the
the receiver. A possible receiver is a PSAP and the recipient of the recipient of the alert may be call taker. In the generic case, there
alert may be call taker. In the generic case, there is very likely is very likely no prior relationship between the originator and the
no prior relationship between the originator and the receiver, e.g. receiver, e.g. PSAP. A PSAP, for example, is likely to receive and
PSAP. A PSAP, for example, is likely to receive and accept alerts accept alerts from entities it cannot authorize. This scenario
from entities it cannot authorize. This scenario corresponds more to corresponds more to the classical emergency services use case and the
the classical emergency services use case and the description in description in [I-D.ietf-ecrit-phonebcp] is applicable.
[I-D.ietf-ecrit-phonebcp] is applicable.
+-----------+ +----------+ +-----------+ +----------+
+--------+ | SIP Proxy | | PSAP as | +--------+ | ESRP | | PSAP |
| Sensor | | as Relay | | Receiver | | Sensor | | | | |
+---+----+ +---+-------+ +---+------+ +---+----+ +---+-------+ +---+------+
| | | | | |
Sensors | | Sensors | |
trigger | | trigger | |
emergency | | emergency | |
alert | | alert | |
| | | | | |
| | | | | |
| MESSAGE with CAP | | | MESSAGE with CAP | |
| (including Service URN, | | (including Service URN, |
| such as urn:service:sos) | | such as urn:service:sos) |
|------------------->| | |------------------->| |
| | | | | |
| SIP Proxy performs | | ESRP performs |
| emergency alert | | emergency alert |
| routing | | routing |
| | MESSAGE with CAP | | | MESSAGE with CAP |
| | (including identity info) | | | (including identity info) |
| |----------------------------->| | |----------------------------->|
| | | | | |
| | PSAP | | PSAP
| | processes | | processes
| | emergency | | emergency
| | alert | | alert
skipping to change at page 7, line 9 skipping to change at page 7, line 9
|<-------------------| | |<-------------------| |
| | | | | |
| | | | | |
Figure 2: Location-Based Emergency Alert Routing Figure 2: Location-Based Emergency Alert Routing
4. Protocol Specification 4. Protocol Specification
4.1. CAP Transport 4.1. CAP Transport
Since alerts structured via CAP require a "push" medium, they SHOULD Since alerts structured via CAP require a "push" medium. The
be sent via the SIP MESSAGE. The MIME type is set to 'application/ following SIP requests MAY carry the CAP payload defined in this
common-alerting-protocol+xml'. document: INVITE [RFC3261], UPDATE [RFC3311], MESSAGE [RFC3428], INFO
[RFC6086], NOTIFY [RFC3265], and PUBLISH [RFC3903]. The MIME type is
set to 'application/cap+xml'.
Alternatively, the SIP PUBLISH mechanism or other SIP messages If the server does not support the functionality required to fulfill
could be used. However, the usage of SIP MESSAGE is a simple the request then a 501 Not Implemented MUST be returned by RFC 3261
enough approach from an implementation point of view. [RFC3261]. This is the appropriate response when a 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 by RFC 3261
[RFC3261] if the 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 field, depending on the specific problem with the
content.
4.2. Profiling of the CAP Document Content 4.2. Profiling of the CAP Document Content
The usage of CAP MUST conform to the specification provided with The usage of CAP MUST conform to the specification provided with
[cap]. For the usage with SIP the following additional requirements [cap]. For the usage with SIP the following additional requirements
are imposed: are imposed:
sender: When the CAP was created by a SIP-based entity then the sender: A few sub-categories for putting a value in the <sender>
element MUST be populated with the SIP URI of that entity. element have to be considered:
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: In
case that the alert was created by a non-SIP based entity and
the identity of this original sender wants to be preserved then
this identity MUST be placed into the <sender> element. In
this category the it is not useful to be explicit about the
author of the alert. The specific type of identity being used
will depends on the technology being used by the original
originator.
Author indication relevant: In case the author is different from
the actual originator of the message and this distinction wants
to be preserved then the <sender> element MUST NOT contain the
SIP URI.
incidents: The <incidents> element MUST be present whenever there is incidents: The <incidents> element MUST be present whenever there is
a possibility that alert information needs to be updated. The a possibility that alert information needs to be updated. The
initial message will then contain an incident identifier carried initial message will then contain an incident identifier carried
in the <incidents> element. This incident identifier MUST be in the <incidents> element. This incident identifier MUST be
chosen in such a way that it is unique for a given <sender, chosen in such a way that it is unique for a given <sender,
expires, incidents> combination. Note that the <expires> element expires, incidents> combination. Note that the <expires> element
is optional and may not be present. is optional and may not be present.
scope: The value of the <scope> element MUST be set to "private" as scope: The value of the <scope> element MUST be set to "Private" as
the alert is not meant for public consumption. The <addresses> the alert is not meant for public consumption. The <addresses>
element is, however, not used by this specification since the element is, however, not used by this specification since the
message routing is performed by SIP and the respective address message routing is performed by SIP and the respective address
information is already available in the geolocation header. information is already available in other SIP headers. Populating
Populating location information twice into different parts of the information twice into different parts of the message may lead to
message can quickly lead to inconsistency. inconsistency.
parameter: The <parameter> element MAY contain additional parameter: The <parameter> element MAY contain additional
information specific to the sensor. information specific to the sensor.
area: It is RECOMMENDED to omit this element when constructing a area: It is RECOMMENDED to omit this element when constructing a
message. In case that the CAP message already contained an <area> message. In case that the CAP message already contained an <area>
element then the specified location information MUST be copied element then the specified location information MUST be copied
into the PIDF-LO structure of the geolocation header element. into the PIDF-LO structure of the 'geolocation' header.
5. Example 5. Error Handling
Figure 3 shows a CAP document indicating a BURLARY alert issued by a This section defines a new error response code and a header field for
sensor with the identity 'sensor1@domain.com'. The location of the additional information.
sensor can be obtained from the attached geolocation information
provided via the geolocation header contained in the SIP MESSAGE
structure. Additionally, the sensor provided some data long with the
alert message using proprietary information elements only to be
processed by the receiver, a SIP entity acting as an aggregator.
This example reflects the description in Figure 1.
MESSAGE sip:aggregator@domain.com SIP/2.0 5.1. 425 (Bad Alert Message) Response Code
Via: SIP/2.0/TCP sensor1.domain.com;branch=z9hG4bK776sgdkse
Max-Forwards: 70
From: sip:sensor1@domain.com;tag=49583
To: sip:aggregator@domain.com
Call-ID: asd88asd77a@1.2.3.4
Geolocation: <cid:abcdef@domain.com>
;routing-allowed=yes
Supported: geolocation
Accept: application/pidf+xml, application/common-alerting-protocol+xml
CSeq: 1 MESSAGE
Content-Type: multipart/mixed; boundary=boundary1
Content-Length: ...
--boundary1 This SIP extension creates a new location-specific response code,
defined as follows,
Content-Type: common-alerting-protocol+xml 425 (Bad Alert Message)
Content-ID: <abcdef2@domain.com>
<?xml version="1.0" encoding="UTF-8"?>
<alert xmlns="urn:oasis:names:tc:emergency:cap:1.1"> The 425 response code is a rejection of the request due to its
<identifier>S-1</identifier> included alert content, indicating that it was malformed or not
<sender>sip:sensor1@domain.com</sender> satisfactory for the recipient's purpose.
<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 A SIP intermediary can also reject an alert it receives from a UA
when it understands that the provided alert is malformed.
Content-Type: application/pidf+xml Section 5.2 describes a AlertMsg-Error header field with more details
Content-ID: <abcdef2@domain.com> about what was wrong with the alert message in the request. This
<?xml version="1.0" encoding="UTF-8"?> header field MUST be included in the 425 response.
<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
xmlns:cl="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr"
xmlns:gml="http://www.opengis.net/gml"
entity="pres:sensor1@domain.com">
<tuple id="12345">
<dm:device id="sensor1">
<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>
<gp:retransmission-allowed>yes
</gp:retransmission-allowed>
<gp:retention-expiry>2010-07-30T20:00:00Z
</gp:retention-expiry>
</gp:usage-rules>
<gp:method>802.11</gp:method>
</gp:geopriv>
<dm:deviceID>mac:1234567890ab</dm:deviceID>
<dm:timestamp>2010-07-28T20:57:29Z</dm:timestamp>
</dm:device>
</tuple>
</presence>
--boundary1-- It is only appropriate to generate a 425 response when the responding
entity has no other information in the request that are usable by the
responder.
Figure 3: Example Message conveying an Alert A 425 response code MUST NOT be sent in response to a request that
lacks an alert message entirely, as the user agent in that case may
not support this extension at all.
6. Security Considerations A 425 response is a final response within a transaction, and MUST NOT
terminate an existing dialog.
This section discusses security considerations when using SIP to make 5.2. The AlertMsg-Error Header Field
data-only emergency alerts utilizing CAP. Location specific threats
are not unique to this document and the discussion in
[I-D.ietf-ecrit-trustworthy-location].
6.1. Forgery The AlertMsg-Error header provides additional information about what
was wrong with the original request. In some cases the provided
information will be used for debugging purposes.
Threat: The AlertMsg-Error header field has the following ABNF [RFC5234]:
An adversary could forge or alter a CAP document to report false message-header /= AlertMsg-Error
emergency alarms. ; (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
Countermeasures: HCOLON, SEMI, and EQUAL are defined in RFC3261 [RFC3261]. DIGIT is
defined in RFC5234 [RFC5234].
To avoid this kind of attack, the entities must assure that proper The AlertMsg-Error header field MUST contain only one ErrorValue to
mechanisms for protecting the CAP documents are employed, e.g., indicate what was wrong with the alert payload the recipient
signing the CAP document itself. Section 3.3.2.1 of [cap] determined was bad.
specifies the signing of CAP documents. This does not protect
against a legitimate sensor sending phrank alerts after being
compromised.
6.2. Replay Attack 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 understandable.
The text string are 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.
Threat: The AlertMsg-Error header field MAY be included in any response as an
alert message was in the request part of the same transaction. For
example, a UA includes an alert in an MESSAGE to a PSAP. The PSAP
can accept this MESSAGE, thus creating a dialog, even though his UA
determined the alert message contained in the MESSAGE was bad. The
PSAP merely includes a AlertMsg-Error header value in the 200 OK to
the MESSAGE informing the UA that the MESSAGE was accepted but the
alert provided was bad.
An adversary could eavesdrop alerts and reply them at a later If, on the other hand, the PSAP cannot accept the MESSAGE without a
time. suitable alert message, a 425 response is sent.
Countermeasures: A SIP intermediary that requires the UA's alert message in order to
properly process the MESSAGE may also sends a 425 with a AlertMsg-
Error code.
A CAP document contains the mandatory <identifier>, <sender>, This document defines an initial list of error code ranges for any
<sent> elements and an optional <expire> element. These SIP response, including provisional responses (other than 100 Trying)
attributes make the CAP document unique for a specific sender and and the new 425 response. There MUST be no more than one AlertMsg-
provide time restrictions. An entity that has received a CAP Error code in a SIP response.
message already 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 [RFC4474], to tie the CAP message
to the SIP message.
6.3. Injecting False Alerts AlertMsg-Error: 100 ; code="Cannot Process the Alert Payload"
Threat: AlertMsg-Error: 101 ; code="Alert Payload was not present or could
not be found"
When an entity receives a CAP message it has to determine whether AlertMsg-Error: 102 ; code="Not enough information to determine the
the entity distributing the CAP messages is genuine to avoid purpose of the alert"
accepting messages that are injected by adversaries. In scenario
Countermeasures: AlertMsg-Error: 103 ; code="Alert Payload was corrupted"
For some types of data-only emergency calls author/originator and Additionally, if an LR cannot or chooses not to process the alert
the receiver/recipient have a relationship with each other and message from a SIP request, a 500 (Server Internal Error) SHOULD be
hence it is possible (using cryptographic techniques) to verify used with or without a configurable Retry-After header field.
whether a message was indeed issued by an authorized entity.
Figure 1 is such an environment. Standard SIP security mechanisms
can be re-used for this purpose. For example, identity based
access control is a viable approach utilizing the asserted
identity of the alert originator using P-Asserted-Identity
[RFC3325] or SIP Identity [RFC4474].
There are, however, other types of data-only emergency calls where 6. Example
there is no such relationship between the author/originator and
the receiver/recipient. Incoming alerts need to be treated more
carefully than multi-media emergency calls that contain additional
information, such as audio, to allow a call taker to sort out
phrank calls.
7. IANA Considerations Figure 3 shows a CAP document indicating a BURLARY alert issued by a
sensor with the identity 'sensor1@domain.com'. The location of the
sensor can be obtained from the attached location information
provided via the 'geolocation' header contained in the SIP MESSAGE
structure. Additionally, the sensor provided some data long with the
alert message using proprietary information elements only to be
processed by the receiver, a SIP entity acting as an aggregator.
This example reflects the description in Figure 1.
7.1. Registration of the 'application/common-alerting-protocol+xml' MESSAGE sip:aggregator@domain.com SIP/2.0
MIME type Via: SIP/2.0/TCP sensor1.domain.com;branch=z9hG4bK776sgdkse
Max-Forwards: 70
From: sip:sensor1@domain.com;tag=49583
To: sip:aggregator@domain.com
Call-ID: asd88asd77a@1.2.3.4
Geolocation: <cid:abcdef@domain.com>
;routing-allowed=yes
Supported: geolocation
Accept: application/pidf+xml, application/cap+xml
CSeq: 1 MESSAGE
Content-Type: multipart/mixed; boundary=boundary1
Content-Length: ...
--boundary1
Content-Type: cap+xml
Content-ID: <abcdef2@domain.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@domain.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@domain.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 3: Example Message conveying an Alert
7. Security Considerations
This section discusses security considerations when SIP user agents
issue emergency alerts utilizing CAP. Location specific threats are
not unique to this document and are discussed in
[I-D.ietf-ecrit-trustworthy-location] and
[I-D.ietf-sipcore-location-conveyance].
The ECRIT emergency services architecture [I-D.ietf-ecrit-phonebcp]
considers classical individual-to-authority emergency calling and 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
will not depend on the identity of the caller. In case of emergency
alerts generated by devices, like sensors, the processing may be
different in order to reduce the number of falsely generated
emergency alerts. Alerts may get triggered based on certain sensor
input that may have been caused by other factors than the actual
occurrence of an alert relevant event. For example, a sensor may
simply be malfunctioning. For this purpose not all alert messages
are directly sent to a PSAP but are rather 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. These two message
routing examples are shown in Figure 1 and in Figure 2.
In any case, for alerts that are initiated by sensors the identity
may 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 purpose it needs to be ensured that no alert messages from an
unknown origin are accepted. Two types of information elements can
be used for this purpose:
1. SIP itself provides security mechanisms that allow the
verification of the originator's identity. These mechanisms can
be re-used, such as P-Asserted-Identity [RFC3325] or SIP Identity
[RFC4474]. The latter provides a cryptographic assurance while
the former relies on a chain of trust model.
2. CAP provides additional security mechanisms and the ability to
carry additional information about the sender's identity.
Section 3.3.2.1 of [cap] specifies the signing algorithms of CAP
documents.
In addition to the desire to perform identity-based access control
the classical communication security threats need to be considered,
including integrity protection to prevent forgery and 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. These attributes make the CAP document
unique for a specific sender and provide time restrictions. An
entity that has received a CAP message already 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 [RFC4474], 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
mandatory.
Note that none of the security mechanism in this document protect
against a compromised sensor sending crafted alerts.
8. IANA Considerations
8.1. Registration of the 'application/cap+xml' MIME type
To: ietf-types@iana.org To: ietf-types@iana.org
Subject: Registration of MIME media type application/ common- Subject: Registration of MIME media type application/ cap+xml
alerting-protocol+xml
MIME media type name: application MIME media type name: application
MIME subtype name: common-alerting-protocol+xml MIME subtype name: cap+xml
Required parameters: (none) Required parameters: (none)
Optional parameters: charset; Indicates the character encoding of Optional parameters: charset; Indicates the character encoding of
enclosed XML. Default is UTF-8 [RFC3629]. enclosed XML. Default is UTF-8 [RFC3629].
Encoding considerations: Uses XML, which can employ 8-bit Encoding considerations: Uses XML, which can employ 8-bit
characters, depending on the character encoding used. See RFC characters, depending on the character encoding used. See RFC
3023 [RFC3023], Section 3.2. 3023 [RFC3023], Section 3.2.
skipping to change at page 13, line 9 skipping to change at page 17, line 5
Published specification: RFC XXX [Replace by the RFC number of this Published specification: RFC XXX [Replace by the RFC number of this
specification]. specification].
Applications which use this media type: Applications that convey Applications which use this media type: Applications that convey
alerts and warnings according to the CAP standard. alerts and warnings according to the CAP standard.
Additional information: OASIS has published the Common Alerting Additional information: OASIS has published the Common Alerting
Protocol at http://www.oasis-open.org/committees/ Protocol at http://www.oasis-open.org/committees/
documents.php&wg_abbrev=emergency documents.php&wg_abbrev=emergency
Person & email address to contact for further information: Hannes Person and email address to contact for further information: Hannes
Tschofenig, Hannes.Tschofenig@nsn.com Tschofenig, Hannes.Tschofenig@nsn.com
Intended usage: Limited use Intended usage: Limited use
Author/Change controller: IETF SIPPING working group Author/Change controller: IETF ECRIT working group
Other information: This media type is a specialization of Other information: This media type is a specialization of
application/xml RFC 3023 [RFC3023], and many of the considerations application/xml RFC 3023 [RFC3023], and many of the considerations
described there also apply to application/ described there also apply to application/cap+xml.
common-alerting-protocol+xml.
8. Acknowledgments 8.2. 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.
8.3. IANA Registration of New AlertMsg-Error Header Field
The SIP AlertMsg-error header field is created by this document, with
its definition and rules in Section 5, to be added to the IANA sip-
parameters registry with two actions:
1. Update the Header Fields registry with
Registry:
Header Name compact Reference
----------------- ------- ---------
AlertMsg-Error [this doc]
2. In the portion titled "Header Field Parameters and Parameter
Values", add
Predefined
Header Field Parameter Name Values Reference
----------------- ------------------- ---------- ---------
AlertMsg-Error code yes [this doc]
8.4. IANA Registration for the SIP AlertMsg-Error Codes
This document creates a new registry for SIP, called "AlertMsg-Error
Codes". AlertMsg-Error codes provide reason for the error discovered
by recipients, categorized by action to be taken by 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.
9. Acknowledgments
The authors would like to thank the participants of the Early Warning The authors would like to thank the participants of the Early Warning
adhoc meeting at IETF#69 for their feedback. Additionally, we would adhoc meeting at IETF#69 for their feedback. Additionally, we would
like to thank the members of the NENA Long Term Direction Working like to thank the members of the NENA Long Term Direction Working
Group for their feedback. Group for their feedback.
9. References Additionally, we would like to thank Martin Thomson, James
Winterbottom, Shida Schubert, Bernard Aboba, and Marc Linsner for
their review comments.
9.1. Normative References 10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", March 1997. Requirement Levels", March 1997.
[cap] Jones, E. and A. Botterell, "Common Alerting Protocol v. [cap] Jones, E. and A. Botterell, "Common Alerting Protocol v.
1.1", October 2005. 1.1", October 2005.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension
for Event State Publication", RFC 3903, October 2004.
[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media [RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Types", RFC 3023, January 2001. Types", RFC 3023, January 2001.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003. 10646", STD 63, RFC 3629, November 2003.
[I-D.ietf-ecrit-trustworthy-location]
Tschofenig, H., Schulzrinne, H., and B. Aboba,
"Trustworthy Location Information",
draft-ietf-ecrit-trustworthy-location-01 (work in
progress), October 2010.
9.2. Informative References
[I-D.ietf-ecrit-phonebcp] [I-D.ietf-ecrit-phonebcp]
Rosen, B. and J. Polk, "Best Current Practice for Rosen, B. and J. Polk, "Best Current Practice for
Communications Services in support of Emergency Calling", Communications Services in support of Emergency Calling",
draft-ietf-ecrit-phonebcp-15 (work in progress), draft-ietf-ecrit-phonebcp-17 (work in progress),
July 2010. March 2011.
[I-D.ietf-sipcore-location-conveyance]
Polk, J., Rosen, B., and J. Peterson, "Location Conveyance
for the Session Initiation Protocol",
draft-ietf-sipcore-location-conveyance-08 (work in
progress), May 2011.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
UPDATE Method", RFC 3311, October 2002.
[RFC3428] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.,
and D. Gurle, "Session Initiation Protocol (SIP) Extension
for Instant Messaging", RFC 3428, December 2002.
[RFC6086] Holmberg, C., Burger, E., and H. Kaplan, "Session
Initiation Protocol (SIP) INFO Method and Package
Framework", RFC 6086, January 2011.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension
for Event State Publication", RFC 3903, October 2004.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
10.2. Informative References
[I-D.ietf-atoca-requirements] [I-D.ietf-atoca-requirements]
Schulzrinne, H., Norreys, S., Rosen, B., and H. Schulzrinne, H., Norreys, S., Rosen, B., and H.
Tschofenig, "Requirements, Terminology and Framework for Tschofenig, "Requirements, Terminology and Framework for
Exigent Communications", draft-ietf-atoca-requirements-00 Exigent Communications", draft-ietf-atoca-requirements-01
(work in progress), September 2010. (work in progress), January 2011.
[I-D.ietf-ecrit-trustworthy-location]
Tschofenig, H., Schulzrinne, H., and B. Aboba,
"Trustworthy Location Information",
draft-ietf-ecrit-trustworthy-location-02 (work in
progress), May 2011.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for [RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006. Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private [RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325, Asserted Identity within Trusted Networks", RFC 3325,
November 2002. November 2002.
 End of changes. 62 change blocks. 
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