One document matched: draft-schulzrinne-ecrit-psap-callback-00.xml
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<front>
<title abbrev="PSAP Callback Marking">Marking of Calls initiated by Public Safety Answering
Points (PSAPs)</title>
<author initials="H." surname="Schulzrinne" fullname="Henning Schulzrinne">
<organization>Columbia University</organization>
<address>
<postal>
<street>Department of Computer Science</street>
<street>450 Computer Science Building</street>
<city>New York</city>
<region>NY</region>
<code>10027</code>
<country>US</country>
</postal>
<phone>+1 212 939 7004</phone>
<email>hgs+ecrit@cs.columbia.edu</email>
<uri>http://www.cs.columbia.edu</uri>
</address>
</author>
<author initials="H." surname="Tschofenig" fullname="Hannes Tschofenig">
<organization>Nokia Siemens Networks</organization>
<address>
<postal>
<street>Linnoitustie 6</street>
<city>Espoo</city>
<code>02600</code>
<country>Finland</country>
</postal>
<phone>+358 (50) 4871445</phone>
<email>Hannes.Tschofenig@gmx.net</email>
<uri>http://www.tschofenig.priv.at</uri>
</address>
</author>
<date year="2009"/>
<workgroup>ECRIT</workgroup>
<abstract>
<t>After an emerency call is completed it is possible that the need for further
communication between the call-taker and the emergency caller arises. For example,
further assistance may be needed but the communication previously got interrupted. A
call-taker may trigger a callback towards the emergency caller using the contact
information provided with the initial emergency call. This callback would then be
treated like any other call. As a consequence, it may get blocked by authorization
policies configured by the person seeking help or may get forwarded to his answering
machine.</t>
<t>The current ECRIT framework document addresses callbacks in a limited fashion and
thereby covers a few scenarios. This document discusses shortcomings and raises the
question whether additional solution techniques are needed.</t>
</abstract>
</front>
<middle>
<!-- ****************************************************************************************** -->
<section anchor="intro" title="Introduction">
<t>Summoning police, the fire department or an ambulance in emergencies is one of the
fundamental and most-valued functions of the telephone. As telephone functionality
moves from circuit-switched telephony to Internet telephony, its users rightfully
expect that this core functionality will continue to work at least as well as it has
for the legacy technology. New devices and services are being made available that
could be used to make a request for help, which are not traditional telephones, and
users are increasingly expecting them to be used to place emergency calls.</t>
<t>Regulatory requirements demand that the emergency call itself provides enough
information to allow the call-taker to initiate a call back to the emergency caller
in case the call dropped or to interact with the emergency caller later in case of
questions. Such a call, referred as PSAP callback subsequently in this document,
may, however, be blocked or forwarded to an answering machine as SIP entities (SIP
proxies as well as the SIP UA itself) cannot associate the potential importantance
of the call based on the SIP signaling.</t>
<t>
<list style="empty">
<t>Note that the authors are, however, not aware of regulatory requirements for
providing preferential treatment of callbacks initiated by the call-taker at
the PSAP towards the emergency caller nor that these calls have to be
treated in any form differently from any other call.</t>
</list>
</t>
<t>Section 10 of <xref target="I-D.ietf-ecrit-framework"/> discusses the identifiers
required for callbacks. Section 13 of <xref target="I-D.ietf-ecrit-framework"/>
provides the following guidance regarding callback handling:</t>
<t>
<list style="empty">
<t> A UA may be able to determine a PSAP call back by examining the domain of
incoming calls after placing an emergency call and comparing that to the
domain of the answering PSAP from the emergency call. Any call from the same
domain and directed to the supplied Contact header or AoR after an emergency
call should be accepted as a call-back from the PSAP if it occurs within a
reasonable time after an emergency call was placed. </t>
</list>
</t>
<t>This approach mimics a stateful packet filtering firewall and is indeed helpful in a
number of cases but it may fail in others. Below, we discuss a few cases where this
approach fails. </t>
<section title="Multi-Stage Resolution">
<t>Consider the following emergency call routing scenario shown in <xref
target="stages"/> where routing towards the PSAP occurs in several stages.
An emergency call uses a SIP UA that does not run LoST on the end point. Hence,
the call is marked with the 'urn:service:sos' Service URN <xref target="RFC5031"
/>. The user's VoIP provider receives the emergency call and determines where to
route it. Local configuration or a LoST lookup might, in our example, reveal
that emergency calls are routed via a dedicated provider FooBar and targeted to
a specific entity, referred as esrp1@foobar.com. FooBar does not handle
emergency calls itself but performs another resolution step to let calls enter
the emergency services network and in this case another resolution step takes
place and esrp-a@esinet.org is determined as the recipient, pointing to an edge
device at the IP-based emergency services network. Inside the emergency services
there might be more sophisticated routing taking place somewhat depending on the
existing structure of the emergency services infrastructure.</t>
<t/>
<t>
<figure anchor="stages" title="Multi-Stage Resolution">
<artwork><![CDATA[
,-------.
+----+ ,' `.
| UA |--- urn:service:sos / Emergency \
+----+ \ | Services |
\ ,-------. | Network |
,' `. | |
/ VoIP \ | |
( Provider ) | |
\ / | |
`. ,' | |
'---+---' | +------+ |
| | |PSAP | |
esrp1@foobar.com | +--+---+ |
| | | |
| | | |
,---+---. | | |
,' `. | | |
/ Provider \ | | |
+ FooBar ) | | |
\ / | | |
`. ,' | +--+---+ |
'---+---' | +-+ESRP | |
| | | +------+ |
| | | |
+------------+-+ |
esrp-a@esinet.org | |
\ /
`. ,'
'-------'
]]></artwork>
</figure>
</t>
</section>
<section title="Call Forwarding">
<t>Imagine the following case where an emergency call enters an emergency network
(state.org) via an ERSP but then gets forwarded to a different emergency
services network (in our example to police-town.org, fire-town.org or
medic-town.org). The same considerations apply when the the police, fire and
ambulance networks are part of the state.org sub-domains (e.g.,
police.state.org). </t>
<t>
<figure anchor="fwd" title="Call Forwarding">
<artwork><![CDATA[
,-------.
,' `.
/ Emergency \
| Services |
| Network |
| (state.org) |
| |
| |
| +------+ |
| |PSAP +--+ |
| +--+---+ | |
| | | |
| | | |
| | | |
| | | |
| | | |
| +--+---+ | |
------------------+---+ESRP | | |
esrp-a@state.org | +------+ | |
| | |
| Call Fwd | |
| +-+-+---+ |
\ | | | /
`. | | | ,'
'-|-|-|-' ,-------.
Police | | | Fire ,' `.
+------------+ | +----+ / Emergency \
,-------. | | | | Services |
,' `. | | | | Network |
/ Emergency \ | Ambulance | | fire-town.org |
| Services | | | | | |
| Network | | +----+ | | +------+ |
|police-town.org| | ,-------. | +----+---+PSAP | |
| | | ,' `. | | +------+ |
| +------+ | | / Emergency \ | | |
| |PSAP +----+--+ | Services | | | ,
| +------+ | | Network | | `~~~~~~~~~~~~~~~
| | |medic-town.org | |
| , | | |
`~~~~~~~~~~~~~~~ | +------+ | |
| |PSAP +----+ +
| +------+ |
| |
| ,
`~~~~~~~~~~~~~~~
]]></artwork>
</figure>
</t>
</section>
<section title="PSTN Interworking">
<t>In case an emergency call enters the PSTN, as shown in <xref target="pstn"/>,
there is no guarantee that the callback some time later does leave the same
PSTN/VoIP gateway or that the same end point identifier is used in the forward
as well as in the backward direction making it difficult to reliably detect PSAP
callbacks.</t>
<t>
<figure anchor="pstn" title="PSTN Interworking">
<artwork><![CDATA[
+-----------+
| PSTN |-------------+
| Calltaker | |
| Bob |<--------+ |
+-----------+ | v
-------------------
//// \\\\ +------------+
| | |PSTN / VoIP |
| PSTN |---->|Gateway |
\\\\ //// | |
------------------- +----+-------+
^ |
| |
+-------------+ | +--------+
| | | |VoIP |
| PSTN / VoIP | +->|Service |
| Gateway | |Provider|
| |<------Invite----| Y |
+-------------+ +--------+
| ^
| |
Invite Invite
| |
V |
+-------+
| SIP |
| UA |
| Alice |
+-------+
]]></artwork>
</figure>
</t>
</section>
</section>
<!-- ****************************************************************************************** -->
<section anchor="terminology" title="Terminology">
<t>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 <xref target="RFC2119"/>.</t>
<t>Emergency services related terminology is borrowed from <xref target="RFC5012"/>.</t>
</section>
<!-- ****************************************************************************************** -->
<section title="Requirements">
<t>From the previously presented scenarios, the following generic requirements can be
crafted:</t>
<t>
<list style="hanging">
<t hangText="Reliable Identification:"><vspace blankLines="1"/> The solution
approach MUST offer a way to reliable detect a PSAP callback in light of the
challenges presented in <xref target="intro"/>. </t>
<t hangText="Resistance Against Security Vulnerabilities:"><vspace
blankLines="1"/> The main possibility of attack involves use of the PSAP
callback marking to bypass blacklists, ignore call forwarding procedures and
similar features to interact with users and to raise their attention. For
example, using PSAP callback marking devices would be able to recognize
these types of incoming messages leading to the device overriding user
interface configurations, such as vibrate-only mode. As such, the
requirement is ensure that only PSAPs can issue callbacks. This may require
secure identification of the calling party. </t>
<t hangText="Fallback to Normal Call"><vspace blankLines="1"/>When the newly
defined extension is not recognized by intermediaries or other entities then
it MUST NOT lead to a failure of the call handling procedure but rather a
fall-back to a call that did not have any marking provided.</t>
</list>
</t>
<t>A further differentiation has to be made with respect to relationship between the
person who previously received the emergency call and the person who triggers the
callback. The choices are: <list style="symbols">
<t>The callback has to be made using the same UA. </t>
<t>The callback has to made by the same user but potentially with a different
UA.</t>
<t>A different user from a different UA can make the callback.</t>
</list>
</t>
<t>[Editor' Note: A requirement has to be formulated.]</t>
</section>
<!-- ****************************************************************************************** -->
<section title="Solution Approaches">
<t>This version of the document does not yet contain any specific solution approaches.
An example solution can be found in an earlier version of <xref
target="I-D.patel-ecrit-sos-parameter"/>. The usage of the In-Reply-To header is
another one.</t>
<t> Solution categories can be clustered into three areas: </t>
<t>
<list style="numbers">
<t>Verify that the caller is a PSAP </t>
<t>Verify that the call is related to an emergency, but not necessarily an
earlier emergency call. This might include public notification
(authority-to-citizen). </t>
<t>Verify that the call is returning an earlier emergency call.</t>
</list>
</t>
<t> These solution differ in their semantics and in the security impact or user
choice.</t>
</section>
<!-- ****************************************************************************************** -->
<section title="Security Considerations">
<t>This document provides discussions problems of PSAP callbacks and lists requirements,
some of which illustrate security challenges. The current version does not yet
provide a specific solution but rather starts with overall architectural
observations.</t>
<t>An important aspect from a security point of view is the relationship between the
emergency services network and the VSP (assuming that the emergency call travels via
the VSP and not directly between the SIP UA and the PSAP). If there is a strong
trust relationship between the PSAP operator and the VSP (for example based on a
peering relationship) without any intermediate VoIP providers then the
identification of a PSAP call back is less problematic than in the case where the
two entities have not entered in some form of relationship that would allow the VSP
to verify whether the marked callback message indeed came from a legitimate
source.</t>
</section>
<!-- ****************************************************************************************** -->
<section title="Acknowledgements">
<t>We would like to thank members from the ECRIT working group, in particular Brian
Rosen and Milan Patel, for their discussions around PSAP callbacks. </t>
</section>
<!-- ****************************************************************************************** -->
</middle>
<back>
<references title="Informative References"> &RFC2119; </references>
<references title="Informative References"> &I-D.patel-ecrit-sos-parameter;
&I-D.ietf-ecrit-framework; &RFC5031; &RFC5012; </references>
</back>
</rfc>
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