ECRIT H. Schulzrinne
Internet-Draft Columbia University
Intended status: Informational H. Tschofenig
Expires: April 29, 2010 Nokia Siemens Networks
M. Patel
Nortel
October 26, 2009
Public Safety Answering Point (PSAP) Callbacks
draft-schulzrinne-ecrit-psap-callback-01.txt
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Abstract
After an emergency call is completed (either prematurely terminated
by the emergency caller or normally by the call-taker) it is possible
that the call-taker feels the need for further communication or for a
clarification. For example, the call may have been dropped by
accident without the call-taker having sufficient information about
the current situation of a wounded person. A call-taker may trigger
a callback towards the emergency caller using the contact information
provided with the initial emergency call. This callback could, under
certain circumstances, then be treated like any other call and as a
consequence, it may get blocked by authorization policies or may get
forwarded to an answering machine.
The IETF emergency services architecture addresses callbacks in a
limited fashion and thereby covers a couple of scenarios. This
document discusses some shortcomings and raises the question whether
additional solution techniques are needed.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Multi-Stage Resolution . . . . . . . . . . . . . . . . . . 4
1.2. Call Forwarding . . . . . . . . . . . . . . . . . . . . . 6
1.3. PSTN Interworking . . . . . . . . . . . . . . . . . . . . 8
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 9
3. Requirements and Design Approaches . . . . . . . . . . . . . . 10
4. Solution Approaches . . . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1. Informative References . . . . . . . . . . . . . . . . . . 16
7.2. Informative References . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
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1. Introduction
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.
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 in case of further 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.
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.
Section 10 of [I-D.ietf-ecrit-framework] discusses the identifiers
required for callbacks, namely AOR URI and a globally routable URI in
a Contact: header. Section 13 of [I-D.ietf-ecrit-framework] provides
the following guidance regarding callback handling:
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.
This approach mimics a stateful packet filtering firewall and is
indeed helpful in a number of cases. Below, we discuss a few cases
where this approach fails.
1.1. Multi-Stage Resolution
Consider the following emergency call routing scenario shown in
Figure 1 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.
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Hence, the call is marked with the 'urn:service:sos' Service URN
[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.
,-------.
+----+ ,' `.
| UA |--- urn:service:sos / Emergency \
+----+ \ | Services |
\ ,-------. | Network |
,' `. | |
/ VoIP \ | |
( Provider ) | |
\ / | |
`. ,' | |
'---+---' | +------+ |
| | |PSAP | |
esrp1@foobar.com | +--+---+ |
| | | |
| | | |
,---+---. | | |
,' `. | | |
/ Provider \ | | |
+ FooBar ) | | |
\ / | | |
`. ,' | +--+---+ |
'---+---' | +-+ESRP | |
| | | +------+ |
| | | |
+------------+-+ |
esrp-a@esinet.org | |
\ /
`. ,'
'-------'
Figure 1: Multi-Stage Resolution
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1.2. Call Forwarding
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).
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,-------.
,' `.
/ 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 +----+ +
| +------+ |
| |
| ,
`~~~~~~~~~~~~~~~
Figure 2: Call Forwarding
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1.3. PSTN Interworking
In case an emergency call enters the PSTN, as shown in Figure 3,
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.
+-----------+
| PSTN |-------------+
| Calltaker | |
| Bob |<--------+ |
+-----------+ | v
-------------------
//// \\\\ +------------+
| | |PSTN / VoIP |
| PSTN |---->|Gateway |
\\\\ //// | |
------------------- +----+-------+
^ |
| |
+-------------+ | +--------+
| | | |VoIP |
| PSTN / VoIP | +->|Service |
| Gateway | |Provider|
| |<------Invite----| Y |
+-------------+ +--------+
| ^
| |
Invite Invite
| |
V |
+-------+
| SIP |
| UA |
| Alice |
+-------+
Figure 3: PSTN Interworking
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Emergency services related terminology is borrowed from [RFC5012].
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3. Requirements and Design Approaches
From the previously presented scenarios, the following generic
requirements can be crafted:
Resistance Against Security Vulnerabilities:
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 to ensure that the
mechanisms described in this document can not be used for
malicious purposes, including SPIT.
Fallback to Normal Call
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.
In addition to the high-level requirements there are a few design
choices.
What is the granularity of the decision making?
There are a few choices that impact the solution mechanism quite
considerably:
* Verify that the caller is a PSAP
* Verify that the call is in response to a previous emergency
call.
* Verify that the call is related to an emergency, but not
necessarily an earlier emergency call. This might include
public notification (authority-to-citizen).
Who calls back?
The relationship between the person who previously received the
emergency call and the person who triggers the callback allows a
couple of choices:
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* The callback has to be made using the same User Agent.
* The callback has to made by the same user but potentially with
a different UA.
* A different user from a different UA can make the callback.
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4. Solution Approaches
This version of the document does not yet contain a fully specified
solution description. Instead, it tries to explore the different
alternatives.
An example solution can be found in an earlier version of
[I-D.patel-ecrit-sos-parameter]. The "sos" URI parameter is appended
to the URI in the Contact header field of the INVITE request for PSAP
call-back establishment. Although this approach can distinguish the
PSAP call-back from other sessions, such a solution is prone to
security vulnerabilities since the insertion of the URI parameter
cannot verify the request was generated from a PSAP rather than a
malicious entity.
The usage of the In-Reply-To header field can provide the capability
to relate the PSAP call-back to a previously made emergency call.
The UA of the emergency caller, as well as enities within the service
provider's network can therefore infer that the request is a PSAP
callback, providing they maintained information pertaining to the
emergency call. This solution also relies on the PSAP call-back
routing over the same entities that the emergency call was routed
over if such a solution is used to provide preferential treatment of
callbacks. A solution based on the inclusion of the In-Reply-To
header would be useful in the case the network or the UA is required
to disable services or features which may prevent the callback from
reaching the UA from which the emergency call was placed.
Furthermore, it may facilitate success of the callback by removing,
for example, incoming call barring restrictions that may have been
enforced for the emergency caller's service.
To fulfill the requirements of verifying the caller is a PSAP,
mechanisms such as those described in RFC 4474 [RFC4474] or in RFC
3325 [RFC3325] are recommended to be used. Such an approach would
mitigate security vulnerabilities, but does not explicitly mark the
request generated from the PSAP as a request for callback.
Additional information, such a PSAP whitelist, would have to be
known. This is, however, only likely to work in a smaller scale
rather than world wide.
The use of the Calling Party's Category URI parameter in the
P-Asserted-Identity [RFC3325], as described in
[I-D.patel-dispatch-cpc-oli-parameter], is one method of a network
asserted identifier, describing the nature of the calling party and
in this case, the PSAP. This approach only works when the entity
that inserts the CPC parameter is trusted by those who verify it.
This relies on a circle of trust similar to the a white list.
Additionally, it has to be mentioned that unlike [I-D.ietf-sip-saml]
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applying SIP Identity over the parameter does not ensure that the
authentication service indeed asserts the validity of the parameter.
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5. Security Considerations
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.
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.
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6. Acknowledgements
We would like to thank members from the ECRIT working group, in
particular Brian Rosen, for their discussions around PSAP callbacks.
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7. References
7.1. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.2. Informative References
[I-D.ietf-ecrit-framework]
Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
"Framework for Emergency Calling using Internet
Multimedia", draft-ietf-ecrit-framework-10 (work in
progress), July 2009.
[I-D.ietf-sip-saml]
Tschofenig, H., Hodges, J., Peterson, J., Polk, J., and D.
Sicker, "SIP SAML Profile and Binding",
draft-ietf-sip-saml-06 (work in progress), March 2009.
[I-D.patel-dispatch-cpc-oli-parameter]
Patel, M., Jesske, R., and M. Dolly, "Uniform Resource
Identifier (URI) Parameters for indicating the Calling
Party's Catagory and Originating Line Identity",
draft-patel-dispatch-cpc-oli-parameter-00 (work in
progress), October 2009.
[I-D.patel-ecrit-sos-parameter]
Patel, M., "SOS Uniform Resource Identifier (URI)
Parameter for Marking of Session Initiation Protocol
(SIP) Requests related to Emergency Services",
draft-patel-ecrit-sos-parameter-06 (work in progress),
May 2009.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC5012] Schulzrinne, H. and R. Marshall, "Requirements for
Emergency Context Resolution with Internet Technologies",
RFC 5012, January 2008.
[RFC5031] Schulzrinne, H., "A Uniform Resource Name (URN) for
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Emergency and Other Well-Known Services", RFC 5031,
January 2008.
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Authors' Addresses
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
US
Phone: +1 212 939 7004
Email: hgs+ecrit@cs.columbia.edu
URI: http://www.cs.columbia.edu
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Milan Patel
Nortel
Maidenhead Office Park, Westacott Way
Maidenhead SL6 3QH
UK
Email: milanpa@nortel.com
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