ECRIT H. Schulzrinne
Internet-Draft Columbia University
Intended status: Informational H. Tschofenig
Expires: May 9, 2011 Nokia Siemens Networks
M. Patel
InterDigital Communications
November 5, 2010
Public Safety Answering Point (PSAP) Callbacks
draft-ietf-ecrit-psap-callback-02.txt
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 illustrates an extension.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 9, 2011.
Copyright Notice
Schulzrinne, et al. Expires May 9, 2011 [Page 1]
Internet-Draft PSAP Callback Marking November 2010
Copyright (c) 2010 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
(http://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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Routing Asymmetry . . . . . . . . . . . . . . . . . . . . 3
1.2. Multi-Stage Resolution . . . . . . . . . . . . . . . . . . 4
1.3. Call Forwarding . . . . . . . . . . . . . . . . . . . . . 5
1.4. PSTN Interworking . . . . . . . . . . . . . . . . . . . . 7
1.5. Network-based Service URN Resolution . . . . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 9
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 10
4. Callback Marking . . . . . . . . . . . . . . . . . . . . . . . 12
4.1. Tel URI . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2. SIP URI . . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Schulzrinne, et al. Expires May 9, 2011 [Page 2]
Internet-Draft PSAP Callback Marking November 2010
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. It is also relatively simple to
implement. Below, we discuss a few cases where this approach fails.
1.1. Routing Asymmetry
In some deployment environments it is common to have incoming and
outgoing SIP messaging to use different routes.
Schulzrinne, et al. Expires May 9, 2011 [Page 3]
Internet-Draft PSAP Callback Marking November 2010
,-------.
,' `.
,-------. / Emergency \
,' `. | Services |
/ VoIP \ I | Network |
| Provider | n | |
| | t | |
| | e | |
| +-------+ | r | |
+--+---|Inbound|<--+-----m | |
| | |Proxy | | e | +------+ |
| | +-------+ | d | |PSAP | |
| | | i | +--+---+ |
+----+ | | | a-+ | | |
| UA |<---+ | | t | | | |
| |----+ | | e | | | |
+----+ | | | | | | |
| | | P | | | |
| | | r | | | |
| | +--------+ | o | | | |
+--+-->|Outbound|--+---->v | | +--+---+ |
| |Proxy | | i | | +-+ESRP | |
| +--------+ | d | | | +------+ |
| | e || | |
| | r |+-+ |
\ / | |
`. ,' \ /
'-------' `. ,'
'-------'
Figure 1: Example for Routing Asymmetry
1.2. Multi-Stage Resolution
Consider the following emergency call routing scenario shown in
Figure 2 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
[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
Schulzrinne, et al. Expires May 9, 2011 [Page 4]
Internet-Draft PSAP Callback Marking November 2010
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 2: Example for Multi-Stage Resolution
1.3. 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).
Schulzrinne, et al. Expires May 9, 2011 [Page 5]
Internet-Draft PSAP Callback Marking November 2010
,-------.
,' `.
/ 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 3: Example for Call Forwarding
Schulzrinne, et al. Expires May 9, 2011 [Page 6]
Internet-Draft PSAP Callback Marking November 2010
1.4. PSTN Interworking
In case an emergency call enters the PSTN, as shown in Figure 4,
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 4: Example for PSTN Interworking
1.5. Network-based Service URN Resolution
The mechanism described in [I-D.ietf-ecrit-framework] assumes that
all devices at the call signaling path store information about the
domain of the communication recipient. This is necessary to match
the stored domain name against the domain of the sender when an
incoming call arrives.
Schulzrinne, et al. Expires May 9, 2011 [Page 7]
Internet-Draft PSAP Callback Marking November 2010
However, the IETF emergency services architecture also considers
those cases where the resolution from the Service URN to the PSAP URI
happens somewhere in the network rather than immediately at the end
point itself. In such a case, the end device is therefore not able
to match the domain of the sender with any information from the
outgoing emergency call.
Figure 5 shows this message exchange graphically.
,-------.
,' `.
/ Emergency \
| Services |
| Network |
|police-town.org|
| |
| +------+ | Invite to police.example.com
| |PSAP +<---+------------------------+
| | +----+------------------+ ^
| +------+ |Invite from | |
| ,police.example.com| |
`~~~~~~~~~~~~~~~ v |
+--------+ ++-----+-+
| | query |VoIP |
| LoST |<-----------------------|Service |
| Server | police.example.com |Provider|
| |----------------------->| |
+--------+ +--------+
| ^
Invite| | Invite
from| | to
police.example.com| | urn:service:sos
V |
+-------+
| SIP |
| UA |
| Alice |
+-------+
Figure 5: Example for Network-based Service URN Resolution
Schulzrinne, et al. Expires May 9, 2011 [Page 8]
Internet-Draft PSAP Callback Marking November 2010
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].
Schulzrinne, et al. Expires May 9, 2011 [Page 9]
Internet-Draft PSAP Callback Marking November 2010
3. Architecture
Section 4 describes how to mark a call as a callback. However, the
pure emergency service callback marking is insufficient since it
lacks any built-in security mechanism. Fortunately, available SIP
security techniques for the purpose of authorization can be re-used,
as described in the rest of the section.
In Figure 6 an interaction is presented that allows a SIP entity to
make a policy decision whether to bypass installed authorization
policies and thereby providing preferential treatment. To make this
decision the sender's identity is compared with a whitelist of valid
PSAPs. The identity assurances in SIP can come in different forms,
such as SIP Identity [RFC4474] or with P-Asserted-Identity [RFC3325].
The former technique relies on a cryptographic assurance and the
latter on a chain of trust.
+----------+
| List of |+
| valid ||
| PSAP ids ||
+----------+|
+----------+
*
* whitelist
*
V
Incoming +----------+ Normal
SIP Msg | SIP |+ Treatment
-------------->| Entity ||=============>
+ Identity | ||(if not in whitelist)
+----------+|
+----------+
||
||
|| Preferential
|| Treatment
++=============>
(in whitelist)
Figure 6: Identity-based Authorization
The establishment of a whitelist with PSAP identities is
operationally complex and does not easily scale world wide. When
there is a local relationship between the VSP/ASP and the PSAP then
populating the whitelist is far simpler.
Schulzrinne, et al. Expires May 9, 2011 [Page 10]
Internet-Draft PSAP Callback Marking November 2010
An alternative approach to an identity based authorization model is
outlined in Figure 7. In fact, RFC 4484 [RFC4484] already
illustrated the basic requirements for this technique.
+----------+
| List of |+
| trust ||
| anchor ||
+----------+|
+----------+
*
*
*
V
Incoming +----------+ Normal
SIP Msg | SIP |+ Treatment
-------------->| Entity ||=============>
+ trait | ||(no indication
+----------+| of PSAP)
+----------+
||
||
|| Preferential
|| Treatment
++=============>
(indicated as
PSAP)
Figure 7: Trait-based Authorization
In a trait-based authorization scenario an incoming SIP message
contains a form of trait, i.e. some form of assertion. The assertion
contains an indication that the sending party has the role of a PSAP
(or similar emergency services entity). The assertion is either
cryptographically protected to enable end-to-end verification or an
chain of trust security model has to be assumed. In Figure 7 we
assume an end-to-end security model where trust anchors are
provisioned to ensure the ability for a SIP entity to verify the
received assertion.
Schulzrinne, et al. Expires May 9, 2011 [Page 11]
Internet-Draft PSAP Callback Marking November 2010
4. Callback Marking
The callback marking is represented as URI parameter for an URI
scheme. The ABNF [RFC5234] syntax is shown below.
4.1. Tel URI
The 'par' production is defined in RFC 3966 [RFC3966]. The "/="
syntax indicates an extension of the production on the left-hand
side:
par /= callback
callback = callback-tag "=" callback-value
callback-tag = "callback"
callback-value = "normal" / "test" /
The semantics of the callback values are described below:
normal: This represents an normal PSAP callback.
test: This is a test callback.
An example of the "callback" parameter is given below:
P-Asserted-Identity: <tel:+17005554141;callback=test>
4.2. SIP URI
The 'uri-parameter' production is defined in RFC 3966 [RFC3261]. The
"/=" syntax indicates an extension of the production on the left-hand
side:
uri-parameter =/ callback
callback = callback-tag "=" callback-value
callback-tag = "callback"
callback-value = "normal" / "test" /
The semantics of the callback values are described below:
normal: This represents an normal PSAP callback.
Schulzrinne, et al. Expires May 9, 2011 [Page 12]
Internet-Draft PSAP Callback Marking November 2010
test: This is a test callback.
An example of the "callback" parameter is given below:
P-Asserted-Identity: <sip:psap@example.com;callback=normal>
Schulzrinne, et al. Expires May 9, 2011 [Page 13]
Internet-Draft PSAP Callback Marking November 2010
5. Security Considerations
This document defines a callback marking scheme using URI parameters
and illustrates how to handle authorization for preferential
treatment. The URI parameter that is included for a URI MUST be used
in concert with either the PAI [RFC3325] or the SIP Identity
[RFC4474] header. A pure From header does not provide security
assurance that the calling party is indeed a PSAP.
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 some form of relationship between the
emergency services operator and the VSP 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.
The main attack surface can be seen in the usage of PSAP callback
marking to bypass blacklists, ignore call forwarding procedures and
similar features to interact with users and to get 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.
A SIP entity MAY treat the call as a normal incoming call if it
considers the request with the included URI parameter to be
fraudulent, i.e. if it does not recognize the originator, or the
domain from where the call originated from as being trusted/owned by
a PSAP. It is NOT RECOMMENDED to drop a call that is marked as PSAP
callback in such a case since this may severely impact the ability
for calltakers at PSAPs to contact emergency callers.
Schulzrinne, et al. Expires May 9, 2011 [Page 14]
Internet-Draft PSAP Callback Marking November 2010
6. IANA Considerations
This document extends the registry of URI parameters for SIP, as
defined in RFC 3969 [RFC3969]. A new SIP URI parameter is defined in
this document as follows:
Parameter Name: callback
Predefined Values: Yes
Reference: This document
This document extends the registry of Tel URI parameters for SIP, as
defined in RFC 5341[RFC5341]. A new Tel URI parameter is defined in
this document as follows:
Parameter Name: callback
Predefined Values: Yes
Reference: This document
Schulzrinne, et al. Expires May 9, 2011 [Page 15]
Internet-Draft PSAP Callback Marking November 2010
7. Acknowledgements
We would like to thank members from the ECRIT working group, in
particular Brian Rosen, for their discussions around PSAP callbacks.
The working group discussed the topic of callbacks at their virtual
interim meeting in February 2010 and the following persons provided
valuable input: John Elwell, Bernard Aboba, Cullen Jennings, Keith
Drage, Marc Linsner, Roger Marshall, Dan Romascanu, Geoff Thompson,
Janet Gunn.
Schulzrinne, et al. Expires May 9, 2011 [Page 16]
Internet-Draft PSAP Callback Marking November 2010
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[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.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
RFC 3966, December 2004.
[RFC3969] Camarillo, G., "The Internet Assigned Number Authority
(IANA) Uniform Resource Identifier (URI) Parameter
Registry for the Session Initiation Protocol (SIP)",
BCP 99, RFC 3969, December 2004.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC5341] Jennings, C. and V. Gurbani, "The Internet Assigned Number
Authority (IANA) tel Uniform Resource Identifier (URI)
Parameter Registry", September 2008.
8.2. 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-12 (work in
progress), October 2010.
[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-08 (work in progress), October 2010.
[RFC4484] Peterson, J., Polk, J., Sicker, D., and H. Tschofenig,
"Trait-Based Authorization Requirements for the Session
Schulzrinne, et al. Expires May 9, 2011 [Page 17]
Internet-Draft PSAP Callback Marking November 2010
Initiation Protocol (SIP)", RFC 4484, 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
Emergency and Other Well-Known Services", RFC 5031,
January 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
Schulzrinne, et al. Expires May 9, 2011 [Page 18]
Internet-Draft PSAP Callback Marking November 2010
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
InterDigital Communications
Email: Milan.Patel@interdigital.com
Schulzrinne, et al. Expires May 9, 2011 [Page 19]
