Internet DRAFT - draft-ietf-ecrit-psap-callback
draft-ietf-ecrit-psap-callback
ECRIT H. Schulzrinne
Internet-Draft Columbia University
Intended status: Standards Track H. Tschofenig
Expires: April 17, 2014 Nokia Solutions and Networks
C. Holmberg
Ericsson
M. Patel
InterDigital Communications
October 14, 2013
Public Safety Answering Point (PSAP) Callback
draft-ietf-ecrit-psap-callback-13.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. 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, 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 specification already offers
a solution approach for allowing PSAP callbacks to bypass
authorization policies to reach the caller without unnecessary
delays. Unfortunately, the specified mechanism only supports limited
scenarios. This document discusses shortcomings of the current
mechanisms and illustrates additional scenarios where better-than-
normal call treatment behavior would be desirable. A solution based
on a new header field value, called "psap-callback", for the SIP
Priority header field is specified to accomplish the PSAP callback
marking.
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/.
Schulzrinne, et al. Expires April 17, 2014 [Page 1]
Internet-Draft PSAP Callback October 2013
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 April 17, 2014.
Copyright Notice
Copyright (c) 2013 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
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Callback Scenarios . . . . . . . . . . . . . . . . . . . . . 4
3.1. Routing Asymmetry . . . . . . . . . . . . . . . . . . . . 5
3.2. Multi-Stage Routing . . . . . . . . . . . . . . . . . . . 5
3.3. Call Forwarding . . . . . . . . . . . . . . . . . . . . . 6
3.4. Network-based Service URN Resolution . . . . . . . . . . 8
3.5. PSTN Interworking . . . . . . . . . . . . . . . . . . . . 9
4. SIP PSAP Callback Indicator . . . . . . . . . . . . . . . . . 10
4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3.1. General . . . . . . . . . . . . . . . . . . . . . . . 10
4.3.2. ABNF . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5.1. Security Threat . . . . . . . . . . . . . . . . . . . . . 10
5.2. Security Requirements . . . . . . . . . . . . . . . . . . 11
5.3. Security Solution . . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . 14
Schulzrinne, et al. Expires April 17, 2014 [Page 2]
Internet-Draft PSAP Callback October 2013
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.
An overview of the protocol interactions for emergency calling using
the IETF emergency services architecture are described in [RFC6443]
and [RFC6881] specifies the technical details. As part of the
emergency call setup procedure two important identifiers are conveyed
to the PSAP call taker's user agent, namely the Address-Of-Record
(AOR), and, if available, the Globally Routable User Agent (UA) URIs
(GRUU). RFC 3261 [RFC3261] defines the AOR as:
"An address-of-record (AOR) is a SIP or SIPS URI that points to a
domain with a location service that can map the URI to another URI
where the user might be available. Typically, the location
service is populated through registrations. An AOR is frequently
thought of as the "public address" of the user."
In SIP systems a single user can have a number of user agents
(handsets, softphones, voicemail accounts, etc.) which are all
referenced by the same AOR. There are a number of cases in which it
is desirable to have an identifier which addresses a single user
agent rather than the group of user agents indicated by an AOR. The
GRUU is such a unique user-agent identifier, which is still globally
routable. RFC 5627 [RFC5627] specifies how to obtain and use GRUUs.
[RFC6881] also makes use of the GRUU for emergency calls.
Regulatory requirements demand that the emergency call setup
procedure itself provides enough information to allow the call taker
to initiate a callback to the emergency caller. This is desirable in
those cases where the call got dropped prematurely or when further
communication need arises. The AOR and the GRUU serve this purpose.
The communication attempt by the PSAP call taker back to the
emergency caller is called 'PSAP callback'.
A PSAP callback may, however, be blocked by user configured
authorization policies or may be forwarded to an answering machine
since SIP entities (SIP proxies as well as the SIP user equipment
itself) cannot differentiate the PSAP callback from any other SIP
Schulzrinne, et al. Expires April 17, 2014 [Page 3]
Internet-Draft PSAP Callback October 2013
call. "Call barring", "do not disturb", or "call diversion"(aka call
forwarding) are features that prevent delivery of a call. It is
important to note that these features may be implemented by SIP
intermediaries as well as by the user agent.
Among the emergency services community there is the desire to offer
PSAP callbacks a treatment such that chances are increased that it
reaches the emergency caller. At the same time a design must deal
with the negative side-effects of allowing certain calls to bypass
call forwarding or other authorization policies. Ideally, the PSAP
callback has to relate to an earlier emergency call that was made
"not too long ago". An exact time interval is difficult to define in
a global IETF standard due to the variety of national regulatory
requirements but [RFC6881] suggests 30 minutes.
To nevertheless meet the needs from the emergency services community
a basic mechanism for preferential treatment of PSAP callbacks was
defined in Section 13 of [RFC6443]. The specification says:
"A UA may be able to determine a PSAP callback 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 callback 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 even though it requires call state to be maintained by the
user agent as well as by SIP intermediaries. Unfortunately, the
solution does not work in all deployment scenarios. In Section 3 we
describe cases where the currently standardized approach is
insufficient.
2. Terminology
Emergency services related terminology is borrowed from [RFC5012].
This includes terminology like emergency caller, user equipment, call
taker, Emergency Service Routing Proxy (ESRP), and Public Safety
Answering Point (PSAP).
3. Callback Scenarios
This section illustrates a number of scenarios where the currently
specified solution, as specified in [RFC6881], for preferential
treatment of callbacks fails. As explained in Section 1 a SIP entity
examines an incoming PSAP callback by comparing the domain of the
Schulzrinne, et al. Expires April 17, 2014 [Page 4]
Internet-Draft PSAP Callback October 2013
PSAP with the destination domain of the outbound emergency call
placed earlier.
3.1. Routing Asymmetry
In some deployment environments it is common to have incoming and
outgoing SIP messaging routed through different SIP entities. Figure
1 shows this graphically whereby a VoIP provider uses different SIP
proxies for inbound and for outbound call handling. Unless the two
devices are synchronized, the callback hitting the inbound proxy
would get treated like any other call since the emergency call
established state information at the outbound proxy only.
,-------.
,' `.
,-------. / 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.
3.2. Multi-Stage Routing
Schulzrinne, et al. Expires April 17, 2014 [Page 5]
Internet-Draft PSAP Callback October 2013
Consider the following emergency call routing scenario shown in
Figure 2 where routing towards the PSAP occurs in several stages. In
this scenario we consider a SIP UA that uses the Location-to-Service
Translation Protocol (LoST) [RFC5222] to learn the next hop
destination, namely esrp@example.net, to get the call closer to the
PSAP. This call is then sent to the proxy of the user's VoIP
provider (example.org). The user's VoIP provider receives the
emergency call and creates state based on the destination domain,
namely example.net. It then routes it to the indicated ESRP. When
the ESRP receives it it needs to decide what the next hop is to get
to the final PSAP. In our example the next hop is the PSAP with the
URI psap@example.com.
When a callback is sent from psap@example.com towards the emergency
caller the call will get normal treatment by the proxy of the VoIP
provider since the domain of the PSAP does not match the stored state
information.
,-----------.
+----+ ,' `.
| UA |--- esrp@example.net / Emergency \
+----+ \ | Services |
\ ,-------. | Network |
,' `. | |
/ VoIP \ | +------+ |
( Provider ) | | PSAP | |
\ example.org / | +--+---+ |
`. ,' | | |
'---+---' | | |
| | psap@example.com |
esrp@example.net | | |
| | | |
| | | |
| | +--+---+ |
+------------+-----+ ESRP | |
| +------+ |
| |
\ /
`. ,'
'----------'
Figure 2: Example for Multi-Stage Routing.
3.3. Call Forwarding
Imagine the following case where an emergency call enters an
emergency network (state.example) via an ESRP but then gets forwarded
Schulzrinne, et al. Expires April 17, 2014 [Page 6]
Internet-Draft PSAP Callback October 2013
to a different emergency services network (in our example to
example.net, example.org or example.com). The same considerations
apply when the police, fire and ambulance networks are part of the
state.example sub-domains (e.g., police.state.example).
Similar to the previous scenario the problem here is with the wrong
state information being established during the emergency call setup
procedure. A callback would originate in the example.net,
example.org or example.com domains whereas the emergency caller's SIP
UA or the VoIP outbound proxy has stored state.example.
,-------.
,' `.
/ Emergency \
| Services |
| Network |
|(state.example)|
| |
| |
| +------+ |
| |PSAP +--+ |
| +--+---+ | |
| | | |
| | | |
| | | |
| | | |
| | | |
| +--+---+ | |
------------------+---+ESRP | | |
esrp-a@state.org | +------+ | |
| | |
| Call Fwd | |
| +-+-+---+ |
\ | | | /
`. | | | ,'
'-|-|-|-' ,-------.
Police | | | Fire ,' `.
+------------+ | +----+ / Emergency \
,-------. | | | | Services |
,' `. | | | | Network |
/ Emergency \ | Ambulance | | (Fire) |
| Services | | | | | |
| Network | | +----+ | | +------+ |
| (Police) | | ,-------. | +----+---+PSAP | |
| | | ,' `. | | +------+ |
| +------+ | | / Emergency \ | | |
| |PSAP +----+--+ | Services | | | example.com ,
| +------+ | | Network | | `~~~~~~~~~~~~~~~
Schulzrinne, et al. Expires April 17, 2014 [Page 7]
Internet-Draft PSAP Callback October 2013
| | | (Ambulance) | |
| example.net , | | |
`~~~~~~~~~~~~~~~ | +------+ | |
| |PSAP +----+ +
| +------+ |
| |
| example.org ,
`~~~~~~~~~~~~~~~
Figure 3: Example for Call Forwarding.
3.4. Network-based Service URN Resolution
The IETF emergency services architecture also considers cases where
the resolution from the Service URN to the PSAP URI does not only
happen at the SIP UA itself but at intermediate SIP entities, such as
the user's VoIP provider.
Figure 4 shows this message exchange of the outgoing emergency call
and the incoming PSAP graphically. While the state information
stored at the VoIP provider is correct the state allocated at the SIP
UA is not.
,-------.
,' `.
/ Emergency \
| Services |
| Network |
| example.com |
| |
| +------+ | Invite to police@example.com
| |PSAP +<---+------------------------+
| | +----+--------------------+ ^
| +------+ |Invite from | |
| ,police@example.com | |
`~~~~~~~~~~~~~~~ | |
v |
+--------+ Query with location +--+---+-+
| | + urn:service:sos | VoIP |
| LoST |<-----------------------|Service |
| Server | police@example.com |Provider|
| |----------------------->| |
+--------+ +--------+
| ^
Invite| | Invite
from| | to
police@example.com| | urn:service:sos
V |
Schulzrinne, et al. Expires April 17, 2014 [Page 8]
Internet-Draft PSAP Callback October 2013
+-------+
| SIP |
| UA |
| Alice |
+-------+
Figure 4: Example for Network-based Service URN Resolution.
3.5. PSTN Interworking
In case an emergency call enters the PSTN, as shown in Figure 5,
there is no guarantee that the callback some time later leaves 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 5: Example for PSTN Interworking.
Schulzrinne, et al. Expires April 17, 2014 [Page 9]
Internet-Draft PSAP Callback October 2013
Note: This scenario is considered outside the scope of this document.
The specified solution does not support this use case.
4. SIP PSAP Callback Indicator
4.1. General
This section defines a new header field value, called "psap-
callback", for the SIP Priority header field defined in [RFC3261].
The value is used to inform SIP entities that the request is
associated with a PSAP callback SIP session.
4.2. Usage
SIP entities that receive the header field value within an initial
request for a SIP session can, depending on local policies, apply
PSAP callback specific procedures for the session or request.
The PSAP callback specific procedures may be applied by SIP-based
network entities and by the callee. The specific procedures taken
when receiving such a PSAP callback marked call, such as bypassing
services and barring procedures, are outside the scope of this
document.
4.3. Syntax
4.3.1. General
This section defines the ABNF for the new SIP Priority header field
value "psap-callback".
4.3.2. ABNF
priority-value /= "psap-callback"
Figure 6: ABNF
5. Security Considerations
5.1. Security Threat
The PSAP callback functionality described in this document allows
marked calls to bypass blacklists, ignore call forwarding procedures
and other similar features used to raise the attention of emergency
callers when attempting to contact them. In the case where the SIP
Priority header value, 'psap-callback', is supported by the SIP UA,
it would override user interface configurations, such as vibrate-only
mode, to alert the caller of the incoming call.
Schulzrinne, et al. Expires April 17, 2014 [Page 10]
Internet-Draft PSAP Callback October 2013
5.2. Security Requirements
The security threat discussed in Section 5.1 leads to the requirement
to ensure that the mechanisms described in this document can not be
used for malicious purposes, including telemarketing.
Furthermore, if the newly defined extension is not recognized, not
verified adequately, or not obeyed by SIP intermediaries or SIP
endpoints then it must not lead to a failure of the call handling
procedure. Such call must be treated like a call that does not have
any marking attached.
The indicator described in Section 4 can be inserted by any SIP
entity, including attackers. So it is critical that the indicator
only lead to preferential call treatment in cases where the recipient
has some trust in the caller, as described in the next section.
5.3. Security Solution
The approach for dealing with implementing the security requirements
described in Section 5.2 can be differentiated between the behavior
applied by the UA and by SIP proxies. A UA that has made an
emergency call MUST keep state information so that it can recognize
and accepted a callback from the PSAP if it occurs within a
reasonable time after an emergency call was placed, as described in
Section 13 of [RFC6443]. Only a timer started at the time when the
original emergency call has ended is required; information about the
calling party identity is not needed since the callback may use a
different calling party identity, as described in Section 3. Since
these SIP UA considerations are described already in [RFC6443] as
well as in [RFC6881] the rest of this section focuses on the behavior
of SIP proxies.
Figure 7 shows the architecture that utilizes the identity of the
PSAP to decide whether a preferential treatment of callbacks should
be provided. To make this policy decision, the identity of the PSAP
(i.e., calling party identity) is compared with a PSAPs white list.
+----------+
| List of |+
| valid ||
| PSAPs ||
+----------+|
+----------+
*
* white list
*
V
Schulzrinne, et al. Expires April 17, 2014 [Page 11]
Internet-Draft PSAP Callback October 2013
Incoming +----------+ Normal
SIP Msg | SIP |+ Treatment
-------------->| Entity ||======================>
+ Identity | ||(if not in white list)
Info +----------+|
+----------+
||
||
|| Preferential
|| Treatment
++========================>
(if successfully verified)
Figure 7: Identity-based Authorization
The identity assurance in SIP can come in different forms, namely via
the SIP Identity [RFC4474] or the P-Asserted-Identity [RFC3325]
mechanisms. The former technique relies on a cryptographic assurance
and the latter on a chain of trust. Also the usage of TLS between
neighboring SIP entities may provide useful identity information. At
the time of writing these identity technologies are being revised in
the Secure Telephone Identity Revisited (stir) working group [STIR]
to offer better support for legacy technologies interworking and SIP
intermediaries that modify the content of various SIP headers and the
body. Once the work on these specifications has been completed they
will offer a stronger calling party identity mechanism that limits or
prevents identity spoofing.
An important aspect from a security point of view is the relationship
between the emergency services network (containing the PSAPs) and the
VoIP provider (assuming that the emergency call travels via the VoIP
provider and not directly between the SIP UA and the PSAP).
The establishment of a white list with PSAP identities may be
operationally complex and dependent on the relationship between the
emergency services operator and the VoIP provider. When there is a
relationship between the VoIP provider and the PSAP operator, for
example when they are both operating in the same geographical region,
then populating the white list is fairly simple and consequently the
identification of a PSAP callback is less problematic compared to the
case where the two entities have never interacted with each other
before. In the end, the VoIP provider has to verify whether the
marked callback message indeed came from a legitimate source.
VoIP providers MUST only give PSAP callbacks preferential treatment
when the calling party identity of the PSAP was successfully matched
against entries in the white list. If it cannot be verified (because
there was no match),then the VoIP provider MUST remove the PSAP
Schulzrinne, et al. Expires April 17, 2014 [Page 12]
Internet-Draft PSAP Callback October 2013
callback marking. Thereby, the callback is degenerated to a normal
call. As a second step, SIP UAs MUST maintain a timer that is
started with the original emergency call and this timer expires
within a reasonable amount of time, such as 30 minutes per [RFC6881].
Such a timer also ensures that VoIP providers cannot misuse the PSAP
callback mechanism, for example to ensure that their support calls
reaches their customers.
Finally, a PSAP callback MUST use the same media as the original
emergency call. For example, when an initial emergency call
established a real-time text communication session then the PSAP
callback must also attempt to establish a real-time communication
interaction. The reason for this is two-fold. First, the person
seeking for help may have disabilities that prevent them from using
certain media and hence using the same media for the callback avoids
unpleasant surprises and delays. Second, the emergency caller may
have intentionally chosen a certain media and does not prefer to
communicate in a different way. For example, it would be unfortunate
if a hostage tries to seek for help using instant messaging to avoid
any noise when subsequently the ring-tone triggered by a PSAP
callback using a voice call gets the attention of the hostage-taker.
User interface designs need to cater to such situations.
6. IANA Considerations
This document adds the "psap-callback" value to the SIP Priority
header IANA registry allocated by [RFC6878]. The semantic of the
newly defined "psap-callback" value is defined in Section 4.
7. Acknowledgements
We would like to thank the following persons for their feedback: Paul
Kyzivat, Martin Thomson, Robert Sparks, Keith Drage, Cullen Jennings
Brian Rosen, Martin Dolly, Bernard Aboba, Andrew Allen, Atle Monrad,
John-Luc Bakker, John Elwell, Geoff Thompson, Dan Romascanu, James
Polk, John Medland, Hadriel Kaplan, Kenneth Carlberg, Timothy Dwight,
Janet Gunn
We would like to thank the ECRIT working group chairs, Marc Linsner
and Roger Marshall, for their support. Roger Marshall was the
document shepherd for this document. Vijay Gurbani provided the
general area review.
During IESG review the document received good feedback from Barry
Leiba, Spencer Dawkins, Richard Barnes, Joel Jaeggli, Stephen
Farrell, and Benoit Claise.
Schulzrinne, et al. Expires April 17, 2014 [Page 13]
Internet-Draft PSAP Callback October 2013
8. References
8.1. Normative References
[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.
[RFC5627] Rosenberg, J., "Obtaining and Using Globally Routable User
Agent URIs (GRUUs) in the Session Initiation Protocol
(SIP)", RFC 5627, October 2009.
[RFC6878] Roach, A., "IANA Registry for the Session Initiation
Protocol (SIP) "Priority" Header Field", RFC 6878, March
2013.
8.2. Informative References
[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.
[RFC5222] Hardie, T., Newton, A., Schulzrinne, H., and H.
Tschofenig, "LoST: A Location-to-Service Translation
Protocol", RFC 5222, August 2008.
[RFC6443] Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
"Framework for Emergency Calling Using Internet
Multimedia", RFC 6443, December 2011.
[RFC6881] Rosen, B. and J. Polk, "Best Current Practice for
Communications Services in Support of Emergency Calling",
BCP 181, RFC 6881, March 2013.
[STIR] IETF, "Secure Telephone Identity Revisited (stir) Working
Group", URL: http://datatracker.ietf.org/wg/stir/charter/,
Oct 2013.
Schulzrinne, et al. Expires April 17, 2014 [Page 14]
Internet-Draft PSAP Callback October 2013
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 Solutions and Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
EMail: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: christer.holmberg@ericsson.com
Milan Patel
InterDigital Communications
EMail: Milan.Patel@interdigital.com
Schulzrinne, et al. Expires April 17, 2014 [Page 15]
