| ECRIT | H. Schulzrinne |
| Internet-Draft | Columbia University |
| Intended status: Standards Track | S. McCann |
| Expires: November 01, 2013 | Research in Motion UK Ltd |
| G. Bajko | |
| Nokia | |
| H. Tschofenig | |
| Nokia Siemens Networks | |
| D. Kroeselberg | |
| Siemens | |
| April 30, 2013 |
Extensions to the Emergency Services Architecture for dealing with Unauthenticated and Unauthorized Devices
draft-ietf-ecrit-unauthenticated-access-06.txt
The IETF emergency services architecture assumes that the calling device has acquired rights to use the access network or that no authentication is required for the access network, such as for public wireless access points. Subsequent protocol interactions, such as obtaining location information, learning the address of the Public Safety Answering Point (PSAP) and the emergency call itself are largely decoupled from the underlying network access procedures.
In some cases, however, the device does not have these credentials for network access, does not have a VoIP service provider, or the credentials have become invalid, e.g., because the user has exhausted their prepaid balance or the account has expired.
This document provides a problem statement, introduces terminology and describes an extension for the base IETF emergency services architecture to address these scenarios.
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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 older 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.
Roughly speaking, the IETF emergency services architecture (see [I-D.ietf-ecrit-phonebcp] and [RFC6443]) divides responsibility for handling emergency calls between the access network (ISP), the application service provider (ASP) that may be a VoIP service provider and the provider of emergency signaling services, the emergency service network (ESN). The access network may provide location information to end systems, but does not have to provide any ASP signaling functionality. The emergency caller can reach the ESN either directly or through the ASP's outbound proxy. Any of the three parties can provide the mapping from location to PSAP URI by offering LoST [RFC5222] services.
In general, a set of automated configuration mechanisms allows a device to function in a variety of architectures, without the user being aware of the details on who provides location, mapping services or call routing services. However, if emergency calling is to be supported when the calling device lacks access network authorization or does not have an ASP, one or more of the providers may need to provide additional services and functions.
In all cases, the end device has to be able to perform a LoST lookup and otherwise conduct the emergency call in the same manner as when the three exceptional conditions discussed below do not apply.
We distinguish between three conditions:
These three cases are not mutually exclusive. A caller in need for help may find himself/herself in, for example, a NAA and NASP situation, as explained in more details in Figure 1. Depending on local policy and regulations, it may not be possible to place emergency calls in the NAA case. Unless local regulations require user identification, it should always be possible to place calls in the NASP case, with minimal impact on the ISP. Unless the ESN requires that all calls traverse a known set of VSPs, it is technically possible to let a caller place an emergency call in the ZBP case. We discuss each case in more details in Section 3.
Note: At the time of writing there is no regulation in place that demands the functionality described in this memo. SDOs have started their work on this subject in a proactive fashion in the anticipation that national regulation will demand it for a subset of network environments.
There are also indications that the functionality of unauthenticated emergency calls (called SIM-less calls) in today's cellular system in certain countries leads to a fair amount of hoax or test calls. This causes overload situations at PSAPs which is considered harmful to the overall availability and reliability of emergency services.
As an example, Federal Office of Communications (OFCOM, Switzerland) provided statistics about emergency (112) calls in Switzerland from Jan. 1997 to Nov. 2001. Switzerland did not offer SIM-less emergency calls except for almost a month in July 2000 where a significant increase in hoax and test calls was reported. As a consequence, the functionality was disabled again. More details can be found in the panel presentations of the 3rd SDO Emergency Services Workshop [esw07].
In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 [RFC2119].
This document reuses terminology from [RFC5687] and [RFC5012], namely Internet Access Provider (IAP), Internet Service Provider (ISP), Application Service Provider (ASP), Voice Service Provider (VSP), Emergency Service Routing Proxy (ESRP), Public Safety Answering Point (PSAP), Location Configuration Server (LCS), (emergency) service dial string, and (emergency) service identifier.
On a very high-level, the steps to be performed by an end host not being attached to the network and the user starting to make an emergency call are the following:
Figure 1 compiles the basic logic taking place during network entry for requesting an emergency service and shows the interrelation between the three conditions described in the above section.
+-----Y
|Start|
`...../
|
| Are credentials
| for network attachment
| available?
|
NO v YES
+----------------------------+
| |
| |
V v
.............. ................
| Idle: Wait | |Execute |
| for ES Call| |LLA Procedures|
| Initiation | "--------------'
"------------' |
Is | +---------->O
emergency | | | Is ASP
service | NO +-----Y | | configured?
network +--->| End | | +---------------+
attachment| `...../ | YES | | NO
possible? | | | |
v | v v
+------------+ | +------------+ +------------+
| Execute | | | Execute | | Execute |
| NAA |--------+ | Phone BCP | | NASP |
| Procedures | | Procedures | | Procedures |
+------------+ +------------+ +------------+
Authorization for| |
Emergency Call? | |
+--------------+ v
| NO | YES +-----Y
| | | Done|
v v `...../
+------------+ +------------+
| Execute | | Execute |
| ZBP | | Phone BCP |
| Procedures | | Procedures |
+------------+ +------------+
| |
| |
v v
+-----Y +-----Y
| Done| | Done|
`...../ `...../
Abbreviations:
LLA: Link Layer Attachment
ES: Emergency Services
Figure 1: Flow Diagram
ZBP includes all cases where a subscriber is known to an ASP, but lacks the necessary authorization to access regular ASP services. Example ZBP cases include empty prepaid accounts, barred accounts, roaming and mobility restrictions, or any other conditions set by ASP policy.
Local regulation might demand that emergency calls are always authorized. An ASP can identify emergency sessions by identifying the service URN [RFC5031] used in call setup. Emergency calls can then be authorized accordingly. The ZBP case therefore only affects the ASP.
Permitting a call with limited authorization could present an opportunity for abuse. The ASP MAY choose to validate session initiation messages for valid destinations, see Section 7.
An ASP without a regulatory requirement to authorize emergency calls can deny emergency call setup. Where an ASP does not authorize an emergency call, the caller can fall back to NASP procedures.
To start the description we consider the sequence of steps that are executed in an emergency call based on Figure 2.
For editorial reasons the end-to-end SIP and media exchange between the PSAP and SIP UA are not shown in Figure 2.
+-------+
| PSAP |
| |
+-------+
^
| (8)
|
+----------+(7) +----------+
| LoST |<-->| ESRP |
| Server | | |
+----------+ +----------+
^ ^
+----------------+----------------|--------------+
| ISP | | |
|+----------+ | | +----------+|
|| LCS-ISP | (3)| | | DHCP ||
|| |<-+ | | | Server ||
|+----------+ | | | +----------+|
+-------^------+-+----------------|-----------^--+
+-------|------+-+----------------|-----------|--+
| IAP | (4) | |(5) | | |
| V | | | | |
|+----------+ | | | | |
|| LCS-IAP | | | +--------+ | | |
|| | | | | Link | |(6) | |
|+----------+ | | | Layer | | | |
| | | | Device | | (2)| |
| | | +--------+ | | |
| | | ^ | | |
| | | | | | |
+--------------+-|-------|--------|-----------|--+
| | | | |
| | (1)| | |
| | | | |
| | | +----+ |
| | v | |
| | +----------+ |
| +->| End |<-------------+
+___>| Host |
+----------+
Figure 2: Architectural Overview
Note: Figure 2 does not indicate who operates the ESRP and the LoST server. Various deployment options exist.
The end host MUST discover a LoST server [RFC5222] using DHCP [RFC5223].
The end host MUST discover the ESRP using the LoST protocol [RFC5222].
The end host MUST support location acquisition and the LCPs described in Section 6.5 of [I-D.ietf-ecrit-phonebcp]. The description in Section 6.5 and 6.6 of [I-D.ietf-ecrit-phonebcp] regarding the interaction between the device and the LIS applies to this document.
The SIP UA in the end host MUST attach available location information in a PIDF-LO [RFC4119] when making an emergency call. When constructing the PIDF-LO the guidelines in PIDF-LO profile [RFC5491] MUST be followed. For civic location information the format defined in [RFC5139] MUST be supported.
To determine which calls are emergency calls, some entity needs to map a user entered dialstring into this URN scheme. A user may "dial" 1-1-2, but the call would be sent to urn:service:sos. This mapping SHOULD be performed at the endpoint device.
End hosts MUST use the Service URN mechanism [RFC5031] to mark calls as emergency calls for their home emergency dial string.
SIP signaling capabilities [RFC3261] are mandated for end hosts.
The initial SIP signaling method is an INVITE. The SIP INVITE request MUST be constructed according to the requirements in Section 9.2 [I-D.ietf-ecrit-phonebcp].
Regarding callback behavior SIP UAs SHOULD place a globally routable URI in a Contact: header.
End points MUST comply with the media requirements for end points placing an emergency call found in Section 14 of [I-D.ietf-ecrit-phonebcp].
The description in Section 15 of [I-D.ietf-ecrit-phonebcp] is fully applicable to this document.
An ISP MUST provision a DHCP server with information about LoST servers [RFC5223]. An ISP operator may choose to deploy a LoST server or to outsource it to other parties.
The ISP is responsible for location determination and exposes this information to the end points via location configuration protocols. The considerations described in [RFC6444] are applicable to this document.
The ISP MUST support one of the LCPs described in Section 6.5 of [I-D.ietf-ecrit-phonebcp]. The description in Section 6.5 and 6.6 of [I-D.ietf-ecrit-phonebcp] regarding the interaction between the end device and the LIS applies to this document.
The interaction between the LIS at the ISP and the IAP is often priorietary but the description in [I-D.winterbottom-geopriv-lis2lis-req] may be relevant to the reader.
The ESRP continues to route the emergency call to the PSAP responsible for the physical location of the end host. This may require further interactions with LoST servers but depends on the specific deployment.
The ESRP MUST understand the Service URN mechanism [RFC5031] (i.e., the 'urn:service:sos' tree).
SIP signaling capabilities [RFC3261] are mandated for the ESRP. The ESRP MUST process the messages sent by the client, according to Section 5.1.5.
Some radio networks have added support for unauthenticated emergency access, some other type of networks advertise these capabilities using layer beacons. The end host learns about these unauthenticated emergency services capabilities either from the link layer type or from advertisement.
This section discusses different methods to indicate an emergency service request as part of network attachment. It provides some general considerations and recommendations that are not specific to the access technology.
To perform network attachment and get access to the resources provided by an IAP/ISP, the end host uses access technology specific network attachment procedures, including for example network detection and selection, authentication, and authorization. For initial network attachment of an emergency service requester, the method of how the emergency indication is given to the IAP/ISP is specific to the access technology. However, a number of general approaches can be identified:
In general, link layer emergency indications provide good integration into the actual network access procedure regarding the enabling of means to recognize and prioritize an emergency service request from an end host at a very early stage of the network attachment procedure. However, support in end hosts for such methods cannot be considered to be commonly available.
No general recommendations are given in the scope of this memo due to the following reasons:
For network attachment in NAA cases, it may make sense to secure the link-layer connection between the device and the IAP/ISP. This especially holds for wireless access with examples being IEEE 802.11 or IEEE 802.16 based access. The latter even mandates secured communication across the wireless link for all IAP/ISP networks based on [nwgstg3].
Therefore, for network attachment that is by default based on EAP authentication it is desirable also for NAA network attachment to use a key-generating EAP method (that provides an MSK key to the authenticator to bootstrap further key derivation for protecting the wireless link).
The following approaches to match the above can be identified:
The security threats discussed in [RFC5069] are applicable to this document.
There are a couple of new vulnerabilities raised with unauthenticated emergency services in NASP/NAA cases since the PSAP operator will typically not possess any identity information about the emergency call via the signaling path itself. In countries where this functionality is used for GSM networks today this has lead to a significant amount of misuse.
In the context of NAA, the IAP and the ISP will probably want to make sure that the claimed emergency caller indeed performs an emergency call rather than using the network for other purposes, and thereby acting fraudulent by skipping any authentication, authorization and accounting procedures. By restricting access of the unauthenticated emergency caller to the LoST server and the PSAP URI, traffic can be restricted only to emergency calls. This can be accomplished with traffic separation. The details, however, e.g. for using filtering, depend on the deployed ISP architecture and are beyond the scope of this document.
We only illustrate a possible model. If the ISP runs its own LoST server, it would maintain an access control list including all IP addresses contained in responses returned by the LoST server, as well as the LoST server itself. (It may need to translate the domain names returned to IP addresses and hope that the resolution captures all possible DNS responses.) Since the media destination addresses are not predictable, the ISP also has to provide a SIP outbound proxy so that it can determine the media addresses and add those to the filter list.
For the ZBP case the additional aspect of fraud has to be considered. Unless the emergency call traverses a PSTN gateway or the ASP charges for IP-to-IP calls, there is little potential for fraud. If the ASP also operates the LoST server, the outbound proxy MAY restrict outbound calls to the SIP URIs returned by the LoST server. It is NOT RECOMMENDED to rely on a fixed list of SIP URIs, as that list may change.
Finally, a number of security vulnerabilities discussed in [RFC6280] around faked location information are less problematic in the context of unauthenticated emergency since location information does not need to be provided by the end host itself or it can be verified to fall within a specific geographical area.
Parts of this document are derived from [I-D.ietf-ecrit-phonebcp]. Participants of the 2nd and 3rd SDO Emergency Services Workshop provided helpful input.
We would like to thank Richard Barnes, Brian Rosen, James Polk, Marc Linsner, and Martin Thomson for their feedback at the IETF#80 ECRIT meeting.
Furthermore, we would like to thank Martin Thomson and Bernard Aboba for their detailed document review in preparation of the 81st IETF meeting.
This document does not require actions by IANA.