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RFC 7406

Informational
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Extensions to the Emergency Services Architecture for Dealing With Unauthenticated and Unauthorized Devices

 


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Internet Engineering Task Force (IETF)                    H. Schulzrinne
Request for Comments: 7406                           Columbia University
Category: Informational                                        S. McCann
ISSN: 2070-1721                                           BlackBerry Ltd
                                                                G. Bajko
                                                                MediaTek
                                                           H. Tschofenig
 
                                                          D. Kroeselberg
                                            Siemens Corporate Technology
                                                           December 2014


   Extensions to the Emergency Services Architecture for Dealing With
                Unauthenticated and Unauthorized Devices

Abstract

   This document provides a problem statement, introduces terminology,
   and describes an extension for the base IETF emergency services
   architecture to address cases where an emergency caller is not
   authenticated, has no identifiable service provider, or has no
   remaining credit with which to pay for access to the network.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7406.

Page 2 
Copyright Notice

   Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Use-Case Categories . . . . . . . . . . . . . . . . . . . . .   5
   4.  ZBP Considerations  . . . . . . . . . . . . . . . . . . . . .  12
   5.  NASP Considerations . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  End-Host Profile  . . . . . . . . . . . . . . . . . . . .  15
       5.1.1.  LoST Server Discovery . . . . . . . . . . . . . . . .  15
       5.1.2.  ESRP Discovery  . . . . . . . . . . . . . . . . . . .  15
       5.1.3.  Location Determination and Location Configuration . .  15
       5.1.4.  Emergency Call Identification . . . . . . . . . . . .  15
       5.1.5.  SIP Emergency Call Signaling  . . . . . . . . . . . .  15
       5.1.6.  Media . . . . . . . . . . . . . . . . . . . . . . . .  16
       5.1.7.  Testing . . . . . . . . . . . . . . . . . . . . . . .  16
     5.2.  IAP/ISP Profile . . . . . . . . . . . . . . . . . . . . .  16
       5.2.1.  ESRP Discovery  . . . . . . . . . . . . . . . . . . .  16
       5.2.2.  Location Determination and Location Configuration . .  16
     5.3.  ESRP Profile  . . . . . . . . . . . . . . . . . . . . . .  16
       5.3.1.  Emergency Call Routing  . . . . . . . . . . . . . . .  16
       5.3.2.  Emergency Call Identification . . . . . . . . . . . .  16
       5.3.3.  SIP Emergency Call Signaling  . . . . . . . . . . . .  17
   6.  Lower-Layer Considerations for NAA Case . . . . . . . . . . .  17
     6.1.  Link-Layer Emergency Indication . . . . . . . . . . . . .  18
     6.2.  Securing Network Attachment in NAA Cases  . . . . . . . .  19
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  22
   Acknowledgments  . . . . . .  . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

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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 telephony 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; those
   devices are not traditional telephones, and users are increasingly
   expecting them to be able to place emergency calls.

   Roughly speaking, the IETF emergency services architecture (see
   [RFC6881] and [RFC6443]) divides responsibility for handling
   emergency calls among the access network (Internet Access Provider
   (IAP) or ISP); the application service provider (ASP), which may be a
   VoIP service provider (VSP); 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 the
   Public Safety Answering Point (PSAP) URI by offering Location-to-
   Service Translation (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.

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   We distinguish among three conditions:

   No Access Authentication (NAA):  In the NAA case, the emergency
      caller does not posses valid credentials for the access network.
      This includes the case where the access network allows
      pay-per-use, as is common for wireless hotspots, but there is
      insufficient time to enter credit card details and other
      registration information required for access.  It also covers all
      cases where either no credentials are available at all or the
      available credentials do not work for the given IAP/ISP.  As a
      result, the NAA case basically combines the No ASP (NASP) and
      zero-balance ASP (ZBP) cases below, but at the IAP/ISP level.
      Support for emergency call handling in the NAA case is subject to
      the local policy of the ISP.  Such policy may vary substantially
      between ISPs and typically depends on external factors that are
      not under the ISP control.

   No ASP (NASP):  The caller does not have an ASP at the time of the
      call.  This can occur in case the caller either does not possess
      any valid subscription for a reachable ASP or does possess a valid
      subscription but none of the ASPs are reachable through the ISP.

      Note: The interoperability need is increased with this scenario
      since the client software used by the emergency caller must be
      compatible with the protocols and extensions deployed by the ESN.

   Zero-balance ASP (ZBP):  In the case of a zero-balance ASP, the ASP
      can authenticate the caller, but the caller is not authorized to
      use ASP services, e.g., because the contract has expired or the
      prepaid account for the customer has been depleted.

   These three cases are not mutually exclusive.  A caller in need of
   help may, for example, be both in an NAA and NASP situation, as
   explained in more detail 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 Voice Service Providers (VSPs), it is
   technically possible to let a caller place an emergency call in the
   ZBP case.  We discuss each case in more detail in Section 3.

   Some of the functionality provided in this document is already
   available in the Public Switched Telephone Network (PSTN).
   Consequently, there is real-world experience available and not all of
   it is positive.  For example, the functionality of calls without
   Subscriber Identity Modules (SIMs) in today's cellular system has
   lead to a fair amount of hoax or test calls in certain countries.

Top      ToC       Page 5 
   This causes overload situations at PSAPs, which is considered harmful
   to the overall availability and reliability of emergency services.

      As an example, the 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
      Standards Development Organization (SDO) Emergency Services
      Workshop [esw07].

2.  Terminology

   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 [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.

3.  Use-Case Categories

   An end host needs to perform the following steps if it is not
   attached to the network and the user is starting to place an
   emergency call:

   Link-Layer Attachment:  Some networks have added support for
      unauthenticated emergency access while others have advertised
      these capabilities using layer beacons (multicast or broadcast
      announcements).  The end host learns about these unauthenticated
      emergency services capabilities from either the link layer type or
      advertisement.

      The end host uses the link-layer-specific network attachment
      procedures defined for unauthenticated network access in order to
      get access to the network.

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   Pre-emergency Service Configuration:  When the link-layer network
      attachment procedure is completed, the end host learns basic
      configuration information using DHCP from the ISP.  The end host
      uses a Location Configuration Protocol (LCP) to retrieve location
      information.  Subsequently, the LoST protocol [RFC5222] is used to
      learn the relevant emergency numbers and to obtain the PSAP URI
      applicable for that location.

   Emergency Call:  In case of the need for help, a user dials an
      emergency number and the SIP User Agent (UA) initiates the
      emergency call procedures by communicating with the PSAP.

   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 earlier.


                         +-----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

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        +------------+        |     +------------+    +------------+
        | Execute    |        |     | Execute    |    | Execute    |
        | NAA        |--------+     | Phone BCP  |    | NASP       |
        | Procedures |              | Procedures |    | Procedures |
        +------------+              +------------+    +------------+
                         Authorization for|                |
                            making an     |                |
                         emergency call   |                |
                         with the ASP/VSP?|                |
                           +--------------+                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: NAA, ZBP, and NSAP Scenarios

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   The diagrams below highlight the most important steps for the three
   cases.

               +-----Y
               |Start|
               `...../
                  |
                  | No
                  | credentials
                  | for network access
                  | available
                  v
            ..............
            | Idle: Wait |
            | for ES Call|
            | Initiation |
            "------------'
                  |
                  |
                  |
                  v
                  --
                //  --
               /      --
             //  Is     --
            /  emergency  --
            |  service     |  NO   +--------+
            |  network     |------>| Call   |
            |  attachment  |         Failed |
            \  possible?   /       `......../
             \           //
              \\       //
                \    //
                 \--/
                  |
                  | YES
                  |
                  |
                  v
            +------------+
            | Execute    |
            | NAA        |
            | Procedures |
            +------------+

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                  |
                  | Network
                  | attachment
                  | in progress
                  v
                /--\  Continue
               |    | with
               |    | application-layer
                \--/  interaction

                   Figure 2: Flow Diagram: NAA Scenario


                        +-----+
           +------------|Start|-----------------+
           |            `...../                 |
           v                                    v
     +------------+                     +----------------+
     | NAA        |                     | Regular        |
     | Procedures |                     | Network Access |
     +------------+                     | Procedures     |
           |                            +----------------+
           |                                    |
           |                                    |
           ----------------o--------------------+
                           |
                           |
                           |
                           |
                       Network
                       Attachment
                       Completed
                           |
                           |
                           |
                           |
                           v

Top      ToC       Page 10 
                     +------------+      +---------+
                     | ASP        |  NO  | See     |
                     | Configured?|----->| main    |
                     +------------+      | diagram |
                           |             `........./
                           |
                           | YES
                           |
                           v
                        //----
                       /      --
                     //         --
                    /              -       +---------+
                    | Authorization|  YES  | See     |
                    | for making   |------>| main    |
                    |   ES call    |       | diagram |
                    \    with      /       `........./
                     \  VSP/ASP? //
                      \\       //
                        \    //
                         \--/
                           |
                           | NO
                           |
                           |
                           v
                     +------------+
                     | Execute    |
                     | ZBP        |
                     | Procedures |
                     +------------+
                           |
                           | Call
                           | in progress
                           |
                           v
                       +--------+
                       | Call   |
                         Success|
                       `......../

                   Figure 3: Flow Diagram: ZBP Scenario

Top      ToC       Page 11 
                              +-----+
                 +------------|Start|-----------------+
                 |            `...../                 |
                 v                                    v
           +------------+                     +----------------+
           | NAA        |                     | Regular        |
           | Procedures |                     | Network Access |
           +------------+                     | Procedures     |
                 |                            +----------------+
                 |                                    |
                 |                                    |
                 ----------------o--------------------+
                                 |
                                 |
                                 |
                                 |
                             Network
                             Attachment
                             Completed
                                 |
                                 |
                                 |
                                 |
                                 v
                           +------------+      +---------+
                           | ASP        |  YES | See     |
                           | Configured?|----->| main    |
                           +------------+      | diagram |
                                 |             `........./
                                 |
                                 | NO
                                 |
                                 v
                           +------------+
                           | Execute    |
                           | NASP       |
                           | Procedures |
                           +------------+
                                 |
                                 | Call
                                 | in progress
                                 |
                                 v
                             +--------+
                             | Call   |
                             | Success|
                             `......../
                   Figure 4: Flow Diagram: NASP Scenario

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   The NAA procedures are described in Section 6.  The ZBP procedures
   are described in Section 4.  The NASP procedures are described in
   Section 5.  The Phone BCP procedures are described in [RFC6881].  The
   LLA procedures are not described in this document since they are
   specific to the link-layer technology in use.

4.  ZBP Considerations

   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 cannot proceed
   without successful service authorization.  In some regulatory
   regimes, however, it may be possible to allow emergency calls to
   continue despite authorization failures.  To distinguish an emergency
   call from a regular call, an ASP can identify emergency sessions by
   inspecting the service URN [RFC5031] used in call setup.  The ZBP
   case, therefore, only affects the ASP.

   Permitting a call despite authorization failures could present an
   opportunity for abuse.  The ASP may choose to verify the destination
   of the emergency calls and to only permit calls to certain,
   preconfigured entities (e.g., to local PSAPs).  Section 7 discusses
   this topic in more detail.

   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 may be able to fall back to NASP
   procedures.

5.  NASP Considerations

   To start the description, we consider the sequence of steps that are
   executed in an emergency call based on Figure 5.

   o  As an initial step, the devices attach to the network as shown in
      step (1).  This step is outside the scope of this section.

   o  When the link-layer network attachment procedure is completed, the
      end host learns basic IP configuration information using DHCP from
      the ISP, as shown in step (2).

Top      ToC       Page 13 
   o  When the end host has configured the IP address, it starts an
      interaction with the discovered LCS at the ISP, as shown in step
      (3).  In certain deployments, the ISP may need to interact with
      the IAP.  This protocol exchange is shown in step (4).

   o  Once location information is obtained, the end host triggers the
      LoST protocol to obtain the address of the ESRP/PSAP.  This is
      shown in step (5).

   o  In step (6), the SIP UA initiates a SIP INVITE request towards the
      indicated ESRP.  The INVITE message contains all the necessary
      parameters required by Section 5.1.5.

   o  The ESRP receives the INVITE and processes it according to the
      description in Section 5.3.3.

   o  The ESRP routes the call to the PSAP, as shown in step (8),
      potentially interacting with a LoST server first to determine the
      route.

   o  The PSAP evaluates the initial INVITE and aims to complete the
      call setup.

   o  Finally, when the call setup is completed, media traffic can be
      exchanged between the PSAP and the SIP UA.

   For brevity, the end-to-end SIP and media exchange between the PSAP
   and SIP UA are not shown in Figure 5.

Top      ToC       Page 14 
                                  +-------+
                                  | 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 5: Architectural Overview

   Note: Figure 5 does not indicate who operates the ESRP and the LoST
   server.  Various deployment options exist.

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5.1.  End-Host Profile

5.1.1.  LoST Server Discovery

   The end host MUST discover a LoST server [RFC5222] using DHCP
   [RFC5223] unless a LoST server has been provisioned using other
   means.

5.1.2.  ESRP Discovery

   The end host MUST discover the ESRP using the LoST protocol [RFC5222]
   unless a ESRP has been provisioned using other means.

5.1.3.  Location Determination and Location Configuration

   The end host MUST support location acquisition and the LCPs described
   in Section 6.5 of [RFC6881].  The description in Sections 6.5 and 6.6
   of [RFC6881] regarding the interaction between the device and the
   Location Information Server (LIS) applies to this document.

   The SIP UA in the end host MUST attach available location information
   in a Presence Information Data Format Location Object (PIDF-LO)
   [RFC4119] when making an emergency call.  When constructing the
   PIDF-LO, the guidelines in the PIDF-LO profile [RFC5491] MUST be
   followed.  For civic location information, the format defined in
   [RFC5139] MUST be supported.

5.1.4.  Emergency Call Identification

   To determine which calls are emergency calls, some entity needs to
   map a user-entered dial string into this URN scheme.  A user may
   "dial" 1-1-2, 9-1-1, etc., 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.

5.1.5.  SIP Emergency Call Signaling

   SIP signaling capabilities [RFC3261] are REQUIRED 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 of [RFC6881].

   To enable callbacks, SIP UAs SHOULD place a globally routable URI in
   a Contact header field.

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5.1.6.  Media

   Endpoints MUST comply with the media requirements for endpoints
   placing an emergency call as described in Section 14 of [RFC6881].

5.1.7.  Testing

   The description in Section 15 of [RFC6881] is fully applicable to
   this document.

5.2.  IAP/ISP Profile

5.2.1.  ESRP Discovery

   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.

5.2.2.  Location Determination and Location Configuration

   The ISP is responsible for location determination and exposes this
   information to the endpoints 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
   [RFC6881].  The description in Sections 6.5 and 6.6 of [RFC6881]
   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
   proprietary, but the description in [LIS] may be relevant to the
   reader.

5.3.  ESRP Profile

5.3.1.  Emergency Call Routing

   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.

5.3.2.  Emergency Call Identification

   The ESRP MUST understand the Service URN mechanism [RFC5031] (i.e.,
   the 'urn:service:sos' tree).

Top      ToC       Page 17 
5.3.3.  SIP Emergency Call Signaling

   SIP signaling capabilities [RFC3261] are REQUIRED for the ESRP.  The
   ESRP MUST process the messages sent by the client, according to
   Section 5.1.5.

   Furthermore, if a PSAP wants to support NASP calls, then it MUST NOT
   restrict incoming calls to a particular set of ASPs.

6.  Lower-Layer Considerations for NAA Case

   Some networks have added support for unauthenticated emergency access
   while others have advertised these capabilities using layer beacons.
   The end host learns about these unauthenticated emergency services
   capabilities either from the link-layer type or from advertisement.

   It is important to highlight that the NAA case is inherently a Layer
   2 problem, and the general form of the solution is to provide an
   "emergency only" access type, with appropriate limits or monitoring
   to prevent abuse.  The described mechanisms are informative in nature
   since the relationship to the IETF emergency services architecture is
   only indirect, namely via some protocols developed within the IETF
   (e.g., EAP and EAP methods) that require extensions to support this
   functionality.

   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:

   Link-layer emergency indication:  The end host provides an
      indication, e.g., an emergency parameter or flag, as part of the
      link-layer signaling for initial network attachment.  Examples
      include an emergency bit signaled in the IEEE 802.16-2009 wireless
      link.  In IEEE 802.11 WLAN [IEEE802.11], an emergency support
      indicator allows the station (i.e., end host in this context) to
      download before association to a Network Access Identifier (NAI),
      which it can use to request server-side authentication only for an
      IEEE 802.1X network.

Top      ToC       Page 18 
   Higher-layer emergency indication:  Typically, emergency indication
      is provided in the network access authentication procedure.  The
      emergency caller's end host provides an indication as part of the
      access authentication exchanges.  Authentication via the EAP
      [RFC3748] is of particular relevance here.  Examples are the EAP
      NAI decoration used in Worldwide Interoperability for Microwave
      Access (WiMAX) networks and modification of the authentication
      exchange in IEEE 802.11 [nwgstg3].

6.1.  Link-Layer Emergency Indication

   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:

   o  Dependency on the specific access technology.

   o  Dependency on the specific access network architecture.  Access
      authorization and policy decisions typically happen at different
      layers of the protocol stack and in different entities than those
      terminating the link-layer signaling.  As a result, link-layer
      indications need to be distributed and translated between the
      different protocol layers and entities involved.  Appropriate
      methods are specific to the actual architecture of the IAP/ISP
      network.

   o  An advantage of combining emergency indications with the actual
      network attachment procedure performing authentication and
      authorization is the fact that the emergency indication can
      directly be taken into account in the authentication and
      authorization server that owns the policy for granting access to
      the network resources.  As a result, there is no direct dependency
      on the access network architecture that otherwise would need to
      take care of merging link-layer indications into the
      authentication, authorization, and policy decision process.

   o  EAP signaling happens at a relatively early stage of network
      attachment, so it is likely to match most requirements for
      prioritization of emergency signaling.  However, it does not cover

Top      ToC       Page 19 
      early stages of link-layer activity in the network attachment
      process.  Possible conflicts may arise, e.g., in case of filtering
      based on Media Access Control (MAC) in entities terminating link-
      layer signaling in the network (like a base station).  In normal
      operation, EAP-related information will only be recognized in the
      Network Access Server (NAS).  Any entity residing between the end
      host and NAS should not be expected to understand/parse EAP
      messages.

   o  An emergency indication can be given by forming a specific NAI
      that is used as the identity in EAP-based authentication for
      network entry.

6.2.  Securing Network Attachment in NAA Cases

   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 access based
   on IEEE 802.11 or IEEE 802.16.  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 a Master Session Key
   (MSK) to the authenticator to bootstrap further key derivation for
   protecting the wireless link).

   To match the above, the following approaches can be identified:

   1) Server-Only Authentication:

      The device of the emergency service requester performs an EAP
      method with the IAP/ISP EAP server that performs server-side
      authentication only.  An example for this is EAP-TLS [RFC5216].
      This provides a certain level of assurance about the IAP/ISP to
      the device user.  It requires the device to be provisioned with
      appropriate trusted root certificates to be able to verify the
      server certificate of the EAP server (unless this step is
      explicitly skipped in the device in case of an emergency service
      request).  This method is used to provide access of devices
      without existing credentials to an IEEE 802.1X network.  The
      details are incorporated in the IEEE 802.11-2012 specification
      [IEEE802.11].

Top      ToC       Page 20 
   2) Null Authentication:

      In one case (e.g., WiMAX), an EAP method is performed.  However,
      no credentials specific to either the server or the device or
      subscription are used as part of the authentication exchange.  An
      example for this would be an EAP-TLS exchange using the
      TLS_DH_anon (anonymous) ciphersuite.  Alternatively, a publicly
      available static key for emergency access could be used.  In the
      latter case, the device would need to be provisioned with the
      appropriate emergency key for the IAP/ISP in advance.  In another
      case (e.g., IEEE 802.11), no EAP method is used, so that empty
      frames are transported during the over-the-air IEEE 802.1X
      exchange.  In this case, the authentication state machine
      completes with no cryptographic keys being exchanged.

   3) Device Authentication:

      This case extends the server-only authentication case.  If the
      device is configured with a device certificate and the IAP/ISP EAP
      server can rely on a trusted root allowing the EAP server to
      verify the device certificate, at least the device identity (e.g.,
      the MAC address) can be authenticated by the IAP/ISP in NAA cases.
      An example for this is WiMAX devices that are shipped with device
      certificates issued under the global WiMAX device public-key
      infrastructure.  To perform unauthenticated emergency calls, if
      allowed by the IAP/ISP, such devices perform network attachment
      based on EAP-TLS with client authentication based on the device
      certificate.

7.  Security Considerations

   The security threats discussed in [RFC5069] are applicable to this
   document.

   The NASP and NAA cases introduce new vulnerabilities since the PSAP
   operator will typically not have any information about the identity
   of the caller via the signaling path.  Today, in countries where this
   functionality is used for Global System for Mobile Communications
   (GSM) networks, 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.  However, the details, e.g., for using filtering,

Top      ToC       Page 21 
   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
   (caching) LoST server, the ISP would maintain an access control list
   populated with IP-address information obtained from LoST responses
   (in the mappings).  These URIs would either be URIs for contacting
   further LoST servers or PSAP URIs.  It may be necessary to translate
   domain names returned in LoST responses to IP addresses.  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.

   RFC 6280 [RFC6280] discusses security vulnerabilities that are caused
   by an adversary faking location information and thereby lying about
   the actual location of the emergency caller.  These threats may be
   less problematic in the context of an unauthenticated emergency when
   location information can be verified by the ISP to fall within a
   specific geographical area.

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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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, <http://www.rfc-editor.org/info/rfc3261>.

   [RFC4119]  Peterson, J., "A Presence-based GEOPRIV Location Object
              Format", RFC 4119, December 2005,
              <http://www.rfc-editor.org/info/rfc4119>.

   [RFC5031]  Schulzrinne, H., "A Uniform Resource Name (URN) for
              Emergency and Other Well-Known Services", RFC 5031,
              January 2008, <http://www.rfc-editor.org/info/rfc5031>.

Top      ToC       Page 22 
   [RFC5139]  Thomson, M. and J. Winterbottom, "Revised Civic Location
              Format for Presence Information Data Format Location
              Object (PIDF-LO)", RFC 5139, February 2008,
              <http://www.rfc-editor.org/info/rfc5139>.

   [RFC5222]  Hardie, T., Newton, A., Schulzrinne, H., and H.
              Tschofenig, "LoST: A Location-to-Service Translation
              Protocol", RFC 5222, August 2008,
              <http://www.rfc-editor.org/info/rfc5222>.

   [RFC5223]  Schulzrinne, H., Polk, J., and H. Tschofenig, "Discovering
              Location-to-Service Translation (LoST) Servers Using the
              Dynamic Host Configuration Protocol (DHCP)", RFC 5223,
              August 2008, <http://www.rfc-editor.org/info/rfc5223>.

   [RFC5491]  Winterbottom, J., Thomson, M., and H. Tschofenig, "GEOPRIV
              Presence Information Data Format Location Object (PIDF-LO)
              Usage Clarification, Considerations, and Recommendations",
              RFC 5491, March 2009,
              <http://www.rfc-editor.org/info/rfc5491>.

   [RFC6881]  Rosen, B. and J. Polk, "Best Current Practice for
              Communications Services in Support of Emergency Calling",
              BCP 181, RFC 6881, March 2013,
              <http://www.rfc-editor.org/info/rfc6881>.

8.2.  Informative References

   [IEEE802.11]
              IEEE, "IEEE Standard for Information Technology -
              Telecommunications and information exchange between
              systems - Local and metropolitan area networks - Specific
              requirements Part 11: Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications", IEEE Std
              802.11-2012, March 2012,
              <http://standards.ieee.org/about/get/802/802.11.html>.

   [LIS]      Winterbottom, J. and S. Norreys, "LIS to LIS Protocol
              Requirements", Work in Progress, draft-winterbottom-
              geopriv-lis2lis-req-01, November 2007.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
              3748, June 2004, <http://www.rfc-editor.org/info/rfc3748>.

Top      ToC       Page 23 
   [RFC5012]  Schulzrinne, H. and R. Marshall, "Requirements for
              Emergency Context Resolution with Internet Technologies",
              RFC 5012, January 2008,
              <http://www.rfc-editor.org/info/rfc5012>.

   [RFC5069]  Taylor, T., Tschofenig, H., Schulzrinne, H., and M.
              Shanmugam, "Security Threats and Requirements for
              Emergency Call Marking and Mapping", RFC 5069, January
              2008, <http://www.rfc-editor.org/info/rfc5069>.

   [RFC5216]  Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
              Authentication Protocol", RFC 5216, March 2008,
              <http://www.rfc-editor.org/info/rfc5216>.

   [RFC5687]  Tschofenig, H. and H. Schulzrinne, "GEOPRIV Layer 7
              Location Configuration Protocol: Problem Statement and
              Requirements", RFC 5687, March 2010,
              <http://www.rfc-editor.org/info/rfc5687>.

   [RFC6280]  Barnes, R., Lepinski, M., Cooper, A., Morris, J.,
              Tschofenig, H., and H. Schulzrinne, "An Architecture for
              Location and Location Privacy in Internet Applications",
              BCP 160, RFC 6280, July 2011,
              <http://www.rfc-editor.org/info/rfc6280>.

   [RFC6443]  Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
              "Framework for Emergency Calling Using Internet
              Multimedia", RFC 6443, December 2011,
              <http://www.rfc-editor.org/info/rfc6443>.

   [RFC6444]  Schulzrinne, H., Liess, L., Tschofenig, H., Stark, B., and
              A. Kuett, "Location Hiding: Problem Statement and
              Requirements", RFC 6444, January 2012,
              <http://www.rfc-editor.org/info/rfc6444>.

   [esw07]    "3rd Standards Development Organziations (SDO) Emergency
              Services Workshop", October 30th - November 1st 2007,
              <http://www.emergency-services-
              coordination.info/2007Nov/>.

   [nwgstg3]  WiMAX Forum, "WiMAX Forum Network Architecture - Detailed
              Protocols and Procedures Base Specification", Stage-3 WMF-
              T33-001-R022V02, April 2014, <http://resources.wimaxforum.
              org/sites/wimaxforum.org/files/technical_document/2014/05/
              WMF-T33-001-R022v02_Network-Stage3-Base.pdf>.

Top      ToC       Page 24 
Acknowledgments

   Parts of this document are derived from [RFC6881].  Participants of
   the 2nd and 3rd SDO Emergency Services Workshop provided helpful
   input.

   We would like to thank Richard Barnes, Marc Linsner, James Polk,
   Brian Rosen, and Martin Thomson for their feedback at the IETF#80
   Emergency Context Resolution with Internet Technology (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.  Alexey Melnikov was the General Area (Gen-Art) reviewer.  A
   number of changes to the document had been made in response to the AD
   review by Richard Barnes.

   Various IESG members provided review comments, including Spencer
   Dawkins, Stephen Farrell, Joel Jaeggli, Barry Leiba, Ted Lemon, and
   Pete Resnick.

Top      ToC       Page 25 
Authors' Addresses

   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027
   United States

   Phone: +1 212 939 7004
   EMail: hgs+ecrit@cs.columbia.edu
   URI:   http://www.cs.columbia.edu


   Stephen McCann
   BlackBerry Ltd
   200 Bath Road
   Slough, Berks  SL1 3XE
   United Kingdom

   Phone: +44 1753 667099
   EMail: smccann@blackberry.com
   URI:   http://www.blackberry.com


   Gabor Bajko
   MediaTek

   EMail: gabor.bajko@mediatek.com


   Hannes Tschofenig
   Hall in Tirol  6060
   Austria

   EMail: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at


   Dirk Kroeselberg
   Siemens Corporate Technology
   Otto-Hahn-Ring 6
   Munich  81739
   Germany

   EMail: dirk.kroeselberg@siemens.com