Possible motivations for attackers could be the following:
■■ Prevent the caller from receiving aid
■■ Gain infomation about the caller
■■ Bypass normal authentication to gain access to free servicesThe reference architecture offers many options for attacks that go beyond
the scope of this chapter and, as a consequence, ECRIT security requirements
are under development as of this writing.Using the PSTN for VoIP Emergency Calls
PSTN-based emergency calling services need, in principle, only to be con-
nected to the Internet to complete emergency calls using IP-IP from end to end.
Existing emergency call services have, however, a different logical structure
and legacy databases that have to be taken into account. This brings along the
associated complexity that will be illustrated here.
We will use as the base reference the Interim VoIP Architecture for Enhanced
911 Services [7], published by National Emergency Number Association
(NENA). This architecture leverages the best insight from the Internet-based
architecture shown in Figure 16.1, while taking into account the constraints
and resources of the PSTN-based emergency call services in most of North
America. A simplified diagram of VoIP emergency calling using the PSTN is
shown in Figure 16.2. This architecture does not use either the sosURI, nor
the DNS-based location information for call routing discussed in the preceding
sections, since they are at present not yet implemented.
The SIP UA or some other SIP endpoint (such as an IP-enabled PBX) can
obtain the location object (LO) document from the location information (LI)
server in the network. The LI originates from a VoIP positioning center (VPC)
using an Emergency Routing database that maps civic addresses to routing
numbers on the PSTN. To avoid errors in the central database for routing num-
bers, the operator of the validation database has access to the MSAG that is
also used by the PSTN-based PSAP. The telephony call routing numbers from
the VPC are used by the emergency SIP proxy to route the call to the PSTN
gateway. The telephony routing number will be used by the emergency switch
for 911 calls to route the call to the correct PSAP. The agent taking the call in the
PSAP can consult the Automatic Line Identification (ALI) database to obtain
the callback number for the caller.
Detailed definitions of the elements in the transition VoIP-PSTN architec-
ture, functions, and call flows are provided in the NENA design that we have
outlined here.280 Chapter 16