\section{User-Visible Emergency Address}
\label{sec:user}

Emergency addresses can be defined at both the network and application
layer.  At the network layer, a designated scoped multicast address
\cite{rfc2365} could reach a local node with knowledge of the iPSAP's
address (see below) that then forwards the request to the appropriate
authority.  Multicast precludes the use of TCP and application-layer
forwarding obliterates the original IP identity.

As an alternative, an IPv6 anycast address can be designated for
emergency communications, but a DHCP option for configuring end systems
would need to be defined.

At the application layer, any protocol designed for user-to-user
communication, including email, SIP, IRC and chat, should have a
designated domain-specific emergency response address, similar to the
current {\tt hostmaster@domain}, {\tt postmaster@domain}, {\tt
webmaster@domain} and similar functional addresses \cite{rfc2142}. 
There is no need to restrict this to one address, however, so that a
message addressed to, say, {\tt 911}, {\tt sos}, or {\tt 110} reach the
correct destination, regardless of the host part of the URL.  It is
highly desirable to standardize this address Internet-wide rather than
leaving this to national authorities, to avoid having to customize
software applications for each national jurisdiction.

It is difficult to provision a proxy for all possible emergency numbers,
as there are at least 60 different ones in use throughout the world (000
01 02 03 04 05 061 07 080 091 100 10022 101 10111 10177 102 107 110 111
112 113 117 118 119 120 122 123 131 132 133 140 144 15 150 155 158 160
166 17 171 175 18 19 190 191 192 193 196 199 911 92 93 94 990 993 994
995 997 998 999).

In addition, callers may also use the \texttt{tel} URL \cite{rfc2806} in
SIP requests with the local emergency number.  However, this does not
simplify the problem since the PSTN gateway still has to determine
whether the caller is within the same PSAP and thus can simply dial 911. 
As discussed in Section~\ref{sec:locating}, PSTN gateways are likely to
yield incorrect location information.

