* Mapping Locations to ECCs

Once a calling user agent knows its location, the emergency call must be
routed to the best ECC for that location.

DNS and SIP proxies share the responsibility for routing emergency
calls, with caching used to minimize the number of queries handled by
DNS.  A new DNS domain, described below, contains the boundaries of the
major (country, A1, A2) administrative divisions and the address (as an
NAPTR record) of a SIP proxy that can route emergency calls for this
political subdivision.

TBD:  Big issue:  how large is the boundary description for a country or
state?  If thousands of line segments, this will not fit into a DNS
record.  Check maximum size of DNS record.

The outbound proxy always first consults DNS, either an actual server or
the appropriate cached data, to find a SIP proxy.  The SIP proxy for
each domain is considered authoritative and meant to perform
fine-grained routing.  Each administrative division SHOULD have a
corresponding ESRP, even though for efficiency reasons, lower-level
proxies are most likely to be consulted.  Thus, using the administrative
divisions of the United States as an example, each state and county (or
parish) has an ESRP.

Normal DNS caching ensures that almost all calls will not incur the
delay of consulting the authoritative DNS server.  Also, caching allows
emergency call routing even if the DNS server for a particular
administrative sub-division is inaccesible or inoperative.

Unlike for regular DNS lookups, it is RECOMMENDED that outbound proxies
maintain stale DNS entries and use those only if the DNS query fails.

As an example, consider a fictitious way of organizing emergency
services in the United States.  We assume that boundary information for
each state and county is in DNS.  An outbound proxy located in New
Jersey does a zone transfer on .sos.arpa to obtain the current list of
countries.  Similarly, a zone transfer of .us.sos.arpa obtains the
states, the District of Columbia and their boundaries.  It then does a
zone transfer on the counties in New Jersey, but not on the other states
since this particular outbound proxy rarely sees emergency calls from
other regions.  (For example, a company where few of its employees
travel out of state may not need to cache a larger dataset.  Most
carriers and enterprises would probably cache county-level information
for the whole United States, a dataset of about XXX entries that needs
to be refreshed infrequently, probably no more than once a month.)

For this example, we chose counties since all areas in the United States
are assigned to a county.  Not all areas map to Also, county-level
information is easily cached in proxies and changes infrequently,
compared to town and cities.  In many cases, the service area of a PSAP
corresponds to a county, although this is merely a convenience and not a
necessity.


TBD:  how many counties are there in the US?  Are there areas that are
not in any county?

Note that multiple administrative divisions can share the same SIP
proxy.  There only needs to be agreement within each administrative
division as to the boundaries for the next lower division.

It remains to be seen whether the national and sub-national divisions
(states, provinces, etc.) are willing to operate ESRPs.  These ESRPs are
very simple, as they just map a set of geographic coordinates to the
next lower administrative layer.  They do not, for example, have to
track or adjudicate the service boundaries of individual ECCs.

* The .sos.arpa Domain

The .sos.arpa domain. 
not
Countries are designated by their ISO XXX names as maintained by IANA.

Administrative subdivisions appear in multiple Iplaces, using both the
UNCTAD (?) listing of administrative subdivisions (see http://....) and
spellings in the local languages.  If names cannot be represented in US
ASCII, internationalized domain names [XXX] are used.

The administration of the .sos.arpa domain is beyond the scope of this
document.  The management of the .e164.arpa provides an example [XXX] of
how responsibility for different administrative domains can be managed.

Note that in some rare cases of contested international boundaries, it
may occur that the same geographic coordinate falls into multiple
countries.  An outbound proxy may make a local policy decision to route
to one of the claimants or fork the call.

TBD:  What about areas outside territorial waters?


* Mapping from Civil Coordinates




* Mapping from Geospatial Coordinates


DNS PTR records
http://www.ietf.org/internet-drafts/draft-cheshire-dnsext-dns-sd-02.txt
http://www.dns.net/dnsrd/rr.html