A number of interesting issues for implementing SIP call routing using
ENUM for number portability are discussed in [18]. New parameters are pro-
posed for the tel URI to carry number portability (NP) related information that
can be passed to the next hop after the NP database dip has been performed
It is important to note that SIP provides the ability of a UA without access to
ENUM to place SIP calls. The configuration of a default SIP outgoing proxy
server allows an extremely simple UA to simply take a telephone number or
URL input from a user and forward the SIP Request to the outgoing proxy. The
proxy then performs DNS and ENUM described in this chapter, and the call
completes in the same way as if the SIP UA had made the ENUM query.
It is a system design choice if the ENUM resolver is placed in the SIP UA,
such as a SIP phone, or in the outgoing SIP proxy. For the sake of end-user con-
trol, we believe that well-designed SIP UAs must also have an integrated
ENUM resolver as part of the DNS resolver module.
The default SIP proxy can be statically configured in a SIP user agent client
(UAC), or automatically configured using Dynamic Host Configuration Proto-
col (DHCP) at the same time the device is assigned an IP address. Work is in
progress for the complete automatic configuration of SIP UA, including with
the address of the outgoing SIP proxy [19].
Outgoing calls can be handled by the SIP UAC several ways using ENUM:
- UAC takes phone number and performs ENUM DNS query to get the
URI, or UAC performs DNS SRV query on the domain in the URI to get
the IP address. - UAC puts phone number in a tel URL and forwards it to a gateway or
proxy, or UAC puts phone number in a SIP URL and forwards it to a
gateway or proxy. - UAC forwards the URL to the default proxy, which performs either
steps 1 and 2, or queries a location database. This last option may be
preferable, because it relieves SIP devices of DNS transactions required
for ENUM and, thus, avoids extra call setup delay.
Application Scenarios for SIP Service Using ENUM
Here, we will provide high-level examples for an end user in an enterprise net-
work (such as a broker in a financial institution) trying the reach a customer
who may be accessible on either the PSTN/mobile telephone network, Inter-
net, or paging network.
DNS and ENUM 73