■■ Real-time updates—Present DNS technology may introduce minute-long
update delays. Current DNS work aims, however, to reduce the update
delay.
■■ Provisioning complexity—The implementation of secure provisioning of
NAPTR records required for ENUM is quite complex and beyond the
scope of this introduction to SIP.
■■ Other considerations—These may include the storage of supplementary
information, such as security data and spoken names (audio files), or
the improved flexibility of queries.
More detailed implementation issues go beyond the scope of this book. The
IETF, in keeping with its tradition of choosing scalability, simplicity, and state-
of-the art technology, together with allowing for only one option, has decided
to use DNS NAPTR RR for the Tier 2 address resolution using ENUM.DNS and SIP User Preferences
Contact addresses for phone, e-mail, and other addresses can also be hosted in
the DNS, though we believe this to be a less scalable approach, since user pref-
erences will slow down such servers.
Besides contact preferences, SIP also allows users to specify preferences so
as to route incoming calls according to who the caller is, the time of day, the
location of the user, and so on. These preferences can be updated or changed
in real time by the user.
DNS servers are less suited for frequent real-time updates. There are, at pre-
sent, no standard facilities to register and authorize users and to provide stan-
dard contact data formats as specified in SIP. By contrast, SIP servers can
accommodate fast-changing user preferences. SIP adds the following value-
added features to ENUM:
■■ Confidentiality (DNS, by contrast, contains public data)
■■ User preferences
■■ Personal, service, and precall mobility (see Chapter 15)
■■ Frequent and secure access by end users.These features simply may not make sense for ENUM lookup.
Implementers of SIP clients can use the preceding features to considerably
enhance the value and indeed “stickiness” of their products. Feature-rich SIP
phones with adequate displays and PC clients, for example, can display the
presence information about the called party after being allowed to be a
watcher (see Chapter 12), and use it in combination with other applications.72 Chapter 4