ߜ Enabling SIP islands to discover the existence of one
another
ߜ Ensuring that the two islands interoperate
ߜ Managing and controlling this interconnection so that
spam and abuse does not become rampant
One answer to all of the above issues is federation.
Federation is the mechanism by which you/your company
can provide end-to-end SIP communications for end-users.
Federation builds a network of open communications within
the ecosystem of SIP communications.
Accomplishing this interconnectivity may require one or
more of these processes:
ߜ A federation service provides discovery services
enabling other SIP islands to be discovered from just a
telephone number or address. A number of databases
and directories are available today. One such database
is ENUM, which is described in the next section,
“Mapping Phone Numbers to URIs.”
ߜ A federation service built on top of SIP provides interop-
erability over a wide range of services and features,
from voice, video, presence, IM, and others. A federation
service may also provide gateways to inter-operate with
other non-SIP devices. For example, a SIP to Jabber gate-
way could interconnect the SIP federation with a Jabber
federation.
ߜ A federation service authenticates all the users and sets
policies for reasonable use. A SIP equivalent of caller ID
is possible in this model, enabling users to trust calling
party indications. This practice avoids the anarchy of
e-mail in which anyone can claim to be anyone else, and
the resulting avalanche of spam. With policies in place,
an enterprise can control how much of its communica-
tion is routed over SIP and to whom.
An alternative to federation is direct peering. In this mode,
two enterprises get together, exchange directory and routing
information between them, and set up policies and proce-
dures for communication. Although this system may work
among a handful of large enterprises, this bilateral arrange-
ment does not work for hundreds and thousands of enter-
prises. It obviously does not scale like a federation does.
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