The points that are the key in our architecture are:

1) The maintenance of a single name space, of URL format, for all
user-to-user communication.  This means that we should be able to use
the same URL for presence and for session initiation.

2) The possibility to implement all services with stateless proxies,
i.e.  proxies that do not need to keep any state information about
ongoing sessions.  This is the very strong point of SIP when it comes to
session handling.

3) The possibility to operate in a strict end-to-end mode once initial
discovery has been accumplished.  SIP provides this through the
"Contact" mechanism.  Indeed, use of that mode can be controlled.

4) The possibility to maintain several identities that resolve to the
same user.  This is a direct consequence of the "stateless proxy" mode. 
A given user may be known by several URL, in different context, and may
use the register function to direct incoming sessions, or buddy
peerings, to the adequate location.

5) The possibility to use UDP as a transport is essential when it comes
to the scaling of servers, since it minimize the amount of state that
need to be kept for any "dormant" association, while not imposing a
three-ways handshake for each message.