Conclusion:
SIP is a simple but powerfull protocoll. Easy to use, easy to program,
easy to debug and as a drawback easy to sniff.
As a countermeasure parts can be encrypted, leading into more
secure call handling. With its open structure layers can
easily be changed and adapted to suit the needs as no specific layer
is a must. Applications easily can be written in any
language you want. SIP-telephone numbers are in the e-mail based form
SIP:homerun@unitalk.com. So you can use
existing schemes and methodes to find, forward and send call invitations.
SIP-URLs can be stored as DNS-entries.
SIP is only used to initiate call, not for transporting the data, so
any protocoll can be used here, leading into more flexibility.
As SIP is pwoperfull controlling protocoll it has drawbacks as well:
It is still a problem to get a phone connection working over a Firewall,
has no specific means to master rights to speak in
a multiparty connection.When used with SDP, SIP messages carry the
IP addresses and ports to be used for the media
sessions. Sooner or later there will be a shortage on IPv4 addresses.