Interoperability
  Any authorized users
    many failures (a
    indication of failure via
      Required
  Fall-back
Separate resource control
  at each layer: IP network, service
Separation of network layer and SIP services
  multiple operators

Limited trust
  leverage is only the value of call or subscription
  information much more valuable
  no reason to reveal Subject or media keys

Defined roles
  for each request, act as either proxy or UA

Proxy transparency
End system services
End-end information
  body
  headers other than Via, Record-Route, Route

Forking is not for establishing multiple calls

Back-to-back UAs
  UAs designed as single points of failure
  loop detection
  service transparency
