Operation Manual

SIP
SIP Software Release 6-0-0
J613-M0524-00
45
The SIP Server element also provides for more than one type of server. There
are effectively three forms of server that can exist in the network - the SIP
stateful proxy server, the SIP stateless proxy server and the SIP re-direct
server. The main function of the SIP servers is to provide name resolution
and user location, since the caller is unlikely to know the IP address or host
name of the called party. What will be available is perhaps an email-like
address or a telephone number associated with the called party. Using this
information, the caller’s user agent can identify with a specific server to
ʺresolveʺ the address information – it is likely that this will involve many
servers in the network.
A SIP proxy server receives requests, determines where to send these, and
passes them onto the next server (using next hop routing principals). There
can be many server hops in the network.
The difference between a stateful and stateless proxy server is that a stateful
proxy server remembers the incoming requests it receives, along with the
responses it sends back and the outgoing requests it sends on. A stateless
proxy server forgets all information once it has sent on a request. This allows
a stateful proxy server to fork requests to try multiple possible user locations
in parallel and only send the best responses back. Stateless Proxy servers are
most likely to be the fast, backbone of the SIP infrastructure. Stateful proxy
servers are then most likely to be the local devices close to the User Agents,
controlling domains of users and becoming the prime platform for the
application services.
A re-direct server receives requests, but rather than passing these onto the
next server it sends a response to the caller indicating the address for the
called user. This provides the address for the caller to contact the called
party at the next server directly.
SIP addresses users by an email-like address. Each user is identified through
a hierarchical URL that is built around elements such as a user’s phone
number or host name (for example, SIP:[e-mail]). Because of this
similarity, SIP URLs are easy to associate with a user’s e-mail address.
SIP provides its own reliability mechanism and is therefore independent of
the packet layer and only requires an unreliable datagram service. SIP is
typically used over UDP or TCP.
SIP provides the necessary protocol mechanisms so that end systems and
proxy servers can provide services:
• User location
• User capabilities
• User availability
• Call set-up
• Call handling
• Call forwarding, including
The equivalent of 700-, 800- and 900- type calls
Call-forwarding no answer
Call-forwarding busy
Call-forwarding unconditional
Other address-translation services