| SIP - Session Initiation Protocol |
SIP - Session Initiation Protocol
- SIP, Session Initiation Protocol is the protocol
used to establish or initiate sessions on IP networks
SIP, the Session Initiation Protocol is used in many
applications and has been adopted as the signalling protocol for use with Voice
over IP ( VoIP ). SIP is a signalling protocol that is used for establishing
sessions on an IP network. The presence of SIP enables sessions to be set up in
a way that enables a host of new services to be made available, thereby allowing
far greater flexibility to be achieved.
SIP, Session Initiation Protocol, is focussed purely on
establishing, modifying and terminating sessions, and has no interest in the
content of the sessions. In view of the focus of SIP, it provides a level of
simplicity that enables to be extensible, and to site easily within different
deployment architectures and scenarios.
SIP is an RFC standard - RFC 3261 from the Internet
Engineering Task Force (IETF). This is the organization that is responsible for
administering and developing the mechanisms that support the Internet. While
other protocols have been used in the past, SIP has now become the protocol of
choice as a result of its flexibility and ability to be updated.
Key functions
There are a number of key functions that SIP provides. It is
able to provide name translation and user location, it negotiates the features
that will be available in a session and it manages the participants in a
session.
- User location and name translation - this function enables data
to reach a party regardless of location. To achieve this SIP, Session
Initiation Protocol addresses are used. These are very similar in format to
email addresses, having elements such as a domain name and a user name or
phone number. Also because of their structure, they are easy to associate
with email addresses.
- Feature negotiation - as different parties may have different
features that are supported it is necessary that both ends communicate in a
way that both can support. For example it would be no use a video enabled
phone trying to sent video to a voice only phone. Thus when a link is set up
all participants negotiate to agree the features that are supported. Also
when one user leaves a session, the remaining ones may renegotiate to
determine whether any new features may be supported.
- Participant management - sessions need to be managed to enable
users to enter or leave sessions. SIP provides this capability.
SIP elements
SIP comprises two basic elements, namely the SIP User Agent
and the SIP Network Server:
- The SIP User Agent This is the component of the protocol that
resides with the user. In turn it consists of two parts: the User Agent
Client (UAC) which initiates the calls and the User Agent Server (UAS) which
answers calls. It allows calls to be made using a peer to peer client server
protocol.
- SIP network server This element contains three basic parts: the
SIP Stateful Server, the SIP Stateless Server, and thirdly the SIP Redirect
Server. These servers act to provide the location of the user and
accordingly direct data to the user, and they also provide name resolution
in a similar way that email addresses and domain names do on the Internet as
it is unlikely that users will remember IP addresses.
SIP also provides its own transfer mechanism which is
independent of the packet layer. This enables it to perform reliably over
protocols such as UDP - a particularly useful feature under some circumstances.
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