Multimedia Session Management IP QoS Technologies

234 Chapter 7 • Networking and Services Aspects of Broadband Wireless

7.2.1 Session Initiation Protocol

SIP is a transaction-oriented text-based application-layer protocol that runs over IP [55]. When compared to H.323, SIP is designed as a flexible, lightweight protocol that is extensible, easy to
implement, and quite powerful. Its design philosophy was to decouple the signaling protocol from the service itself and thereby make it useful for a range of unknown future services as well.
A partial list of currently supported services includes multimedia call establishment, user mobil- ity, conference call, multicast, call redirection and other supplementary services, unified mes-
saging, presence detection, and instant messaging. SIP integrates well with other IP-based protocols to provide full multimedia session capabilities. For example, it may use RTP for media
exchange, transport-layer security TLS for security, session description protocol SDP for ses- sion description, and DNS for discovery. SIP can run over a variety of transport protocols: TCP,
user datagram protocol UDP, stream control transport protocol SCTP, and TLS over TCP. Obviously, media streams, such as voice and video for real-time communications, use UDP
rather than TCP owing to delay constraints.
An important feature of SIP is its programmability. SIP follows the HTTP programming model, which allows users and third-party providers to develop SIP-based customized services
rather easily. Many arbitrary services can be built on top of SIP. For example, one could build a service to redirect calls from unknown callers during office hours to a secretary or reply with a
Web page if unavailable. Call-control services, such as third-party call control, that are very dif- ficult to implement in traditional intelligent network IN–based circuit-switched networks are
very easy to set up using SIP. SIP programming may be done using call-processing language CPL, common gateway interface CGI, and application programming interfaces APIs, such
as JAIN
8
and ParlayOSA.
9
The callsession processing logic in SIP may live in either the net- work or the end devices, depending on the particular application. For example, call distribution
may be implemented in the network, distinctive ringing may be implemented in the end device, and forward-on-busy may be implemented in both places.
7.2.1.1 SIP Components and Architecture The basic components of a SIP architecture are illustrated in Figure 7.4.
• SIP end points are called user agents UA and are responsible for making or responding to
calls on behalf of a SIP user or application. Every SIP user or application is given a SIP URL universal resource locator that resembles an e-mail address: sip: user-namedomainname.
The UA acts as either a client or a server, depending on whether it is generating requests or responding to requests on behalf of the user. The UA is typically implemented in the sub-
scriber terminal but may also be on an application server located elsewhere—for example, a video server in the network.
8. JAIN is Java for Advanced Intelligent Network. 9. OSA is Open Systems Access.

7.2 Multimedia Session Management 235

• A SIP proxy server relays session signaling and acts as both client and server. The proxy
server typically operates in a transactional manner and does not keep the session-state information. This makes it extremely scalable and robust. It may, however, be required to
keep state information for certain applications. Proxy servers may rewrite parts of a SIP message before relaying, if required. For example, if a user has moved to a new location,
the proxy server may need to change the destination address. The proxy server determines the current location of the user by querying the location server. The proxy server may also
provide authentication and security services as needed and interact with other proxies belonging to different SIP domains.
• A SIP redirect server responds to a UA request with a redirection response indicting the
current location of the called party. The UA has to then establish a new session to the indi- cated location. This function is analogous to that of a DNS server, which provides the cur-
rent IP address for a given URL.
• A registrar server is where a SIP UA registers its current location information and prefer-
ences. The location information typically includes the current IP address of the SIP UA but may also have additional link-layer-specific details, such as base station identity or
access router identity. The registration message also includes the transport protocol to be used—such as TCP or UDP—port number, and optional fields, such as timestamp, valid-
ity period, and contact preferences.
• The redirect or proxy server contacts the location server to determine the current location
of the user. The location server may be colocated with other servers, such as the registrar server. As shown in Figure 7.4, SIP users may initiate a session by directly contacting one
another or via the proxy server.
Figure 7.4 Basic SIP architecture
Network B Network A
User Agent
User Agent
Location Server
Redirect Server
Direct Call User
Registration Location
Update Location
Request Loca
tion Req
uest
Cal l via
Ser ver
User Agent
Proxy Server
Registrar Server
Redirect Server
Location Server
Forwarded Call
Location Update
Location Request
U se
r R eg
is tra
tio n
L o cat
io n Re
q u est
Proxy Server
Registrar Server
Forward Call