Dialogic 6.2 Manuale Utente

Understanding the SIP Protocol

November 2009

246

SIP is not a vertically integrated communications system. SIP is 

rather a component that is used with other IETF protocols to build a 

complete multimedia architecture. Typically, these architectures 

include protocols such as the Real-time Transport Protocol (RTP) 

(RFC 1889 [28]) for transporting real-time data and providing QoS 

feedback, the Real-Time streaming protocol (RTSP) (RFC 2326 [29]) 

for controlling delivery of streaming media, the Media Gateway 

Control Protocol (MEGACO) (RFC 3015 [30]) for controlling 

gateways to the Public Switched Telephone Network (PSTN), and 

the Session Description Protocol (SDP) (RFC 2327 [1]) for describing 

multimedia sessions. Therefore, SIP should be used in conjunction 

with other protocols in order to provide complete services to the 

users. However, the basic functionality and operation of SIP does not 

depend on any of these protocols.

SIP does not provide services. Rather, SIP provides primitives that 

are used to implement different services. For example, SIP can 

locate a user and deliver an opaque object to the current location. If 

this primitive is used to deliver a session description written in SDP, 

for instance, the endpoints can agree on the parameters of a session. 

If the same primitive is used to deliver a photo of the caller as well as 

the session description, a “caller ID” service is easily implemented. 

As this example shows, a single primitive is typically used to provide 

several different services. 

SIP does not offer conference control services such as floor control or 

voting and does not prescribe how a conference is to be managed. SIP 

is used to initiate a session that uses some other conference control 

protocol. Since SIP messages and the sessions they establish can 

pass through entirely different networks, SIP cannot, and does not, 

provide any kind of network resource reservation capabilities.

The nature of the services provided make security particularly 

important. To that end, SIP provides a suite of security services, 

which include denial-of-service prevention, authentication (both user 

to user and proxy to user), integrity protection, and encryption and 

privacy services.

SIP works with both IPv4 and IPv6.