SIP Overview

The Session Initiation Protocol (SIP) is enabling the initiation, management, and termination of real-time sessions across IP networks. This protocol, foundational to the operation of Voice over Internet Protocol (VoIP) and unified communications, facilitates a vast array of communications processes including call setups, presence information, location services, and secure transmissions. At its core, SIP uses a universal request-response transaction mechanism to allow devices to find each other and efficiently negotiate session parameters, thereby supporting many forms of communications beyond simple voice calls, such as multimedia sessions and conferencing. 

Operating as an application-layer protocol, SIP boasts transport layer independence, capable of functioning across Transmission Control Protocol (TCP), User Datagram Protocol (UDP), or Stream Control Transmission Protocol (SCTP). This flexibility ensures SIP's seamless integration into existing network infrastructures, enhancing its universal applicability and adoption. Its inherently end-to-end design facilitates direct peer-to-peer communications, although intermediate servers often play a critical role in session establishment, management, and routing, demonstrating SIP's adaptable architecture. 

A key feature of SIP is its support for user mobility, leveraging proxying and redirection to accommodate users' current locations, with SIP Uniform Resource Identifiers (URIs) providing a user-friendly means of identification reminiscent of email addresses. The protocol's extensibility, outlined in RFC 3261, underpins its broad range of capabilities, including call forwarding, transfer, hold, and the inclusion of new participants in ongoing sessions, making SIP an indispensable protocol in the telecommunications industry. 

Also, the Session Description Protocol (SDP) works hand in hand with the SIP to establish and manage communication sessions. SDP is used within SIP in several ways to complement the setup and management of communication sessions: Session Description, Negotiation, Media Establishment, Modification and Adaptation. 

SDP is used to convey information about multimedia sessions to the participants. Its primary role is to describe the parameters of the communication session, such as the media format, codecs, media transport protocols, and session timing, among others. SDP itself does not deliver media or initiate sessions. Instead, it is used within SIP for session negotiation.