Session Initiation Protocol (SIP) in the IMS architecture

Session Initiation Protocol (SIP) is a critical building block for service delivery in the Next Generation Networks. It has a prominent role in IMS architecture as the new framework for service delivery and management over fixed and mobile networks.


The adoption of SIP by the 3rd Generation Partnership Project (3GPP) as a call control protocol was a major milestone for the standardization world as the IMS architecture became the first SIP-based standard commercial system, setting the guideline for following developments. Driven by the demand for multimedia communication services, the industry had to search for an efficient way to deploy those services in the existing and new networks. Wireless telecommunication operators and vendors had to adapt quickly, enabling a rapid introduction of IP and SIP services into their network through the IMS. The standardization effort has been driven by the 3GPP in close work with the Internet Engineering Task Force (IETF) to ensure the harmonization of standards.


SIP standards set the baseline for the creation of CSCF and other functional elements in the IMS network, ensuring the interoperability of various network elements (UE, CSCF, etc), defining basic communication rules between the elements, and setting the guidelines for interconnecting with other network architectures. The standard body comprises the standards for data plane, user plane as well as session control in the SIP network.

Why is Session Initiation Protocol (SIP) so important?

Ensuring standards compliance helps to keep the number of implementation- and vendor-specific workarounds in the core network to a bare minimum, resulting in a more stable and better-performing network. Below is a list of some of the SIP standards which are essential for the stable operation of all network elements.

RFC 3261 SIP: Session Initiation Protocol

RFC 3262 Reliability of provisional responses in SIP

RFC 3263 Locating SIP servers

RFC 3265 SIP-specific event notification

RFC 3311 The SIP UPDATE Method

RFC 3325 Private Extensions to the SIP for asserted identity within trusted networks

RFC 3326 The Reason Header field for the SIP

RFC 3329 Security-Client, Security-Server and Security-Verify headers

RFC 3515 The SIP Refer method

RFC 3581 The SIP Extension for Symmetric Response Routing

RFC 3824 Using E.164 numbers with the SIP

RFC 3891 The SIP Replace header

RFC 3903 SIP Extension for event state publication

RFC 4032 Update to the SIP preconditions framework

RFC 4353 A framework for conferencing with the SIP

RFC 4579 Call Control – Conferencing for User Agents

RFC 3680 A SIP event package for registrations

RFC 3856 A presence event package for the SIP


RFC 3310 AKA Authentication ( HTTP Digest Authentication

Using Authentication and Key Agreement)

RFC 4169 AKAv2

RFC 4566 – defines basic SDP protocol.

RFC 3264 – Offer/Answer model for SDP media negotiation: defines how SDP is used to negotiate media, e.g. call hold/retrieve.


Standards are defining service functions that enable the creation of new services, content and media distribution, and various formats of multimedia services. They are wide-ranging, defining everything from the network interoperability functions, legacy functions such as SMS to particular aspects of IMS function such as QoS reservation or voice call continuity (where 3GPP extensions rely on particular SIP messages and headers for the call handover). Standards also define particular aspects of IMS function that form the end-user experience and fulfill the mission-critical role, such as enabling emergency calling.

SIP standards as common enablers of new services on IMS networks

The availability of service enablers ensures service compliance and lowers the cost of implementing the more complex end-user services in terms of both capital expenses (CAPEX) and operational expenses (OPEX). To ensure simple and rapid integration of services, the network must support open standard-based interfaces and apply service-oriented architecture principles. Even though many IMS equipment vendors are capable of creating new services, that is just the first of many steps required to get a given service operational and ensure the best customer experience. Successful integration requires taking service applications from diverse sources and integrating them in an innovative way. SIP provides session establishment capabilities for application, using SDP protocol offer/answer (RFC3264) mechanisms.

ng-voice’s fully containerized and cloud-native IMS core is 100% standard-compliant and SIP-based, having been successfully integrated both with legacy structure and innovative open-source applications. To know more about our solution, contact us at

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Andrii Pogrebennyk

Software engineer and team lead

Publisher Bio

After years of experience in VoIP in Ukraine and Austria, Andrii joined ng-voice as a Software Engineer and is using his vast knowledge of telco-specific protocols and Kamailio programming to further develop our fully cloud-native and standard-based IMS core.

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