What Is Session Initiation Protocol? Here’s What You Need To Know

session initiation protocol

From voice calls to video conferences and beyond, the ways businesses connect are evolving at an astonishing pace. One of the technologies driving this evolution is Session Initiation Protocol (SIP). As the internet protocol responsible for maintaining real-time communications, SIP solutions have become increasingly popular. In fact, the global SIP services market is expected to reach nearly $44 billion by 2031.1

So, what exactly is SIP? Read on to learn how it works, why it’s important, and how you can leverage it to modernize your business communications today.

First Things First: What Is an Internet Protocol?

An internet protocol (IP) is a set of rules that manage how data is sent and received over the internet. As the core protocol for sending data packets between devices over the internet, IP handles addressing and routing packets to their destinations.

Each internet-connected device has at least one IP address that differentiates it from other devices on the internet. This system allows devices to locate and exchange data with each other, facilitating internet connectivity.

Internet protocols work together using a layered approach, which consists of four main layers:

  • Application Layer – The application layer is where applications, such as web browsers and email clients, interact with protocols like HTTP, FTP, and SMTP.
  • Transport Layer – Protocols like TCP and UDP operate at the transport layer, managing end-to-end communication and data reliability.
  • Internet Layer – IP operates here, handling routing and addressing, ensuring that data packets reach the correct destination.
  • Link Layer – The link layer deals with the physical connection between devices, such as Ethernet or Wi-Fi, and includes protocols that govern data transfer over these physical links.

Key functions of internet protocols include:


Internet protocols enable devices and computers on an IP network to identify each other using unique addresses. The most common example is the IP address, which is used to locate and route data to specific devices on the internet.


Internet protocols define how information should be divided into data packets, which allow it to be easily transported and reassembled at its destination.


Internet protocols determine the path packets should take to reach their destination. This involves routing decisions made by routers and switches at two or more endpoints on IP networks.

Error Handling

Internet protocols include error-checking mechanisms to identify and correct errors that may occur in transit, ensuring data integrity.

What Is SIP?

Session Initiation Protocol is a specific type of IP used for initiating, maintaining, and terminating real-time communications. It’s the technology behind many of today’s most commonly used communication applications, such as VoIP calls, video conferencing, and instant messaging.

This protocol works by sending SIP messages between two or more parties to establish a direct connection, control the communication, and eventually terminate it. This makes it possible for users to share multimedia content seamlessly over the internet, regardless of their geographical location.

SIP has become an integral part of modern communication infrastructure thanks to its flexibility and adaptability. The primary components of SIP are:


SIP allows users to initiate multimedia sessions, whether they’re voice calls, video calls, or instant messages. Users can be individuals, devices, or user agents, which are software or hardware components that act on behalf of users or devices to make calls. The two primary types of user agents in SIP are:

  • User Agent Client (UAC) – A UAC initiates a SIP session, such as making a VoIP phone call. It sends a SIP request, such as an INVITE message, to establish the session.
  • User Agent Server (UAS) – A UAS receives the SIP request and generates a response to it. For example, when you receive a phone call, your VoIP phone acts as a UAS by accepting the call and responding with SIP messages like Trying, Ringing, or OK.


A SIP endpoint is a device or application used for communication. These endpoints can be smartphones, computers, SIP phones, or any other internet-connected device.

SIP Servers

SIP relies on servers to facilitate IP-based communications. The most common SIP server types include:

  • Proxy Servers – A proxy server forwards SIP requests and responses between users or other SIP servers.
  • Registrar Servers – SIP registrar servers maintain user location information to help route calls to the right destination.
  • Redirect Servers – A redirect server provides routing instructions to redirect SIP requests to the appropriate location.

How Does the SIP Protocol Work?

Session Initiation Protocol works using a request-response model. When a communication session is initiated, SIP follows the following steps:

User Initiation

A user initiates communication by sending a SIP request, which can be as simple as a request to start a voice call or a more complex request for a multi-party video conference call. These requests use the Session Description Protocol (SDP) to describe the parameters of the SIP session, offering a standard representation of session information independent of the real-time transport protocol or network.

Server Processing

SIP servers – including a proxy, registrar, and redirect server – process the pending request. The proxy server, in particular, plays a critical role in routing SIP requests to the recipient.

Session Establishment

Once the SIP request reaches the recipient’s device or application, they can accept, reject, or redirect the session. If accepted, the SIP session is established, enabling real-time communication.

Session Management

SIP continues to manage the session, allowing users to modify or terminate it as needed. For example, during a SIP call, users can add participants, mute, or end it altogether.

SIP Features Businesses Should Look For

When deciding on a SIP provider for your business, there are several features you should look out for to maximize your investment. Here are some of the most important:


SIP is vendor-agnostic, meaning it’s not tied to a specific vendor or service provider. This promotes interoperability, which means devices, applications, and systems from different vendors can communicate effectively. Before selecting a SIP service, make sure it can work smoothly with your existing equipment to save you from potential costly upgrades.

Presence Information

SIP includes presence information and location services, allowing users to see their contacts’ availability status. This feature is invaluable for efficient communication since your teams can easily determine if a colleague is available for a call or if they’re in a meeting, reducing unnecessary interruptions.

User Mobility

An estimated 22% of the U.S. workforce will work remotely by 2025,2 making solutions that promote user mobility essential for keeping your teams connected regardless of where they’re working. This feature is especially important today, where remote work and mobile devices are increasingly prevalent. SIP ensures your staff can switch between devices, such as moving from a desk phone to a mobile phone, without disrupting ongoing communications.

Security and Encryption

Given the sensitive nature of information shared over SIP calls, secure transmissions are crucial. Your SIP service should offer encryption and authentication mechanisms to protect your employees’ communication sessions from eavesdropping and unauthorized access, ensuring your sensitive conversations and data remain confidential.

Quality of Service (QoS)

QoS is a measure of the performance of SIP networks, reflecting the service’s transmission quality and availability. A high QoS means fewer disruptions and better SIP call quality, which are essential for real-time business communications.

Reliability and Redundancy

SIP is designed for high reliability – a must for businesses that rely on uninterrupted communication. It supports failover and redundancy, ensuring communications continue even during server failures or IP network issues.

Unified Communications

Session Initiation Protocol is at the heart of unified communications (UC) solutions, which integrate various communication channels and tools into a single platform. UCaaS solutions streamline communications, increasing employee productivity by 32%.3

SIP vs. VoIP: What’s the Difference?

When searching for a modern business communications system, the terms SIP and VoIP come up often. While these technologies have similarities and are used together in many cases, it’s important to note that they aren’t identical.

VoIP, or voice over internet protocol, enables voice communication using the internet. It transforms analog voice signals into digital data packets that can be sent over the internet, enabling users to make calls from their computer, a SIP phone, or other devices. Basically, VoIP is a broad term that represents any internet telephony.

The biggest differences between SIP and VoIP technology include:

Scope of Services

The SIP protocol establishes and manages multimedia communication sessions, including voice, video, and messaging. It focuses on session initiation, modification, and termination. In contrast, VoIP encompasses a broader range of services. This includes SIP and other voice-related protocols like H.323.


Since SIP is a standardized protocol, it offers a higher degree of interoperability when compared to VoIP. It allows users to initiate and receive communication from any device, including smartphones, VoIP phones, and computers.

Use Cases

SIP is typically used to enable multimedia communication and collaboration applications, such as web conferencing and instant messaging. VoIP is used primarily for voice communication, such as making telephone calls over the internet. It’s the technology behind services like Skype, Zoom, and most phone systems that use the internet for calls.

session initiation protocol

How Do VoIP Calls Use the SIP Protocol?

VoIP uses Session Initiation Protocol to establish and maintain multimedia sessions, enabling businesses to save 30% to 50% on communication costs.4 When a VoIP call is initiated, SIP sends an invite message to the recipient containing data such as the sender and receiver’s SIP addresses, session descriptions, and multimedia data types.

Once the recipient receives the invite, the SIP protocol manages the dialogue between the caller and the receiver to negotiate the terms of the communication. Upon agreement, the call is established.

During a VoIP call, the SIP client allows modifications to the session, like adding other participants or changing the type of multimedia being shared. When the call ends, SIP is responsible for terminating the session.

Modernize Your Business Communications With Netlink Voice

Session Initiation Protocol empowers workers to connect with anyone, anywhere, and on any device. SIP makes it easy to support your remote or hybrid teams, whether they need to answer customer phone calls, join a video conference, or collaborate with coworkers.

Our simple cloud communication solution, ConnectWare, leverages SIP to help your employees communicate even better than they did in the office. ConnectWare delivers voice, text, chat, video, presence, real-time wallboards, and reporting – all from a single, integrated internet telephony solution.

If you’re ready to upgrade your outdated phone system, Netlink Voice can help. Schedule a call to get started with our hassle-free business technology solution today.


  1. https://www.transparencymarketresearch.com/sip-trunking-services-market.html
  2. https://www.forbes.com/advisor/business/remote-work-statistics
  3. https://www.uctoday.com/unified-communications/the-best-unified-communications-statistics-for-2022
  4. https://www.founderjar.com/voip-statistics/