Christian M. 4 min read

What is Session Initiation Protocol (SIP)?

In this blog post, we explain the Session Initiation Protocol (SIP) in the simplest way possible, including real-life examples so that even your 90 year-old grandma gets it. Let’s go!


What is Session Initiation Protocol (SIP)?

Ever pondered how our gadgets, from mobiles to laptops, know when and how to chat with each other? Enter SIP, the behind-the-scenes conductor of digital communication.

SIP stands for Session Initiation Protocol. Think of it as the rulebook that apps like Skype or Facebook Messenger follow when they want to start, maintain, or end a chat. It’s the digital equivalent of the social cues we Brits use daily.

Imagine you’re on a bustling high street and spot a friend. You don’t just shout across the crowd; you make eye contact, maybe give a cheeky wave. That’s your human ‘protocol’ at work, signalling, “Fancy a chat?” Once you’ve got their attention, you can dive into the latest gossip or ask where they got that snazzy hat.

Similarly, when your mobile sends a voice note to a friend’s mobile, it doesn’t just send the message into the digital ether. It uses SIP to give a little digital wave, saying, “I’ve got an audio message here!” Your mate’s mobile, understanding the SIP ‘lingo’, knows how to receive, decrypt, and play that message.

Without these rules, our devices would be like two people from different planets trying to chat. Imagine tossing a banana at someone to ask for directions. Confusing, right? That’s why we have SIP, ensuring our gadgets always have meaningful, glitch-free conversations.

Note: Modern apps like WhatsApp, Instagram, and Facebook Messenger don’t use SIP but use their proprietary protocols akin to SIP that are customised for their applications.

How does your device find another device?

Each device is assigned an IP address, which acts like an online postcode.

Devices use another protocol, TCP/IP, to determine the location of the device they want to communicate with. It all starts when a device hops onto the internet and is automatically assigned a static IP or dynamic IP address, which indicates where it is located within the internet. Think of this as a device’s postcode but applied to the online realm.

When your device wants to talk to another, it uses TCP/IP like a map to find the best route to its public IP address, like when your postman uses Google Maps to navigate the roads and deliver its letters. Devices come with TCP/IP pre-installed so that any messaging app can just use this built-in route to find instructions on where to send and receive messages.

Say one of your messaging apps wants to send a message to another device with IP address “”. Your device will first use TCP/IP to find the route to the device, and then SIP will step in, acting like a digital postman. It will send a digital knock on the device’s door, signalling the start, continuation, or end of a chat and what kind of communication is taking place.

If your device wants to send a text message using Mandarin characters, it will tell the other device it is doing so so that the 1s and 0s being sent are not decoded as gibberish. This is why protocols (i.e. rule books) are so important; they make sure devices don’t get lost in digital translation.

What are some typical SIP messages?

There are several signals in the SIP rulebook that are used by devices that are communicating with each other. Let’s say Chris video calls Ben. Hi, the device sends an “INVITE” that prompts Ben’s phone to ring.

If Ben wants to answer, he’ll press the green button, and his device will answer with a “200 OK” in SIP language, confirming that he can answer.

If Ben is busy or doesn’t fancy a chat with Christian, he’ll reject the call, prompting his device to send a “486 Busy Here”, prompting Christian’s phone to display “Call Ended”.

Now, SIP isn’t just about hellos and hold-ons. If Chris’s phone detects a bad connection, it might send an “OPTIONS” request, asking Ben’s phone about its capabilities and if he can change something to improve it.

If Ben misses Chris’s call, his phone will send a “NOTIFY”, letting Chris’s phone know in SIP that Ben’s currently unavailable.

But before they can even hit a stride in their conversation, their phones use SIP to decide on the tone (codec) and the frequency (IP address and port) for their chat. This mutual understanding is facilitated by another protocol, the Real-time Transport Protocol (RTP), ensuring their life-changing conversation doesn’t get lost in the aether.

What apps use Session Initiation Protocol (SIP)?

While SIP (Session Initiation Protocol) is a robust and widely adopted standard for live telecommunications, the most popular communication apps like WhatsApp, Telegram or Instagram use their own proprietary protocols and are only compatible with SIP for certain niche cases.

This is because proprietary standards allow companies to have:

  • Customisation: Can customise the communication experience precisely to their needs, ensuring optimal performance, features, UX and seamless integrations between the company’s own apps (Think Apple!).
  • Security: Can introduce security measures that are unique, to make it harder for malicious actors to exploit vulnerabilities.
  • Control: Owning the protocol means the company has full control over its development direction, updates, and modifications. They aren’t reliant on third-party changes or developments.

However, devices specifically being used in the business and telecom sectors are still using the more interoperable SIP rulebook. VoIP phones, certain video conferencing systems like Cisco, and some call centre applications are still very much in the traditional SIP standards, and for a good reason: they prioritise remaining understood by all parties!

Who develops and maintains SIP?

SIP (Session Initiation Protocol) is maintained and standardised by the Internet Engineering Task Force (IETF), an open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet.

Specifically, the development and maintenance of the SIP protocol falls under the purview of the IETF’s SIP Working Group, which is responsible for addressing any issues, updates, or enhancements related to SIP. They produce documents, known as Request for Comments (RFCs), which detail the specifications and updates to the protocol.

It’s worth noting that while the IETF standardises the protocol, the actual implementation and use of SIP in various products and services are up to individual developers and companies. They often refer to the IETF’s RFCs to ensure they adhere to the standard.

Is Session Initiation Protocol (SIP) open-source?

SIP is an “open standard”, which means its specifications are publicly available, and anyone can implement them without facing licensing restrictions from the IETF.

As SIP is a set of rules (protocol) and not software, it cannot have “open-sourced code”! However, there are many open-source software implementations of SIP. Examples include Asterisk, FreeSWITCH, and Kamailio.

Does my broadband have SIP?

This question is akin to asking: “Does my road have the communications rule book written on it?”.

Broadband internet is the physical infrastructure that allows the transfer of data across devices connected to it, while SIP is the specific standard which devices use to understand each other when establishing live text, voice or media communication.

They are completely different things, and it is certainly not your business broadband provider that gives you a SIP. Instead, SIP files may come directly pre-installed in devices like VoIP phones or in certain niche operating systems.

In normal devices like smartphones, tablets or computers, SIP capabilities are normally introduced through specific applications. When you download a SIP-based app (like a VoIP calling app), it comes with its SIP stack that allows the app to handle SIP communications.

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