SIP Protocol Guide: Session Initiation Protocol and Its Uses

Phil Henken
Phil Henken
Updated May 9 2022
webrtc video streaming - business man conducts virtual meeting
Phil Henken
Phil Henken
Updated May 9 2022

SIP protocol is one of the most common protocols used in VoIP technology. That is, “Voice Over IP” which is one of the cheapest ways to communicate any place the Internet is available. VoIP allows users to deliver voice, videos, pictures, and other content.


Be careful not to confuse your acronyms or use them interchangeably, though: between VoIP, SIP, and IP, the letters stand for different things!


IP most often refers to Internet Protocol, which is used to communicate between nodes (machines) on a network. IP is also most often talked about in reference to internet communication–however, IP can also be implemented on enterprise and local area networks (as can the SIP protocol). Essentially, it’s a network layer communications protocol in its own right; IP routes and internetworks, allowing networks to communicate with other networks, essentially establishing the Internet.


When discussing VoIP, the “IP” is referring to Internet Protocol. However, with SIP as our headline points out, the “IP” stands for “initiation protocol”, and the three technologies have markedly different functions.


To better understand SIP protocol, how it’s differentiated from IP and VoIP, and how it affects and improves networked communication, continue below!



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What is the SIP protocol (Session Initiation Protocol)?

SIP protocol stands for “Session Initiation Protocol”–this ironically means that when using the phrase “SIP protocol” you are actually saying “Session Initiation Protocol protocol”, but we’ll just acknowledge that and move on.


As mentioned, SIP is commonly used in VoIP (more about that below). It’s a signaling protocol used for initiating, maintaining, and ending real-time sessions that include voice, video, and messaging. It’s a text-based protocol incorporating elements of HTTP and SMTP and works in conjunction with several other protocols that specify and carry media for the session.


SIP is an application layer protocol: it’s at the top level of the conceptual model describing the flow of data through a computing or telecommunication system. As opposed to physical hardware (the bottom “layer”), the top layer describes user-facing software and interfaces and communication with hosts.


SIP Protocol


Where & When is Session Initiation Protocol Used?

The SIP protocol is used, as you might guess, at the beginning of a session. It can be used to establish two-party or multiparty sessions. It can also modify existing calls: changing addresses or ports, inviting additional participants, and adding and deleting media streams. It’s also useful for messaging applications such as event subscriptions and notifications and instant messaging. As mentioned, SIP functions at the application layer and other protocols handle the media that will be carried in the session; more details in the next section.




SIP Protocol: Key Features & Elements

When SIP is being used in a call or transmission, SDP (Session Description Protocol) most commonly manages the media payload. SIP is independent of transport layer protocols, so underneath the SIP session UDP (User Datagram Protocol), TCP (Transmission Control Protocol), or SCTP (Stream Control Transmission Protocol) could be at work. When dealing with media streams such as a voice and video transmission, SIP typically employs the Real-time Transport Protocol (RTP) or Secure RTP (SRTP). We’ve mentioned these protocols before around here and while RTP and SRTP are old news as far as longer-form online video streaming, they’re still viable in the context of audio and video calls. While not necessarily cutting edge on their own they’re nonetheless a vital part of an overall technology stack that’s used for video calling or conferencing. We’ll give an example or two further along in the post.


SIP protocol is distinguished for being a project originally developed by the Internet community rather than the telecommunications industry and was standardized by the Internet Engineering Task Force (EITF). It was designed as a signaling and call setup protocol for IP-based communication that supported the call processing features and functions that existed in the old public switched telephone network, but with a vision of supporting multimedia applications. It would therefore be extended for video conferencing, streaming media, messaging, file transfer, online games, and several other online applications.


SIP is intertwined in some ways with VoIP technology. We note distinctions between the two in the sections below. In short, SIP is one of the most popular means of delivering VoIP, while VoIP is a broader term applying to any form of calls made over the internet.


SIP Protocol


SIP, VoIP, Video; Differences & Similarities

SIP is a protocol used for handling a media session. SIP includes rules for the establishment and termination of each session and can be used to transmit information between two endpoints or many.


VoIP is a separate individual technology used to establish voice calls over the internet. It’s also worth noting that VoIP is not a protocol itself. SIP can be used in VoIP technology, although VoIP can also use different protocols.


Video is most likely part of your content package (most likely alongside audio since these technologies are used in “calling”). SIP and VOIP are technologies that facilitate video calling. The video itself will be compressed using a codec to travel over the internet.




Pros & Cons of Using SIP

There are alternatives to the SIP protocol, however, it’s the protocol that’s most widely used for internet calling. Here’s a quick look at some main pros and cons.



  • VoIP calls keep costs the same no matter where and how often you call.
  • Communications that are easy to manage. SIP messaging is also available.
  • Flexible for all users, regardless of software and hardware capabilities. Audio-only calling can be used by users with no video.



  • Reliant on an Internet connection for any communications.
  • Weak security features.
  • Internet calling and messages take up bandwidth.

SIP Protocol


Examples of SIP Protocol Use

Typical SIP-based applications are used for internet telephony and multimedia distribution. In the case of mobile and smartphones, an advantage of SIP protocol is that due to its design specifications it can easily operate between VoIP devices and conventional landline-based telephone networks (yes, they’re still out there). It’s also very frequently found “under the hood” (so to speak) of online meeting applications.


Here’s a concrete example: if you start digging into Apple’s technical specifications you’ll discover that Apple’s popular FaceTime video calls use SIP as part of its service for creating encrypted streams.


Essentially, any type of internet calling, from popular apps to enterprise-level calling and conferencing, can potentially incorporate SIP protocol. Many applications that see everyday use both in personal communication and the increasingly virtualized workplace can leverage SIP protocol as an aspect of their call functionality.


Professional example number two: Kaltura has traditionally used SIP as part of our self-serve live broadcasting Video Conferencing Integrations. We build our solutions to be as convenient and seamless as possible to incorporate into your existing workflows, including making it easy for our customers to integrate with meeting applications they already use and are familiar with, if they choose.




Future Developments in Voice & Video Protocol

Technology, and particularly areas like VoIP and telecom, can be unpredictable and subject to a lot of rumors and contrary predictions. However, by some experts’ estimation, the VoIP industry (as it currently exists) will likely continue growing until at least 2025. Certainly, a globalized economy and workforce, and trends toward WFH and hybrid workplaces, give these technologies more room and more cause to expand.


There have been some developments, though. WebRTC emerged in the last decade as an alternative to SIP. While there continues to be a growing demand for video calling, virtual meetings, and real-time communication, additional innovations are likely. Further, the wide adoption and range of use cases, including in professional settings, will no doubt demand the development of more secure tools and protocols for an ever more networked world.




Final Thoughts

SIP protocol is currently a bedrock technology for how we communicate over voice services and the internet. Understanding the fundamentals of communications protocols, how internetworking functions, and the ways real-time voice and video are carried online is a hefty subject area. And it’s not always front of mind among your day-to-day concerns!


At the same time, understanding core components of how your business or organization communicates will also provide a fuller understanding of your technology infrastructure and give a basis for understanding when a better innovation comes along. We hope this article has both expanded your knowledge base and sparked your curiosity to explore SIP protocol, VoIP, and the technologies behind online communication further.

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