SIP protocol: what it is, what it stands for and what it's used for
The protocol that starts, maintains and ends voice and video calls over the internet
The Session Initiation Protocol (SIP) is a signaling protocol used to initiate, maintain, and terminate communication sessions over IP networks.
These sessions can include voice calls, videoconferencing, instant messaging, and other multimedia services.
SIP is an open standard protocol defined by the Internet Engineering Task Force (IETF) in RFC 3261.
What does SIP stand for? It is short for Session Initiation Protocol. That’s why SIP is described as the protocol that opens the call: it connects the participants, agrees on how they will communicate and closes the session when they’re done.
How a SIP call works
A SIP call always follows the same exchange of messages between the caller and the recipient:
- INVITE: the caller’s phone or app sends an invitation to the recipient’s SIP address.
- 100 Trying and 180 Ringing: the network confirms it is looking for the recipient and reports that their phone is ringing.
- 200 OK: the recipient picks up and accepts the call.
- ACK: the caller confirms it has received the response. The session is now open.
- Conversation: voice and video travel over a different protocol, RTP. SIP only sets up and manages the call.
- BYE and 200 OK: one side hangs up and the other confirms the session is closed.
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Relevant historical data
SIP was developed in the 1990s, a time when the telecommunications industry was seeking more flexible and scalable solutions for IP communication.
Before SIP, the most commonly used signaling protocols were H.323 and MGCP, which had certain limitations in terms of flexibility and scalability.
In 1996, Henning Schulzrinne and Mark Handley published the first draft of the SIP protocol, which was quickly adopted by the Internet engineering community.
In 1999, SIP was formally adopted by the IETF as a standard with the publication of RFC 2543. Later, in 2002, it was updated and improved in RFC 3261, which is the most widely used version today.
Key considerations
One of the most important features of SIP is its ability to integrate different types of communication into a single session.
This is achieved through the use of SIP addresses, which work similarly to email addresses.
For example, a SIP address can take the form [email protected], making it easy to integrate with other online services, including email marketing.
SIP operates on a client-server model, where SIP devices can act as clients, servers, or both.
This model allows for the creation of highly flexible and scalable communication networks.
In addition, SIP uses a decentralized architecture, which means it does not rely on a single control point, thereby increasing network resilience.
SIP methods
Methods are the types of request sent in a SIP session. The first 6 are part of the RFC 3261 standard. The rest were added later in other IETF documents.
| Method | What it’s for |
|---|---|
| INVITE | Start a session, for example a call |
| ACK | Confirm that the final response to an INVITE has been received |
| BYE | End an open session |
| CANCEL | Cancel a request that has not been answered yet, such as a call that is still ringing |
| REGISTER | Register a user’s location with the server |
| OPTIONS | Ask which features the other device supports |
| PRACK | Confirm provisional responses, such as 180 Ringing |
| UPDATE | Change a session’s parameters before it is established |
| INFO | Send information during the call, such as keypad tones |
| MESSAGE | Send instant text messages |
| SUBSCRIBE and NOTIFY | Subscribe to an event (for example, a user’s status) and receive notifications |
| REFER | Request that another destination be contacted, for example to transfer a call |
| PUBLISH | Publish a user’s status to the server |
SIP response codes
Every request gets a response with a 3-digit code, similar to the ones used on the web:
| Code | Type | Examples |
|---|---|---|
| 1xx | Provisional | 100 Trying, 180 Ringing |
| 2xx | Success | 200 OK |
| 3xx | Redirection | 302 Moved Temporarily |
| 4xx | Request error | 404 Not Found, 486 Busy Here |
| 5xx | Server error | 503 Service Unavailable |
| 6xx | Global failure | 603 Decline |
Why it matters: benefits
The SIP protocol plays a major role in modern communications thanks to several benefits:
Flexibility
SIP allows the integration of different communication media into a single platform.
This includes voice, video and instant messaging.
This ability to integrate multiple services into one platform simplifies communication management and improves operational efficiency.
Scalability
Since SIP is an open standard protocol, it is compatible with a wide variety of devices and applications.
This allows companies to scale their communication systems without worrying about compatibility issues.
Furthermore, SIP’s decentralized architecture makes it easy to add new users and services without needing to make significant changes to the existing infrastructure.
Cost-effectiveness
Using SIP can result in significant communication cost savings.
By using IP networks for voice transmission and other services, companies can reduce or eliminate the costs associated with traditional telephone networks.
Additionally, SIP enables the implementation of software-based communication solutions, which are usually more affordable than hardware-based solutions.
Improved collaboration
SIP facilitates real-time collaboration by integrating videoconferencing, voice calls, and instant messaging.
This is especially relevant for remote work and global operations, where the ability to communicate effectively and efficiently is key.
Interoperability
As an open standard, SIP ensures interoperability between different systems and vendors.
This means that companies are not tied to a single provider and can choose the solutions that best suit their needs.
In addition, interoperability makes it easier to integrate with other business systems, such as customer relationship management (CRM) systems and email marketing platforms.
How Mailrelay helps you
In short, the Session Initiation Protocol (SIP) is a key component of modern communications.
Its ability to integrate multiple communication services into a single platform, its flexibility, scalability, cost-effectiveness, and improved collaboration make it an attractive option for businesses of all sizes.
Additionally, the interoperability offered by SIP ensures that companies can adapt their communication systems to their specific needs without worrying about compatibility issues.
Mailrelay is not a phone service and does not use SIP. What you can do is combine calls with other channels to stay in touch with your customers:
- Email marketing for newsletters, offers and automated emails.
- SMS for urgent alerts or reminders, with your name as the sender.
- WhatsApp for messages with images and buttons.
- Automations that combine email and SMS, such as a reminder before an appointment.
The free account includes 80,000 emails a month and up to 20,000 contacts.
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Frequently asked questions
These are the most common questions about the SIP protocol.
What does SIP stand for?
It stands for Session Initiation Protocol.
What is the SIP protocol used for?
To start, modify and end real-time communication sessions, such as voice calls, video calls or messaging, over the internet. It is the foundation of most VoIP phone services.
What are the SIP methods?
The basic ones are INVITE (start a session), ACK (confirm it), BYE (end it), CANCEL (cancel a request), REGISTER (register a user) and OPTIONS (check capabilities). Others were added later, such as INFO, UPDATE, MESSAGE, SUBSCRIBE, NOTIFY and REFER.
Which ports does SIP use?
Usually 5060 for unencrypted connections and 5061 for TLS-encrypted connections.
What is the difference between SIP and VoIP?
VoIP is the technology that lets you make calls over the internet. SIP is one of the protocols that make it possible: it handles setting up and managing the calls.