{"id":85532,"date":"2025-07-04T17:51:17","date_gmt":"2025-07-04T17:51:17","guid":{"rendered":"https:\/\/mailrelay.com\/glossary\/sip-protocol\/"},"modified":"2026-10-23T11:14:56","modified_gmt":"2026-10-23T11:14:56","slug":"sip-protocol","status":"publish","type":"glossary","link":"https:\/\/mailrelay.com\/en\/glossary\/sip-protocol\/","title":{"rendered":"SIP Protocol"},"content":{"rendered":"\n

The Session Initiation Protocol (SIP)<\/strong> is a signaling protocol used to initiate, maintain, and terminate communication sessions over IP networks. <\/p>\n\n

These sessions can include voice calls, videoconferencing, instant messaging<\/strong>, and other multimedia services. <\/p>\n\n

SIP is an open standard protocol<\/strong> defined by the Internet Engineering Task Force (IETF)<\/strong> in RFC 3261.<\/p>\n\n

What does SIP stand for?<\/strong> It is short for Session Initiation Protocol<\/em>. 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.<\/p>\n\n

How a SIP call works<\/h2>\n\n

A SIP call always follows the same exchange of messages between the caller and the recipient:<\/p>\n\n

    \n
  1. INVITE:<\/strong> the caller’s phone or app sends an invitation to the recipient’s SIP address.<\/li>\n\n\n\n
  2. 100 Trying and 180 Ringing:<\/strong> the network confirms it is looking for the recipient and reports that their phone is ringing.<\/li>\n\n\n\n
  3. 200 OK:<\/strong> the recipient picks up and accepts the call.<\/li>\n\n\n\n
  4. ACK:<\/strong> the caller confirms it has received the response. The session is now open.<\/li>\n\n\n\n
  5. Conversation:<\/strong> voice and video travel over a different protocol, RTP. SIP only sets up and manages the call.<\/li>\n\n\n\n
  6. BYE and 200 OK:<\/strong> one side hangs up and the other confirms the session is closed.<\/li>\n<\/ol>\n\n
    \"Infographic<\/figure>\n\n
    \n

    Reach your customers in their inbox too<\/strong><\/p>\n\n\n\n

    Besides calls and video calls, email lets you stay in touch with your customers. Get started for free with Mailrelay.<\/p>\n\n\n\n

    \n
    Create a free account<\/a><\/div>\n<\/div>\n<\/div><\/div>\n\n

    Relevant historical data<\/h2>\n\n

    SIP was developed in the 1990s, a time when the telecommunications industry was seeking more flexible and scalable solutions for IP communication. <\/p>\n\n

    Before SIP, the most commonly used signaling protocols were H.323 and MGCP, which had certain limitations in terms of flexibility and scalability.<\/strong><\/p>\n\n

    In 1996<\/strong>, Henning Schulzrinne and Mark Handley published the first draft of the SIP protocol<\/strong>, which was quickly adopted by the Internet engineering community. <\/p>\n\n

    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. <\/p>\n\n

    Key considerations<\/h2>\n\n

    One of the most important features of SIP is its ability to integrate different types of communication into a single session.<\/strong> <\/p>\n\n

    This is achieved through the use of SIP addresses, which work similarly to email addresses. <\/p>\n\n

    For example, a SIP address can take the form user@domain.com<\/strong>, making it easy to integrate with other online services, including email marketing<\/a>.<\/p>\n\n

    SIP operates on a client-server model<\/strong>, where SIP devices can act as clients, servers, or both. <\/p>\n\n

    This model allows for the creation of highly flexible and scalable communication networks. <\/p>\n\n

    In addition, SIP uses a decentralized architecture, which means it does not rely on a single control point, thereby increasing network resilience.<\/p>\n\n

    SIP methods<\/h2>\n\n

    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.<\/p>\n\n

    Method<\/th>What it’s for<\/th><\/tr><\/thead>
    INVITE<\/td>Start a session, for example a call<\/td><\/tr>
    ACK<\/td>Confirm that the final response to an INVITE has been received<\/td><\/tr>
    BYE<\/td>End an open session<\/td><\/tr>
    CANCEL<\/td>Cancel a request that has not been answered yet, such as a call that is still ringing<\/td><\/tr>
    REGISTER<\/td>Register a user’s location with the server<\/td><\/tr>
    OPTIONS<\/td>Ask which features the other device supports<\/td><\/tr>
    PRACK<\/td>Confirm provisional responses, such as 180 Ringing<\/td><\/tr>
    UPDATE<\/td>Change a session’s parameters before it is established<\/td><\/tr>
    INFO<\/td>Send information during the call, such as keypad tones<\/td><\/tr>
    MESSAGE<\/td>Send instant text messages<\/td><\/tr>
    SUBSCRIBE and NOTIFY<\/td>Subscribe to an event (for example, a user’s status) and receive notifications<\/td><\/tr>
    REFER<\/td>Request that another destination be contacted, for example to transfer a call<\/td><\/tr>
    PUBLISH<\/td>Publish a user’s status to the server<\/td><\/tr><\/tbody><\/table><\/figure>\n\n
    \"Infographic<\/figure>\n\n

    SIP response codes<\/h2>\n\n

    Every request gets a response with a 3-digit code, similar to the ones used on the web:<\/p>\n\n

    Code<\/th>Type<\/th>Examples<\/th><\/tr><\/thead>
    1xx<\/td>Provisional<\/td>100 Trying, 180 Ringing<\/td><\/tr>
    2xx<\/td>Success<\/td>200 OK<\/td><\/tr>
    3xx<\/td>Redirection<\/td>302 Moved Temporarily<\/td><\/tr>
    4xx<\/td>Request error<\/td>404 Not Found, 486 Busy Here<\/td><\/tr>
    5xx<\/td>Server error<\/td>503 Service Unavailable<\/td><\/tr>
    6xx<\/td>Global failure<\/td>603 Decline<\/td><\/tr><\/tbody><\/table><\/figure>\n\n

    Why it matters: benefits<\/h2>\n\n

    The SIP protocol plays a major role in modern communications thanks to several benefits:<\/p>\n\n

    Flexibility<\/h3>\n\n

    SIP allows the integration of different communication media into a single platform. <\/p>\n\n

    This includes voice, video and instant messaging. <\/p>\n\n

    This ability to integrate multiple services into one platform simplifies communication management and improves operational efficiency.<\/p>\n\n

    Scalability<\/h3>\n\n

    Since SIP is an open standard protocol, it is compatible with a wide variety of devices and applications. <\/p>\n\n

    This allows companies to scale their communication systems<\/strong> without worrying about compatibility issues. <\/p>\n\n

    Furthermore, SIP’s decentralized architecture makes it easy to add new users and services without needing to make significant changes to the existing infrastructure.<\/p>\n\n

    Cost-effectiveness<\/h3>\n\n

    Using SIP can result in significant communication cost savings. <\/p>\n\n

    By using IP networks for voice transmission and other services, companies can reduce or eliminate the costs associated with traditional telephone networks. <\/p>\n\n

    Additionally, SIP enables the implementation of software-based communication solutions, which are usually more affordable than hardware-based solutions.<\/p>\n\n

    Improved collaboration<\/h3>\n\n

    SIP facilitates real-time collaboration by integrating videoconferencing, voice calls, and instant messaging. <\/p>\n\n

    This is especially relevant for remote work and global operations, where the ability to communicate effectively and efficiently is key.<\/p>\n\n

    Interoperability<\/h3>\n\n

    As an open standard, SIP ensures interoperability between different systems and vendors. <\/p>\n\n

    This means that companies are not tied to a single provider and can choose the solutions that best suit their needs. <\/p>\n\n

    In addition, interoperability makes it easier to integrate with other business systems, such as customer relationship management (CRM) systems and email marketing platforms<\/a>.<\/p>\n\n

    How Mailrelay helps you<\/h2>\n\n

    In short, the Session Initiation Protocol (SIP) is a key component of modern communications. <\/p>\n\n

    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. <\/p>\n\n

    Additionally, the interoperability offered by SIP ensures that companies can adapt their communication systems to their specific needs without worrying about compatibility issues.<\/p>\n\n

    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:<\/p>\n\n