Digital Communication


Digital Communication

SIP protocol: how does it work?


The Session Initiation Protocol (SIP) manages the setting up and tearing down of audio and video connections in real time. It is used in particular in VoIP telephony.

What is the SIP protocol?

In the office as at home, daily life is increasingly punctuated by videoconferences, instant messages, shared files, telephone calls via VoIP or any other form of real-time communication. A single and unique technology allows all these applications: the Session Initiation Protocol or SIP protocol. This network protocol is very important for establishing communication, controlling and later deleting audio or video conversations via VoIP (Voice over IP) with at least two members. The SIP protocol, a leading component in the field of real-time communication, takes into account the particularities of IP networks.

The introduction of the SIP protocol, specified in RFC 3261, made Internet telephony a real alternative to traditional phone calls by hardware telephone system. With the SIP protocol, everyone benefits from much greater mobility and significant financial savings. These two key advantages have allowed SIP to play an increasing role since its introduction in 2004 until today. It even goes so far as to almost completely replace fixed telephone lines.

The Session Initiation Protocol is text-based, similar to HTTP (Hypertext Transfer Protocol) for the Web and SMTP (Simple Mail Transfer Protocol) for electronic communication.

What are the functions of the SIP protocol?

Like the other two protocols, SIP operates on the fifth layer of the OSI model, namely the Session Layer. The task of the SIP protocol resembles that of a early telephone switchboard. At the time, the role of switchboard operators was first to establish the line for a call between two people. The open line was maintained during the conversation and then interrupted at the end of the discussion between the two parties, leaving it free again for other calls. This role is today fulfilled by the SIP. In contrast, the Session Initiation Protocol does not handle any other facet of communication.

The SIPS (Session Initiation Protocol Secure) extension allows the SIP protocol to establish lines for secure and encrypted conversations. Since session and media are two different things, their respective data streams can in theory be independently encrypted. A few other protocols allow the transmission of only voice data, for example the Real Time Transport Protocol (RTP) and the Session Description Protocol (SDP), which provides IP addresses.

How does the SIP protocol work?

The SIP protocol uses a client-server architecture traditional. The basic protocol works by requests and responses, the Session Initiation Protocol acting as an intermediary between the connected terminals. This can affect almost any internet-connected device. SIP then receives requests from clients or user agent (UAC) clients and responses from servers or user agent (UAS) servers and the interface [Trunk SIP] provides phone numbers. However, the other protocols mentioned allow the actual exchange of data. Proxy servers and other gateways are also part of the components for communication with the SIP protocol.

To establish the connection, the Session Description Protocol determines the type of possible connection and regulates its modalities. These different methods are also called codec. The network addresses that should be used are also defined by the SDP. Once this question is settled, a protocol such as RTP ensures the transmission of the actual data. When the session is terminated, the SIP protocol terminates the connection.

SIP protocol: which addressing system?

For correct addressing, the SIP protocol uses the Uniform Resource Identifier (URI) and the Domain Name System (DNS). The structure of the addresses thus assigned to each participant resembles that of ordinary e-mail addresses. As with an email address, a SIP address consists of two elements: a user name or phone number at the beginning and the corresponding network behind. Phone numbers are especially common on devices that interface with phone networks.

What are the different SIP requests?

The SIP protocol works with different requests (or Requests) to which it then reacts with responses (or Responses). These responses use HTTP status codes. SIP protocol requests are divided into basic SIP requests and advanced SIP requests. Here they are in detail:

Basic SIP requests

  • ACK: confirms receipt of a request or response;
  • BYE: properly ends an active session;
  • SIP CANCEL: cancel a pending request;
  • GUEST:* sends a request to a server to open a session;
  • OPTIONS: tells terminals the specifications of other devices involved;
  • REGISTER: registers a device with the service provider.

Advanced SIP Requests

  • INFORMATION: transmits information not directly related to the SIP session;
  • MESSAGE: sends a text message to a device;
  • NOTIFY: checks the connection status and sends notifications in case of change;
  • PRACK: confirms a request provisionally;
  • REFER: forwards an existing connection to another person;
  • SUBSCRIBE: sends a message on the occurrence of a particular event;
  • UPDATE: changes the status of a call.

What are the different SIP responses?

SIP replies are used to respond to the SIP requests listed above. They are divided into six categories:

  • 1xx: indicates that the server has successfully received the request and also provides provisional status information;
  • 2xx: indicates the success of the request;
  • 3xx: informs about possible or necessary redirections;
  • 4xx: indicates that a request could not be processed;
  • 5xx: informs of a failure on the server side;
  • 6xx: indicates that the server could be contacted, but the transaction could not take place for other reasons.

What is the difference between SIP and VoIP?

While it is increasingly common to see the terms used together and these two protocols are closely related, SIP is not synonymous with VoIP. The SIP protocol establishes a connection, maintains it and terminates it. The VoIP protocol remains necessary to transmit data packets via the different types of networks and servers. He therefore begins his work after SIP has prepared the ground for him.

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