SIP or Session Initiation Protocol
The SIP Initiation Protocol (SIP) is a signaling protocol for controlling and establishing multimedia calls and meetings over the IP network introduced by the IETF.
The SIP protocol is a client-server connection in which messages (responses and requests) are textual. SIP is a Peer-to-Peer protocol and is a protocol for creating, managing and completing meetings in the NGN network. This session can be text, audio, video or a combination of these. This protocol determines the location and availability of the user. SIP sessions can be shared between one or more users depending on whether they are Unicast or Multicast.
Terminal: Supports real-time two-way communication with another SIP kernel. Supports both media and signaling and includes UAC.
Proxy Server: Connects to one or more clients or Next Hop servers and forwards call requests forwarding, including UAC and UAS.
Redirect Server: Accepts SIP requests, redirects addresses to new addresses, and returns these addresses to the Client. Of course, the Redirect Server does not initiate SIP requests and does not accept calls.
Location Servers: Provides information about the possible location of the caller to the passing server and proxy server.
There are several other servers that include feature servers and are required to support general capabilities and development requirements such as policies, authentication, security, and more.
SIP protocol message structure
Each SIP message includes a start line, a header, a blank line, and a credit line.
The main messages in the SIP protocol
• Requests
It is sent from the customer to the server.
1. INVITE: The user or service is invited to attend the meeting.
2. ACK: The client has received the final response to an invitation request.
3. OPTION: The server asks for capabilities.
4. BYE: The client indicates to the server that the call has been released.
5. CANCEL: Cancels unfinished requests.
6. REGISTER: The client registers the address by a SIP Server.
•Responses
It is sent from the server to the customer.
1. 1X: Information - The request is received and the request process continues.
2. 2XX: Success - Operations are successfully received and analyzed.
3.XX: Routing - Further operations are required to complete the request.
4. 4XX: Client Error - The request contains Bad Syntax or is not executed by the Server.
5. 5XX: Server Error Archives seemingly valid and executable requests.
6. 6XX: Global Error - The request could not be executed on every server.
The following figure shows the conversion of SIP and CCS7 messages.
