The Telecom Protocols

SIP responses generated by the UAS or SIP servers. SIP Response Classes: 1xx - Informational - Indicates the status of call prior to the completion. 2xx - Success -  Request has succeeded. 3xx - Redirection - The client should try to complete the request at another server. 4xx - Client Error - The request has failed due to error due to the client.  The client may retry. 5xx - Server failure - The request has failed due to server error. request may retry to another server. 6xx - Global failure - the request has failed and can not retry to any server. 1xx --Informational : 100 Trying : This response is used to indicate the next node receives the request and stop the retransmission. This response is sent if there is delay in sending the final response more the 200ms. 180 Ringing: The response is generated if UA receives the INVITE and started the ringing. It may used to initiate local ring back. 181 Call is being Forwarded: This response is indication of ca

SIP Servers: Proxy Servers:  - A stateless proxy server processes each SIP request or response based solely on the message contents. Once the message has been parsed, processed, and forwarded or responded to,no information about the message is stored—no dialog information is stored. A stateless proxy never re-transmits a message, and does not use any SIP timers. - A stateful proxy server keeps track of requests and responses received in the past and uses that information in processing future requests and responses. For example, a stateful proxy server starts a timer when a request is forwarded. If no response to the request is received within the timer period, the proxy will re-transmit the request, relieving the user agent of this task. Also, a stateful proxy can require user agent authentication. Back-to-Back User Agents (B2BUA): An B2BUA is a type of SIP device that receives the SIP request, that reformulates the request and send it out as new request. Response to the