Knowledgebase
SIP Messages and Response codes
Posted by on 09 December 2011 01:45 AM

The Session Initiation Protocol (SIP) is a text based protocol used by ESI Hosted Services to manage call flows and provide telephony service over the internet. SIP is an RFC2833 text message standard; SIP messaging is structured in the format of messages from servers and responses from clients.

There are several basic message types used for controlling calls:

Type
Message
Action or Indication
Request

INVITE

Invites a user or service to participate in a call session

ACK

Confirms that the client has received a final response to an INVITE request

BYE

Terminates a call and can be sent by either the caller or the called party

CANCEL

Cancels any pending searches but does not terminate a call that has already been accepted

OPTIONS

Queries the capabilities of servers

REGISTER

Registers the address listed in the To header field with a SIP server

These messages are one-half of the client-server communication format of SIP messages. There are also response codes sent as replies to the messages; here is a general outline of SIP response codes:

Type
Message
Action or Indication
Response

SIP 1xx

Informational

SIP 2xx

Successful

SIP 3xx

Redirection

SIP 4xx

Client Failure

SIP 5xx

Server Error

SIP 6xx

Global failure                                                                                                                                        


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