Request Messages

Request messages, also known as SIP requests, are sent between parties to invoke specific actions. There are several types of request messages, each designed for different communication scenarios: 

INVITE – is used to initiate a session between two or more participants. It's the most commonly used method for starting calls, video conferences, or any multimedia communication. INVITE carries session descriptions that allow participants to agree on a set of compatible media types.   

ACK – is sent in response to a successful INVITE request, specifically after receiving a final response. It signifies that the client has received the final response to an INVITE, completing the transaction and establishing the session.  

BYE – terminates an existing session between two parties. It can be initiated by either the caller or the callee to end the communication. Upon receiving a BYE, the session is considered closed.   

CANCEL – aborts a pending request. It is used to cancel any pending INVITE requests but does not terminate a session if it has already been established. CANCEL requests must have the same parameters as the INVITE request, they are attempting to cancel. 

REGISTER – allows a user agent to register its current location (IP address and port) with a SIP registrar. This is crucial for the SIP system to route incoming calls and messages to the user agent's current location.   

OPTIONS – queries the capabilities of a server or user agent, without initiating a call or transaction. It's often used for capability negotiation or to check the availability of a server or user agent. 

SUBSCRIBE – is used by a user agent to request current state and state updates from a notifier.    

NOTIFY – is used by the notifier to send the user agent information about the state of a resource, in response to a SUBSCRIBE request.   

PUBLISH – uploads the current state of a resource to the server. It's often used in conjunction with the presence information to publish the current status of a user agent.   

INFO – is used for mid-session signaling that does not modify the session state. This can include sending DTMF tones or other information.   

REFER – requests that the recipient REFER to another resource, often used for call transfer or initiating a call on behalf of another party.   

MESSAGE – sends an instant message in the context of a session or as a standalone message outside of a session. It's part of SIP's extension for instant messaging. 

PRACK – (Provisional Acknowledgement) is sent in response to provisional responses (1xx responses except for 100) to an INVITE request. It is used to provide reliable provisional response signaling.   

UPDATE – modifies the state of a session without changing the session's state. It can be used for session refresh, updating media streams, or any other mid-session changes that do not warrant a re-INVITE.

These request messages are crucial for initiating and managing SIP-based communications, allowing for a wide range of functionalities from session establishment to real-time data exchange.