![]() PUBLISH is used by a user agent to send event state information to a server. NOTIFY contain an Event header field indicating the event and a subscriptionstate header field indicating the current state of the subscription.Ī NOTIFY is always sent at the start and termination of a subscription. Usually a NOTIFY will trigger within a dialog when a subscription exists between the subscriber and the notifier.Įvery NOTIFY will get 200 OK response if it is received by notifier. NOTIFY is used by user agents to get the occurrence of a particular event. Users can unsubscribe by sending another SUBSCRIBE method with Expires value 0(zero). You can re-subscription again by sending another SUBSCRIBE within the dialog before the expiration time.Ī 200 OK will be received for a subscription from User. ![]() Subscription establishes a dialog between the user agents. It contains an Expires header field that indicates the duration of a subscription.Īfter the time period passes, the subscription will automatically terminate. SUBSCRIBE is used by user agents to establish a subscription for the purpose of getting notification about a particular event. A proxy never generates an OPTIONS request. The response to a request lists the capabilities of the user agent or server. OPTIONS method is used to query a user agent or a proxy server about its capabilities and discover its current availability. An ACK always goes in the direction of INVITE.ACK may contain SDP body (media characteristics), if it is not available in INVITE.ĪCK may not be used to modify the media description that has already been sent in the initial INVITE.Ī stateful proxy receiving an ACK must determine whether or not the ACK should be forwarded downstream to another proxy or user agent.įor 2xx responses, ACK is end to end, but for all other final responses, it works on hop by hop basis when stateful proxies are involved. It can be sent either by a user agent or a proxy server.ĬANCEL is a hop by hop request, i.e., it goes through the elements between the user agent and receives the response generated by the next stateful element.ĪCK is used to acknowledge the final responses to an INVITE method. User agents use this request to cancel a pending call attempt initiated earlier. Here, the From tag contains the URI of the party submitting the registration on behalf of the party identified in the To header.ĬANCEL is used to terminate a session which is not established. This is known as third-party registration. One user agent can send a REGISTER request on behalf of another user agent. REGISTER request contains the time period (3600sec). It carries the AOR (Address of Record) in the To header of the user that is being registered. The REGISTER request may be forwarded or proxied until it reaches an authoritative registrar of the specified domain. This request is sent by a user agent to a registrar server. REGISTER request performs the registration of a user agent. This is a SIP request that can be sent by either the caller or the callee to end a session.īYE request normally routes end to end, bypassing the proxy server.īYE cannot be sent to a pending an INVITE or an unestablished session. ![]() ![]() O = Alice 2890844526 2890844526 IN IP4 īYE is the method used to terminate an established session. Via: SIP/2.0/TLS :5061 branch = z9hG4bK74bf9Īllow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY The following code shows how INVITE is used. Re-INVITE is used to change the session characteristics or refresh the state of a dialog. INVITE can contain the media information of the caller in the message body.Ī session is considered established if an INVITE has received a success response(2xx) or an ACK has been sent.Ī successful INVITE request establishes a dialog between the two user agents which continues until a BYE is sent to terminate the session.Īn INVITE sent within an established dialog is known as a re-INVITE. In other words, an INVITE method is used to establish a media session between the user agents. INVITE is used to initiate a session with a user agent. There are six core methods as discussed below. METHODS are distinguished into two types − METHODS can be regarded as SIP requests, since they request a specific action to be taken by another user agent or server. These SIP requests which are known as METHODS make SIP message workable. To complement them, there are SIP responses that generally indicate whether a request succeeded or failed. SIP requests are the codes used to establish a communication. Similarly, the opening line of a response contains a response code. The opening line of a request contains a method that defines the request, and a Request-URI that defines where the request is to be sent. SIP messages are of two types − requests and responses.
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