Starts a dialog or changes session parameters when the dialog already exists. This is the main call-setup method in IMS voice traces.
IMS / SIP Requests
This page groups the SIP request methods most often seen in IMS traces. The goal is simple: identify what the endpoint or network is trying to do before you dive into headers, SDP, authentication, or policy details.
In practice, request methods fall into a few repeatable jobs: session setup, session maintenance, registration, service control, subscription handling, and messaging.
Request Message Pages
Open the individual IMS / SIP request references below.
ACK
ACK is the SIP request that confirms a final response to INVITE.
BYEBYE is the SIP request that terminates an established dialog.
CANCELCANCEL is the SIP request that stops a pending INVITE transaction before final acceptance.
INFOINFO is the SIP request used to carry mid-session information inside an existing dialog.
INVITEINVITE is the SIP request that starts or modifies the main session dialog in IMS.
MESSAGEMESSAGE is the SIP request used to carry standalone message content without creating a full session dialog.
NOTIFYNOTIFY is the SIP request that delivers state updates for an active subscription.
OPTIONSOPTIONS is the SIP request used to query capabilities or liveness without creating a full session.
PRACKPRACK is the SIP request that acknowledges a reliable provisional response.
PUBLISHPUBLISH is the SIP request used to publish event state toward a server that manages subscriptions.
REFERREFER is the SIP request used to ask the far side to contact a third target or follow a transfer-related action.
REGISTERREGISTER is the SIP request that creates, refreshes, or removes IMS service reachability for the UE.
SUBSCRIBESUBSCRIBE is the SIP request that creates or refreshes a subscription to an event package.
UPDATEUPDATE is the SIP request used to modify session parameters inside an existing early or confirmed dialog.
Core Session Control
These methods create, confirm, modify, or end the SIP session itself.
Confirms the final response to an INVITE transaction. In traces, it tells you the INVITE exchange really closed successfully.
Ends an existing dialog. Use it when reading call release or session teardown.
Stops an INVITE transaction before a final response is received.
Changes session parameters inside an existing dialog without ending it.
Acknowledges a reliable provisional response, usually when early-session handling matters.
Registration and Capability
These methods matter most when the UE or network is establishing service reachability rather than an active voice session.
Registers the UE contact and service reachability with the IMS core. This is the anchor request in IMS registration procedures.
Queries capabilities or liveness without starting a full session.
Publishes presence-related state toward the service layer when that feature is in use.
Messaging and Mid-Call Service Control
These methods carry application or service information without behaving like the main session-setup request.
Carries standalone SIP message content.
Transfers mid-session service information inside an existing dialog.
Requests call transfer or related redirection behavior.
Subscription and Event Signaling
These methods are common when IMS services depend on event packages or notification-based state updates.
Creates or refreshes a subscription to an event package.
Carries state change notifications for an active subscription.