REGISTER for IMS registration and authentication flow.
IMS SIP Request Reference
IMS SIP Request Methods
IMS SIP request methods are used to register users, establish sessions, manage call progress, modify media, exchange service information, and release calls in IMS networks. This page is the main reference hub for SIP request messages used in VoLTE, VoNR, and IMS troubleshooting.
How SIP requests work
SIP request methods initiate or modify actions. They tell the network or peer what the UE or service is trying to do, such as register, start a session, confirm setup, update media, send service information, or release a call.
Requests and responses work together: the request starts the transaction, and the response chain shows whether the request progressed, succeeded, or failed. In IMS, the same request methods appear repeatedly across registration, call setup, early media handling, session updates, release, messaging, and supplementary services.
Request function overview
SIP request methods are used to initiate, modify, and terminate actions in IMS signaling. Grouping them by function helps engineers move quickly from a protocol method to the registration, call flow, or troubleshooting context where it appears.
INVITE and ACK for session establishment.
BYE and CANCEL for call teardown and pre-answer cancellation.
PRACK for reliable provisional response handling.
UPDATE and re-INVITE for media and session changes.
MESSAGE, REFER, SUBSCRIBE, NOTIFY, INFO, and OPTIONS for service signaling.
Featured request pages
These are the SIP request methods most commonly seen in IMS registration, VoLTE call setup, early media handling, session updates, service signaling, and call release.
REGISTER
Initial SIP registration request toward the IMS core.
registration INVITEPrimary session-establishment request for voice and multimedia calls.
call setup ACKConfirms the final response to a successful INVITE transaction.
call setup BYENormal request used to release an established dialog.
release CANCELStops a pending INVITE before final answer.
release PRACKAcknowledges a reliable provisional response.
early media UPDATEModifies the session before final answer or mid-dialog.
session update MESSAGEPager-mode SIP messaging request.
messaging REFERTriggers transfer-related or redirection-oriented service behavior.
supplementary service SUBSCRIBEStarts or refreshes an event subscription.
supplementary service NOTIFYCarries event state updates for an existing subscription.
supplementary service OPTIONSQueries capabilities or service reachability.
service signaling INFOCarries mid-session service information within an existing dialog.
service signalingFull request catalog
Registration
Registration requests establish service reachability and identity binding between the UE and IMS core.
Session setup
These requests are at the center of session creation and successful completion of call setup.
Release and cancellation
These requests stop or tear down sessions either before answer or after a dialog is established.
Reliable provisional / early media
These requests matter when the network uses reliable provisional signaling and early media handling.
Session modification and capability handling
These requests are used for media changes, capability checks, and service information exchange. re-INVITE is a procedure variant of INVITE, but it is still a useful separate navigation target for telecom readers.
Messaging and supplementary services
These requests support messaging, transfer, subscription, event reporting, and related supplementary-service signaling.
Requests by IMS procedure
Many users search for SIP requests in the context of a procedure rather than as standalone protocol items. Use the groups below to jump directly to the request methods typically seen in registration, call setup, session modification, release, and supplementary services.
IMS registration
Mobile originated / mobile terminated call setup
Early media / reliable provisional handling
Session modification
Call release
Call cancellation before answer
Messaging / pager mode / service control
Supplementary services / event handling
Common SIP Request Methods in IMS Troubleshooting
How to read a SIP request page
Each child request page should help you move from the method name into its exact role in the procedure and the most useful inspection points in a trace.
- Purpose
- Where it appears in call flow
- Direction of the message
- Common trigger conditions
- Typical SIP headers to inspect
- Whether SDP or body is common
- Normal responses
- Common failures
- Troubleshooting checklist
- Related pages
Related pages
FAQ
What are SIP request methods in IMS?
SIP request methods in IMS are the protocol actions used to register users, establish dialogs, acknowledge successful setup, release calls, update session parameters, exchange service information, and trigger supplementary-service behavior.
What is the difference between SIP requests and SIP responses?
A SIP request starts or modifies an action, such as registration, call setup, update, or release. A SIP response reports what happened to that request, such as progress, success, authentication challenge, or failure.
What is the difference between INVITE, ACK, BYE, and CANCEL?
INVITE starts session setup, ACK confirms a successful final response to INVITE, BYE releases an established dialog, and CANCEL stops a pending INVITE before final answer is completed.
Why does IMS registration use REGISTER?
REGISTER is the SIP request that binds the UE contact and service reachability to the IMS core so voice, messaging, and supplementary-service signaling can be delivered correctly.
Why is PRACK important in IMS?
PRACK is important when reliable provisional responses are used, because it confirms early-dialog signaling and helps the network continue controlled early media and session progression safely.
What is the difference between UPDATE and re-INVITE?
Both can change session parameters, but UPDATE is often used before final answer or mid-dialog without reopening the full INVITE transaction pattern, while re-INVITE is a new INVITE sent inside an existing dialog to change media or session state.
Which SIP request methods are most important in VoLTE and VoNR call flows?
The most important request methods are REGISTER, INVITE, PRACK when reliable provisional handling is used, ACK, UPDATE, BYE, and CANCEL, with MESSAGE, INFO, REFER, SUBSCRIBE, and NOTIFY appearing in service-specific flows.