IMS SIP REFER | IMS Message

Message Fact Sheet

Protocol	ims	Network	5G and LTE
Spec	RFC 3515 / 3GPP TS 24.229	Spec Section	SIP REFER method and transfer-oriented signaling
Direction	UE <-> IMS network	Message Type	SIP request method

Full message name	IMS SIP REFER
Protocol	IMS
Technology	5G and LTE
Common deployment	VoNR and VoLTE
Direction	UE <-> IMS network
Interface	Gm (UE <-> P-CSCF) with onward IMS routing
Signaling bearer / channel	IMS SIP signaling / Transfer-oriented in-dialog SIP transaction
Typical trigger	A service or user action requires call transfer or referral handling.
Main purpose	Triggers referral or transfer behavior by telling the far side what new target or request should be followed.
Main specification	RFC 3515 / 3GPP TS 24.229, SIP REFER method and transfer-oriented signaling
Release added	See specification history
Procedures where used	IMS Transfer Handling

What is IMS SIP REFER in simple terms?

REFER is the SIP request used to ask the far side to contact a third target or follow a transfer-related action.

Triggers referral or transfer behavior by telling the far side what new target or request should be followed.

Why this message matters

REFER asks the far side to follow a new target or transfer-related action.

Where this message appears in the call flow

IMS transfer handling

Call flow position: Service-control step that redirects later signaling toward another target.

Typical state: The session exists and a transfer-like action is being requested.

Preconditions:

A valid dialog or control context exists.

Next likely message: 202 Accepted and follow-up NOTIFY

Blind transfer request

IMS SIP REFER flow requesting a blind call transfer through the IMS core — In a blind transfer path, REFER asks the remote side to start a new call leg toward a different target.

Attended transfer request

IMS SIP REFER flow requesting an attended transfer while an existing call context is already known — In an attended transfer path, REFER uses existing call context to move the session toward the new target.

Call flow position

Previous message(s): Active dialog or service-control context

Next message(s): 202 Accepted, NOTIFY

Message direction and transport

Sender and receiver: UE <-> IMS network

Interface: Gm (UE <-> P-CSCF) with onward IMS routing

Domain: IMS service control

Signaling bearer: IMS SIP signaling

Logical channel: Transfer-oriented in-dialog SIP transaction

Transport / encapsulation: SIP over UDP, TCP, or TLS

Security context: Usually sent inside an existing dialog or service-control context.

Message Structure Overview

Refer-To is the first field to inspect because it explains what referral behavior is being requested.

ASN.1 Message Syntax for IMS SIP REFER

REFER sip:user@example.net SIP/2.0
Via:
From:
To:
Call-ID:
CSeq:
Refer-To:
Referred-By:                       OPTIONAL
Content-Length:

How to read this message syntax

REFER is SIP referral syntax. The target in Refer-To usually tells the core story.

IMS SIP REFER - Example Dump

REFER sip:bob@example.net SIP/2.0
Via: SIP/2.0/UDP ue.example.net;branch=z9hG4bK-ref1
From: <sip:alice@example.net>;tag=mo1
To: <sip:bob@example.net>;tag=term1
Call-ID: call-001@example.net
CSeq: 4 REFER
Refer-To: <sip:carol@example.net>
Content-Length: 0

How to read this dump

Read Refer-To first because it explains the intended referral outcome.

Important Information Elements

IE	Required	Description
`Refer-To`	Yes	Identifies the referral target or requested follow-up address.
`Referred-By`	Optional	Identifies who initiated the referral.
`Call-ID`	Yes	Correlates REFER to the active dialog.

Detailed field explanation

`Refer-To`

Identifies the referral target or requested follow-up address.

Presence: Required

In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.

`Referred-By`

Identifies who initiated the referral.

Presence: Optional

In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.

`Call-ID`

Correlates REFER to the active dialog.

Presence: Required

In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.

What to check in logs and traces

Inspect Refer-To and any Referred-By context.
Check whether REFER belongs to the expected dialog.
Look for the follow-up NOTIFY branch.

Common Issues and Troubleshooting

Transfer signaling starts but the outcome is unclear.

Likely cause: The referral target may be wrong or the notification branch may not complete.

What to inspect: Check Refer-To and the later NOTIFY status reporting.

Next step: Trace REFER together with the resulting NOTIFY updates.

FAQ

What does REFER do in IMS?

It requests referral or transfer behavior, usually by pointing the far side toward another target.

Related Procedures

Related Tools

Use This Reference in Practice

Decode this message with the 3GPP Decoder, inspect the related message database, or open the matching call flow to see where this signaling step fits in the full procedure.