| Protocol | ims | Network | 5G and LTE |
|---|---|---|---|
| Spec | RFC 3261 / 3GPP TS 24.229 | Spec Section | SIP 400 Bad Request failure response |
| Direction | IMS network -> UE | Message Type | SIP request-failure response |
| Full message name | IMS SIP 400 Bad Request |
|---|---|
| Protocol | IMS |
| Technology | 5G and LTE |
| Common deployment | VoNR and VoLTE |
| Direction | IMS network -> UE |
| Interface | Gm with onward IMS routing |
| Signaling bearer / channel | IMS SIP signaling / Final request-failure response |
| Typical trigger | A request field, header, or body is missing, malformed, or structurally invalid. |
| Main purpose | Stops processing when the request itself is malformed or inconsistent. |
| Main specification | RFC 3261 / 3GPP TS 24.229, SIP 400 Bad Request failure response |
| Release added | See specification history |
| Procedures where used | IMS Request Validation |
400 Bad Request means the request could not be understood or accepted because of syntax or structural problems.
Stops processing when the request itself is malformed or inconsistent.
400 means the request itself was malformed or structurally wrong.
Previous message(s): IMS SIP INVITE, IMS SIP REGISTER
Next message(s): Corrected retry if applicable
Security context: Uses the same protected IMS signaling path as the original request.
SIP/2.0 400 Bad Request
Via:
From:
To:
Call-ID:
CSeq:
Warning: OPTIONAL
Content-Length: 0
This is SIP response syntax rather than ASN.1.
SIP/2.0 400 Bad Request
Via: SIP/2.0/UDP ue.example.net;branch=z9hG4bK-reg1
From: <sip:alice@example.net>;tag=reg1
To: <sip:alice@example.net>
Call-ID: reg-001@example.net
CSeq: 2 REGISTER
Content-Length: 0
| IE | Required | Description |
|---|---|---|
Status-Line | Yes | Shows the 400 failure code. |
Warning | Optional | Can provide extra parse or validation context. |
Status-LineShows the 400 failure code.
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.
WarningCan provide extra parse or validation context.
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.
Likely cause: The request format, headers, or body were invalid.
What to inspect: Check the exact request syntax, header completeness, and body formatting.
Next step: Compare the failed request against a known-good message of the same type.
It means the SIP request could not be accepted because its format or structure was invalid.
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.