SIP 183 Session Progress vs 421 Extension Required
SIP 183 (Session Progress) is a 1xx Provisional response, while 421 (Extension Required) is a 4xx Client Failure response. 183 indicates that used to convey information about the progress of the call that is not otherwise classified. Often carries early media (ringback tones, announcements) via SDP. In contrast, 421 means that the UAS needs a particular extension to process the request but the extension is not listed in the Supported header.
Description
Used to convey information about the progress of the call that is not otherwise classified. Often carries early media (ringback tones, announcements) via SDP.
Quand vous le voyez
During call setup, especially in PSTN interworking scenarios. The response may contain SDP for early media like custom ringback tones or IVR prompts.
Comment résoudre
No fix needed — this is informational. If early media is not playing, check codec negotiation and SDP handling in your UA.
Description
The UAS needs a particular extension to process the request but the extension is not listed in the Supported header.
Quand vous le voyez
When the server requires a specific SIP extension that the client did not indicate support for.
Comment résoudre
Add the required extension to the Supported header and ensure your UA implements it. Check the response for which extension is needed.
Différences clés
183 is a 1xx Provisional response, while 421 is a 4xx Client Failure response.
SIP 183: Used to convey information about the progress of the call that is not otherwise classified. Often carries early media (ringback tones, announcements) via SDP.
SIP 421: The UAS needs a particular extension to process the request but the extension is not listed in the Supported header.
You encounter 183 when during call setup, especially in PSTN interworking scenarios. The response may contain SDP for early media like custom ringback tones or IVR prompts.
You encounter 421 when when the server requires a specific SIP extension that the client did not indicate support for.
Quand utiliser lequel
For 183 (Session Progress): No fix needed — this is informational. If early media is not playing, check codec negotiation and SDP handling in your UA. For 421 (Extension Required): Add the required extension to the Supported header and ensure your UA implements it. Check the response for which extension is needed.