SIP 100 Trying vs 421 Extension Required
SIP 100 (Trying) is a 1xx Provisional response, while 421 (Extension Required) is a 4xx Client Failure response. 100 indicates that the request has been received by the next-hop server and an unspecified action is being taken to process it. The UAC should expect additional responses. 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
The request has been received by the next-hop server and an unspecified action is being taken to process it. The UAC should expect additional responses.
Quand vous le voyez
Immediately after sending an INVITE. The proxy has received the call and is searching for the callee. This is the SIP equivalent of a dial tone.
Comment résoudre
No fix needed — this is normal call setup behavior. If stuck on 100 Trying for too long, check network connectivity and proxy configuration.
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
100 is a 1xx Provisional response, while 421 is a 4xx Client Failure response.
SIP 100: The request has been received by the next-hop server and an unspecified action is being taken to process it. The UAC should expect additional responses.
SIP 421: The UAS needs a particular extension to process the request but the extension is not listed in the Supported header.
You encounter 100 when immediately after sending an INVITE. The proxy has received the call and is searching for the callee. This is the SIP equivalent of a dial tone.
You encounter 421 when when the server requires a specific SIP extension that the client did not indicate support for.
Quand utiliser lequel
For 100 (Trying): No fix needed — this is normal call setup behavior. If stuck on 100 Trying for too long, check network connectivity and proxy configuration. 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.