SIP 100 Trying vs 491 Request Pending
SIP 100 (Trying) is a 1xx Provisional response, while 491 (Request Pending) 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, 491 means that the request was received by a UAS that had a pending request within the same dialog. The UAS cannot process both requests simultaneously.
Descripción
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.
Cuándo lo verás
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.
Cómo solucionarlo
No fix needed — this is normal call setup behavior. If stuck on 100 Trying for too long, check network connectivity and proxy configuration.
Descripción
The request was received by a UAS that had a pending request within the same dialog. The UAS cannot process both requests simultaneously.
Cuándo lo verás
When sending a re-INVITE while another re-INVITE is still being processed. Common during hold/unhold or codec change sequences.
Cómo solucionarlo
Wait for the pending transaction to complete before sending a new one. Implement proper glare handling (RFC 3261 Section 14.1).
Diferencias clave
100 is a 1xx Provisional response, while 491 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 491: The request was received by a UAS that had a pending request within the same dialog. The UAS cannot process both requests simultaneously.
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 491 when when sending a re-INVITE while another re-INVITE is still being processed. Common during hold/unhold or codec change sequences.
Cuándo usar cada uno
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 491 (Request Pending): Wait for the pending transaction to complete before sending a new one. Implement proper glare handling (RFC 3261 Section 14.1).