SIP 200 OK vs 491 Request Pending
SIP 200 (OK) is a 2xx Success response, while 491 (Request Pending) is a 4xx Client Failure response. 200 indicates that the request has succeeded. For INVITE, the call has been answered. For other methods, the action has been completed successfully. 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 succeeded. For INVITE, the call has been answered. For other methods, the action has been completed successfully.
Cuándo lo verás
When a call is answered (INVITE 200 OK), a registration succeeds (REGISTER 200 OK), or a subscription is accepted (SUBSCRIBE 200 OK).
Cómo solucionarlo
No fix needed — this is a success response. The caller should send an ACK to complete the three-way handshake for INVITE transactions.
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
200 is a 2xx Success response, while 491 is a 4xx Client Failure response.
SIP 200: The request has succeeded. For INVITE, the call has been answered. For other methods, the action has been completed successfully.
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 200 when when a call is answered (INVITE 200 OK), a registration succeeds (REGISTER 200 OK), or a subscription is accepted (SUBSCRIBE 200 OK).
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 200 (OK): No fix needed — this is a success response. The caller should send an ACK to complete the three-way handshake for INVITE transactions. 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).