SIP 180 Ringing vs 491 Request Pending
SIP 180 (Ringing) is a 1xx Provisional response, while 491 (Request Pending) is a 4xx Client Failure response. 180 indicates that the UA receiving the INVITE is trying to alert the user. This response is used to initiate local ringback tone at the caller's end. 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.
説明
The UA receiving the INVITE is trying to alert the user. This response is used to initiate local ringback tone at the caller's end.
このコードが表示される場合
During call setup when the callee's phone is ringing. You should hear a ringback tone after receiving this response.
解決方法
No fix needed — the callee's device is ringing. If the call never connects after ringing, check the callee's device or call forwarding settings.
説明
The request was received by a UAS that had a pending request within the same dialog. The UAS cannot process both requests simultaneously.
このコードが表示される場合
When sending a re-INVITE while another re-INVITE is still being processed. Common during hold/unhold or codec change sequences.
解決方法
Wait for the pending transaction to complete before sending a new one. Implement proper glare handling (RFC 3261 Section 14.1).
主な違い
180 is a 1xx Provisional response, while 491 is a 4xx Client Failure response.
SIP 180: The UA receiving the INVITE is trying to alert the user. This response is used to initiate local ringback tone at the caller's end.
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 180 when during call setup when the callee's phone is ringing. You should hear a ringback tone after receiving this response.
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.
どちらをいつ使うか
For 180 (Ringing): No fix needed — the callee's device is ringing. If the call never connects after ringing, check the callee's device or call forwarding settings. 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).