本文介紹IP電話機RTP串流穿透的背景、工作原理、設定方式與封包擷取驗證,說明在NAT環境中如何透過RTP Keep Alive維持路由器映射,避免跨網通話、長時間Hold後Resume以及X6視訊通話中的媒體串流中斷問題,適合維護人員排查語音與視訊通訊故障。
當話機跨越NAT進行通訊時,外部媒體串流可能無法穿越內部網路,從而導致無法通訊。這種情況在兩台已經通訊的設備長時間處於Hold狀態後也會出現。由於外部路由器保存的NAT映射資訊逾時,導致Resume後仍然無法通訊。
為了保證NAT環境下的正常通訊,確保RTP串流穿透顯得尤為重要。
根據RFC6263,在INACTIVE和RECVONLY狀態下工作時,需要採用規範中推薦的一種方式來定期發送RTP封包。規範推薦使用RTCP複用RTP的方法,但考慮到很多終端可能沒有實作,基於相容性考量決定採用其他方式。
參考規範第四節,可透過定時發送錯誤Payload Type的RTP封包來保持通訊。
1.3.1設定
開啟上圖設定後,會啟用RTP串流穿透,並在以下情況發送RTP Keep Alive封包:
1話機撥通電話後會發送RTP封包來打通NAT通道(應用於X6視訊通話)
2話機通話Hold後會定時發送RTP來保持NAT連線
1.3.2封包擷取
下圖顯示了發送的RTP Keep Alive 封包,可以看到Wireshark解析出的封包,正常通話的封包編碼為G.711 PCMU,與發送出去的RTP Keep Alive封包編碼並不一致。
下圖顯示一個完整的RTP Keep Alive封包
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