RTSP

RTSP是1996年由 RealNetworks, Netscape, 哥倫比亞大學開發，提交草案給 IETF，1998年發布為 RFC2326，2016年RTSP 2.0 發布於 RFC 7826。RTSP 是串流媒體伺服器的控制協定，可建立與控制終端設備跟伺服器之間的多媒體 session。

RTSP 協定本身看起來跟 HTTP 類似，但 HTTP 本身是 stateless，而RTSP 是 stateful，故需要追蹤 session。RTSP 是用 TCP 連線，而多媒體本身，是用 RTP 傳輸，可以用 TCP 或 UDP，常見狀況是為求傳輸速度快，使用 UDP。

節錄一個 RTSP client 取得 RTSP 的封包過程

1. Options

   查詢 RTSP server 支援的 command

   Client -> Server

   Request: OPTIONS rtsp://192.168.1.11:8554/mystream RTSP/1.0\r\n
   CSeq: 2\r\n
   User-Agent: LibVLC/3.0.20 (LIVE555 Streaming Media v2016.11.28)\r\n
   \r\n

   Server -> Client

   Response: RTSP/1.0 200 OK\r\n
   CSeq: 2\r\n
   Public: DESCRIBE, ANNOUNCE, SETUP, PLAY, RECORD, PAUSE, GET_PARAMETER, TEARDOWN\r\n
   Server: gortsplib\r\n
   \r\n

2. Describe

   查詢可處理的多媒體資料格式，server 以 SDP 方式回覆

   Client -> Server

   Request: DESCRIBE rtsp://192.168.1.11:8554/mystream RTSP/1.0\r\n
   CSeq: 3\r\n
   User-Agent: LibVLC/3.0.20 (LIVE555 Streaming Media v2016.11.28)\r\n
   Accept: application/sdp\r\n
   \r\n

   Server -> Client

   Response: RTSP/1.0 200 OK\r\n
   CSeq: 3\r\n
   Content-Base: rtsp://192.168.1.11:8554/mystream/\r\n
   Content-length: 560
   Content-type: application/sdp
   Server: gortsplib\r\n
   \r\n
   Session Description Protocol Version (v): 0
   Owner/Creator, Session Id (o): - 0 0 IN IP4 127.0.0.1
   Session Name (s): test
   Connection Information (c): IN IP4 0.0.0.0
   Time Description, active time (t): 0 0
   Media Description, name and address (m): video 0 RTP/AVP 96
   Media Attribute (a): control:rtsp://192.168.1.11:8554/mystream/trackID=0
   Media Attribute (a): rtpmap:96 H264/90000
   Media Attribute (a): fmtp:96 packetization-mode=1; profile-level-id=640032; sprop-parameter-sets=Z2QAMqzIUB4AiflwEQAAAwPpAAC7gA8YMZY=,aOk4XLIs
   Media Description, name and address (m): audio 0 RTP/AVP 97
   Media Attribute (a): control:rtsp://192.168.1.11:8554/mystream/trackID=1
   Media Attribute (a): rtpmap:97 mpeg4-generic/48000/2
   Media Attribute (a): fmtp:97 config=1190; indexdeltalength=3; indexlength=3; mode=AAC-hbr; profile-level-id=1; sizelength=13; streamtype=5

3. Setup

   要求 server 設定傳送某一個 media stream，setup 要在 play 之前完成

   Client -> Server

   Request: SETUP rtsp://192.168.1.11:8554/mystream/trackID=0 RTSP/1.0\r\n
   CSeq: 4\r\n
   User-Agent: LibVLC/3.0.20 (LIVE555 Streaming Media v2016.11.28)\r\n
   Transport: RTP/AVP/TCP;unicast;interleaved=0-1
   \r\n

   Server -> Client

   Response: RTSP/1.0 200 OK\r\n
   CSeq: 4\r\n
   Server: gortsplib\r\n
   Session: 262553a7d5084e8fb57f9d8f485c89f4
   Transport: RTP/AVP/TCP;unicast;interleaved=0-1;ssrc=03DAD41B
   \r\n

4. Setup

   client 透過 setup 跟 server 確認 stream session

   Client -> Server

   Request: SETUP rtsp://192.168.1.11:8554/mystream/trackID=1 RTSP/1.0\r\n
   CSeq: 5\r\n
   User-Agent: LibVLC/3.0.20 (LIVE555 Streaming Media v2016.11.28)\r\n
   Transport: RTP/AVP/TCP;unicast;interleaved=2-3
   Session: 262553a7d5084e8fb57f9d8f485c89f4
   \r\n

   Server -> Client

   Response: RTSP/1.0 200 OK\r\n
   CSeq: 5\r\n
   Server: gortsplib\r\n
   Session: 262553a7d5084e8fb57f9d8f485c89f4
   Transport: RTP/AVP/TCP;unicast;interleaved=2-3;ssrc=294122AE
   \r\n

5. Play

   在 setup 設定的 session 裡面，開始播放 media

   Client -> Server

   Request: PLAY rtsp://192.168.1.11:8554/mystream/ RTSP/1.0\r\n
   CSeq: 6\r\n
   User-Agent: LibVLC/3.0.20 (LIVE555 Streaming Media v2016.11.28)\r\n
   Session: 262553a7d5084e8fb57f9d8f485c89f4
   Range: npt=0.000-\r\n
   \r\n

   Server -> Client

   Response: RTSP/1.0 200 OK\r\n
   CSeq: 6\r\n
   RTP-Info: url=rtsp://192.168.1.11:8554/mystream/trackID=0;seq=10453;rtptime=1024864644,url=rtsp://192.168.1.11:8554/mystream/trackID=1;seq=4824;rtptime=3779844790\r\n
   Server: gortsplib\r\n
   Session: 262553a7d5084e8fb57f9d8f485c89f4
   \r\n

6. RTP

   用 RTP 的格式，傳送 media 內容

   Server -> Client

   10.. .... = Version: RFC 1889 Version (2)
   ..0. .... = Padding: False
   ...0 .... = Extension: False
   .... 0000 = Contributing source identifiers count: 0
   0... .... = Marker: False
   Payload type: DynamicRTP-Type-96 (96)
   Sequence number: 10453
   Timestamp: 1024871144
   Synchronization Source identifier: 0x03dad41b (64672795)
   Payload: 09f0

7. Teardown

   client 通知 server 停止播放 media

   Client -> Server

   Request: TEARDOWN rtsp://192.168.1.11:8554/mystream/ RTSP/1.0\r\n
   CSeq: 7\r\n
   User-Agent: LibVLC/3.0.20 (LIVE555 Streaming Media v2016.11.28)\r\n
   Session: 262553a7d5084e8fb57f9d8f485c89f4
   \r\n

References

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