RFC4317 Session Description Protocol (SDP) Offer/Answer Examples 第二章节阅读笔记

在RFC4317中枚举了很多例子，对于常见的不再解释，这里只选择部分记录

（1）2.4.  Two Audio Streams

In this example, Alice wishes to establish separate audio streams, one for normal audio and the other for telephone-events.  Alice offers two separate streams, one audio with two codecs and the other with RFC 2833 [4] tones (for DTMF).  Bob accepts both audio streams choosing the iLBC codec and telephone-events.

    [Offer]
      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0 97
      a=rtpmap:0 PCMU/8000
      a=rtpmap:97 iLBC/8000
      m=audio 49172 RTP/AVP 98
      a=rtpmap:98 telephone-event/8000

      a=sendonly

这里将普通的音频流和DTMF按键流分成两个m行，在实际应用中，它们经常合在一起

      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0 97 98
      a=rtpmap:0 PCMU/8000
      a=rtpmap:97 iLBC/8000
      a=rtpmap:98 telephone-event/8000

      a=sendonly

（2） 2.5.  Audio and Video 4

Alice and Bob establish an audio and video session with a single audio and video codec.  In a second exchange, Bob changes his address for media and Alice accepts with the same SDP as the initial exchange  (and as a result does not increment the version number).

    [Offer]
      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC/8000
      m=video 51372 RTP/AVP 31

      a=rtpmap:31 H261/90000

    [Answer]
      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49174 RTP/AVP 97
      a=rtpmap:97 iLBC/8000
      m=video 49170 RTP/AVP 31

      a=rtpmap:31 H261/90000

    [Second-Offer]
      v=0
      o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 newhost.biloxi.example.com
      t=0 0
      m=audio 49178 RTP/AVP 97
      a=rtpmap:97 iLBC/8000
      m=video 49188 RTP/AVP 31
      a=rtpmap:31 H261/90000

    [Second-Answer]
      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 97
      a=rtpmap:97 iLBC/8000

      m=video 51372 RTP/AVP 31                                                                                                                                                  a=rtpmap:31 H261/90000

可以看到二次能力协商Offer中Session Number递增1，Answer中Session Number保持不变。Session Number表示当SDP能力发生变更时，则递增，接受方来决定是否解析能力。所以当会话保活选择re-Invite方式进行时，则Session Number保持不变。

（3）2.7.  Audio and Video 5

Alice and Bob establish an audio and video session in the first exchange with a single audio and video codec.  In the second exchange, Alice adds a second video codec, which Bob accepts.  This allows Alice and Bob to switch between the two video codecs without another offer/answer exchange. 

[Offer]
      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 99
      a=rtpmap:99 iLBC/8000
      m=video 51372 RTP/AVP 31
      a=rtpmap:31 H261/90000

[Answer]
      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 99
      a=rtpmap:99 iLBC/8000
      m=video 51374 RTP/AVP 31

      a=rtpmap:31 H261/90000

    [Second-Offer]
      v=0
      o=alice 2890844526 2890844527 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 99
      a=rtpmap:99 iLBC/8000
      m=video 51372 RTP/AVP 31 32
      a=rtpmap:31 H261/90000
      a=rtpmap:32 MPV/90000

    [Second-Answer]
      v=0
      o=bob 2808844564 2808844565 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49172 RTP/AVP 99
      a=rtpmap:99 iLBC/8000
      m=video 51374 RTP/AVP 31 32
      a=rtpmap:31 H261/90000
      a=rtpmap:32 MPV/90000

不理解：因为在视频会议场景中，都只能协商唯一能力。如果m行中支持多种能力，那么认为第一个能力是作为最终能力的。

（4）2.8.  Audio and Video 6  --允许不同能力的C行地址不同

   This example shows an audio and video offer that is accepted, but the answerer wants the video sent to a different address than that of the audio.  This is a common scenario in conferencing where the video and audio mixing utilizes different servers.  In this example, Alice offers audio and video, and Bob accepts.

    [Offer]
      v=0
      o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
      s=
      c=IN IP4 host.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0 8 97
      a=rtpmap:0 PCMU/8000
      a=rtpmap:8 PCMA/8000
      a=rtpmap:97 iLBC/8000
      m=video 51372 RTP/AVP 31 32
      a=rtpmap:31 H261/90000

      a=rtpmap:32 MPV/90000

    [Answer]
      v=0
      o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
      s=
      c=IN IP4 host.biloxi.example.com
      t=0 0
      m=audio 49174 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=video 49172 RTP/AVP 32
      c=IN IP4 otherhost.biloxi.example.com
      a=rtpmap:32 MPV/90000