该系列⽂章主要介绍，上级域对国标平台进⾏预览/回看等流程的信令交互。 ⼀个简单的gb28181直播预览流程：
1. 接收下级域的注册请求
3. 选择设备进⾏预览
invite流程：
1. 向下级域发送invite请求
2. 下级域回复100
3. 下级域回复200
4. 上级域回复ack
信令流程：
向下级域发送invite请求：
注：此时⽬的域id不再是下级平台的域id，⽽是要预览的设备id了。
INVITE sip:34020000001320000003@3402000000 SIP/2.0
Via: SIP/2.0/UDP 192.168.30.173:5060;rport;branch=SrsGbB87213477
From: <sip:34020000002000000001@3402000000>;tag=SrsGbF30139209
To: <sip:34020000001320000003@3402000000>
Call-ID: 202096464045
CSeq: 101 INVITE
Content-Type: Application/SDP
Contact: <sip:34020000002000000001@3402000000>
Max-Forwards: 70
User-Agent: SRS/4.0.26(Leo)
饮用水过滤器Subject: 34020000001320000003:0009880078,34020000002000000001:0
Content-Length: 216
v=0
o=34020000002000000001 0 0 IN IP4 192.168.30.173
s=Play
u=34020000001320000003:0
c=IN IP4 192.168.30.173
t=0 0
m=video 9000 RTP/AVP 96
a=recvonly
a=rtpmap:96 PS/90000
y=0009880078
下级域回复100：
SIP/2.0 100 Trying
小型生物反应器
Via: SIP/2.0/UDP 192.168.30.173:5060;rport;branch=SrsGbB87213477
From: <sip:34020000002000000001@3402000000>;tag=SrsGbF30139209 To: <sip:34020000001320000003@3402000000>
Call-ID: 202096464045
CSeq: 101 INVITE
User-Agent: IP Camera
Content-Length: 0
下级域回复200：
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.168.30.173:5060;rport=5060;branch=SrsGbB87213477 From: <sip:34020000002000000001@3402000000>;tag=SrsGbF30139209 To: <sip:34020000001320000003@3402000000>;tag=1263794727
Call-ID: 202096464045
CSeq: 101 INVITE
Contact: <sip:34020000001320000003@192.168.30.228:5060>
Content-Type: application/SDP
氨基酸螯合物User-Agent: Embedded Net DVR/NVR/DVS
Content-Length: 264
v=0
o=34020000001110000001 0 0 IN IP4 192.168.30.228
s=EZVIZ X5S
c=IN IP4 192.168.30.228
t=1591259324 1591269113
m=video 62172 RTP/AVP 96剥离力测试方法
a=sendonly
a=rtpmap:96 PS/90000
a=username:34020000001110000001
a=password:12345678
a=filesize:1
y=0009880078
f=
上级域回复ack：
ACK sip:34020000001320000003@3402000000 SIP/2.0
Via: SIP/2.0/UDP 192.168.30.173:5060;rport;branch=SrsGbB87213477
From: <sip:34020000002000000001@3402000000>;tag=SrsGbF30139209 To: <sip:34020000001320000003@3402000000>;tag=1263794727
Call-ID: 202096464045
CSeq: 101 ACK
Max-Forwards: 70
User-Agent: SRS/4.0.26(Leo)
Content-Length: 0柴油机免摇启动器
invite流程⾛完后，会根据sdp中协商的传输协议，ip，端⼝发送⾳视频数据
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