简介：
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：
-
接收下级域的注册请求
-
发送catalog请求，查看设备信息
-
选择设备进行预览
invite流程：
- 向下级域发送invite请求
- 下级域回复100
- 下级域回复200
- 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)
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
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:[email protected]:5060>
Content-Type: application/SDP
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
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
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