在GB28181的2016版中，对于媒体流的传输在原有UDP传输的基础中，增加了主动tcp和被动tcp的方式。

   1.UDP的传输

    这个是普遍的传输方式，GB28181服务端在发invite时，在携带的SDP中包含了接收媒体的端口，设备端（被呼叫端）收到invite后，解析该端口，通过UDP将媒体流发向该端口。

   2.TCP被动

   GB28181服务端在发invite时，在携带的SDP中包含了接收媒体的端口，并监听该端口的媒体数据，设备端（被呼叫端）收到invite后，解析该端口，通过TCP将媒体流发向该端口。

   3.TCP主动

   设备端（被呼叫端）告知服务端自己的媒体流tcp端口，服务端主动去连接设备端（被呼叫端）的该端口，获取数据。

 

基于新的2016版GB28181协议，我们在设备端开发时除了传统的udp发流外，还要考虑tcp主动和tcp被动两种方式，在代码的实现中，我们通过被服务端invite所携带的sdp进行区分，代码如下：

enum SYMediaProtocol
{
	SYUdp = 0,  //UDP传输
	SYTcpActive, //tcp主动
	SYTcpPassive  //tcp被动
};

SYMediaProtocol CGbtDeviceAgent::getMediaProtocol(const sstd::string szProtocol )
{
    SYMediaProtocol protocol = kUdp;
    if (szProtocol == MediaMedium::RTP_AVP_TCP || szProtocol == MediaMedium::TCP_RTP_AVP)
    {
        if (medium.setup == MediaMedium::active)
        {
            protocol = SYTcpActive;
        }
        else
        {
            protocol = SYTcpPassive;
        }
    }
    return protocol;
}

我们通过 sdp的信息解析后可以判断具体的媒体流传输协议，以下是tcp主动呼叫时服务端和设备端的GB28181报文，为了突出重点，限制篇幅，只截取invite和200OK的报文：

INVITE sip:53000000441329000515@120.229.43.100:14940 SIP/2.0
Via: SIP/2.0/UDP 106.58.169.248:5066;branch=z9hG4bK-524287-1---96235649734f9450;rport
Max-Forwards: 70
Contact: <sip:11010200082006180524@106.58.169.248:5066>
To: <sip:53000000441329000515@120.229.43.100:14940>
From: <sip:11010200082006180524@106.58.169.248:5066>;tag=4452781c
Call-ID: TReMwjEl4tWDd-O-uEJ9vw..
CSeq: 1 INVITE
Subject: 53000000441329000515:0,11010200082006180524:0
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, NOTIFY, SUBSCRIBE, INFO, MESSAGE, ACK, CANCEL, OPTIONS, MESSAGE, SUBSCRIBE, INVITE, REGISTER
Content-Type: application/sdp
Supported: timer, outbound, path, path, 100rel
Content-Length: 190

v=0
o=53000000441329000515 0 0 IN IP4 106.58.169.248
s=Play
c=IN IP4 106.58.169.248
t=0 0
m=video 20000 TCP/RTP/AVP 96
a=rtpmap:96 PS/90000
a=recvonly
y=0999999999
a=setup:passive
SIP/2.0 100 Trying
Via: SIP/2.0/UDP 106.58.169.248:5066;branch=z9hG4bK-524287-1---96235649734f9450;rport=5066
From: <sip:11010200082006180524@106.58.169.248:5066>;tag=4452781c
To: <sip:53000000441329000515@120.229.43.100:14940>
Call-ID: TReMwjEl4tWDd-O-uEJ9vw..
CSeq: 1 INVITE
User-Agent: GbtDeviceKit
Content-Length: 0



SIP/2.0 200 OK
Via: SIP/2.0/UDP 106.58.169.248:5066;branch=z9hG4bK-524287-1---96235649734f9450;rport=5066
From: <sip:11010200082006180524@106.58.169.248:5066>;tag=4452781c
To: <sip:53000000441329000515@120.229.43.100:14940>;tag=1591516984
Call-ID: TReMwjEl4tWDd-O-uEJ9vw..
CSeq: 1 INVITE
Contact: <sip:53000000441329000515@120.229.43.100:14940>
Content-Type: Application/SDP
User-Agent: GbtDeviceKit
Content-Length:   216

v=0
o=53000000441319000515 0 0 IN IP4 192.168.137.180
s=Play
c=IN IP4 192.168.137.180
t=0 0
a=setup:active
m=video 12006 TCP/RTP/AVP 96
a=sendonly
a=rtpmap:96 PS/90000
y=0999999999
f=v/2/15/2/524288/a/1/8
ACK sip:53000000441329000515@120.229.43.100:14940 SIP/2.0
Via: SIP/2.0/UDP 106.58.169.248:5066;branch=z9hG4bK-524287-1---38246b00e73cc219;rport
Max-Forwards: 70
Contact: <sip:11010200082006180524@106.58.169.248:5066>
To: <sip:53000000441329000515@120.229.43.100:14940>;tag=1591516984
From: <sip:11010200082006180524@106.58.169.248:5066>;tag=4452781c
Call-ID: TReMwjEl4tWDd-O-uEJ9vw..
CSeq: 1 ACK
Content-Length: 0

从上图可以看到，TCP/RTP/AVP表示请求tcp进行流媒体的传输，a=setup:active表示主动tcp，根据sdp的内容，这样大家就可以很清楚的了解GB28181的媒体的3种传输方式了。

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