我们先看看官方规范针对TCP协议的视音频传输描述：

实时视频点播、历史视频回放与下载的 TCP媒体传输应支持基于RTP封装的视音频PS流，封装格式参照IETFRFC4571。

流媒体服务器宜同时支持作为TCP媒体流传输服务端和客户端。默认情况下，前端设备向流媒体服务器发送媒体流时前端设备应作为TCP媒体流传输客户端,流媒体服务器作为 TCP媒体流传输服务端;同级或跨级流媒体服务器间基于 TCP协议传输视频流时,媒体流的接收方宜作为TCP媒体流传输服务端。

媒体流的发送方和接收方可扩展SDP参数进行TCP媒体流传输服务端和客户端的协商,协商机制参考附录 F及IETFRFC4571的定义。

这里我们看个INVITE信令交互示例：

INVITE sip:34020000001320000001@3402000000 SIP/2.0
Via: SIP/2.0/TCP 192.168.0.105:15060;rport;branch=z9hG4bK630055772
From: <sip:34020000002000000001@3402000000>;tag=562055772
To: <sip:34020000001320000001@3402000000>
Call-ID: 589055668
CSeq: 183 INVITE
Content-Type: APPLICATION/SDP
Contact: <sip:34020000002000000001@192.168.0.105:15060>
Max-Forwards: 70
User-Agent: LiveGB28181
Subject: 34020000001320000001:0200000001,34020000002000000001:0
Content-Length: 222v=0
o=34020000001320000001 0 0 IN IP4 192.168.0.105
s=Play
c=IN IP4 192.168.0.105
t=0 0
m=video 30076 RTP/AVP 96 97 98
a=recvonly
a=rtpmap:96 PS/90000
a=rtpmap:97 MPEG4/90000
a=rtpmap:98 H264/90000
y=0200000001

判断媒体流走TCP还是UDP，主要看这里：

m=video 30076 RTP/AVP 96 97 98

传输方式采用“RTP/AVP”标识传输层协议为 RTP over UDP，采用“TCP/RTP/AVP”标识传输层协议为 RTP over TCP，需要注意的是，我们实际对接的时候，部分厂商SDP非常随意，有的甚至直接标记个tcp，这让我们对接的时候，很困惑。

技术实现：

 

本文以大牛直播SDK的Android平台GB28181设备接入端为例，启动GB28181，完成注册、catalog等交互后，Invite上来后，设置媒体流通过TCP还是UDP发送出去：

@Override
public void ntsOnInvitePlay(String deviceId, SessionDescription session_des) {handler_.postDelayed(new Runnable() {@Overridepublic void run() {// 先振铃响应下gb28181_agent_.respondPlayInvite(180, device_id_);MediaSessionDescription video_des = null;SDPRtpMapAttribute ps_rtpmap_attr = null;// 28181 视频使用PS打包Vector<MediaSessionDescription> video_des_list = session_des_.getVideoPSDescriptions();if (video_des_list != null && !video_des_list.isEmpty()) {for(MediaSessionDescription m : video_des_list) {if (m != null && m.isValidAddressType() && m.isHasAddress() ) {video_des = m;ps_rtpmap_attr = video_des.getPSRtpMapAttribute();break;}}}if (null == video_des) {gb28181_agent_.respondPlayInvite(488, device_id_);Log.i(TAG, "ntsOnInvitePlay get video description is null, response 488, device_id:" + device_id_);return;}if (null == ps_rtpmap_attr) {gb28181_agent_.respondPlayInvite(488, device_id_);Log.i(TAG, "ntsOnInvitePlay get ps rtp map attribute is null, response 488, device_id:" + device_id_);return;}Log.i(TAG,"ntsOnInvitePlay, device_id:" +device_id_+", is_tcp:" + video_des.isRTPOverTCP()+ " rtp_port:" + video_des.getPort() + " ssrc:" + video_des.getSSRC()+ " address_type:" + video_des.getAddressType() + " address:" + video_des.getAddress());long rtp_sender_handle = libPublisher.CreateRTPSender(0);if ( rtp_sender_handle == 0 ) {gb28181_agent_.respondPlayInvite(488, device_id_);Log.i(TAG, "ntsOnInvitePlay CreateRTPSender failed, response 488, device_id:" + device_id_);return;}gb28181_rtp_payload_type_  = ps_rtpmap_attr.getPayloadType();gb28181_rtp_encoding_name_ =  ps_rtpmap_attr.getEncodingName();libPublisher.SetRTPSenderTransportProtocol(rtp_sender_handle, video_des.isRTPOverUDP()?0:1);libPublisher.SetRTPSenderIPAddressType(rtp_sender_handle, video_des.isIPv4()?0:1);libPublisher.SetRTPSenderLocalPort(rtp_sender_handle, 0);libPublisher.SetRTPSenderSSRC(rtp_sender_handle, video_des.getSSRC());libPublisher.SetRTPSenderSocketSendBuffer(rtp_sender_handle, 2*1024*1024); // 设置到2MlibPublisher.SetRTPSenderClockRate(rtp_sender_handle, ps_rtpmap_attr.getClockRate());libPublisher.SetRTPSenderDestination(rtp_sender_handle, video_des.getAddress(), video_des.getPort());if ( libPublisher.InitRTPSender(rtp_sender_handle) != 0 ) {gb28181_agent_.respondPlayInvite(488, device_id_);libPublisher.DestoryRTPSender(rtp_sender_handle);return;}int local_port = libPublisher.GetRTPSenderLocalPort(rtp_sender_handle);if (local_port == 0) {gb28181_agent_.respondPlayInvite(488, device_id_);libPublisher.DestoryRTPSender(rtp_sender_handle);return;}Log.i(TAG,"get local_port:" + local_port);String local_ip_addr = IPAddrUtils.getIpAddress(context_);MediaSessionDescription local_video_des = new MediaSessionDescription(video_des.getType());local_video_des.addFormat(String.valueOf(ps_rtpmap_attr.getPayloadType()));local_video_des.addRtpMapAttribute(ps_rtpmap_attr);local_video_des.setAddressType(video_des.getAddressType());local_video_des.setAddress(local_ip_addr);local_video_des.setPort(local_port);local_video_des.setTransportProtocol(video_des.getTransportProtocol());local_video_des.setSSRC(video_des.getSSRC());if (!gb28181_agent_.respondPlayInviteOK(device_id_,local_video_des) ) {libPublisher.DestoryRTPSender(rtp_sender_handle);Log.e(TAG, "ntsOnInvitePlay call respondPlayInviteOK failed.");return;}gb28181_rtp_sender_handle_ = rtp_sender_handle;}private String device_id_;private SessionDescription session_des_;public Runnable set(String device_id, SessionDescription session_des) {this.device_id_ = device_id;this.session_des_ = session_des;return this;}}.set(deviceId, session_des),0);
}

接口设计如下：

/***设置 RTP Sender传输协议** @param rtp_sender_handle, CreateRTPSender返回值* @param transport_protocol, 0:UDP, 1:TCP, 默认是UDP** @return {0} if successful*/public native int SetRTPSenderTransportProtocol(long rtp_sender_handle, int transport_protocol);

以上是GB28181基于TCP协议的视音频媒体传输探究及实现，感兴趣的开发者，可以查看相关协议规范，根据需求实现自己的业务逻辑即可。