GB28181中SSRC的使用和语音广播流程浅析

今天主要聊聊GB28181中，SSRC的作用，从我们之前跟第三方厂商的对接来看，好多厂商对SSRC的处理，并不符合规范。

举个典型的操作：语音广播时带的SSRC和发送RTP包时的SSRC并不一致，然后厂商一开始给出来的结论是，不一致也不影响使用，实则按照规范来看，SSRC还是至关重要的，想想看，如果SSRC不重要的话，SDP携带的SSRC的意义在哪里？如果接入端，不对SSRC做判断，假设有多台设备向Android端GB28181设备接入设备（如执法记录仪、智能头盔等）发送语音广播RTP包，如何过滤哪个设备发过来的数据？再比如，第三方恶意冲击系统，给监听的端口乱发RTP包，如何规避？

咱们先来仔细看看GB/T28181-2016规范里面，是怎么描述SSRC的使用的：

SSRC值由媒体流发送设备所在的SIP监控域产生,作为媒体流的标识使用。点播域内设备、点播外域设备媒体流SSRC的处理方式分别说明如下：

a) 点播域内设备媒体流SSRC处理方式

点播域内设备媒体流时，SSRC值由本域监控系统产生并通过Invite请求发送给设备使用，设备在回复的200 OK消息中携带此值，设备在发送的媒体流中使用此值作为RTP的SSRC值。流程图见图 F.1。

b) 点播外域设备媒体流SSRC处理方式

点播外域设备媒体流时，SSRC由被点播域产生并在被点播域回复的200 OK SDP消息体中携带，被点播域发送的RTP码流使用该值作为SSRC值。流程图见图 F.2。

注5:错误响应补充说明

当设备收到无法满足的SDP时,向发送的Invite请求方发送488错误响应消息;当设备不能满足更多的呼叫请求时,向发送的Invite请求方发送486错误响应消息。

以下就以Android平台GB28181设备接入模块，语音广播这块为例：

当收到GB28181平台端的语音广播请求后，客户端做出响应，并在ntsOnNotifyBroadcastCommand()回调做出相应的处理，调用respondBroadcastCommand()回复平台端，并使能GB28181语音按钮。

/*
 * MainActivity.java
 * GitHub: https://github.com/daniulive/SmarterStreaming
 */
@Override
public void ntsOnNotifyBroadcastCommand(String fromUserName, String fromUserNameAtDomain, String sn, String sourceID, String targetID) {
  handler_.postDelayed(new Runnable() {
    @Override
    public void run() {
      Log.i(TAG, "ntsOnNotifyBroadcastCommand, fromUserName:"+ from_user_name_ + ", fromUserNameAtDomain:"+ from_user_name_at_domain_
            + ", SN:" + sn_ + ", sourceID:" + source_id_ + ", targetID:" + target_id_);

      if (gb28181_agent_ != null ) {
        gb28181_agent_.respondBroadcastCommand(from_user_name_, from_user_name_at_domain_,sn_,source_id_, target_id_, true);
        btnGB28181AudioBroadcast.setText("收到GB28181语音广播通知");
      }
    }

    private String from_user_name_;
    private String from_user_name_at_domain_;
    private String sn_;
    private String source_id_;
    private String target_id_;

    public Runnable set(String from_user_name, String from_user_name_at_domain, String sn, String source_id, String target_id) {
      this.from_user_name_ = from_user_name;
      this.from_user_name_at_domain_ = from_user_name_at_domain;
      this.sn_ = sn;
      this.source_id_ = source_id;
      this.target_id_ = target_id;
      return this;
    }

  }.set(fromUserName, fromUserNameAtDomain, sn, sourceID, targetID),0);
}

然后，在ntsOnAudioBroadcast()回调处理语音广播，创建RTP链路接收数据。

@Override
public void ntsOnAudioBroadcast(String commandFromUserName, String commandFromUserNameAtDomain, String sourceID, String targetID) {
  handler_.postDelayed(new Runnable() {
    @Override
    public void run() {
      Log.i(TAG, "ntsOnAudioBroadcastPlay, fromFromUserName:" + command_from_user_name_
            + " FromUserNameAtDomain:" + command_from_user_name_at_domain_
            + " sourceID:" + source_id_ + ", targetID:" + target_id_);

      stopAudioPlayer();
      destoryRTPReceiver();

      if (gb28181_agent_ != null ) {
        String local_ip_addr = IPAddrUtils.getIpAddress(context_);

        boolean is_tcp = true; // 考虑到跨网段, 默认用TCP传输rtp包
        rtp_receiver_handle_ = lib_player_.CreateRTPReceiver(0);
        if (rtp_receiver_handle_ != 0 ) {
          lib_player_.SetRTPReceiverTransportProtocol(rtp_receiver_handle_, is_tcp?1:0);
          lib_player_.SetRTPReceiverIPAddressType(rtp_receiver_handle_, 0);

          if (0 == lib_player_.CreateRTPReceiverSession(rtp_receiver_handle_, 0) ) {
            int local_port = lib_player_.GetRTPReceiverLocalPort(rtp_receiver_handle_);
            boolean ret = gb28181_agent_.inviteAudioBroadcast(command_from_user_name_,command_from_user_name_at_domain_,
                                                              source_id_, target_id_, "IP4", local_ip_addr, local_port, is_tcp?"TCP/RTP/AVP":"RTP/AVP");

            if (!ret ) {
              destoryRTPReceiver();
              btnGB28181AudioBroadcast.setText("GB28181语音广播");
            }
            else {
              btnGB28181AudioBroadcast.setText("GB28181语音广播呼叫中");
            }
          } else {
            destoryRTPReceiver();
            btnGB28181AudioBroadcast.setText("GB28181语音广播");
          }
        }
      }
    }

    private String command_from_user_name_;
    private String command_from_user_name_at_domain_;
    private String source_id_;
    private String target_id_;

    public Runnable set(String command_from_user_name, String command_from_user_name_at_domain, String source_id, String target_id) {
      this.command_from_user_name_ = command_from_user_name;
      this.command_from_user_name_at_domain_ = command_from_user_name_at_domain;
      this.source_id_ = source_id;
      this.target_id_ = target_id;
      return this;
    }

  }.set(commandFromUserName, commandFromUserNameAtDomain, sourceID, targetID),0);
}

如有异常或timeout，处理相关回调：

@Override
public void ntsOnInviteAudioBroadcastException(String sourceID, String targetID, String errorInfo) {
  handler_.postDelayed(new Runnable() {
    @Override
    public void run() {
      Log.i(TAG, "ntsOnInviteAudioBroadcastException, sourceID:" + source_id_ + ", targetID:" + target_id_);

      destoryRTPReceiver();
      btnGB28181AudioBroadcast.setText("GB28181语音广播");
    }

    private String source_id_;
    private String target_id_;

    public Runnable set(String source_id, String target_id) {
      this.source_id_ = source_id;
      this.target_id_ = target_id;
      return this;
    }

  }.set(sourceID, targetID),0);
}

@Override
public void ntsOnInviteAudioBroadcastTimeout(String sourceID, String targetID) {
  handler_.postDelayed(new Runnable() {
    @Override
    public void run() {
      Log.i(TAG, "ntsOnInviteAudioBroadcastTimeout, sourceID:" + source_id_ + ", targetID:" + target_id_);

      destoryRTPReceiver();
      btnGB28181AudioBroadcast.setText("GB28181语音广播");
    }

    private String source_id_;
    private String target_id_;

    public Runnable set(String source_id, String target_id) {
      this.source_id_ = source_id;
      this.target_id_ = target_id;
      return this;
    }

  }.set(sourceID, targetID),0);
}

ntsOnInviteAudioBroadcastResponse()回调处理如下：

@Override
public void ntsOnInviteAudioBroadcastResponse(String sourceID, String targetID, int statusCode, PlaySessionDescription sessionDescription) {
  handler_.postDelayed(new Runnable() {
    @Override
    public void run() {
      Log.i(TAG, "ntsOnInviteAudioBroadcastResponse, statusCode:" + status_code_ +" sourceID:" + source_id_ + ", targetID:" + target_id_);

      boolean is_need_destory_rtp = true;

      if (gb28181_agent_ != null ) {
        boolean is_need_bye = 200==status_code_;

        if (200 == status_code_ && session_description_ != null && rtp_receiver_handle_ != 0 ) {
          MediaSessionDescription audio_des = session_description_.getAudioDescription();

          SDPRtpMapAttribute audio_attr = null;
          if (audio_des != null && audio_des.getRtpMapAttributes() != null && !audio_des.getRtpMapAttributes().isEmpty() )
            audio_attr = audio_des.getRtpMapAttributes().get(0);

          if ( audio_des != null && audio_attr != null ) {
            lib_player_.SetRTPReceiverSSRC(rtp_receiver_handle_, audio_des.getSSRC());

            lib_player_.SetRTPReceiverPayloadType(rtp_receiver_handle_, audio_attr.getPayloadType(),
                                                  audio_attr.getEncodingName(), 2, audio_attr.getClockRate());

            // 如果是PCMA, SDK会默认填 采样率8000, 通道1, 其他音频编码需要手动填入
            // lib_player_.SetRTPReceiverAudioSamplingRate(rtp_receiver_handle_, 8000);
            // lib_player_.SetRTPReceiverAudioChannels(rtp_receiver_handle_, 1);

            lib_player_.SetRTPReceiverRemoteAddress(rtp_receiver_handle_, audio_des.getAddress(), audio_des.getPort());
            lib_player_.InitRTPReceiver(rtp_receiver_handle_);

            if (startAudioPlay()) {
              is_need_bye = false;
              is_need_destory_rtp = false;

              gb_broadcast_source_id_ = source_id_;
              gb_broadcast_target_id_ = target_id_;
              btnGB28181AudioBroadcast.setText("终止GB28181语音广播");
              btnGB28181AudioBroadcast.setEnabled(true);
            }
          }

        } else {
          btnGB28181AudioBroadcast.setText("GB28181语音广播");
        }

        if (is_need_bye)
          gb28181_agent_.byeAudioBroadcast(source_id_, target_id_);
      }

      if (is_need_destory_rtp)
        destoryRTPReceiver();
    }

    private String source_id_;
    private String target_id_;
    private int status_code_;
    private PlaySessionDescription session_description_;

    public Runnable set(String source_id, String target_id, int status_code, PlaySessionDescription session_description) {
      this.source_id_ = source_id;
      this.target_id_ = target_id;
      this.status_code_ = status_code;
      this.session_description_ = session_description;
      return this;
    }

  }.set(sourceID, targetID, statusCode, sessionDescription),0);
}

ntsOnByeAudioBroadcast()回调处理如下：

@Override
public void ntsOnByeAudioBroadcast(String sourceID, String targetID) {
  handler_.postDelayed(new Runnable() {
    @Override
    public void run() {
      Log.i(TAG, "ntsOnByeAudioBroadcast sourceID:" + source_id_ + " targetID:" + target_id_);

      gb_broadcast_source_id_ = null;
      gb_broadcast_target_id_ = null;
      btnGB28181AudioBroadcast.setText("GB28181语音广播");
      btnGB28181AudioBroadcast.setEnabled(false);

      stopAudioPlayer();
      destoryRTPReceiver();
    }

    private String source_id_;
    private String target_id_;

    public Runnable set(String source_id, String target_id) {
      this.source_id_ = source_id;
      this.target_id_ = target_id;
      return this;
    }

  }.set(sourceID, targetID),0);
}

ntsOnTerminateAudioBroadcast()回调处理如下：

@Override
public void ntsOnTerminateAudioBroadcast(String sourceID, String targetID) {
  handler_.postDelayed(new Runnable() {
    @Override
    public void run() {
      Log.i(TAG, "ntsOnTerminateAudioBroadcast sourceID:" + source_id_ + " targetID:" + target_id_);

      gb_broadcast_source_id_ = null;
      gb_broadcast_target_id_ = null;
      btnGB28181AudioBroadcast.setText("GB28181语音广播");
      btnGB28181AudioBroadcast.setEnabled(false);

      stopAudioPlayer();
      destoryRTPReceiver();
    }

    private String source_id_;
    private String target_id_;

    public Runnable set(String source_id, String target_id) {
      this.source_id_ = source_id;
      this.target_id_ = target_id;
      return this;
    }

  }.set(sourceID, targetID),0);
}

以上是GB28181关于SSRC和语音广播的一点经验，感兴趣的开发者可酌情参考。