code: private void trialConnect() throws IOException, RetryDirectly, IllegalAccessException,
              FileDownloadSecurityException {
          FileDownloadConnection trialConnection = null;
          try {
              final ConnectionProfile trialConnectionProfile;
              if (isNeedForceDiscardRange) {
                  trialConnectionProfile = ConnectionProfile.ConnectionProfileBuild
                          .buildTrialConnectionProfileNoRange();
              } else {
                  trialConnectionProfile = ConnectionProfile.ConnectionProfileBuild
                          .buildTrialConnectionProfile();
              }
              final ConnectTask trialConnectTask = new ConnectTask.Builder()
                      .setDownloadId(model.getId())
                      .setUrl(model.getUrl())
                      .setEtag(model.getETag())
                      .setHeader(userRequestHeader)
                      .setConnectionProfile(trialConnectionProfile)
                      .build();
              trialConnection = trialConnectTask.connect();
              handleTrialConnectResult(trialConnectTask.getRequestHeader(),
                      trialConnectTask, trialConnection);
  
          } finally {
              if (trialConnection != null) trialConnection.ending();
          }
      }
docs: the trial connection is for: 1. etag verify; 2. partial support verify.