The coordinator of zstack 3.0 defaults to a time limit for device join, and how to keep it in a permit join forever？

void ZDO_ProcessMgmtPermitJoinReq( zdoIncomingMsg_t *inMsg )
{
  uint8 stat;
  uint8 duration;
#if (ZG_BUILD_COORDINATOR_TYPE)
  if(ZG_DEVICE_COORDINATOR_TYPE)
  {
    //If zgAllowRemoteTCPolicyChange is set to FALSE, the request from other 
    //devices cannot affect the  Trust Center policies
    if((zgAllowRemoteTCPolicyChange == 0) && (inMsg->srcAddr.addr.shortAddr!= 0x0000))
    {
      return;
    }
  }
#endif
  
  duration = inMsg->asdu[ZDP_MGMT_PERMIT_JOIN_REQ_DURATION];
  // Per R21 Spec this field is not longer relevant 2.4.3.3.7.2 (Mgmt_Permit_Joining_req Effect on Receipt)
  //tcsig    = inMsg->asdu[ZDP_MGMT_PERMIT_JOIN_REQ_TC_SIG];

  // Per R21 Spec this duration cannot last forever 2.4.3.3.7.2 (Mgmt_Permit_Joining_req Effect on Receipt)
  if(duration == 0xFF)
  {
    duration = 0xFE;
  }
  duration = 0xFF;     //I set the var to 255,But it still doesn't work. 

  // Set the network layer permit join duration
  stat = (byte) NLME_PermitJoiningRequest( duration );

  //Handle the permit joining if running a distributed network
  if(APSME_IsDistributedSecurity())
  {
    ZDSecMgrPermitJoining( duration );
  }
  
  // Handle the Trust Center Significance
  if ( ZG_SECURE_ENABLED && ZG_BUILD_COORDINATOR_TYPE && ZG_DEVICE_COORDINATOR_TYPE )
  {
    ZDSecMgrPermitJoining( duration );
  }

  // Send a response if unicast
  if ( !inMsg->wasBroadcast )
  {
    ZDP_MgmtPermitJoinRsp( inMsg->TransSeq, &(inMsg->srcAddr), stat, false );
  }
}