Using iOS9 Network Extension Framework to implement VPN , the iPhone can't access Internet

I'm implementing a custom VPN by using iOS9 NetworkExtension framework. The VPN connection has established between my iPhone and the VPN server. But my iPhone still can't access Internet.

I did the following steps:

1. Reading IP packets from TUN, and sent it to VPN server using UDP:

   - (void)startReadingTunPackets{ [self.packetFlow readPacketsWithCompletionHandler:^(NSArray<NSData*> *packets, NSArray<NSNumber*> *protocols) { for(NSData *data in packets){ [self.UpdSession writeDatagram:data completionHandler:^(NSError * error){
}]; } [self startReadingTunPackets]; }];

2. The VPN server received the UDP data, decoded them to IP packet (eg. a TCP SYN packet) and changed its source address to the VPN server's address. Send the fake SYN packet to remote site.

3. The remote site responded a TCP SYN/ACK packet to the VPN server.

4. The VPN server modified the SYN/ACK packet's destination address to iPhone's TUN address(eg. 10.0.1.100), sent it to iPhone.

5. iPhone used the following code to receive UDP data that contains SYN/ACK packet and sent them to TUN:

       [udpSession setReadHandler:^(NSArray<NSData *> * datagrams, NSError * error) {
       for(NSData *data in datagrams){
           NSArray *packet = [[NSArray alloc] initWithObjects:data,nil];
           NSArray *protocols = [[NSArray alloc] initWithObjects:@(AF_INET),nil];
           //Write to TUN
           BOOL r = [self.packetFlow writePackets:packet withProtocols:protocols];
           }} maxDatagrams:4096];
   

I thought an ACK packet can be read from TUN in order to complete 'three-way handshake'. But this never happened. Instead, a SYN packet was read from TUN, like TUN has never received the SYN/ACK packet, which as I described in step 5.

I guess something might went wrong with the packet format in step 5. Should it to be a complete ip packet or just payload? Or should I not modify the SYN/ACK's destination address in step 4?