snat_main_t snat_main;

typedef struct snat_main_s
{
  /* Per thread data */
  snat_main_per_thread_data_t *per_thread_data;

  /* Find a static mapping by local */
  clib_bihash_8_8_t static_mapping_by_local; 

  /* Find a static mapping by external */
  clib_bihash_8_8_t static_mapping_by_external; 
  /* Static mapping pool */
  snat_static_mapping_t *static_mappings;

  /* Interface pool */
  snat_interface_t *interfaces;
 
  /* vector of outside fibs */
  nat_outside_fib_t *outside_fibs; 
    
  // other filed..  
} snat_main_t;

int snat_interface_add_del (u32 sw_if_index, u8 is_inside, int is_del)
{
  // code omitted...
  // 找到该 interface 关联的 fib  
  u32 fib_index = fib_table_get_index_for_sw_if_index (FIB_PROTOCOL_IP4, sw_if_index);

  // 由于我们添加的是 outside interface，所以这里得到的是 "nat44-out2in"
  feature_name = is_inside ? "nat44-in2out" : "nat44-out2in";
  
  // 由于我们添加的是 outside interface，这里将 interface 对应的 fib 添加到 outside_fibs vector 中
  if (!is_inside)
    {
      // code omitted
      if (!is_del)
		{
	  		vec_add2 (sm->outside_fibs, outside_fib, 1);
	  		outside_fib->refcount = 1;
	  		outside_fib->fib_index = fib_index;
		}
    }
  // 接下来进行 feature set，我们假设 G1 之前没有使能过 NAT 功能
  // 这里分配一个 snat 的 interface    
  pool_get (sm->interfaces, i);  // allocate a snat_interface_t
  i->sw_if_index = sw_if_index;  // 
  i->flags = 0;

  // 启动 "ip4-unicast" 这个 arc 上的 "nat44-out2in" 特性.
  vnet_feature_enable_disable ("ip4-unicast", feature_name, sw_if_index, 1, 0, 0);
}

static inline uword
snat_in2out_node_fn_inline (vlib_main_t * vm,
			    vlib_node_runtime_t * node,
			    vlib_frame_t * frame, int is_slow_path,
			    int is_output_feature)
{
    // code omitted
    while (n_left_from > 0)
    {
        // 解析出输入的报文的 IP 头和 transport header 以及输入的 interface 和关联的 fib
        ip0 = (ip4_header_t *) ((u8 *) vlib_buffer_get_current (b0) +
			      iph_offset0);
        udp0 = ip4_next_header (ip0);
        tcp0 = (tcp_header_t *) udp0;
        icmp0 = (icmp46_header_t *) udp0;
        
        sw_if_index0 = vnet_buffer (b0)->sw_if_index[VLIB_RX];
        rx_fib_index0 = vec_elt (sm->ip4_main->fib_index_by_sw_if_index, sw_if_index0);
        
        proto0 = ip_proto_to_nat_proto (ip0->protocol);
      
        // 根据 sip+sport+fib+proto 计算一个 key 
        init_nat_k (&kv0, ip0->src_address, vnet_buffer (b0)->ip.reass.l4_src_port, rx_fib_index0,
		            proto0);
        // 用该 key 搜索是否存在现成的 snat_session，
        if (clib_bihash_search_8_8(&sm->per_thread_data[thread_index].in2out, &kv0, &value0))
		{ 
           // 如果没有找到snat session，由于当前我们不是处于 slow path，则这里我们将报文传递给 slow path
	  	   if (is_slow_path)
	    	{
	      		next0 = slow_path (sm, b0, ip0, ip0->src_address, 
                   			   vnet_buffer (b0)->ip.reass.l4_src_port,
                   			   rx_fib_index0, proto0, &s0, node, next0,   
                   			   thread_index, now);
	    	}
	  		else
	    	{
	      		next0 = SNAT_IN2OUT_NEXT_SLOW_PATH;
	      		goto trace0;
	    	}
		}
      else
         // 如果找到，则得到现成的 snat_session
		 s0 = pool_elt_at_index (sm->per_thread_data[thread_index].sessions, value0.value);
    
    }
    
    // code omitted... 
}

VLIB_CLI_COMMAND (set_interface_snat_command, static) = {
  .path = "set interface nat44",
  .function = snat_feature_command_fn,
  .short_help = "set interface nat44 in <intfc> out <intfc> [output-feature] "
                "[del]",
};