# 從應用層程式取得傳輸層 TCP 資訊
Aaron Liao，2014\
目的：\
\
當設計伺服器網路應用程式時，由於應用層本身並沒有網路底層的資訊，因此當有異常發生時，無法立即直接知道連線的資訊，通常需要藉由評估資料收送狀況、逾時（timeout）等方式間接得知。若伺服器應用程式可以取得得知目前 TCP 的狀態，就能知道連線是否還存在著，即使只是 TCP 連線狀態的改變，都能清楚知曉，而伺服器能提早對該連線進行處理。\
\
我們可以在 netinet/tcp.h 表頭檔案找到 TCP 資訊的定義。（在 Linux 系統的絕對路徑位於 /usr/include/netinet/tcp.h）。\
\
若你想要取得 TCP 連線的狀態，你可以引用這個表頭檔。\
```c
#include
50 #define TCP_INFO 11 /* Information about this connection. */
```
\
這邊是 TCP 資訊的資料結構 \
```c
199 struct tcp_info
200 {
201 u_int8_t tcpi_state;
202 u_int8_t tcpi_ca_state;
203 u_int8_t tcpi_retransmits;
204 u_int8_t tcpi_probes;
205 u_int8_t tcpi_backoff;
206 u_int8_t tcpi_options;
207 u_int8_t tcpi_snd_wscale : 4, tcpi_rcv_wscale : 4;
208
209 u_int32_t tcpi_rto;
210 u_int32_t tcpi_ato;
211 u_int32_t tcpi_snd_mss;
212 u_int32_t tcpi_rcv_mss;
213
214 u_int32_t tcpi_unacked;
215 u_int32_t tcpi_sacked;
216 u_int32_t tcpi_lost;
217 u_int32_t tcpi_retrans;
218 u_int32_t tcpi_fackets;
219
220 /* Times. */
221 u_int32_t tcpi_last_data_sent;
222 u_int32_t tcpi_last_ack_sent; /* Not remembered, sorry. */
223 u_int32_t tcpi_last_data_recv;
224 u_int32_t tcpi_last_ack_recv;
225
226 /* Metrics. */
227 u_int32_t tcpi_pmtu;
228 u_int32_t tcpi_rcv_ssthresh;
229 u_int32_t tcpi_rtt;
230 u_int32_t tcpi_rttvar;
231 u_int32_t tcpi_snd_ssthresh;
232 u_int32_t tcpi_snd_cwnd;
233 u_int32_t tcpi_advmss;
234 u_int32_t tcpi_reordering;
235
236 u_int32_t tcpi_rcv_rtt;
237 u_int32_t tcpi_rcv_space;
238
239 u_int32_t tcpi_total_retrans;
240 };
```
\
// Enum of TCP status\
```c
136 enum
137 {
138 TCP_ESTABLISHED = 1,
139 TCP_SYN_SENT,
140 TCP_SYN_RECV,
141 TCP_FIN_WAIT1,
142 TCP_FIN_WAIT2,
143 TCP_TIME_WAIT,
144 TCP_CLOSE,
145 TCP_CLOSE_WAIT,
146 TCP_LAST_ACK,
147 TCP_LISTEN,
148 TCP_CLOSING /* now a valid state */
149 };
```
\
這邊用一個簡單的函式範例來說明如何取得 TCP 資訊。如果你的程式中的 socket descriptor 名為 sd，那麼你就能用 getsockopt() 取得 TCP 的資訊。這邊我將 getsockopt() system call 包到 get\_tcp\_state() 函式，而 get\_tcp\_state() 函式就會將 sd socket descriptor 的 TCP 的狀態傳回。\
```c
int get_tcp_state(int sd)
{
struct tcp_info t_info; /* the data structure of the TCP information */
socklen_t t_info_len = sizeof(t_info);
if( getsockopt( sd, IPPROTO_TCP, TCP_INFO, &t_info, &t_info_len) != -1) {
return t_info.tcpi_state; /* the state of the 'sd' TCP connection */
}
return -1;
}
```
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