本文的起因是周师傅早上突然问起为何本应四次挥手的TCP，抓包发现只有三个包，看图显示为客户端主动发送FIN的情况下，少了一个服务器对客户端的ACK回复，而是直接发送了FIN+ACK。
此时我还不知道是因为Linux的Delay ACK机制造成的。于是针对这个现象展开了一波研究。
首先找到定义TCP协议的RFC793的文档 https://tools.ietf.org/html/rfc793 的3.5部分，文档表示
TCP A TCP B
1. ESTABLISHED ESTABLISHED
2. (Close)
FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT
3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT
4. (Close)
TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK
5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED
6. (2 MSL)
CLOSED
Normal Close Sequence
Figure 13.
TCP A TCP B
1. ESTABLISHED ESTABLISHED
2. (Close) (Close)
FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> ... FIN-WAIT-1
<-- <SEQ=300><ACK=100><CTL=FIN,ACK> <--
... <SEQ=100><ACK=300><CTL=FIN,ACK> -->
3. CLOSING --> <SEQ=101><ACK=301><CTL=ACK> ... CLOSING
<-- <SEQ=301><ACK=101><CTL=ACK> <--
... <SEQ=101><ACK=301><CTL=ACK> -->
4. TIME-WAIT TIME-WAIT
(2 MSL) (2 MSL)
CLOSED CLOSED
Simultaneous Close Sequence
显然不管是单方发起还是双方同时发起关闭连接，都是有四次挥手的。
只好从源代码下手去寻找答案。由于周师傅抓包的是HTTP的情况，于是从服务器端的nginx源码研究起。
nginx/src/os/unix/ngx_socket.h里显示
ioctl(FIONBIO) sets a non-blocking mode with the single syscall
while fcntl(F_SETFL, O_NONBLOCK) needs to learn the current state
using fcntl(F_GETFL).
ioctl() and fcntl() are syscalls at least in FreeBSD 2.x, Linux 2.2
and Solaris 7.
ioctl() in Linux 2.4 and 2.6 uses BKL, however, fcntl(F_SETFL) uses it too.
nginx是使用系统函数来管理TCP连接的。所以这不是nginx的锅，于是找到Linux内核源码来研究系统对TCP的管理。
首先找到linux/net/ipv4/tcp.c来观察系统对于TCP_ESTABLISHED时收到FIN包如何切换状态。
在tcp_set_state函数中找到了linux对tcp状态的变化维护表
static const unsigned char new_state[16] = {
/* current state: new state: action: */
[0 /* (Invalid) */] = TCP_CLOSE,
[TCP_ESTABLISHED] = TCP_FIN_WAIT1 | TCP_ACTION_FIN,
[TCP_SYN_SENT] = TCP_CLOSE,
[TCP_SYN_RECV] = TCP_FIN_WAIT1 | TCP_ACTION_FIN,
[TCP_FIN_WAIT1] = TCP_FIN_WAIT1,
[TCP_FIN_WAIT2] = TCP_FIN_WAIT2,
[TCP_TIME_WAIT] = TCP_CLOSE,
[TCP_CLOSE] = TCP_CLOSE,
[TCP_CLOSE_WAIT] = TCP_LAST_ACK | TCP_ACTION_FIN,
[TCP_LAST_ACK] = TCP_LAST_ACK,
[TCP_LISTEN] = TCP_CLOSE,
[TCP_CLOSING] = TCP_CLOSING,
[TCP_NEW_SYN_RECV] = TCP_CLOSE, /* should not happen ! */
};
而对FIN包的处理是由linux/net/ipv4/tcp_input.c里面的tcp_fin函数负责
/*
* Process the FIN bit. This now behaves as it is supposed to work
* and the FIN takes effect when it is validly part of sequence
* space. Not before when we get holes.
*
* If we are ESTABLISHED, a received fin moves us to CLOSE-WAIT
* (and thence onto LAST-ACK and finally, CLOSE, we never enter
* TIME-WAIT)
*
* If we are in FINWAIT-1, a received FIN indicates simultaneous
* close and we go into CLOSING (and later onto TIME-WAIT)
*
* If we are in FINWAIT-2, a received FIN moves us to TIME-WAIT.
*/
void tcp_fin(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
inet_csk_schedule_ack(sk);
sk->sk_shutdown |= RCV_SHUTDOWN;
sock_set_flag(sk, SOCK_DONE);
switch (sk->sk_state) {
case TCP_SYN_RECV:
case TCP_ESTABLISHED:
/* Move to CLOSE_WAIT */
tcp_set_state(sk, TCP_CLOSE_WAIT);
inet_csk(sk)->icsk_ack.pingpong = 1;
break;
case TCP_CLOSE_WAIT:
case TCP_CLOSING:
/* Received a retransmission of the FIN, do
* nothing.
*/
break;
case TCP_LAST_ACK:
/* RFC793: Remain in the LAST-ACK state. */
break;
case TCP_FIN_WAIT1:
/* This case occurs when a simultaneous close
* happens, we must ack the received FIN and
* enter the CLOSING state.
*/
tcp_send_ack(sk);
tcp_set_state(sk, TCP_CLOSING);
break;
case TCP_FIN_WAIT2:
/* Received a FIN -- send ACK and enter TIME_WAIT. */
tcp_send_ack(sk);
tcp_time_wait(sk, TCP_TIME_WAIT, 0);
break;
default:
/* Only TCP_LISTEN and TCP_CLOSE are left, in these
* cases we should never reach this piece of code.
*/
pr_err("%s: Impossible, sk->sk_state=%d\n",
__func__, sk->sk_state);
break;
}
/* It _is_ possible, that we have something out-of-order _after_ FIN.
* Probably, we should reset in this case. For now drop them.
*/
skb_rbtree_purge(&tp->out_of_order_queue);
if (tcp_is_sack(tp))
tcp_sack_reset(&tp->rx_opt);
sk_mem_reclaim(sk);
if (!sock_flag(sk, SOCK_DEAD)) {
sk->sk_state_change(sk);
/* Do not send POLL_HUP for half duplex close. */
if (sk->sk_shutdown == SHUTDOWN_MASK ||
sk->sk_state == TCP_CLOSE)
sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_HUP);
else
sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
}
}
函数做了状态转换和乱序包的处理。而在TCP_ESTABLISHED这里发现一个有趣的现象，没有马上调用tcp_send_ack进行ACK回复，而是执行了
inet_csk(sk)->icsk_ack.pingpong = 1;
这个icsk_ack.pingpong是什么呢，顺藤摸瓜的找到了include/net/inet_connection_sock.h
@icsk_ack: Delayed ACK control data
struct {
__u8 pending; /* ACK is pending */
__u8 quick; /* Scheduled number of quick acks */
__u8 pingpong; /* The session is interactive */
__u8 blocked; /* Delayed ACK was blocked by socket lock */
__u32 ato; /* Predicted tick of soft clock */
unsigned long timeout; /* Currently scheduled timeout */
__u32 lrcvtime; /* timestamp of last received data packet */
__u16 last_seg_size; /* Size of last incoming segment */
__u16 rcv_mss; /* MSS used for delayed ACK decisions */
其中pingpong的作用是表明这个session是交互式。这个标志位是delay ack的一个control位。查阅了一下icsk_ack.pingpong和delay ack的资料，发现正常的挥手流程是这样的
- client: FIN (will not send more)
- server: ACK (received the FIN)
… server: sends more data…, client ACKs these data
- server: FIN (will not send more)
- client: ACK (received the FIN)
而下面有说
If the server has no more data to send it might close the connection also. In this case steps 2+3 can be merged, e.g. the server sends a FIN+ACK, where the ACK acknowledges the FIN received by the client.
也就是说如果服务器端在客户端发出FIN以后，如果有数据要发送，需要先ACK这个FIN，然后再进行数据发送。但是如果服务器端没有更多数据发送，也要关闭连接的情况下，很可能ACK包就跟随FIN一起发出。其中ACK为确认客户端的FIN包。
查询RFC1122的 4.2.3.2 When to Send an ACK Segment 得知
4.2.3.2 When to Send an ACK Segment
A host that is receiving a stream of TCP data segments can
increase efficiency in both the Internet and the hosts by
sending fewer than one ACK (acknowledgment) segment per data
segment received; this is known as a "delayed ACK" [TCP:5].
A TCP SHOULD implement a delayed ACK, but an ACK should not
be excessively delayed; in particular, the delay MUST be
less than 0.5 seconds, and in a stream of full-sized
segments there SHOULD be an ACK for at least every second
segment.
原来：
TCP采用两种方式来发送ACK：快速确认和延迟确认。
在快速确认模式中，本端接收到数据包后，会立即发送ACK给对端。
在延迟确认模式中，本端接收到数据包后，不会立即发送ACK给对端，而是等待一段时间，如果在此期间：
- 本端有数据包要发送给对端。就在发送数据包的时候捎带上此ACK，如此一来就节省了一个报文。
- 本端没有数据包要发送给对端。延迟确认定时器会超时，然后发送纯ACK给对端。
具体实现上面用
icsk->icsk_ack.pingpong == 0，表示使用快速确认。
icsk->icsk_ack.pingpong == 1，表示使用延迟确认。
而对周师傅遇到的情况的FIN包的处理刚好是在icsk->icsk_ack.pingpong == 1的场景。于是服务端的FIN和ACK合并发送了。
参考资料：
https://github.com/torvalds/linux/
http://blog.csdn.net/wdscq1234/article/details/52430382
http://blog.csdn.net/dog250/article/details/52664508
http://stackoverflow.com/questions/21390479/fin-omitted-fin-ack-sent