Question

我创建了一个带有backlog 1的侦听套接字，并连接了64个套接字。

socket.c中

#define _POSIX_C_SOURCE 199309L

#include <arpa/inet.h>
#include <stdio.h>
#include <sys/socket.h>
#include <sys/time.h> 
#include <time.h>

int main() {
    int server_socket = socket(AF_INET, SOCK_STREAM, 0);
    struct sockaddr_in server_address = {0};
    server_address.sin_family = AF_INET;
    server_address.sin_addr.s_addr = htonl(INADDR_ANY);
    server_address.sin_port = htons(8000); 
    bind(server_socket, (struct sockaddr*)&server_address, sizeof(server_address)); 
    if (listen(server_socket, 1) == 0) {
        printf("Listening\n");
    } else {
        printf("Failed to listen\n");
        return 1;
    }

    for (int i = 0; i < 64; i++) {
        int client_socket = socket(AF_INET, SOCK_STREAM, 0);
        struct timespec start;
        clock_gettime(CLOCK_MONOTONIC, &start);
        if (connect(client_socket, (struct sockaddr*)&server_address, sizeof(server_address)) == 0) {
            struct timespec end;
            clock_gettime(CLOCK_MONOTONIC, &end);
            double elapsed = end.tv_sec - start.tv_sec + (end.tv_nsec - start.tv_nsec) / 1e9;
            printf("Connected socket #%d in %.3lfs\n", i, elapsed);
        } else {
            printf("Failed to connect socket #%d\n", i);
        }
    }
}

输出：

$ gcc -o socket -std=c99 socket.c
$ ./socket
Listening
Connected socket #0 in 0.000s
Connected socket #1 in 0.000s
Connected socket #2 in 0.000s
Connected socket #3 in 0.000s
Connected socket #4 in 1.000s
Connected socket #5 in 0.000s
Connected socket #6 in 3.004s
Connected socket #7 in 0.000s
Connected socket #8 in 3.004s
Connected socket #9 in 0.000s
Connected socket #10 in 3.004s
Connected socket #11 in 0.000s
Connected socket #12 in 3.004s
Connected socket #13 in 0.000s
Connected socket #14 in 3.004s
Connected socket #15 in 0.000s
Connected socket #16 in 3.004s
Connected socket #17 in 0.000s
...

前四个连接是瞬时的，第五个连接是1秒，然后连接在0到3秒之间振荡，直到所有插座连接并且程序退出。此行为是100％可重复的。

我已经广泛阅读了有关TCP连接，Linux网络文档和other resources的内容。

但我仍然无法解释发生了什么。

为什么连接时间变化如此之大？

系统信息

$ uname -a
Linux paul 3.19.0-51-generic #58~14.04.1-Ubuntu SMP Fri Feb 26 22:02:58 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

（/etc/sysctl.conf为空。）

Answer 1

listen的文档解释了这一点，＆＃34;如果底层协议支持重传，则可以忽略该请求，以便稍后在连接时重新尝试成功＆＃34;。如果您提高积压，您将看到延迟开始变化的点。

Answer 2

正如David Schwartz所说，这是由于你的积压。处理SYN请求的TCP代码认为接受队列已满并且还有SYN队列。见here

1287         /* Accept backlog is full. If we have already queued enough
1288          * of warm entries in syn queue, drop request. It is better than
1289          * clogging syn queue with openreqs with exponentially increasing
1290          * timeout.
1291          */
1292         if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1) {
1293                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
1294                 goto drop;
1295         }

达到上述条件后，您的SYN数据包将被丢弃。客户端的重传在3秒（初始SYN）之后发生，这被接受

如果将backlog设置为65

，则不会看到连接有任何延迟

在Linux中非常奇怪的连接时间

2 个答案: