UDP 是 IPC 的可靠协议吗? [英] Is UDP a reliable protocol for IPC?

问题描述

如果我纯粹将 UDP 用于进程间通信(即在 1 个系统中，不涉及网络)，我可以认为它是可靠的吗?还是我还需要担心丢包等问题?

If I use UDP purely for inter-process communication (i.e., in 1 system, with no network involved), can I consider it to be reliable? Or do I still need to worry about packets getting dropped, etc.?

注意这是一个实践问题，而不是理论问题.如果答案因操作系统而异，请说明具体情况，尤其包括 Windows、Linux 和 Mac.

Note that this is a practical question, not a theoretical one. If the answer differs across OSes, please explain how, in particular including Windows, Linux, and Mac.

感谢当前答案为我指明了正确的方向.
此代码在 Windows 8.1 上丢弃了一个数据包(我得到 Received: 18432 (DROPPED PACKET)).
(我不确定为什么它不能在 Linux 上运行，但它应该接近工作了.)

Thanks to the current answer for pointing me in the right direction.
This code drops a packet on Windows 8.1 (I get Received: 18432 (DROPPED PACKET)).
(I'm not sure why it doesn't run on Linux, but it should be close to working.)

#include <stdio.h>
#ifdef _WIN32
#include <ws2tcpip.h>
#else
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#endif

int main()
{
#ifdef _WIN32
    typedef int socklen_t;
#else
    typedef int SOCKET;
#endif
    SOCKET r = socket(AF_INET, SOCK_DGRAM, 0);
    struct sockaddr_in addr = { AF_INET };
    addr.sin_addr.s_addr = htonl(0x7F000001);
    {
        socklen_t addrlen = sizeof(addr);
        if (bind(r, (struct sockaddr *)(&addr), addrlen) == -1 ||
            getsockname(r, (struct sockaddr *)(&addr), &addrlen) == -1)
        {
            return 1;
        }
    }
    SOCKET s = socket(AF_INET, SOCK_DGRAM, 0);
    int tids = 0;
    for (long c = 0, i = 0, j = 0; c < 1000000; ++c)
    {
        if ((c + 1) % 10)
        {
            int n = sendto(s, (char const *)(&i), sizeof(i), 0, (struct sockaddr const *)(&addr), sizeof(addr));
            if (n != sizeof(i)) { return 1; }
            // else { fprintf(stderr, "Sent:     %lu
", i); }
            ++i;
        }
        else
        {
            struct sockaddr temp;
            socklen_t templen = sizeof(temp);
            long v;
            int n = recvfrom(r, (char *)(&v), sizeof(v), 0, (struct sockaddr *)(&temp), &templen);
            if (n != sizeof(v)) { return 2; }
            else if (v != j) { fprintf(stderr, "Received: %lu (DROPPED PACKET)
", v); return 3; }
            // else { fprintf(stderr, "Received: %lu
", v); }
            ++j;
        }
    }
}

推荐答案

如果我纯粹使用 UDP 进行进程间通信(即在 1系统，不涉及网络)，我可以认为它是可靠的吗?

If I use UDP purely for inter-process communication (i.e., in 1 system, with no network involved), can I consider it to be reliable?

没有.即使所有通信都在同一主机上完成，UDP 数据包仍可能(有时会)被丢弃.

No. UDP packets can (and sometimes will) still be dropped even when all communication is being done over the same host.

如果你愿意，你可以自己演示一下；在同一主机上设置两个使用 UDP 套接字的程序，程序 A 将 UDP 数据包发送给程序 B，程序 B 接收并记录它们.(在 UDP 数据包中包含序列号，以便程序 B 可以轻松判断何时没有收到数据包.

You can demonstrate this for yourself, if you want; set up two UDP-socket-using programs on the same host, with program A sending UDP packets to program B that receives them and records them. (Include sequence numbers in the UDP packets so that program B can easily tell when a packet was not received).

一旦它开始工作并且数据包以合适的速率传输，将一些代码放入程序 B 中，以便它经常调用 sleep(5) (或类似的，这样程序 B 无法在其上调用 recv()UDP 套接字很长一段时间).您可能会看到，在 sleep() 调用返回后，程序 B 报告某些数据包被跳过——因为当 B 处于睡眠状态时，其 UDP 套接字的传入数据包缓冲区已满，然后网络丢弃了一些数据包堆栈，因为没有地方可以放置它们.

Once that is working and packets are being transmitted at a decent rate, put some code into program B so that every so often it calls sleep(5) (or similar, so that program B fails to call recv() on its UDP socket for a significant amount of time). You'll likely see that after the sleep() call returns, program B reports that some packets were skipped -- because while B was asleep, the incoming-packets buffer for its UDP socket became full and then some packets were dropped by the networking stack because there was no place to put them.

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