部分TCP封包遗失

Question

I was writing a TCP server. I works fine on localhost. When I deploy it remotely some bytes are lost on their way to the destination.

I used netcat on localhost and remotely and made a diff of the two outputs and the lost bytes are deterministic.

So I doubt it is packet loss since it's very unlikely the same packet would get lost.

I tried to reduce the size of my writes on the socket to 1000 bytes but I still get this problem.

Are there any common reasons for this kind of errors ?

I could post the code but it's just a socket.send from the Boost asio library. I'm not sure the error comes from the code otherwise it would not work on localhost.

Thank you in advance for your help

Answer 1

When sending data using a socket and function send() it returns the number of bytes actually sent, which may be less than the amount you pass to the function.

If you use blocking I/O, sometimes a function like this is used:

ssize_t send_all(int socket, const void *data, size_t len)
{
    while (len > 0)
    {
        ssize_t r = send(socket, data, len, 0);
        if (r < 0)
        { //you could also return -1 on EINTR
            if (errno == EINTR)
                continue;
            else
                return -1;
        }
        len -= r;
        data = (const char*)data + r;
    }
    return len;
}

Since you are using boost::asio , not raw sockets that approach is not useful. But at the async_send documentation you get exactly what you need:

The send operation may not transmit all of the data to the peer. Consider using the async_write function if you need to ensure that all data is written before the asynchronous operation completes.