Reduce TCP maximum segment size (MSS) in Linux on a socket

Question

In a special application in which our server needs to update firmware of low-on-resource sensor/tracking devices we encountered a problem in which sometimes data is lost in the remote devices (clients) receiving packets of the new firmware. The connection is TCP/IP over GPRS network. The devices use SIM900 GSM chip as a network interface.

The problems possibly come because of the device receiving too much data. We tried reducing the traffic by sending packages more rarely but sometimes the error still occured.

We contacted the local retailer of the SIM900 chip who is also responsible for giving technical support and possibly contacting the chinese manufacturer (simcom) of the chip. They said that at first we should try to reduce the TCP MSS (Maximum Segment Size) of our connection.

In our server I did the following:

static int
create_master_socket(unsigned short master_port) {

    static struct sockaddr_in master_address;
    int master_socket = socket(AF_INET,SOCK_STREAM,0);
    if(!master_socket) {
            perror("socket");
            throw runtime_error("Failed to create master socket.");
    }

    int tr=1;
    if(setsockopt(master_socket,SOL_SOCKET,SO_REUSEADDR,&tr,sizeof(int))==-1) {
            perror("setsockopt");
            throw runtime_error("Failed to set SO_REUSEADDR on master socket");
    }

    master_address.sin_family = AF_INET;
    master_address.sin_addr.s_addr = INADDR_ANY;
    master_address.sin_port = htons(master_port);
    uint16_t tcp_maxseg;
    socklen_t tcp_maxseg_len = sizeof(tcp_maxseg);
    if(getsockopt(master_socket, IPPROTO_TCP, TCP_MAXSEG, &tcp_maxseg, &tcp_maxseg_len)) {
            log_error << "Failed to get TCP_MAXSEG for master socket. Reason: " << errno;
            perror("getsockopt");
    } else {
            log_info << "TCP_MAXSEG: " << tcp_maxseg;
    }
    tcp_maxseg = 256;
    if(setsockopt(master_socket, IPPROTO_TCP, TCP_MAXSEG, &tcp_maxseg, tcp_maxseg_len)) {
            log_error << "Failed to set TCP_MAXSEG for master socket. Reason: " << errno;
            perror("setsockopt");
    } else {
            log_info << "TCP_MAXSEG: " << tcp_maxseg;
    }
    if(getsockopt(master_socket, IPPROTO_TCP, TCP_MAXSEG, &tcp_maxseg, &tcp_maxseg_len)) {
            log_error << "Failed to get TCP_MAXSEG for master socket. Reason: " << errno;
            perror("getsockopt");
    } else {
            log_info << "TCP_MAXSEG: " << tcp_maxseg;
    }
    if(bind(master_socket, (struct sockaddr*)&master_address,
                            sizeof(master_address))) {
            perror("bind");
            close(master_socket);
            throw runtime_error("Failed to bind master_socket to port");

    }

    return master_socket;
}

Running the above code results in:

I0807 ... main.cpp:267] TCP_MAXSEG: 536
E0807 ... main.cpp:271] Failed to set TCP_MAXSEG for master socket. Reason: 22 setsockopt: Invalid argument
I0807 ... main.cpp:280] TCP_MAXSEG: 536

As you may see, the problem in the second line of the output: setsockopt returns "Invalid argument".

Why does this happen? I read about some constraints in setting TCP_MAXSEG but I did not encounter any report on such a behaviour as this.

Thanks, Dennis

Answer 1

除了 xaxxon 的回答之外，我只想记录一下我试图强制我的 Linux 仅发送特定大小的最大 TCP 段（低于它们通常的大小）的经验：

• 我发现这样做的最简单方法是使用 iptables：

sudo iptables -A INPUT -p tcp --tcp-flags SYN,RST SYN --destination 1.1.1.1 -j TCPMSS --set-mss 200

这会覆盖出站连接上的远程传入 SYN/ACK 数据包，并强制 MSS 为特定值。

注1：您在wireshark 中看不到这一点，因为wireshark 在此之前捕获。

注意 2：Iptables 不允许你 - 增加 - MSS，只是降低它

• 或者，我也尝试设置套接字选项 TCP_MAXSEG，就像丹尼斯所做的那样。从 xaxxon 修复后，这也有效。

注意：您应该在建立连接后读取 MSS 值。否则它返回默认值，这让我（和丹尼斯）走错了路。

现在终于，我还遇到了其他一些事情：

• 我遇到了TCP 卸载问题，尽管我的 MSS 设置正确，但发送的帧仍然被wireshark 显示为太大。您可以通过以下方式禁用此功能：sudo ethtool -K eth0 tx off sg off tso off。这花了我很长时间才弄明白。

• TCP 有很多奇特的东西，比如 MTU 路径发现，它们实际上试图动态地增加 MSS。有趣又酷，但显然令人困惑。虽然在我的测试中我没有遇到问题

希望这有助于有一天尝试做同样事情的人。