Question

I’m using a TCP library with a Server and Client class called Lacewing http://lacewing-project.org/

I have noticed that when I send a client several separate messages, it sometimes bundles them and the only way I can separate them is by parsing them again. When sending binary data, it is very hard to parse though.

Does anyone know why it might do this? I tried the DisableNagling() on both client and server but it still does it.

Here is an example of something I might send:

void ServerCore::loginRequestC( const std::string& userName, const std::string& password )
{
    std::cout << "Got request" << std::endl;
    ServerPlayer* player = (ServerPlayer*)getServerClient()->Tag;
    player->setUsername(userName);

    for (size_t i = 0; i < m_players.size(); ++i)
    {
        if(m_players[i] != player)
        {
            std::cout << "Telling new about " << m_players[i]->getUsername() << std::endl;
            m_enc.playerJoinedS(m_players[i]->getUsername());
            player->getClient()->Send(m_enc.getLastMessage().c_str());
        }
    }

    m_enc.loginResultS(true,"");

    player->getClient()->Send(m_enc.getLastMessage().c_str());

    m_enc.playerJoinedS(userName);
    for (size_t i = 0; i < m_players.size(); ++i)
    {
        if(m_players[i] != player)
        m_players[i]->getClient()->Send(m_enc.getLastMessage().c_str());
    }

}

So if I intended to have:

MSG_A
MSG_B
MSG_C

It might send:

MSG_A
MSG_BMSG_C

The messages get randomly bundled together. It is not my code that is the problem. I have checked.

Although it is useful to bundle messages, I want to control when it happens, not the library.

Thanks

Answer 1

This is not the library, but the TCP protocol itself that is doing it. TCP socket is a bi-directional stream of bytes. One write might result in multiple reads on the other side, and vise versa.

You have to design you application-level protocol so that it’s easy to split that stream into messages. Common approaches are to use a length prefix or a message delimiter.