The functions in the game DLL to engine interface that are used for sending network messages are:
pfnMessageBegin()
pfnWriteByte()
pfnWriteString()
pfnWriteAnythingTheCrapElseOrWhateverAndTheLikes()
pfnMessageEnd()
Every message must begin with a call to pfnMessageBegin() - or the MESSAGE_BEGIN macro - and end with a call to MESSAGE_END. These functions tell the engine to build a header and a footer for the message before and after sending bytes on the wire. Then, according to the syntax of the message, you either send a byte, a string, a float, whatever, provided it's shaped like a genuine game message so that the client, at the other end of the wire, will understand it. To learn about the syntax of the different network messages, look for examples of them through the SDK.
BTW. I believe there's a typo in your signature
