Commands names are sent over the ssh pipe as plain text followed by newline. Arguments are sent as "[argname] [value length]\n" followed by the value. Responses are "[response length]\n" followed by the response.
Examples:
<pre> send: heads response: 41 482d3fb47d80693f929101f95944bf019009dd79 </pre>
Unknown commands elicit no response in early hg versions and zero length.
Version detection:
Because older Mercurial versions have no response to unknown commands, to detect a version you must send the hello command followed by a command with known output and then determine if the hello command responded before the known output was sent.