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: 

send:
heads
response:
41
482d3fb47d80693f929101f95944bf019009dd79

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.