This should have gone in with the multiplexing merge, but I dropped it
at the time.
diff --git a/PROTOCOL.mux b/PROTOCOL.mux
new file mode 100644
index 0000000..d22f737
--- /dev/null
+++ b/PROTOCOL.mux
@@ -0,0 +1,196 @@
+This document describes the multiplexing protocol used by ssh(1)'s
+ControlMaster connection-sharing.
+
+Most messages from the client to the server contain a "request id" field.
+This field is returned in replies as "client request id" to facilitate
+matching of responses to requests.
+
+1. Connection setup
+
+When a multiplexing connection is made to a ssh(1) operating as a
+ControlMaster from a ssh(1) in multiplex slave mode, the first
+action of each is to exchange hello messages:
+
+ uint32 MUX_MSG_HELLO
+ uint32 protocol version
+ string extension name [optional]
+ string extension value [optional]
+ ...
+
+The current version of the mux protocol is 4. A slave should refuse
+to connect to a master that speaks an unsupported protocol version.
+Following the version identifier are zero or more extensions
+represented as a name/value pair. No extensions are currently
+defined.
+
+2. Opening sessions
+
+To open a new multiplexed session, a client may send the following
+request:
+
+ uint32 MUX_C_MSG_NEW_SESSION
+ uint32 request id
+ string reserved
+ bool want tty flag
+ bool want X11 forwarding flag
+ bool want agent flag
+ bool subsystem flag
+ uint32 escape char
+ string terminal type
+ string command
+ string environment string 0 [optional]
+ ...
+
+To disable the use of an escape character, "escape char" may be set
+to 0xffffffff. "terminal type" is generally set to the value of
+$TERM. zero or more environment strings may follow the command.
+
+The client then sends its standard input, output and error file
+descriptors (in that order) using Unix domain socket control messages.
+
+The contents of "reserved" are currently ignored.
+
+If successful, the server will reply with MUX_S_SESSION_OPENED
+
+ uint32 MUX_S_SESSION_OPENED
+ uint32 client request id
+ uint32 session id
+
+Otherwise it will reply with an error: MUX_S_PERMISSION_DENIED or
+MUX_S_FAILURE.
+
+Once the server has received the fds, it will respond with MUX_S_OK
+indicating that the session is up. The client now waits for the
+session to end. When it does, the server will send an exit status
+message:
+
+ uint32 MUX_S_EXIT_MESSAGE
+ uint32 session id
+ uint32 exit value
+
+The client should exit with this value to mimic the behaviour of a
+non-multiplexed ssh(1) connection. Two additional cases that the
+client must cope with are it receiving a signal itself and the
+server disconnecting without sending an exit message.
+
+3. Health checks
+
+The client may request a health check/PID report from a server:
+
+ uint32 MUX_C_ALIVE_CHECK
+ uint32 request id
+
+The server replies with:
+
+ uint32 MUX_S_ALIVE
+ uint32 client request id
+ uint32 server pid
+
+4. Remotely terminating a master
+
+A client may request that a master terminate immediately:
+
+ uint32 MUX_C_TERMINATE
+ uint32 request id
+
+The server will reply with one of MUX_S_OK or MUX_S_PERMISSION_DENIED.
+
+5. Requesting establishment of port forwards
+
+A client may request the master to establish a port forward:
+
+ uint32 MUX_C_OPEN_FORWARD
+ uint32 request id
+ uint32 forwarding type
+ string listen host
+ string listen port
+ string connect host
+ string connect port
+
+forwarding type may be MUX_FWD_LOCAL, MUX_FWD_REMOTE, MUX_FWD_DYNAMIC.
+
+A server may reply with a MUX_S_OK, a MUX_S_PERMISSION_DENIED or a
+MUX_S_FAILURE.
+
+5. Requesting closure of port forwards
+
+A client may request the master to establish a port forward:
+
+ uint32 MUX_C_OPEN_FORWARD
+ uint32 request id
+ uint32 forwarding type
+ string listen host
+ string listen port
+ string connect host
+ string connect port
+
+forwarding type may be MUX_FWD_LOCAL, MUX_FWD_REMOTE, MUX_FWD_DYNAMIC.
+
+A server may reply with a MUX_S_OK, a MUX_S_PERMISSION_DENIED or a
+MUX_S_FAILURE.
+
+6. Requesting stdio forwarding
+
+A client may request the master to establish a stdio forwarding:
+
+ uint32 MUX_C_NEW_STDIO_FWD
+ uint32 request id
+ string reserved
+ string connect host
+ string connect port
+
+The client then sends its standard input and output file descriptors
+(in that order) using Unix domain socket control messages.
+
+The contents of "reserved" are currently ignored.
+
+A server may reply with a MUX_S_SESSION_OPEED, a MUX_S_PERMISSION_DENIED
+or a MUX_S_FAILURE.
+
+7. Status messages
+
+The MUX_S_OK message is empty:
+
+ uint32 MUX_S_OK
+ uint32 client request id
+
+The MUX_S_PERMISSION_DENIED and MUX_S_FAILURE include a reason:
+
+ uint32 MUX_S_PERMISSION_DENIED
+ uint32 client request id
+ string reason
+
+ uint32 MUX_S_FAILURE
+ uint32 client request id
+ string reason
+
+7. Protocol numbers
+
+#define MUX_MSG_HELLO 0x00000001
+#define MUX_C_NEW_SESSION 0x10000002
+#define MUX_C_ALIVE_CHECK 0x10000004
+#define MUX_C_TERMINATE 0x10000005
+#define MUX_C_OPEN_FORWARD 0x10000006
+#define MUX_C_CLOSE_FORWARD 0x10000007
+#define MUX_S_OK 0x80000001
+#define MUX_S_PERMISSION_DENIED 0x80000002
+#define MUX_S_FAILURE 0x80000003
+#define MUX_S_EXIT_MESSAGE 0x80000004
+#define MUX_S_ALIVE 0x80000005
+#define MUX_S_SESSION_OPENED 0x80000006
+
+#define MUX_FWD_LOCAL 1
+#define MUX_FWD_REMOTE 2
+#define MUX_FWD_DYNAMIC 3
+
+XXX TODO
+XXX extended status (e.g. report open channels / forwards)
+XXX graceful close (delete listening socket, but keep existing sessions active)
+XXX lock (maybe)
+XXX watch in/out traffic (pre/post crypto)
+XXX inject packet (what about replies)
+XXX server->client error/warning notifications
+XXX port0 rfwd (need custom response message)
+XXX send signals via mux
+
+$OpenBSD: PROTOCOL.mux,v 1.1 2010/01/26 01:28:35 djm Exp $