DSSM Protocol Specification
Basics
DSSM is a protocol for multiplexing 256 bidirectional byte streams over a single one.
DSSM does not provide any error detection, it depends on a reliable, ordered transport.
DSSM does not provide encryption. If this is required, the underlying transport must provide it.
DSSM uses a minimal set of control characters from the C0 ASCII character set for stream multiplexing, keep alive signaling and stream termination indication.
For DSSM, streams are stateless. Error signalling is out of scope of DSSM. Senders and receivers have to agree on stream and stream state semantics as well as on error signalling.
Signaling
The control characters ENQ, ACK, NAK, SOH and EM are used for signaling. The ESC control character is used as a prefix for transmitting any of these control characters or for transmitting itself inside a stream.
Initial State
Upon connection establishment of the underlying transport any data transmitted is considered to be sent by stream 0.
Stream switching
The character SOH signals a switch to another stream. The byte following the SOH character is interpreted as the stream number (0..255). All following characters up to the next signal are part of the respective stream.
Stream termination indication
A stream sender can signal the "end" of the current stream by sending the EM character followed by a status byte.
To DSSM there is no meaning of "end" of a stream or the stream "status". Especially, if a sender "ends" a stream and then continues sending data to the same stream, DSSM does not consider this an error.
Keep alive signaling
Whenever the ENQ character is received, the receiver should send back an ACK or NAK character before sending the next byte of the stream.
Whether ACK/NAK is considered a good/bad status indication, or whether the failure to respond an ENQ with ACK/NAK has any meaning is completely up to the implementation of sender and receiver.
Unsolicited ACK/NAK signals are not considered an error and must be discarded by the receiver.
Notes
Periodic ENQ/ACK/NAK signals can be used to keep an idle connection alive or to sense operational status of the opposite side.
ENQ/ACK can be used by a sender to meter round trip delay, transmission queue sizes or to flush remote buffers.
ENQ/NAK can be used to indicate fatal conditions or request restart of the underlying connection.
The receiver can choose to interpret a sequence of multiple ENQ ACK or NAK characters or unsolicited ACK/NAK characters as a denial of service attack. The same applies to sequences of stream switch or stream end signalling without interspersed stream data.
If the underlying transport is packet oriented, senders might decide to split the streams into pieces fitting into one packet.
Implementers must take care not to intermingle ACK/NAK signals into an ESC, SOH or EM sequence.
The rationale for defining DSSM is to provide the stderr backchannel for rush, the 'remote unsecure shell', inside a
single TCP connection in a leaner way than done by SSH.