E.11 Packet Acknowledgment

By default, when either the host or the target machine receives a packet, the first response expected is an acknowledgment: either `+' (to indicate the package was received correctly) or `-' (to request retransmission). This mechanism allows the gdb remote protocol to operate over unreliable transport mechanisms, such as a serial line.

In cases where the transport mechanism is itself reliable (such as a pipe or TCP connection), the `+'/`-' acknowledgments are redundant. It may be desirable to disable them in that case to reduce communication overhead, or for other reasons. This can be accomplished by means of the `QStartNoAckMode' packet; see QStartNoAckMode.

When in no-acknowledgment mode, neither the stub nor gdb shall send or expect `+'/`-' protocol acknowledgments. The packet and response format still includes the normal checksum, as described in Overview, but the checksum may be ignored by the receiver.

If the stub supports `QStartNoAckMode' and prefers to operate in no-acknowledgment mode, it should report that to gdb by including `QStartNoAckMode+' in its response to `qSupported'; see qSupported. If gdb also supports `QStartNoAckMode' and it has not been disabled via the set remote noack-packet off command (see Remote Configuration), gdb may then send a `QStartNoAckMode' packet to the stub. Only then may the stub actually turn off packet acknowledgments. gdb sends a final `+' acknowledgment of the stub's `OK' response, which can be safely ignored by the stub.

Note that set remote noack-packet command only affects negotiation between gdb and the stub when subsequent connections are made; it does not affect the protocol acknowledgment state for any current connection. Since `+'/`-' acknowledgments are enabled by default when a new connection is established, there is also no protocol request to re-enable the acknowledgments for the current connection, once disabled.