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<h3 class="section">E.11 Packet Acknowledgment</h3>

<p><a name="index-acknowledgment_002c-for-_0040value_007bGDBN_007d-remote-3592"></a><a name="index-packet-acknowledgment_002c-for-_0040value_007bGDBN_007d-remote-3593"></a>By default, when either the host or the target machine receives a packet,
the first response expected is an acknowledgment: either &lsquo;<samp><span class="samp">+</span></samp>&rsquo; (to indicate
the package was received correctly) or &lsquo;<samp><span class="samp">-</span></samp>&rsquo; (to request retransmission). 
This mechanism allows the <span class="sc">gdb</span> remote protocol to operate over
unreliable transport mechanisms, such as a serial line.

   <p>In cases where the transport mechanism is itself reliable (such as a pipe or
TCP connection), the &lsquo;<samp><span class="samp">+</span></samp>&rsquo;/&lsquo;<samp><span class="samp">-</span></samp>&rsquo; 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
&lsquo;<samp><span class="samp">QStartNoAckMode</span></samp>&rsquo; packet; see <a href="QStartNoAckMode.html#QStartNoAckMode">QStartNoAckMode</a>.

   <p>When in no-acknowledgment mode, neither the stub nor <span class="sc">gdb</span> shall send or
expect &lsquo;<samp><span class="samp">+</span></samp>&rsquo;/&lsquo;<samp><span class="samp">-</span></samp>&rsquo; protocol acknowledgments.  The packet
and response format still includes the normal checksum, as described in
<a href="Overview.html#Overview">Overview</a>, but the checksum may be ignored by the receiver.

   <p>If the stub supports &lsquo;<samp><span class="samp">QStartNoAckMode</span></samp>&rsquo; and prefers to operate in
no-acknowledgment mode, it should report that to <span class="sc">gdb</span>
by including &lsquo;<samp><span class="samp">QStartNoAckMode+</span></samp>&rsquo; in its response to &lsquo;<samp><span class="samp">qSupported</span></samp>&rsquo;;
see <a href="qSupported.html#qSupported">qSupported</a>. 
If <span class="sc">gdb</span> also supports &lsquo;<samp><span class="samp">QStartNoAckMode</span></samp>&rsquo; and it has not been
disabled via the <code>set remote noack-packet off</code> command
(see <a href="Remote-Configuration.html#Remote-Configuration">Remote Configuration</a>),
<span class="sc">gdb</span> may then send a &lsquo;<samp><span class="samp">QStartNoAckMode</span></samp>&rsquo; packet to the stub. 
Only then may the stub actually turn off packet acknowledgments. 
<span class="sc">gdb</span> sends a final &lsquo;<samp><span class="samp">+</span></samp>&rsquo; acknowledgment of the stub's &lsquo;<samp><span class="samp">OK</span></samp>&rsquo;
response, which can be safely ignored by the stub.

   <p>Note that <code>set remote noack-packet</code> command only affects negotiation
between <span class="sc">gdb</span> and the stub when subsequent connections are made;
it does not affect the protocol acknowledgment state for any current
connection. 
Since &lsquo;<samp><span class="samp">+</span></samp>&rsquo;/&lsquo;<samp><span class="samp">-</span></samp>&rsquo; 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.

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