<p>The latest hardening of the gnrc_tcp option parser was a bit too restrictive.</p>
<p>Packets with unknown options were dropped by gnrc_tcp. This leads to problems then communicating with feature complete tcp implementations, since valid options like SACK are often announced in first SYN Packet. Without this PR such a SYN would be dropped by gnrc_tcp.</p>
<p>This PR allows packets with unknown options to be valid as long as they respect the boundaries of the option field and the option minimal length.</p>

<hr>

<h4>You can view, comment on, or merge this pull request online at:</h4>
<p>  <a href='https://github.com/RIOT-OS/RIOT/pull/12298'>https://github.com/RIOT-OS/RIOT/pull/12298</a></p>

<h4>Commit Summary</h4>
<ul>
  <li>gnrc_tcp: allow unknown options</li>
</ul>

<h4>File Changes</h4>
<ul>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12298/files#diff-0">sys/net/gnrc/transport_layer/tcp/gnrc_tcp_option.c</a>
    (15)
  </li>
</ul>

<h4>Patch Links:</h4>
<ul>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/12298.patch'>https://github.com/RIOT-OS/RIOT/pull/12298.patch</a></li>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/12298.diff'>https://github.com/RIOT-OS/RIOT/pull/12298.diff</a></li>
</ul>

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