<p><b>@smlng</b> requested changes on this pull request.</p>

<p>first review round mostly on documentation, there should also be a reference to RFC 6690 which this is implementing</p><hr>

<p>In <a href="https://github.com/RIOT-OS/RIOT/pull/11189#discussion_r292785170">sys/clif/Makefile</a>:</p>
<pre style='color:#555'>> @@ -0,0 +1,3 @@
+MODULE = clif
</pre>
<p>not needed, module name is autogenerated from path in <code>sys</code></p>

<hr>

<p>In <a href="https://github.com/RIOT-OS/RIOT/pull/11189#discussion_r292788147">sys/clif/include/clif_internal.h</a>:</p>
<pre style='color:#555'>> +#define LF_LINK_SEPARATOR_C ','
+
+/**
+ * @brief link format parameter separator character
+ *
+ */
+#define LF_PARAM_SEPARATOR_C ';'
+
+/**
+ * @brief link format attribute separator character
+ */
+#define LF_PARAM_ATTR_SEPARATOR_C '='
+
+/**
+ * @{
+ * @brief link format anchor parameter string and size
</pre>
<p>this grouping looks odd, is this correct doxygen? IIRC grouping should use <code>@name</code> instead of <code>@brief</code> and should like:</p>
<pre><code>/**
 * @name link format anchor parameter string and size
 * @{
 */
#define LF_PARAM_ANCHOR      "anchor"  /**< param name */
#define LF_PARAM_ANCHOR_S    (6)       /**< param name length */
/** @} */
</code></pre>

<hr>

<p>In <a href="https://github.com/RIOT-OS/RIOT/pull/11189#discussion_r292788227">sys/clif/include/clif_internal.h</a>:</p>
<pre style='color:#555'>> +#define LF_LINK_SEPARATOR_C ','
+
+/**
+ * @brief link format parameter separator character
+ *
+ */
+#define LF_PARAM_SEPARATOR_C ';'
+
+/**
+ * @brief link format attribute separator character
+ */
+#define LF_PARAM_ATTR_SEPARATOR_C '='
+
+/**
+ * @{
+ * @brief link format anchor parameter string and size
</pre>
<p>I think the extra documentation on each define is optional</p>

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