title: Are utm_source and utm_medium always required for link tagging in Google Analytics?
body: <p>According to every source I've found, in order to manually tag links for campaign tracking in Google Analytics 3 fields are required - <code>utm_campaign</code>, <code>utm_source</code>, and <code>utm_medium</code>. However, I get mixed results when I don't use the latter two.<br><br>
  If I set <code>utm_campaign</code> and <code>utm_source</code>, the campaign is tracked in GA with a medium value of <code>(not set)</code>. This seems to work dependably.</p>
  
  <p>If I set <code>utm_campaign</code> in the href of an anchor tag on another site and leave out <code>utm_source</code>, the campaign source shows up as the referring domain when using the <a href="http://code.google.com/apis/analytics/docs/tracking/gaTrackingTroubleshooting.html#gaDebug" rel="nofollow noreferrer">GA debugging script</a> but no campaign name is listed nor does not seem to track properly when I look at the reports in Google Analytics. For example, if on www.referringsite.com i have an anchor that looks like 
  <br><code>&lt;a href="http://www.mysite.com?utm_campaign=test"&gt;click me&lt;/a&gt;</code><br>
  The debug tool will show <code>Campaign Source: referringsite</code> and the parameter <code>Campaign Name</code> is not listed.</p>
  
  <p>What is going on here? When do I need to use these parameters and when can I leave them out? My ultimate goal is to allow the referrer to be used as the campaign source when <code>utm_source</code> is not explicitly set, and to leave <code>utm_medium</code> out altogether (which it appears I can do without issue). Can someone explain concretely what has to be used when and where, as well as how to achieve the desired effect of using httpReferrer as the implicit campaign source?</p>