title: When is IPPROTO_UDP required?
body: <p>When is IPPROTO_UDP required?</p>
  
  <p>Is there ever a case where UDP is <strong>not</strong> the default protocol for SOCK_DGRAM? (real cases, not hypothetical "it might be", please")</p>
  
  <p>i.e., what are the situations where the following two lines would <strong>not</strong> produce identical behavior?</p>
  
  <pre><code>if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1)
  if ((s=socket(AF_INET, SOCK_DGRAM, 0))==-1)
  </code></pre>