title: Why should I use, or not use, MSG_CONFIRM?
body: <p>I am acquainting with BSD sockets, and flicking through the man page of <code>sendto</code>, I bumped into <code>MSG_CONFIRM</code> flag, which is quite mysterious to me at the moment.</p>
  
  <p>The description says:</p>
  
  <blockquote>
    <p>Tell the link layer that forward progress happened: you got a
    successful reply from the other side. If the link layer doesn't get
    this it will regularly reprobe the neighbor (e.g., via a unicast ARP).
    Only valid on SOCK_DGRAM and SOCK_RAW sockets and currently
    implemented only for IPv4 and IPv6.</p>
  </blockquote>
  
  <p>After a quick look at the man page of <code>arp</code>, I understand that flagging something <code>MSG_CONFIRM</code> prevents the ARP mapping MAC address ↔ IP address of the remote machine from being considered stale.</p>
  
  <p>Now I am puzzled because I can’t see any reason why I should <strong>not</strong> put it, and therefore, why didn’t they enforce that directly in the library. Why is the application layer expected to deal with anything that happens down there at the link layer.</p>
  
  <p>So did I miss anything? when should I set it, or not set it?</p>