title: What is the use of from-tag in SIP request?
body: <p>I have a trivial doubt with respect to SIP. 
  I tried googling and referring many books, but still I am not able to find a solid reason for adding from-tag in SIP request.</p>
  
  <p>Example SIP request (Snapshot from rfc-3261)</p>
  
  <pre><code>INVITE sip:bob@biloxi.com SIP/2.0
  Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bK776asdhds
  Max-Forwards: 70
  To: Bob &lt;sip:bob@biloxi.com&gt;
  From: Alice &lt;sip:alice@atlanta.com&gt;;tag=1928301774
  Call-ID: a84b4c76e66710@pc33.atlanta.com
  CSeq: 314159 INVITE
  Contact: &lt;sip:alice@pc33.atlanta.com&gt;
  Content-Type: application//sdp
  Content-Length: 142
  </code></pre>
  
  <p>As per rfc-3261:</p>
  
  <p>Dialog can be identified using from-tag, to-tag and call-id.</p>
  
  <p>I know the precise reason for adding to-tag and call-id. What I don't know is,</p>
  
  <ol>
  <li>Why do we need from-tag ?  </li>
  <li>What would be the impact if from-tag was absent (assuming that
  rfc-3261 doesn't mandates it)?</li>
  </ol>