title: Why is 200 OK retransmission handled by UAS in SIP Protocol?
body: <p>Initially I was looking for an answer to why ACKs to 2xx responses form a separate SIP Transaction, while ACKs to non 2xx responses don't. Google gave me this: </p>
  
  <p>"When the UAC receives 200 OK, the client transaction is destroyed at TL.</p>
  
  <p>This is done because, the TL is unaware of the above core. The above
  core can be a Proxy or an UAC core.</p>
  
  <p>In case of proxy, the 200 OK is forwarded whereas in case of UAC, an
  ACK is generated. Now this ACK has to
  create a new transaction (transaction created by INVITE had been
  destroyed)at TL for its transmission, hence the ACK for 200 OK is
  outside the INVITE transaction.</p>
  
  <p>For other non-2xx final responses, the client transaction at TL is not
  destroyed and the ACK is generated by TL.</p>
  
  <p>Hence in this case, the ACK is within the transaction."</p>
  
  <p><a href="http://sipforum.org/pipermail/discussion/2011-June/008499.html" rel="nofollow">http://sipforum.org/pipermail/discussion/2011-June/008499.html</a></p>
  
  <p>Now my next question was why is the client transaction destroyed at TL upon reception of 200 Ok. Is it because ACKs are sent directly to UAS without passing intermediate proxies? (so proxies will never know if the INVITE transaction is actually completed).</p>
  
  <p>Another possibly related issue I am having is that I don't understand why retransmission of 200 OK is done end-by-end. Is there a reason why retransmission is not done in hop-by-hop manner?</p>