title: Is UPDATE allowed before PRACK is not received from the network to the earlier 18x(Reliable Provisional Response)
body: <p>Invite carries offer from the network.
  Can we send Update with sdp before the PRACK for first 18x is pending from the network, 18x sent was reliable provisional response?</p>
  
  <p>RFC 3261 says 
    "After the first reliable provisional response for a request has been
     acknowledged, the UAS MAY send additional reliable provisional
     responses.  The UAS MUST NOT send a second reliable provisional
     response until the first is acknowledged.  After the first, it is
     RECOMMENDED that the UAS not send an additional reliable provisional
     response until the previous is acknowledged.  The first reliable
     provisional response receives special treatment because it conveys
     the initial sequence number.  If additional reliable provisional
     responses were sent before the first was acknowledged, the UAS could
     not be certain these were received in order."</p>