title: H.225 User Information Packet Parsing
body: <p>I'm writing some code using PacketDotNet and SharpPCap to parse H.225 packets for a VOIP phone system. I've been using Wireshark to look at the structure, but I'm stuck. I've been using <a href="http://www.ethereal.com/lists/ethereal-users/200311/msg00001.html" rel="nofollow noreferrer">This</a> as a reference.</p>
  
  <p>Most of the H.225 packets I see are user information type with an empty message body and the actual information apparently shows up as a list of NonStandardControls in Wireshark. I thought I'd just extract  out these controls and parse them later, but I don't really know where they start.</p>
  
  <p>In almost all cases, the items start at the 10th byte of the H.225 data. Each item appears to begin with the length which is recorded as 2 bytes. However, I am getting a packet that has items starting at the 11th byte.</p>
  
  <p>The only difference I see in this packet is something in the message body supposedly called open type length which has a value of 1, whereas the rest all appear to be 0. Would the items start at 10 + open type length? Is there some document that explains what this open type length is for?</p>
  
  <p>Thanks.</p>