title: Why am I getting SIP response 400: Bad Session Description?
body: <p>I dumped The following SIP INVITE datagram from Linphone to a file with CR-LF line breaks, using wireshark:</p>
  
  <pre><code>INVITE sip:1002@172.16.76.21 SIP/2.0 
  Via: SIP/2.0/UDP 172.16.76.21:5060;rport;branch=z9hG4bK1936726928 
  From: &lt;sip:1555@172.16.76.21&gt;;tag=1350138383 
  To: &lt;sip:1002@172.16.76.21&gt; 
  Call-ID: 1393698667 
  CSeq: 20 INVITE 
  Contact: &lt;sip:1555@172.16.76.20&gt; 
  Content-Type: application/sdp 
  Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY, MESSAGE, SUBSCRIBE, INFO 
  Max-Forwards: 70 
  User-Agent: Linphone/3.5.2 (eXosip2/3.6.0) 
  Subject: Phone call 
  Content-Length:   205 
  
  v=0 
  o=1555 1125 1125 IN IP4 172.16.76.21 
  s=Talk 
  c=IN IP4 172.16.76.21
  t=0 0 
  m=audio 7078 RTP/AVP 8 0 101 
  a=rtpmap:8 PCMA/8000 
  a=rtpmap:0 PCMU/8000 
  a=rtpmap:101 telephone-event/8000 
  a=fmtp:101 0-11
  </code></pre>
  
  <p>I wrote a simple Python script that reads the file binary, puts in a UDP datagram and sends through a a socket bound to port 5060. When I send this to a client running user agent, I get 200 OK. When I try to send it to our SIP proxy, FreeSwitch, I get 400 Bad Session Description. </p>
  
  <p>FreeSwitch responded with 200 OK when this message was originally sent by Linphone.</p>